SEDIMENTARY EVOLUTION OF WILLISTON BASIN AT THE ONSET OF HIRNANTIAN GLACIATION

The Williston Basin is a large epicratonic basin underlying the great plains of North Dakota, South Dakota, Montana, Saskatchewan and Manitoba. During the Late Ordovician the basin was located in the interior of the shallow and very broad, tropical, arid carbonate shelf that developed along the western passive margin of North America. This study focuses on the Red River Formation in North Dakota, an up to 210 m thick succession of predominantly shallow-marine carbonates that record the Upper Ordovician Katian greenhouse to icehouse transition. The study is based on all available combined neutron porosity-bulk density logs from the North Dakota Geological Survey, as well as on detailed analysis of 27 cores.

The upper Red River Formation is made up of three complete 3^rd order sequences (RR1, RR2, and RR3) composed of lowstand (LST), transgressive (TST), and highstand systems tracts (HST). Subaqueous anhydrites within each sequence represent evaporite LSTs that formed during relative sea-level lows that lead to precipitation of gypsum (later pseudomorphed by anhydrite) in a quiet, lagoonal environment. TSTs are commonly thin and composed of subtidal, lagoonal facies of predominantly skeletal mudstone to wacke-packstone with abundant burrow mottling. HSTs are generally thicker than TSTs and composed of peritidal, porous laminated dolomite. Sequence boundaries are characterized by onlap of shallow subaqueous evaporites, with locally developed subaerial and submarine erosional truncation marked by intraclast breccia interbedded with dololaminites below sequence boundaries.

The evaporite-bounded 3rd–order sequences are composed of relatively few parasequences whose duration seems to be within the eccentricity band. This is compatible with the transitional eustasy, and the onset of icehouse conditions in Late Ordovician Hirnantian. The low-amplitude sea-level changes and arid climate promoted regional early dolomitization during late highstands and subsequent gypsum precipitation as evaporative drawdown lead to third-order lowstand systems. By the Late Ordovician Richmondian the sea level fluctuations seem to have changed from moderate back to low-amplitude, prior to Hirnantian glaciation and the development of Ordovician-Silurian unconformity during the sea-level lowstand.