Geology of northwestern Baja California, Field Trip no. 5

Abstract

This trip will offer an opportunity to examine the coastal segment of the Peninsular Range Province in an area of Baja California where the stratigraphic and tectonic history of the province is far clearer than in the more highly deformed areas north of the international border. Three major rock units characterize this province: (1) pre-batholithic eugeosynclinal accumulations of volcanic and sedimentary rocks, in part metamorphosed; (2) plutonic rocks of the batholith of southern and Baja California (the Peninsular batholith); and (3) Post-batholithic sedimentary and volcanic rocks of Upper Cretaceous and Cenozoic age. In general, the high "backbone" of Baja California is underlain by massive batholithic rocks, whereas the foothills between the high ranges and the Pacific coast are underlain by pre-batholithic volcanic, sedimentary and metamorphic rocks. The pre-batholithic rocks have been intruded by plutons that decrease in size and abundance toward the west. Relatively unmetamorphosed prebatholithic rocks and batholithic rocks are truncated and overlain by Upper Cretaceous and Cenozoic rocks in a thin strip along the Pacific coast. Inasmuch as this trip is limited primarily to the coastal area, the batholithic rocks will be examined only in some of these smaller plutons, but the pre- and post-batholithic rocks exposed in this coastal strip are among the most revealing that are present anywhere within the Peninsular Range province.