Sub-La Jolla Unconformity in Vicinity of San Diego, California

Abstract

Rock units of the San Diego area include two basic divisions separated by a high-relief unconformity: (1) an intensely deformed late Mesozoic basement complex and (2) an essentially flat-lying succession of latest Cretaceous and Cenozoic sedimentary rocks. The lower part of the latter division in the northern part of the San Diego area consists of several Upper Cretaceous rock units and the Eocene La Jolla Formation. Upper Cretaceous (Campanian-Maestrichtian) marine sandstone and shale of the Rosario formation crop out along the coastal area at Point Loma, La Jolla, and near Carlsbad. The Rosario is overlain unconformably by the La Jolla Formation. Another Upper Cretaceous unit crops out in the vicinity of Rancho Santa Fe east of the Rosario outcrop belt; in marked contrast to the Rosario, it is nonmarine and is dominantly a massive reddish boulder conglomerate. This unnamed unit unconformably overlies the basement complex and is separated from the La Jolla Formation by a high-relief (up to 450 ft) disconformity. The unnamed boulder conglomerate at Rancho Santa Fe apparently correlates with a sub-Rosario conglomeratic redbed unit reported from several wells in the San Diego area, and a conglomerate-sandstone unit beneath the Rosario near Carlsbad. If this correlation is correct, and if the contact between the Rosario and the unnamed unit is essentially planar, these relations indicate a post-Rosario uplift and slight westward tilting, an episode of erosion exposing both the Rosario and the sub-Rosario conglomerate, and subsequent La Jolla deposition on either of the Cretaceous formations, or on the basement complex, but without total destruction of the pre-La Jolla topography.