Abstract

Between Indian Pass and Picacho State Recreation Area, southeastern California, the Neogene Bear Canyon conglomerate crops out in and around the Chocolate Mountains anticlinorium, and can be subdivided into three unconformity-bounded sequences that are designated from oldest to youngest I, II, and III. Each of these sequences is composed of interstratified debris flow, channelized-stream-flow, and sheet flood deposits. Debris flow deposits are poorly sorted inversely graded pebble to boulder paraconglomerates with subangular clast populations. Large clasts extruding into overlying layers are a characteristic of such beds. In contrast, sheet flood and channelized-stream-flow deposits are well sorted orthoconglomerates with subrounded to rounded clasts.

The Bear Canyon conglomerate unconformably overlies a lower Miocene volcanic and epiclastic unit, metasedimentary and metavolcanic rocks of the Jurassic Winterhaven Formation, a probable Jurassic orthogneiss, and the Upper Cretaceous to Paleogene Orocopia Schist. Geochemical, paleocurrent, and clast-count data suggest that alluvial material in the Bear Canyon conglomerate was derived from local exposures underlain by one or more of these units, and then was funneled into an approximately north–south-trending structural depression in the axial trace of the Chocolate Mountains anticlinorium.

Though the exact timing of deposition of the three sequences in the Bear Canyon conglomerate is difficult to constrain, stratigraphic and published geochronological data, when viewed in the context of regional tectonic patterns, suggest that sequence I probably was deposited after ~ 17 Ma and before ~ 9.45 Ma. In contrast, sequence II was deposited after sequence I and then was folded either sometime prior to or after ~ 9.45 Ma. Sequence III is interstratified with the ~ 9.45 Ma basalts of Black Mountain, and was folded prior to deposition of flat-lying Quaternary alluvial sediments.

The east–west-trending Copper basin reverse fault cuts sequence III. In addition, key components of a conjugate set of northwest- and northeast-trending dextral- and sinistral-slip faults, respectively, transect the Copper basin fault, the axis of the Chocolate Mountains anticlinorium, and the ~ 9.45 Ma basalts of Black Mountain. However, neither the Copper basin nor the conjugate faults disrupt flat-lying Quaternary alluvial sediments.

Episodic growth of the east–west-trending Chocolate Mountains anticlinorium during deposition of the Neogene Bear Canyon conglomerate, and subsequent formation of the east–west-trending Copper basin fault and the conjugate set of dextral and sinistral faults all record transient north–south shortening within the margin of the North American plate. These transient periods of north–south shortening followed extensional deformation of lower Miocene continental-arc volcanic rocks, and are strain compatible with the ~ 12–10 Ma to ~ 6 Ma development of the eastern California shear zone. Numerous other examples of Late Cenozoic east–west-trending folds and reverse faults are reported in the literature to have formed during development of this 100–110 km wide zone of dextral shear. We, therefore, conclude that the geology and development of the Chocolate Mountains anticlinorium between Indian Pass and Picacho State Recreation Area reflect changing continental-margin strain patterns as the boundary between the Farallon and North American plates was transformed from a Paleogene convergent margin to a wholly Neogene transform setting.