The Oligocene Goodsight-Cedar Hills half graben near Las Cruces and its implications to the evolution of the Mogollon-Datil volcanic field and to the southern Rio Grande rift
Greg H. Mack, Alice L. Nightengale, William R. Seager, and Russell E. Clemons
The Goodsight-Cedar Hills (GCH) depression near Las Cruces, New Mexico, was a north-northeast- trending, nonmarine, elliptical basin of Oligocene age that developed along the southeastern margin of the Mogollon-Datil volcanic field. Within the basin were deposited six ash-flow tuff outflow sheets, as well as sedimentary rocks composed of interbedded syneruption and post-eruption lithofacies. The syneruption lithofacies consists of light-colored, medium-grained tuffaceous sandstones that represent reworked fallout tephra. Epiclastic conglomerates and coarse sandstones constitute the post-eruption lithofacies, with clast composition reflecting derivation from adjacent bedrock uplifts. An asymmetrical distribution of post-eruption lithofacies, based on provenance and sediment dispersal data, suggests that the GCH basin was an eastward-tilted half graben, with the Cedar Hills vent zone as the footwall and the Ancestral Goodsight uplift as the hanging wall. The GCH half graben provides important information about the Mogollon-Datil volcanic field, not only because it was a depository for several regional outflow sheets, but also because syneruption sandstones may record explosive eruptions that occurred during or between major ignimbrite-generating events. Furthermore, initial development of the half graben at 35 Ma precedes by 2 to 7 million years previous interpretations of the onset of regional crustal extension in the southern Rio Grande rift.
- Mack, Greg H.; Nightengale, Alice L.; Seager, William R.; Clemons, Russell E., 1994, The Oligocene Goodsight-Cedar Hills half graben near Las Cruces and its implications to the evolution of the Mogollon-Datil volcanic field and to the southern Rio Grande rift, in: Mogollon Slope, west-central New Mexico, Chamberlin, Richard M.; Kues, Barry S.; Cather, Steven M.; Barker, James B.; McIntosh, William C., New Mexico Geological Society, Guidebook, 45th Field Conference, pp. 135-142.