POST-9.45 MA DEPOSITION AND STRUCTURAL HISTORY OF THE BEAR CANYON CONGLOMERATE, SOUTHEASTERN CALIFORNIA, USA

The region between Indian Pass and Picacho State Recreation Area (PSRA) in southeastern California records complex deformation related to the transition from a Paleogene convergent margin to a Neogene transform setting, although the timing and nature of youngest deformation in this region remains incompletely known. We focus on a 1.7 km-thick section of Miocene Bear Canyon conglomerate (BCC) in order to constrain this young period of deformation. These rocks lie unconformably upon 23 Ma volcanic and epiclastic rocks and a Mesozoic metamorphic basement terrane. Near PSRA the Bear Canyon conglomerate contains locally-derived clasts derived from the underlying units and can be further subdivided into three unconformity-bounded sequences, referred to as sequences I, II, and III from oldest to youngest. At Indian Pass, the 9.45 ± 0.27 Ma basalt of Black Mountain is interstratified with the upper part of the BCC, but it is unknown how these relationships extend to PSRA where detailed mapping has established the stratigraphic architecture of this unit.

The BCC is cut by numerous relatively small-scale normal and strike-slip faults and preserves a set of EW-trending gentle to open folds. The most prominent structure to cut the BCC is an EW-trending, south-dipping reverse fault known as the Copper Basin fault, which places a Mesozoic orthogneiss on top of sequence III of the BCC. To build upon previous results, we conducted a sedimentological analysis to determine provenance and paleoflow directions of the BCC. A primary focus of our research is continued mapping of this unit in an effort to more tightly document its relationship to the basalt of Black Mountain. Regional relationships suggest that a link between the Gulf of California and the Eastern California Shear Zone was active during the late Miocene and early Pliocene, and this deformation may therefore be recorded in the BCC. These observations suggest that the entire BCC is an important record of deformation related to continued development of the San Andreas transform margin.