Abstract

Beneath the shallowly submerged coastal mangrove forest (paralic mangrove swamps) of southwestern Florida, marine and brackish-water sediments of Recent age overlie fresh-water calcitic mud that was deposited on bedrock or fresh-water peat about 4,000 years ago. This sedimentary succession is thought to be the record of a marine inundation of the western margin of the extensive fresh-water swamps (Everglades) of southern Florida. To map the extent of the submergence a stratigraphic study was made of piston core samples of unconsolidated sediments underlying waterways dissecting the coastal forest and intra-forest bays enclosed within it. These cores were primarily taken in the vicinity of Whitewater Bay and in the Ten Thousand Islands area. The latter region forms the northern end of approximately 50 nautical miles of swamps and coastal mangrove forest; this belt of paralic swamps is typically 1–3 miles broad, although it is as much as 10 miles wide in some areas. Whitewater Bay is situated at the southern terminus of these swamps. The sequence of transgressive sediments consists of a basal unit of autochthonous (in situ) fibrous peat, largely derived from mangrove and other rooted halophytic plants, and an overlying allochthonous unit of peaty and calcareous shell debris (Whitewater Bay) or shelly quartz-rich sand and silt (Ten Thousand Islands area). Judging from radiocarbon dates, the basal peat unit began to form 3,000–3,400 years ago after cessation of calcitic mud formation. Within a period of a few hundred to a thousand years formation of in situ fibrous peat in areas which are now waterways and intra-forest bays gave way to the deposition of shelly brackish-water and marine sediments of the upper member of the transgressive sequence. The environmental shift from fresh-water to brackish-water and marine milieus came about in response to a more or less steady rise in sea level and marine inundation of former mainland paludal swamps. Based on the age and elevation of fibrous peat overlying bedrock and fresh-water calcitic sediment, the rise in sea level across southwestern Florida 3,000–4,000 years ago was approximately 0.5 ft./century. Relative to its present stand, sea level ca. 4,000 B.P. stood 9–11 ft. lower; about 3,000 B.P. it stood only 4.5 ft. lower. Since ca. 3,000 B.P. sea level has risen to its present elevation at a steadily diminishing rate. This is interpreted from the rate of clastic sedimentation during the last three millennia in areas near the seaward edge of the swamps. Because much geologic and geomorphic evidence attests to the tectonic stability of peninsular Florida since the last interglacial stage, the rise in sea level is regarded as eustatic in cause and related to post-Valderan melting of continental ice masses. Sea level has evidently not stood appreciably higher than its present position during the last 5,000 years. This means that the + 10 ft. Silver Bluff shoreline recognized along the eastern coast of the United States, and its equivalent mapped elsewhere in the world, is not of Recent age but probably of Sangamon (last interglacial) or mid-Wisconsinan age.

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