Sequence Correlations and Stacking Patterns of Lower Mississippian Strata in Montana and Idaho: Evidence for Eustatic Controls on Sequence Development and Platform Evolution

REID, S. K., and S. L. DOROBEK, Texas A&M University, College Station, TX

The Lower Mississippian Mission Canyon Formation and stratigraphic equivalents in southwestern Montana and east-central Idaho consist of stacked sequences of shallowing-upward platform carbonates that grade westward into correlative slope and basinal sequences. Deeper water facies were deposited in a northern extension of the Antler foreland basin. Detailed biostratigraphic and lithostratigraphic correlations across the platform and into the Antler foredeep document regional stacking patterns of third- to fourth-order depositional sequences across this wide area. Twelve to fourteen (depending on amount of erosion/karstification on top of the Mission Canyon Formation) sequences have been identified and correlated across the platform and slope. Major flooding surfaces can be identified nd correlated from the Antler foredeep across the Mission Canyon platform and into stratigraphic equivalents in the Williston basin. These regional flooding surfaces, along with biostratigraphic boundaries, allow detailed correlation of individual sequences and sequence sets from the Williston basin to slope environments of the Mission Canyon platform in east-central Idaho, a palinspastic distance of over 1500 km.

Progressive shallowing of facies within stacked slope sequences indicates progradation. Platform progradation occurred during deposition of two progradational, stacked sequence sets that comprise a large-scale highstand systems tract. An areally restricted platform margin carbonate wedge (McKenzie Canyon Formation) apparently developed during subaerial exposure and karstification of most of the Mission Canyon platform in Montana.

This study illustrates the application of sequence stratigraphy in a tectonically shortened, carbonate platform-to-basin setting where lateral correlations must be made between isolated and widely separated exposures. The correlation of sequences with very similar stacking patterns from the Williston basin and into the Antler foredeep also suggests that these sequences were caused by eustatic sea-level oscillations.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)