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The AAPG/Datapages Combined Publications Database
Williston Basin Symposium
Abstract
MTGS-AAPG
Seventh International Williston Basin Symposium, July 23,
Sequence Stratigraphy of the Bakken and Exshaw Formations: A Continuum of Black Shale Formations in the Western Canada Sedimentary Basin
ABSTRACT
Late Devonian and Early Mississippian Bakken and Exshaw formations form part of a continuum of regionally correctable, organic-rich, black shale units covering much of the western interior of North America. In the Williston Basin, the Bakken Formation is composed of three members: 1.) a black mudstone Lower Member; 2.) a gray mudstone/sandstone Middle Member; and 3.) a black mudstone Upper Member. On the cratonic platform and in the Prophet Trough to the west, the Exshaw Formation is composed of two members: 1.) a lower Black Shale Member; and 2.) an upper Siltstone Member. The black shale unit at the base of the overlying Early Mississippian Banff Formation constitutes a second organic-rich interval. Biostratigraphic analyses show the Exshaw and Bakken formations are diachronous. The Bakken and Exshaw formations and basal black shale unit of the Banff Formation are subdivided into three successive systems tracts: 1.) a transgressive systems tract; 2.) a low stand systems tract; and 3.) a second transgressive systems tract. A sequence boundary is identified between the Lower and Middle Bakken members. Variations in the internal composition of the systems tracts imply that two depositional sites: 1.) Williston Basin and 2.) cratonic platform and Prophet Trough, responded separately to sea-level change because of different water depths during black shale deposition.
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