Origin of Shell Beds and Evolution of a Shelly Sand Spit, Bahia la Choya, Northern Gulf of California

Abstract

An actively prograding shelly sand spit forms the eastern margin of Bahia la Choya, northern Gulf of California. Shell-rich deposits exposed on the eastern side of the spit record previous phases of spit growth. Analysis and comparison of faunal composition, biofabric and shell preservation from the beach, estero mouth bar, and the fossil spit deposits suggest 3 major phases of spit evolution: (1) southward progradation of the spit tip/estero mouth bar (lower shell bed), (2) deposition of washover lobes during a major storm (upper shell bed), and (3) eolian accretion (spit crest). A 1300 ± 50 y.b.p. radiocarbon date on a bivalve shell from the storm shell bed provides a maximum age for the storm event.