Description
Sedimentary rocks of Late Cretaceous through Pleistocene age are exposed as a southwest dipping homocline along the western flanks of the northern Peninsular Ranges Province of southern California. Within Camp Pendleton, the sedimentary sequence overlies a crystalline basement complex of Late Cretaceous plutonic rocks comprised primarily of tonalite, and lesser amounts of older metasedimentary and metavolcanic units. This basement complex served as the source terrane for all of the pre-Miocene sedimentary units. The Lusardi Formation of Late Cretaceous age is comprised of cobble to boulder conglomerate that was deposited as high gradient alluvial fans on the exposed, high relief, crystalline basement complex. The Lusardi Formation occurs only as scattered erosional remnants. Lateritic Paleosols of early Tertiary age are locally preserved within Camp Pendleton, and represent soil profiles developed by severe leaching during an early Cenozoic tropical climate. These paleosols are developed directly on the exposed basement complex. Middle Eocene arkosic sandstone, sub-arkosic siltstone, and sandy mudstone of the Santiago Formation dominate the sedimentary sequence exposed in southern Camp Pendleton with nearly 1,800 meters of stratigraphic section. The Santiago Formation is a fining upward sequence divided into three lithofacies: the Crossbedded Sandstone Lithofacies, the Pebbly Sandstone Lithofacies, and the Siltstone Lithofacies. These lithofacies represent a transgressive succession of facies in response to a middle Eocene eustatic sealevel rise. The Crossbedded Sandstone Lithofacies is the lowest unit of the Santiago Formation and consists of 700 meters of fine-to coarse-grained crossbedded arkose, and interbedded thin mudstone that were deposited in paralic environments including tidal flats, tidal channels, and salt marshes. The Pebbly Sandstone Lithofacies overlies the Crossbedded Sandstone Lithofacies and is comprised of 580 meters of massive, slightly gravelly, coarse-grained arkose interbedded with massive sandy mudstone. This unit represents deposition of shoaling sand and protected bay facies. The Siltstone Lithofacies is the uppermost unit of the Santiago Formation and consists of 520 meters of siltstone, mudstone, and minor fine-grained arkose that was deposited in marine shelf environments. Lower Miocene rocks of the San Onofre Breccia consist of lithic sandstone and pebble breccia shed from a now submerged western source terrane of blueschist and greenschist grade metamorphic rock. Pleistocene marine terrace deposits locally overlie both the Eocene and Miocene units, and attest to continued regional uplift.
