Sedimentology and structural geology of the Housetop Mountains-Castle Mountain area, Marathon Basin, Trans-Pecos, Texas
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In many sedimentary sections recording extreme soft-sediment and tectonic deformation, the origin of outcrop-scale structures is commonly difficult to resolve. Numerous soft-sediment deformational features developed in the Mississippian-Pennsylvanian Tesnus Formation in the Marathon Basin in response to rapid sedimentation rates and tectonic instability. Soft-sediment structures were overprinted during Late Pennsylvanian to Early Permian compressional deformation that emplaced allochthonous and parautochthonous rocks of the Marathon Basin. Because of its sedimentary and tectonic history, the Tesnus Formation provides an ideal section for evaluating the role of soft-sediment and tectonic processes in producing outcrop-scale structures. The Tesnus Formation was rapidly deposited in deep water in an unstable, migrating foredeep. Paleocurrent data derived from sole marks on turbidite sandstone beds in the study area indicate that the overall trend of sediment dispersal during Tesnus deposition was from southeast to northwest. The most well-developed trend is in an arc between N20°W to N30°W. Extensive synsedimentary and early-post-depositional deformation, particularly sediment liquefaction and clastic intrusion, accompanied sedimentation. The most common clastic injection features in the Tesnus Formation are dikes, which are interpreted to be coeval with previously undescribed clastic intrusions including sills in excess of 2.5 m thick, cylindrical clastic "plugs", and composite structures that are partly concordant and partly discordant. Clastic injections are interpreted to have formed under conditions of essentially non-deviatoric stress, and commonly in response to bedding-parallel (horizontal) pressure gradients. Clastic injections are not systematically oriented locally or regionally, provide no information regarding paleogeography (paleoslope), and are not related to any later structural trends. "Decompaction" of ptygmatically folded dikes has commonly been used to determine the extent of post-injection compaction, but data from the Tesnus Formation indicate that the method conventionally used for determining sediment compaction and dike injection depth is unreliable. Tectonically generated structures post-date all soft-sediment deformation and are here interpreted to have formed in a uniformly-oriented, compressional stress field with a maximum compression direction oriented between approximately Nl9°W and N22°W. The Dagger Flat Anticlinorium, a major structural culmination west and southwest of the study area, acted as a structural buttress that greatly influenced structural evolution in the eastern part of the Marathon Basin. Compression and rotation of the Tesnus/Dimple/Haymond flysch package against and around the Dagger Flat Anticlinorium modified the orientation of pre-rotation structures and changed the orientation of rocks in the map area with respect to prevailing stresses. The sequence of Paleozoic tectonic events affecting rocks in the study area is interpreted as follows. 1. Generation of first-stage joints. 2. Rotation of the Tesnus/Dimple/Haymond flysch package around the Dagger Flat Anticlinorium. 3. Flexural-slip folding of the flysch package. 4. Formation of major thrust faults and related tear faults. Thrust sheet rotation around the Dagger Flat Anticlinorium. Formation of first-stage extensional microfaults with northeasterly trends in major thrust décollement zones. 5. Space constraints along the northeast margin of the Dagger Flat Anticlinorium prevent further thrust sheet rotation. 6. Uniform compression continues. Thrust motion is no longer influenced by the Dagger Flat Anticlinorium, but is directed parallel with maximum compression direction (Nl9°W to N22°W). Second-stage joints form. Incremental strike-slip motion along major thrusts causes formation of east-northeasterly striking contraction microfaults and second-stage extensional microfaults.