PROBABLE UNROOFING OF THE CENTRAL BELT OF THE FRANCISCAN COMPLEX RECORDED BY NEWLY RECOGNIZED UPPER CRETACEOUS TO LOWER TERTIARY(?) FOREARC DEPOSITS WEST OF UKIAH BASIN, CALIFORNIA

West of the Ukiah basin and the Maacama fault in northern California, new geologic mapping focuses on the tectonostratigraphy and accretionary history of the Cretaceous and Tertiary Central and Coastal belts of the Franciscan Complex. The mapping has revealed previously unrecognized, distinctive Upper Cretaceous and younger non-Franciscan marine rocks that apparently overlie and are partly in fault contact with the Central belt. The lower part of this stratal sequence consists of relatively intact to somewhat broken turbiditic graywacke, argillite and conglomerate. These strata contain minor veins of laumontite, 3-7 percent detrital K-feldspar, conspicuous minor detrital biotite, and exhibit abundant sedimentary structures. Late Cretaceous (Campanian to Santonian) fossils (Inoceramus schmidti) are present at one locality. We posit that these Upper Cretaceous rocks might represent Great Valley forearc strata faulted above the Franciscan Complex, but note that no remnants of the Jurassic Coast Range ophiolite intervene anywhere between them and the underlying Central belt. Alternatively, the fossils and lithologic character of the turbidite unit suggest correlation of these rocks with the lithologically similar Novato Quarry terrane of the Central belt (also containing Inoceramus schmidti), mapped ~ 75 km to the south.

A massive, cavernous-weathering marine arkose unit unconformably overlies the Cretaceous rocks. The arkose contains 15-30 percent detrital K-feldspar, minor detrital biotite, and local minor veins of laumontite but lacks neoblastic minerals. No age-definitive fossils have yet been found in the arkosic sandstone, but a ~2 m thick interbed of siliceous mudstone (altered tuff or diatomite) contains detrital zircons that in aggregate indicates an 86 Ma maximum depositional age (MDA), likely reflecting youngest pluton source ages in the Sierra Nevada arc. More zircon dating is underway in search of a possibly younger MDA. Tentatively, this sandstone and the underlying Cretaceous strata are interpreted to record deposition along the Late Cretaceous to Paleocene margin, in the upper reaches of a submarine fan or canyon cut into the Central belt during or following its unroofing.