THE KEY LARGO LIMESTONE REVISITED: PLEISTOCENE SHELF-EDGE FACIES, FLORIDA KEYS, USA

The southern margin of the Pleistocene Florida Platform is well known from the coral-bearing Key Largo Limestone and the ooid-rich Miami Limestone that crop out along the island chain of the Florida Keys between Miami and Key West. These units were deposited during the last interglacial highstand of sea level ca. 125 yr BP (oxygen-isotope stage 5e). Based on sedimentological and chronological (U-series dating) investigations of 12 long and 57 short cores, the youngest of the well known Q units (Q1-Q5) of the Key Largo Limestone of south Florida was further subdivided into Q5e and post-Q5e. Individual units correspond to highstands of sea level, and units are, to a large part, separated from each other by subaerial exposure horizons reflecting sea level lowstands of the Pleistocene. Units Q1 and Q2 are characterized by abundant quartz and to a lesser extent by skeletal fragments of molluscs and foraminifera. We speculate that units Q1 and Q2 may have been deposited during the high sea levels of oxygen-isotope stage 11 between 420-360 ka. Abundant carbonate production and reef development occurred during deposition of unit Q3, presumably during isotope stage 9. The abundance of corals and coral boundstone decreases in unit Q4 (corresponding to isotope stage 7), which can be subdivided into a lower quartz-rich and an upper carbonate-rich succession. Unit Q5e (equivalent to isotope stage 5e), which forms the present day emergent Florida Keys, is again rich in massive corals (Montastrea annularis) and reefs. The seaward-dipping geometry of this unit and the scarcity of the Atlantic breakwater Acropora palmata support the contention that this Q5e platform margin had a ramp-type character. Shelf-margin reefs with Acropora palmata developed during deposition of post-Q5e units and correspond to highstands of sea level during isotope stages 5a and 5c. These deposits, which exhibit shelf margin wedge and offshore outlier reef geometries, act as Pleistocene foundations of the Holocene bank barrier reefs at the modern south Florida shelf edge.