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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 44 (1960)

Issue: 4. (April)

First Page: 433

Last Page: 457

Title: Tarzana Fan, Deep Submarine Fan of Late Miocene Age Los Angeles County, California

Author(s): Harold H. Sullwold, Jr. (2)


The sandstone beds in the Modelo formation (upper Miocene) exposed on the north flank of the Santa Monica Mountains were for the most part deposited from turbidity currents. Evidence of rapid deposition is provided by poor sorting, high clay-silt content, grain angularity, high feldspar content, and load deformation. Evidence for great depth of water (about 3,000 feet) is provided by abundant foraminifers and fish remains in the interbedded shales. That the mechanism for transporting this poorly sorted sand-silt-clay mixture into waters of this depth was that of turbidity currents is borne out by the multitude of syngenetic textures and structures which these rocks have in common with accepted turbidites and with structures which have been produced in laboratory studies.

Some of these structures are oriented with respect to the direction of travel of the current and thierefore provide a means of determining the direction of bottom slope along which the currents moved. When plotted on a map these directions reveal a pattern which is unmistakably that of a fan with its apex toward the north. The area studied constitutes a 16-mile-long section through this outcropping north-tlted fan. The point source was most logically the mouth of a submarine canyon. The canyon itself is thus far unrevealed, being concealed beneath the alluviated San Fernando Valley, but the eroding source area is tentatively identified on the basis of mineralogical studies in the San Gabriel Mountains about 22 miles northeast of the fan's apex.

Further studies of this sort can not fail to provide valuable data on earth history and thus aid petroleum exploration in similar rock types elsewhere in California.

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