--> ABSTRACT: Synsedimentary Deformation and Erosion of the Bakken/Sappington Formation in West-Central Montana: Evidence for a Brief Basin Polarity Switch and Development of Paleohighs along the Western Bakken Fairway, by Adiguzel, Zeynep; Grader, George W.; Doughty, Ted. P.; and Pope, Michael C.; #90142 (2012)

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Synsedimentary Deformation and Erosion of the Bakken/Sappington Formation in West-Central Montana: Evidence for a Brief Basin Polarity Switch and Development of Paleohighs along the Western Bakken Fairway

Adiguzel, Zeynep1; Grader, George W.*2; Doughty, Ted. P.2; and Pope, Michael C.1
(1) Texas A & M University
(2) PRISEM Geoscience, Spokane, WA.

The Bakken-equivalent Sappington Formation in west-central Montana provides good evidence for tectonically-driven instability and an explanation for why part of the Devonian-Mississippian rock record is locally absent. Similar to the Bakken, the Sappington Formation comprises an organic-rich basal black shale, a shallowing-upward Siltstone Member, and a younger upper black shale that correlates with the basal Lodgepole Formation. In south-central Montana, north of Yellowstone Park, all three members are well developed. The facies and stacking patterns record an east-to-west, down-to-basin polarity for the incipiently subsiding Central Montana Trough. To the northwest in the Three Forks area, the lower black shale is locally eroded below the basal part of the Siltstone Member (e.g. Logan Type Section) before thickening substantially to the west. The upper black shale displays a nearly opposite distribution. It is missing west of Three Forks and in other parts of northwestern Montana, like along the southern Sweetgrass Arch (Alberta Bakken Fairway), where the Siltstone Member is also missing.

West of Three Forks at Milligan Canyon, the Siltstone Member is deformed into meter-scale detached slump folds that moved down a southeast-oriented paleoslope. After slumping, the folds were sharply truncated by erosive currents and mid to distal ramp encrinite beds of the lower Lodgepole. A southeast paleoslope and widespread submarine erosion demonstrate that basin polarity switched to a west-to-east orientation during the Early Mississippian in western Montana. Paleohighs such as the Lemhi Arch (southwest Montana) and the paleo Sweetgrass Arch and ‘Montania’ (northwestern Montana) were active at this time and subjected to erosion or nondeposition. Basin polarity reverted back again to an east-to-west orientation after filling of accommodation space, continued Mississippian subsidence and westward progradation of the Madison Group.

We conclude that the polarity reversal and erosion of the western upper black shale provides evidence for basin inversion, indicative of a complexly loaded plate margin reactivated across shelf paleohighs and cratonic intrashelf basins (migration of an Antler forebulge?). Although strata often look more subtly disconformable, our observations explain why there is an erosive submarine hiatus in the earliest Mississippian and why key Bakken reservoir units were locally thinned or removed prior to Lodgepole deposition.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California