New Mexico Geological Society Annual Spring Meeting — Abstracts

Many of the breccias in the southern Franklin Mountains that have been interpreted to be karst in origin appear to be Laramide tectonic breccias. A thrust fault on the west side of the mountain, above the Coronado Golf Course, cuts out section and brecciated Montoya and Fusselman rocks have been thrust to the northeast over various El Paso Group formations. As the fault is followed to the southeast up and over the mountain it becomes the EI Paso-Montoya thrust fault contact. The brecciated EI Paso Scenic Drive Formation (Ranger Peak) beneath this thrust appears to be a tectonic breccia rather than a karst breccia.

Montoya rocks are repeated four times by northeast vergent thrust faults at the south end of Sugarloaf Mountain northwest of Beaumont Hospital Beneath the lowest Montoya rocks the more competent Scenic Drive Formation, lying between the thin bedded shaley Florida Mountain and McKelligon Canyon Formations, has been brecciated along the entire length of Sugarloaf Mountain. Also, a flat thrust parallel to the bedding between the McKelligon Canyon Formation and the overlying Scenic Drive Formation can be traced into the brecciated Scenic Drive Formation above Beaumont Hospital.

Numerous minor flat thrust faults and breccia parallel to the bedding zones, occur in the Bliss, EI Paso, Montoya and Fusselman Group rocks. All tectonic deformation occurs only in the lower Paleozoic sedimentary section which overlies the flat unfaulted Precambrian basement surface. In several areas the sedimentary section is detached from the Precambrian basement along thrust faults.