Genetic Sequence Relationships of Winnipegosis Platform Carbonates, Southern Elk Point Basin, North Dakota

Keith W. Shanley, Timothy A. Cross

Examination of cores and well-log data from the Winnipegosis Formation (Givetian) within a study area of approximately 11,500 mi² (30,000 km²) in northern North Dakota allows recognition of seven time-stratigraphic progradational units within the Winnipegosis Formation. Together with the underlying Ashern Formation, these units are arranged in landward-stepping, vertical stacking, and seaward-stepping geometric patterns, which reflect changes in relative sea level. Abrupt juxtaposition of shallow over deeper water lithologies, evidence for subaerial exposure, and onlap geometries further suggest that these progradational units form two larger Vail-type sequences separated by regionally persistent unconformities or their correlative conformities.

Sea level rise during the early Eifelian caused southeastward onlap of the Ashern Formation onto Middle Silurian carbonates of the Interlake Formation. Maximum flooding, expressed by deepest marine facies and a hardground surface, suggests the existence of a condensed section at the top of the Ashern Formation. This section was developed during the maximum rate of sea level rise. A decrease in the rate of sea level rise resulted in aggradation of lower Winnipegosis units on a gently dipping ramp. These units are represented by nodular and burrowed open-marine limestones with scattered stromatoporoid patch reefs and grainstone shoals. During the subsequent sea level fall, represented by Temple units, a shelf margin with pronounced depositional topography and adjacent starved basin were developed. Temple strata include coral-brachiopod-stromatoporoid reefs and productive fore-reef talus deposits along the shelf-margin rim. With increased rates of sea level fall, the platform interior and shelf margin were subaerially exposed, slope carbonates were dolomitized, and the E-shale was deposited as a lowstand wedge. As the rate of sea level fall diminished, tidal flat and shallow marine units prograded and basinal laminites were deposited. Understanding time-stratigraphic geometries and the distribution of depositional facies within that context allows accurate prediction of reservoir quality and distribution.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988