OVERVIEW OF PACIFIC ISLAND CARBONATE BEACH SYSTEMS

摘要

The geologic evolution of tropical oceanic, intraplate volcanic islands is reasonably well understood and follows a succession from island initiation through volcanism followed by denudation, subsidence, and reef formation. The end-member island form is the atoll. There is also a general progression in the development of beach systems as the islands grow and decay. Using examples from Hawaii and Micronesia this paper discusses: a) the variation in beach system size with island age (and reef development), and, b) a brief description of typical beach systems. The main Hawaiian Islands increase in age from Hawaii towards Kauai. Associated with this increase in island age is a related increase in percentage of shoreline fronted by beach, beach volume per kilometer of coast, and total beach sand reservoirs. A similar trend is evident between islands in Micronesia, however, there are additional variations caused by reef development and vertical tectonic history. For example, within the Federated States of Micronesia, the Kosrae-Pohnpei-Chuuk island chain increase in age towards Chuuk but beach system development on Pohnpei is limited because the wide barrier reef and deep lagoon prohibit reef sediment from reaching the coast. Uplifted islands with fringing reefs, such as Guam and Saipan, display beach systems analogous to slowly subsiding islands with fringing reefs. Although the beach systems examined are Holocene in age, it is clear from the examination of a wide variety of islands that the geologic framework is critical to beach system development. It takes longer than 6,000 years (i.e. the Holocene) to develop extensive beach systems. There are a number of distinct physiographic settings where beaches occur in Pacific Islands. These include: drowned river valley with bayhead beach; structural embayment with beach; pocket beach; coastal plain/cuspate foreland beach; delta beach; fringing beach; and, perched storm-beach. In addition to these beach systems of high islands, there are atolls which are almost entirely encompassed by beaches along their shorelines. Atoll islet beaches can be further classified by islet type and exposure.