The Flooding of a Carbonate Platform: the Eastern Yucatán Platform as a Model for Transgressive Carbonates
The dynamic relationships between marine and freshwater systems on carbonate platforms and their respective responses to sea level rise remain poorly understood. This is surprising given the frequency of platform exposure and flooding events seen in the stratigraphic record, noting also that occurrences first of freshwater and later of brackish water carbonates are typically some of the first indicators of transgression within carbonate platform successions. The Sian Ka’an Wetlands (SKW) of the eastern Yucatán Peninsula, a spectacular and sedimentologically little studied example of a transgressive sedimentary system, comprise a 4,000 km2 low-relief complex of groundwater-fed freshwater marsh, lake and brackish coastal lagoons. The SKW record a history of Quaternary sea level fall and subsequent rise which is now resulting in the progressive encroachment of freshwater and shallow marine environments onto the platform top. Deep karstification of the platform and the formation of sinkholes (cenotes) and regional cave systems at low sea level have provided effective conduits for extensive recharge from the pediment. Rising groundwater during late Quaternary and Holocene transgression has led to partial flooding and the emergence of groundwater onto the platform top, leading to the formation of extensive palustrine environments in low-lying areas behind the reef and beach, across a vast freshwater carbonate factory which is dynamically linked with the marine carbonate factory offshore. As aquifer levels progressively rise, the marsh areas are shown to flood and migrate successively further landwards. At the coast, breaches of the beach barrier have led to the formation of brackish lagoons. Further sea level rise has locally seen progressive destruction of the barrier and partial marine flooding of coastal palustrine sloughs and lagoons to form seagrass embayments, in which groundwater is still emergent at the seafloor. The deposition and preservation of thick freshwater-brackish and palustrine successions during greenhouse intervals in the Mesozoic and Cenozoic of Iberia, Southern France and Switzerland likely reflect the tectonic creation of accommodation space in extensional and intramontane depocentres developed on regionally extensive carbonate pediments. By contrast, the preservation potential of platform top carbonates is in many cases likely to be low in today’s icehouse interval where eustatic sea level rise is relatively rapid in relation to subsidence.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019