The Triassic-Jurassic Hartford Rift Basin, Connecticut and Massachusetts: Evolution, Sandstone Diagenesis, and Hydrocarbon History (1)

Abstract

During initial subsidence in the Late Triassic, rivers flowed from both sides of the subtropical Hartford basin to deposit the red beds of the lower part of the 2-km-thick New Haven Arkose. Subsequently, the fault-bounded eastern highlands became the major source of detritus for the fluvial red beds of the upper New Haven Arkose. Extensional crustal thinning increased dramatically in the Early Jurassic, producing volcanism and a closed, asymmetrical half-graben in which accumulated Milankovitch-type cycles of oligomictic gray/black mudstones and playa red beds. These strata form most of the 150-m-thick Shuttle Meadow, 170-m-thick East Berlin, and lower half of the 2-km-thick Portland formations. Rivers and sheetfloods from the western hinged margin spread sediment over th valley floor, up to small alluvial fans along the eastern highlands. As extension eased, the fault-bounded eastern highlands shed detritus for the fluvial and alluvial-fan red beds of the upper Portland Formation. Basinal sandstones are oligoclase-rich arkoses and lithic arkoses whose compositions reflect erosion of the eastern and western highlands. Ubiquitous albite overgrowths were precipitated in the sands, followed by ferroan calcite/calcite in fluvial sands and mostly ferroan dolomite/dolomite in playa and lacustrine sands. Na{+} for albite cement came from weathering of oligoclase-rich gneisses and schists around the basin. Topography-driven flow circulated groundwater through the sands, precipitating albite and locally albitizing feldspar grains. In the Early to Middle Jurassic high heat flow generated hydrocarbons in the lacustrine, organic-rich mudstones. These source rock mudstones are locally mature for hydrocarbons and are interbedded with or adjacent to potential reservoirs of lacustrine and playa sandstones. In the Middle Jurassic, hydrocarbons migrated into secondary porosity in some of these sandstones.