SURFICIAL GEOLOGY OF THE PONDEROSA, NM 7.5’ QUADRANGLE: IMPLICATIONS FOR REGIONAL HYDROLOGY, WATER RESOURCES, AND LANDSCAPE EVOLUTION IN THE SOUTHERN ROCKY MOUNTAINS

The Ponderosa 7.5’ quadrangle is located in north-central New Mexico near the northern boundary of the Albuquerque basin, on the south side of the Valles caldera, and along the structural western flank of the Rio Grande rift. EDMAP supported mapping of this quadrangle has helped understand the hydrology and landscape evolution of the northwest corner of the Albuquerque basin, a region that is coming under increasing land development pressure. Two large sources of surface water, the Jemez River and Vallecito Creek, both of which directly recharge the Sante Fe Group aquifer in the Albuquerque basin, flow through the quadrangle. Surface water flow to the Santa Fe Group is only part of the aquifer’s hydrologic budget. The remainder is largely apportioned to subsurface flow through surficial deposits, which have a highly variable physical and hydrologic connection to the unconsolidated Santa Fe Group, providing the up-dip recharge and conduits of groundwater flow into the aquifer. The surficial deposits form a complicated mantle of sediment that unconformably overlies Paleozoic sedimentary and Quaternary volcanic bedrock. These early Pleistocene (?) to late Holocene deposits have been mapped as 42 allostratigraphic units at 1:24,000 scale based on composition, sedimentology, landscape position, and genesis. Mapping has also extended a pre-existing fluvial terrace stratigraphy along the Jemez River, helped redefine rates and processes of Jemez River and Vallecito Creek incision, and provided insights into landscape evolution in the southern Rocky Mountains. Incision rates range from 0.17 - 0.19 mm/yr along the middle Jemez River and 0.1 - 0.2 mm/yr along lower and middle Vallecito Creek based on correlative and absolute terrace ages. Paleo river long profiles, reconstructed from at least 9 terraces along the Jemez River, diverge downstream. This suggests that exhumation of the Jemez Mountains and adjacent areas of the southern Rocky Mountains is being dominated by upstream knickpoint migration resulting from distal base level fall of the Rio Grande over the past ~ 1 m.y.