The dynamic interaction of bathymetry and hydrography on the narrow, reef-bounded, continental shelf of Belize produces a complex pattern of Holocene sediment types and biotic assemblages. Basically, the Belize shelf consists of a seaward reef-cay complex and a shoreward lagoon system. The mainland of northern Belize is a low karst surface. The adjacent northern shelf is shallow and is mantled by a suite of almost wholly carbonate sediments. The southern Belize mainland is mountainous and drained by numerous streams transporting large volumes of clastic sediment seaward. The corresponding southern shelf is a well-developed karst surface characterized by considerable bathymetric relief evidenced by numerous steep-sided patch reefs and shelf atolls surrounded by deep channe s. The diverse sediment types of the southern shelf range from nearshore quartzose sands and deltaic deposits to a suite of lagoonal marls distinguished from one another by the dominant biotic constituents, to wholly carbonate reef and platform bioclastic sediments. The overall distribution of Holocene sediment reflects a structural influence and is closely related to the historical development of the Belize shelf.

The diversity, abundance, and composition of the benthonic biotic assemblages of the shelf vary systematically in response to bathymetry, hydrography, and sediment types. This is most evident in the coral reef development. Reefs occur in a myriad of sizes, shapes, and environmental settings, but the most obvious and impressive constructional feature is the nearly continuous barrier reef developed on the seaward shelf edge. The diversity and distribution of mollusk assemblages demonstrate the strong influence of substrate, bathymetry, and hydrography on the benthonic fauna. The diverse epifaunal opportunities offered by marine grass (Thalassia) are especially evident.