OLIGOCENE ROSILLOS MOUNTAINS LACCOLITH, BIG BEND NATIONAL PARK, TEXAS

Local structures observed during detailed mapping and petrographic data seem to suggest that two laccoliths may exist in the Rosillos Mountains instead of one, yet regional stratigraphic, structural, and geophysical evidence leads us to conclude that only one faulted, fayalite-bearing, granitic laccolith holds up the Rosillos. A large semicircular laccolith (10.5 km long x 8 km wide, > 600 m thick) exposes its base in the lower Aguja Formation and forms the higher elevations of the Rosillos. To the NE, across a NW-trending, 1-km-wide bench of Cretaceous Aguja and underlying Pen Formations, a NW-trending, 130-m-thick slice of laccolith (1.5 km wide x 7.5 km long), is bounded on the SW by a large fault and on the NE by its pinchout into lower Aguja. The larger SW body has less K-feldspar and a more granular texture; in contrast, the narrow NE slice is more K-feldspar rich and felty. In the sedimentary bench SW of the fault, dips in the gray mudstone of the Pen increase from near horizontal 0.5 km from the fault to over 75 degrees close to the fault to form a structure similar to Basin-Range rollovers, suggesting that the fault is a SW-dipping normal fault. However, this fault interpretation is negated by the presence of 1) lower Aguja at both the base of the SW laccolith and the base of the NE slice, 2) the large NE-dipping Chalk Draw normal fault NW of the Rosillos that trends under surficial deposits toward the faulted part of the Rosillos, 3) a positive magnetic anomaly coincides perfectly with laccolithic slice exposures, indicating that there is no burial of the slice or offset body at depth. The SE end of the slice has no offset relative to main Rosillos laccolith exposures or aeromagnetic signature, suggesting that only normal dip-slip occurred. Adding the slice to a restored Rosillos body creates a symmetrical 10.5-x-10.5-km laccolith. These relations compel us to propose that a splay of the Chalk Draw fault dropped the NE edge of the Rosillos laccolith about 300 m at the slice's NW end, but that fault offset decreased significantly toward the SE end of the slice. Thus, the apparent "rollover" probably is a drag zone related to a NE-dipping normal fault, the felty texture is likely related to more abrupt cooling, and observed subtle color layering in the laccolith may be related to chemical zoning and K-feldspar layering.