63. Kent, BAP; Dashtgard, SE; Huang, CQ; MacEachern, JA; Gibson, HD; Cathyl-Huhn, G.Initiation and early evolution of a forearc basin: Georgia Basin, Canada.Basin Res., 2020, 32: 163-185 Initiation and early evolution of a forearc basin: Georgia Basin, Canada
coal; forearc basins; Late Cretaceous; sequence stratigraphy; stratigraphy; T-R sequences; transgression
The lower Nanaimo Group was deposited in the (forearc) Georgia Basin, Canada and records the basin's initiation and early depositional evolution. Nanaimo Group strata are subdivided into 11 lithostratigraphic units, which are identified based on lithology, paleontology, texture and position relative to both the basal nonconformity and to each other. Significant topography on the basal nonconformity, however, has resulted in assignment of lithostratigraphic units that are not time correlative, and hence, cannot reliably be used to accurately reconstruct basin evolution. Herein, we present a sequence stratigraphic framework for lower Nanaimo Group strata in the Comox Sub-Basin (northern Georgia Basin) that integrates both facies analysis and maximum depositional ages (MDAs) derived from detrital zircon. This stratigraphic framework is used to define significant sub-basin-wide surfaces that bound depositional units and record the evolution of the basin during its early stages of development. Seven distinct depositional phases are identified in the lower 700 m of the lower Nanaimo Group. Depositional phases are separated by marine flooding surfaces, regressive surfaces, or disconformities. The overall stratigraphy reflects net transgression manifested as an upwards transition from braided fluvial conglomerates to marine mudstones. Transgression was interrupted by periods of shoreline progradation, and both facies analysis and MDAs reveal a disconformity in the lowermost part of the Nanaimo Group in the Comox Sub-Basin. Stratigraphic reconstruction of the Comox Sub-Basin reveals two dominant depocenters (along depositional strike) for coarse clastics (sandstones and conglomerates) during early development of the Georgia Basin. The development and position of these depocenters is attributed to subduction/tectonism driving both subsidence in the north-northwest and uplift in the central Comox Sub-Basin. Our work confirms that in its earliest stages of development, the Georgia Basin evolved from an underfilled, ridged forearc basin that experienced slow and stepwise drowning to a shoal-water ridged forearc basin that experienced rapid subsidence. We also propose that the Georgia Basin is a reasonable analogue for ridged forearc basins globally, as many ridged forearcs record similar depositional histories during their early evolution. DOI
60. Chateau, CCF; Dashtgard, SE; MacEachern, JA; Hauck, TE.Parasequence architecture in a low-accommodation setting, impact of syndepositional carbonate epikarstification, McMurray Formation, Alberta, Canada.Mar. Pet. Geol., 2019, 104: 168-179 Parasequence architecture in a low-accommodation setting, impact of syndepositional carbonate epikarstification, McMurray Formation, Alberta, Canada
Parasequences; Depositional units; Accommodation space; Syndepositional epikarst associated subsidence
The Sparrow Paleovalley (SPV) is situated in the southwest corner of the McMurray Sub-Basin, Alberta, Canada, and drained a portion of the Grosmont Highlands on a sub-Cretaceous paleotopographic surface. The regional stratigraphic architecture of the Lower Cretaceous McMurray Formation in SPV comprises a series of regressive parasequences (PS) organized into depositional units (DU), the latter of which are separated by regionally mappable flooding surfaces. Isopach maps of individual DUs show that in general they have a consistent thickness, although stratal thickening of DUs occurs in the landward part of the SPV and extends over an area of 400 km(2). This stratal thickening ranges from +21% to +45% and is accompanied by increases in both the abundance of soft-sediment deformation structures (SSD) and normal micro-faults in PSs, and an increase in the number and thickness of PSs. The thickness disparity between the over-thickened zone compared to the rest of the SPV is statistically significant and points to an external control on the accumulation of DUs. The McMurray Formation unconformably overlies a complex distribution of Devonian strata that subcrop at the sub-Cretaceous unconformity below the SPV. Devonian strata include, from east to west (oldest to youngest), the Beaverhill Lake, Woodbend, and Winterburn groups. In the western reaches of the SPV, McMurray Fm deposits overlie and are flanked by strata of the Woodbend and Winterburn groups, the latter of which experienced erosion and carbonate epikarstification when subaerially exposed. Subsidence through epikarstification of Devonian carbonates generated localized deepening in the McMurray Sub-Basin, which was infilled during deposition of DUs and resulted in over-thickening of McMurray strata. The correspondence of topographic lows on the sub-Cretaceous unconformity and over-thickening of DUs overlying these topographic lows indicates that active carbonate epikarstification of Devonian strata was contemporaneous with deposition of the McMurray Formation. DOI
59. Huang, C; Dashtgard, SE; Kent, BAP; Gibson, HD; Matthews, WA.Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon.Sci Rep, 2019, 9: 15360 Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon
Convergent-margin basins (CMBs) are commonly associated with active arcs, and hence are rich in detrital zircon (DZ) whose ages closely reflect the timing of deposition. Consequently, maximum depositional ages (MDA) from DZ geochronology can be employed to resolve the stratigraphy and evolution of CMBs. Herein, we use DZ to revise the internal architecture of the lower Nanaimo Group, which partially comprises the fill of the (forearc) Georgia (or Nanaimo) Basin. Maximum depositional ages and multi-dimensional scaling of DZ age distributions are employed to determine chronologic equivalency of strata and assess sediment provenance variability within the pre-existing lithostratigraphic framework. The results are compared to a recently developed sequence stratigraphic framework for the lower Nanaimo Group. The basal lithostratigraphic unit of the Nanaimo Group, the Comox Formation (Fm), comprises strata that are neither time correlative nor genetically related. The three lithostratigraphic units directly overlying the Comox Fm (Haslam, Extension, and Protection formations) comprise strata with similar genetic affinities and MDAs that indicate deposition of these units was not always sequential and locally was contemporaneous. Through this work, we provide an example of how MDAs from DZ geochronology in CMBs can resolve basin-scale stratigraphic relations, and identify chronological changes in sediment provenance. DOI PubMed
58. La Croix, AD; Dashtgard, SE; MacEachern, JA.Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: Example from the McMurray Formation, Canada.Geosci. Front., 2019, 10: 2219-2238 Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: Example from the McMurray Formation, Canada
Fluvial-tidal transition; McMurray Formation; Fraser river; Inclined heterolithic stratification; Modern analogue; Quantitative sedimentology and ichnology
The fluvial-tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FIT generally consists of three broad zones: (1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial-tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FIT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley ("A" Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FIT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial-tidal channels. Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north. Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the "A" Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FIT zone appears to have extended for several hundred kilometers from south to north. Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems. (C) 2019. China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. DOI
57. Lei, ZH; Dashtgard, SE; Wang, J; Li, M; Feng, QL; Yu, Q; Zhao, AK; Du, LT.Origin of chert in Lower Silurian Longmaxi Formation: Implications for tectonic evolution of Yangtze Block, South China.Paleogeogr. Paleoclimatol. Paleoecol., 2019, 529: 53-66 Origin of chert in Lower Silurian Longmaxi Formation: Implications for tectonic evolution of Yangtze Block, South China
Microcrystalline silica; Productivity; Hydrothermal plume; Extensional tectonics; Compressional tectonics; Rhuddanian
Upper Ordovician (Katian) to Lower Silurian (Rhuddanian) strata in the Middle-Upper Yangtze Block, South China contain extensive chert beds, although the origin of microcrystalline silica in these cherts is largely unknown. In this study, the mineralogy and structure of chert as well as major, trace, and rare earth elements are used to determine the origin of microcrystalline silica in 29 chert samples from Longmaxi Formation (Rhuddanian) in Baizitian outcrop, west of Kangdian Uplift (KU). Results show that 21 of the 29 chert samples have a biogenic origin, 2 indicate a hydrothermal origin, and 6 show a mixed origin. Cherts from Baizitian outcrop are compared to contemporaneous cherts deposited east of KU. Cherts both east and west of KU are mainly biogenic in origin, which is attributed to high productivity in the paleo-seaway. Hydrothermal microcrystalline silica is relatively uncommon in the Longmaxi Formation across the Middle-Upper Yangtze Block, and only occurs in Baizitian outcrop. Terrigenous influx contributed to chert accumulation both east and west of KU; however, cherts east of KU received much more clastic material than those in Baizitian outcrop. A comparison of chert composition and geochemistry to geological reconstructions of the Middle-Upper Yangtze Block reveal that accumulation of hydrothermal chert is closely linked to tectonic evolution. During the Early Silurian, the Middle-Upper Yangtze Block was in a compressional tectonic regime with limited heat supply, and hence, low hydrothermal circulation; this is manifested in the paucity of hydrothermal microcrystalline silica in cherts east of KU. In contrast, hydrothermal microcrystalline silica in cherts west of KU is interpreted to have formed in an extensional tectonic regime with sufficient heat supply. This research demonstrates that chert source is, in part, tectonic controlled, and chert geochemistry can be used in reconstructing paleoenvironments. DOI
56. Morshedian, A; MacEachern, JA; Dashtgard, SE; Bann, KL; Pemberton, SG.Systems tracts and their bounding surfaces in the low- accommodation Upper Mannville group, Saskatchewan, Canada.Mar. Pet. Geol., 2019, 110: 35-54 Systems tracts and their bounding surfaces in the low- accommodation Upper Mannville group, Saskatchewan, Canada
Mannville group; Sparky formation; Waseca formation; McLaren formation; Sequence stratigraphy; Facies analysis; Ichnology; Systems tract; Autogenic surfaces; Allogenic surfaces
Sequence stratigraphic interpretations of paralic successions can be challenging, because such settings are commonly controlled by a combination of allogenic controls and autogenic processes, leading to complex depositional architectures. In low-accommodation systems where parasequences are less than 15 m thick, the challenges in recognizing discrete stratigraphic surfaces are exacerbated by limited vertical separation between surfaces or their outright erosional amalgamation, requiring high-resolution facies analysis to resolve their geometries. Herein, we present criteria derived from core data and well logs to differentiate between allogenic and autogenic surfaces in a low-accommodation paralic setting, using the Lower Cretaceous Mannville Group of west-central Saskatchewan, Canada as a case study. The upper part of the Mannville Group (Sparky, Waseca and McLaren formations) displays complex stratigraphic relationships related to base-level changes. Marine flooding surfaces (MFS) and transgressive surfaces of erosion (TSE) of 3rd and 4th order record allogenic changes (e.g., basin subsidence and/or eustatic changes in sea level), and are used to define parasequence boundaries of strandplain-shoreface successions. Deltaic deposits not only possess these allogenic transgressive surfaces, but autogenic 5th-order flooding surfaces, which are broadly similar in appearance, as well. These autogenic surfaces record responses to channel avulsion or lobe abandonment, separate discrete delta lobes, and display limited lateral extents. Shoreface-related flooding surfaces, by contrast, bound true allogenic parasequences and, therefore, have regional correlatability. The Waseca Formation also possesses surfaces related to base-level fall. To the north, a subtle regressive surface of marine erosion (RSME) marks the base of the falling stage systems tract. Elsewhere, a widespread subaerial unconformity (SU) separates the Waseca into lower and upper members. In most areas, the SU is amalgamated with surfaces generated by later base-level rise. Upper Mannville strata in the study area, therefore, can be separated into parts of two depositional sequences. The main deposits of the lower sequence comprise a highstand systems tracts (HST) in the Sparky Formation. The base of the Lower Waseca Member marks the onset of a transgressive systems tract (TST), associated with widespread shoreline retreat. A 3rd-order maximum flooding surface (MxFS) marks the end of transgression and resumption of normal progradational regression in a highstand systems tract associated with the Lower Waseca Member. Following highstand normal regression, a major base-level fall initiated a subaerial unconformity that marks the base of the upper sequence. Fluvial valley incision led to sediment bypass and deposition of forced regressive and lowstand shoreface and delta complexes of the falling stage systems tract (FSST) and lowstand systems tract (LST), respectively, in the northern part of the study area. Pedogenic modification of subaerially exposed sediment occurred in interfluve areas. Ensuing TST accumulation was initially confined to the tidal-fluvial estuarine infill of the incised valleys and transgressive bay deposits of the Upper Waseca Member. The Upper Waseca is capped by a 3rd-order maximum flooding surface (MxFS), which separates it from the overlying McLaren Formation. This marks the return to regional shoreline progradational regression and corresponds to the highstand systems tract (HST) of the upper sequence. DOI
55. Niu, FF; Liu, ZY; O'Neil, D; Dashtgard, S; Liu, JT.Study of a novel density well-logging tool using A position-sensitive detector.Appl. Radiat. Isot., 2019, 154 Study of a novel density well-logging tool using A position-sensitive detector
Density logging tool; Monte Carlo simulation; Vertical resolution; Position-sensitive detector
Density well-logging tool is an essential instrument used in the petroleum industry for determining formation density, porosity, and lithology of subsurface strata. To improve the vertical resolution and accuracy of density measurements in thin-bedded sedimentary rocks, we proposed an innovative density well-logging tool in which the traditional short- and long-spacing detectors are replaced with a position-sensitive detector (PSD). This replacement enables the measurement of the continuous gamma-ray flux in the area near the PSD. To demonstrate the feasibility of this new tool, we first simulated the performance of a PSD with NaI(Tl) crystal by using the Monte Carlo simulation method (GEANT4). The outcome shows that both gamma-ray deposited energy and interaction position can be determined by the photoelectron yield at each end of the detector, and the resolutions for energy and position are 19.0% and 1.3 cm, respectively, when the gamma-ray energy is 100 keV. Next, we created a Monte Carlo model of a density well-logging tool using such a PSD and simulated its response in a geologic model of thin-bedded strata. The result proves that the proposed tool can yield a good density evaluation when the thickness of the layer is 5 cm, which is significantly better than that obtained with the 16 cm-thick layer of traditional density tools. This study confirms the feasibility of using a PSD in density well-logging tool in theory and indicates that the proposed tool has a promising application in measuring the density of thin-bedded strata. DOI PubMed
54. Rinke-Hardekopf, L; Dashtgard, SE; MacEachern, JA.Earliest Cretaceous Transgression of North America Recorded in Thick Coals: McMurray Sub-Basin, Canada.Int. J. Coal Geol., 2019, 204: 18-33 Earliest Cretaceous Transgression of North America Recorded in Thick Coals: McMurray Sub-Basin, Canada
Coal; Terrestrial stratigraphy; Petrography; Coastal plain; Relative sea-level; CA-TIMS
Strata of the Lower Cretaceous McMurray Formation in the McMurray Sub-Basin (MSB) of Alberta and Saskatchewan, Canada, comprise marginal marine, tidal-fluvial, fluvial and interfluve deposits. In the Firebag Tributary located in the northeastern portion of the sub-basin, a thick coal seam (up to > 20 m) forms a semi continuous bed at the top of the lowermost McMurray Fm deposits (herein referred to as Lower McMurray). In coastal plain settings, peat-forming mires are highly sensitive archives of base-level. We investigate the stratigraphic significance of coals in the Firebag Tributary by analyzing coal petrography (4 cores), coal geochemistry (2 cores), and coal distribution in similar to 4500 wells. Additionally, we record the first absolute age for the McMurray Fm through the dating of an ash layer within the coal seam. The petrographic character and distribution of coals in the northeastern part of the MSB suggest a complex history of base-level rise at the end of Lower McMurray deposition. Wetting-upward character coal seams record initial transgression of the Boreal Sea during a 4th order sea level rise (Milankovitch-scale) within lowstand to early transgressive systems tract conditions. Sea-level rise during Lower McMurray times was estimated based on temperate climate peat formation rates, as well as peat-to-coal compaction factors, and probably occurred on a scale of 0.5-3 mm a(-1). Seam-internal wetting-upward cycles suggest that base-level rise was punctuated. Locally, compound and drying-upward character coals represent zones of high accommodation, wherein peat accumulation in mires was influenced by phases of syn-depositional subsidence and karst collapse. The thickest coals in the study area form an arc on a coastal plain as the Boreal Sea inundated east- to southeastward into the Firebag Tributary. Within the arc of thick coals, a north-south decrease in coal thickness is interpreted to reflect the general southward inundation of the Boreal Sea across the MSB. Exceptionally thick coals (> 10 m) likely formed where basement processes (e.g., karsting in Devonian carbonates) provided preferential groundwater pathways. Chemical abrasion thermal ionization mass spectrometry dating of 5 sharply faceted zircons recovered from an ash bed within the coal provides the first absolute age (121.39 +/- 0.2 Ma - Aptian) to the southward transgression of the Boreal Sea. We hypothesize that initial inundation of the MSB was related to tectonic activity in the Canadian Cordillera and the onset of magmatic flow in the western Coast Plutonic Complex. DOI
53. Weleschuk, ZP; Dashtgard, SE.Evolution of an ancient (Lower Cretaceous) marginal-marine system from tide-dominated to wave-dominated deposition, McMurray Formation.Sedimentology, 2019, 66: 2354-2391 Evolution of an ancient (Lower Cretaceous) marginal-marine system from tide-dominated to wave-dominated deposition, McMurray Formation
Fluvio-tidal; McMurray; oilsands; parasequences; tide-dominated; wave-dominated
Regionally extensive parasequences in the upper McMurray Formation, Grouse Paleovalley, north-east Alberta, Canada, preserve a shift in depositional processes in a paralic environment from tide domination, with notable fluvial influence, through to wave domination. Three stacked parasequences form the upper McMurray Formation and are separated by allogenic flooding surfaces. Sediments within the three parasequences are grouped into three facies associations: wave-dominated/storm-dominated deltas, storm-affected shorefaces to sheltered bay-margin and fluvio-tidal brackish-water channels. The two oldest parasequences comprise dominantly tide-dominated, wave-influenced/fluvial-influenced, shoreface to bay-margin deposits bisected by penecontemporaneous brackish-water channels. Brackish-water channels trend approximately north-west/south-east, which is perpendicular to the interpreted shoreline trend; this implies that the basinward and progradational direction was towards the north-west during deposition of the upper McMurray Formation in Grouse Paleovalley. The youngest parasequence is interpreted as amalgamated wave-dominated/storm-dominated delta lobes. The transition from tide-dominated deposition in the oldest two parasequences to wave-dominated deposition in the youngest is attributed mainly to drowning of carbonate highlands to the north and north-west of the study area, and potentially to relative changes in accommodation space and deposition rate. The sedimentological, ichnological and regional distribution of the three facies associations within each parasequence are compared to modern and Holocene analogues that have experienced similar shifts in process dominance. Through this comparison it is possible to consider how shifts in depositional processes are expressed in the rock record. In particular, this study provides one of few ancient examples of preservation of depositional process shifts and showcases how topography impacts the character and architecture of marginal-marine systems. DOI
52. Jones, MT; Dashtgard, SE; MacEachern, JA.A conceptual model for preservation of thick, transgressive shoreline successions with examples from a forearc basin: Nanaimo Basin, British Columbia, Canada.J. Sediment. Res., 2018, 88: 811-826 A conceptual model for preservation of thick, transgressive shoreline successions with examples from a forearc basin: Nanaimo Basin, British Columbia, Canada
Sequence stratigraphic models derived from passive margins and foreland basins characterize transgressive shorelines (excluding incised valleys) as recording periods of sediment starvation, and are commonly represented as thin (< 1 m) coarse-grained lags overlain by offshore mudstones. Preservation of thick, transgressive successions is relatively uncommon, but is possible in settings characterized by high sedimentation rates coupled with rapid base-level rise. Examples of such settings are found in deposits of the Upper Cretaceous Nanaimo Group, exposed on Vancouver Island and the Gulf Islands of British Columbia, Canada. The Nanaimo Basin is interpreted as a rapidly subsiding forearc basin that formed as a result of accretion of Wrangellia Terrane during subduction of the Farallon Plate beneath North America. The transgressive units of the Comox and Protection formations in the Nanaimo Basin display thick shoreline successions ranging from 3 m to 66 m, wherein base-level changes were dominantly controlled by tectonics. By comparing our results to previously studied transgressive successions and to mechanisms leading to transgression, a conceptual model for predicting transgressive architectures is proposed. Thick transgressive shoreline successions (> 10 m) are more likely to be developed in active tectonic settings (e.g., forearc and rift basins), which generally experience high sedimentation rates and relatively rapid changes in base level. Settings with high sedimentation rates and moderate base-level rise lead to progradational trends despite overall transgression of the shoreline. In settings where changes in eustatic sea level mainly control the resulting stratigraphic architecture (e.g., passive margins), thin transgressive successions (1-10 m) are more likely to be produced. Finally, in settings defined by relatively slow base-level change (e.g., foreland basins), the resultant transgressive succession typically comprises a lag (< 1 m) overlain by offshore mudstones. This conceptual model highlights the importance of the interplay between sedimentation rates and base-level rise, and is intended as a guide for the predictive modeling of sedimentary basins. DOI
51. Brekke, H; MacEachern, JA; Roenitz, T; Dastgard, SE.The use of microresistivity image logs for facies interpretations: An example in point-bar deposits of the McMurray Formation, Alberta, Canada.AAPG Bull., 2017, 101: 655-682 The use of microresistivity image logs for facies interpretations: An example in point-bar deposits of the McMurray Formation, Alberta, Canada
Well logs such as spontaneous potential and gamma ray historically have been the only tools available for facies evaluation of noncored wells in the McMurray Formation. The addition of microresistivity image logs has greatly improved facies identifications and interpretations, particularly when integrated with core data sets. In the case of McMurray channel complexes, core descriptions provide detail about bedding contacts, sedimentary texture, stratification, bioturbation intensity, and trace fossil diversity. Image logs provide texture, fabric, bedding contact style, dip directions and angles, and bedding architecture information, yielding paleoflow and lateral accretion directions. This study characterizes facies by integrating interpretations from 414 image logs with core-based descriptions from 138 of these wells. The reservoir targets, and most prolific depositional facies in this study, are associated with channel systems and their associated point-bar deposits. Facies identifications are based on several image log criteria. Mud clast breccias display variable dip angles and dip directions. Cross-stratified sands comprise vertical successions of stacked, internally consistent bedsets with high dip angles (>15 degrees) that indicate paleoflow direction. Lateral accretion beds show consistent dip directions with a progressive change from shallow-to-steep-to-shallow dip angles (e.g., <4 to 15 degrees to <4 degrees) from the base to the top of the succession, as well as beds that dip toward the thalweg of the paleochannel. Flat-lying (<4 degrees) mud records vertical accretion associated with point-bar tops or channel abandonment. Although this facies classification is specific to the McMurray Formation in the study area, the principles provided here are applicable to other subsurface studies and demonstrate the enhanced reliability of integrated core-image log data sets. DOI
50.Dashtgard, SE; Ainsworth, RB; MacEachern, JA.Preferential orientation of arthropod-generated Diplocraterion parallelum and their statistical reliability as paleocurrent indicators.Palaios, 2017, 32: 429-438 Preferential orientation of arthropod-generated Diplocraterion parallelum and their statistical reliability as paleocurrent indicators
Arthropods from the late Campanian (Late Cretaceous) of the Western Interior Seaway produced U-shaped Diplocraterion parallelum in mudstones along two closely spaced surfaces (10 cm apart), one of which corresponds to a maximum transgressive surface. Diplocraterion parallelum are widely distributed across both surfaces, with substantial variations in burrow orientations. Based on a comparison of paleocurrent indicators to burrow orientations, we demonstrate that D. parallelum are preferentially oriented parallel to the prevailing fair-weather wave propagation direction (wave-forced currents) that acted upon the colonized surfaces. There is an apparent maximum (34% above uniformity) preferential orientation of the burrows, attributed to the fact that wave-forced currents represent only one of several factors controlling shrimp burrowing behaviors. Based on this study, we propose that in the absence of paleocurrent data, Diplocraterion and other U-shaped burrows can be used to resolve flow directions, and that preferential burrow orientations >= 2% above uniformity are significant. However, it is noted that it is only possible to resolve 2-way flow directions (trend) from U-shaped burrows, as there is no way to determine vector directions. DOI
49. Friesen, OJ; Dashtgard, SE; Miller, J; Schmitt, L; Baldwin, C.Permeability heterogeneity in bioturbated sediments and implications for waterflooding of tight-oil reservoirs, Cardium Formation, Pembina Field, Alberta, Canada.Mar. Pet. Geol., 2017, 82: 371-387 Permeability heterogeneity in bioturbated sediments and implications for waterflooding of tight-oil reservoirs, Cardium Formation, Pembina Field, Alberta, Canada
Pembina Field; Cardium Formation; Ichnology; Permeability; Bioturbation; Stratigraphy
Bioturbated sediments recording distal expressions of paralic depositional environments are increasingly being exploited for hydrocarbons in the super-giant Pembina Field (Cardium Formation), Alberta, Canada. These strata were previously considered unproductive due to limited vertical and horizontal connectivity between permeable beds. In these "tight oil" plays (0.1-10 mD), pressure decay profile permeametry (micropermeability) data indicate that sand-filled burrows provide vertical permeable pathways between bioturbated and parallel-laminated sandstone beds in the central, northeast and northwest parts of the field. This relationship enables the economic exploitation of hydrocarbons via horizontal drilling and multi-stage hydraulic fracturing. As the exploitation of bioturbated strata progresses in the Pembina Field, additional primary targets are being sought out, and horizontal waterflooding is being considered in areas where horizontal wells exist. Proximal to historical produced conventional targets, reservoir analyses indicate that areas where the bioturbated facies average permeability lies between 035 mD and 0.85 mD and sandstone isopach thicknesses are between 0.25 m and 2.5 m should be targeted in east central Pembina. Micropermeability values enable correlation of bulk permeability from plugs and full-diameter samples to the heterogeneous permeability distributions in intensely bioturbated strata. Bulk and micro permeability data are graphically compared, and permeability distributions are mapped across the field. Using isopach thickness of bioturbated facies, production data, and permeability data, "sweet spots" are identified for placement of effective waterfioods. Production information for recently drilled horizontal wells in the Pembina Field demonstrate that bioturbated muddy sandstones and sandy mudstones in paralic environments can be economically exploited when sand-filled burrows provide connectivity between sand beds. However, well performance within these poorly understood unconventional tight oil plays can better be predicted with an in-depth characterization of their fades and complex permeability heterogeneities. Based on our results, it is clear that micropermeability analysis can be effectively employed to differentiate between economic and sub-economic plays, identify areas with high effective permeability, and high-grade areas for enhanced oil recovery schemes. (C) 2017 Elsevier Ltd. All rights reserved. DOI
48. La Croix, AD; MacEachern, JA; Ayranci, K; Hsieh, A; Dashtgard, SE.An ichnological-assemblage approach to reservoir heterogeneity assessment in bioturbated strata: Insights from the Lower Cretaceous Viking Formation, Alberta, Canada.Mar. Pet. Geol., 2017, 86: 636-654 An ichnological-assemblage approach to reservoir heterogeneity assessment in bioturbated strata: Insights from the Lower Cretaceous Viking Formation, Alberta, Canada
Viking Formation; Ichnofacies; Ichnological assemblage; Facies analysis; Shoreface; Porosity; Permeability
Facies-scale trends in porosity and permeability are commonly mapped for reservoir models and flow simulation; however, these trends are too broad to capture bed and bed-set heterogeneity, and there is a need to up-scale detailed, bed-scale observations, especially in low-permeability reservoir intervals. Here we utilize sedimentology and ichnology at the bed- and bedset-scale to constrain the range of porosity and permeability that can be expected within facies of the Lower Cretaceous Viking Formation of south-central, Alberta, Canada. Three main fades were recognized, representing deposition from the middle shoreface to the upper offshore. Amalgamated, hummocky cross-stratified sandstone fades (Facies SHcs) consist of alternations between intensely bioturbated beds and sparsely bioturbated/laminated beds. Trace fossil assemblages in bioturbated beds of Facies SHcs are attributable to the archetypal Skolithos Ichnofacies, and are morphologically characterized by vertical, sand-filled shafts (VSS). Bioturbated beds show poor reservoir properties (max: 10% porosity, mean: 85.1 mD) compared to laminated beds (max 20% porosity, mean: 186 mD). Bioturbated muddy sandstone fades (Facies S-B) represent trace fossil assemblages primarily attributable to the proximal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur in varying proportions, namely sediment-churning assemblages (SC), horizontal sand-filled tube assemblages (HSF), VSS assemblages, and mud-filled, lined, or with spreiten (MLS) assemblages. Ichnological assemblages containing horizontal (max: 30% porosity, mean: 1.28 mD) or vertical sand-filled burrows (max: 10% porosity, mean: 2.2 mD) generally have better reservoir properties than laminated beds (max: 20% porosity, mean: 0.98 mD). Conversely, ichnological assemblages that consist of muddy trace fossils have lower porosity and permeability (max 10% porosity, mean: 0.89 mD). Highly bioturbated, sediment churned fabrics have only slightly higher porosity and permeability overall (max: 15% porosity, mean: 1.29 mD). Bioturbated sandy mudstone facies (Facies M-B) contain ichnofossils representing an archetypal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur throughout Facies MB that are similar to Facies SB; SC, HSF, VSS, and MLS assemblages. The SC (max: 15% porosity, mean: 21.67 mD), HSF (max: 20% porosity, mean: 3.79 mD), and VSS (max: 25% porosity, mean: 7.35 mD) ichnological assemblages have similar or slightly lower values than the laminated beds (max: 20% porosity, mean: 10.7 mD). However, MLS assemblages have substantially lower reservoir quality (max: 10% porosity, mean: 0.66 mD). Our results indicate that the most likely occurrence of good reservoir characteristics in bioturbated strata exists in sand-filled ichnological assemblages. This is especially true within the muddy upper offshore to lower shoreface, where vertically-oriented trace fossils can interconnect otherwise hydraulically isolated laminated sandstone beds; this improves vertical fluid transmission. The results of this work largely corroborate previous findings about ichnological impacts on reservoir properties. Unlike previous studies, however, we demonstrate that the characteristics of the ichnological assemblage, such as burrow form and the nature of burrow fill, also play an important role in determining reservoir characteristics. It follows that not all bioturbated intervals (attributed to the same facies) should be treated equally. When upscaling bed-scale observations to the reservoir, a range of possible permeability porosity values can be tested for model sensitivity and to help determine an appropriate representative elementary volume. (C) 2017 Elsevier Ltd. All rights reserved. DOI
46. Martinius, AW; Fustic, M; Garner, DL; Jablonski, BVJ; Strobl, RS; MacEachern, JA; Dashtgard, SE.Reservoir characterization and multiscale heterogeneity modeling of inclined heterolithic strata for bitumen-production forecasting, McMurray Formation, Corner, Alberta, Canada.Mar. Pet. Geol., 2017, 82: 336-361 Reservoir characterization and multiscale heterogeneity modeling of inclined heterolithic strata for bitumen-production forecasting, McMurray Formation, Corner, Alberta, Canada
Bitumen pool; Reservoir characterization; Inclined heterolithic stratification; Point bar geomodel; McMurray Formation
Bitumen reservoirs dominated by inclined heterolithic stratification (IHS) formed in large point bars of the Aptian (Lower Cretaceous) McMurray Formation in the northwestern part of the Corner oil sand lease (Alberta, Canada) were investigated to establish their value. Hybrid production technologies were applied to thin pay, typified by homogeneous reservoir sand units thicker than 5 m at the base overlain by IHS (so-called 'thin pay'), as well as IHS-dominated reservoirs in which the IHS extends down to the base of the reservoir. High-resolution seismic data and well data (core, dipmeter, HMI) were used to map four facies associations, comprising a total of 16 sedimentary facies, as well as various fluid contacts to assist in reservoir characterization and risk assessment. The conceptual depositional model was based on the analysis of the migration and re-orientation history of the IHS-dominated point bars reflecting lateral accretion, downstream migration, rotation and relocation of the bars. Multiple reactivation events, which control the heterolithic nature and reservoir quality of the deposits, create developable "pools". Seven electrofacies (with generally increasing mud content) were defined and used as input to construct vertical proportion curves that relate the electrofacies distribution to geomodel statistics in the main reservoir zone. At the electrofacies scale, numerical effective porosity-permeability models were created using micromodeling and minimodeling concepts. The geometrical shape of the electrofacies in the geomodel was investigated using non-stationary Truncated Gaussian (TG) facies simulation to enforce the stacking patterns. Each geomodel area was characterized using one variogram to efficiently compute the horizontal and vertical variogram ranges and average azimuths. Sequential Gaussian simulation (SGS) was used to map the distribution of key petrophysical parameters such as effective porosity, effective water saturation and V-shale Empirically derived saturation versus elevation profiles for each electrofacies were included in the modeling. The distributions from the micro- and mini-modeling were introduced using probability field (P-field) simulation. To investigate the amount of connected resources (the degree of connectivity of good sand as well as IHS) were extractable flow simulation studies were performed at the pad scale. In preparation for reservoir simulation, connectivity calculations within the local pool geomodel realizations were tailored for the reservoir heterogeneities (i.e., IHS) that are expected to have a major impact on the specific thermal and gravity drainage extraction processes. The geomodel realizations were ranked by expected pseudo-dynamic behaviour with connected exploitable pay as a critical parameter. (C) 2017 Elsevier Ltd. All rights reserved. DOI
45. Ainsworth, RB; Vakarelov, BK; MacEachern, JA; Nanson, RA; Lane, TI; Rarity, F; Dashtgard, SE.Process-driven architectural variability in mouth-bar deposits: A case study from a mixed-process mouth-bar complex, Drumheller, Alberta, Canada.J. Sediment. Res., 2016, 86: 512-541 Process-driven architectural variability in mouth-bar deposits: A case study from a mixed-process mouth-bar complex, Drumheller, Alberta, Canada
Mouth bars can develop in locations where fluvial distributary channels deliver coarse sediment to the shoreline. As a consequence, they are typically considered, by default, to be fluvially dominated. However, depending upon the relative balance of fluvial, wave, and tidal energies at any given shoreline location, the preserved sedimentary structures, geometry, and internal architecture of the mouth bar will reflect the relative balance of these energies. Hence, mouth bars can be fluvial-, wave-, or tide-dominated and can also exhibit sedimentary structures formed by more than one of these processes. The ability to recognize, and to predict the architecture of these mixed-process systems is important for accurately interpreting the ancient record and has implications for hydrocarbon exploration and exploitation strategies. A detailed description and interpretation is given herein of a mixed-process mouth-bar complex which forms part of an asymmetrical delta system from the Campanian of Alberta, Canada. This mouth-bar complex is interpreted as a mixed-influence, wave-dominated system that transitions laterally into a mixed-influence, fluvially dominated system. The mouth-bar complex represents a single progradational phase of deposition, is up to 8 m thick, and can be traced over an area of approximately 60 km(2) in both outcrop and the subsurface. It comprises multiple, kilometer-scale lobe geometries (elements, element sets, and element complexes) with internal dipping, mudstone- and carbonaceous-draped clinoform surfaces at two scales (intra-element and inter-element set). The lobe geometries are oriented subparallel to the coastline. The influence of tides on the system is illustrated by tidal bundle sequences in intra-element mouth-bar clinoform bedsets, networks of tidally overdeepened, mudstone-filled distributary channels, and the development of significant thicknesses (up to 3.5 m) of swash-zone deposits at the tops of the wave-dominated mouth-bar successions. Depositional architectural units of the mouth-bar complex are estimated to have been generated by autogenic events from daily (tidal couplets), through decadal (elements) to centennial (element sets) and millennial (element complex and element complex set) time frames. A Holocene partial analog for these types of depositional systems is suggested from the Gulf of Carpentaria, Australia. Geometries, aspect ratios, and facies relationships of these mixed-process mouth-bar complexes vary significantly from the archetypal fluvial-dominated mouth bars described in the published literature. DOI
44. Ayranci, K; Dashtgard, SE.Asymmetrical deltas below wave base: Insights from the Fraser River Delta, Canada.Sedimentology, 2016, 63: 761-779 Asymmetrical deltas below wave base: Insights from the Fraser River Delta, Canada
Asymmetrical delta; Fraser River Delta; ichnology; neoichnology; sedimentology; tides
The Fraser River Delta exhibits distinct asymmetry in the sedimentological and neoichnological characteristics of the updrift (south) and downdrift (north) sides of the main distributary channel in water depths below storm-wave base. The asymmetry is the result of net northward tidal flow. Tides erode sediments across the updrift delta front, whereas the downdrift delta front is an area of net deposition. A submarine channel prevents sand eroded from the updrift delta front from reaching the downdrift delta. The updrift delta front and updrift upper prodelta are composed of sand or heterolithic sand and mud that show a low density of burrowing (Bioturbation Index 0 to 3) and are dominated by simple traces. The downdrift delta front and prodelta, and the updrift lower prodelta are composed of homogeneous muds with significantly higher bioturbation intensities (Bioturbation Index 3 to 6), and a more diverse suite of traces akin to Cruziana Ichnofacies. Using the Fraser River Delta as an archetype and comparing the Fraser to the Amazon River Delta, a preliminary model for deep-water (below storm-wave base: ca 20 m) asymmetrical deltas is proposed. Firstly, deep-water asymmetrical deltas are recognized from sediments deposited below storm-wave base. At these depths, tidal and ocean currents are more likely to impact sediment transport, but wave processes are less effective as a sediment transport mechanism. Sediments deposited below storm-wave base in deep-water asymmetrical deltas will display the following: (i) the updrift delta front will be coarser-grained (for example, sand-dominated or heterolithic sand and mud), than the downdrift delta front (for example, mud-dominated); and (ii) the updrift delta front should show low-diversity suites of simple burrows. Depending on sedimentation rates, the downdrift delta front and prodelta may show either high diversity suites of traces that are dominated by both complex and simple burrows (low sedimentation rates) or low density and diversity suites akin to the updrift delta front (high sedimentation rates). DOI
43.Dashtgard, SE; MacEachern, JA.Unburrowed mudstones may record only slightly lowered oxygen conditions in warm, shallow basins.Geology, 2016, 44: 371-374 Unburrowed mudstones may record only slightly lowered oxygen conditions in warm, shallow basins
Unbioturbated mudstones and highly bioturbated silty and sandy mudstones from the late Albian of Alberta, Canada, are characterized by their ichnological, foraminiferal, and geochemical signatures. A comparison of these data sets is undertaken to isolate the dissolved oxygen (DO) conditions that led to the preservation of unbioturbated mudstones versus highly bioturbated silty and sandy mudstones. Highly diverse and abundant benthic foraminiferal assemblages, coupled with conclusive geochemical signatures, indicate that unbioturbated mudstones were deposited under oxic bottom waters. The paucity of bioturbation in these rocks is attributed to the persistence of low-oxic conditions (5 > DO > 2 mg L-1) at the seafloor, comparable to the present-day Gulf of Mexico. We assert that unbioturbated mudstone should not automatically be attributed to oxygen deficiency (< 2 mg L-1). Instead, it may reflect oxygenation sufficient to support benthic microfauna (foraminifera) but insufficient to sustain a diverse ecosystem of macrofauna (burrowing fauna). Moreover, we propose that the distribution of unburrowed mudstones deposited below low-oxic waters is predictable. A paucity of bioturbation is normal in shallow marine (below fair-weather wave base to similar to 200 m water depth) deposits of subtropical to tropical ocean basins and/or semienclosed seaways. DOI
41. Ainsworth, RB; Vakarelov, BK; Lee, C; MacEachern, JA; Montgomery, AE; Ricci, LP; Dashtgard, SE.Architecture and evolution of a regressive, tide-influenced marginal-marine succession, Drumheller, Alberta, Canada.J. Sediment. Res., 2015, 85: 596-625 Architecture and evolution of a regressive, tide-influenced marginal-marine succession, Drumheller, Alberta, Canada
Tide-dominated and tide-influenced clastic depositional environments are typically interpreted to be associated with landward-stepping or transgressive systems. Here, we present an example of an overall regressive succession dominated by high-frequency sequences that exhibit abundant sedimentary and ichnological evidence of tidal processes. These strata comprise the Campanian Bearpaw-Horseshoe Canyon Formation transition exposed near Drumheller, Alberta, Canada. The clastic marine to marginal-marine sediments were deposited in mixed-process (wave-, tide-, and fluvial-influenced) depositional environments along the western margin of a foreland basin. The deposits are subdivided into six relatively thin (10 m on average), high-frequency (similar to 140,000 years), transgressive-regressive (T-R) sequences (A to F). Outcrop sedimentary logs and hand-held gamma-ray profiles were correlated to subsurface data (4 cored wells and 75 wells with wireline logs) within an area of 22 km x 16 km in order to generate a 3D geocellular computer model. This model along with paleocurrent data from outcrop were used to analyze the stratigraphic architecture and evolution of the succession. The evolution of the successive T-R sequences was mapped on a subregional scale using the 3D geocellular model. The orientations of the paleoshorelines rotate in a predictable manner during transgression and regression. Transgressive shorelines are oriented approximately S-N and rotate clockwise during regression to approximately SW-NE. The rotations are attributed to the paleogeography of the region, with the study area interpreted to sit on the western flank of a large structurally controlled embayment. The progradational high-frequency sequence set is dominated by tide-influenced strata and clearly demonstrates that tidal deposits can be preserved in both regressive as well as transgressive successions. Tidal influence in this regressive setting is attributed to shallow water depths on a wide shelf, which resulted in amplification of the tidal wave as it moved across the shelf. The embayed character of the coastline may also have augmented this effect. Tidal range is estimated to have been mesotidal to macrotidal. A modern partial analogue for the Horseshoe Canyon systems is identified from the tropical Mitchell River Delta in the Gulf of Carpentaria, Queensland, Australia. This delta system is analogous in terms of its tide-influenced facies, mesotidal range, low-gradient shelf with accompanying shallow water depths, the mixed-process character of its shoreline systems, and its horizontal to falling shoreline trajectory. DOI
39.Dashtgard, SE; Snedden, JW; MacEachern, JA.Unbioturbated sediments on a muddy shelf: Hypoxia or simply reduced oxygen saturation?Paleogeogr. Paleoclimatol. Paleoecol., 2015, 425: 128-138 Unbioturbated sediments on a muddy shelf: Hypoxia or simply reduced oxygen saturation?
Hypoxia; Anoxia; Shale; Bioturbation; Ichnology; Shelf
X-radiographs of sediment box cores acquired from the western Gulf of Mexico reveal limited bioturbation in sediment deposited in bathymetries greater than 35 m. Between 15 and 35 m, sediments are thoroughly bioturbated with averaged bioturbation indices (for all beds in a core) between 2.1 and 5.6, and trace diversities between 2 and 9 distinct burrow forms. Below 35 m water depth, box cores exhibit trace diversities of 1-3, and core-averaged bioturbation indices range between 0.3 and 3.6. There is an overall decrease in trace diversity and bioturbation indices in the offshore direction. Cross-shore ichnological trends are compared to dissolved oxygen contents of bottom waters. Above 35 m, dissolved oxygen (DO) contents show pronounced variability, ranging from 100% DO saturation through to hypoxia (<2.0 mg l(-1)), and reflect the periodic introduction of oxygen-depleted waters into otherwise fully oxygensaturated seawater. Below 35 m, DO content of bottom waters is consistently at 60-75% saturation. DO decreases by an average of 0.117 mg l(-1) per one-meter increase in water depth, such that bottom waters in 100 m water depth contain an average of 4.55 mg l(-1) DO. The data reveal a direct correlation between: a) the density of infauna and the diversity and density of burrows, and b) DO of bottom water. The degree of bioturbation is significantly reduced in waters that are oxic, but below 80% DO saturation. Based on these observations, we suggest that it is inappropriate to link low bioturbation intensities and diversities to hypoxia (<2.0 mg l(-1)), and by extension, anoxia (0 mg l(-1)). Instead, reduced oxygen contents (4.3-5.3 mg l(-1)) that lie well above hypoxic levels have a dramatic impact on the health of infaunal communities, and this is reflected by severe reductions in the ichnological character of sediments. (C) 2015 Elsevier B.V. All rights reserved. DOI
38. La Croix, AD; Dashtgard, SE.A synthesis of depositional trends in intertidal and upper subtidal sediments across the tidal-fluvial-transition: Fraser River, Canada.J. Sediment. Res., 2015, 85: 683-698 A synthesis of depositional trends in intertidal and upper subtidal sediments across the tidal-fluvial-transition: Fraser River, Canada
Sedimentological, neoichnological, palynological, and geochemical trends from upper subtidal and intertidal positions on channel bars in the lower Fraser River, Canada are synthesized into a single, coherent framework. From these data we define criteria for determining depositional position in shallow water depths in tide-influenced rivers. Three sedimentological trends are observed from the river mouth, through the locus of mud deposition (within the turbidity maximum zone (TMZ)), and into the freshwater-tidal zone. (1) The recurrence (per meter) and thickness of mud beds increase towards the TMZ and tapers in both the landward and seaward directions. (2) Muddy current ripples and graded current ripples are most abundant in the TMZ; they are less common with decreasing brackish-water influence and are absent in the freshwater river reach. (3) Heterolithic bedding (i.e., flaser, wavy, and lenticular) is common in the TMZ, less common seaward, and absent from the freshwater realm. In addition to the sedimentological trends, four ichnological trends parallel decreasing water salinity. With decreasing salinity, there is: (1) a decrease in bioturbation intensity from BI 2-3 to BI 0-1; (2) a decrease in the abundance of bioturbated beds; (3) a marked decrease in the diversity of traces from 5-6 forms to 1-2 forms; and, (4) a decrease in the diameter and length of traces. Traces are rare to absent in the tidal freshwater zone. Palynological and geochemical trends generally follow ichnological trends but are less obvious. Neither dinocyst abundance nor geochemical signature can be used to determine relative position in a tide-influenced river channel, although dinocyst abundances greater than 1% indicate significant marine influence, and 0-1% marine dinocysts indicate tidal influence. Although it is not feasible to determine exact depositional position within the tidal-fluvial transition, our results suggest that it is possible to determine where sediments were deposited relative to the TMZ. In turn, predicting relative depositional position can assist in unraveling stratigraphy and in recognizing nested channels in architecturally complex sedimentary successions. DOI
37. La Croix, AD; Dashtgard, SE; Gingras, MK; Hauck, TE; MacEachern, JA.Bioturbation trends across the freshwater to brackish-water transition in rivers.Paleogeogr. Paleoclimatol. Paleoecol., 2015, 440: 66-77 Bioturbation trends across the freshwater to brackish-water transition in rivers
Brackish-water ichnological model; Neoichnology; Trace fossils; Freshwater; Brackish water; Deltas; Estuaries
Most existing ichnological models predict an increase in burrow density and diversity from the zone of persistent brackish-water into the realm of persistent freshwater. Herein we compare the neoichnology of five tidal-fluvial channels with varying tidal magnitudes, saltwater incursion distances, and river discharge. The results indicate that there is a noticeable diminution in the sizes of marine traces and a corresponding decrease in their distribution (reduced abundance of burrowed versus unburrowed beds) with decreasing salinity. From the landward limit of saltwater incursion into the wholly freshwater tidal backwater, there is no concurrent increase in terrestrial or freshwater burrow forms; rather, burrow diversities remain low (range: 10 to 35% of the "open marine" signature) and burrow densities decrease from BI 0-3 to BI 0-1 in both sand and mud units. The five modern systems described in this study do not support the hypothesis that there is an increase in bioturbation from the brackish-water reaches of channels into freshwater reaches. In fact, freshwater channels, including those in the tidal backwater zone, are characterized by very low trace-fossil diversities (1 to 2 forms) as well as low and sporadically distributed bioturbation intensities (BI 0-1). (C) 2015 Elsevier B.V. All rights reserved. DOI
36. Ayranci, K; Dashtgard, SE; MacEachern, JA.A quantitative assessment of the neoichnology and biology of a delta front and prodelta, and implications for delta ichnology.Paleogeogr. Paleoclimatol. Paleoecol., 2014, 409: 114-134 A quantitative assessment of the neoichnology and biology of a delta front and prodelta, and implications for delta ichnology
echinoderm Deltas; Ichnology; Fraser River delta; Asymmetric deltas
The Fraser River delta front exhibits asymmetry with respect to its neoichnological, biological, and sedimentological characteristics on opposing sides of the main distributary channel, as well as the submarine canyon seaward of the channel. This phenomenon is attributed to asymmetric tidal flow, wherein there is a net northward tidal flow along the delta front. The southern delta front lying updrift of the Main Channel, is primarily composed of sandy beds and bedsets that exhibit low bioturbation intensities (BI 0-3) and limited biodiversity (H 0.9-1.4). Tracemakers in the updrift delta front predominantly consist of polychaetes and bivalves that produce suites dominated by simple vertical and horizontal traces attributable to the Skolithos Ichnofacies. The suites consist of traces comparable to Thalassinoides, Planolites, Skolithos, rare Conichnus, and Cylindrichnus. The northern delta front lying downdrift of the Main Channel, and the prodelta consist of muddy beds and bedsets that display high bioturbation intensities (BI 3-6) and diverse infaunal populations (H 1.4-23). Polychaetes, bivalves, and echinoderms comprise the main tracemakers, and these infauna produce complex and robust ichnological suites typical of the Cruziana Ichnofacies. The downdrift delta front and prodelta trace assemblage includes traces comparable to Skolithos, Planolites, Thalassinoides, Artichnus, Palaeophycus, Conichnus, Asterosoma, Rosselia, Cylindrichnus, Gyrolithes, Teichichnus, Phycosiphon, Arenicolites, Polykladichnus, and Scolicia. The spatial distribution (area and volume) of biogenically induced sediment disruption indicates that the updrift delta front is significantly less bioturbated than its downdrift counterpart. Additionally, the prodelta does not exhibit evidence of asymmetry with respect to its neoichnological character; all spatial variability is recorded in delta front positions. (C) 2014 Elsevier B.V. All rights reserved. DOI
35. Czarnecki, JM; Dashtgard, SE; Pospelova, V; Mathewes, RW; MacEachern, JA.Palynology and geochemistry of channel-margin sediments across the tidal-fluvial transition, lower Fraser River, Canada: Implications for the rock record.Marine and Petroleum Geology, 2014, 51: 152-166 Palynology and geochemistry of channel-margin sediments across the tidal-fluvial transition, lower Fraser River, Canada: Implications for the rock record
Dinoflagellate cyst; Pollen; Carbon isotopes; Estuary; Delta; Marginal-marine; McMurray Formation
In the tidally influenced Fraser River, Canada, palynological and carbon isotope (delta C-13(org)) signatures of channel-margin sediments are compared to environmental parameters (e.g., grain size, water salinity) to establish how the signatures vary across the tidal-fluvial transition. Palynological assemblages in the Fraser River are dominated by tree pollen, which constitutes between 85% and 95% of all assemblages. Dinocyst abundances do not exceed 2% of the total palynological assemblage, and the number and diversity of dinocysts gradually decreases landward. The calculated landward limit for dinocysts is at approximately 83 river km, which is relatively close to the upstream limit of the tidal backwater (at similar to 100 km). delta C-13(org) values show minimal variability across the tidal fluvial transition, and the average value is approximately -26 parts per thousand. The delta C-13(org) signature of river sediments indicates a dominance of terrestrially sourced organic matter regardless of brackish-water and tidal influence on sediment deposition. Six palynological and geochemical trends are identified as relevant to the rock record. 1) In deltaic environments, palynological and geochemical characteristics are less useful than sedimentological and ichnological characteristics for establishing depositional conditions. 2) In marginal-marine settings, low abundances and low species diversities of dinocysts, coupled with a "terrestrial" geochemical signature (delta C-13(org) < -25 parts per thousand.) do not necessarily indicate deposition in a terrestrial environment. 3) Dinocyst abundances above 1% of the total palynomorph population can indicate a significant marine influence on sediment deposition. 4) Mud beds, preferably bioturbated, should be preferentially sampled in order to maximize palynomorph recovery. 5) Marine palynomorphs can occur, albeit in very low concentrations, to the landward limit of the tidal-backwater zone. 6) Palynological and geochemical data should be compared across the paleo-depositional environment in order to establish general trends and remove local variations caused by biases such as grain size. (C) 2013 Elsevier Ltd. All rights reserved. DOI
33. Johnson, SM; Dashtgard, SE.Inclined heterolithic stratification in a mixed tidal-fluvial channel: Differentiating tidal versus fluvial controls on sedimentation.Sediment. Geol., 2014, 301: 41-53 Inclined heterolithic stratification in a mixed tidal-fluvial channel: Differentiating tidal versus fluvial controls on sedimentation
IHS; Inclined heterolithic stratification; Mixed tidal-fluvial; Fraser River; Channel bars
Tidal and fluvial processes control deposition and determine the sedimentological and ichnological character of sediments in the mixed tidal fluvial Middle Arm, lower Fraser River, Canada. Sedimentological trends that define the mixed tidal fluvial zone include: 1) mud beds present from the intertidal zone to the base of the channel, 2) a downstream increase in the number of mud beds, and 3) a lateral mud sand mud profile developed in the intertidal zone of each bar. Non-rhythmic deposition of sand under conditions of elevated river discharge is apparent, although sand beds are interbedded with cm- to dm-scale mud beds deposited during periods of low river discharge and increased tidal influence. In rare cases, mm- to cm-scale rhythmically alternating sand and mud lamina are deposited in successions of 12-14 beds (tidal rhythmites). Ichnologically, burrow density, trace size, and homogeneity in the vertical distribution of traces increase in the seaward direction. Burrows in all of the bars are evenly distributed in the vertical profile through substrates of similar grain size; however, infaunal distributions vary as a function of grain size and subaerial exposure. Muddier sediments and substrates that experience less subaerial exposure display a higher degree of bioturbation (bioturbation index [BI] 2-5). Sand beds are generally bioturbated to a lesser degree (BI 0-1) except in the lower delta plain, where higher degrees of bioturbation (BI 3-5) can be produced. The sedimentological and ichnological character of sediments in the tidal fluvial Middle Arm showcases subtle, but important differences between tide-influenced, river-dominated inclined heterolithic stratification (IHS) and mixed tidal fluvial IHS. In particular, more homogeneous and diverse burrowing in both mud and sand beds, more rhythmic sand mud interbedding, mud deposition to the base of the channel, and the development of a mud sand mud profile along the length of the bar are considered indicative of heightened tidal influence and sustained brackish-water conditions. (C) 2013 Elsevier B.V. All rights reserved. DOI
32. La Croix, AD; Dashtgard, SE.Of sand and mud: Sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river.Sedimentology, 2014, 61: 1961-1981 Of sand and mud: Sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river
Backwater effect; bed-thickness distribution; Fraser River; inclined heterolithic stratification; intertidal zone; saltwater incursion; tidal-fluvial transition; turbidity maximum
The thickness and lateral distribution of sand and mud beds and bedsets on channel bars from the tidally influenced Fraser River, British Columbia, Canada, are quantitatively assessed. Fifty-six vibracores totalling ca 114m of vertical section are used to tabulate bed thicknesses. Statistical calculations are undertaken for nine channel bars ranging from the freshwater and tidal zone, to the sustained brackish water and tidal zone. The data reveal that thickness trends can be organized into three groups that broadly correspond to time-averaged hydrodynamic and salinity conditions in the various distributary channels. Thick sand beds (up to 30cm) and thin mud beds (up to 5cm) characterize the freshwater tidal zone. The tidal and freshwater to brackish-water transition zone comprises thin sands (up to 10cm) and thicker muds (up to 19cm), and the sustained brackish water tidal zone consists of thin muds (up to 6cm) with relatively thicker sands (up to 25cm). The results suggest that the locus of mud deposition occurs in the tidal freshwater to brackish-water zone, probably reflecting mud flocculation and deposition at the turbidity maximum. Landward of the turbidity maximum, mud deposition is linked to tidal influence (tidal backwater effect and reverse eddy currents on channel margins) as mud beds thin in the landward direction. These results support the hypothesis that mud deposition is greatest at the turbidity maximum and decreases in both the seaward and landward direction. This study also showcases that mud-bed thicknesses are greatest towards the turbidity maximum and thin in both the landward and seaward direction. In the rock record, the apex of mud deposition probably marks the position of the palaeo-turbidity maximum. DOI
31. Ayranci, K; Dashtgard, SE.Infaunal holothurian distributions and their traces in the Fraser River delta front and prodelta, British Columbia, Canada.Paleogeogr. Paleoclimatol. Paleoecol., 2013, 392: 232-246 Infaunal holothurian distributions and their traces in the Fraser River delta front and prodelta, British Columbia, Canada
Neoichnology; Ichnology; Sea cucumber; Molpadiida; Apodida; Dendrochirotida
Holothutians (sea cucumbers) constitute one of the largest and most abundant infaunal groups inhabiting the Fraser River delta front (0 to 150 m water depth) and prodelta (greater than 150 m water depth). The spatial distribution and density of animals reveal their preferred substrate grain sizes, and their tolerance or sensitivity to various stress factors, including salinity fluctuations, available food resources, sedimentation rate, and pollution. In the delta front, below 25 m water depth (below storm-wave base), holothurians are asymmetrically distributed with respect to the main distributary channel. Population densities primarily reflect asymmetry in grain-size distributions across the delta front Holothurians are absent or occur in low density in the sand-dominated southern delta front (SDF) whereas the mud-dominated northern delta front (NDF) is more heavily colonized. Sedimentation rate and nutrient content are equally important in holothurian distribution. Population densities of certain holothurians show significant increase where there are moderate sedimentation rates (2-4 cm yr(-1)), and river-derived nutrient supply. Several subsurface traces and their surface expressions are attributed to holothurians. Subsurface traces include diminutive- and robust-Artichnus, Thalassinoides, and U-shaped bio-deformation structures. On the sediment surface, bio-topographic structures such as mounds and funnel-shaped depressions are produced by the activity of sea cucumbers. Of the holothurian-generated traces, only Artichnus can conclusively be attributed to sea cucumbers. Based on the distributions of the trace makers of Artichnus, we hypothesize that in the rock record this trace fossil can be used as evidence of stable euhaline salinity conditions, and sediment deposition and colonization below storm-wave base. Artichnus occurs mainly in mud-dominated (>55% mud) substrates. (C) 2013 Elsevier B.V. All rights reserved. DOI
30. Ayranci, K; Lintern, DG; Hill, PR; Dashtgard, SE.Tide-supported gravity flows on the upper delta front, Fraser River delta, Canada.Mar. Geol., 2012, 326: 166-170 Tide-supported gravity flows on the upper delta front, Fraser River delta, Canada
deltas; hyperpycnal; salinity; turbidites; VENUS; gravity flow
Three anomalous events (AEs) were recorded off the Fraser River, Canada during the freshet of 2008, and are ascribed to quasi-continuous gravity flows. These flows transport warm, low-salinity Fraser River waters to the upper delta front of the Fraser delta to at least 50 m water depth. All flows occurred during the freshet and spring ebbing tide, and were characterized by deposition of fine-grained sand and silt beds with likely high-water contents. These sediments were susceptible to resuspension by the subsequent flood-tide current, producing very high near bed suspended sediment concentrations (SSC) with enough excess density to generate hyper-concentrated flows. Our observations show that these episodic events result from the combination of high river discharge, high SSC, and strong tides, and therefore, are likely to occur on the delta fronts of other large tide-dominated and tide-influenced rivers. In shallow-water basins (< 50 m water depth), these tide-supported gravity flows can affect sediment deposition on both the delta front and prodelta. (C) 2012 Elsevier B.V. All rights reserved. DOI
29.Dashtgard, SE; Gingras, MK.Chapter 10: Marine invertebrate neoichnology.Trace Fossils as Indicators of Sedimentary Environments (Knaust, D; Bromley, RG (eds.)), 2012, Developments in Sedimentology 64: 273-295 Chapter 10: Marine invertebrate neoichnology
DOI
28.Dashtgard, SE; MacEachern, JA; Frey, SE; Gingras, MK.Tidal effects on the shoreface: Towards a conceptual framework.Sediment. Geol., 2012, 279: 42-61 Tidal effects on the shoreface: Towards a conceptual framework
Tides; Beach; Facies models; Sedimentology; Ichnology; Storm-dominated
Tidal processes can have a significant impact on the sedimentological and ichnological character of wave-dominated shoreface deposits. As the influence of tides increases, the resulting shoreface successions begin to depart markedly from those postulated by the conventional, wave-dominated shoreface model, which was built upon essentially non-tidal shoreline settings. In shoreface settings subject to stronger tidal flux, tides can be manifest either directly or indirectly. Direct tidal effects refer to those characteristics imparted by tidal energy (e.g., tidal currents) per se, and are best expressed in offshore and lower shoreface positions. Key evidence of direct tidal control includes uniform sediment calibres from the upper shoreface to the offshore, and little or no mud preserved in the lower shoreface. Additionally, sands in the lower shoreface and offshore tend to be intensely bioturbated. Where primary stratification is preserved, it largely comprises current-generated structures. Such shoreface deposits are referred to herein as "tide-influenced shorefaces", and are expected in settings with low storm-wave input coupled with strong tidal currents (e.g., straits). Indirect tidal influences are manifest by the lateral translation of wave zones across the shoreface profile owing to changes in water depth during the tidal cycle. This is best developed in macrotidal to megatidal settings. Indirect tidal influences are more pronounced in the upper and lower shoreface, and are recorded through the interbedding of sedimentary structures produced by shoaling waves, breakers and surf, swash-backwash, and surface runoff. The boundaries between shoreface subenvironments are correspondingly poorly defined. The foreshore in settings of elevated tidal range is also generally much thicker (typically 4 to 5 m). Bioturbation tends to be patchy in distribution across the shoreface, and dominated by vertical structures. Such systems are defined as "tidally modulated shorefaces". Using well-established sedimentological and ichnological criteria for recognizing wave-dominated (nontidal) shorefaces - wherein sediment deposition is nearly wholly controlled by fair-weather wave and storm-wave processes - a conceptual model is developed for discriminating fair-weather (non-tidal) shorefaces, storm-influenced (non-tidal) shorefaces, and tidally influenced shorefaces. Five shoreface archetypes are defined: storm-affected, storm-influenced, storm-dominated, tide-influenced, and tidally modulated. (C) 2010 Elsevier B.V. All rights reserved. DOI
26. Frey, SE; Dashtgard, SE.Seaweed-assisted, benthic gravel transport by tidal currents.Sediment. Geol., 2012, 265: 121-125 Seaweed-assisted, benthic gravel transport by tidal currents
Seaweed Kelp; Gravel; Tidal currents; Shoreface
Traction transport of pebbles and cobbles occurs subtidally at current velocities below 0.5 m s(-1) when seaweed attached to clasts provides additional lift and drag to the clast In the Juan de Fuca Strait, British Columbia, Canada, the seaweed Cymanthere triplicata commonly attaches to pebbles and provides sufficient additional surface area for tidal currents to drag the clast along the seafloor. Using in situ measurements of current velocities at 13 m water depth, the threshold for initiation of motion of a 30 mm pebble with attached seaweed is 0.3 m s(-1). This is approximately one order of magnitude less than the activation velocity for a 30 mm pebble without attached seaweed. In addition to kelp-rafted (floated) gravel, seaweed-assisted, benthic gravel transport is possible in marine settings where unidirectional currents (e.g., tidal currents, storm-induced bottom currents) are sufficient to transport pebbles alongshore, and into and across the offshore (below fairweather wave base). If preserved in the rock record, deposits of algal-enhanced gravel deposited via unidirectional, subtidal currents will likely appear as isolated gravel clasts encased in sandstone, reflecting the similar current velocities required to transport these two clast groups. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved. DOI
25. Gingras, MK; MacEachern, JA; Dashtgard, SE.The potential of trace fossils as tidal indicators in bays and estuaries.Sediment. Geol., 2012, 279: 97-106 The potential of trace fossils as tidal indicators in bays and estuaries
Ichnology; Trace fossil; Tubular tidalite; Intertidal flat; Tidal sedimentary structures
identification of trace-fossil assemblages characteristic of brackish-water settings. However, this limited means of (ichnologically) identifying tidal settings is unsatisfying and the establishment of ichnological-tidal relationships for the purpose of discerning tidal depositional processes is desirable. We suggest that several types of ichnological data can be used to identify tidal influence: (1) ichnological response to rhythmic sedimentation; (2) the presence of tubular tidalites (i.e. biogenic structures infilled by tidally derived sediments); (3) the identification of anisotropic resource exploitation associated with food-rich slack-tide laminations; (4) the identification of tidal-flat deposits through bioturbation intensity and ichnological composition; (5) the recognition of brackish-water trace fossils; and potentially (6) system-scale ichnological distributions (which are presently poorly understood). As with tidal sedimentary structures, the inference of tidal processes from biogenic sedimentary structures should depend on identifying as many of the above characteristics as possible. (C) 2011 Elsevier B.V. All rights reserved. DOI
24. Gingras, MK; MacEachern, JA; Dashtgard, SE; Zonneveld, J-P; Schoengut, J; Ranger, MJ; Pemberton, SG.Chapter 16: Estuaries.Trace Fossils as Indicators of Sedimentary Environments (Knaust, D; Bromley, RG (eds.)), 2012, Developments in Sedimentology 64: 471-514 Chapter 16: Estuaries
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23. La Croix, AD; Gingras, MK; Dashtgard, SE; Pemberton, SG.Computer modeling bioturbation: The creation of porous and permeable fluid-flow pathways.AAPG Bull., 2012, 96: 545-556 Computer modeling bioturbation: The creation of porous and permeable fluid-flow pathways
Computer modeling of trace fossils (Skolithos, Thalassinoides, Planolites, Zoophycos, and Phycosiphon) and ichnofacies (Skolithos, Cruziana, and Zoophycos ichnofacies) is undertaken to assess the impact of bioturbation on porosity and permeability trends in sedimentary media. Model volumes are randomly populated with the digitally modeled trace fossils to test for connectivity between burrows. The probability of vertical and lateral interconnections is compared with bioturbation intensity. The results of the simulations indicate that biogenic flow networks develop at low bioturbation intensity, between 10 and 27.5% bioturbation (BI-2). However, the efficiency of connectivity is controlled by the architecture of the burrows. For all trace-fossil and ichnofacies models, regardless of trace-fossil orientation, continuous horizontal and vertical connectivity across the sediment volume is achieved within a 0 to 10% range in bioturbation. In subsurface aquifers and petroleum reservoirs, the presence of bioturbation can significantly influence fluid flow. In particular, for marine sedimentary rocks, where burrows are more permeable than the surrounding matrix, a greater degree of three-dimensional burrow connectivity can produce preferred fluid-flow pathways through the rock. Recognizing these flow conduits may enable optimization of resource exploitation or may contribute to increasing reserve estimates from previously interpreted nonreservoir rock. DOI
22. MacEachern, JA; Bann, KL; Gingras, MK; Zonneveld, J-P; Dashtgard, SE; Pemberton, SG.Chapter 4: The Ichnofacies Paradigm.Trace Fossils as Indicators of Sedimentary Environments (Knaust, D; Bromley, RG (eds.)), 2012, Developments in Sedimentology 64: 101-136 Chapter 4: The Ichnofacies Paradigm
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21. MacEachern, JA; Dashtgard, SE; Knaust, D; Catuneanu, O; Pemberton, SG.Chapter 6: Sequence stratigraphy.Trace Fossils as Indicators of Sedimentary Environments (Knaust, D; Bromley, RG (eds.)), 2012, Developments in Sedimentology 64: 155-192 Chapter 6: Sequence stratigraphy
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19. Pemberton, SG; MacEachern, JA; Dashtgard, SE; Bann, KL; Gingras, MK; Zonneveld, J-P.Chapter 19: Shorefaces.Trace Fossils as Indicators of Sedimentary Environments (Knaust, D; Bromley, RG (eds.)), 2012, Developments in Sedimentology 64: 585-625 Chapter 19: Shorefaces
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18. Sisulak, CF; Dashtgard, SF.Seasonal controls on the development and character of inclined heterolithic stratification in a tide-influenced, fluvially dominated channel: Fraser River, Canada.J. Sediment. Res., 2012, 82: 244-257 Seasonal controls on the development and character of inclined heterolithic stratification in a tide-influenced, fluvially dominated channel: Fraser River, Canada
Inclined heterolithic stratification (interbedded sand and mud with depositional dip; IHS) is developed on an in-channel bar in the tide-influenced, fluvially dominated (brackish water) reach of the Fraser River, British Columbia, Canada. The vertical bar succession is characterized by a fining-upward profile with an increase in mud content and mud-bed thickness from the shallow subtidal zone to the upper intertidal zone. There is also an increase in the number and lateral continuity of mud beds from the upstream side to the downstream side of the bar. Sand beds are dominated by current ripples in the intertidal zone, and by current ripples and trough cross-beds in the shallow subtidal zone. The channel base is mantled by downstream (ebb)-oriented dune-scale bedforms. Mud beds are dominantly parallel laminated, although current ripples may develop in silt-rich and sand-rich mud beds. Current-generated bedforms are predominantly ebb-oriented. Sediment deposition is seasonally controlled. Sand deposition occurs during periods of high discharge (snowmelt-induced freshet), and mud is deposited during waning freshet flow and low discharge (base flow). In mud-dominated deposits, current ripples may develop in sand-rich mud beds deposited during the freshet. Seasonal cyclicity in sediment deposition is also recorded in the ichnological characteristics of the IHS. Bioturbation is significantly more common in base-flow deposits (mud beds) than freshet deposits (sand beds). Burrows in sand beds typically subtend from overlying mud beds. Diminutive, vertical burrows dominate the trace suite, reflecting a very low diversity of infauna. Bioturbation is more common on the downstream side of the bar, associated with the thicker and more laterally contiguous mud beds. Based on these observations, seasonal cyclicity in channel-bar deposits, formed in tide-influenced, fluvially dominated channels, can be identified using a combination of sedimentological and ichnological characteristics, particularly in mud-dominated successions and in IHS. In mud-dominated deposits, seasonal controls are best-expressed ichnologically through the interbedding of bioturbated and unbioturbated intervals. The dominance of a low-diversity suite of mainly vertical traces can also be considered indicative of brackish-water conditions in the channel. In IHS successions, interbedded sand and mud beds are the best indicators of seasonal cyclicity: sand beds are typically unburrowed, and mud beds are burrowed. In sand-dominated successions that lack mud interbeds, it is difficult to recognize seasonal cyclicity. DOI
17.Dashtgard, SE.Linking invertebrate burrow distributions (neoichnology) to physicochemical stresses on a sandy tidal flat: implications for the rock record.Sedimentology, 2011, 58: 1303-1325 Linking invertebrate burrow distributions (neoichnology) to physicochemical stresses on a sandy tidal flat: implications for the rock record
Boundary Bay; British Columbia; Fraser River Delta; grain size; ichnology; neoichnology; polyhaline; sedimentation rate; Strait of Georgia; tidal flat
Invertebrate burrow distributions (neoichnology) across a modern tidal flat are presented as an analogue for interpreting the ichnology of palaeo-tidal-flat successions. Burrow distributions are linked to physical and chemical (physicochemical) stresses to establish the main controls on the distribution of biosedimentary structures. Across the tidal flat, there is clear heterogeneity in both the diversity of traces and the intensity of burrowing. This heterogeneity reflects a myriad of physicochemical stresses, with the sedimentation rate dominating burrow distributions. Across all substrates, the total area occupied by organisms rarely exceeds 3% of the tidal-flat surface, and is commonly <1%; this equates to a bioturbation index value of one. To reach bioturbation index values of two to six, sediments must be available to biogenic reworking and/or recolonization. With an increasing sedimentation rate, substrates are rapidly buried and re-exposed, which limits the time when a substrate is available to colonization. For palaeo-ichnological studies, this research presents several key results. (i) Burrow cross-cutting relationships in tidal-flat successions commonly reflect natural heterogeneities in the areal distribution of infaunal communities, rather than infaunal tiering. (ii) Ichnofabric analysis of palaeo-tidal flats with a high sedimentation rate would yield fabrics that reflect heterogeneities in the areal distribution of infaunal communities rather than variability in the physicochemical stresses of the environment. (iii) The composite trace-fossil assemblage of tidal flats cannot be attributed to a single ichnofacies, but instead comprises elements typical of multiple ichnofacies. (iv) The main controls on trace assemblages across tidal flats in fully marine settings are sedimentological and include the sedimentation rate and, to a lesser extent, grain size. DOI
16.Dashtgard, SE.Neoichnology of the lower delta plain: Fraser River Delta, British Columbia, Canada: Implications for the ichnology of deltas.Paleogeogr. Paleoclimatol. Paleoecol., 2011, 307: 98-108 Neoichnology of the lower delta plain: Fraser River Delta, British Columbia, Canada: Implications for the ichnology of deltas
Ichnology; Tidal flats; Turbidity; Salinity; Trace fossil; Infauna
Neoichnological (i.e., burrows, tracks and trails made by extant organisms) distributions are mapped across the active lower delta plain of the Fraser River Delta, British Columbia, Canada. The neoichnological character of the lower delta plain is best described as a moderate diversity trace assemblage mainly comprising simple, vertical burrows of filter-feeding animals and fewer dwellings of surface deposit feeders. Neoichnological trends generally parallel the coast, and bivalve-generated structures dominate in both muddy and sandy sediments. Given that all of the bivalves in sand-dominated sediments of the lower delta plain tidal flats are filter-feeding animals, the occurrence of large numbers of bivalve-generated structures in deltaic tidal flats may be indicative of at least moderate turbidity and possibly hypopycnal flow. These observations are not applicable to muddy substrates, however, as bivalve-generated structures in mud are made by both filter-feeding and deposit-feeding animals. Neoichnological trends on the active lower delta plain are compared to the neoichnological character of the effectively abandoned part of the lower delta plain at Boundary Bay (considered to be a fully marine equivalent of the lower delta plain). In comparison to Boundary Bay, there is a significant decrease in the diversity of infauna from Boundary Bay to the tidal flats on the active lower delta plain. Bivalves thrive in the more turbid waters of the active lower delta plain, whereas many large polychaetes cannot survive the low salinity conditions of this subenvironment. The Fraser delta is a tidally asymmetric delta with a net northward transport of sediment. However, at shallow depths (less than 10 m water depth relative to low low tide), the delta exhibits a reversed asymmetry in its ichnological character. Waves exert a greater control on sediment and animal distributions in shallow water, such that the updrift tidal flats (those that are south of the main distributary channel) experience higher sedimentation, salinity, and turbidity stresses than the downdrift tidal flats. This is manifested by a reduced diversity of traces and intensity of burrowing on the tidal flats updrift of the main Fraser distributary. In addition, there is a higher proportion of filter-feeding animal burrows on the downdrift side of the main distributary. (C) 2011 Elsevier B.V. All rights reserved. DOI
15. Frey, SE; Dashtgard, SE.Sedimentology, ichnology and hydrodynamics of strait-margin, sand and gravel beaches and shorefaces: Juan de Fuca Strait, British Columbia, Canada.Sedimentology, 2011, 58: 1326-1346 Sedimentology, ichnology and hydrodynamics of strait-margin, sand and gravel beaches and shorefaces: Juan de Fuca Strait, British Columbia, Canada
Beach; shoreface; ichnology; tides; clastic; strait; hydrodynamics; tide-influenced
On the south-west coast of Vancouver Island, Canada, sedimentological and ichnological analysis of three beach-shoreface complexes developed along a strait margin was undertaken to quantify process-response relations in straits and to develop a model for strait-margin beaches. For all three beaches, evidence of tidal processes are expressed best in the lower shoreface and offshore and, to a lesser extent, in the middle shoreface. Tidal currents are dominant offshore, below 18 m water depth (relative to the mean spring high tide), whereas wave processes dominate sediment deposition in the nearshore (intertidal zone to 5 m water depth). From 18 to 5 m water depth, tidal processes decrease in importance relative to wave processes. The relatively high tidal energy in the offshore and lower shoreface is manifest sedimentologically by the dominance of sand, of a similar grain size to the upper shoreface/intertidal zone and, by the prevalence of current-generated structures (current ripples) oriented parallel to the shoreline. In addition, the offshore and lower shoreface of strait-bound beach-shoreface complexes are recognized ichnologically by traces typical of the Skolithos Ichnofacies. This situation contrasts to the dominantly horizontal feeding traces characteristic of the Cruziana Ichnofacies that are prevalent in the lower shoreface and offshore of open-coast (wave-dominated) beach-shorefaces. These sedimentological and ichnological characteristics reflect tidal influence on sediment deposition; consequently, the term 'tide-influenced shoreface' most accurately describes these depositional environments. DOI
14. Gingras, MK; MacEachern, JA; Dashtgard, SE.Process ichnology and the elucidation of physico-chemical stress.Sediment. Geol., 2011, 237: 115-134 Process ichnology and the elucidation of physico-chemical stress
Ichnology; Ichnofacies; Ichnofabrics; Process ichnology; Trace fossil
This paper sets out a philosophical approach to ichnological (trace fossil) analysis, which focuses on the interpretation of trace fossils as sedimentary structures rather than as paleontological entities per se. Using wide-ranging datasets and a large number of observations and interpretations, a "Process Ichnology" framework is proposed. This interpretive framework provides an improved means of estimating the presence and magnitude of various physical and chemical (i.e., physico-chemical) depositional stresses (e.g., water turbidity, sedimentation rates, substrate consistency, salinity, and oxygenation) in ancient sedimentary environments. Ichnological datasets that are considered include: 1) trace-fossil distributions; 2) ethological diversity and the range of diversity; 3) the significance of burrow linings; 4) trace-fossil size; and, 5) post-depositional compaction of trace fossils. From these data, higher-resolution estimates can be made for the determination of sedimentation rates, temporal variation in sedimentation rate, sediment consistency, and aspects of the bottom- and interstitial-water chemistries. Additionally, the character of depositional bypassing of sediment grains can be determined. The methodologies and interpretations herein are intended for use by non-ichnologists in a manner akin to the interpretation of physical sedimentary structures. However, the outlined framework is complementary to other methods of ichnological analysis, such as ichnofacies- or ichnofabric-analysis, and can be applied as such. Indeed, this method is a derivative of these and other earlier techniques, and should be employed where a systematic approach to obtaining high-resolution sedimentological interpretations is a required aspect of the study. (C) 2011 Published by Elsevier B.V. DOI
13.Dashtgard, SE; Gingras, MK; MacEachern, JA.Tidally modulated shorefaces.J. Sediment. Res., 2009, 79: 793-807 Tidally modulated shorefaces
Tidally modulated shorefaces (TMS) are wave-dominated, but differ from conventional shorefaces in that sediments deposited in water depths equivalent to the upper, middle, and lower shoreface (depending upon the tidal range) are regularly subjected to variable wave processes, including swash-backwash, breaking-wave and surf processes, and shoaling waves; during the tidal cycle. In upper macrotidal and megatidal settings, it is also possible that these shoreface deposits are subaerially exposed during low tide. TMS exhibit the morphology, seaward decrease in sediment caliber, and dominance of wave-generated sedimentary structures consistent with beach-shoreface settings. However, the sedimentological and ichnological structures of deposits exposed in the laterally extensive intertidal zone are more akin to those of the upper and lower shoreface, and not the beach. Four major differences permit the ready differentiation of tidally modulated shoreface successions from conventional shorefaces. (1) Sedimentary structures generated by swash-backwash (plane beds), surf and breaking waves (current ripples and trough cross-beds), shoaling waves (oscillation ripples), and storm waves (hummocky and swaly cross-stratification) are interbedded with one another across the shoreface. (2) Ebb-oriented tidal currents and surface runoff during the failing tide and at low tide deposit offshore-directed current ripples and combined-flow ripples in sandy sediments, or trough cross beds in gravel-dominated sediments. (3) Ichnologically, TMS exhibit a reduction in both the diversity of ichnogenera and density of burrowing across the entire shoreface profile. However, the incipient-trace associations are most similar to the Skolithos Ichnofacies in the upper shoreface-equiva lent zone, and to the Cruziana Ichnofacies in the lower shoreface-equiva lent zone. (4) The sedimentological and ichnological criteria commonly employed to identify the middle shoreface are spread out across the upper and lower shoreface, making this subenvironment difficult to differentiate. DOI
12. Frey, SE; Gingras, MK; Dashtgard, SE.Experimental studies of gas-escape and water-escape structures: Mechanisms and morphologies.J. Sediment. Res., 2009, 79: 808-816 Experimental studies of gas-escape and water-escape structures: Mechanisms and morphologies
Water-escape and gas-escape structures were generated in the laboratory to assess variations in deformational structures generated during these events. Particulate fluidization was observed to be the method by which grains were redistributed during water escape. Resulting structures of particulate fluidization comprised disrupted and downward-deflected primary laminations, and a massive-appearing column of sand above the point source. Gas experiments, with a low flow rate (6.167 X 10(-6) m(3)/s) allowed gas to disperse through fractures, and only subtle deformation was achieved. Aggregative fluidization (i.e., fluid movement through sediment as bubbles) was the mechanism for grain displacement during gas escape. With increased flow rate (7.67 X 10(-5) m(3)/s) aggregative fluidization occurred and particles were carried upward in the wake of the passing gas bubble. Aggregative fluidization resulted in broken, upwards-warping laminations and convoluted bedding. Experiments illustrate a significant morphological difference between water escape and gas escape. Water-escape structures were hounded by downwards-warping laminations, consistent with water-escape structures encountered in the rock record. Gas-escape structures displayed upwards-warping laminations, and could represent a formation mechanism for convolute bedding. DOI
11. Hauck, TE; Dashtgard, SE; Pemberton, SG; Gingras, MK.Brackish-water ichnological trends in a microtidal barrier island-embayment system, Kouchibouguac National Park, New Brunswick, Canada.Palaios, 2009, 24: 478-496 Brackish-water ichnological trends in a microtidal barrier island-embayment system, Kouchibouguac National Park, New Brunswick, Canada
A complex variety of marginal-marine microtidal environments from Kouchibouguac Bay, New Brunswick, Canada, present an opportunity to ichnologically and sedimentologically characterize microtidal settings in a high-latitude, temperate subarctic climate. Variations in bioturbate fabrics and distribution of infauna, analysis of the distributions of sediments and physical sedimentary structures, and the distribution of total organic carbon (TOC) can be associated with characteristic depositional processes. From these data typical sedimentary facies associations are produced. In outer estuary tidal inlets and areas of the flood-tidal deltas, strong currents and wave action eradicate the ichnological signature, resulting in variably laminated and bedded sand. In the central estuary, infauna activity coupled with generally low hydraulic energy levels lead to an absence of primary sedimentary structures. The inner estuary near bay-head deltas experiences riverine currents and freshwater influence. As a consequence, primary sedimentary structures are preserved. Mapping of infauna, sediment texture, TOC, and salinity reveals strong links between animal distribution and these three physicochemical parameters. Consequently, the distribution and type of bioturbation observed is at least passively related to grain size, TOC, and salinity. In outer estuaries and lower-central estuaries, salinity is near marine levels and fluctuates minimally. The distribution of infauna in these areas corresponds directly to sediment texture and TOC. Further up the estuaries, lower and fluctuating salinities-in addition to sediment texture and TOC content-control the distribution and diversity of infauna. Mapping of diversity and infaunal size up-estuary reveals two significant trends attributable to salinity stresses: (1) vermiform diminution, and (2) a significant decrease in infaunal diversity. DOI
10.Dashtgard, SE; Buschkuehle, MBF; Fargrieve, B; Berhane, H.Geological characterization and potential for carbon dioxide (CO2) enhanced oil recovery in the Cardium Formation, central Pembina Field, Alberta.Bull. Can. Pet. Geol., 2008, 56: 147-164 Geological characterization and potential for carbon dioxide (CO2) enhanced oil recovery in the Cardium Formation, central Pembina Field, Alberta
Injection of CO2 into the Cardium Formation, central Pembina Field, Alberta was initiated in 2004 in two inverted 5-spot waterflood patterns. The purpose of this flood was to assess the economic viability of enhanced oil recovery (FOR) using CO2. To study CO2 behaviour within the reservoir and assess long-term risks of CO2 leakage and storage potential, a geological and hydrogeological model was developed for the Cardium Formation, and for both the underlying and overlying strata. The injection zone, Cardium Formation, was sub-divided into four permeable and three impermeable reservoir units. The underlying and overlying strata were subdivided based on their geological potential to contain CO2 over the long-term (5,000 years). Permeable reservoir units in the Cardium Formation include the lower, middle and upper sandstones, and the conglomerate. Most of the storage capacity of the Cardium Formation is contained within the three sandstone beds, which exhibit average porosities between 14.8 percent and 16.4 percent. Permeability in the sandstones increases predictably with increasing porosity, but is variable from sandstone to sandstone. The lower sandstone has the lowest average permeability (9.5 md), which is about half the average permeability of the middle and upper sandstones (21.4 and 19.8 md, respectively). There is no relation between porosity and permeability in the conglomerate. The conglomerate on average is 50 percent more permeable (33 md) than the middle and upper sandstones; although, permeability may exceed one Darcy in individual beds. The higher permeability of the conglomerate results in it acting as a "thief" zone for injected CO2. This tendency is further compounded by both the endemic use of hydraulic fractures (which propagate vertically) to stimulate oil production, and the positive buoyancy of CO2 relative to water. The Cardium Formation at Pembina occurs within the middle of the 650 m thick Colorado Group shale. The effectiveness and integrity of the overlying shale succession, as a seal for the Cardium Formation, is vital to the long-term storage of CO2. Leakage through this shale is very unlikely as it is thicker than 300 m and separates two aquifers with significantly different regional pressure regimes (the Cardium Formation was originally over-pressured whereas the overlying Wapiti Formation is sub-hydrostatically pressured). DOI
9.Dashtgard, SE; Gingras, MK; Pemberton, SG.Grain-size controls on the occurrence of bioturbation.Paleogeogr. Paleoclimatol. Paleoecol., 2008, 257: 224-243 Grain-size controls on the occurrence of bioturbation
ichnology; neoichnology; bioturbation; Bay of Fundy; grain size; intertidal; gravel; conglomerate
Grain size and grain-size related stresses impart a significant influence on the ichnological character of marginal-marine deposits. This is evident on the New Brunswick coastline of the Bay of Fundy, Canada, where three coarse-grained marginal-marine deposits are studied to assess grain-size controls on the occurrence and type of bioturbation. Firm mud and sand substrates exhibit the greatest diversity and density of bioturbation (i.e., bioturbation intensity). The types of organisms colonizing sands and firm-mud substrates are variable; however, the resultant trace assemblages are similar. Thixotropic muds exhibit significantly reduced trace diversity and density relative to firm mud, reflecting the additional stress placed on the organisms by the relatively soupy consistency of the sediment. A significant change in the trace assemblage occurs when sediment caliber passes the gradational sand-fine gravel boundary. Four main conclusions can be drawn from this study. First, for mixed sand and gravel, fine gravel, and coarse-gravel deposits, the degree of bioturbation (diversity * density) decreases more rapidly onshore (across the intertidal zone) than is noted in sand or mud deposits. Second, there is a decrease in the degree of bioturbation with increasing grain size for substrates composed of sand-sized and larger clasts. Third, burrows in gravels tend to be lined and/or robust, likely to maintain a stable environment within the burrow. Fourth, in coarse-gravel substrates or substrates with a significant component of coarse gravel, burrows are developed between the clasts and tend to be more permanent structures (than those developed in sand or mud), which are generally continuously occupied. The degree of burrowing noted in these modem gravel deposits contrasts with the relative paucity of biogenic structures reported in conglomerates preserved in the rock record. Based on the intensity of burrowing observed in the gravels, we hypothesize that ancient marginal-marine conglomerates are likely bioturbated, but that these burrows are likely distorted during burial and compaction. (C) 2007 Elsevier B.V. All rights reserved. DOI
8. Gingras, MK; Dashtgard, SE; MacEachern, JA; Pemberton, SG.Biology of shallow marine ichnology: a modern perspective.Aquat. Biol., 2008, 2: 255-268 Biology of shallow marine ichnology: a modern perspective
ichnology; neoichnology; traces; crustacea; vermiforms; bivalves; echinoderms; anemones
This study considers the construction and functionality of biogenic structures made by marine, vermiform nemerteans, polychaetes and hemichordates; marine crustaceans; motile bivalves; motile echinoderms; and sponges and sea anemones. We report on a range of modern biogenic structures similar to several known ichnogenera. Vermiform animals dominantly occupy vertical burrows that range from simple through helical shafts to Y- and U-shapes. Horizontal traces made by worms range in form, but are dominated by branching and variably sinuous to meandering burrows. Crustaceans primarily excavate open burrow systems that possess a range of architectures that are similar to either Thalassinoides or Psilonichnus. Smaller crustaceans, such as amphipods, mix the sediment. Bivalve traces vary in form, but generally preserve evidence of vertically oriented filter or interface-deposit feeding from a stationary location, rapid vertical escape, or horizontal grazing. Echinoderms dominantly preserve body impressions and motility traces, such as Asteriacites. An important class of biogenic structure, Scolicia and Bichordites, are made by urchins. Finally, sea anemones can generate large, penetrative, conical biogenic structures. Large, open horizontal networks serve as domiciles and deposit-feeding structures for crustaceans, but with worms similar burrow types are used more for passive carnivory and establishing an interface-feeding network. We report that the trace fossil Gyrolithes potentially represents mechanical ramps for shrimp, but is used as a sediment holdfast when made by worms. Finally, Y-shaped burrows are used for filter feeding in shrimp, and interface-deposit feeding in worms. These examples emphasize that inferences of behavior in the rock record are interpretive. DOI
7. Gingras, MK; Pemberton, SG; Dashtgard, S; Dafoe, L.How fast do marine invertebrates burrow?Paleogeogr. Paleoclimatol. Paleoecol., 2008, 270: 280-286 How fast do marine invertebrates burrow?
Burrowing rates; Bivalve traces; Arthropod burrows; Echinoderm traces
The burrowing of bivalves, arthropods, and echinoderms collected from tidal flats and shallow subtidal sediments of the Ogeechee estuary, Georgia, U.S.A was analyzed using time-elapse, X-ray analysis of thin-walled aquaria. The rate of sediment intrusion was determined for each animal. Burrowing rates ranged between 0.01 and 0.15 cm(3)/h for suspension-feeding animals. Deposit-feeding animals moved between 1 and 10 cm(3) of sand per hour, approximately 10 to 100 times more sediment than the suspension feeders moved over similar times. Neoichnological experiments show that ten filter-feeding individuals could take as long as 115 yr to churn a 1 m(2) plot of sediment, by indexing the measured burrowing rates to realistic animal population densities. Ten such mobile deposit feeders as irregular echinoderms could bioturbate the same sediment in just 42 days. Under the maximum population densities modeled, the animals could bioturbate the sediment plot in 61 min. Given the reported results, qualitative interpretation of the rock record is possible: highly burrowed examples of the Skolithos; Ichnofacies reflect high population densities and at least seasonal time spans. Highly burrowed examples of the Cruziana Ichnofacies may represent moderate population densities and short time spans. (C) 2008 Published by Elsevier B.V. DOI
6. Hauck, TE; Dashtgard, SE; Gingras, MK.Relationships between organic carbon and pascichnia morphology in intertidal deposits: Bay of Fundy, New Brunswick, Canada.Palaios, 2008, 23: 336-343 Relationships between organic carbon and pascichnia morphology in intertidal deposits: Bay of Fundy, New Brunswick, Canada
Grazing trails of the Valviferan isopod Chiridotea coeca are examined to test the relationships between trail morphology and the distribution of food (organic carbon). These isopods burrow up to I cm beneath the surface within ripple troughs and planar-bedded sand in the upper intertidal zone. The burrows are grouped into three forms based on the tortuosity of their course and degree of looping and trail crossover in plan view. Sediment samples taken directly from the trail furrows are used to establish the total organic-carbon content associated with each burrow morphology. There is an increase in organic-carbon content from burrows of low tortuosity (linear burrows) to burrows of higher tortuosity (convolute burrows with many crossovers), suggesting that benthic food content directly influences the behavior of C. coeca. Detailed study of trace emplacement further reveals a relationship between C. coeca and food content leading to the recognition of three grazing styles. These are directly related to the plan-view morphology of the trail and reflect the depth at which the isopod tunnels in the sediment. Graphical analysis of weight percent organic carbon against grazing style shows a positive correlation between deep burrowing and high benthic food content. The morphology of C. coeca trails differs from deep-sea turbidite and flysch deposits. In the deep-sea environment resources are replaced slowly, and food distribution is comparatively low and uniform, leading to regular meanders and complex traces that do not crossover existing trails. In the intertidal zone, C. coeca encounters sporadically distributed resources that are replenished semidiurnally. The resultant burrows exhibit increasing crossovers with increasing food content, representing a once-over feeding strategy designed to rapidly harvest a high-value, renewable resource. DOI
5.Dashtgard, SE; Gingras, MK.Tidal controls on the morphology and sedimentology of gravel-dominated deltas and beaches: Examples from the megatidal Bay of Fundy, Canada.J. Sediment. Res., 2007, 77: 1063-1077 Tidal controls on the morphology and sedimentology of gravel-dominated deltas and beaches: Examples from the megatidal Bay of Fundy, Canada
Comparison of two gravel deltas and a gravel beach in the megatidal Bay of Fundy provides insights into tidal influences on gravel-dominated coastlines. Tidal effects on the morphology and sedimentology of the three systems are manifest as: a consistent onshore to offshore topography of the intertidal zone; deep incision of fluvial and tidal channels at the landward end of each system; and the occurrence of these deposits with salt marshes. Indirect morphological and sedimentological manifestations of tides include control on the height of surf-zone bars developed within the intertidal zone; extensive mud deposition (passive association with surf zone bars); and the potential preservation of thick gravel deposits, particularly in transgressive systems. The only direct sedimentological indicators of tides are bidirectional bedforms developed where onshoredirected high-angle cross bedding and offshore-directed trough cross bedding are interbedded. The presence of mud in relation to these bidirectional bedforms provides further evidence of tidal influence within conglomerate systems. Interestingly, no sedimentological characteristics of megatidal conditions are recognized. The results of this research suggest that, although there are a few morphological and sedimentological characteristics that are indicative of tidal deposition, the reliable recognition of tidal control on the deposition of conglomerates requires identifying at least two or three sedimentological characteristics that may have developed as a result of tidal depositional processes. These include: (1) a sharp change upwards in bedding dips, in a vertical succession of upper-shoreface to foreshore deposits; (2) anomalously thick gravel sequences, particularly in transgressive settings; (3) association of gravel beaches and deltas with saltmarsh deposits; and (4) an increase in the amount and extent of mud deposition in conglomeratic systems. DOI
4.Dashtgard, SE; White, RO; Butler, KE; Gingras, MK.Effects of relative sea level change on the depositional character of an embayed beach, Bay of Fundy, Canada.Mar. Geol., 2007, 239: 143-161 Effects of relative sea level change on the depositional character of an embayed beach, Bay of Fundy, Canada
sequence stratigraphy; Bay of Fundy; embayment; beach; sedimentology; ichnology; seismic profiling
Sedimentological mapping and interpretation of shallow-seismic profiles collected at Waterside Beach, Bay of Fundy, Canada reveal a varied post-glacial depositional history for the shore and near-shore system. Four sedimentologically and seismically distinct units are identified: I) rounded pebbles and cobbles; 2) laminated silty sand; 3) rooted, laminated silt and clayey silt; and 4) mixed sand and gravel. These four units were deposited at different stages of relative sea level (RSL) fall and rise and under significantly diverse hydraulic conditions. The rounded pebbles and cobbles unit is glacial outwash deposited into a marine setting during rapidly falling RSL. This was followed by a seaward shift of the shoreline and development of intertidal and subtidal mud flats (laminated silty sand unit) during continued sea level fall. Salt marshes (rooted laminated silt and clayey silt unit) developed landward of the mud flats upon subsequent sea level rise, and finally the beach (mixed sand and gravel) developed as sea level rise exposed the rounded pebble and cobble unit to wave reworking and erosion. The relatively complex nature of the deposits and depositional processes, and the significant changes in the sedimentological and geomorphological character of the Waterside coastline during RSL fall, lowstand, and rise elucidates the variability that may be observed in sediments over relatively short time frames (< 15 000 yr) - particularly for sediments deposited in embayments. Within embayed settings, relative sea level change does not only imply a shift of the shoreline, but may also result in significant changes in depositional environments, basin configuration, and/or hydraulic conditions. This is the case for Waterside Beach, where the post-glacial depositional character of the coastline developed in response to three main factors: 1) notable increase in tidal range; 2) large-scale changes in the configuration of the upper Bay of Fundy; and, 3) availability of coarse-grained sediment and a mechanism to deliver that sediment to the coastline. (c) 2007 Elsevier B.V. All rights reserved. DOI
