PALEOREDOX INDICATORS IN THE UPPER ORDOVICIAN MAQUOKETA FORMATION OF EASTERN IOWA

Geochemical parameters, including degree-of-pyritization (DOP_T), trace-element indices Ni/Co, V/Cr, and V/(V+Ni), and enrichment factors, were used to assess paleoredox depositional conditions for the Maquoketa Formation (Upper Ordovician, Richmondian) in eastern Iowa. Sampling included the Clermont-Brainard Shales (mostly medium to light gray shale), the underlying brownish-black, laminated shales of the Elgin Member, and a basal phosphorite layer. TOC increases with depth, with the upper gray shales containing between 0.2 and 2%, whereas the brown shales range between 1 and 9%. Rock-Eval pyrolysis indicates primarily Type II kerogen for the more carbon-rich intervals: leaner intervals have lower HI and higher OI. T_max values of around 430° indicate that these shales are immature to just within the oil window.

Redox indicators suggest variable conditions during accumulation of these shales. DOP_T indicates primarily oxic conditions in the upper part of the core and primarily dysoxic conditions for the Elgin Member. Trace-metal redox indices do not provide consistent results. Ni/Co ratios indicate an anoxic environment for the laminated brown shales, and dysoxic to oxic environments for the upper part of the core. By contrast, V/Cr ratios indicate oxic environments and V/(V+Ni) ratios suggest dysoxic to anoxic conditions throughout the core. In general, trace metals show little enrichment in the upper part of the core (Clermont-Brainard Shales) compared to average shales. However, the Elgin Member shows enrichment levels for Mo, Pb, and Ni that are comparable to or exceed those in average black shales. Despite the relative enrichment, the Mo levels for the Elgin Member are still low, being in the 10-20 ppm range, and as such suggest conditions were not strongly anoxic. Although there are disparities in these indicators, it is clear that redox conditions changed during accumulation of these sediments: the lower, brownish-black shales of the Elgin Member were likely deposited under dysoxic to anoxic conditions, whereas the less-organic-rich shales in the upper part of the core were deposited in an environment that was mainly oxic.