Stratigraphy and Depositional History of the Deadwood Formation (Upper Cambrian and Lower Ordovician), Williston Basin, North Dakota

Based on gamma-ray characteristics, the Deadwood Formation is divided into six informal, attribute-defined units, members A through F. Members A and Bare Late Cambrian in age; members C through Fare Early Ordovician in age. The type section of the Deadwood, in the northern Black Hills, is dominated by Upper Cambrian strata. Much thicker Lower Ordovician rocks occur in the deeper portion of the Williston Basin. The Deadwood comprises the Sauk sequence and represents the initial Paleozoic transgression or the seas on the North American craton. Eight smaller-scale transgressive-regressive events occurred on the craton during the Late Cambrian and the Early Ordovician. Three of these are represented by the Upper Cambrian portion of the Deadwood; three are represented by the Lower Ordovician strata. The lithotypes deposited during the Early Ordovician differ from those deposited during the Late Cambrian. Basal member A consists of conglomerates and cross-bedded quartz arenites. This unit was deposited in high-energy, nearshore settings during the Dresbachian. The upper portion of this member has been highly oxidized due to weathering during the withdrawal of the seas at the end of the Dresbachian. Member B represents two later Late Cambrian transgressiveregressive events. In the subsurface, this member is dominated by highly glauconitic, bioturbated to laminated, fine-grained, siliciclastic rocks. Toward the eastern margin of the basin, the grain size coarsens and a decrease in the glauconite content occurs. This member was deposited in low-energy subtidal shelf, possibly representing offshore to lower shoreface settings. Members C through F consist of three asymmetrical, progradational successions deposited during three Early Ordovician transgressiveregressive events. In ascending order, each succession consists of: 1) a mixed sandstone-limestone lithotype; 2) limestone lithotypes ranging from mudstone to grainstone; 3) bioturbated, peloidal, · calcareous, siliciclastic mudstone and siltstone; 4) bioturbated to planar-laminated, peloidal, calcareous siltstone and sandstone; 5 ) Skolithos-bored, cross-bedded to planar-laminated quartz arenite; 6) bioturbated, dolomite- and anhydrite-cemented, fossiliferous quartz wacke; and 7) silty, lamin~ted dolomudstone. The asymmetrical successions represent progradation of a siliciclastic shoreline, and back-barrier lagoon and intertidal algal flat over a shallow siliciclastic shelf and a distal carbonate shoal. The present distribution of the members and the individual lithotypes in the Williston Basin is a function of: 1) the limited eastward advance of the carbonate shoal during transgression, 2) the limited westward advance of the shoreline during progradation, 3) deep shoreface erosion of the previous succession during rapid transgression, and 4) Middle Ordovician erosion. Middle Ordovician erosion may have removed strata deposited during two later Early Ordovician transgressive-regressive events