Sandstone Lithofacies Within the Icebox Fm (Ordovician), Williston Basin, North Dakota and Montana

Aaron J. Ulishney, Department of Geology & Geological Engineering, University of North Dakota, Grand Forks, ND 58202, [email protected] and Richard D. LeFever, Department of Geology & Geological Engineering, University of North Dakota, Grand Forks, 58202.

The Icebox Fm (Ordovician) is the middle of three formations within the Winnipeg Group. It is a regionally extensive shale, typically dark green to black, and reaches a maximum thickness in excess of 200 ft (61 m)in North Dakota. The Icebox contains significant organic carbon and is considered to be an important source rock for the Lower Paleozoic in the Williston Basin.

Wireline log gamma-ray traces were digitized and converted to a uniform scale and format. Traceable coarser facies and discontinuities were established by correlating the digitized wireline log curves across North Dakota and Montana. Forty traceable sandy lithofacies were identified. Five have regional extent, and a maximum thickness of at least 20 ft (6 m); one is 72 ft (22 m) at its thickest. Five additional sand bodies each extend across several tens of miles, and have maximum thicknesses ranging from 10 ft (3 m) to 23 ft (7 m).

In North Dakota, six cores have been taken through the sandy intervals, all in the lower part of the Icebox Fm. Based on the existing cores, it appears that the sandy lithofacies represent intervals of bioturbated sandstone and siltstone within the shalier Icebox Fm.

The Icebox Fm in the Williston Basin was deposited in the epeiric sea distal to shore, but not in deep water The sandy bodies represent environments that were somewhat shallower, perhaps even marginal marine. Many of the less extensive bodies appear to represent deposition on top of local topographic or structural features.