STERANE AND TRITERPANE BIOMARKERS IN THE PRECAMBRIAN NONESUCH FORMATION, NORTH-AMERICAN MIDCONTINENT RIFT

Molecular fossils and organic matter from microbial communities living around 1.1 Ga ago are well preserved in the Nonesuch Formation of the US Midcontinent Rift System. Although some compound classes of biological markers have been recognized repeatedly in previous studies of Nonesuch rocks and associated petroleums, steranes and triterpanes have not been detected. Failure to detect these compounds was due primarily to instrumental limitations and the relatively high thermal maturity of samples collected within zones of copper mineralization. Our analyses of Nonesuch samples from the White Pine Mine and nonmineralized localities in Michigan and Wisconsin reveal suites of C26-C30 steranes and C27-C35 pentacyclic triterpanes similar to those found in Phanerozoic oils and sedimentary rocks of marine origin. Stereoisomeric distributions in the Nonesuch samples are consistent with moderate thermal maturity except in the mineralized zone near White Pine.