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 de posits 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 Gul f) and beach ridges (Vizcaino Peninsula). They formed during Pleistoce ne 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 wa ve-dominated western Vizcaino Peninsula differ in underlying lithology , size, profile, and shell fragmentation. Northern Gulf ridges prograd ationally overlie deltaic or marsh deposits, are smaller (median thick ness 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 cont rast, Vizcaino ridges disconformably overlie bedrock on wave-cut terra ces, are larger (median thickness 3.3 m; median width 100 m; median le ngth 2000 m), typically have a landward-dipping upper surface and a li near crest on the seaward side, and contain beds of extensively fragme nted large bivalve shells. These differences are interpreted to reflec t contrasting processes of ridge formation in the two regions. The rid ges of the tide-dominated northern Gulf are true cheniers-transgressiv e beach deposits produced during episodes of elastic starvation associ ated with shifts in Colorado River delta deposition. They are modified primarily through episodic washover processes during high tides and s torms. The ridges of the wave-dominated Vizcaino Peninsula are regress ive beach ridges developed on highstand wave-cut terraces. They are mo dified primarily through continuous wave accretion to the beach face, and secondarily by washover processes.