Not all unconformities are alike, and for carbonates, their discrimination can be difficult. In this regard, one particular discontinuity surface, among many, which is exposed as a laterally persistent surface in the Pleistocene Key Largo Limestone of the classic Florida Windley Key Quarry outcrop is problematic. Attempts to date the surface have been inconclusive. Ur234-Th230 dating by previous researchers reveals older ages (130 ka) above the surface and younger (126 ka) below. Is this discontinuity surface allocyclic (eustatic sea level fall), autocyclic (biologic, climatic, or storm changes) or diagenetic (phreatic-vadose fluctuations)? In order to address this problem, a novel approach incorporating high resolution Light Detection and Ranging (LIDAR) and X-Ray Fluorescence (XRF) is employed on a cm scale. LIDAR reflectivity is correlated to XRF mineralogical constructed compositions and quantitatively mapped using color extraction, which highlights the amount of Aragonite, Calcite and Hi Mg Calcite. The precise location of the XRF scan is matched to the LIDAR high resolution reflectivity image and run through a color extraction algorithm. The colors are then correlated to the specific carbonate minerals.
Observations reveal the main reef builder to be massive colonies of Montastraea annularis accompanied by Diploria labyrubthiformis, Diploria strigosa, Diploria clivosa, Porites astreoides, Porites porites, Montastraea cavernosa, Siderastrea radians and Siderastrea siderea. Syndepositional porosity is common within the colonies from boring Pholads and Clionid sponges. In between the colonies are disarticulated mollusks, Porites sp., and abundant Halimeda flakes and carbonate mud. Examination of fossil coral-algal species above the discontinuity surface reveals post-mortem dislocation and dislodgement of corals as well as occasional planar surfaces. While the corals are composed of various stages of aragonite inverting to calcite with low Mg calcite carbonate mud filled cavities, the undulating surface when cutting through corals is coated by Hi Mg calcite coralline algae. Repatriation/regrowth of Montastraea annularis above the surface on top of older Montastraea annularis is observed. The dislodgement, destruction and repatriation/regrowth of these colonies without significant differences in diagenesis above and below the discontinuity suggests a storm event of large magnitude rather than a sea level fall and subaerial exposure unconformity or groundwater fluctuations. To break a Montastraea annularis at depths of 10m, a maximum inferred depth for this Key Largo Pleistocene reef, takes considerable wave energy to fracture these massive corals (greater than 44 meganewtons/meters2 ), e.g. a hurricane or tsunami. Such an event would also allow the ripping up of older rocks and depositing them on top of and adjacent to younger rocks (e.g. the observed Ur234-Th230 vertical stratigraphic age inversion) and would be consistent with the observed cobble to boulder-sized allochem rubble above the discontinuity surface. These observations appear to support the hypothesis that this discontinuity surface represents the autocyclic paleotempestite signature of a Pleistocene hurricane or tsunami.