THE NATURE OF TRANSGRESSION: CAPE SABLE, FLORIDA

Rapid dramatic changes to Cape Sable over the past 75 years provide insight into how transgression occurs in coastal and shallow marine sedimentary environments. Cape Sable, on the southwestern Florida coast in Everglades National Park, is defined by low marl and sand ridges, which separate freshwater and saline lakes, lagoons and wetlands from the marine environment. The 22 cm relative rise in sea level in south Florida over the past 75 years has partly inundated the low-lying ridges and caused coastal erosion. As a result, small natural tidal channels and narrow human-constructed canals, together with tidal overtopping of low marl ridges, have initiated rapid transgressive changes in the system.

Changes include: rapid and persistent erosive widening of channels as they become exposed to the coast or connected to interior lakes; rapid sediment filling of connected interior lakes and constriction of tidal flow through filling lakes to narrow channels; climax capping of mud-filled lakes by mangrove communities; and construction of tidal mud levees on the new mangrove-lined channels. As interior lakes fill and are channel constructed, sediment is transported progressively inland, initiating deltaic filling yet landward.

As the marl ridges are inundated by rising sea level, interior freshwater marshes have collapsed, evolving into shallow open water areas. It is thought that saline conditions have limited effective vegetative growth and initiated peat decay.

Shorelines exposed to the west and northwest (to winter storms) are persistently erosional with rates ranging from small to 4 m per year. As the eroding sequence is mud dominated, the eroded sediment does not remain in the shore system but is dispersed offshore or well into the channel system. South-facing mangrove capped marl shorelines record stepped erosion from major hurricanes.