Date(s) - 06/09/2020
11:30 am - 1:00 pm
Tectonic evolution of the Pennsylvanian-early Permian Orogrande basin revealed through backstripped subsidence curves and structural analysis
The tectonic evolution of the Orogrande basin is commonly reported in broad qualitative terms. This study conducts rigorous subsidence analysis through backstripping of three measured sections through the Pennsylvanian-early Permian strata, and ties this subsidence evolution to structures manifested in the Pennsylvanian strata to more fully assess basin evolution. Backstripped curves, or geohistory curves, demonstrate that subsidence largely began in the late Morrowan and peak subsidence interval occurred during Virgilian time across all three sections. However, in the latest Virgilian to earliest Permian, subsidence ceased in the northern part of the study area, while increasing uplift and subsequent erosion occurred progressively to the south in the study area. This period of basin history was also associated with folding and faulting of Pennsylvanian strata. The shortening event manifested largely as folds with amplitudes ranging greater than 575 m and generally north-south trends. It is likely that further to the south the folding is a response to subsurface thrusting, as folding here is geometrically consistent with fault-bend folds. In the middle part of the early Permian, subsidence renewed across the area accommodating deposition of the Abo Formation. The angular unconformity at the base of the Abo Formation overlies progressively older Pennsylvanian units southward to form a buttress unconformity with a deformed Pennsylvanian landscape. Throughout the range, Pennsylvanian strata of the eastern Orogrande basin are folded and faulted, dipping in excess of 80 degrees in some locations. In stark contrast, lower Permian strata commonly dip only a few degrees, overlying and directly adjacent to steeply-dipping Pennsylvanian beds of various age. Because the Pedernal uplift is also overlain by the Abo Formation, this period of renewed subsidence in the middle part of the early Permian was a regional event, and thus not related to general offset of the Pedernal uplift-Orogrande basin system. Ultimately, these results indicate that classical ancestral Rocky Mountain tectonism, of which the Orogrande basin-Pedernal uplift system is typically considered, ceased in the study area in the early Permian.
John Brotherton is a Ph.D. candidate at Texas Tech University. A Texas native, prior to studying geology he guided hiking, backpacking, and river trips throughout the American West. He graduated from Western Colorado University with a B.Sc. in geology with an emphasis in petroleum geology. His research is focused on the evolution of the ancestral Rocky Mountains in modern southern New Mexico. He enjoys camping in the Southwest with his dog, for research and/or recreation.