DISCOVERY OF UPPER PLEISTOCENE REEFAL SPONGE BIOSTROMES WITHIN RELICT TIDAL CHANNELS OF THE MIAMI LIMESTONE, SOUTHEASTERN FLORIDA

Four ancient, tropical-marine, reefal biostromes, built of a new ceractinomorph demosponge that ranges from barrel to vase shape, recently have been found in SE Florida. These sponge-bearing biostromes occur in outcrops alongside canals that parallel four relict tidal channels within an Upper Pleistocene oolitic tidal-bar belt in the Miami Limestone. The reefal biostromes conceptually have a ribbon-like geometry. In one area, densely spaced demosponges populate a paleo-channel along a 3.5-km length of its paleo-reach and a 2.2-km length of another channel. Some of these sponges are very large—up to 2 m tall and 1.2 m in diameter. At the 3.5-km-length site, the density of sponge occurrence varies throughout the span of the biostrome, with the sponges uncommonly absent and as many as 1.8 visible occurrences per meter of outcrop (mean = 0.5 sponges/m). Scarce, small Schizoporella bryozoan bioherms and biostromes, and rare instances of a few other small sponges of differing morphology, contribute minimal paleo-biomass to the mostly monospecific demosponge biostromes. Fill between demosponges is commonly ooid-bearing grainstones. Bifurcating canals extend downward from dermal walls of some sponges and probably functioned as holdfasts to steady the large sponges in soft sediment and strong current. In the absence of predatory competition, the sponges may have flourished in an environment stressed by daily or diurnal salinity changes, strong currents, and frequently shifting submarine sand dunes. The sponges trapped mostly quartz sand in their walls to create a rigid framework able to withstand strong currents. Rapid cementation also may have contributed to sponge wall strengthening. The walls of the fossil sponges within relict tidal channels may provide a good record of by-passing sediments within the channels.

Modern shallow tropical reefs with a framework constructed primarily by sponges are relatively rare because most of these reefs are built principally by corals. Large, tropical, shallow-marine, carbonate-platform demosponge reefs in the geologic record are also relatively uncommon. To our knowledge, the occurrence of sponge reefs in modern or paleo-tidal channels between ooid shoals is unique, and the SE Florida examples provide an alternative reef system for comparative paleoenvironmental studies.