CARBON CHEMOSTRATIGRAPHY AND DEPOSITIONAL SEQUENCES OF THE LATE ORDOVICIAN KATIAN RED RIVER FORMATION, NORTH DAKOTA

The Upper Ordovician Katian Red River Formation is a carbonate-evaporite supersequence that formed within the intracratonic Williston basin, in the interior of the shallow and very broad, tropical, arid carbonate shelf that developed along the western passive margin of North America. It is an overall shallowing (and “brining”) upward supersequence up to ~210 meters thick, composed of three 3rd-order depositional sequences. Each sequence is characterized by an upward progressively shallower, more restricted and hypersaline facies. The transgressive systems tracts of all three sequences consist of predominantly skeletal mudstone-wackepackstone with abundant burrow mottling (subtidal, lagoonal facies). Sequences 1 and 2 have early highstand systems tracts composed of porous, peritidal laminated dolomites, whereas late HSTs consist of anhydrites (subaqueous and supratidal setting). Sequence 3 has late HST composed of peritidal laminites; anhydrite is present only in the basin depocenter and pinches out laterally. Lowstand systems tracts are not preserved in the Williston Basin because of its updip position on the broad shallow shelf.

Samples for δ¹³C analysis were collected from three cores in western North Dakota, including: Simpson #1 (Williams County), Federal #10-1 (Dunn County), and Urlacher State Unit #1 (Hettinger County). Carbon isotope values (PDB) of dolomite vary from -3‰ to +1.7‰ in limestone, and from -0.7 to 2‰ in dolomite. On average the oxygen isotope value of dolomites are 1.26‰ heavier than limestones (mean δ¹⁸O in dolomites -6.1‰ PDB vs. -4.87‰ PDB in limestones). A small positive shift in oxygen isotope values between dolomite and limestone suggests that the dolomites picked up a slightly lighter signature due to burial (2,700 to 4,300 meters). We document three δ¹³C positive excursions in the studied Late Ordovician succession of the Williston Basin; these excursions are stratigraphically above the Guttenberg (GICE) and below the Hirnantian (HICE) carbon isotope excursions. The positive excursions in carbon isotopes and their possible relation to North American carbon chemostratigraphy are tested, and used to propose an age for the Red River Formation in the absence of high-resolution paleontological data.