ASPECTS OF THE EMPLACEMENT, EVOLUTION AND C-14 CHRONOLOGY OF RIDGES ON A COASTAL SPIT, GROOTE EYLANDT, NORTHERN AUSTRALIA

A coastal spit on Groote Eylandt in the western Gulf of Carpentaria, Northern Australia, began to form between 3000 and 2000 yr B.P. in response to the migration of a large parabolic dune across the coastline, Systematic discrepancies in the C-14 chronology and the remarkable preservation of fragile shelly material indicated a complex origin for a series of concentric ''beach'' ridges. One of the shell species deposited in the beach ridges (Clypeomorus bifasciata Sowerby) was being continuously incorporated through normal wave action while a second species (Fragum whitleyi Iredale) was introduced only under storm conditions. The importance of selecting a single shell species for C-14 dating and understanding the environmental tolerance of the species is highlighted. A conceptual model for the emplacement of surficial ridge material under bedload flow during storm set-up is proposed. It is argued that these ridges act as offshore bars rather than storm beaches during storm events. This would have significant implications for the environmental interpretation of some types of spits and beach ridges. In particular, sea-level estimates based on the assumption that these types of ridges are formed as beaches, are open to question.