The Miami Oölite, named by Sanford (1909) for the oölitic limestone of Pleistocene age which covers a large part of the southern tip of Florida, has been found to consist of two separate units—an upper unit, herein designated the oölitic facies, and a lower unit, called here the bryozoan facies. In this paper the two units are combined as the Miami Limestone,1 a formational name which now seems more appropriate than the Miami Oölite. The bryozoan facies, the dominant constituents of which are massive compound colonies of the cheilostome bryozoan Schizoporella floridana Osburn surrounded by ooids and pellets, covers the greater part of Dade County and extends in places into adjoining counties—a total area of about 2000 square miles. It averages 10 feet in thickness in southeastern Florida and thins to 1 foot or so westward to the Gulf of Mexico. It is the surface rock of the southern Everglades and is one of the most extensive bryozoan limestones in the country. In southeastern Florida it is covered by an elongated mound of cross-bedded oölitic limestone, the upper unit or oölitic facies. This is the rock of the southern end of the Atlantic Coastal Ridge, with a maximum thickness of 35 feet under the Ridge summit thinning westward toward the low-lying Everglades as it encroaches over the bryozoan facies.

Interest in the origin of the two units has been heightened recently by the recognition of similar deposits that are being actively produced in a nearby area. Immediately east of Miami on the western edge of the Great Bahama Bank, strung in a north-south line, are the islands of Bimini, Cat Cay, Sandy Cay, etc., the region described by Newell and others (1959). East of the Cays and parallel to them, a large underwater mound of unstable oölite is forming, and east of the mound in the shallow lagoon, massive, tubular bryozoans (Schizoporella flondana Osburn) are growing. The oölite from the mound is slowly encroaching over the bryozoan beds. The bathymetric and ecologic conditions now extant in this area are probably similar to those which existed during the Pleistocene to form the units of the Miami Limestone.

The eastern slope of the unstable oölite mound of the Cat Cay and Sandy Cay area is cut by tidal channels which run normal to the direction of the mound itself. Narrow valleys, similar to these channels, can be found in the indurated rock of the oölitic facies of the Atlantic Coastal Ridge. The valleys probably had their origin as channels produced by tidal currents at the time the oölitic mound of the Ridge was in an unstable condition. It is also believed that the shape and orientation of the Lower Keys of Florida originated in a similar fashion.

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