Bed Forms on West Florida Shelf as Detected with Side-Scan Sonar: ABSTRACT

Abstract

A Side-Scan Sonar investigation on the west Florida shelf reveals a multitude of bed-form types. A nongenetic classification was devised based on apparent wavelength and ripple index (R.I. = Wavelength/wave height). This system divides the observed features into four groups: Giant--wavelength greater than 30 m, R.I. 30 to 100; Large--wavelength less than 30 m but greater than 1 m, R.I. 15 to 30; Small--wavelength less than 1 m, R.I. 5 to 15; and low-relief swells--wavelength greater than 300 m and relief only a few meters. Five major zones roughly parallel to the coast are delineated according to the distribution of bed-form types. Zone A, parallels the coastline out to approximately 20 m depth and is characterized by giant to large-scale bed forms. These features are observed on the sonographs as long, sinuous, and sometimes bifurcating, troughs of high reflectivity (coarse-grained?) sediment, interspaced with mounds of presumably finer grained material. Similar bed forms described in the literature have been labeled lineations. Zone B extends out to mid-shelf depth (40 to 100 m) and is characterized by low-relief swells and a few patches of giant to large-scale features. The low-relief swells at times correlate with large elliptical patches of apparently fine sand on a relatively coarser grained, flat, sea floor. Zone C is centered around the Florida Middle Grounds region and is characterized by small-scale bed forms and low-relief swells. These small-scale bed forms observed on the sonographs resemble current ripples. The orientation of these ripples varies from predominantly north-south across the Florida Middle Grounds to an east-west orientation in areas farther south. Zone D is situated offshore Cape San Blas along the Florida panhandle. The bed forms in this zone are characterized by high relief (2.0 to 8.0 m) giant-scale features. Superimposed on the giant-scale bed forms and on the sea floor fringing this zone are small-scale bed forms resembling current ripples. Zone E encompasses the outer shelf and is generally void of bed forms. However, a few unusual giant to large-scale features are observed. Most of the giant, giant to large, and large-scale bed forms on the west Florida shelf are considered to be storm-related features. Some giant-scale features and the low-relief swells may be relict structures left over from times of lowered sea level. The small-scale bed forms within Zone C are possibly the results of either internal waves or tides set up on the summer thermocline and/or currents created by Loop Current intrusion on the shelf. The latter event may also generate strong shelf-edge currents creating the bed forms in Zone E. End_of_Article - Last_Page 1687------------