POST-MIOCENE STRATIGRAPHY OF THE ALBUQUERQUE BASIN, RIO GRANDE RIFT, NEW MEXICO: IMPLCATIONS FOR GEOMORPHIC AND HYDROGEOLOGIC DEVELOPMENT OF EXTENSIONAL BASINS

The large-scale depositional architecture of basin fill influences aquifer heterogeneity and anisotropy with implications for conceptualizing aquifer structure in fluvially dominated rift basins. Recent refinements to the stratigraphy of the Santa Fe Group in the tectonically active Albuquerque basin of the Rio Grande rift delineate an important tectonostratigraphic boundary between Miocene and Pliocene deposits. The Rincones surface, a soil-bounded disconformity exposed along the northwestern Albuquerque basin, locally contains paleosols that exhibit Stage III to V pedogenic carbonate morphology. It is buried by fluvial deposits of the Pliocene Ceja Formation, which is capped by the ~2-2.5 Ma Llano de Albuquerque surface, a local depositional top of the Santa Fe Group. Down-dip correlation of the Rincones surface and bounding deposits, using surficial and subsurface data, constrains the distribution of component depositional belts across much of the basin. Soils of the Rincones surface are not recognized in drillholes towards the basin center, and basinward thickening of the overlying stratigraphic succession suggests nearly continuous basinward sedimentation. Pliocene deposits on the distal hangingwall-ramp record upsection coarsening and basinward progradation that culminated in amalgamated fluvial channel gravels. Subsurface control is currently insufficient to establish direct physical correlation across the basin, but the presence of Mio-Pliocene erosional surfaces along the eastern footwall-margin of the basin suggests they might be (at least in part) coeval with the Rincones surface. The axial river (ancestral Rio Grande) occupied a position near the rift-bounding (Sandia) master fault during most of Pliocene time. The axial-river subsequently migrated towards the hangingwall ramp before incising into basin fill during early Pleistocene time. Ongoing work will examine spatial and temporal trends of texture, channel abundance and stacking patterns within different facies belts in order to elucidate hydrologic and tectonic controls on the development of facies belts within asymmetrically subsiding rift basins. Continuing magnetostratigraphic work will constrain temporal trends in basin fill and document sediment bypass across the distal hangingwall ramp.