LATE CRETACEOUS-EOCENE LANDSCAPE EVOLUTION AND DRAINAGE REORGANIZATION ALONG THE SOUTHWEST EDGE OF THE CORDILLERA; INSIGHT FROM VOLCANIC CLASTS IN CONGLOMERATES OF THE CABRILLO FORMATION AND POWAY-LA JOLLA GROUPS, SAN DIEGO COUNTY, CALIFORNIA

The Late Cretaceous Cabrillo Formation records a major pulse of forearc sedimentation derived from rapid unroofing of the adjacent Peninsular Ranges batholith (PRB) and contains an abundance of silicic volcanic clasts. Unconformably overlying fluvial-deltaic strata of late Paleocene to latest-middle Eocene age also contain abundant conglomerate with silicic volcanic clasts, but include the distinctive “Poway-type” clasts that have been recognized by southern California geologists for over a century and have been matched to Jurassic bedrock sequences in Sonora, Mexico. Despite distinctions based on hand specimens, the Cretaceous and Eocene volcanic clasts (n = 53) are all similar in terms of whole rock major and trace element geochemistry determined by XRF. All but 5 of the analyzed rocks are high SiO₂ (69-76%), low Sr (~160 ppm), high Ba (~1100 ppm), high-K calc-alkaline (HKCA) rhyolites ; there is no clear basis for distinguishing the two groups of rocks on the basis of available whole rock geochemistry. However, zircon U-Pb ages for the Cretaceous vs. Eocene clast suites are clearly distinct. Ten Cabrillo Formation rhyolites, which include several welded tuffs, yield zircon ages that cluster tightly from ~97 to 104 Ma. Of the 14 dated Eocene clasts, 12 samples range narrowly from ~166 to 175 Ma overlapping Sonoran bedrock rhyolite ages of 168.8 ± 1.1 & 168.0 ± 2.6 Ma; the remaining two Eocene clasts yielded ages of ~142 and 150 Ma. The Cabrillo rhyolites may represent supracrustal volcanic cover of the eastern PRB (90-100 Ma) which have been completed eroded away, although HKCA chemical affinities and low Sr characteristic of the clasts is not a good match for the eastern PRB. The Eocene clasts are clearly extraregional and can be matched Sonoran bedrock as proposed by previous workers.