Stratigraphic architecture and facies analysis of the Lower Cretaceous Dina Member of the Mannville Group in Northwest Saskatchewan.
Kohlruss, Daniel Jonathon
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The Dina Member in northwest Saskatchewan was deposited unconformably on top of the underlying Devonian Elk Point Group with the thickest Dina sandstones residing within paleo-topographic lows on the unconformity surface. The Dina Member was extensively eroded by Pleistocene glacial processes and is unconformably overlain by Pleistocene glacial tills. Analysis of 83 stratigraphic test hole drill cores and 255 geophysical well log suites has revealed 8 recurring facies and 5 facies associations. The facies are comprised of siliciclastic sediments, including sandstones, siltstones, mudstones and in rare instances, coal. These facies are predominantly non-marine in origin, including fluvial sediments and associated over-bank deposits. Many fluvial facies exhibit a significant tidal and/or seasonal brackish water influence. Tidal indicators are manifested as rhythmic grain size striping, reactivation surfaces and co- and back-flow structures. Brackish conditions are indicated by impoverished, mono-specific assemblages of diminutive trace fossils in mud beds and discrete sand layers, along with marine palynmorphs and microfossils. Deposition of the Dina Member is interpreted to have occurred within an incised valley system formed as a result of a relative base-level drop, which initiated trenching and deepening of pre-existing valleys. Braided channel deposits proceeded to fill the lowest portions of the valley during the lowstand and part of the subsequent transgression. As the valley was gradually filled and the lateral accommodation space increased, fluvial style changed from braided to meandering, with deposition of laterally accreted point-bar deposits. These manifest as inclined stratification (IS) and inclined heterolithic stratification (IHS) overlying basal trough cross-bed sets. Mud filled, oxbow lake deposits were also observed in close association with the point-bar deposits. Bitumen distribution throughout the study area is not uniform and has several controls. Bitumen saturation is highest where braided fluvial and IS deposits are found, especially when stacked. Conversely, IHS deposits and mud filled oxbow lake deposits restrict oil flow and in cases act as flow barriers to oil migration. Bitumen is trapped laterally by the incised valley walls created by the sub-Cretaceous unconformity, where bitumen saturated sand pinches out against the impermeable carbonates. The Dina sandstones, in turn, were sealed above by shales of the Clearwater Formation/ Cummings Member of the Mannville Group.