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Lacustrine Strata of Early Jurassic Age, Hartford and Deerfield Rift Basins, Massachusetts and Connecticut: Deposition, Maturation of Black Mudstones, Sandstone Diagenesis, and Hydrocarbon Occurrences

Paul E. Meriney, John F. Hubert, Michael A. Smith

Lacustrine cycles occur in the 100-m Shuttle Meadow, 170-m East Berlin, and lower part of the 1,200-m Portland Formations in the Hartford basin and in the 600-m Turners Falls Sandstone in the Deerfield basin. Eleven exposed cycles range in thickness from 3 to 10 m. Black mudstones are mostly 1-2 m thick, deposited in the deeper parts of oligomictic perennial lakes. Gray sandstones accumulated on delta lobes, fan deltas, and from sheetfloods that spread across the valley floor to cover exposed lacustrine littoral muds. Both rift-basin fills are dominated by alluvial-fan, fluvial, and playa red beds.

The lacustrine sandstones are arkoses and lithic arkoses (72 modal analyses). Omitting minor cements, diagenetic events are (1) compaction, (2) quartz and albite overgrowths with albitization of detrital plagioclase, (3) dolomite and ferroan dolomite cements, and (4) hydrocarbon migration with creation of secondary porosity. Consistently large intergranular volumes in the lacustrine sandstones, as high as 40%, suggest that cementation began prior to significant compaction. Porosity in the sandstones ranges from 0 to 11%, averaging 2%.

Organic geochemical data for black mudstones in the East Berlin, Portland, and Turners Falls Formations, combined with published and USGS open-file data, show that organic richness and oil proneness increase southward in the Hartford basin whereas thermal maturity increases northward. Oil generation has occurred in the East Berlin Formation, as evidenced by relatively large amounts of extractable bitumen, high-molecular-weight hydrocarbons, and hydrocarbons present in the sandstones. The Deerfield basin is overmature for oil.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.