SHALLOW STRATIGRAPHY OF LAKE OKEECHOBEE, FLORIDA - A PRELIMINARY RECONNAISSANCE

The shallow stratigraphy of Lake Okeechobee in south-central Florida h as been studied on a preliminary basis using a high resolution seismic profiling technique combined with vibracoring. The main purpose was t o investigate the unconsolidated sediment infill, but incidental infor mation on the first few meters of the underlying rockhead has been obt ained. The deepest reflector is from a surface exhibiting basins separ ated by ''reefs'' or ridges and cut by V-shaped channels. The basins a re filled by thin calcareous and similarly channelled horizons, probab ly the Caloosahatchee-Fort Thompson Formation. The overlying sediments form an incomplete cover and are developed m different areas, having limited overlap. There are three contrasted deposits. Rooted peats occ ur extensively in the shallow margins at the northeastern and southern ends of the lake. In the northwest a fan of quartz sand reaches its m aximum thickness off the Kissimmee River, from which it apparently was derived. It extends over the peats in the northeast quadrant. The top most deposit occupies about a third of the lake bed and involves about 0.2 km3 of organic-rich black mud. The mud lies mainly in the norther n half of the lake and has an eccentric location with greatest depths to the northeast of the central axis of the lake and an inclined surfa ce sloping down to the southwest. The mud thickness is frequently less than 30 cm, but reaches 80 cm in one area. The older, deeper part of the mud succession is differentiated into various colored clays interb edded with sand and shells, whereas younger mud is undifferentiated an d spreads over the peats of the shallow periphery. The lower sections of clays show submillimeter lamination, which is evidence that neither storms nor bioturbation have reworked these deposits. Density and she ar strength profiles confirm that only the thin surficial flocculent l ayer evident in some places is liable to periodic reworking by wave-in duced currents.