COMPETING INFLUENCES ON NEOGENE PALEOCLIMATE CHANGE AND RHYTHMITE FORMATION IN THE SALTON TROUGH REGION, CALIFORNIA

Miocene-Pliocene marine mudrocks exposed in Anza Borrego Desert State Park, California provide evidence that the Salton Trough region experienced abrupt paleoclimatic and paleoenvironmental change. Possible factors include shifts in ocean circulation, the closure of the Isthmus of Panama and cooling of Pacific surface waters, availability of a Gulf of California derived moisture source, uplift of the coastal range, establishment of a rain shadow, and arrival of the Colorado River. Conformable fossiliferous bioturbated units and unbioturbated, unfossiliferous rhythmites indicate rapid increase in salinity and stratification of the water column, a shutdown of the local ecosystem and cessation of bioturbation in the deeper basin, representing a shift in basin conditions that supported marine fauna to those that were non-conducive to burrowing organisms and fossil preservation. Similarity of mineralogy and grain size between units suggests tectonically driven basin restriction and climate changes had greater effect than changes in sediment source or sedimentation rate. Equivalent proximal units to bioturbated and unbioturbated distal units are fossiliferous and bioturbated, suggesting basin margin conditions supported a diversity of organisms throughout the entire period of deposition.

Quantitative XRD on the £ 2 mm fraction examined mineralogic changes occurring laterally and vertically. Comparison between bioturbated and unbioturbated units in vertical succession showed no significant changes while lateral variation within units and individual marker beds was notable. Deeper basin deposits are dominated by 2:1 smectitic clays and quartz, with minor amounts of gypsum and very little carbonate, while basin-margin units contain significantly less clay, and an abundance of quartz, feldspar, biotite and carbonate, suggesting that proximity to the basin margin and input of local sources had far greater influence on composition than changes in clay source.