Biological Zonation on a Rocky-Shore Boulder Deposit: Upper Pleistocene Bahia San Antonio (Baja California Sur, Mexico)

Abstract

During Late Pleistocene time, a shallow embayment covering more than six hectares near Punta San Antonio (Baja California Sur, Mexico) was flooded from the Gulf of California to a depth of about 14 m. More than 400 m of shore-line along the inner perimeter of this bay consists of fitted boulders derived from adjacent tectonic blocks of Cretaceous granodiorite and Miocene andesite. In most places, the 1-m-thick boulder bed unconformably blankets Miocene tuff related to the Upper Comondu Group. Elsewhere it sits on Lower Pliocene limestone belonging to the San Marcos Formation. Stabilized through extensive cementation by coralline red algae, the rim of the boulder bed forms a steep rocky shoreline reposed between 50 degrees and 70 degrees . Biological zonation is expressed both laterally and vertically by the remains of more than 30 species of marine intertidal organisms that colonized the uneven face of the boulder bed under varying conditions of wave activity and shore orientation. An exposed outer shore preserves vertical zonation among in situ mollusks differentiated between an upper zone, characterized by Modiolus capax, and a lower zone of Codakia distinguenda. A more sheltered inner shore shows vertical differentiation between an upper zone, including Ostrea palmula and Crucibulum scutellatum, and a lower zone still featuring Codakia distinguenda (but with different associates). At the rear of the embayment, a small protected cove was well sheltered from wave activity. Here an upper zone is dominated by large coral colonies of Porites californica, together with diverse mollusks, in contrast to a lower zone depleted in corals but retaining much the same mollusk assemblage. Upper Pleistocene marine terraces correlated with oxygen isotope substage 5e follow elevations close to 12 m above present sea level throughout the surrounding gulf-coast region. The top of the extensive boulder bed at Punta San Antonio is 27.5 m above present sea level, but its seaward margin is connected by a continuous ramp to a 13-m level on the present shoreline. The boulder bed represents a local anomaly in average coastal uplift since the last interglacial epoch (corrected for eustasy) that is three and a half times the normal regional rate (17 cm/1,000 yr as opposed to 5 cm/1,000 yr). Ongoing uplift associated with the adjacent tectonic block that contributed granodiorite to the Punta San Antonio boulder bed is implicated.