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AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 1. (January)

First Page: 83

Last Page: 96

Title: Structural Controls on Submarine-Fan Geometry and Internal Architecture: Upper La Jolla Fan System, Offshore Southern California

Author(s): S. A. Graham (2), S. B. Bachman (3)

Abstract:

La Jolla fan, offshore of San Diego, California, is a well-studied example of submarine-fan sedimentation, yet the internal architecture of the fan has remained poorly known. High-resolution seismic data, recorded in a 1 by 2 mi (1.6 by 3.3 km) grid, over much of the fan, allow better understanding of upper and middle fan features and processes, and of structural controls on fan sedimentation.

Three bathymetrically prominent conduits supply sediment to the upper La Jolla fan system from stream and nearshore littoral drift-cell sources. La Jolla canyon (and contiguous La Jolla fan valley) is the main feeder to the fan. Seismic profiling data confirm the previously reported erosional character of the channel and constructional nature of flanking levees. These data also reveal that the position of the channel is controlled by the geometry of a buried, hard-rock structure.

Newport canyon-channel, northerly feeder to the upper La Jolla fan system, is a single well-defined channel with flanking constructional levees where it lies in a structurally controlled trough. In contrast, the channel splays into multiple broad, poorly defined channels where sedimentation is unconstrained. A buried Pliocene-Pleistocene ancestral version of Newport channel displays the same structural control.

The position of Loma sea valley, southern feeder to La Jolla fan system, is tightly controlled by the structure of the steep flank of Coronado Bank. Thus, Loma sea valley trends parallel with the shore and is fed by an orthogonal set of tributary channels which drains the San Diego shelf to the east. At intersections with these tributaries, abnormal thicknesses of sediment partially clog Loma sea valley. These channel fills may be analogous to those occurring in terrestrial trunk stream-intermittent tributary systems like the Colorado River in the Grand Canyon.

Seismic data demonstrate that the La Jolla fan system comprises a complex interleaved set of sediment wedges derived from multiple sources and woven around the wrench tectonic fabric of uplifts and basins of the southern California borderland. Thus, La Jolla fan system presents an expansion from the simple radial growth pattern of fan sedimentation to a complex fan system built of a number of smaller interwoven radial growth components. Despite these complexities, lithofacies patterns are in part predictable for the La Jolla fan system. Fault-bounded uplifts form long-lived barriers to sediment dispersal and enhance channel development along their flanks. Multistory channel complexes, detectable seismically, commonly occur in these structurally controlled positions adjacent to wrench elated uplifts.

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