Barren inter-tidal mud and salt flats comprise a low-lying coastal plain of 2,000 sq. km. extending southward from the Colorado River estuary to near the town of San Felipe, Baja California. The region is arid and characterized by maximum tides of 10 m. Examination of surface sediments in terms of sedimentary structures, texture, composition, and color reveals a repetitious sediment zonation across the plain related to varying exposure to subaerial drying, tidal inundation, and the activity of burrowing organisms. In a seaward direction or with decreasing elevation, the sedimentary sequence includes: (1) chaotic muds and evaporites; (2) medium brown, laminated clayey silts; (3) brown to gray, mottled silty clays; and (4) gray, laminated silty clays. Borings reveal a similar sequence stratigraphically disposed beneath the mud-flats, indicating their development through depositional regression.

Seaward growth was initiated during the late stages of post-Wisconsin sea level rise when accretion of tidally supplied muds from the Colorado River out-stripped the ability of small gulf waves to rework. Subsequent up-growth has resulted in onlap of tidal muds across the Pleistocene piedmont plain toward the west. Mud supply diminishes toward the south and wave effects are thus accentuated in the upper inter-tidal zones. Waves have truncated the piedmont plain. Coarse sands derived thereby are carried northward to form elongate beach ridges which finger out into the intertidal muds. Limited depositional regression by strand plain development has occurred in the southern area in response to mud-flat encroachment from the north.

Supply of river suspended load has been negligible for the past 50 to 60 years. As a consequence, waves have winnowed the former poorly segregated mud-flat deposits, piling coarse mollusk remains into beach ridges and spreading very fine sands as a sheet over the intertidal flats. Older beach ridges, now encased by intertidal muds, may represent earlier periods of low supply when the Colorado River was diverted into the Salton basin toward the north.

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