A1.07 Ma Change from Persistent Lakes to Intermittent Flooding and Desiccationin the San Felipe Hills, Salton Trough, Southern California

Document Type

Report

Journal/Book Title/Conference

USGS Open File Report 2005-1404

Publisher

United States Department of Agriculture

Publication Date

4-15-2005

First Page

18

Last Page

18

Abstract

Sedimentary rocks of Pliocene to Pleistocene age in the San Felipe Hills (SFH), Salton Trough, record an abrupt change from the older, perennial lake beds of the Borrego Formation to cyclic alluvial and marginal lacustrine deposits at 1.07 Ma. The ~1680-m-thick lacustrine claystone, mudstone and sandstone of the Borrego Formation exhibit few marginal lacustrine facies in the San Felipe Hills, 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 Formation from the overlying conglomeratic Ocotillo Formation and its fine-grained equivalent, the Brawley Formation. The disconformity is dated paleomagnetically at 1.07 Ma and records a permanent drop in base level as alluvial to eolian deposition replaced open lacustrine deposition. The Brawley Formation 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 dessication 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.and ~40% sublitharenite derived from the Colorado Plateau. Sediment transport was to the E to NNE. Sedimentation rates range from 1.1 mm/yr to 1.8 mm/yr.

The disconformity between the Borrego and Brawley Formations, at 1.07 Ma, indicates that the lake margin shifted ~25 km to the NE, the lake was reduced in size, and near modern depositional environments were established across a large area. This abrupt change reflects reorganization of the basin due to initiation or reorganization of the San Jacinto fault zone in the SFH. Brawley Formation sediments accumulated in an ephemeral stream and delta system on the western margin of the Salton Trough while evaporites accumulated offshore. Flooding of the basin occurred when channel switching in the Colorado River delta delivered water N into the Brawley basin. Although water in the Brawley lake was derived from the Colorado River to the SE, sand was derived from local sources in the W and SW. Colorado River derived sand was recycled from uplifted Pliocene Diablo Formation, which consists of older delta-plain deposits of the Colorado River. Folding and uplift in the SFH began after the end of deposition of the Brawley Formation (0.61 ± 0.02 to 0.52 Ma ± 0.03 Ma) and shifted the depocenter farther to the east. this latter transition records the initiation of the Clark strand of the San Jacinto fault SE of the Santa Rosa Mountains.

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