Abstract
The Ten Thousand Islands region of southwest Florida is composed of three subtidal subenvironments: (1) shallow coastal lagoons; (2) tidal channels that connect lagoons to the adjacent Gulf of Mexico through a maze of mangrove islands; and (3) small, protected bays formed by the irregular shapes and distribution of these islands. Coring and probing studies in each of the subenvironments have revealed that subsurface sediments are primarily alternating zones of bioturbated, shelly, sandy mud, 5-50 cm in thickness, and coarse layers of quartz sand and shell material, 1-23 cm in thickness.In each subenvironment, the surface sediments exhibit a characteristic texture and composition caused by the local variations in (1) hydraulic energy associated with prevailing conditions and winter storms and (2) shell production. Subsurface bioturbated sediments are, in general, slightly coarser than local surface sediments and are interpreted to be the result of the biological reworking of material deposited by a combination of prevailing conditions, winter storms and minor hurricanes. Coarse layers exhibit significantly coarser textures and contain a larger component of shell material and/or quartz sand, than surface and burrowed sediments. Layers commonly show internal laminae and fining-upwards grading, and contain shells oriented convex side up. Coarse layers are interpreted to be deposited during major hurricanes.Coarse layers can exhibit variations in texture, composition, internal structure, bed contact and bed thickness, but are consistent within each subenvironment. These characteristics are determined by the proximity to tidal channels and local sediment sources, and the hydraulic energy during deposition. Cores taken in transect indicate that layers are the coarsest and thickest, contain the most shell material and exhibit the most internal structure in flood deltas located at channel inlets into lagoons. Coarse layers become finer, thinner and exhibit less internal structure away from inlets into the adjacent lagoons, and gradually pinch out. Four basic types of coarse layers have been recognized: (1) sandy to quartz-rich skeletal grainstones and packstones, (2) quartz grainstones, (3) shelly quartz packstones and wackestones, and (4) diffuse quartz packstones and wackestones.