Distribution, Sediment Source, and Coastal Erosion of Fan-Delta Systems on Isla Cerralvo (Lower Gulf of California, Mexico)

Abstract

BACKUS, D.H.; JOHNSON, M.E., and RIOSMENA-RODRÍGUEZ, R., 2012. Distribution, sediment source, and coastal erosion of fan-delta systems on Isla Cerralvo (lower Gulf of California, Mexico).Located near the tip of the Baja California peninsula, Isla Cerralvo is the sixth largest island in the Gulf of California. Although surrounded by some of the most productive waters in the world, field surveys show that Isla Cerralvo's shelf is largely devoid of biogenic carbonates, especially rhodolith beds, which are found in abundance elsewhere within the region. In counterpoint, a series of prominent fan deltas extend from the mouths of arroyos on Isla Cerralvo, despite the fact that the island has a granitic core, suggesting that the island's bedrock is severely weakened. Field observations suggest that the fracture pattern (submeter), hydrothermal alteration, as well as the orientations of metamorphic foliation, fracture sets, and fault planes all play a role in the accelerated rate of erosion on the island. The role of hydrothermal alteration is illustrated by a principal components analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Short Wave Infrared (SWIR) bands, which links heavily eroded areas at the south end of the island to areas with high concentrations of clays.A synthetic drainage system created using a 30-m-resolution digital elevation model (DEM) generated from an ASTER image was used to model the Isla Cerralvo drainage basins and stream channel networks. Analyses of basin and stream network information, including basin slope values and channel slope values, were used to identify islandwide differences in basin morphology and erosion characteristics. Stream channel profiles and slope-area data supported by limited uplift data indicate that Isla Cerralvo has not been uplifted as a single block, but it is broken into at least two major structural blocks with different uplift histories.Due to the arid climate and low annual precipitation, we find that sediment removal from the interior of Isla Cerralvo can only be accomplished by episodic, but very short (hours to days), catastrophic rainfall events caused by hurricanes or chubascos (winter storms). Subsequently, the sediment is eroded from fan deltas and transported southward by longshore currents and wind-generated waves, choking carbonate production along Isla Cerralvo's shore and shelf.