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ABSTRACT: Origin of Barrier Islands on Sandy Coasts

William F. Tanner

Many barrier islands on the coasts of the Gulf of Mexico contain one or more nuclei; island growth has taken place more or less seaward from these nuclei, which are the oldest parts of the islands. The nuclei were, at one time, separate islands; the oldest beach ridges "wrap around" them on two or three sides, showing that they were not remnants of spits or earlier larger features. The nuclei grew larger with time because of a local "equilibrium of abundance" of sand, rather than a regime of erosion.

The younger "growth" areas are commonly marked by sequences, or sets, of beach ridges; such features are not visible in the nuclei. The question of the origin of many barrier islands on sandy coasts must be closely related to the question of the origin of the nuclei. But the nuclei appear to have no distinguishing marks that in themselves might help explain their origin.

Johnson Shoal (Lee County, on the lower west coast of the Florida peninsula) may provide some insight into the origin of nuclei. It appeared for the first time, more recently than 1853, in water 2-6 m deep, and has been migrating landward (eastward) ever since. Seven maps and charts from various dates and many sets of black-and-white aerial photographs have been used to produce a history of shoal migration. By December 1988, welding of the remnants of the shoal onto the shore of Cayo Costa island was already under way.

The migrating shoal contained more than 10 million m3 of sand (13 million yd3), with a mass of 10 trillion kg. Known nuclei on other islands are commonly about this size, or smaller.

The moving shoal did not develop from dredge spoil (there has been no dredging of this magnitude in the area), a spit, a drowned dune, or a fault. It must have been a natural nontectonic event: emergence of a shoal since 1853 without notable changes in sea level or wave climate. Perhaps such events, common a few thousand years ago, account for barrier island nuclei.

A small sea-level drop of a meter or two would be enough to convert such a shoal into a small island, which would then show no distinctive markings, until later growth added beach ridges and/or dunes. Small drops like this, in middle to late Holocene time, are now known. Johnson Shoal is being welded against a pre-existing shoreline and will not form a new island. This is because there has been no suitable sea-level drop in the last 130 years.

The central idea here is that a shoal can emerge and become an island by means of modest water level changes. Such a change in water level may be small and perhaps short-lived, as during a storm, or may last for a longer period of time. In the Gulf of Mexico area, sea level dropped a meter or more at least twice in middle to late Holocene time: once about 4000 B.P., and a second time about 2000 B.P. Many barrier islands may have been initiated then with obvious growth histories starting a century, or a few centuries, later.

This hypothesis may explain the origin of many coastal island nuclei, which formed offshore without any help from dredge spoil, drowned dunes, growing spits, faulting, warping, or other such popular explanations. Once such a nucleus has been formed, there will be no problem in enlarging it, if there is a local excess of sand ("equilibrium of abundance").

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990