ABSTRACT

Seismic and borehole data in the southern Laramie basin of southeastern Wyoming outline a fault-bordered pop-up structure that is central to the Quealy wrench duplex and the site of the Quealy Dome oil field. This contractional duplex formed at a restraining fault offset between two parallel, laterally extensive, northeast-trending fault zones—the South and North Quealy faults. Spaced about 2 mi (3.2 km) apart, these parallel fault zones are interpreted as wrench straights, equivalent to the floor and roof thrusts of a duplex in a fold and thrust belt, but described in map view rather than in cross section.

The Quealy wrench duplex is comprised of three east-vergent, basement-involved thrust imbricates and a west-vergent, antithetic thrust, detached in Permian shales. Duplex thrusts strike at high angles to the bordering South and North Quealy fault zones. Measurements based on interpretive piercing-point evidence and fault-parallel, differential shortening indicate that net slip on the dominant South Quealy fault zone is dextral-oblique. A component of dextral slip is also indicated on the North Quealy fault zone.

The South Quealy fault and North Quealy/Overland fault trends can be traced southwesterly into the footwall of the north-trending Arlington (basin-boundary) thrust and projected into the Precambrian Cheyenne belt of the Medicine Bow Mountains in the hanging wall of the Arlington thrust. It is proposed that the Quealy wrench duplex is a product of Precambrian shear zone reactivation along the Cheyenne belt under Permian and Laramide (Tertiary) transpression.

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