PLEISTOCENE CORAL REEF DESTRUCTION IN THE FLORIDA KEYS: PALEOTEMPESTITE EVIDENCE FROM A HIGH RESOLUTION LIDAR XRF ANALYSIS OF WINDLEY KEY QUARRY

Not all unconformities are alike, and for carbonates, their delineation and underlying cause can be difficult to resolve. One particularly problematic discontinuity surface, among several, is a laterally persistent ~1 m amplitude undulating sharp surface within the Pleistocene Key Largo Limestone of Windley Key Quarry. ²³⁴Ur-²³⁰Th dating by previous researchers reveals older ages (130 ka) above the surface and younger (126 ka) below. Is this discontinuity surface allocyclic, autocyclic or diagenetic? To address this problem, a novel approach incorporating high resolution Light Detection and Radar (LIDAR) and X-Ray Fluorescence (XRF) is employed. LIDAR reflectivity is correlated to XRF mineralogical constructed compositions and quantitatively mapped using color extraction, which highlights aragonite, Lo Mg calcite, Hi Mg calcite, and minor clastic silt-clay occurrences.

Below the discontinuity surface the corals (dominant is Montastrea annularis with Diploria labyrubthiformis, Diploria strigosa, Porites astreoides, Porites porites, Montastraea cavernosa, Siderastrea radians and Siderastrea sidereal) are in growth position and exist today in various stages of aragonite to calcite inversion and solution-reprecipitation. Above the surface are tumbled and dislodged corals of similar diagenetic state, poorly sorted broken sand to cobble-sized rubble, disarticulated molluscs, and low Mg calcite mud. When cutting through coral heads the discontinuity surface is coated by Hi Mg calcite coralline algae. Now to break or fracture the observed Montastrea annularis species would take considerable wave energy (greater than 44 meganewtons/meters² reported). If due to a hurricane or tsunami, such an event would allow the observed subsequent regrowth or repatriation of the same species under continuous subaqueous conditions and help explain the observed ²³⁴Ur-²³⁰Th vertical stratigraphic age inversion. These observations suggest this discontinuity surface within the Key Largo Limestone to represent an autocyclic paleotempestite signature of a Pleistocene hurricane or tsunami rather than a short-lived Stage 5e eustatic sea level fall.