SHALLOW SPELEOGENESIS IN A COASTAL KARST CARBONATE PLATFORM: QUINTANA ROO, MEXICO

Many existing models of speleogenesis in carbonate platforms assume extensive cave development occurs at the freshwater and saltwater mixing zone, and that cave formation occurs most readily along the lower boundary of the mixing zone. In coastal carbonate platforms where the hydraulic gradient is near zero, mixing dissolution should result in formation of near-horizontal cave passages at or just below this zone during periods of sea level stability. Additionally, many models assume rapid rates of dissolution and cave formation, sometimes resulting in caves in as little as 10,000 years.

In Quintana Roo, Mexico, extensive underwater caves are coincident with the modern mixing zone, and equally extensive dry caves occur at or above modern sea level and above recent sea-level high stands. We hypothesize that mixing dissolution alone is insufficient to explain the location and extent of observed dry cave passage, especially given the relatively brief periods of time at which these elevations could have been in a mixing zone. Other mechanisms, namely inputs of soil CO₂ along the water table, and aquatic microbial influences, merit further investigation. Both the amount of time spent at each sea level and the past climate control the volume and rate of conduit formation. Existing sea level records and proxies suggest that the water table has not been at or above modern sea level long enough to produce the caves we observe. In order to better constrain the timing of cave development, U-Th dates will be obtained from speleothems and folia in dry caves. Narrow zones of folia are laterally extensive in some sections of the caves and are assumed to represent calcite deposition at an ancient water table under tidal influence.

Preliminary U-Th ages obtained from one stalactite in Jaguar Claw Cave yields an age of ~252 ka BP, and folia overgrowth (~7.2m above modern water table) of ~49 ka BP. Current sea level proxies suggest that the latter was a time of sea level low stand (-40m). Folia from another sample ~1.4m above the water table indicate that a sea level high stand occurred ~252 ka BP. If supported with further data, this may be the first record of a high stand at these times recorded in the Caribbean, and suggests that sea level curves for the Yucatan Peninsula require some revision.