The Geologic Effects of Hurricane Donna in South Florida

The passage of hurricane Donna (September 9-10, 1960) across south Florida caused storm effects in an area where detailed data on prestorm sea-floor conditions existed. The amount of boulder-sized rubble formed by hurricane surf on platform-edged reefs far exceeded the amount produced by day-to-day processes of death and deterioration. Each large storm affecting the reef tract adds a small increment to the building of the rubble accumulations. Large quantities of skeletal sand on the shoals behind the reefs and linear rock patches were transported and redeposited during the hurricane. Cross-bedding dip directions in storm deposits are like those in deposits of normal tidal and surf currents, i.e., platformward on shoals and seaward in lows on the platform edges, because the direction of storm currents was controlled by the same regional and local topography that controls normal currents; but scale of cross-bed sets and maximum size of sediments in storm deposits are larger. The ebb of the storm tides left large amounts of layered lime mud stranded on the supratidal flats (above the normal high-tide line). The large extent of the supratidal flats results from (1) the ability of storm tides to strand sediment over large areas, (2) the inaccessibility of the tidal flat to processes that could rework its sediment into adjacent marine environments, and (3) the supply of the tidal-flat sediments at the expense of adjacent marine fades that compete with tidal-flat sediments for a place in the geologic record. These factors probably explain the large amount of tidal-flat facies in ancient rocks. Mound-type accumulations of muddy sediment were not eroded by storm-wave or tidal currents. The main conclusion of our study is that a bias exists for preservation of effects of higher-energy events. Although such events may be catastrophic in terms of a man's longest period of observation, his lifetime, they are nevertheless only commonplace events in terms of geologic time.