PALEOECOLOGY OF THE NEOGENE IMPERIAL GROUP OF THE FISH CREEK VALLECITO BASIN; IMPLICATIONS FOR THE ORIGIN OF COARSENING UPWARD SUCCESSIONS IN THE DEGUYNOS AND CAMELS HEAD FORMATIONS

The Fish Creek Vallecito Basin, located on the western side of the Salton Trough, contains the most complete statigraphic section of Neogene Colorado River deltaic sediments.  The Pliocene Imperial Group is a marine succession which is divided into two distinct sandstone suites.  The first consists of immature micaceous sandstone of local origin while the second is a mature quartz arenite derived from the Colorado River.  The upper formations of this Group, namely the Deguynos and Camels Head Formations, are dominated by coarsening upward cycles of silty mud, capped by “coquinas” and fossiliferous sandstones.  Determining the origin of these coquina-capped coarsening upward cycles has proven problematic due, in part, to structural complexities arising from Quaternary tectonics.  Process oriented interpretations include in-situ reefs, tidal shoals, point bars, and lateral accretion surfaces.  Many of these interpretations were based on single localities and did not take into account within-basin lateral variation.  We have conducted detailed geologic mapping of the resistant cycle-capping units within the Fish Creek-Vallecito Basin.  At each outcrop, we collected data on fragmentation, orientation, abrasion, and articulation to assess taphonomic grade.  Our preliminary analyses show that there are four dominant facies in the Deguynos and Camels Head Formations: (1) Silty-mud—predominant, laminated to fully bioturbated (2) Sand—massive, laterally continuous (km scale), unfossiliferous, exhibiting dewatering structures (3) Channel lags—not laterally continuous (10-100s m) with both local and Colorado River origin, and varying degrees of fossil content (4) Coquinas—laterally continuous (km scale) yet varying sedimentologically, taxonomically and taphonomically.  Future work includes, (1) the analysis of taxonomic diversity and abundance data (2) a thorough investigation of the silty-mud facies, in order to further constrain the possible sources of the bioclastic material that composes the coquina and to gain further insight  into the environment of deposition of these units.