DETAILED MAPPING AND STRUCTURAL ANALYSIS OF THE SOUTH PERSIMMON GAP LACCOLITH HELP TO CONSTRAIN LOCATION AND TIMING OF REGIONAL FAULTING, NORTHERN BIG BEND NATIONAL PARK, TEXAS

We report on field mapping (1:12,000-scale) of the South Persimmon Gap laccolith (SPGL). Preliminary analysis of the SPGL map area relates the intrusion to deformation of adjacent Cretaceous sedimentary rocks in the Dog Canyon map area (Satterfield and Dyess, 2007). Work to date suggests that the SPGL and Dog Canyon area plutons were intruded after Laramide deformation and before the latest Basin and Range faulting.

Tertiary intrusive rocks (Ti) of the SPGL and Dog Canyon plutons are phaneritic, quartz-poor, and mafic to felsic in composition. The contact between the Cretaceous Aguja Formation (Kag) and Tertiary intrusive rocks (Ti) of the SPGL is sharp with a well exposed contact metamorphic zone of <1 meter. The SPGL is crosscut by at least two NW-trending high-angle faults. Slickenlines on fault surfaces indicate mostly normal offsets. Sparse slickenlines on NW-striking fault surfaces on both faults indicate nearly pure strike-slip. We hypothesize that these faults are a result of Basin and Range extension in this area, and are related to a large, covered normal fault that separates the SPGL from the Dog Canyon map area, which we infer from stratigraphic relationships between these two map areas.

The SPGL interfingers with Kag at map scale and outcrop scale. Kag – Ti contacts are subparallel to bedding in Kag. Furthermore, Kag in the vicinity of the SPGL consistently dips gently northwest, suggesting that intrusion of the SPGL did not deform the surrounding Kag during emplacement. These observations suggest that the SPGL is actually a series of sills within Kag, which is consistent with observations made by Scott and others (2004).