Over 1500 m thickness of tertiary continental and marine sedimentary and volcanic rocks crop out on Santa Rosa Island. The Oligocene Sespe and Miocene Vaqueros Formations were studied to determine depositional environment and provenance. 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. Sedimentary features suggest an alluvial environment for all but the easternmost rocks which are shallow marine. The Vaqueros Formation is a buff to yellow-green, fossiliferous, fine to medium sandstone interbedded with fossiliferous mudstone and rare pebble conglomerate. Sedimentary features such as foreset crossbedding and climbing ripples suggest a nearshore environment for the deposition of the lower Vaqueros Formation, and sheet sands interbedded with fossiliferous mudstone suggest shallow shelf deposition for the upper Vaqueros Formation. Paleocurrent directions are consistently to the north throughout Sespe and Vaqueros time. Sespe and Vaqueros conglomerates both contain clasts apparently derived from Eocene rocks similar to those present on the island. The source is postulated to be an uplift of Eocene sedimentary rocks and metavolcanics of indeterminate age immediately to the south. The sequence of rock studied shows a transgression onto this uplift during the late Oligocene to early Miocene time.
