New Mexico Geological Society Annual Spring Meeting — Abstracts

Fourteen geologic maps by the NM Bureau of Geology and Mineral Resources and USGS, and drillhole data were compiled into a 1:50,000-scale map encompassing 2200 km2 of the Albuquerque-Rio Rancho metropolitan area and vicinity. Sedimentdistribution patterns and paleocurrent data document SSE-flowing paleorivers associated with the ancestral Rio Puerco fluvial system (Arroyo Ojito Fm, To) and provide geologic constraints on groundwater-flow anisotropy. The To dominates the upper Santa Fe Group basin fill. To drainages flowed obliquely towards the basin axis where they entered the SSW -flowing axial Rio Grande fluvial system (ARG). Basin-margin unconformities generally become gradational toward depocentral areas. Northwestern basin-margin faults may have been active prior to 19 Ma, resulting in local stripping of formerly widespread Oligocene strata. The lack of middle-member (To) deposits on the footwall of the San Ysidro fault indicates that erosion of the northwestern basin margin occurred between late Miocene and Pliocene time. Widespread basinal aggradation diachronously ceased ~2.5-0.8 Ma, probably as a consequence of differential tilting and changing sedimentation rates and patterns within the basin. Conglomeratic deposits were largely restricted to basin margins prior to Pliocene time. The coarse-grained upper member (To) subsequently prograded across much of the western basin. The ARG flowed within 2 km of the Sandia Mountains frontal fault system prior to Pleistocene time. Piedmont deposits derived from rift-flank uplifts prograded west and buried much of the ARG succession by early Pleistocene time. Between 1.2-0.8 Ma, the ARG had migrated towards the basin center when incision began forming the Rio Grande Valley.