UPPER PLEISTOCENE ROCKY SHORES AND INTERTIDAL BIOTAS AT PLAYA-LA-PALMITA (BAJA-CALIFORNIA-SUR, MEXICO)

Four examples of Upper Pleistocene, rocky shores and associated biotas are preserved in ecological detail at Playa La Palmita in Bahia Santa Ines on the Gulf of California (Baja California Sur, Mexico). A 2 m-h igh sea cliff eroded from Miocene andesite reaches 6.25 m above presen t sea level and retains 15 species of encrusting organisms. Dominant s pace occupiers are the bivalves Pseudochama janus, Ostrea fischeri, O. palmula, Area pacifica, Cardita affinis, and Chama mexicana together with coralline red algae. Carbonate sediments burying the sea cliff in clude rhodoliths, 15 additional species of free-dwelling bivalves and 15 species of gastropods, most of which are intertidal to shallow-subt idal in habit. At the top of the sea cliff is a hydrothermal deposit o f dolomite, which formed a broad abrasion platform extensively bored b y a bivalve species attributed to Thracia curta. An andesite shoal 2,6 00 m(2) in area occurs at a slightly higher elevation 7.75 m above pre sent sea level and preserves zoned populations of Glans affinis and Pe riglypta multicostata (bivalves), as well as vermetid ''worm-shell'' c olonies. A flat andesite island 12,500 m(2) in area with an elevation of 11 m is completely surrounded but not covered by carbonates. Nestle d among boulders in the lee of the paleo-island are bivalve population s of Chione californiensis, P. multicostata and Codakia distinguenda. Based on previous studies employing U-series analyses at Playa Palmita and nearby Mulege, these features can be correlated to oxygen isotope substage 5e, that is, developed about 125,000 years ago as a facies o f the Mulege Formation.