Carbonate islands are obvious features of modern shelf seas, and the same was probably true of the extensive shallow seas of the past. The purpose of this investigation was to recognize and describe those attributes of island sediments that distinguish them from adjacent submarine deposits and to indicate the processes responsible for these features.

The most important characteristics of shelf islands are that (1) they strongly influence the depositional environments of adjacent submarine areas, (2) they comprise distinctive and potentially large bodies of carbonate sediment, and (3) sediments of such islands may be the key to environmental interpretation of much larger areas because of their definite connotation of water depth and position near a shelf edge.

Ambergris Cay, Belize, is a modern shelf island that has formed by Holocene sediment accumulation on and around a partly exposed, karstic limestone surface under conditions of rising sea level. The configuration of the limestone surface has had an important effect on the distribution of the Holocene sediment.

This island consists of a narrow, windward, sand beach ridge, leeward of which is a broad expanse of supratidal flats interspersed with intra-island lagoons and areas of exposed rock. The windward ridge is a storm deposit of coralgal sand derived from the adjacent reef lagoon. Its geometry and the importance of coral and calcified algal constituents distinguish this sand deposit from other island sediments. The supratidal-flat sediments are predominantly magnesium calcite, and the most important constituents are foraminifers and mollusks. These deposits are further characterized by their lateral and vertical variability, relatively high content of mud-size carbonate material, and assemblages of the following features: laminations, stromatolites, desiccation cracks, intraformational co glomerates, and root casts. Intra-island-lagoon sediments are typically nonlaminated, contain a high percentage of mud-size carbonate material, and have a restricted faunal assemblage.

In addition to these primary characteristics, island sediments are distinguished by early diagenetic modifications that reflect their subaerial exposure. Solution destruction and early cementation are results of exposure of the sediments to meteoric water. In contrast, early dolomitization affects supratidal-flat sediments associated with ponds of hypersaline water.

Thus, island sediments form a distinctive, locally "regressive" sequence within a transgressive marine situation. That is, the island's land area is growing at the expense of its submarine area, increasing its sheltering effect on surrounding submarine environments.