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Stratigraphy and Depositional Environment of Unnamed (Lower Miocene) Submarine-Fan Sandstone Unit in Sierra Madre and San Rafael Mountains, Northeastern Santa Barbara County, California

Garold D. Thomas, A. Eugene Fritsche, Michael W. Condon

A relatively thick and extensive, previously unnamed, lower Miocene sandstone unit occurs in the central Sierra Madre and in the Hurricane Deck area of the San Rafael Mountains of northeastern Santa Barbara County, California. It is underlain conformably and interfingers with a dark mudstone that correlates with the Soda Lake Shale Member of the Vaqueros Formation; it is overlain conformably and interfingers with a brown shale that correlates with the Saltos Member of the Monterey Shale.

Northeastern exposures along the north flank of the Sierra Madre are almost exclusively medium to coarse-grained, structureless sandstone with scattered pebbly conglomerate beds. Thickness ranges from zero in the southeastern part of the Sierra Madre to 70 m in the northeast, 75 m in the northwest, and 600 m in the central part of the range. Toward the southwest in the Hurricane Deck area of the San Rafael Mountains, the unit becomes thicker and more extensively interbedded with mudstone. Lithology of the unit consists of 0.3-3.5 m thick beds of medium to coarse-grained, structureless to vaguely graded sandstone with scoured contacts at the base. Sandstone beds 0.3-3.0 m thick, which are more distinctly graded from coarse to very fine are also present. The interbedded mudstone commonly is bioturbated, so bedding is indistinct. Thickness ranges from 1,020 m in the central part of the area to 750 m toward the southwest and 92 m toward the northwest.

In the northeastern Sierra Madre, the unit contains an excellent macrofossil fauna that indicates shallow-water shoreface deposition. In the San Rafael Mountains to the southwest, the interbedded mudstone contains a lower bathyal microfauna. The unit most likely represents deposition in a submarine-canyon and fan complex that had its channel head in the northeast and spread southwestward into a thick sequence of submarine-fan sandstone lobes, which were confined in a narrow west-trending trough.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.