Geochemical relationships of Late Paleozoic carbon-rich shales of the Midcontinent, USA: a compendium of results advocating changeable geochemical conditions

Abstract

Previous studies have shown that Pennsylvanian black shales of the Midcontinent US possess a more complex geochemical history than was once thought. Vine and Tourtelot [Econ. Geol. 65 (1970) 253] and Coveney and Glascock [Appl. Geochem. 4 (1989) 347] note that the widespread nature of metalliferous Pennsylvanian black shales and the diverse geological environments across the Midwestern US pose the likelihood that metals contained in the black shales are of complex origins. Detailed geochemical and statistical representations using characteristics of numerous black shale members have shown that water depth may not have been uniformly “deep” as previously inferred by numerous workers. Earlier suggestions that these black shales were associated exclusively with anoxic depositional conditions have been challenged by a more realistic theory of multifaceted, fluctuating environmental conditions. A wide range of major and trace element concentrations, combined with widely varying characteristics and diverse types of organic matter, indicate the presence of fluctuating geochemical settings. The presumption is that numerous associated chemical reactions occurred syngenetically. One geochemical index, degree of pyritization (DOP), suggests a dysaerobic to aerobic setting for many of the shales, whereas some other geochemical proxies (i.e., V/(V+Ni)) indicate punctuated episodes of bottom water anoxia. Schieber [Abstr. Programs-Geol. Soc. Am. 33/6 (2001) A102] notes that in some black shales, pyrite enrichment due to intermittent organic reworking explains the high DOP values, once thought to equate with extreme euxinic depositional conditions. It seems likely, therefore, that even during apparent euxinic periods as indicated by bulk geochemistry, oxygen-deprived events were apparently short lived. The seemingly euxinic “fingerprint” as shown by geochemical characteristics, may result through “time averaging” [Abstr. Programs-Geol. Soc. Am. 33/6 (2001) A102] of intermittent euxinic “events”, providing a misleading indication of dominant euxinic conditions. Thus, detailed microstratigraphic sampling is necessary to delineate short, punctuated occurrences of intermittent sub-oxic episodes. The Midcontinent Pennsylvanian shales may reflect a complex array of bottom water oxygenation variations and water depth fluctuations in response to climatic and localized events [Hatch, J.R., and Leventhal, J.S., (1992). Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, USA. In: Meyers, P.A., Pratt, L.M., Nagy, B. (Eds.), Geochemistry of Metalliferous Black Shales. Chem. Geol. 99, 65–82.], complicated by post-depositional events [Appl. Geochem. 4 (1989) 347]. This research examines the previous “exclusively anoxic” origin for Midcontinent Pennsylvanian black shales from a geochemical perspective and offers an alternative supported by detailed statistical presentations with the hypothesis that events of anoxia were punctuated by short-lived dysaerobic or even near-oxic events.