Atolls are roughly circular in plan with a central lagoon that contains no significant land mass. The central lagoon is often deep (less than 25 m), but this is not a prerequisite. If land does exist, it sits atop a part of the encircling reef and is comprised solely of carbonate material derived from the reef. As originally defined for Pacific reefs, the term implies a specific genetic origin around a volcanic island. Caribbean and Atlantic atoll-like reefs are not of this type, and tend to form around isolated highs formed by local tectonics.
A barrier reef is a morphologic entity, separate from its tectonic regime. A limit should be set on lagoonal dimensions in fringing vs. barrier reefs. Implied in Darwin's definition of reef types is the idea that the lagoon is sufficiently large to permit open circulation behind the reef. The reef serves as a "barrier" that clearly separates lagoonal processes from those of the open ocean. Based on this, we pragmatically make the split between barrier and fringing reefs at a point when the lagoon reaches 500 m in width and 5 m in depth. In a natural setting, and in the absence of significant upland clearing of vegetation, a lagoon of this magnitude can substantially isolate the reef from direct impact by terrestrial runoff. Furthermore, circulation within the lagoon is distinctly removed from that of the open ocean beyond. Classification criteria are summarized in the Table.
In the fossil record, many of the Silurian reefs of the Michigan and Illinois Basins appear pinnacle-like when viewed in stratigraphic cross section (Fig. 7.9). However, this characterization is largely an artifact of the vertical exaggeration required to fit these sections onto a published page. When plotted at equal horizontal and vertical scales, these reefs appear much broader in cross section. While it is difficult to determine precisely their actual relief at any one time, it is likely that their pinnacle-like character is in large part due to the sequential stacking of one reef atop another.
Submerged shelf-edge reefs are Caribbean platform margins that presently sit in water depths greater than 10-15 meters after being flooded by rising sea level 6,000 - 10,000 years ago. Since then, they have not been able to offset the effects of ever-deepening water, and many of them have been left behind. While coral and other calcifying organisms occur along most of these margins, they are not producing carbonate at a rate sufficient for the reef to "catch up" with sea level.
Equally problematic are reefs that occur on wider shelves (>5 km), and fall between the criteria for either barrier or patch reefs. They are similar to patch reefs in shape, but they are usually larger, more linear, and are aligned in roughly shore-parallel. They exist near sea level and, in some instances, have shoaled enough that islands can form. The sediments behind the reef (landward) are similar to those seaward reflecting the absence of lagoonal conditions. Because they usually occur along either insular or continental shelves, they are classified as shelf reefs.
The nature of shelf reefs changes from shore to the shelf edge. More-seaward reefs are exposed to higher wave energy. Those closer to shore come more under the influence of terrestrial sedimentation. For example, on the southern coast of Puerto Rico, the inner-shelf reefs are often subjected to fine-grained sediments derived from the adjacent hillsides . As a result, they are mostly mud mounds with scattered corals. In some instances, they have been stabilized by mangroves and have built small islands. The mid-shelf reefs are subject to the effects of open-ocean circulation and more wave action. Accordingly, coral cover is higher and the benthic-community structure is more complex.
A similar pattern occurs across the Queensland shelf from inner reefs and islands under the influence of heavy runoff to the mid-shelf and outer-shelf reefs that occur in increasingly clearer but rougher water. This is reflected in higher growth rates by the coral Porites lutea at similar water depths along the more-seaward reefs. Isdale, 1984 Paralleling this pattern, photosynthetic species of sponges increase in abundance as the water clears near the shelf edge. Wilkinson, 1987