Abstract
Abstract Precambrian, unmetamorphosed, organic-rich clastic rocks, whilst uncommon in the geologic record, are of interest with respect to their potential for hydrocarbon generation. The ∼ 1.1 billion year old Nonesuch Formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central North America. Previous organic geochemical investigations of the Nonesuch Formation have been limited to isolated samples from the basal portion of the Nonesuch Formation at a copper mine (White Pine) in northern Michigan. In the present study, 183 outcrop and core samples of the Nonesuch Formation from northern Wisconsin and Michigan (Upper Peninsula) were collected for sedimentologic and organic geochemical analyses. Total organic carbon (TOC) values for the samples ranged from 0.0 to 2.5% and showed a strong correlation between organic richness and depositional environment. Detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies (1 and 2) which could be further distinguished using combined pyrolysis-gas chromatography/mass spectrometry (PY/GC/MS). Whereas the composition of Organic Petrography 1 kerogens is primarily aliphatic, the composition of Organic Petrography 2 kerogens is dominated by aromatic and/or phenolic constituents. The geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source-input or preservation, superimposed over maturity differences, is responsible for both organic petrographic and kerogen pyrolysate compositions.
Published Version
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