SLOPPY PANCAKE STACKS: OLIGOCENE LACCOLITHS OF NORTHERN BIG BEND NATIONAL PARK, TEXAS

Detailed geologic mapping and new high-resolution aeromagnetic maps of 6 major alkalic granitic laccolith and sill complexes show a variety of geometries ranging from single to stacked, largely concordant, intrusive bodies. The Rosillos laccolith, which intrudes Cretaceous (K) strata is a circular (10.5 km wide, >600 m thick) single body that upholds the Rosillos Mts. and is cut by the major NE-dipping Tertiary Chalk Draw normal fault (CDf). 3 km NE of the Rosillos body, the SW-dipping South Persimmon Gap laccolith (6.5 x 3 km, ~350 m thick, NE elongate) has its SW half buried below K strata and alluvium and is cut by NW-trending normal faults as young as Quaternary. Another NE-elongate (5 x 2 km), barely exposed, body lies 12 km N of the Rosillos and E of CDf. These two NE trending intrusive bodies suggest a Marathon tectonic emplacement control. Several small granitic laccoliths and basaltic sills intrude the CDf. The Nine Point Draw stacked laccolith and sill complex (9 x 6 km, NW elongate) lies just NW of landslides at the N end of the Rosillos laccolith; in contrast to common dark-gray laccolithic bodies with positive anomalies in the area, one 1.5-km-wide circular body has a sharp negative anomaly, and a different body is a thin, 60-m-thick, red-weathering sill. 15 km SE of the Rosillos Mts., the McKinney Springs laccolith complex displays 3 sloppily stacked bodies: The upper McKinney Springs laccolith (10 x 6 km, ~750 m thick, NW elongate) intrudes K strata and dips under alluvium and K strata at ~ 15 degrees for 1.5 km to the W and even more to the NW. Only a few sills mark the top of the buried, mid-level Javalina Creek laccolith (~15 x 5 km, N elongate), the S end of which is tucked under the N end of the upper laccolith. The lowest aeromagnetic anomaly is larger (~21 x 12 km, E elongate), underlies the eastern edge of the higher laccoliths, and lies at depths of ~ 0.3 km to ~ 1 km. Depths to this body are based on fault-induced magnetic lineament gradients. About 4 km S of the Rosillos, the base of the single, smaller, circular Grapevine Hills laccolith is 3.5 km wide, based on its aeromagnetic signature. This laccolith is >200 m thick and is partially roofed by K strata. No evidence of eruption from any of these laccoliths had been found. Faults cutting the bodies are generally NW-trending regional structures with no indication of collapse features.