Preliminary Geologic Map of the Socorro Quadrangle, Socorro County, New Mexico

Richard M. Chamberlin

New Mexico Bureau of Geology and Mineral Resources, Open File Digital Map Series OF-DM-34, 1999

SUMMARY¾ Major topographic features of the Socorro quadrangle include the west-tilted Socorro Peak block and west flank of the Socorro Basin; both are relatively young features of the central Rio Grande rift. The Socorro Basin is defined by dissected and faulted piedmont slopes that descend toward the Rio Grande at the east edge of the quadrangle.

Geologic mapping and ⁴⁰Ar/ ³⁹Ar dating of volcanic strata, in and adjacent to the quadrangle, have demonstrated the following major structural features and evolution. 1) The NE margin of the 31.9 Ma Socorro caldera (source of Hells Mesa Tuff) is exposed on the east face of Socorro Peak. 2) Locally erupted tuffs and lavas (Luis Lopez Fm.) back filled the caldera between 30.04 and 28.77 Ma. 3) Early rift alluvial conglomerates and playa deposits (Popotosa Fm.) buried the caldera margin from about. 17 to 6.88 Ma. 4) A synextensional cluster of dacite to rhyolite lava domes was erupted onto the playa floor (above the buried caldera) between 9.51and 7.02 Ma. 5) Late-rift extension caused subsidence of the Socorro Basin contemporaneous with transition to a through going axial-river depositional system (ancestral Rio Grande, Sierra Ladrones Fm.), sometime after 6.88 Ma and prior to 3.73 Ma. 6) The ancestral Rio Grande aggraded in the subsiding basin until after 1.2 Ma; since about 0.7 Ma the Rio Grande has mostly cut down through its own deposits, thereby forming an eroded inner valley.

Pliocene to Pleistocene sands and gravels of the ancestral Rio Grande form a major aquifer under the city of Socorro. Deep monitoring wells (e.g. 2000 ft) will be needed to determine the maximum thickness and storage capacity of this important source of potable groundwater. Numerous late Quaternary fault scarps in the quadrangle (Socorro Canyon fault zone) demonstrate a significant seismic risk; however, earthquake recurrence intervals are presently not well constrained. Seismic risk may be increased by the presence of a geophysically defined mid-crustal magma body at 12 miles depth below the Socorro region. Numerous microearthquake clusters occur at depths of 6-7 miles below the Sedillo Hill area (9 miles SW of Socorro); this suggests the presence of slowly rising cupolas from the mid-crustal magma body. More work is warranted to evaluate the long-term probability of a volcanic eruption near Sedillo Hill and its potential impact to the Socorro area.