EBTAG Annual Workshop and Field Trip
May 16-17, 2012

¹NM Bureau of Geology and Mineral Resources, 801 Leroy Place, Socorro, NM, 87801, dkoning@nmbg.nmt.edu

²Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM, 87545

³NM Bureau of Geology and Mineral Resources, 2808 Central Avenue, SE, Albuquerque, NM, 87106

We present late Oligocene-Quaternary paleogeographic maps that illustrate the depositional, tectonic, and volcanic history of the Española Basin.  These maps are largely a result of recent geologic mapping, stratigraphic correlations, and inspection of subsurface well data. Previous studies using geochemical correlation and radiometric dating of tephra beds, biostratigraphic correlation of mammalian fossils, and paleomagnetic-polarity studies of basin-fill also proved indispensible in constructing these maps.

The Española Basin consists of a general half-graben to the east, which is tilted westward towards a central fault system (CFS). Locally within this half-graben are smaller grabens with thicker basin fill. West of the CFS, thinner basin-fill deposits overlie a faulted structural platform (Abiquiu embayment and northern Jemez Mountains), separated from the Colorado Plateau by a 7-13 km-wide zone of east-down normal faults. We discuss 6 important depositional belts in the Santa Fe Group of the Española Basin, which we list from east to west: 1) an eastern alluvial slope deposited by west-flowing streams that drained the Sangre de Cristo Mountains; 2) an ancestral Rio Embudo that flowed southwest into the basin from the Peñasco embayment; 3) a river draining the southeastern San Luis Basin (<18 Ma); 4) a western alluvial slope deposited by streams sourced in volcanic terrain in the southern San Luis Basin; 5) a river draining the Colorado Plateau (<14 Ma); and 6) alluvial fans along the eastern Jemez Mountains (<12 Ma).

Several paleogeographic changes occurred after 16 Ma. The ancestral Rio Embudo and the toe of the eastern alluvial-slope shifted westward starting at 16 Ma, with deposits of the ancestral Rio Embudo onlapping the CFS footwall during 11-10 Ma. This progradation coincided with high throw rates along the CFS between 15 and 10 Ma, but was also influenced by paleoclimatic changes at 14-13 Ma. A river system draining the Colorado Plateau flowed into the southwest part of the basin starting at ~14 Ma. This paleoriver carried a sandy bedload and displaced south-flowing streams originating from the San Luis Basin. Northeastward eolian transport of sand from this paleoriver resulted in formation of an extensive eolian dune field during 13.5-11 Ma. Coarse ash and lapilli beds (radiometrically dated at 13-12 Ma), intercalated in basin fill, mark the beginning of felsic-to-intermediate volcanism in the Jemez Mountains. The increase in topographic relief from Jemez volcanism, together with elaboration of paleovalleys, resulted in the integration of the Colorado Plateau drainage into a narrower river capable of carrying gravel. Erosion of these volcanic edifices produced east-sloping alluvial fans in the Los Alamos area. A hiatus in deposition, between ~6-5 Ma, was followed by coarse-grained deposition adjacent to the CFS. Erosion dominated the Española Basin since 1.2 Ma, with particularly rapid incision in the last 0.6 Ma.