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The Effects of Sea-Level Fluctuations on Prograding Shorelines and Estuarine Valley-Fill Sequences in the Glauconitic Member, Medicine River Field and Adjacent Areas
The Glauconitic Member of the middle Mannville Group consists of four major lithological successions in the Medicine River Field and adjacent areas: 1) the Medicine River sheet sands; 2) Hoadley bar sands, representing prograding shoreline deposits; 3) the pre-Glauconitic coal; and 4) post-Glauconitic coal, valley-fill successions. The lowering of relative sea level resulted in the seaward progradation of shorelines and the development of deeply incised river valleys cutting unconformably into previously deposited shoreface sediments. The return of shoreline sedimentation to the area and the deposition of valley-fill successions was associated with the subsequent rise in sea level.
The Medicine River sheet sands succession consists of offshore marine mudstones at the base, passing upward through interbedded sandstone, siltstone and mudstone of the lower shoreface, to middle and upper shoreface amalgamated sandstones, to a lagoonal mudstone and overlying coal. This succession was deposited during the northward progradation of a laterally extensive shoreline complex oriented southwest-northeast. Considerable variation in thickness and minimal reworking of sediments indicate localized deposition. Brackish water indicators include syneresis cracks, Gyrolithes sp., and pyritic and sideritic concretions.
The Hoadley bar sands succession lies north of and parallel to the Medicine River sheet sands. It displays a similar shallowing upward trend but consists of 10 to 25 m of laterally extensive, highly reworked, fine to medium grained shoreface sandstones. A widespread coal zone, the Glauconitic coal, directly overlies these two successions.
Valley fill successions in the study area are 10 to 50 m thick, 1 to 4 km in width and tens of kilometres in length. The mud dominated valley fill deposits commonly form the updip seal for hydrocarbon reservoirs in the Hoadley bar sands and the Medicine River sheet sands successions. Examples include the Ferrier, Hoadley and Medicine River fields. Sand dominated valley-fill deposits locally form highly permeable reservoirs, capped updip and laterally by surrounding muddy estuarine deposits. One example from this study is the Sylvan Lake Field. To date, all known producing fields of the Glauconitic member in the study area are associated with either the pre-Glauconitic coal, or post-Glauconitic coal, valley-fill successions.
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