MIDDLE–LATE MIOCENE GROWTH OF A DEPRESSION IN THE CHOCOLATE MOUNTAINS ANTICLINORIUM, AS RECORDED BY THE BEAR CANYON CONGLOMERATE, SE CALIFORNIA

Between Indian Pass and Picacho State Recreation Area (PSRA), the EW trending Chocolate Mountains anticlinorium forms a structural depression bounded by two culminations. The depression in the axis of the anticlinorium occurs between PSRA and Carrizo Wash. To the east and west of the depression the 28-24 Ma Sortan fault, a package of ~23 Ma volcanics, and the Bear Canyon conglomerate are folded about the anticlinorium, which plunges westward and eastward into the depression.

Detailed clast analysis within the depression indicates derivation from the underlying volcanic sequence, as a result of the progressive unroofing of the anticlinorium. Moreover, the Bear Canyon conglomerate can be subdivided into three unconformably bound members. The lower member is composed of clasts of locally derived volcanics, and generally dips to the east along the western margin of the depression. The middle member contains clasts of the underlying volcanics and basement rocks, and occurs as NS oriented panels along both the E and W sides, dipping inward toward the axis of the depression. Within the structural depression, the middle member forms an extensive unit which records the growth of the anticlinorium as well as the progressive deepening of the depression. The upper member contains clasts of local volcanics, basement, and pre-existing polymictic conglomerate. Slightly tilted gravels of the upper member record the final phases of fold growth. To the west of the structural depression, interstratified within the Bear Canyon conglomerate are the 13.4-9.6 Ma basalts of Black Mountain, which dip ~26°S along the southern limb of the anticlinorium at Indian Pass. Additionally, along the southern margin of the depression, the upper member Bear Canyon conglomerate is truncated by the informally recognized EW trending Copper Basin reverse fault.

These data, in conjunction with previously published results, imply that in the region between Indian Pass and PSRA, ~NS shortening resulted in episodic growth and denudation of the Chocolate Mountains anticlinorium and the deposition of gravels into a deepening depression during the Early to Middle Miocene. Hence, the formation of these structures may be directly related to the early formation of the San Andreas fault system.