LAKES OF THE JURASSIC EAST BERLIN FORMATION, HARTFORD BASIN, NEWARK SUPERGROUP: RE-EVALUATING THE SEDIMENTOLOGY, STRATIGRAPHY, AND HYDROCARBON POTENTIAL

The Mesozoic Hartford Basin of southern New England has been studied since the 1700s, but limited outcrop and extensive faulting have restricted research in the central region of the basin. Past studies identified six complete lake cycles, or source rocks with black shale, in the southern and northern extents of the upper 107 m of the lacustrine, playa, and alluvial Jurassic East Berlin Formation; no exposures of the entire East Berlin, however, have been analyzed to confirm the exact number and thickness of lakes preserved. A new sedimentologic and stratigraphic analysis of nine cores collectively penetrating the entire East Berlin for the first time in the central region of the basin has identified (1) two additional lower lakes separated by a tufa horizon, (2) displacive chicken-wire evaporites in the lowermost lake, and (3) a 10-m-thick paleosol and lacustrine sequence, known as the South Hartford Member, sandwiched within the Holyoke Basalt. These data, unknown to previous researchers, provide new insight into the paleoenvironments of the Hartford Basin during East Berlin time.

To understand paleolacustrine environments, and therefore hydrocarbon potential of a basin, the lake-basin type model derived by Bohacs, Carroll, and others identifies a predictable evolution of facies from fluvial basin to over-filled, balance-filled, and under-filled lacustrine before grading back into balance-filled lacustrine through fluvial basin during a complete accommodation space cycle of a tectonic basin. Fluvial basin facies at the top and bottom of the Hartford rift infill enclose lake deposits. Past studies hypothesized over-filled lacustrine conditions in the lower Shuttle Meadow with rare, dispersed evaporites suggesting balance-filled conditions in the East Berlin and lower Portland Formations. Despite this pattern, identifying the predictable evolution of lake types remains nebulous as the balance between tectonics and climate shifted during development of the basin. New biomarker analyses, however, identify lake types of the eight East Berlin lacustrine cycles to (1) refine hydrocarbon data in the previously unexposed central region of the basin and (2) provide insight into the evolution of lakes throughout the lifespan of the Hartford rift.