Geomorphic Expression of Last Interglacial Maximum Reef Complexes of the Florida Keys: New Insights from LiDAR Data

Abstract:

Pleistocene carbonates of the Bahamas-Florida archipelago are crucial archives of past sea levels (SL) and paleo-seascapes formed by physical and biological processes. In this study, we use new high-resolution LiDAR data obtained from the National Oceanic and Atmospheric Administration (NOAA) to shed light on the spatial distribution of a fossilized barrier reef and its relationship to two surrounding tidally-dominated ooid shoal complexes along the Florida Keys and Miami area.

By utilizing the LiDAR-derived digital elevation model (DEM) three phases of carbonate accumulation are recognized for the Key Largo (KL) and Miami Limestones (ML). These phases were constrained by integrating existing sedimentologic and geochronologic data with the LiDAR model of the region. We follow a well-recognized two-stage rise of the last interglacial (LIG) with the first phase between ~132 and 126 ka largely not recorded at surface in the Keys, and the second pulse of relative rise occurring between 126 and 117 ka.

Development of the KL Limestone reefs occurred during the initial phase of the late-stage MIS 5e SL rise, with the LiDAR-calculated average elevation of the reef crests at +2m. This average elevation suggests a paleo-water depth range of +4 to +5m for the deposition of these reefs (assuming a +2 to +3m addition to the average elevation).

In comparison, the ooid shoal complex in the Miami area illustrates an average elevation of +3.5m at the shoal crests. Distinct geomorphic features identified by previous authors such as polygonal bars, spits, and beach ridges within this complex supports the interpretation of a shoreline elevation of +3.5 to +4.5m. These elevations indicate synchronous deposition of the ooid shoals and the KL reefs in the early stages of the late MIS 5e rise, while postdating them when the LIG SL reached its peak near +6m.

The third discrete unit is the ML forming the dip-elongate tidal bars that now make up the islands from Big Pine Island to Key West. A distinctly lower average elevation of +1m as derived from the DEM for this complex suggests deposition at a lower paleo-SL relative to either the Miami region shoals or the KL reefs. This downstepping pattern during late-stage MIS 5e is recognized widely throughout the Bahamas and places these units on the falling limb of the MIS 5e curve with ages between 118 and 116 ka