We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
The undifferentiated Sespe and Vaqueros Formations are exposed over a large part of the Santa Ana Mountains and San Joaquin Hills in southern California and provide an unbroken record of the Late Eocene (?) to early Miocene history of the area and its subsequent transition from marine to continental sedimentation. The principal rock types in the Sespe Formation are conglomerate, conglaneratic sandstone, arkosic sandstone, and minor siltstone. The arkoses were derived from highlands within an orogenic belt, reflecting a mixture of both plutonic and tectonic source areas. A distinct suite of clasts ("Sespe suite") occurs in the Sespe Formation which is indigenous to the northern Peninsular Ranges. These clasts give the most direct evidence for provenance, and many of the lithologies seem to be common to the Jurassic Sonoran Arc of northwestern Sonora, Mexico, and southwestern Arizona. Relative proportions of the different clast types through the measured sections facilitated a definition of the stratigraphy and allowed correlation between widely dispersed geographic areas. The Sespe Formation in the Santa Ana Mountains consists of a 1,600-foot section of non-marine red beds representing a variety of fluviatile conditions from braided channel to low sinuosity stream and alluvial plain environments. The formation has been divided into three distinct members based on lithology and sedimentologic characteristics of the rocks. The lower member of the Sespe Formation crops out in western Gypsum Canyon and at Santiago Creek, representing locally derived distal braided and sand-dominated low sinuosity river deposits which are conformable with underlying non-marine deposits of the Santiago Formation. This member contains a high proportion of granitic clasts but a relatively low percentage of exotic "Sespe suite" clasts. The middle member of the Sespe Formation is a thick, massive to crudely stratified conglomerate, represented as a proximal braided river channel deposit in eastern Gypsum Canyon and as correlative distal braided fluvial deposits at Santiago Creek and Santiago-Aliso Divide. This southward-flowing, perennial channel system cut and infilled older deposits in Gypsum Canyon and spilled out across the ancestoral Cristianitos Fault zone as an alluvial fan-delta system in the Santiago Creek area. Channeling may have been structurally controlled along a northern extension of the Cristianitos Fault Zone. Sespe sediments were shed into and infilled the graben, eventually reaching the paleotopographic elevation of the upthrown eastern block, where it represents an unconformable "basal conglomerate" on the Santiago and older formations of the Santiago-Aliso Divide area. This member is marked by a high percentage of a "Sespe suite" rhyolite porphyry (both "Poway"- and "Pseudo Poway"-type clasts), metavolcanic and quartzite clasts. The upper member of the Sespe Formation crops out in the Santiago-Aliso Divide section and is widespread along the southwestern margin of the Santa Ana Mountains. It is represented by gravelly alluvial plain and low sinuosity stream deposits along a sand-dominated coastal alluvial plain. These deposits suggest a waning of the perennial fluvial-deltaic system and progradation of non-marine fluvial deposits out onto a former marine basin. Upper portions of this member contain a Peninsular Range suite of clasts with no "Sespe suite" clasts. A transgressive sequence of elastic marine shoreline deposits characterize the final phases of deposition in the Santa Ana Mountains. Variegated sandstones and siltstones of the Vaqueros Formation overlapped and interfingered with continental red beds of the Sespe Formation in earliest Miocene time, marking the end of non-marine deposition in the area. The "Sespe suite" of clasts entered the borderland as a single pulse and should be recognized as a stratigraphic unit of limited temporal extent instead of as the thin distal end of the thicker Eocene-Miocene Sespe Formation. A major Oligocene drop in sea level may have facilitated the entrance of these clasts as a pulse into the continental borderland. "Sespe suite" clasts form a significant percentage of the clasts in the Oligocene Vaqueros Formation on southern Santa Cruz Island. The southern portion of the island lay off San Diego in the Eocene, where it received the "Poway" clasts contained in the Jolla Vieja Formation. If the southwesterly direction of transport of the "Sespe suite" of clasts in Orange County continued offshore, the clasts could have been deposited on the Poway fan without northward translation of the island block. More probably, Santa Cruz Island lay off the Orange County coast in the Oligocene where it received the "Sespe suite" of clasts. This position could have resulted from a constant northward rate of motion of southern Santa Cruz Island from the Eocene to the present.
