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Description
The late Oligocene to middle Miocene Bear Canyon fanglomerate is a widespread unit in southeasternmost California exposed on the margins of the Chocolate Mountains and adjacent mountain ranges. The tilted crustal blocks that compose this region formed the source areas for the deposit, which forms a complex network of semi-arid alluvial fans shed largely off the Chocolate Mountains, but also from the Peter Kane and Trigo Mountains. The Bear Canyon fanglomerate records both the cessation of regional detachment faulting and the initiation of San Andreas-related opening of the Gulf of California and Salton Trough. The locally-derived heterolithic fanglomerate and interbedded 13 ± 2 Ma olivine basalts were deposited in low-relief regions formed by intense shattering and basin formation related to regional mid-Tertiary detachment faulting. While older formations in the study area are moderately to intensely deformed by mid-Tertiary and older events, the Bear Canyon fanglomerate is largely flat-lying. North-south trending faults in the fanglomerate in parts of the study area seen both in outcrop and in seismic lines are extension fractures related to motion on the San Andreas fault. Slickensided zeolites on these fault surfaces indicate predominantly dip-slip rather than strike-slip displacement. The absence of faults in younger rocks is evidence that these faults have been inactive since the early Pliocene and is consistent with the findings of other studies which show that motion on this portion of the San Andreas fault has been transferred to other segments of the San Andreas system. The Bear Canyon fanglomerate can be divided into an upper member composed largely of Tertiary volcaniclastics and a lower member composed largely of Mesozoic plutonic and metamorphic rocks. The lower member was deposited in the late Oligocene, prior to the deposition of large quantities of rhyolitic to andesitic pyroclastics in the region. The Bear Canyon fanglomerate is not considered to contain economic quantities of gold. Gold-bearing rocks underlying the fanglomerate are so friable that any gold eroded from them and transported in the fanglomerate is likely to be extremely finely disseminated and contained in downfan portions of the deposit which are much finer-grained than those in the study area.
