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Early Mesozoic Rift Basins of Eastern United States

Steven Schamel, Robert Ressetar, Susan Gawarecki, G. Kendall Taylor, Alfred Traverse, Hugh F. Houghton, Peter Letourneau

The exposed Triassic-Liassic rift basins in the eastern United States are half-grabens filled with up to several kilometers of continental sediments. The location and sense of asymmetry of the half-grabens are closely tied to the structural grain of the Appalachian crystalline terranes on which they have formed. In many instances, the faulted margins of the basins are older thrusts or terrane boundaries reactivated as listric normal faults. The sediment fill of the basins reflects the structural asymmetry of the half-grabens. Coarse alluvial-fan deposits along the main border faults pass basinward into a complex array of fluvial, paludal, and lacustrine facies. The oldest sediment fill in the rift basins is dated palynologically as late Ladinian to late Carnian. Perhaps r flecting the northward opening of the central Atlantic, the youngest rift-fill sediments are older in the southern basins than in the northern-Carnian in the Richmond basin versus Toarcian in the Hartford-Deerfield basin. Floral evidence points to a tropical to near-tropical environment, with severe oscillations between xerophytic (dry) and hygrophytic (wet) conditions. The degree of thermal maturation, as estimated from vitrinite reflectance and clay mineralogy, varies widely from basin to basin. The basins with highest thermal maturities are those having large volumes of diabase intrusives and presumed higher paleogeothermal gradients.

In general, all of the exposed basins have adequate clastic reservoirs; however, an insufficient volume of source rock and the paucity of traps severely limit the hydrocarbon potential of many of the basins.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.