NEW GEOLOGIC MAP OF PALEOZOIC ROCKS IN THE MOUNT MORRISON PENDANT, EASTERN SIERRA NEVADA, CALIFORNIA

We have completed a new geologic map of Paleozoic (Pz) rocks in the Mount Morrison pendant, to be published at a scale of 1:24,000 as California Division of Mines and Geology Map Sheet 53 (Greene and Stevens, 2002).

In the Mount Morrison pendant, locally highly deformed, predominantly Cambrian through Devonian metasedimentary rocks are juxtaposed against less deformed Mississippian through Permian metasedimentary rocks across the Triassic Laurel-Convict fault. East of the Laurel-Convict fault, lower Pz rocks are generally north-striking and overturned, dipping steeply to the east. The section is imbricated by north-striking overturned thrust faults and deformed by vertically plunging, map-scale folds. West of the Laurel-Convict fault upper Pz rocks are generally west-dipping, and characterized by outcrop- and map-scale open folds with horizontal, northwest-trending hinge lines and little or no significant faulting.

The Laurel-Convict fault is northwest-striking and very steeply dipping. The fault cuts bedding in the upper Paleozoic rocks at a low angle, and truncates folds and faults in the lower Paleozoic rocks. At least 5 km of left-lateral strike-slip displacement occurred after deposition of middle(?) Permian rocks and before intrusion of a felsite dike dated at 225 +/- 16 Ma.

The lower Pz section has been thrust over rocks on the east edge of the pendant, formerly mapped as Cambrian Hilton Creek Marble, that we identify as Pennsylvanian Mount Baldwin Marble. Orogenic activity that produced imbrication of the lower Paleozoic section and folding of the upper Paleozoic section is constrained to the period between late Early Permian (the age of the youngest rocks affected) and Late Triassic time (the age of the cross-cutting felsite dike). We have referred to this event as the Morrison orogeny.