Three Scales Of Late Paleozoic Structures In The West Texas Basin –Description And Genesis
Thomas E. Ewing
Frontera Exploration Consultants, San Antonio TX 78259
The West Texas Basin displays three mappable scales of Late Paleozoic deformation: a macroscale (field-scale) web of structures (spaced 2-20 km), a megascale (trend-scale) pattern of uplift and subsidence (spaced 20-200 km), and a gigascale basinwide subsidence. The three scales are remarkably separable, although they affect the same area.
Major faulting, folding and tilting during the Pennsylvanian and Early Permian (Atoka to mid-Wolfcamp) created a web of macroscale (2-20 km half-wavelength) structural features including faulted uplifts, broad and narrow faulted ridges, flower structures and strike-slip and oblique-slip faults. Most features are consistent with distributed WNW-directed left-lateral strike-slip and ENE-directed compression; NE to N trending (right-lateral) strike-slip may also be present. Most structures involve basement, but may be detached in the upper crust; complex crustal layering is a feature of the region.
The macroscale web occurs both within zones of megascale (20-200 km half-wavelength) subsidence (Delaware and Midland Basins) and also in zones of relative megascale uplift (Central Basin and Diablo axes). The megascale pattern may represent lower crustal thrusting along a Precambrian feature.
Both the macroscale and megascale uplifts occurred within a basinwide (gigascale) subsidence that outlasted macroscale deformation, continuing into late Permian time (covering the structures with 2-3 km of sediment). This apparent bowl-shaped subsidence is similar to other major intracratonic basins, and may share a common origin (e.g., sinking of a mafic crustal root).
On its southern margin, the basin abuts and underlies the Marathon fold and thrust belt, an allochthonous assemblage of Paleozoic oceanic/slope and foredeep deposits. Foredeep and forebulge features (Val Verde, Ozona) developed in the adjacent parts of the southern West Texas Basin, but did not extend farther north. The stress patterns indicated by macroscale and megascale uplifts are inconsistent with NNW-directed Marathon thrusting as an important driver of the deformation. The features form part of the Ancestral Rocky Mountains (ARM). The West Texas Basin is anomalous in the ARM in its abundant detached macroscale structures, and the presence of a major intracratonic basin that buries and seals the uplifts.
AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013