Basement faults in the East Tennessee seismic zone : observations from the area of the Swan Creek field

Basement structures and their relationship to the Alleghanian thrust system in the vicinity of the New York - Alabama lineament are revealed by new seismic reflection and well data from the Swan Creek oil and gas field. A system of normal faults and monoclinal flexures, having an aggregate ~800 m of vertical offset down to the southeast, changes the depth to Precambrian crystalline basement here. Structural relationships between the basement offset and the overlying Pine Mountain and Wallen Valley thrust sheets show that the basement faults predate the Alleghanian orogeny, and possibly originated during Eocambrian rifting. A mushwad (ductile shale duplex) containing Rome and Conasauga rocks formed beneath the structurally stiff units during Alleghanian thrusting, and arched the overriding Wallen Valley thrust sheet. Decoupling of the Wallen Valley thrust near the base of the stiff Maynardville limestone allowed the duplex to form in the underlying weak Conasauga strata without internal shortening of the structurally stiff Maynardville and Knox Group. The Swan Creek anticline formed where the Knox carbonates in the Wallen Valley thrust sheet arched over the mushwad, but without the basement buttress that is an integral part of mush wad formation elsewhere in the southern Appalachians. Enhanced porosity and permeability, probably due to outer arc extension in the uppermost Knox and higher strata where they arch over the mushwad, are a major factor contributing to oil and gas production from the Swan Creek field.