MIDDLE MIOCENE (CA. 13-14 MA) CHANGES IN RIFT TECTONISM INFERRED FROM SEDIMENTATION PATTERNS IN THE FLUVIALLY DOMINATED ESPAÑOLA BASIN OF THE RIO GRANDE RIFT, NORTH-CENTRAL NEW MEXICO

Exposures of middle and upper Miocene strata of the Tesuque Formation provide a record of fluviatile sedimentation and tectonic deformation in the west-tilted Española basin of the Rio Grande rift. In the north-central part of this half-graben, strata deposited on the hangingwall ramp include alluvial slope deposits that flank the Sangre de Cristo Mountains to the east. Basinward of the alluvial slope deposits are fluvial and eolian sediments associated with basin-floor depositional environments. We used six distinctive tephra zones and limited fossil data to correlate and date these strata. Two independent data sets (bedding attitudes and thickness trends of stratigraphic intervals across the study area) were used to calculate rates of stratal tilt and sediment accumulation through time. An upward decrease in stratal dips and a basinward thickening of tephra-bounded stratigraphic intervals document syntectonic deposition. There is a noteworthy decrease in the rate of stratal tilting and a corresponding reduction in sediment accumulation after ~13-14 Ma. This decrease in rate roughly coincides with an overall coarsening of the stratigraphic succession and a basinward progradation of alluvial slope deposits, which resulted in the preservation of coarse-grained deposits towards the basin center. The decrease in stratal tilt rate indicates diminished asymmetric tectonic subsidence in the Española basin after ~13-14 Ma. Decreased sediment accumulation and the upward coarsening character of deposits could be caused by an increase in the sedimentation/subsidence ratio in the basin, perhaps during a transition from an underfilled to an overfilled condition. Nontectonic factors that could influence up-section coarsening and basinward progradation include a middle Miocene change in climate as inferred from marine proxy records. The emplacement of an intrabasinal dune field (Ojo Caliente Sandstone) between 12.5 and 13.5 Ma may be consistent with this climatic event. Other possible influences are geomorphic changes in streams draining the Sangre de Cristo Mountains that could have increased stream power and sediment delivery into the basin. Understanding the causes of coarsening and progradation has important predictive and hydrogeologic implications for this and other rift basins.