ABSTRACT

A total of 144 bottom dredge samples were collected on Cape San Bias Shoal and vicinity, west of Apalachicola Bay, Florida, in the Northeast Gulf of Mexico. On the basis of this investigation, four bathymetric-sedimentary provinces can be distinguished:

(1) The Cape San Blas Shoal extends southwest approximately 14.5 km into the Gulf from the Cape. Characterized by a very rough ridge-and-swale topography, it can be divided into two general units by sediment type, mean grain size, and standard deviation.

(2) A basin, located northeast of Cape San Blas Shoal and extending south to about 12.8 km offshore, contains very fine-grained sediments derived from the Apalachicola River.

(3) To the south of this basin, and southeast of Cape San Blas Shoal is the ridge-and-swale province composed of a mixture of shell hash, medium to coarse sand and a low percentage of silt.

(4) The shelf to the west of the shoal is basically composed of very-well sorted fine sand and shelly fine sand with some coarse sand on a smooth, gently seaward sloping topography. Thin mud layers, originating in Apalachicola Bay and transported around the shoal, overlie the sand in certain areas of the shelf.

The doublet of geomorphic provinces consisting of Cape San Blas Shoal and the ridge-and-swale province are interpreted as a shoal-retreat massif. It was initiated as a cuspate delta of an Apalachicola River distributary during a low stand of seal level. At the present time it is maintained 'in line' through wave refraction, although sediment-budget calculations indicate that a shift to the east is taking place. The shoal grows landward through accretion at the base. Prominent cross-cutting depressions were initiated as spillways by hurricane-generated currents. Hurricane waves and surge-generated currents largely determine the ultimate shape of these provinces.

Surface features of selected quartz grains were studied by differential-interference and scanning electron microscopy. The coarser grains of the shoal, ridge-and-swale and north shelf provinces, show V-shaped impact features and grooves characteristic of high-energy environments. The fine grained shelf and basin sands are smooth and relatively featureless.

The sediments in the study area generally consist of a minor contribution of modern mud and shell hash, but mainly of quartz-sand which was originally laid down as a subaerial deposit during a low stand of the sea. Nearshore processes have produced from the original material the nearshore modern well-sorted, fine sand. This material is most prominent in, but not restricted to either the nearshore or the shoal regions of the study area.

Lower Cape San Blas Shoal and the ridge-and-swale province consist of coarser sands with notable admixtures of shell hash. Although such sand is often referred to as relict, textural considerations and some (ill-developed) associations between textural parameters and topography suggest that the sediment continues to undergo transportational and depositional episodes, largely in response to hurricane conditions. The term 'palimpsest' is more properly applied to this kind of material.