Geochemistry of the Niobrara Formation and oceanic anoxic event III (OAE III): a regional study in the Western Interior Seaway

Abstract

The Coniacian – Santonian (C – S) time interval displays positive enrichment of stable carbon isotopes (CIE) globally, and elevated organic carbon burial under oxygen depleted conditions in the Western Interior Seaway (WIS) and proto-equatorial Atlantic regions. The CIE associated with C – S has been described as the Oceanic Anoxic Event III (OAE III) with a duration of approximately 3 my and high organic carbon potential. The OAE III in the WIS correlates to the Niobrara Formation C marl and C chalk intervals and Scaphites Depressus biozone in the Denver Basin and is represented by the Razor 25-2514H core (Redtail Field) in this study. Redox sensitive trace elements (molybdenum (Mo), vanadium (V), and iron (Fe)) indicate a range of redox conditions from dysoxia to persistent euxinia were present during the regional OAE III in the Denver Basin. Water-column stratification and deep-water mass restriction were established and displayed varying degrees at the onset and during the event. The OAE III is continuous across the WIS based on redox sensitive Mo trends indicating positive enrichment, except in the Piceance Basin. The analysis of the high-resolution bulk sedimentary XRF results indicate three distinct anoxic phases which are informally named OAE IIIa, OAE IIIb, and OAEIIIc. OAE IIIa and OAE IIIb displays the strongest anoxic conditions, while OAE IIIc marks the waning period of anoxia in the WIS. Each of the subdivisions are separated by intervals that display low enrichments of redox sensitive trace elements that indicate periodic oxygenation of the water column in between anoxic phases. Based on the acyclic, saturates, and aromatics biomarkers results of the Niobrara Formation, the organic matter (OM) is primarily composed of algae remains and terrestrial woody material. The relative concentration of the algal and woody organic matter depends on the paleogeography where western margins display more woody organic matter content whereas eastern parts show more algal input. The OM in the Niobrara Formation was deposited in epeiric seaway conditions. Ocean salinity levels in the WIS differed from the Gulfian Regions based on the distribution of Oleanane (Ol/H) and Moretane (Mo/H). Gammacerane (Ga/H) index indicates anoxic conditions were stronger in the Powder River Basin than the Cañon City Embayment and the Piceance Basin. The presence of tricyclic terpanes is either related to presence of Tasmanites algae or formation of tricyclic terpanes through cracking of tetracyclic terpanes. Dinosterane index readings from the Cañon City Embayment and Piceance Basin indicate that dinoflagellates were not present in the western parts. In the OAE III, trends in the relative abundances of n-alkanes, pristane and phytane indicate the organic matter composition changed and became more algal compared to the Fort Hays member. Source rock analysis (SRA) results show substantial increase in hydrogen index (HI) and decrease in oxygen index (OI) values at the onset of the OAE III supporting an increase in algal contribution to the OM composition as well as formation of oxygen depleted conditions. During the OAE III, phosphorus (P), barium (Ba), cadmium (Cd), nickel (Ni) and copper (Cu) were recycled and led elevated production rates. Ni and Cu trends with low values in the OAE IIIa and OAE IIIb, indicate lack of free hydrogen-sulfide (H¬2S) in the water column. Elevated amounts of Ni and Cu deposition took place during the OAE IIIc. A sharp positive increase in the concentrations of redox sensitive trace elements that correlate to the B chalk, A marl, and A chalk intervals indicate another episode of oxygen depletion during the deposition of Smoky Hill Member under high marine productivity rates. Silicon (Si) and titanium (Ti) indicate major changes in the paleoclimate that occurred with the onset of OAE III. Dry climate conditions transitioned to more wet/humid climate promoting elevated continental weathering rates and subsequently larger quantities of nutrient transportation to the WIS. Potassium (K) and magnesium (Mg) show a period of steady stream influx followed by quiescence at the onset of the OAE III. The low riverine influx rates are due to the initial deepening of the WIS which is followed by an increase in the stream influx rates during the deposition of the C marl interval. Paleoproductivity, nutrient recycling, paleoclimate, and riverine influx trends across the WIS show drastic changes associated with the paleogeography. In the western margins, trends are cyclic due to the proximity to sediment source areas whereas in deeper parts of the WIS, the trends better show influence of anoxia on trace metal sequestration. Overall, the results from this study indicate that the OAE III in the Denver Basin include three discrete anoxic intervals and was initially formed during the deepening of the WIS above the Fort Hays member, and was prolonged due to the elevated production rates that took place under wet/humid climate conditions and varying degrees of riverine influx during the C marl and C chalk intervals. The OM composition changed from more terrestrial to more marine algal at the onset of the OAE III and persisted throughout the Niobrara Formation. Biomarker results provided important knowledge on the WIS-wide changes in the OM composition. The regional correlation of the OAE III shows the paleogeography of the study sites were the determining factors in terms of anoxia versus oxygen rich conditions. The paleoproductivity, nutrient recycling, riverine influx rates, and paleoclimate varied across the WIS.