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The Oligocene Sespe and Miocene Vaqueros Formations on Santa Rosa Island represent a transgressive sequence composed of locally derived sediments deposited in environments including alluvial, braided stream, nearshore marine, and shallow shelf.

The Sespe Formation consists of up to 150 m of red to brown, poorly sorted, fine to medium lithic arkose interbedded with mudstone and conglomerate, deposited in a nearshore alluvial or braided stream environment. The Sespe shows a transition to marine deposition at the eastern end of its outcrop.

The Vaqueros Formation is made up of light brown to yellow-green, fossiliferous, fine to medium lithic arkose to litharenite, interbedded with mudstone and rare pebble conglomerate. The lower Vaqueros was deposited in a nearshore environment, while the upper facies was deposited on a shallow shelf.

Paleocurrent directions are consistently to presentday north for both units.

Sespe and Vaqueros lithologies are generally sourced from magmatic arc rocks of the Southern California Batholith and may have been recycled through an uplift of Eocene sedimentary rocks located immediately to the present-day south of Santa Rosa Island.

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