Abstract

The Paleogene tectonic setting of California remains uncertain even though the general nature of earlier and later events is now understood. Cretaceous subduction (Hamilton, 1969) gave rise to a marginal arc-trench system whose over-all geometry is well defined (Dickinson, 1970; Ingersoll, 1978c, 1979). The over-all kinematics of Neogene motion along the coastal San Andreas transform (Atwater, 1970) are also well known (Dickinson and others, 1972; Atwater and Molnar, 1973; Graham, 1978). In this paper, we use data on sediment dispersal to define the depositional framework and tectonic setting of northern California during the intervening Paleogene.

Northern California is an especially attractive region for the study of Paleogene rocks near the edge of the continent because it spans the present tectonic transition from subduction to transform slip along the continental margin (Fig. 1). Farther north, the contiued evolution of an arc-trench system in the Pacific North-west during Neogene subduction has partly obscured Paleogene relations. Large areas are covered by young volcanic rocks of the Cascades (McBirney and others, 1974), and intense deformation continues near the active subduction zone at the foot of the continental slope (Kulm the Fowler, 1974). Farther south, Neogene displacement of structural slivers along multiple strands of the San Andreas transform fault system has broken the continuity of Paleogene relations across central and southern California. In Particular, an internally deformed slice of the continent including Baja. California and the Salinian block has been transferred to the Pacific plate and offset obliquely with respect to the continental margin (Carfunkel, 1973). Although many Paleogene exposures in northern California are restricted and isolated, the rocks have not been translated long distances by Neogene tectonic movements. Neither have they been widely hidden beneath younger strata except within the Great Valley, where they have been penetrated extensively by drilling. Reconstructions of regional dispersal paths for Paleogene sedimentation thus can be documented for northern California with less ambiguity than for regions to the north and south.

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