Abstract: Sedimentology and Geomorphology of a Modern Fan Delta, Loreto, Baja California Sur, Mexico

The de Arce delta is an emergent, wave-dominated shoalwater-profile, coarse-grained fan delta located 5 km north of Loreto, Baja California Sur. Sediment is dominated by pebble size gravel, with clasts ranging up to approx. 1.5 meters in diameter. There is a distinct down-fan decrease in grain size, and the lower delta plain contains abundant fine sand. Fluvial supply to the delta is dominated by stream flow and occurs primarily during intense tropical storms in late summer. High-energy storm wave reworking has produced a 2 - 3 meter high cobbly berm at the delta front.

As a result of persistent uplift at approx. 10 cm/ka, the entire delta plain (more than 3 km{2}) has been reworked in the last few thousand years. Storm berms are perhaps the most long-lived feature on the subaerial delta because they are not easily breached and high rates of littoral transport stabilize channel outlets once established. A series of stranded storm berms have thus recorded recent uplift and progradation at the delta front.

Measurements of transported beach clasts yielded an average transport rate of 1 m/month for clasts with a b-axis diameter of 10 cm. High rates of littoral transport have resulted in the progressive development of cobble spits on the southern portion of the delta, in the lee of the prevailing north wind. Progradation of the delta front is closely related to the development of these spits. Protected bays behind the spits are progressively filled by well-sorted marine sands, stream flow deposits, and impoundment fines, and the delta front is thus extended.  

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid