ANALYSIS OF THE ANZA AND ALVERSON FORMATIONS, COYOTE MOUNTAINS, SOUTHERN CALIFORNIA: IMPLICATION FOR MIOCENE SEDIMENTATION

The objective of this geologic study was to perform an analysis of the Miocene stratigraphy in the Coyote Mountains and to address the relative age relations between the sedimentary and volcanic units which occur in the study area. The basement consists of Cretaceous age metamorphic and intrusive rocks which are unconformably overlain by the fluvially deposited sandstone and conglomerate of the Anza Formation, and the basalt flows and related volcaniclastic deposits of the Alverson Formation. In detail, the focus was the analysis of the nature of the relation between the fluvial Anza Formation of unknown age and the volcanic and volcaniclastic Alverson Formation of middle Miocene age. Previous geologic work interpreted this contact as a conformable contact with the upper sediments of the Anza Formation intertongueing with the volcanics of the Alverson Formation. An age of lower to middle Miocene was assigned to the Anza Formation based on this intertongueing relationship, as the Alverson Formation had been radiometrically dated as being middle Miocene. New observations suggest an unconformity exists between the two units. These new observations include thickness variations in both the Anza and Alverson formations, the potential of an erosional surface at the base of the Alverson Formation, sedimentologic and petrologic differences between the Anza and Alverson formations and studies of the sediments (previously called Anza sediments) interbedded in the basalts at the base of the Alverson Formation. Together, these new data suggest that the two formations are unrelated, do not intertongue with one another, and are separated by a significant unconformity.