THE CONTACT BETWEEN ORDOVICAN VIOLA GROUP - BROMIDE FORMATION AND THE COLLINGS RANCH CONGLOMERATE: REFINING THE PULL-APART BASIN MODEL

We have examined the details of geological structures at the contacts between the Ordovician Viola Group - Bromide Formation and the Collings Ranch Conglomerate using exposures of the Arbuckle Mountains in Southern Oklahoma to understand the varying bedding orientations of the Viola Limestone and the Bromide Formation and the geometrical nature of the unconformity between the Viola Limestone – Bromide Formation and the Collings Ranch Conglomerate. The Arbuckle Mountains are part of the Arbuckle-Wichita Trend, and are dominated by Cambrian-Ordovician sedimentary rocks, which were intensively deformed during the Pennsylvanian Wichita and Arbuckle Orogenies. A widely accepted model suggests that the Collings Ranch Conglomerate was formed as a syn-tectonic sediment infill in a NW-SE trending pull-apart basin, formed by left-lateral strike-slip movement along the Washita and Collings Ranch Faults. The contact between the Ordovician carbonates and the conglomerate is accepted as a sharp angular unconformity and varying nature of orientation of bedding planes within the Ordovician carbonates has been interpreted as a NW-SE trending syncline. Our study refines the pull-apart basin model in three ways: (1) The Viola Group – Bromide Formation, close to the contact with the Collings Ranch Conglomerate is deformed by numerous NW-SE trending left-lateral strike-slip faults that variably dip to the NE and SW rather than a single normal fault as has been previously mapped. (2) The varying bedding plane orientations in the Viola Group are the result of rotation of blocks during left-lateral shearing associated with NW-SE trending strike-slip faults. Hence, this bedding plane variation is not a manifestation of a NW-SE trending syncline. (3) The contact between the Viola Group – Bromide Formation and the Collings Ranch Conglomerate is an unconformity that was subsequently modified by layer-parallel growth faults. The weight of the conglomerate, resting atop sub-vertical bedding planes of the Viola Limestone and Bromide Formation, has caused the compression stress semi-principal axis to be vertical and oriented sub-parallel to the dips of the bedding planes. This resulted in the activation of bedding planes into normal faults with increasing displacement with increased accumulation of the Collings Ranch Conglomerate.