ABSTRACT

Deposition in the northern Gulf of California is a battle between two giants: (1) the Colorado River which supplies approximately 150 tons of mud and sand a year to the area, and (2) the Gulf with its strong tidal currents which control depositional patterns in the coastal and marine environments. The river is winning; during the Quaternary it has prograded basinward a cone of sediment covering more than 4,000 square miles of which the recent forms only a broad lens in the southern margin of the sedimentary wedge. Additional sources of sand and gravel are the mountainous areas to the west and the eroded margin of a desert to the east.

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

The Late Recent depositional record is typically 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 the river shifting. The regressive sections either overlie a thin transgressive sand deposited during an Early Recent rising sea level stage or rest directly on the Pleistocene.

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 (1) coalescing, upward-fining channel (estuarine, tidal, and fluvial) deposits in the northern and central parts, (2) upward-coarsening coastal barrier sands along the eastern margin, and (3) mud flat and coarse alluvial fan deposits on the western margin of the basin.