Masters Thesis

Stratigraphic analysis of the Middle Miocene Modelo formation at Lower Piru Creek, Ventura County, California

Six hundred and eighty meters of the Modele Formation in the vicinity of lower Piru Creek, Ventura County, California is divided into seven lithosomes (I through VII). Lithesome I consists of a chaotic conglomerate with isolated sandstone lenses and is interpreted to have been deposited in a shallow, steeply sloping, shelf-like environment. Lithesome II consists of a structureless sandstone interbedded with small siltstone rip-up clast layers. This lithesome is interpreted to have been deposited on the lower middle area of a submarine fan. Lithosomes IIIa and IIIb are made up of brown and black shales, respectively, and have both been assigned to interchannel areas of a submarine fan. Lithesome IV consists of interbedded shale and sandstone and is interpreted to have been deposited on channel levees of a submarine fan. Lithesome V is made up of cyclic-bedded pebble conglomerate, sandstone, and siltstone layers that were deposited in channels in the middle submarine-fan area. Lithesome VI consists of regularly bedded pebble conglomerate, graded sandstone, and laminated siltstone beds that were deposited in the lower-upper submarine-fan channel. Lithesome VII is composed of a matrix-supported cobble conglomerate with isolated sandstone lenses and is interpreted to have been deposited in the upper submarine-fan channel. The fan deposits are assigned to the Relizian and Luisian (medial Miocene) benthonic foraminiferal stages based on microfossils found in several shale units. Age-diagnostic benthonic foraminifera include Bolivina advena, Uvigerinella obesa obesa, Siphogenerina cf. S. collomi, Valvulineria miocenica and Valvulineria californica. During the Relizian and Luisian Stages (medial Miocene), sediment was eroded from the Rincon and older formations exposed northeast of lower Piru Creek and was deposited in the area of lower Piru Creek. In addition to this sediment, minor contributions may have been made by the San Gabriel Mountains which were north and northeast of lower Piru Creek during the Miocene. Sediment was deposited across a steep (5 ° to 10 °), narrow, shelf-like environment which deepened to bathyal depths over a horizontal distance of 1.6 to 4.8 km (1 - 3 mi). The submarine-fan deposits prograded seaward to the southeast. The Modelo Formation is interpreted to be a prograding, steeply sloping, submarine-fan complex deposited adjacent to a hilly shoreline.

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