Sedimentary Facies and Distribution of Biota in Coupon Bight, Lower Florida Keys

Abstract

Coupon Bight is a 9-sq-km bay separated from an offshore Inner Reef Tract by low-lying islands and a bay-mouth bar. Coupon Bight has less wave activity and less complete tidal exchange than Inner Reef Tract waters, and is no more than 250 cm deep. Temperatures and salinities in Coupon Bight are more variable and usually higher than those in waters of the Inner Reef Tract. The harsher environmental conditions of Coupon Bight are reflected in its less diverse biota, compared to that of similar areas in the Inner Reef Tract. Environmental gradients persisting between the various parts of Coupon Bight produce more pronounced faunal variations than are found in the more homogenous Inner Reef Tract. Physical and chemical conditions, sedimentary facies, and biofacies of the bight are closely related. Several major environments can be recognized in Coupon Bight, each characterized by a particular assemblage of organisms. Open bay, nearshore, restricted bay, baymouth bank, and tidal channel environments are present. The unconsolidated sediment of Coupon Bight is mainly autochthonous skeletal debris, with minor contributions washed in from the Inner Reef Tract and from eroded limestone bedrock. The texture of bight sediments ranges from sand to mud, with most of the mud facies found in bank-like deposits associated with thick growths of the marine grass Thalassia. Most of the fine-grained sediment in Coupon Bight is the product of disintegration of coarse shell debris. Fine materials are winnowed out by waves and currents and either trapped on Thalassia covered mud banks or carried out of the bight. The variability of and relations between environments, sedimentary facies, and biofacies that can occur in a small area such as Coupon Bight are clarified by the data collected during this study. As a result, similar variability in shallow-water marine carbonate rocks can be better interpreted.