PRELIMINARY INVESTIGATION OF PRE-EARLY PERMIAN DEFORMATION ASSOCIATED WITH THE ANCESTRAL ROCKY MOUNTAINS OROGENY, SACRAMENTO MOUNTAINS, NEW MEXICO

Tectonic origins of the Ancestral Rocky Mountains (ARM) remain enigmatic owing to their intraplate location, distance from coeval plate margins, and orthogonal trend to the shortening direction of the commonly invoked driver along the Pangean suture. The modern Sacramento Mountains of south-central New Mexico preserve siliciclastic sediment shed westward from the Pedernal uplift of the ARM and deposited in the adjacent Orogrande basin. This region has been affected by younger episodes of deformation, including the Sevier and Laramide orogenies as well as Rio Grande rifting, that have acted to obscure the primary signal of ARM orogenesis and its tectonic driver. However, late Paleozoic strata of the Sacramento Mountains expose apparent original ARM-related structures that have experienced minimal tectonic overprinting.

Pennsylvanian strata of the eastern Orogrande basin are folded and faulted, and dip up to 70 degrees, with strikes commonly subparallel to the trend of the Pedernal uplift. Hinge lines trend NNW and fold amplitudes range up to 575 m. The early Permian Abo Formation commonly exhibits dips of only a few degrees directly overlying or adjacent to steeply-dipping Pennsylvanian beds. Thus, the deformation is constrained below the Abo Formation. In the southern Sacramento Mountains, a ­pronounced angular unconformity marks lowermost Permian sedimentation. Here, pre-unconformity erosion variably removed uppermost Pennsylvanian units, and the Abo Formation forms a buttress unconformity with the preexisting landscape composed of syntectonic Pennsylvanian strata. In the northern Sacramento Mountains, the latest Pennsylvanian-earliest Permian Laborcita Formation exhibits growth strata adjacent to the Fresnal fault indicated by thickening in the core of a footwall syncline. The Laborcita Formation is overlain by undeformed Wolfcampian Abo Formation strata. The Virgilian Holder Formation is the youngest unit incorporated in the folds in the southern Sacramento Mountains, whereas, the Laborcita Formation is the youngest deformed unit in the northern part of the range. However, the Laborcita Formation tapers out to the south. We infer that the deformation occurred near the Pennsylvanian-Permian boundary and ceased by deposition of the early Permian (Wolfcampian) Abo Formation.