Abstract

One generally accepted rule of thrust belt geometry is that folding does not occur before thrust faulting; thrust faults do not cut across folds in cross-section view. Original mapping in the Marathon basin documents that the Hell’s Half Acre thrust sheet was emplaced after an episode of large-scale folding.

The Hell’s Half Acre thrust fault is a shallow, low-angle fault which separates the northeasterly trending, broad (4-km or 2.5-mi wavelength), open folds on the north from the imbricately thrust faulted and overturned, tightly folded (1-km or 0.6-mi wavelength) Paleozoic rocks of the thrust sheet. The thrust fault trends east and truncates the structures on the north. Within the thrust sheet, folds plunge into and are truncated by thrust faults. The Tesnus Formation (Mississippian) is the principal formation of the sheet, but large blocks of younger Dimple Limestone (Morrowan) and Haymond Formation (Atokan) are lodged within fault planes bounded by Tesnus, thereby documenting folding before thrust faulting.

This nonclassic geometry is due to lithology. Classic thrust belt style was developed for a miogeoclinal sequence. The Ouachita stratigraphy is a eugeoclinal sequence composed of lower and middle Paleozoic, thin limestones and cherts with numerous shale interbeds and an upper Paleozoic flysch sequence with a low sand/shale ratio and numerous thick shale horizons. The many thick, incompetent horizons allowed the thrust fault to cut across preexisting structures.

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