Modern fan deltas of the west coast of the Gulf of California, Mexico.

摘要

Six fan deltas distributed along the southern peninsular margin of the Gulf of California were studied. They are wave-dominated to different degrees. The whole fluvial system, from the drainage basin to the receiving basin, was considered. The drainage basins have high relief and some gentle depositional surfaces in the transfer zone; the bed rock is easy to erode and is composed mostly of volcanic and sedimentary rocks. Precipitation is associated with tropical storms and hurricanes; it is higher in the southern portion (300 mm/year) and gradually decreases northwards (100 mm/year). The alluvial fans are stream-flow dominated; however, debris-flows are also observed. They are in an erosional regime dictated probably by changes in sea-level.;Stream-flows in the alluvial fans turn into friction-dominated effluents once in the sea-water, where coarse sediments are trapped within the surf zone. Debris-flows behave as inertia-dominated effluents which are turbulent jets that keep flowing for long distances. The 'flashy' fluvial discharge takes place in periods of hours, while the related high energy waves could last for days. Thus, waves are allowed to destroy or mask the fluvial deposits during starvation periods that could last for days, between storms, or months between wet seasons and even years between ENSO phenomena.;There are morphologies of ancient fan deltas associated with sea-level changes, which include: (1) fan forms at 35 to 50 m water-depth and drowned fluvial valleys that are cutting the peninsular shelf of possible isotopic stages 2 and 3 and (2) above sea-level, Pleistocene marine terraces at elevations of 6 to 8 m of possible stages 5e or 5c.;Waves are the dominant factor affecting the transitional zone and delta front. The delta slope and prodelta are also affected in shallow-water deltas. The delta front is wide and has a lower limit with variable depth; in the southern portion this depth is deeper and gradually shallows towards the north. Differences in morphology and grain-size distribution along the coast are associated with the wave-driven longshore drift.