Introduction

Modern reefs are important for their control of nearshore ecology, their role as natural breakwaters and their value as fishery and recreational resources. They take on added significance as models for ancient reef systems and their links to natural-resource distribution as well as questions about the evolution of marine life through geologic time.
















Coral reefs are biologically produced structures that in time exert control on the surrounding environments. Modern reefs are a focal point for study by varied groups of researchers, each of which has its own perspectives and priorities. Biologists are primarily concerned with individual organisms or interactions among them (ecology). Marine geologists study relatively young (less than 10,000 yr) reef structures that have formed since the latest glaciation, whereas stratigraphers focus on the three-dimensional nature of features developed over much longer periods of time, and often built during multiple episodes of sea-level rise and fall. And finally, natural-resource managers attempt to apply basic scientific principles to specific problems in their area of responsibility. This focuses on reef types, biological and physical zonation, controls of reef development, and the formation of reefs.

The term "reef" was originally applied by mariners to anything hard that might damage their vessel. As such, reefs included the biologically produced entities discussed here as well as any pile of rocks in shallow water, be they deposited by the last glacial advance or the U.S. Army Corps of Engineers. Literally volumes have been dedicated to the subject of reef classification, and the broad overview that follows is an attempt to provide some perspective on the problem.

We start with the elements upon which there is general agreement. First, coral reefs owe their origins to the biological secretion of calcium carbonate by living organisms, and they are rigid structures that stand above the surrounding sea floor. As a result, they exert some control on local oceanographic processes. Also important as reef builders, at different times in geological history, are serpulid worms (i.e. central Florida shelf), oysters, and coralline algae (the algal ridges of the Indo-Pacific and Caribbean Sea), as well as fossil bivalves, sponges, stromatoporoids (probably massive, calcareous sponges), and stromatolites (algal precursors that occupied the niche of modern corals).

The primary disagreement concerns the concept of framework in reef building. The reef-forming organisms generally possess massive skeletons that are the building blocks of the reef. Many workers hold that in a "true reef" these are organized into an in-place and interlocking meshwork that provides the rigidity of the reef. Conversely, any feature lacking this interconnecting framework is not a reef (see Fagerstrom 1987 for an excellent summary).

The problem is that when ancient reefs are examined, many of them are not the organized structures that are central to this model. They have recognizable organisms, but these are loosely packed and often "floating" in a detrital matrix. In these cases it is assumed that either:

This pattern has occurred with sufficient regularity to raise questions about how (and why) modern and ancient reefs might be fundamentally different from one another. Reef organisms have certainly evolved through time; perhaps this has changed the ways in which reefs have functioned. Sea-water chemistry and the shape of ocean basins have likewise changed - many ancient reefs existed in broad, shallow seaways and intracontinental seas, in contrast to the more-exposed and steep platform margins of today. If all these factors have conspired to fundamentally change reefs over the span of geologic time, then modern reefs become very poor models for their ancient counterparts. We will show that this is not the case. In an analogy by Ginsburg, reefs are likened to Shakespearean plays in which an ever-changing cast acts out a timeless plot. The following discussion builds on this theme.