Modern Carbonate Shelf-Slope Breaks
Modem carbonate shelf-slope breaks are highly variable, complex features that are morphologically distinct from their siliciclastic counterparts because of the dominance of in situ organic sediment production and early diagenesis. Carbonate shelf-slope breaks are, in reality, carbonate margins commonly characterized by abrupt and rapid transitions of sedimentary facies, biological communities and physical energy. We recognize four major types of modern carbonate shelf-slope break margins: (1) reef-dominated rimmed margins; (2) atoll margins; (3) sand-shoal-dominated rimmed margins; and (4) non-rimmed margins.
Reef-dominated margins usually occur along windward, open-ocean settings, and their associated shelf-slope breaks tend to be abrupt and precipitous, with steep, seaward slopes. Such margins are characterized by distinct morphological and ecological zonation as well as spur and groove structure. Atoll shelf-slope break margins are circular to elliptical in map view and overlie oceanic, volcanic basement. These margins are the most precipitous and reef-dominated of all carbonate margins. Distinctly zoned reefs, with algal ridges at their seaward edges, generally separate deep, open- marine lagoons from steep (>50°), seaward slopes mantled with reef debris and talus. Sand shoal-dominated margins may also be abrupt and precipitous, but are essentially devoid of reefs at the surface. Such margins typically occur along leeward settings and are characterized by bank-parallel sand bodies composed mostly of non-skeletal and degraded skeletal grains which have buried earlier Holocene reefs. Similar shelf-slope breaks are also found in tidedominated settings where strong tidal and storm currents flow on and off the shelfedge. These margins are characterized by bank-perpendicular sand bodies consisting mostly of oolitic grains. Shelf-slope breaks along non-rimmed carbonate margins are broad, subtle, non-reef features that occur in deeper waters (100–500 m). These carbonate shelfedges are characterized by a mix of non-skeletal and skeletal grains that grade up-dip into molluscan calcarenites and/or bioherms and down-dip into pelagic oozes.
Four primary processes control the location and gross geomorphology of carbonate shelf-slope breaks: (1) tectonism; (2) physical energy flux; (3) antecedent topography; and (4) sealevel history. Secondary processes such as biogenic barrier development, in situ sediment production, sediment transport and cementation serve to modify the gross structure of carbonate shelf-slope breaks. Analogous shelf-slope breaks should be recognizable in the rock record. Rimmed margins will be the easiest to identify, particularly on seismic reflection profiles, because of their abrupt changes in depth, sediment facies and biological communities. However, non-rimmed margins should also be recognizable on the basis of careful examination of lateral and vertical facies relationships.
Figures & Tables
The shelfbreak is that point where the first major change in gradient occurs on the outermost edge of the continental shelf. Although this environment delimits the boundary between two principal and well-defined provinces, the continental shelf and slope - and thus is of the first order of importance on continental margins - it has received surprisingly little specific attention in either modern oceans or in the rock record. This volume, the first compendium dedicated specifically to the shelfbreak, was derived from an SEPM Research Symposium convened at the joint Annual Meeting of SEPM and AAPG on June 2, 1981. The material is organized in a manner to illustrate examples of the shelfbreak in both modern oceans and the rock record.