PLEISTOCENE SHORELINE RIDGES FROM TIDE-DOMINATED AND WAVE-DOMINATED COASTS - NORTHERN GULF OF CALIFORNIA AND WESTERN BAJA-CALIFORNIA, MEXICO

The arid climate of northwestern Mexico preserves Pleistocene beach deposits forming linear, shoreline-parallel ridges on the coastal plains of Sonora, northern Gulf of California, and the Vizcaino Peninsula of Baja California. Beach deposits represent both cheniers (northern Gulf) and beach ridges (Vizcaino Peninsula). They formed during Pleistocene sea-level highstands, most commonly during oxygen-isotope stage 5e (similar to 125 ka). The ridges are 0.5 to 5.0 m thick, < 10 to > 100 m wide, and < 100 to > 1000 m long, coarse-grained with common low- to high-angle cross stratification, and are dominated by bivalve shells. Ridges from the tide-dominated northern Gulf of California and the wave-dominated western Vizcaino Peninsula differ in underlying lithology, size, profile, and shell fragmentation. Northern Gulf ridges progradationally overlie deltaic or marsh deposits, are smaller (median thickness 1.4 m; median width 25 m; median length 300 m), typically have a seaward-dipping upper surface and a linear crest on the landward side, and have low levels of fragmentation of large bivalve shells. In contrast, Vizcaino ridges disconformably overlie bedrock on wave-cut terraces, are larger (median thickness 3.3 m; median width 100 m; median length 2000 m), typically have a landward-dipping upper surface and a linear crest on the seaward side, and contain beds of extensively fragmented large bivalve shells. These differences are interpreted to reflect contrasting processes of ridge formation in the two regions. The ridges of the tide-dominated northern Gulf are true cheniers-transgressive beach deposits produced during episodes of elastic starvation associated with shifts in Colorado River delta deposition. They are modified primarily through episodic washover processes during high tides and storms. The ridges of the wave-dominated Vizcaino Peninsula are regressive beach ridges developed on highstand wave-cut terraces. They are modified primarily through continuous wave accretion to the beach face, and secondarily by washover processes.