--> Preliminary Sequence Stratigraphic Framework for a Mississippian Madison Group Shelf Margin, South Boulder Canyon, Southwest Montana

AAPG ACE 2018

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Preliminary Sequence Stratigraphic Framework for a Mississippian Madison Group Shelf Margin, South Boulder Canyon, Southwest Montana

Abstract

A well exposed, near depositional-dip oriented Madison Group outcrop was studied at South Boulder Canyon in southwest Montana. These strata were deposited on the south flank of the central Montana trough and are placed into a sequence stratigraphic framework for the first time. The outcrop, nearly 300 meters thick, comprises moderately well-exposed Lodgepole Formation calcareous shales and argillaceous limestone, which grades upward into more resistant Mission Canyon Formation limestones. Thin, recessive-weathering beds of late Mississippian - early Pennsylvanian Amsden Formation cap the exposure.

Two complete and several partial Madison Group sections were measured, with bedding surfaces walked laterally and traced onto photopans. A series of polished outcrop slabs and thin sections aided in facies descriptions.

The Madison Group at South Boulder Canyon is interpreted in the context of Sonnenfeld's (1996) regional Mississippian sequence stratigraphic framework. This defined the Madison and related strata as six third-order depositional sequences within a second-order sequence. Parts of four of these third-order sequences are recognized at this outcrop, with bounding surfaces and internal facies architecture described for each. Individual sequences comprise lithofacies representing carbonate platform top, slope and basinal environments and range from less than 75 to over 100 meters in thickness. Proportions of platform top, slope, and basinal lithofacies between third order sequences varies relative to position within the second-order sequence. These sequences identified in outcrop were correlated to the adjacent subsurface and related to a relative sea level curve modified from Sonnenfeld (ibid.). Both long and short term sea level fluctuations are important in determining the relationship between accommodation space and sediment supply. The interplay of these factors influenced the change in Madison Group deposition, from ramp (Sequences 1 and 2) to flat-topped platform (Sequences 3 to 6).

Recognizing the facies architecture and offset within higher order sequences in outcrop provides important insights into the distribution of reservoir and non-reservoir lithologies at the oilfield scale. Our characterization of third order sequences from outcrop and subsurface in southwest Montana supports the regional Mississippian sequence stratigraphy of Sonnenfeld (ibid.).