Paper No. 5
Presentation Time: 9:00 AM
A 1.07 MA CHANGE FROM PERSISTENT LAKES TO INTERMITTENT FLOODING AND DESICCATION IN THE SAN FELIPE HILLS, SALTON TROUGH, SOUTHERN CALIFORNIA
Plio-Pleistocene sedimentary rocks in the San Felipe Hills (SFH), Salton Trough, record an abrupt change from the older, perennial Borrego Fm lake beds to cyclic alluvial and marginal lacustrine deposits at 1.07 Ma. The ~1680 m thick lacustrine claystone, mudstone and sandstone of the Borrego Fm contain few marginal facies, and formed in a large perennial lake. A regional disconformity and laterally equivalent angular unconformity at the crest of a bedrock-cored anticline separate the Borrego Fm from the overlying Ocotillo Fm and its fine-grained equivalent, the Brawley Fm. The disconformity is dated paleomagnetically at 1.07 Ma and records a permanent drop in base level. The Brawley Fm shows evidence for repeated drying of intermittent lakes (> 30 times). It consists of 3 interbedded lithofacies: (1) fluvial to deltaic sandstone with cross-bedding and weak calcic paleosols; (2) lacustrine mudstone, claystone, and marlstone with 0.5 to 1.5 m deep desiccation cracks, rare evaporite minerals, and locally abundant microfossils; and (3) eolian sandstone with large scale (~ 3-4 m high) high-angle cross stratification. Microfossils include marine to lagoonal forams, ostracods, micromollusks, and charophytes. Sandstones include ~60% arkose derived from local tonalite sources (L-suite), and ~40% sublitharenite derived from the Colorado Plateau (C-suite). Sediment transport was to the E to NNE. Sedimentation rates range from 1.1 mm/yr to 1.8 mm/yr. Starting at 1.07 Ma the lake margin shifted ~25 km to the SE, the lake was reduced in size, and near modern depositional environments were established. This abrupt change reflects reorganization of the basin due to initiation or reorganization of the San Jacinto fault zone in the SFH. Brawley sediments accumulated in an ephemeral stream and delta system on the western margin of the Salton Trough while evaporites accumulated offshore. Water in the Brawley lake was derived from the Colorado River to the SE, but sand was derived from local sources in the W and SW. C-suite sand was recycled from uplifted Pliocene Diablo Fm. Flooding of the basin occurred when channel switching in the Colorado River delta delivered water N into the Brawley basin, as it does today. Folding and uplift in the SFH began at the end of Brawley deposition (0.57-0.39 Ma) and shifted the depocenter further to the east.