New Mexico Geological Society Annual Spring Meeting — Abstracts

The Santa Fe Group of late Oligocene to middle Pleistocene age is the major syn-rift deposit of the Rio Grande rift. Santa Fe Group deposits from the Gabaldon badlands, Bobo Butte area, and from the Humble Santa Fe Pacific #1 and Shell Santa Fe Pacific #2 oil test wells were analyzed to determine sandstone detrital modes and provenance.

Most samples are poorly sorted lithic arenites that were deposited in an alluvial-fan/playa complex. Point counts of 34 stained thin sections show an average composition of 40% quartz, 44% feldspar, and 16% lithics. PIagioclase and monoorystalline quartz are the dominant detrital grains. Lithic grains average 92% volcanic, 8% sedimentary, and <1% metamorphic in origin. Conglomerate in the Gabaldon badlands and Bobo Butte area contains from 50-88% Oligocene ash-flow tuff. Conglomerate in the Bobo Butte area also ns about 25% Cretaceous mudstone and sandstone.

These data indicate that the source area for the Santa Fe Group deposits contained ash-flow tuff and Cretaceous sedimentary rocks. Imbricated clasts show a flow direction eastward away from the Lucero uplift. This suggests that the Lucero uplift was the source area for these deposits. No ash-flow tuffs or cretaceous sedimentary rocks are now present in the Lucero uplift. Thus, sometime in the past, the Lucero uplift must have contained at least one ash-flow tuff sheet and some cretaceous sedimentary rocks that have subsequently been removed by erosion.