nois basin in a core from the New Jersey Zinc 1 Par- rish well of Fulton County, Illinois. Organic carbon. (Corg), total sulfur, 13C content of the organic carbon.
Geochemical Indicators of Depositional Environment and Source-Rock Potential for the Upper Ordovician Maquoketa Group, Illinois Basin1 John M. Guthrie and Lisa M. Pratt2
ABSTRACT Two depositional cycles are recognized within the Upper Ordovician Maquoketa Group of the Illinois basin in a core from the New Jersey Zinc 1 Parrish well of Fulton County, Illinois. Organic carbon (Corg), total sulfur, 13C content of the organic carbon (δ13Corg ), hydrogen and oxygen indices (HI and OI) from Rock-Eval pyrolysis, and yields of extractable organic matter (EOM) vary through the cycles. Darkbrown to black, laminated shales are present in the lower portion of each cycle and have high values of C org (1.0–3.0%), HI (500–1000 mg hydrocarbon [HC]/g total organic carbon[TOC]), and EOM (500–2500 ppm), and more negative δ13Corg values (δ 13C org = –30 to –30.5‰). Gray to greenish-gray, bioturbated shales are present in the upper portion of each cycle and have low values of Corg (C20), and small amounts of pristane and phytane (Fowler and Douglas, 1984; Foster et al., 1986; Reed et al., 1986; Longman and Palmer, 1987; Jacobson et al., 1988; Ryder et al., 1991). This type of organic matter has been recognized in the Middle Ordovician Guttenberg Member of the Decorah Formation in Iowa (Fowler and Douglas, 1984; Hatch et al., 1987b; Jacobson et al., 1988), in organic-rich laminae of the Upper Ordovician Trenton Limestone in the Michigan basin (Reed et al., 1986), in Middle Ordovician kukersite beds of Estonia (Foster et al., 1989, 1990), in the Upper Ordovician Yeoman Formation of the Williston basin in Saskatchewan (Macauley et al., 1990; Fowler, 1992), and in the Early-Middle Ordovician Goldwyer Formation of western Australia (Foster et al., 1986; Hoffman et al., 1987). In contrast, shale enriched in assemblage B organic matter is characterized by moderate Corg contents (2.0 mL oxygen/L water), heterotrophs (mainly aerobic bacteria) use oxygen to degrade organic matter in the water column and the sediments. After consumption of oxygen (
