Deposition in the northern Gulf of California is a battle between 2 giants: the Colorado River which supplies approximately 150 million tons of mud and sand a year to the area, and the Gulf with its strong tidal currents (up to 4 knots) which control depositional patterns in the coastal and marine environments. The river is winning; during the Quaternary a cone of sediment has prograded basinward covering more than 4,000 sq mi, of which Holocene sediments form only a broad lens in the southern margin of the delta wedge.

A group of 14 continuously cored borings supplemented by surface observations of sediment distribution and processes document both sedimentologic attributes and facies relations of genetic-sand types in the Holocene. These sand facies include (1) tidal bars in the marine environment; (2) barriers, cheniers, sand-tidal flats, tidal deltas, and tidal and estuarine channels in the coastal environment; and (3) fluvial channels, alluvial bars, and dunes in the continental environment.

The late Holocene depositional record typically is characterized by a single regressive section except along the western margin of the basin where multiple regressive sequences, each separated by a transgressive sand, are common as a result of river shifting. The regressive sections either overlie a thin transgressive sand deposited during an early Holocene rising sea-level stage or lie directly on Pleistocene strata.

In a complete offlap sequence the lower part is characterized by marine bar sands and/or marine clays. This lower marine part of the sequence thins northward under the deltaic cone and is absent in the northernmost areas. The upper part of the sequence is much more variable, consisting predominantly of coalescing, upward-fining channel (estuarine, tidal, and fluvial) deposits in the northern and central parts, upward-coarsening coastal-barrier sands along the eastern margin, and mud-flat and coarse alluvial-fan deposits on the western margin of the basin.

End_of_Article - Last_Page 1834------------