Sequence Evolution and Hierarchy Within the Lower Mississippian Madison Limestone of Northwest Wyoming

Abstract

The Madison Limestone forms a 2nd-order sequence bounded by regional angular unconformities. Six 3rd-order sequences stack into 2 composite sequences intermediate in scale between the entire 2nd-order Madison and its component 3rd-order sequences. This long-term pattern not only highlights a ramp-to-shelf evolution, but is also key to regional physical stratigraphic correlation between the Wyoming Shelf Province and much higher subsidence areas such as the Central Montana Trough and the Williston Basin. 4 out the 5 internal SB’s also show evidence of subaerial exposure capping progradational cycle sets. Sequence boundaries capping sequences III, IV, and locally V occur at the bases of prominent solution collapse breccias representing former evaporites interbedded with restricted argillaceous dolomudstones and micropeloidal grainstones. During early phases of 3rd-order transgressive systems tracts, sedimentation rates kept up with increasing accommodation, fostering shallow water, hypersaline conditions prior to onset of open-marine conditions. This stratigraphic context for evaporite occurrences contrasts with the common paradigm of asymmetric, evaporite-capped cycles.

The lower composite sequence is correlative to the Lodgepole Formation and contains 2 3rd-order sequences (I, II) that are prograding ramp systems containing “shore-attached” grainstone complexes directly overlain by thin peritidal to sabkha deposits, generally without intervening lagoonal rocks. Sequence III is limited to the Big Goose Member of the Mission Canyon Formation, forms the high-accommodation portion of the upper Madison composite sequence, and contains the majority of the Bighorn Basin’s dolomitic reservoir facies. The onset of “detached” grainstone barrier complexes at the outset of Sequence III coincided with aggradation of thick, lagoonal deposits marking the advent of a much flatter, shelf-like morphology and the thickest shallow water facies accumulation of all Madison sequences. Peak transgression of the upper Madison composite sequence is represented by the widespread skeletal lime packstone and grainstone of the Little Tongue Member (Seq. IV). Upper Osagean to lower Meramecian sequences IV-VI thin upward, involve karsted SB’s, and are associated with increasing argillaceous influxes above SB’s. This is indicative of the long-term decrease in accommodation coupled with increasingly humid conditions, culminating with the top Madison karsted, angular unconformity.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018