9. 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
8. 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
7. 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
6.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
5. 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
4.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
3.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
2.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
1.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
