The Pliocene Reefs of Miami: Their Geomorphological Significance in the Evolution of the Atlantic Coastal Ridge, Southeastern Florida, U.S.A.

Abstract

Construction excavations in western Metropolitan Miami (Metropolitan Dade County), Florida have recently dredged up coral reef deposits from depths of only 15-20 m below the Atlantic Coastal Ridge. Based upon index fossils, these richly fossiliferous sediments are referrable to the Buckingham Formation (="Pinecrest Beds or Formation" of Olsson) and are dated as early Pliocene. A distinctive Pliocene coral fauna was present, including Diploria sarasotana Weisbord, Septastrea marylandica (Conrad), Septastrea crassa (Holmes), Porites furcula Lamarck, and the hydrocoral Millepora alcicomin Linnaeus, and over three hundred species of mollusks along with several species of echinoids and crustaceans (collected as fragments). Although being more biohermal in form. the Pliocene reef system under Miami appeared to have distinct zonation, with evidence of four ecological zones being collected at the sampling sites. These zones included the high-energy, reef crest Millepora-Trochita Zone (Zone 1), the reef platform Diploria-Gorgonian Zone (Zone 2), the back reef Porites-Septastrea Zone (Zone 3), and a lagoonal Thalaesio- Turritella Zone (Zone 4) to the west of the reef tract. Based upon both the extent of the Atlantic Coastal Ridge and the implied faunal zonation of the reefs, it is conjectured that the Pliocene reef tract produced the original topographic high along the southeastern coast of Florida. This was later covered by a crust of oolitic limestone and sand in the late Pleistocene to produce the Atlantic Coastal Ridge. Similar Pliocene reef tracts are known from southwestern and southern Florida and these linked with the eastern reef tract to form an atoll-like structure around a central, lagoon-like basin. The southern part of this basin infilled with carbonate sediments and during subaerial exposure in the early Pleistocene, produced the Tamiami Formation. Together, the atoll-like Pliocene reef tracts and the central lagoon-like basin laid down the geomorphological framework for the formation of the Everglades in the Holocene. The Everglades is now seen to be reef-controlled. Fourteen new gastropod species. all index fossils for the reef facies of the Buckingham Formation, and a new genus of hard substrate

muricid gastropods, are proposed.