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

Authors

Michael A. Kruge, Montclair State UniversityFollow
John F. Hubert, University of Massachusetts Amherst
David F. Bensley, Southern Illinois University CarbondaleFollow
John C. Crelling, Southern Illinois University CarbondaleFollow
R Jay Akes, Southern Illinois University Carbondale
Paul E. Meriney, University of Massachusetts AmherstFollow

Document Type

Article

Publication Date

1990

Journal / Book Title

Advances in Organic Geochemistry 1989

Abstract

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.

DOI

https://doi.org/10.1016/0146-6380(90)90110-L

Book Publisher

Pergamon Press

Book Editor(s)

B. Durand and F. Behar

MSU Digital Commons Citation

Kruge, Michael A.; Hubert, John F.; Bensley, David F.; Crelling, John C.; Akes, R Jay; and Meriney, Paul E., "Organic Geochemistry of a Lower Jurassic Synrift Lacustrine Sequence, Hartford Basin, Connecticut, U.S.A." (1990). Department of Earth and Environmental Studies Faculty Scholarship and Creative Works. 95.
https://digitalcommons.montclair.edu/earth-environ-studies-facpubs/95

Published Citation

Kruge, M.A., Hubert, J.F., Bensley, D.F., Crelling, J.C., Akes, R.J. and Meriney P.E., 1990, Organic Geochemistry of a Lower Jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, USA. In, B. Durand and F. Behar, eds., Advances in Organic Geochemistry 1989, Pergamon Press, Oxford, p. 689-701. (Organic Geochemistry 16:689-701).