Radiolarians and conodonts from pebbles in the Franciscan assemblage and the Great Valley sequence of the California Coast Ranges

Abstract

Radiolarians from pebbles in chert-pebble conglomerates that occur in blocks in the Franciscan assemblage are similar to the radiolarians in lithologically similar conglomerate of the Toro Formation and correlative strata of the Great Valley sequence. The blocks may have been derived from the Toro or from a stratigraphic unit with the same source as the Toro. Most radiolarians are Middle and Late Triassic and in part may have been eroded from the Calaveras Formation of the Sierra Nevada. Radiolarians in Upper Cretaceous Franciscan conglomerates are Late Jurassic, which suggests that they were reworked from the Franciscan itself or from chert overlying the Coast Range ophiolite. Triassic conodonts found in Upper Jurassic and Lower Cretaceous conglomerates have low color-alteration indices indicative of low temperature, whereas those found in an Upper Cretaceous conglomerate have high indices indicative of biotite-zone metamorphism. These data thereby bracket the time of metamorphism of the source area. The color index of conodonts in a Franciscan melange suggests a maximum temperature of about 60 °C.