A NEW OCCURRENCE OF THE BOUSE FORMATION IN JOSHUA TREE NATIONAL PARK

A new record of the latest Miocene or early Pliocene Bouse Formation is located in eastern Joshua Tree National Park (JOTR) near the Coxcomb Mountains. In outcrop, the rocks have similar lithologies, lithostratigraphic succession, and topographic position (~330 m ASL) as other known occurrences in the Blythe Basin, and are stratigraphically below a dated ~4.5 Ma olivine basalt. The basalt and the ~20 m section of putative Bouse Formation are enclosed by old fanglomerate deposits shed from the Coxcomb Mountains. The entire package has variable dips to the SW and is adjacent to the NW-trending Sheep Hole fault (SHF), the eastern boundary of the sinistral domain of the Eastern Transverse Ranges (ETR). The outcrop’s position at 330 m is significant, as this is the maximum elevation of other outcrops of Bouse Formation in the Blythe Basin: confirmation of the new occurrence extends the northwestern-most arm of the Mio-Pliocene Bouse water body. The JOTR Bouse exhibits basal fine-grained, laminated marl and carbonate cemented interbeds, transitioning to thick green mudstones, followed by brown oxidized mudstones and sandstones that exhibit large (1.6 m high) z-folds that are clearly seismites. After a sharp transition to oxidized muds, the section is capped by gypsum. Previous museum collections verified the presence of Chara sp. and smooth shelled ostracodes. Additional samples have been collected for strontium and microfaunal analyses. Given that the origin of the Bouse Formation in its southern reaches is hotly debated as to a lacustrine or estuarine setting, corroboration that the JOTR outcrop indeed is the Bouse Formation does not simply expand the Bouse footprint: rather, it informs a larger discussion of the evolution of the lower Colorado River and landscape response to the San Andreas fault system and opening of the Gulf of California. For e.g., geophysical data indicate that the SHF has significant vertical displacement and that deep basins exist at the eastern end of the sinistral faults in the ETR (Blue Cut and Pinto Mountain faults), specifically where they intersect with SHF. Thus, the Bouse water body of may have extended farther west than the outcrop reported here, possibly reaching well beyond the eastern end of Pinto Basin in JOTR.