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The Vaqueros Formation is a sedimentary geologic unit primarily of Upper Oligocene and Lower Miocene age, which is widespread on the California coast and coastal ranges in approximately the southern half of the state. It is predominantly a medium-grained sandstone unit, deposited in a shallow marine environment. Because of its high porosity and nearness to petroleum source rocks, in many places it is an oil-bearing unit, wherever it has been configured into structural or stratigraphic traps by folding and faulting. Being resistant to erosion, it forms dramatic outcrops in the coastal mountains. Its color ranges from grayish-green to light gray when freshly broken, and it weathers to a light brown or buff color.〔Dibblee (1966) 40-42〕 ==Type locality and deposition environment== The type locality of the Vaqueros is from Vaqueros Canyon in the Santa Lucia Mountains, about eight miles southwest of Greenfield. The formation was first described by Homer Hamlin in 1904, as part of a report on the water resources of the Salinas Valley.〔Dibblee (1966) 40〕 The sandstone unit consists of well-sorted grains, averaging medium-size, typically quartz and feldspar with some black flecks, and in form it ranges from cross-bedded to massive and thick-bedded. Occasionally it contains pebbles, especially near its base where it sits on the red non-marine Sespe Formation. Some fossils – including mollusks and barnacles – can be found in the Vaqueros, also near the base of the unit where the depositional environment was nearest shore.〔Dibblee (1966) 42〕〔Minor et al., map legend〕 The unit was deposited by runoff from highlands to the east into a shallow, warm marine environment, as the ocean transgressed on the subsiding floodplain containing the Sespe in the late Oligocene age, between 26 to 28 Ma (million years before present) to 24 to 25 Ma.〔Gregory A. Miles and Catherine A. Rigsby. "Lithostratigraphy and Depositional Environments of the Vaqueros and Upper Sespe/Alegria Formations, Hondo Field, Santa Barbara Channel, California." SEPM Core Workshop No. 14. San Francisco, June 3, 1990. ISBN 0-918985-84-6 p. 46〕 As the land continued to subside, the ocean depth increased with a corresponding drop in grain size in higher strata. The topmost part of the Vaqueros contains interbedded mudstones, silstones, and fine-grained sandstones, representing this shift.〔 The unit above the Vaqueros, the Rincon Formation, consists of deepwater shales.〔 The Vaqueros weathers to a clayey soil which supports chaparral, and on the southern slopes of the Santa Ynez Mountains in southern Santa Barbara County, its contact with the Rincon Formation is easily visible for it correlates closely to the line where the grassland or coastal sage scrub, nearer the coast, abruptly changes to dense chaparral on the mountainside.〔(Hollister Ranch: Environmental Setting )〕〔Dibblee (1966) 41〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vaqueros Formation」の詳細全文を読む スポンサード リンク
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