Biological Markers in Lower Jurassic Synrift Lacustrine Black Shales, Hartford Basin, Connecticut, U.S.A.

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 arc each about a meter thick and were deposited in subtropical, thermally stratified, 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, arc 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 arc either not detectable or present only in subordinate quantities relative to the tricyclic terpanes. The samples arc 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

Organic Geochemistry of a Lower Jurassic Synrift Lacustrine Sequence, Hartford Basin, Connecticut, U.S.A.

Synrift terrestrial strata of the Lower Jurassic East Berlin Formation (Hartford basin, Connecticut, U.S.A.) record cyclical expansion and contraction of major lakes, six of which were deep enough to develop anoxic bottom waters. We have studied one representative lacustrine sequence in detail, sampling a new roadcut near the village of East Berlin. The section examined is 4 m thick, with a gray siltstone at the base, deposited in shallow water, overlain by an organic-rich black shale (deep water), succeeded in turn by another gray siltstone, deposited as the lake waters gradually receded. The upper gray siltstone is chemically distinct from the lower siltstone, as it contains small amounts of corrensite, analcime and gypsum, reflecting the increasing salinity and alkalinity of the contracting lake. The samples in the center of the black shale unit contain laminae of thermally-altered, yellowish orange-fluorescing, mottled telalginite. The fluorescence properties indicate a peak oil generation maturity level, confirmed by a vitrinite reflectance of 1.13% and a Methylphenanthrene Index of 1.08. The other samples have less organic matter, becoming increasingly lean towards the top and bottom of the sequence. Samples in the middle of the black shale unit are distinguished by the presence of an homologous series of tricyclic terpanes extending from C20 to at least C41 , and by the near absence of hopanes and steranes. Moving upsection into the gray siltstone, the samples contain markedly less extractable organic material (EOM) and the concentration of tricyclic terpanes relative to hopanes steadily decreases. In the uppermost sample, hopanes are the predominant terpanes. Moving downsection from the black shale into the lower gray siltstone, EOM and the ratio of tricyclic terpanes also decrease, except in the lowermost samples, which contain terpane distributions like those of the middle part of the black shale. This likely is migrated material, because bitumen fills microfractures and extensive megafractures. The lack of hopanes and steranes in the black shales cannot simply be a maturation effect, as these biomarkers appear in the adjacent beds. Instead, the unusual terpane distributions may indicate a changing depositional environment, documenting the geochemical evolution of the lake. Or, more likely, they may result from fractionation during expulsion of petroleum from these mature source rocks