Stable isotope (C, S, N) and molecular studies on the Precambrian nonesuch Shale (Wisconsin-Michigan, U.S.A.): Evidence for differential preservation rates, depositional environment and hydrothermal influence

Abstract

The organic-rich, Middle Proterozoic Nonesuch Formation is part of a thick volcaniclastic rift-fill sequence (Mid-Continent Rift System, northern Wisconsin and Upper Peninsula Michigan) with a mild thermal history. Despite stratigraphic/sedimentologic similarities between the east (Michigan) and west (Wisconsin) study areas, distinctions in organic petrologic, elemental and stable isotopic parameters are noted. Geologic arguments and predictable relationships among organic carbon content, bulk and molecular pyrolysate composition and petrologic parameters are sufficient to invoke differential preservation of the same (or similar) primary producers as the principal cause of variability. Isotopic signals indicating precipitation of calcite via intense organic production (planktonic blooms) is evident among carbonate laminites. Incorporation or concentration of organic degradation by-products is evident, particularly in the west, from carbon and nitrogen stable isotope data. It is proposed that whereas preservation of organic productivity of a particular organism may be nearly complete in one organic facies in the east and west, organic remains in other intervals in the west were subject to extensive degradation and reconstitution to form protokerogens. Implications of the study include establishing parameters for recognizing pre-Devonian lacustrine vs. marine euxinic systems, clues to the complexity of Middle Proterozoic aquatic systems and in recognizing the input of hydrothermal fluids to water bodies and early sediments.