WHAT BECAME OF THE OUACHITA OROGEN BEYOND THE MARATHON SALIENT? MUSINGS ON THE SOUTHWESTERN MARGIN OF LAURENTIA

Two alternative models exist for the deformed Paleozoic margin that flanked Gondwana beyond exposures in the Big Bend region: (1) It ran directly south from West Texas; or (2) it continued west along the southern edge of Laurentia to Sonora. Deformation is generally accepted to have ended in West Texas in middle Wolfcampian (Sakmarian) time, whereas inferred collisional deformation in Sonora continued into Wordian (middle Guadalupian) time. Aside from the Sonora orogen, how was potential collision expressed west of Texas?

Detrital zircon data* tie Cambrian grains in coarse-grained sandstone of the Roadian (lower Guadalupian) Brushy Canyon Formation exposed in the western Delaware basin to the Florida Mountains batholith (FMB; ~517–497 Ma) of SW New Mexico, then a part of the late Paleozoic Florida uplift. Geologic relations in the Florida Mountains indicate possible late Paleozoic deformation of the FMB, making it a plausible middle Permian source of coarse sediment. North-vergent, basement-involved structures in the range, although generally interpreted as Laramide, are rather loosely constrained as to age. They cut strata as young as lower Permian and are overlapped by unfolded Paleogene strata containing little coarse granitic detritus. The South Florida Mountains reverse fault emplaces interlayered felsic and mafic rocks of the deep FMB over Cambrian syenite and lower Paleozoic strata. In the northern part of the range, an undated matrix-supported conglomerate contains boulders of alkali-feldspar granite and gabbro derived from the FMB and is apparently overthrust by Lower Ordovician Bliss Sandstone.

The possibility of middle Permian uplift in SW New Mexico apparently corroborates westward continuation of collision along the southern margin of Laurentia. The mechanism of deformation, whether by collision with Gondwana itself or outboard oceanic terranes, such as the Cortés and El Fuerte terranes, remains debated.

* George, S., 2014, Wellesley College senior thesis