THE BEAR CANYON CONGLOMERATE AND THE TIMING OF FORMATION OF THE CHOCOLATE MOUNTAINS ANTICLINORIUM, SE CALIFORNIA

The Chocolate Mts. anticlinorium has been interpreted to be an ~50-60 Ma structure related to the Laramide orogeny or a post-20 Ma Miocene fold of unknown tectonic affinity. It extends from the San Andreas fault eastward for over ~150 km into Arizona. At Picacho State Recreation Area, SE California, the anticlinorium forms an upright W-trending fold plunging to the west. Folded about the anticlinorium are the Quechan volcanics (~33-27 Ma), trachyte of Marcus Wash, trachyte of Rojo Grande, pyroclastic material, trachyte of White Wash, Walker andesite, and ignimbrite of Ferguson Wash (IFW) (~25 - ~27 Ma). Analysis of 73 samples indicates that each of these Oligocene units has distinctive trace element chemistry. On the western limb of the Chocolate Mts. anticlinorium, the Bear Canyon conglomerate unconformably overlies the IFW, and is subdivided into three unconformably bounded members. Member 1, the lowest unit, is tilted 21^o NW. An embayment formed by the contact between member 1 and the IFW represents a feeder canyon branching SE toward the trace of the Chocolate Mts. anticlinorium. The greatest proportions of clasts in the lower part of member 1 are texturally and chemically similar to the IFW. However, the clast population in the middle and upper parts of member 1 contains progressively greater proportions of the Quechan volcanics, along with clasts derived from the Marcus Wash trachyte and Walker andesite. Sandstone samples collected from the middle and upper parts of member 1 on a plot of TiO₂/Zr versus Nb/Y spread between the compositional fields defined by the Quechan volcanics, Marcus Wash trachyte, Walker andesite, and IFW. Though member 2 has not yet been studied in detail it dips ~15^o NW, and is dominated by conglomerates with dioritic and mylonitic dioritic clasts along with fragments of the Orocopia Schist. Member 3 is flat lying and polymictic. It contains clasts derived from both Mesozoic rocks and Tertiary volcanics. At Black Mountain member 3 is interstratified with basalts that have reported K-Ar ages of 9.6 and 13.4 Ma. In short, new data indicate that the Chocolate Mts. anticlinorium formed sometime between 25 and 9.6-13.4 Ma, and was likely growing during deposition of members 1 and 2. Hence, it can not be a Laramide structure and instead is likely related to the early evolution of the Easter California Shear Zone.