New Mexico Geological Society Annual Spring Meeting — Abstracts

Coarse-grained sedimentary rocks of the Bell Top Formation (Oligocene) suggest that crustal extension was contemporaneous with bimodal volcanism. The Bell Top Formation includes interbedded conglomerate and tuffaceous sandstone which stratigraphically overlie ash flow tuff 5 or the Kneeling Nun tuff and are 40 to 230 meters thick. The conglomerates were derived from three distinct source areas: 1) Cedar Hills vent zone, located along the southeastern margin of the Goodsight-Cedar Hills depression, which supplied clasts of flow-banded rhyolite, 2) Goodsight arch, located along the western margin of the Goodsight-Cedar Hills depression, which supplied andesite clasts from the Palm Park and Rubio Peak Formations and rhyolite clasts from Bell Top ash flow tuffs, and 3) an unnamed uplift that is assumed to be presently buried in the southern part of the Palomas basin, which supplied a mixture of volcanic clasts derived from the Kneeling Nun tuff and Palm Park Formations, and clasts of Paleozoic carbonate and siltstone and Precambrian granite and metamorphic rocks derived from the Love Ranch Formation (Eocene). Tuffaceous sandstones contain numerous glass shards and pumiceous grains and represent redeposition of pyroclastic debris that mantled the region following explosive volcanic eruptions.

Coarse grain size and the presence of debris-flow deposits in Bell Top conglomerates suggest that the Cedar Hills vent zone and the unnamed source area may have been block uplifts that were associated with the earliest phase of extension in the southwestern Rio Grande rift. If so, the initiation of rifting in south-central New Mexico may be as old as 35 Ma.