The Duperow (Frasnian) sediments of the Williston basin consist of approximately 12 regular cycles. Stratigraphic and petrographic studies were made of the lower 300 ft (91 m) of these strata using cores, cuttings, and radioactivity logs from selected locations in North Dakota, Montana, and Saskatachewan. Each cycle consists of three members. The lower member consists of either dark-brown, burrowed, lithoclastic-bioclastic brachiopod-crinoid limestone with a mud matrix, or a stromatoporoid boundstone. A middle member consists of brown lime mudstone with a restricted microfauna of ostracods and calcispheres interbedded with unfossiliferous pelletoid beds or laminated lime mudstone. Bedded anhydrite and gray-green, silty, very fine-grained dolomite displaying intertidal and supratidal sedimentary structures cap each cycle.

End_Page 870------------------------------

The Duperow cycles are widespread and constituent beds only 10-15 ft (3-5 m) thick can be traced completely across the Williston basin. Deposition occurred within a vast back-reef 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 organisms to flourish; the sea then gradually shallowed as sediments filled the basin.

Desiccation produced extensive tidal flats and evaporitic sabkhas and was perhaps responsible for 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 that may have speeded up reef growth and periodically choked off seawater 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 that permitted rapid flooding certainly would have aided in development of such cycles.

End_of_Article - Last_Page 871------------