Effects of Periodic Burial and Exhumation by Storms on Endolithic Communities, Northern Gulf of California

Ralph Stearley, A. A. Ekdale

Rocky coast endolithic communities that are subject to periodic burial by storm deposits exhibit features that have important paleoecologic implications for the study of storm-influenced coastlines in the ancient record. Diversity, abundance, and population density of endoliths in the northern Gulf of California are controlled not only by the composition and texture of the indurated substrate in which they bore, but also by the extent of unconsolidated sediment that may cover the hard substrate.

Bioeroding endolithic organisms include some forms (especially among bivalves and sipunculans) that successfully inhabit sand-covered rock.

Periodic migration of local sand bodies, resulting from seasonal storm activity, occurs in areas of the rocky intertidal zone. Where no such migration is common and sediment onlap is fairly constant year-round, boring bivalve species that produce permanent chimneys (Lithophaga spatiosa and Gastrochaena ovata) predominate. The chimneys allow them to extend their siphons through the soft sand to the water-sediment interface. In more dynamic environments, where migrating sand bodies provide a variable amount of sand cover, nonchimney-producing endoliths (Lithophaga aristata and L. attenuata) dominate.

Under conditions of variable sediment cover, endolithic communities display higher diversity, greater equitablity, and lower population densities than those living under relatively static conditions. Typically, openings to the sediment rock surface have different orientations. Based on size-frequency determinations, endolith populations in the more dynamic environments appear to be less mature. With adequate sampling of the local environment, these features provide reliable evidence of the dynamics of sand-body migration.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.