Abstract

Scattered boulders of sedimentary rock rest on the Tesnus Formation in the Payne Hills, which are the surface expression of a series of imbricate thrust slices that are floored by the Dugout Creek overthrust. Evidence that the boulders belong to a sedimentary unit within the Tesnus rather than being klippen or klippen lag boulders includes the facts that (1) most boulders are unlike rocks cut by local thrusts, (2) boulders are not brecciated, (3) several boulders demonstrably occur within Tesnus shale, (4) ignoring float, boulders occur within the same 13-m-thick lithosome in the Tesnus, and (5) most boulders do not occur along the surface trace of thrust faults.

The boulders were deposited as submarine rock falls or as clasts (olistoliths) in debris flows during one or possibly several separate events. Boulders were derived from a positive element that probably was at least 12 km to the northwest of the site of deposition. About 49% of the boulders are exotic shelf carbonate rocks of Cambrian-Ordovician age, 10% are identical to indigenous basinal rocks, and 41% are probably facies equivalents of indigenous basinal rocks. Although this western source area contributed much detritus (largely carbonate) before and after Tesnus deposition, the olistostrome is the only record of a western source in the Tesnus Formation; previous work shows that most detritus in the Tesnus came from a source area to the east.

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