MORPHOLOGY AND SEDIMENTOLOGY OF 2 CONTEMPORARY FAN DELTAS ON THE SOUTHEASTERN BAJA-CALIFORNIA PENINSULA, MEXICO

The San Juan de Los Planes drainage basin is located on the southeaste rn margin of the Baja California Peninsula, approximately 20 km southe ast of the city of La Pat. Pleistocene alluvial fans have been built i nto the basin from the surrounding areas of high relief and are presen tly being eroded. At the fan apex, the dissection depth ranges from 20 to 40 m, and is 6 to 15 m at the coast.The sediments have been redepo sited in several active modern fan deltas built out into the narrow sh elf of La Ventana Bay. Shelf width ranges from 0 to 5 km (average 3 km ). Wave energy is delivered from southeasterly storms and from the loc al sea breeze winds; the respective wave heights average 2 and 1 m. Th e Agua Caliente and Las Canoas fan deltas, the subjects of this paper, are located on the active El Sargento Fault Zone and are fed by chann els that deeply dissect the older fan surfaces. The Agua Caliente fan delta is a classic Gilbert-type delta with a steeply inclined delta fr ont. Wave reworking and redistribution of the sediments at the delta f ace are important and a wave-cut terrace has developed at about 6 m de pth. The Las Canoas fan delta is a shoal-water-profile, Hjulstrom type with a gently inclined delta front. Wave energy is less important. Th e alignments of the coasts of both fans are controlled by the fault li ne. Sediment supply is largest for the Las Canoas fan delta. The fan d eltas are examples of this depositional form as it has developed in an arid, tectonically active, high-relief and narrow neo-trailing edge m argin.