Abstract

Deposition in the arid northern Gulf of California is a battle between two giants: (1) the Colorado River which, prior to man's interference, supplied over 150 million tons of sediment per year to the area and (2) the Gulf with its enormous tidal range (over 30 feet) and strong tidal currents. The river empties into a constricted, orogenically active basin where additional sediment is supplied to the dispersal system operative in the delta area from the adjacent uplifted margins, the Sonoran Desert on the east and the Peninsular Range to the west.

The Holocene is characterized by three major depositional environments — shallow marine, coastal, and continental — each comprising several distinct facies as a result of the varied processes operative in each. Fourteen cores supplemented by surface sediment distribution mapping and a description of sedimentation processes, document both the sedimentological attributes and facies relationships of the major genetic sand types within these environments.

The sands group into three main types based on the internal organization of the unit:

(1) Those with upward-fining sequences: estuarine, tidal, and meander belt distributary channel sands;

(2) Those with upward-coarsening sequences: marine tidal bars and coastal barrier sands; and

(3) Those showing no distinct vertical sequence: chenier and alluvial fan deposits.

The wedge of Holocene sediments in the northern Gulf consists of a thin basal transgressive unit deposited during the rising sea level stage overlain by a thicker regressive section which represents the basinward progradation of sediments into the Holocene sea. Locally, on the western margin of the basin, multiple regressive sequences, each separated by a transgressive sand, result from Holocene delta shifting. The lower part of the regressive unit is marine muds and sands which thin northward into the delta and laterally toward the basin margins. The upper part of the sequence is much more variable across the basin and may consist of (1) coalescing channel deposits in the northern and central parts of the basin, (2) coastal barriers along the eastern margin, and (3) tidal flat muds and coarse alluvial fan deposits along the western margin of the basin.