Systems tracts and their bounding surfaces in the low- accommodation Upper Mannville group, Saskatchewan, Canada
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| Authors: | Morshedian, A; MacEachern, JA; Dashtgard, SE; Bann, KL; Pemberton, SG |
| Year: | 2019 |
| Journal: | Mar. Pet. Geol. 110: 35-54 Article Link (DOI) |
| Title: | Systems tracts and their bounding surfaces in the low- accommodation Upper Mannville group, Saskatchewan, Canada |
| Abstract: | 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. |
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