Despite the abundance of Paleozoic epeiric sea carbonates and vast research on various aspects of their stratigraphy, sequence stratigraphic, chemostratigraphic and diagenetic studies of mixed carbonate-evaporite cratonic successions are relatively scarce. The Williston Basin of North America is a large interior cratonic basin underlying the great plains of North Dakota, South Dakota, Montana, Saskatchewan, and Manitoba. The Williston Basin province contains ten petroleum systems, with 13 conventional and six unconventional assessment units spanning the Upper Cambrian through Tertiary. During the Late Ordovician, oceanic communication with the Williston Basin was periodically limited, which during sea-level lows and under a tropical semi-arid climate resulted in the development of basin-central evaporitic units as parts of shallowing and brining-upward, anhydrite-bound sequences of the Red River Formation, the 2nd major conventional reservoir in the basin. Sequences record a change from semi-arid transgressive (TST) and early highstand (HST) to arid late highstand (HST) and lowstand systems tracts (LST). The small number of parasequences (3-15 per sequence) and their durations are suggestive of eccentricity and obliquity forcing, which is compatible with transitional eustasy during the Late Ordovician global cooling and further suggests the existence of moderate ice sheets and the onset of icehouse conditions in Late Ordovician.
In the absence of ocean floor sediments, the epeiric sea carbonates are the major recorders of the Paleozoic global carbon cycle. However, the restricted circulation coupled with local (regional) environmental influences can result in geographic variations in C-isotope record, and possibly excursions that are unrelated to the pelagic surface ocean. Deep burial commonly results in multiple replacement episodes of metastable, early formed dolomite crystals by later, more stable phases. This also may reset the stable isotope signature of dolomite at progressively higher temperatures. Although deeply buried (up to 4.9 km), the Upper Ordovician dolomites of the Williston Basin, for the most part, exhibit no multiple events of post-depositional recrystallization, and both dolomite and limestone have similar carbon-isotope values, suggesting that carbon was rock buffered, and could be representing the original marine C-isotope record. This further suggests that the Upper Ordovician mixed carbonate-evaporite Red River Fm. of the Williston Basin provides a superb example of a large, periodically isolated evaporitic basin in which the carbon isotope signal has been greatly influenced primarily by local (regional) environmental conditions, and stabilized by later burial diagenesis, but without greatly resetting the original reflux dolomite petrography.
Dr. Antun Husinec is an Associate Professor of Geology at St. Lawrence University, New York. He received his Ph.D. from the University of Zagreb, Croatia, and was a Fulbright postdoctoral fellow at Virginia Tech, USA. His current research uses the carbonate sedimentary record, coupled with the integration of stratigraphic analysis, modeling, and isotopic approaches to track the evolution of climates and oceans. He also is focused on high-resolution sequence stratigraphy and its application to carbonate petroleum reservoirs. Recent research projects include Mesozoic Adriatic platform carbonates (Croatia), Lower Paleozoic mixed carbonate-evaporite succession of the Williston Basin (USA); Upper Paleozoic Devonian mixed carbonate-siliciclastic succession (Poland); Upper Paleozoic Permian Zagros Basin (Persian Gulf); Lower Paleozoic mixed siliciclastic-carbonate succession, St. Lawrence Lowlands (USA); and modern carbonate environments of the Caribbean Region (Bahamas, Jamaica). Dr. Husinec has published extensively and regularly conducts field and laboratory courses, and technical seminars, primarily in the areas of carbonate sedimentology and sequence stratigraphy as applied to petroleum exploration. He has also worked as a consultant for petroleum companies in Europe and North America, and has received over $1 million in research grants. Dr. Husinec also is an avid diver with a special interest in the Caribbean reefs.