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Upper Jurassic Peñasquitos Formation—Forearc basin western wall rock of the Peninsular Ranges batholith

By
David L. Kimbrough
David L. Kimbrough
Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
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Patrick L. Abbott
Patrick L. Abbott
Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
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Duane C. Balch
Duane C. Balch
U.S. Army Corps of Engineers, 1325 J St., Sacramento, California 95814-2922, USA
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Sarah Hosken Bartling
Sarah Hosken Bartling
GPS River Rock Products Company, 1333 Kern St., Taft, California 93268, USA
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Marty Grove
Marty Grove
Department of Geological & Environmental Sciences, Stanford University, 397 Panama Mall, Stanford, California 94305-2210, USA
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J. Brian Mahoney
J. Brian Mahoney
Department of Geology, University of Wisconsin–Eau Claire, Eau Claire, Wisconsin 54702-4004, USA
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Robert F. Donohue
Robert F. Donohue
Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA
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Published:
January 01, 2014

Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206*Pb/238U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks.

The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils.

Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

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GSA Memoirs

Peninsular Ranges Batholith, Baja California and Southern California

Douglas M. Morton
Douglas M. Morton
U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, California 92521, USA
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Fred K. Miller
Fred K. Miller
U.S. Geological Survey, 904 West Riverside Ave., Spokane, Washington 99201, USA
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Geological Society of America
Volume
211
ISBN print:
9780813712116
Publication date:
January 01, 2014

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