2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 4
Presentation Time: 8:45 AM


CONNELL, Sean D., New Mexico Bureau of Geology and Mineral Rscs, New Mexico Institute of Mining and Technology, 2808 Central Av. SE, Albuquerque, NM 87106, SMITH, Gary A., Earth and Planetary Sciences, University of New Mexico, MSC03 2040, Albuquerque, NM 87131 and GEISSMAN, John, Earth and Planetary Sciences, University of New Mexico, MSC03-2040, Northrop Hall 141, Albuquerque, NM 87131, connell@gis.edu

Synthesis of geomorphic, geochronologic, magnetostratigraphic, sedimentologic and geophysical data elucidates the evolution of landforms and basin-fill preserved within the Albuquerque basin and Rio Grande rift in New Mexico and Colorado. During Miocene, time a low ratio of sedimentation to subsidence rates promoted internal surface drainage within the rift. By late Miocene, the axial river (ancestral Rio Grande) terminated in the southern Albuquerque basin. By early Pliocene time, this ratio inverted, causing spillover and fluvial integration through a structurally linked chain of previously closed basins to the Mesilla basin in southern New Mexico. Continued tectonic activity is, however, documented by numerous faults that cut Pleistocene sediments. An angular unconformity separates tilted upper Miocene strata from subhorizontal Plio-Pleistocene fluvial sediments, indicating that most stratal rotation (and basin deformation) occurred before Pliocene time. This surface represents a datum used to delineate depositional and deformational patterns in the region. Erosional benches along basin margins and in the southeastern Colorado Plateau are likely related to this surface. By ~3 Ma much of this erosional surface was buried by fluvial sediments within rift basins or became deeply incised in extrabasinal regions.

Basin subsidence patterns controlled the position of the axial river, which flowed near rift-flank uplifts during Pliocene time and migrated towards the present position of the Rio Grande valley by early Pleistocene time, probably in response to slowing subsidence and/or increased piedmont-stream competence. Plio-Pleistocene deposits generally coarsen upsection, suggesting increased stream discharge or decreased basin accommodation. This coarsening trend culminated with the incision of the Rio Grande Valley during early Pleistocene time (near the Brunhes/Matuyama boundary) and is approximately coeval with increased northern hemisphere climatic cyclicity. Globally recognized increases in sediment accumulation rates since ~5 Ma may be represented by alluvial spillover in basins of the Rio Grande rift during times of diminished basin subsidence. These temporal correlations support dominantly climatic influences on drainage integration within tectonically active basins.