Devonian System in Williston Basin and Central Montana

Abstract

Knowledge of Devonian stratigraphy in the southern part of the Williston basin and adjacent areas has increased greatly since extensive deep drilling began in 1951. In recent years many formation names have been borrowed from the Alberta basin, the Canadian part of the Williston basin, and the northern Rocky Mountains of western Montana and applied with varying degrees of success but little uniformity to the buried Devonian rocks of central Montana and the United States part of the Williston basin. This paper summarizes the stratigraphy and geologic history of the Devonian system and, based on the facts and interpretations presented, proposes a uniform subsurface nomenclature for Devonian rocks in North Dakota, South Dakota, eastern and central Montana east of the 111 ; meridian, and the adjacent part of Wyoming. The Middle Devonian subsurface nomenclature introduced by Baillie in 1953 is followed with the exception of the Manitoba group and the Ashern formation, which is not a continuous stratigraphic unit in the report area and is included at the base of the Winnipegosis formation. The Middle Devonian series consists in ascending order of the Winnipegosis and Prairie formations of the Elk Point group and the Dawson Bay formation. The Upper Devonian series consists in ascending order of the Souris River formation, the Jefferson group made up of the Duperow and Birdbear formations, and the Three Forks formation, overlain by the Bakken formation of Late Devonian(?) and Early Mississippian age. (Lower Devonian rocks either have not been recognized or ha e not been penetrated in the subsurface, although local channel-fill deposits, assigned by Dorf in 1934 to the Beartooth Butte formation of Early Devonian age, have been mapped in the mountains of northern Wyoming and south-central Montana.) The Mobil Producing Company's Birdbear well No. 1 in Dunn County, North Dakota, near the geographical center of the Williston basin is proposed as the standard subsurface section for the Souris River, Duperow, and Three Forks formations. A standard subsurface section, as used here, replaces an inadequately defined or unpublished type section or supplements a distant type section that is not representative of the formation in the area under study. The same well serves as the type subsurface section of the Birdbear formation, which is named herein. The Birdbear replaces the Nisku formation of Baillie, which can not be correlated with the type Nisku formation of the Alberta basin. The subsurface Upper Devonian formations are correlated with the Devonian outcrop at the type locality of the Jefferson and Three Forks formations near Logan, Montana. The original contact between the Jefferson and Three Forks formations as defined by Peale in 1893 is accepted in preference End_Page 2293------------------------------ to the higher boundary proposed by Sloss and Laird at a much later date. The subsurface Jefferson group, consisting of the Duperow and Birdbear formations, is the stratigraphic equivalent of the Jefferson formation, and the uppermost part of the subsurface Souris River formation is correlative with part of the Maywood formation, which underlies the Jefferson near Logan. The potentialities of Devonian rocks as petroleum reservoirs in the southern part of the Williston basin and in central Montana are still relatively underdeveloped. Minor oil production has been developed in the Winnipegosis, Dawson Bay, Duperow, and Three Forks formations and many scattered shows of oil and gas have been found in the porous Birdbear formation.