Preliminary geologic map of the Loma Creston quadrangle, Sandoval County, New Mexico

Richard M.Chamberlin, Frank J. Pazzaglia, Karl W. Wegmann and Gary A. Smith

New Mexico Bureau of Geology and Mineral Resources, Open File Digital Map Series OF-DM-25, 1999

SUMMARY--The Loma Creston quadrangle is located on the SE margin of the Jemez volcanic field and the NW margin of the Albuquerque Basin. Both are major features of the Neogene Rio Grande rift. Geologic mapping and ⁴⁰Ar/ ³⁹Ar dating of volcanic strata in the quadrangle demonstrate rift-basin subsidence was contemporaneous with late Miocene volcanism in the Jemez field. Subsidence was controlled by north-trending, dip-slip, dominantly down-to-the-east, high-angle normal faults. South-facing relay ramps (downwarps) are common where north and south propagating faults were deflected by, or terminated in, pre-existing NE-trending basement shear zones. Miocene basin-fill units thicken eastward and southward across the longitudinal and transverse fault zones.

Volcanic strata record: 1) basaltic eruptive events at 9.8, 9.07 and 2.44 Ma; 2) rhyolitic events at 9.66-9.48, 6.97-6.85, and 6.25 Ma; 3) a trachydacitic event at about 9.2 Ma; and 4) a trachyandesite event at 6.99 Ma. Silicified and jasperized footwalls of normal faults are spatially related to 9.6 and 6.9-6.25 Ma intrusive centers, respectively. Hydrothermal adularia from a jasperoidal zone is dated at 5.28 Ma.

Eolian sands and minor fluvial sands of the lower Santa Fe Group (Zia Fm) are older than 9.8 Ma. Volcanic-rich alluvial gravels of the middle Santa Fe Group (lower Arroyo Ojito Fm.?) were shed southward from growing andesitic/dacitic volcanoes at about 9 Ma. An easterly flowing granite/chert-rich fluvial fan system (upper Arroyo Ojito Fm.?) then overlapped and partly intertongued with the volcanic-rich alluvial gravels prior to 6.99 Ma. Strong subsidence then shifted east and southward; most of the volcanic-rich gravels of the Cochiti Formation (upper Santa Fe Gp.) accumulated in deep basins east of the Cocida-Oso fault zone and the southern segment of the Santa Ana fault zone. West of these faults, eroded remnants of the Cochiti Formation fill paleovalleys cut into older Santa Fe beds and volcanic units. Localized minor subsidence, faulting and sediment accumulation continued until about 2.44 Ma; since then the quadrangle has been deeply eroded by drainages tributary to the Rio Grande.