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The Antigua Limestones

A deposit of gravels, including pebbles of lava and petrified wood, is placed by Earle between the tuffs of the medial lowland and the heavy series of overlying marls and limestone. "The heterogeneity of the deposit would point to its being a beach deposit laid down under the influence of strong currents and tides in an epoch of subsidence."

Then come the heavy calcareous beds, the so-called Antigua formation, at least 1500 feet thick. It is said to be separated from the gravels by a slight unconformity. "These beds contain a fossil fauna as noted for its variety as for its abundance... This fauna is essentially a marine one and consists of compound corals, lamellibranchs, foraminifera, calcareous algae, gasteropods, and echinoderms... The commonest and most characteristic fossil is Orbitoides mantelli, which with other species often forms the larger part of the deposit. The commonest lamellibranchs are species of Ostrea and Pecten, the former often reaching a very large size. The algae are chiefly represented by species of Lithothamnion... while the compound corals are represented by genera too numerous to mention here."

According to Vaughan, a coral reef at the base of these limestones "grew upon a basement that had been subaerially eroded and was later depressed below sea level;" and the limestones were "deposited in shoal water on a flattish floor." The corals here must have formed a lagoon reef, for they do not stand at the margin of their area of limestone accumulation. As now eroded the calcareous beds exhibit a number of discontinuous cuestas and intermediate subsequent valleys which occupy the northeastern third of the island. All the valleys of the limestone area as well as the medial subsequent lowland are cultivated, mostly in sugar-cane plantations.

The manifest lesson of this instructive rock series is that it was accumulated during a time of longcontinued subsidence; and, if the lower agglomerates are rightly interpreted as of subaerial origin, the total measure of the subsidence must be 6000 or 8000 feet at least. It must be inferred from the nearly conformable sequence of the volcanic series, as well as from their position with respect to the entire area of the Antigua-Barbuda bank, that the lavas and tuffs are peripheral beds, like those associated with the southwestern mud flows of St. Lucia; hence, if there was a higher central area of the original volcanic island, it is very likely buried nearer the center of the bank.

But, on the other hand, in view of the late date of eruption and the eccentric position of Grenada with respect to the long bank on which it is believed to have been built up, it is possible that the volcanic rocks of southwestern Antigua represent an eccentric addition made to a bank of earlier beginning and later completion. In any case, the occurrence of the pure limestones, with abundant reef-making corals and other fossils, in the upper part of the series suggests that those strata were deposited in a reefencircled lagoon and that the encircling reef was at first a barrier and later an atoll.

The barrier-reef stage of Antigua was clearly recognized by Purves 31 40 years ago, although he did not explicitly suggest that the barrier reef later became an atoll. He wrote: "La puissance de cette vaste formation calcaire indique qu'elle s'est déposée pendant une longue période d'affaissement du sol qui a suivi l'extinction de l'activité volcanique et pendant laquelle des récifs de coraux très-étendus se sont librement établis autours des bancs formés par les matériaux volcaniques éjectés lors des dernières éruptions. Ces roches, actuallement visibles, ne représentent pas la substance même du récif, car, pendant la formation du dépôt, ce récif devait être situé à une distance considérable de la côte. Ces amas de marnes et de calcaires avec leur masse de débris de coraux détachés mais très-bien conservés, de bois flotté et échoué, de coquilles et d'orbitoïdes, représentent évidemment le dépôt particulier que l'on voit encore de nos jours se former par l'accumulation entre la barrière de récifs et les côtes d'une île affectée d'un mouvement d'affaissement lent et continu."

This interpretation of the early or first-cycle history of Antigua seems to be essentially correct; for in view of the abundance of reef-making corals preserved in the limestones of that cycle and in view of the evidence given by the later or second-cycle history of Antigua, as stated below, as well as that given by other islands already described for the former occurrence of protecting barrier reefs, the growth at first of a barrier and later of an atoll reef around the margin of the first-cycle limestone area of Antigua seems wholly reasonable.

THE SECOND CYCLE OF ANTIGUA'S DEVELOPMENT

The continued upbuilding of the atoll was interrupted by the uptilting of its southwestern part, and the uptilting was accompanied and followed by prolonged erosion. So extensive, indeed, has the erosion been that the inferred atoll reef is not discoverable today; it must have been, after tilting, completely removed by erosion over the uplifted southwestern part of Antigua, as Purves implies, and it must be submerged in the bank on the east and west. It may well be that the disappearance of the reef by erosion will be taken by some readers as a reason for doubting that it ever existed; but that it did exist is made extremely probable by the abundance of reef-building corals in the limestone strata; for if corals grew so well on the bank they must have grown still better around its edge.

And as to the disappearance of the reef, it would be, in view of the great amount of erosion that has taken place since the uptilting of the island, and more particularly in view of the situation of the atoll reef with respect to the part of the uplifted island that has been worn away, and still more particularly in view of the submergence the island has suffered since it was well worn down in its secondcycle erosion, as unreasonable to expect to find the reef visible today as to expect still to see the crater rim preserved in a deeply dissected volcano. A visit to the island would doubtless make this clear, even if reading a description of it does not.

Thus interpreted, the deep-lying, central volcanic foundation of Antigua may be of late Eocene date, and its lagoon limestones and marls were deposited in Oligocene time, these being the events of the first cycle: the deformation and degradation of the second cycle have taken place in later Tertiary time and thereafter. Certain of the second-cycle physiographic features next to be considered repeat those of the first-cycle volcanic islands above described so closely as to indicate that the second-cycle sequence is much like the first.

The subdued forms of insequent dissection in the southwestern volcanic district as well as the alternating cuestas and lowlands carved on the strong and weak belts of sedimentary strata prove that a long period of subaerial erosion has elapsed since the firstcycle atoll was uptilted. The coast is now well embayed, and the distance to which the embayments, now more or less filled with delta plains, originally entered the valleys indicates that a considerable subsidence took place in the latter part of the long second-cycle period of subaerial erosion, if not in the earlier part also. Bendel River, the longest stream of the island, flows sluggishly westward for five or six miles on a smooth flood plain, which occupies a subsequent valley next north of the hilly volcanic district, and mouths in Five Island Harbor on the west coast. When the valley was in process of erosion the island must have stood higher than now, and the mouth of the river must then have been two or three miles west of where it is now. The depth of that mouth of the valley below present sea level must be several hundred feet.

The subsidence by which the coastal embayments were produced must have caused a significant diminution of the island's extent where the weak sedimentary strata had been previously reduced to lowlands, as appears to have been the case in the area of the bank on the northwest, northeast, and southeast of the present island. Hence, the foundation of this bank of second generation adjoining an island of strong and weak structures may very possibly be a worn-down and slightly submerged area of weak rocks, chiefly limestones, of an earlier generation, although such a possibility was rejected for the bank of first generation around St. Lucia.

The distinction here made between banks of first generation formed around islands of simple structure, and banks of second generation formed around islands of compound structure, is of significance in estimating the measure of upgrowth accomplished by the reefs within which the strata of the banks are supposed to have been accumulated, or the equivalent measure of island subsidence during reef upgrowth. In the case of an island of compound structure, it may be imagined that after its weaker rocks are worn down to low relief, the resulting lowland is slightly submerged so that a barrier reef grows up from small depth around its outer margin and encloses a lagoon, above which the resistant rocks will stand in submountainous relief. Evidently such a reef may be of small thickness.

On the other hand, in the case of an island of simple structure, like a volcanic cone, no part of it can be worn down to a lowland until all the rest of it is reduced to small relief; hence, if a well-offset barrier reef is found around a volcanic island that is dissected only to the stage of maturity, such a reef cannot be reasonably explained as based upon a slightly submerged lowland and as therefore having but small thickness; it should be explained as based on the submarine slopes of the island, which are probably not unlike the supermarine slopes; and hence the best estimate of the thickness of such a reef, and of the measure of the subsidence which has permitted its upgrowth, is to be made by prolonging the visible slopes of the island downward beneath the reef. In many Pacific islands the thickness of their barrier reefs thus estimated may be 1000 or 2000 feet at least.

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