AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Sequence Stratigraphic Framework, Trace Element and Stable Isotope Geochemistry of the Late Ordovician Red River Formation, Williston Basin, North Dakota
(1) Geology, St. Lawrence University, Canton, NY.
The Upper Ordovician Red River Formation is an overall shallowing (and “brining”) upward supersequence reaching more than 215 meters in the intracratonic Williston Basin depocenter in North Dakota. The supersequence is composed of three third order depositional sequences that formed in the interior of the very broad and shallow, tropical, arid carbonate shelf that developed along the western passive margin of North America.
Based on detailed bed-by-bed analysis and combined neutron porosity-bulk density logs from of 29 cores, three large, 25 to 80 ft thick 3rd order depositional sequences are identified that can be traced regionally over western and central North Dakota. The lowstand systems tracts are not preserved in the Williston Basin because of its updip position on the broad shallow shelf. The transgressive systems tracts consist of subtidal, lagoonal facies of predominantly skeletal mudstone-wackepackstone with abundant burrow mottling. The lower two sequences (sequences 1 and 2) have early highstand systems tracts composed of porous, peritidal laminated dolomites, whereas late HSTs consist of supratidal anhydrites. The late HST of Sequence 3 consists of peritidal laminites; anhydrite is present only in the basin depocenter and pinches out laterally.
Three cores from western North Dakota were sampled for stable isotopes at 3 feet (0.9 meters) intervals; 22 dolomite samples were analyzed for trace elements by ICP-MS. The mean δ18O values are -6.1‰ PDB (limestone) and -4.87‰ PDB (dolomite); a small positive shift suggests that the dolomites picked up a slightly lighter signature due to burial (2,700 to 4,300 meters). The Red River dolomite Sr-values overlap the strontium compositions of the Tertiary dolomites, but the oxygen isotopes are much lighter (5.5 to 10‰), reflecting light oxygen isotope values of later Ordovician marine calcites, and some recrystallization of the dolomite in the presence of isotopically more negative burial fluids. The high Mn2+ (mean value 151 ppm) and high Fe2+ (mean value 1,082 ppm) in the dolomites indicate that the pore waters were reducing and that a significant source of iron and magnesium was present; these values support the idea that some of the early dolomites have been overprinted with burial. The three positive excursions in carbon isotopes and their possible relation to North American carbon chemostratigraphy are tested.