Historical Geology/Reefs

In this article we shall consider the formation of reefs, and discuss how we can recognize reefs in the geological record.

A blue starfish rests on hard Acropora coral.

What is a reef?

A reef, to a geologist, is what you get when macroscopic organisms which secrete skeletal matter (hard corals, for example, or oysters) grow on top of one another, forming a mass of skeletal material in relief from the sea bed.

Since the skeletons of reef-forming organisms are invariably made of calcium carbonate, reefs are by definition limestone.

Note that this definition of "reef" is different from that which would be used by sailors, to whom a reef is any submerged hazard.

Note also that the definition does not just include coral, the main group of reef-producing organisms today, but rather includes any organism with this form of growth, including a number of extinct organisms, some of which we shall list below.

Reef-forming organisms

• Archaecyathids are the first known reef-builders: goblet-shaped organisms typically a few centimeters in size which should probably be classed as calcite-secreting sponges. They flourished in the early Cambrian, began to decline by the middle Cambrian, and appear to have become completely extinct by the end of the Cambrian period.

• Stromatoporoids are a group of hard-bodied (i.e. calcium carbonate secreting) sponges which were important reef-formers from the Ordovician to the Silurian periods. While they are not extinct, they have been displaced to marginal habitats by later and more successful groups of reef-forming organisms.

• Rudist bivalves are a group of molluscs that flourished in the Jurassic and Cretaceous periods. During the Cretaceous they replaced corals in many environments as reef-builders, forming reefs sometimes a hundred meters high and hundreds of kilometers long, until they went extinct, like so many other organisms, at the Cretaceous-Tertiary boundary.

• Corals, despite their plantlike appearance, are animals closely related to sea anemones. Many of them secrete a skeleton of calcium carbonate, forming the main constituent of most modern reefs.

• Oysters are the bivalves familiar to gourmets and pearl-divers; they are capable of forming reefs, although not so large and spectacular as those formed by coral.

Ancient reefs: how do we know?

Fossilized tabulate coral.

It is easy to recognize a former reef in the geological record, since they are still clearly formed from the skeletal remains of coral, bivalves, stromatoporoids, etc. Their biological origin is therefore indisputable.

We might, however, if we were exceptionally cautious, ask ourselves whether the reef-shaped deposits of such remains are really reefs. Conceivably, they are piles of debris transported from elsewhere and then deposited in these formations as what we might call "pseudoreefs".

Like everything else in geology, this question had to be thought about at one point. In the days of Leonardo da Vinci, it was suggested that the oyster reefs and corals discovered inland in his native Italy had been transported there by Noah's Flood (da Vinci disagreed); and if no sensible person today entertains that conjecture, this is only because it has been considered and found to be wrong. For a number of objections occur:

(1) The positions in which the reef-forming organisms are found are the same as they would have in life. Now this is a powerful objection: there is no reason why any sort of transport or deposition forming pseudoreefs should have deposited the skeletal remains in their natural poses.

(2) There is no known mechanism by which water can pile up debris in the shape of a reef rather than spread it over a wider area (let alone give the transported sediment the configurations found in living organisms).

While the absence of a mechanism is not always a fatal blow to a hypothesis, it is certainly a point against it, especially as we have a perfectly good mechanism of reef formation to back up the theory that the things that look like reefs are in fact reefs.

(3) This hypothetical mechanism would have to be curiously selective in its action. We find apparent reefs built from oysters and corals, which we know from direct observation build reefs. We do not find "pseudoreefs" built out of (for example) the shells of crabs, or the cuttlebones of cephalopods. That is, we never find that this hypothetical mechanism has built reefs out of any organism known not to build reefs. This must cast doubt on the existence of such a mechanism.

A puzzle and a solution

Most varieties of coral are constrained by their biology to be shallow-water organisms, which cannot survive if they are more than a few meters below the surface of the sea. For this reason, living corals of this type are invariably found, not at the bottom of the ocean, but at the margins of continents or islands.

But what initially presents a paradox is this: sometime the islands on which shallow-water coral reefs grow are themselves made of (dead) shallow-water coral, and to a very great depth. For example, when the U.S. Atomic Energy Commission drilled at Eniwetok Atoll they found 1405 meters of shallow-water coral reef before striking basalt.

This seems, on the face of it, biologically impossible. These kinds of coral cannot grow at such depths, and therefore shouldn't be there. This puzzle was solved by a young man named Charles Darwin, later to achieve greater eminence in the field of biology. His solution seems obvious in retrospect: the islands must have originally been either above the surface of the sea, or within a few meters of it, and must then have sunk beneath the sea at a rate slower than that at which coral can grow. This seems obvious now, as I say, but it was less obvious in Darwin's time, when the idea that geological phenomena were produced by gradual changes over long periods was a new and revolutionary concept.

In our articles on plate tectonics (chapter 3 of this textbook) we shall explain why we would expect oceanic islands to sink gradually beneath the surface; for now we shall merely observe that by all appearances Darwin was correct.