Abstract

Lower Cretaceous through Paleocene strata are exposed in a small outlier within the Eastern Belt of the Franciscan Complex (Berkland and others, 1972) near Rice Valley in the northern Coast Range of California. This outlier, and three others known in the region, are synclinally folded, downfaulted remnants of an upper thrust plate of Great Valley sequence rocks.

The Rice Valley synclinal remnant is separated by faults from surrounding rocks of the Franciscan Complex. Two belts of serpentinized dunite and harzburgite, converging toward the south, border the Rice Valley sequence on the east, west, and south. The northern border is a reverse fault that brings blueschist facies metaclastic rocks of the Eastern Belt over unmetamorphosed, mildly deformed strata of the Rice Valley sequence.

Basal beds of the sequence are Lower Cretaceous (Hauterivian) sandstone, shale, and limestone that are steeply tilted and faulted against the ultramafic rocks. About 3,500 ft of Cretaceous marine strata are unconformably overlain by more than 1,150 ft of shallow-marine lower Tertiary beds. Fossils are generally scarce throughout the sequence, but are abundant in limestone-rich horizons of Hauterivian and Cenomanian age as well as in a calcareous sandstone bed with more than 30 genera of late Paleocene (Meganos Stage) invertebrates.

Special significance is attached to a 150-ft-thick bed of polished-pebble conglomerate which underlies the Paleocene fossils. The conglomerate is composed chiefly of Franciscan detritus, with pebbles of red radiolarian chert, lawsonitic metagraywacke, and serpentinite. This is the earliest known appearance of coarse Franciscan debris in an upper plate (Great Valley sequence) environment. Time restrictions on underthrusting and isostatic rebound of deeply subducted Franciscan rocks imply rapid rates of uplift and denudation during early Paleocene time. The suggested rate of 240 cm per 1,000 yrs is comparable with the highest known rates in the world today.

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