The Stratigraphy And Depositional History Of The Deadwood Formation, With A Focus On Early Paleozoic Subsidence In The Williston Basin

The Deadwood Formation is an assemblage of siliciclastic, carbonate, and evaporite sedimentary rocks in North Dakota, South Dakota, Montana, Wyoming, Manitoba, and Saskatchewan. The majority of the lateral extent of the Deadwood Formation is in the subsurface of the Williston Basin, where it is the basal lithostratigraphic unit. Deposition began roughly 501 million years ago, as the Sauk sequence reached the exposed Precambrian igneous and metamorphic rock of the North American Craton. Six identifiable and widespread gamma ray markers occur in the well logs, dividing the formation into six informal units, label members A through F in ascending order. The initial deposits on the craton were conglomerates and sandstones of the Cambrian Member A. These sediments were overlain by glauconite rich, siltstones and fine-grained sandstones of the Cambrian and Ordovician Member B. After the deposition of Member B, three regressive-transgressive sequences took place, depositing a succession of sandstones, limestones, dolomudstones, siliciclastic mudstones, and calcareous siltstones. These deposits represent the Ordovician members, C, D, E and F. Using the thickness, depositional environments, age of each member, and other well information, tectonic subsidence values were determined using backstripping analysis. This analysis was completed by inputting all of the information into Novva®, a 1D geological modeling software released by Sirius Exploration Geochemistry Inc. Data collected from well logs and core, other data researched by the author, and information from previous works was combined with information and calculations supplied by Novva®. The results produce an accurate computation of the depositional history for the seven wells that penetrated all six members of the Deadwood Formation and the Precambrian basement. Prior to and at the start of Deadwood deposition the Williston Basin did not exist. Evidence from isopach maps created for each member of the Deadwood Formation and the results from Novva® concluded that subsidence in the area, now known as the Williston Basin, did not begin until Member C was being deposited. This places the initiation of the Williston Basin to be roughly 485 to 482 million years ago