Stratigraphy and Structure of Mountains Northeast of Santa Barbara, California

Abstract

The district discussed in this paper includes parts of the Santa Ynez and San Rafael ranges near the coast of Southern California. The oldest rocks belong to the Franciscan group (Jurassic?) and consist chiefly of structurally disorganized dark shale with isolated bodies of sandstone, contorted chert, and other types of rock. The stratigraphic section is remarkable for its great thickness (more than 28,000 feet) of Cretaceous and Eocene sandstone and shale. These sediments, which are marine but only scantily fossiliferous, greatly predominate over the other rocks in areal extent. In this paper the Cretaceous strata are divided into three formations: the Undifferentiated Cretaceous shale, the Debris Dam sandstone, and the Pendola shale, the two latter formations being newly named. Lower Eocene rocks are lacking. There is an important angular unconformity at the base of the middle Eocene section. The well known Sierra Blanca limestone of middle Eocene age locally comprises the basal unit of the Eocene rocks. It is overlain by the thick Juncal formation (middle Eocene shale and sandstone) which is here defined for the first time. Above this are the Matilija sandstone, Cozy Dell shale, and Coldwater sandstone,--all upper Eocene in age, and all previously described by others. The Oligocene is represented by the non-marine Sespe formation, and the Miocene rocks include Temblor sandstone and Monterey shale. At the base of the Miocene section, there is a striking angular unconformity. The Santa Ynez fault, which extends east and west across the area (and beyond), is the most important structural feature. It is a steeply dipping fault of Quaternary age, probably with both a reverse and a strike-slip component of displacement. North of this fault the rocks are folded along northwest-southeast axes in accordance with the general pattern of California Coast Range structure. South of the fault, however, the formations in general strike east-west in harmony with the Transverse Ranges, of which the Santa Ynez Mountains are a member. The rocks of the Santa Ynez Range in this area comprise a locally overturned limb of a large syncline whose trough lies south of the mountains. On the north the limb may have once been shared by an anticline, but the Santa Ynez fault now occupies the supposed axial region. Near the fault the Cretaceous and Eocene beds are interrupted and truncated by a body of Franciscan rocks which, although mainly sedimentary, have apparently been forced upward in the manner of an intrusive mass. The writers describe other structural features and give a chronology of tectonic events. End_Page 1727------------------------------