Ophiomorpha and Gyrolithes inchnofaunal associations, Early-Mid Eocene Delmar Formation, Del Mar North Beach, San Diego, California
Nearshore to estuarine deposits of the Eocene San Diego Embayment are characterized by shallow marine and intertidal facies of the Delmar and Torrey Sandstone Formations, exposed along sea cliffs 15 km north of San Diego. The Delmar Fm. represents a lagoonal environment and exhibits three facies that include small tidal channels, tidal flats, and oyster reefs in addition to a diverse assemblage of well-exposed trace fossils. This research includes outcrop identification and literature review of the ichnotaxa Ophiomorpha spp. and Gyrolithes spp (spiralling burrow), both found in abundance in the heavily bioturbated interbedded muddy siltstones and sandstones of the Delmar Fm.These biogenic sedimentary structures suggest deposition in lagoonal, littoral to sublittoral environments. Burrows observed in sandy units were lined with mud from underlying muddy siltstone beds. The abundant traces of Ophiomorpha and Gyrolithes were created by a species of thalassinidian crustacean, similar to the extant burrowing shrimp genera Callianasa. The Torrey Sandstone Fm. is a higher energy environment, lacks the interbedded siltstone units characteristic of the Delmar Fm., and is considerably more sandy with large vertical dwelling burrows of several Ophiomorpha species. Integrated use of ichnofossils in paleoecology aids geologic interpretations of depositional environments.