2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 4
Presentation Time: 1:30 PM-5:30 PM


SCOTT, Robert B., U.S. Geol Survey, MS 980, Denver, CO 80225, SNEE, Lawrence W., USGS, Box 25046, MS 974, Federal Center, Denver, CO 80225, DRENTH, Ben, Geological Sciences, Universtiy of Texas at El Paso, El Paso, 79968, ANDERSON, Eric D., US Geol Survey, Denver Federal Center, Bldg 20, MS 964, Denver, CO 80225 and FINN, Carol A., U.S. Geol Survey, Box 25046, M.S. 964, Denver Federal Center, Denver, CO 80225, rbscott@usgs.gov

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.