Biological markers in Lower Jurassic synrift lacustrine black shales, Hartford basin, Connecticut, U.S.A.

Abstract

The East Berlin Formation (Lower Jurassic, Hartford basin, Connecticut, U.S.A.) is distinctive for its six cyclic units of lacustrine black shale and gray mudstone, separated by playa and fluvial redbeds. The black shales are each about a meter thick and were deposited in subtropical, thermally strastified, oligomictic lakes, the youngest of which (lakes 3 through 6) were large enough to flood most of the basin and attained depths of several tens of meters.The saturate fractions of solvent extracts of organic-rich black shales from each of the six lakes, collected at fresh roadcuts near East Berlin, are dominated by extended homologous series of n-alkanes, alkylcyclohexanes, and branched chain alkanes. A striking feature of the black shales is the presence of a series of extended tricyclic terpanes from C20 to at least C41. Hopanes are either not detectable or present only in subordinate quantities relative to the tricyclic terpanes. The samples are depleted in hopanes in part because of the elevated maturity level (mid to late oil window). Tricyclic terpane concentrations may also have been enhanced by fractionation effects related to oil expulsion out of the black shales. In addition, the original organic matter may have been exceptionally rich in tricyclic terpane precursors, i.e. fossil lipids of prokaryotes present in anoxic, moderately saline, alkaline lakes.