EVIDENCE FOR SYN-DEPOSITIONAL FOLDING OF IMPERIAL-AGE SYNRIFT DEPOSITS ABOVE THE WEST SALTON DETACHMENT FAULT, BORREGO MOUNTAIN AREA, SOUTHERN CALIFORNIA

Uplift at Borrego Mountain along the San Jacinto fault exposes the oldest basin fill in the San Felipe-Borrego basin in the southwestern Salton Trough. The late Miocene-Pliocene Hawk Canyon beds consist of locally derived grus at the base, a 12-26 m marine mudstone tongue, and a 13-55 m fluvial upper sandstone and mudstone. Hawk Canyon beds reveal an up-section change in provenance from locally derived grus near the bottom to a southern, non-local source. The overlying West Butte conglomerate (formerly Canebrake Fm. of Dibblee, 1996) has the same southern source. The West Butte cgl. is up to 320 m of conglomeratic sandstone that prograded >12 km from the basin bounding West Salton detachment fault (WSDF) and contains many conspicuous clasts of brecciated, chloritically altered granitoids, but little mylonite. North-directed paleocurrents show that the source of these clasts was located south of Borrego Mt., possibly in the damage zone of the WSDF. Stratigraphic relationships, clast types, and paleocurrents suggest that the West Butte cgl. is the fluvial equivalent of locally derived marine grits, sandstones, coquinas, and rare conglomerates of the marine Imperial Group near Squaw Peak to the east, and sandy to muddy marine Colorado River delta deposits farther east near Shell Reef. This >12 km wide Imperial-age continental shelf at Borrego Mt. contrasts sharply with narrow rocky shorelines of the Fish Creek basin.

Buttress unconformities and onlap relationships define a WNW-trending paleohigh during deposition of the Hawk Canyon beds. The contact with the West Butte cgl. changes across the paleohigh from a disconformity to an angular unconformity. Angular unconformities on the southwest and northeast side of Borrego Mountain show opposite senses of stratigraphic truncation. A subcrop map of Hawk Canyon beds reveals an overall W- to NW-trending bedrock high coincident with the >2 km wide WNW-trending Borrego Mountain Anticline (BMA) where the West Butte cgl. shows dramatic but poorly understood WNW-ward thickening along trend. These stratigraphic relationships define a progressive unconformity produced by tilting on the flanks of the BMA during deposition of both units and suggest that this fold was actively deforming the hanging wall of the West Salton detachment fault in late Miocene to early Pliocene time.