Carbonate-Evaporite Cycles in Lower Duperow Formation of Williston Basin

Abstract

ABSTRACT The Duperow (Frasnian) sediments of the Williston basin consist of about twelve regular cycles. Stratigraphic and petrographic study was made of the lower 300 ft. of these strata using cores and cuttings from 45 wells and about 200 radioactivity logs from selected locations in North Dakota, Montana, and Saskatchewan. Each cycle consists of a lower member, either dark brown, burrowed, lithoclastic-bioclastic brachiopod-crinoid limestone with a mud matrix, or a stromatoporoid biostrome and a middle part of brown lime mudstone with a restricted microfauna of ostracods and calcispheres interbedded with unfossiliferous pelletoid beds or laminated lime mudstone. Bedded anhydrite and greygreen, silty, very fine-grained dolomite displaying intertidal and supratidal sedimentary structures cap the cycle. The Duperow cycles are amazingly widespread and constituent beds only 10-15 ft. thick can be traced completely across the Williston basin, a distance of several hundred miles. Deposition occurred within a vast backreef lagoon lying south of the Woodbend reef platform of southern Alberta and stretching to a sandy shore in South Dakota and northern Wyoming. This lagoon was periodically and rapidly flooded with normal marine water, permitting benthonic organisms to flourish; the sea then gradually shallowed as sedimentation filled the basin. Desiccation resulted in extensive tidal flats and evaporitic sabkhas and perhaps in some dolomitization of the carbonates on shelves outside the basin. The cause of such cyclic sedimentation might have been slow, steady subsidence of the basin with a superimposed climatic rhythm which may have speeded up reef growth and periodically choked off sea water from the basin. Perhaps this process operated coincidentally with sporadic eustatic sea level fluctuation or with abrupt periodic subsidence of the whole basin. The low bathymetric relief which permitted rapid flooding certainly would have aided in development of such cycles. Comparison of patterns of replacement dolomite in lower Duperow sediments of the basin itself with lateral and vertical changes in interpreted depositional facies and with thickness variations of the cycles leads to the conclusion that most replacement dolomitization occurred in the formation before complete lithification, probably not during Duperow time, but later in Devonian time during shelfward evaporite deposition and tectonic adjustment preceding the Mississippian transgression.