MARISCAL CANYON, SOUTHERNMOST BIG BEND NATIONAL PARK: CONTRACTIONAL AND EXTENSIONAL DEFORMATION AND A RECENTLY EXHUMED RHYOLITE IGNIMBRITE

Transecting the western Mariscal-San Vicente mountain mass, Mariscal Canyon reveals structural/lithostratigraphic relations that enable integration of observations at crestal and structurally deeper levels of Mariscal anticline. Both contractional and extensional events have deformed LK Del Carmen (Kdc) through Santa Elena (Kse) strata in the western to central canyon. Thin Sue Peaks (Ksp) limestone/shale beds separate thick cliff-forming Kdc and Kse limestones; mechanical contrasts are well expressed in contractional structures. At mid-canyon Kdc through Kse are thrust and overturned to the east. The N-trending thrust is cut by a N-striking normal fault that juxtaposes thin-bedded, overturned Ksp (west) and massive Kse (east). The Rio Grande there follows an ENE-striking normal fault that brings thrusted Kdc to river level in the upthrown southern block, opposite Ksp on the north. East of that nexus Kdc is not exposed in structurally lower segments of the mountain.

At Cross Canyon farther east, Boquillas (Kbo) and Buda (Kbu) Fms. have been folded in the thrust footwall; flaggy argillaceous Boquillas is more tightly folded than is massive Buda limestone. In discordant relation to the underlying folded and faulted K strata is a densely welded rhyolite tuff (undated), exhumed by recent Rio Grande floods. The south-dipping rhyolite ignimbrite is likely a remnant of a more extensive deposit that was preserved in a depression in K rocks (D. S. Barker, pers. comm.). At present contacts with the K are obscured by the river (south) and by thick sand/gravel (north). The tuff may have foundered into a solution-collapse cavity or may have been let down due to undercutting by the river and removal of fractured, less resistant K strata.

Transpressional deformation is recorded in a flower structure ~2 km downstream (east) from Cross Canyon. Striae on the near-vertical eastern fault wall rake 40º SE and attest to left-oblique displacement. A low-angle contractional fault is exposed near river level, and slabs of Kse between near-vertical shears exhibit variable dips. Exiting the canyon, the Mariscal Mt.-front normal fault drops Kbo against Kse, both of which dip gently eastward (Kse - 12º E, Kbo - 14º E). Quaternary extension just east of Mariscal Mt. produced the Solis graben (Dickerson et al., 2009, BEG Misc. Map 48).