Abstract The Upper Cretaceous strata of the Western Interior of North America contain a rich molluscan fauna and abundant intercalated bentonites. This combination has permitted the development of a detailed time scale for the Late Cretaceous. Volcanic rocks are rare in the Lower Cretaceous of North America. Several isolated time-scale points have been reported for Lower Cretaceous rocks, but the Early Cretaceous time scale is not well defined. Nearly all of the available isotopic ages are K-Ar mineral ages. Data have been recalculated using decay constants for K 40 based on the refined activity data for K 40 and the isotopic abundance of K 40 . The best current estimates for system and series boundaries are: Cretaceous-Tertiary boundary65 to 66 m.y.; Lower Cretaceous-Upper Cretaceous boundary96 m.y.; Jurassic-Cretaceous boundary138 ± 3 m.y.

Abstract In northern California, chaotic Franciscan melange occurs beneath the overlying ophiolite and Great Valley Sequence. Identical melanges occur to the west, separating well-bedded, coherent Franciscan units that differ markedly in age. Detailed studies in several places indicate that these melanges mark the boundaries of imbricate thrust sheets, and they appear to occur at several discrete structural horizons. The melange comprises blocks of graywacke, greenstone, chert, serpentinite, and isolated so-called knockers of high-grade blueschist and eclogite set in a matrix of sheared and quartz-veined mudstone and minor sandstone. Except for the blocks of high-grade schist, these rocks are similar to, but more deformed than, the orderly sedimentary, volcanic, and other rocks that occur immediately above the Coast Range thrust at the base of the Great Valley Sequence. Unlike the other Franciscan units, the melanges contain relatively abundant fossils, mainly Buchia, radiolarians, and dinoflagellates. Significantly, all of these fossils are of Tithonian to Valanginian age. We suggest, on the basis of similarity of lithology and fossil content, that the matrix of the melanges represents a distal, or seaward, portion of the basal sediments of the Great Valley Sequence and that the abundant greenstone, chert, and serpentinite found as tectonic blocks within the melanges were derived from the underlying oceanic crust and upper mantle. Formation of the melanges must be related to multiple subduction of separate plates, the melange beinggenerated repeatedly from the ultramafic-mafic-chert and Buchia-hearing shale and minor graywacke sequence that constitutes the oldest rocks of the Coast Ranges. This process of subduction probably began in the Early Cretaceous and continued into the Tertiary, as Eocene fossils have been found recently in deformed Franciscan (coastal belt) rocks structurally below the melanges. The tectonic blocks of high-grade blueschist and eclogite were formed during an earlier period of subduction, then embedded in serpentinite and carried westward by flow in the upper mantle. During subsequent subduction, the serpentinite and embedded blocks of schist were tectonically mixed with the overlying volcanic rocks, chert, graywacke, and fossiliferous shale.