Modern Fan Deltas of the West Margin of the Gulf of California, Mexico

Enrique H. Nava-Sanchez and Donn S. Gorsline

Modern fan deltas are common along the semi-arid, high-relief cliffed west margin of the Gulf of California. Flash floods during summer storms are responsible for the coarse influx. These fan deltas are all wave-dominated to varying degrees. Six fan deltas are being studied.

Subaerial characteristics such as lagoons, distributary-streams, floodplains, erosional or depositional beaches, grain-size, and beach steepness, depend on factors such as width and steepness of the hydrologic basin, sedimentation rate, sea-bottom morphology, wave energy, littoral currents, and sea-level changes. The subaqueous fan delta is affected by fluvial discharge energy, wave energy, littoral currents, and sea-bottom morphology. Broad delta fronts are related to low fluvial discharge energy, high wave energy, and low sea-bottom gradients; narrow delta fronts are related to high fluvial discharge energy, low wave energy and high sea-bottom gradients. There are two kinds of delta slopes: (1) related to fault scarps and submarine canyons, which are steep; and (2) depositional in c aracter that are steep where fluvial stream are high energy or gentle if growing on wide shelves. The prodelta is not always connected to the delta slope, because of the presence of fault scarps or submarine canyons. Then, prodelta sediments are being deposited at the base of scarps, and they could be affected by active faults. Pelagic sedimentation is also important for prodelta environments.

Ancient fan deltas have been inferred from the prodelta bathymetry, that supposedly were built at low sea-level stands at depths of about 35 to 40 m. Ancient fan deltas are also present adjacent to the modern fan deltas and in interior basins, which are filled with Miocene and Pliocene marine rocks.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California