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Masters Thesis

Stratigraphy and depositional environments of the lower miocene vaqueros formation Santa Ana Mountains, California

The lower Miocene Vaqueros Formation is exposed along the western flank of an arc extending from the northeast to the southeast corners of the northern and central Santa Ana Mountains, California. Eight stratigraphic sections were measured and their lithologies described in detail, which resulted in the recognition of 18 lithosomes. The lithosomes consist of structureless, bioturbated, well-sorted, cross-bedded, flaser-laminated, parallel-laminated, and fossiliferous sandstone; structureless, parallel-laminated, lenticular laminated, and variegated siltstone; fossiliferous siltstone; pebble to cobble conglomerate; and fossiliferous, structureless, and nodular limestone. The dominant fossil types include Arca santana, Turitella inezana, Kewia fairbanksi, and Terebra santana. Measured stratigraphic sections indicate that the formation is approximately 200 m thick in the north and approximately 40 m thick in the south. Provenance data indicate a dominant granitoid source area with a less-dominant reworked sedimentary source. The granitoid source terrain is interpreted to have been from the northern Penninsular Ranges, which indicates a westerly paleocurrent direction. The environments of deposition of the Vaqueros Formation include delta-front, interdistributary-bay, distributary-mouth-bar, crevasse-splay, distributarychannel, and overbank and marsh deposits of a riverdominated deltaic system; lower shoreface through backshore deposits of a wave-dominated deltaic system; interdistributary-bay deposits of a tide-dominated deltaic system. Vaqueros deposition in the Santa Ana Mountains began with a rise in sea level. Fluctuating sedimentation rates and sea level in the study area resulted in small transgressions and regressions at the onset of Vaqueros deposition. Thus, the lowermost deposits consist of interbedded marine Vaqueros and non-marine Sespe Formation deposits. The sea's farthest transgression is marked by delta-front deposits in the river-dominated delta system and lower shoreface deposits in the wave-dominated delta system. Toward the end of Vaqueros deposition, sea level fell, which resulted in delta-plain deposits of the river-dominated delta system and backshore deposits of the wave-dominated delta system at its farthest regression.

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