Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name


Department or School/College

Department of Geosciences

Committee Chair

Marc S. Hendrix

Commitee Members

Christopher P. Palmer, Michael H. Hofmann


Bakken, organic-rich mudstone


University of Montana


Over the past decade, exploration and development of the Devonian Bakken petroleum system in North Dakota and Montana has grown to become the top producing petroleum play in the United States. Despite this remarkable economic boom, relatively little is known about the stratigraphy and sedimentology of equivalent Devonian strata exposed at the surface to the west in the Bridger and Tobacco Root Ranges of southwestern-central Montana. There, the Bakken-equivalent Sappington Formation has not been stratigraphically evaluated since the 1950's and 1960's, long before the widespread application of sequence stratigraphy. This thesis provides an outcrop-based sequence stratigraphic analysis of the Sappington Formation in order to better understand the depositional history of these strata, while offering a set of predictive tools for evaluation of time-equivalent rocks more regionally. The main objectives of this study are the following: 1) to develop a facies model for the Sappington Formation in southwestern-central Montana; 2) to establish a sequence stratigraphic framework for the Sappington Formation in the same area; and 3) to extend this interpretation into the subsurface more regionally, beyond the 'Sappington depositional basin'. To meet these objectives, an outcrop investigation was conducted including sedimentologic, petrographic, petrophysical, and geochemical analyses. These results were used to inform a more regional well log investigation of time-equivalent strata in Montana. The following eight lithofacies characterize the Sappington Formation in southwestern-central Montana: 3) 1) organic-rich mudstone; 2) bioturbated, calcareous/dolomitic muddy siltstone; bioturbated, wavy-laminated/lenticular silty dolostone with interbedded/interlaminated mudstone; 4) hummocky cross-stratified silty dolostone; 5) bioturbated, ripple-laminated, calcareous/dolomitic siltstone-silty dolostone; 6) ripple-laminated silty dolostone; 7) planar-bedded silty dolostone; and 8) oncolitic, fossil-bearing floatstone. From these facies, the following four sedimentary environments are interpreted based on facies associations: 1) partly restricted offshore marine environment; 2) open marine carbonate build-up environment; 3) open marine offshore to offshore transition environment; and 4) open marine storm-dominated shoreface environment. Based on a depositional sequence stratigraphic approach, the Sappington Formation is interpreted to include two higher order sequences with an additional sequence continuing into the overlying Lodgepole limestone. Log-based analysis of depositional patterns for the Sappington Formation and its equivalents in southwestern-central Montana are consistent with sedimentary environments interpreted from outcrop work in suggesting that the lower and upper Sappington mudstone units were deposited in semi-isolated, restricted marine basins, whereas the middle Sappington member carbonate units were deposited in an open marine basin.



© Copyright 2014 Tetsuro Nagase