INTRODUCTION

The Red Bluff 30'x 60' quadrangle includes rocks of four major geologic provinces: the Klamath Mountains, the Coast Ranges, the Great Valley, and the Cascade Range (see fig. 1 on map sheet). These rocks and their geologic histories differ greatly from one another. The Klamath Mountains are mainly an accumulation of large tectonic slices of oceanic crust, upper mantle, and volcanic island arcs that range in age from early Paleozoic to Late Jurassic. The rocks of the Coast Ranges present in the Red Bluff quadrangle consist of oceanic crustal rocks that are somewhat similar lithologically to those of the Klamath Mountains but are Early Jurassic to Cretaceous in age. The Great Valley province includes a thick deposit of moderately deformed Jurassic and Cretaceous marine strata that consist of detrital materials derived from uplifted basement rocks of the Klamath Mountains and the Sierra Nevada. In the western foothills of the Sacramento Valley, these strata rest partly on the Coast Range ophiolite and partly on the Klamath Mountains basement rocks and thus pose a complex tectonic question. In the central part of the Sacramento Valley, a mantle of Tertiary and Quaternary detrital continental deposits overlies the Great Valley sequence; these deposits, which are derived from the west, grade eastward into coeval volcanic materials derived from the Cascade Range province. The stratigraphic and structural relations shown on the map provide a key to understanding how the rocks of these various provinces were formed and were subsequently assembled. For the compilation of this geologic map, Irwin is responsible for the rocks of the Klamath Mountains province; Blake, Jayko, and Jones, the pre-Cenozoic rocks of the Coast Ranges province; Helley, the late Cenozoic deposits of the Sacramento Valley; and Harwood, the Cenozoic rocks of the Cascade Range province. Previously published geologic mapping used in this compilation is summarized in figure 2 (on map sheet).