Sedimentary facies and trace fossils in the Eocene Delmar Formation and Torrey Sandstone, California

[摘要] The Delmar Formation and Torrey Sandstone were studied in seacliff outcrops at Solana Beach, about 15 km north of San Diego. There, they represent lagoonal and barrier bar or shoal deposits, respectively. Five subfacies were recognized in these outcrops, utilizing observations on sediments, physical sedimentary structures, body fossils and trace fossils. The Delmar exhibits three subfacies that formed as oyster reefs, tidal flats, and sublittoral tidal channels and ponds. The Torrey contains two subfacies, representing subaqueous dunes and tidal channels on a tidal delta or interior side of a barrier bar or s.hoal, and large, temporary channels generated by drainage of the lagoon after periods of high run-off or storms. Trace fossils contribute significantly to the description and interpretation of these subfacies. Their density and diversity indicate brackish to marine conditions. The abundant lebensspuren Ophiomorpha nodosa and Gyrolithes indicate deposition in littoral to inner sublittoral zones; Gyrolithes is especially common in brackish environments of the Delmar lagoon. Sandy, high-energy facies of the Torrey Sandstone are characterized by large, verticallyoriented dwelling burrows and by vertical locomotion traces generated by infauna migrating up and down in response to sedimentation and erosion. Muddy, more protected environments of the Delmar Formation exhibit lese robust, horizontally-oriented dwelling burrows and an abundance of feeding burrows constructed by animals mining the organic-rich sediment for food. Sedimentation and physical reworking were very active in Torrey environments, so trace fossils there distort but do not obliterate physical sedimentary structures and bedding characteristics. In contrast, much of the Delmar has been heavily bioturbated, indicating predominance of biological reworking over physical processes.

[发布机构] Rice University