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HARWOOD, DAVID S., U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Mesozoic Geology of Mt. Jura, Northern Sierra Nevada: A Progress Report

Mt. Jura is composed of shallow-water marine volcaniclastic rocks, minor limestone, and volcanic tuff breccia and flows that range in composition from rhyolite to andesite. The volcaniclastic rocks are fossiliferous, some richly so, and they range in age from late Sinemurian or early Pleinsbachian to Callovian and possibly into the Oxfordian. Diller's stratigraphic section is modified to place the Trail Formation at or near the top of the Jurassic sequence, following Crickmay and McMath, and to abandon the Hull Formation, which is actually a mixture of other units.

With few exceptions, rocks on Mt. Jura are overturned to the east. Map units are repeated in east-verging recumbent folds and imbricate thrust slices that rest on a structural basement of Paleozoic rocks, which in turn is thrust eastward over west-facing and dipping Jurassic rocks of the Kettle Rock sequence. The dominant west-over-east shortening deforms rocks as young as Callovian. Relatively minor northwest-over-southeast thrusting and folding is superimposed on the east-verging structures in Foreman Ravine and probably represents deformation related to movement of the Keddie Ridge block which was thrust over the northern end of Mt. Jura.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.