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Description
The Calabasas Formation in the Big Mountain area consists of massive sandstone, conglomerate, and pebbly sandstone with a maximum thickness of 70 m. A basal sandstone and conglomerate one-half to two meters thick rests unconformably on the early Miocene Vaqueros Formation. The remainder of the Calabasas has been informally divided here into three members. The lower member consists of massive fossiliferous sandstone containing abundant shell fragments. Scattered conglomeratic lenses are present through much of this member. The middle member consists of an unfossiliferous tan to white sandstone which contains many lutite clasts. This member as well as the upper member contain bioturbation structures and sections of pebbly sandstones. The upper member is a red-brown- to gray-colored fossiliferous sandstone. Pecten andersoni is extremely abundant in the upper portion of this member. The Calabasas Formation is Relizian (middle Miocene) in age. The depositional environment is interpreted as being a combination of proximal turbidite deposits and mass-gravity flow deposits that were laid down in outer neritic to upper bathyal water depths. The Modelo Formation in the Big Mountain area unconformably overlies the Calabasas Formation. It has been divided in this study into three members: a basal sandstone, a diatomaceous siltstone, and a siliceous shale. The basal sandstone member is a fine-grained yellow sandstone containing abundant foraminifera. The sandstone is poorly bedded and reaches a maximum thickness of 50 m. Foraminifera indicate that this member is Relizian in age. The contact between the basal sandstone member and the diatomaceous siltstone member is gradational. The diatomaceous siltstone member reaches a maximum thickness of 250 m with a major portion of the member being rhythmically bedded. This rhythmic bedding represents seasonal changes in the supply of material brought in for deposition of this member. Foraminifera, radiolarians, diatoms, sponge spicules, and fish remains are abundant. Foraminifera indicate an age of late Relizian to late Luisian. The upper portion of this member is devoid of foraminifera but radiolarians and diatoms indicate that this upper portion is Mohnian in age. The siliceous shale member is less than 10 m thick and is composed of siliceous shale, diatomaceous shale, chert, fine-grained sandstone, and calcite. Diatoms and radiolaria are abundant and indicate a Mohnian age. The depositional environment of the Modelo Formation is interpreted as having been deposited in an irregular deep basin containing shallower marine sills. Sedimentation was essentially by siliceous phytoplankton and zooplankton with minor amounts of terrigenous material. The basin was not far removed from land but microfossils indicate that deposition was at middle to lower bathyal depths. This was an area of high organic productivity and relatively low rates of terrigenous sedimentation.
