Abstract

In the central coastal region of California, conglomerate that is rich in chert and quartzite clasts occurs as channel deposits in thin-bedded turbidites of the lower part of the Great Valley sequence of Late Jurassic and Early Cretaceous age (Tithonian to Valanginian). Very similar conglomerate occurs in displaced blocks in melange of the Franciscan assemblage. The Franciscan conglomerate, however, is richer by about 8% in vein-quartz pebbles and requires a slightly different source area. Thick-bedded lithic sandstone in Franciscan melange is also petrographically similar to sandstone of the lower part of the Great Valley sequence, but it is less rich in lithic grains, and, likewise, requires a slightly different source area. The Franciscan conglomerate and sandstone compare most closely with the southernmost exposures of the lower part of the Great Valley sequence near Santa Barbara and may have had a more southerly source area. Younger Franciscan sedimentary rocks are less similar to rocks of the Great Valley sequence, possibly because of the influence of an enlarging subduction complex.

Correlative conglomerates of the Great Valley sequence of the Sacramento Valley are very rich in chert clasts, whereas those of the San Joaquin Valley contain more clasts of other sedimentary and volcanic rocks. A speculative model, similar to one proposed previously, suggests that the Great Valley forearc basin formed in an embayment with promontories north and south. The promontories contributed pebbles rich in chert that were transported longitudinally north and south into the basin, then westward. Less extensive, volcanic-rich conglomerates were derived from the east. Comparisons of clast compositions suggest that the chert-rich conglomerates of the Great Valley sequence of the central coast area were deposited about 300 km north of their present positions. Deposition was followed by 700 to 800 km of mid-Cretaceous sinistral strike slip on the Sur-Nacimiento fault and 400 to 500 km of Cenozoic dextral strike slip on the San Andreas fault.

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