Gregg R. Brooks, Department of Marine Science, Eckerd College, St. Petersburg, FL
A total of 80 vibracores and 498 surface sediment samples have been collected on the inner west Florida continental shelf between Anclote Key and Venice Inlet in order to establish the geologic framework and the recent geologic development of coastal west-central Florida. All vibracores have been split, described, photographed and subsampled. All vibracore bsamples and surface sediment samples are being analyzed for grain size, calcium carbonate content, total organic content and mineralogy. Sediment constituent analysis is being conducted on some samples and, where appropriate material is present, radiometric age dating is being conducted.
Eight sedimentary facies have been identified. The quartz sand facies is the most common, consisting of relatively clean, fine-grained quartz sand. Quartz sands commonly occupy the tops of cores and are most abundant immediately seaward and to the north of Egmont Key. The quartz sand facies is interpreted to have been deposited under open marine conditions. The marine shell facies consists predominantly of molluscan shell material that may occur alone or in a mixture with quartz sands or muds. Shells may be finely or coarsely fragmented, or remain unfragmented. Shell-rich units can be found at any depth throughout the core and range from a few cm to over 1 m in thickness. Shell-rich sediments are interpreted to have been deposited under a variety of environmental conditions. Thin layers of finely fragmented shells may represent storm deposits. Thicker layers of coarsely fragmented shells may represent a channel lag or ravinement surface. Layers of unfragmented shells may have been produced in situ. The black sand facies consists of a mixture of quartz sands with black particles. X-ray diffraction shows black grains may be carbonate or phosphate (carbonate fluorapatite). Black sands are common in short cores, or where sedimentary cover is thin. They are especially common in the southern part of the study area. Black sands are interpreted to represent old sediments. The phosphatic fraction may have winnowed out of the underlying limestone. The organic-rich mud facies consists of dark brown to black muds containing in excess of 10% organic material. Radiocarbon age dates show a range from 5,900 to >35,000 years. Organic-rich muds are found near the bases of cores, sometimes at the base of unconsolidated sediments, and are interpreted to represent back barrier, lagoonal or estuarine deposits. The olive-gray mud facies is a low-organic, siliciclastic-rich mud. It commonly contains whole shells and is generally found near core bases where it often reaches 1 m in thickness. Occasionally it is found at or near the surface in thin layers. The olive-gray mud facies is interpreted to represent a back barrier environment similar to the organic-rich muds. The lime mud facies consists of a white to light gray mud containing up to 70% calcium carbonate. Lime muds are commonly found near core bases and may contain whole, unfragmented mollusc shells. Lime muds are similar in appearance to many modern Florida Bay carbonate muds. No modern analogous environments have been reported in the west-central Florida area. The blue-gray clay facies consists of a highly compacted blue-gray clay that often constituted refusal during the coring process. It is always found near the base of unconsolidated sediments and, when present, lies immediately over the limestone surface. The blue-gray clay facies has been reported to be associated with the Miocene Hawthorne Formation and is interpreted to represent an alteration product of the underlying limestone. The carbonate lithoclast facies is always found at the base of cores and constituted refusal during the coring process. It is interpreted to consist of fragments of the underlying limestone surface.
Based upon facies associations and stratigraphic relationships, an idealized stratigraphic section is presented. The carbonate lithoclastic facies occupies the base, overlain by the blue-gray clay facies, followed by the organic-rich mud facies and/or the olive-gray mud facies, a thin unit of coarsely fragmented marine shells, and capped by the quartz sand facies. This idealized section represents the landward migration of coastal environments, which would be expected during a relative sea level rise. Preservation of underlying units is discontinuous and is interpreted to be related, at least in part, to the irregular nature of the underlying limestone surface. Correlation of units between cores is difficult, due also to the irregularity of the underlying surface as well as the discontinuous nature of the sediment deposits themselves.
Sediments to the north of Tampa Bay appear to be distinctly different from sediments to the south. Near shore surface sediments to the south of the Bay are coarser grained and have a higher carbonate content than those to the north. Additionally, sediment cover to the south is much thinner than sediment cover to the north. Sedimentological differences are interpreted to be due to the lack of appreciable quantities of quartz sand to the south of the Bay, which may have implications concerning sediment dispersal mechanisms.
Future efforts will focus on determining environments of deposition, factors governing sediment distribution and precise timing of events. Information will be integrated with the seismic network and modern barrier island studies.
Larry J. Doyle, Department of Marine Science, University of South Florida, St. Petersburg, FL
Cores were opened, described and sampled at each lithologic change at Eckerd College. Splits were sent to the USF X-ray laboratory for x-ray diffraction analyses. Samples were ground to a powder and mounted on glass slides coated with Vaseline. Standard x-ray diffraction techniques were utilized. Standard slides of pure aragonite and calcite and mixtures at 25% aragonite, 50% aragonite, and 75% aragonite were made up and basal reflector peak areas of each mineral are computed by the computer. Unknown peak areas are compared by the computer with reference to the standards. This method has been standard for years, although prior to automation, peak areas were computed by hand, by polar planimeter, or even by cutting them out and weighing them. High magnesium calcite is assumed to respond like calcite. The method is semi-quantitative. Results were tablized, and then plotted on the core logs which record lithologic description, Total Organic Content, Grain Size and Percent Carbonate Minerals by weight.
Sixty surface samples were examined for carbonate constituents. 100 grains were counted for each sample.
Results and Discussion
Figures 1, 2, 3, 4, 5 show some representative cores. Several points stand out. The most striking is the variety of sedimentary environments present within a few meters of the surface. Indeed, most of the cores contain a surface layer of quartz sand or shell hash, typical for the area but in many places it is only a few centimeters thick.. Beneath are layers representing a number of back barrier, probably estuarine, and even paludal that are described in Part A. In nine of the vibra-cores, from north to south, USGS 40, 21, 46, 11, 67, 124, 114, 121, and 112, a layer of mud, sometimes quite soupy was present on the top surface of the core. Surficial muds are found scattered throughout the study area. Seven lie between the 12 m and 20 m isobaths. All cores with surficial muds contain other mud layers. All surficial muds are followed by layers of shelf sands, mostly quartz, between 30 cm and 200 cm thick.
There is a distinct difference between the grain sizes of the whole sample containing both carbonates and quartz and the insoluble portion of the sample, composed primarily of quartz. Figure 6 illustrates this difference, with the black bar representing the grain size distribution of the whole sample and the checkered pattern that of the insoluble fraction. Most of the quartz is concentrated in the fine and very fine sand fractions, between 62 and 250 microns.
Another interesting feature of many of the cores is the presence of what appears to be phosphatic sands just below the surface layers. These are found in cores to the north as well as south of Tampa Bay. They may represent the top of the Hawthorne Formation or they may have winnowed from deeper deposits. Phosphate is common in some of the beaches of Sarasota County, but much less common in the surface sediments offshore.
Figures 1, 2, 3, 4, 5 show some of the typical distributions of carbonate minerals in the cores. In most instances carbonate mineralogy is dominated by calcite and aragonite with smaller amounts of high magnesium calcite. High magnesium calcite seems to increase in the layers of shell hash. Toward the base of some of the cores, where aragonite abruptly goes to 0 (see Figure 5). It is probable that the aragonite has reordered to calcite and this change probably represents a much older deposit. Cores where this takes place will be good candidates for radio-dating.
Table I shows the carbonate constituents of the surface sediments of the inner west Florida shelf as presented at the last seminar. Table II shows the carbonate constituents of 60 surface samples for this years effort. Mollusc fragments dominate both with benthic foraminifera making up the second largest component.
Table 1: Composition of Sediments in West Florida Shelf (%).
We believe that the remainder of this study should focus on the vibra-cores already taken. Most significant will be determining the nature of the varied depositional environments present in the subsurface. Specifically, we would like to concentrate on the mineralogy, especially the clay mineralogy, of the mud layers and the layers high in organics in order to determine provenance. In addition, we propose to conduct a micro-paleontological study under the direction of Ms. Pamella Hallock Muller of the various facies in conjunction with above, in order to help determine the environment of deposition, such as back barrier, estuarine or even paludal and the age of the deposits.