Summary

A Holocene carbonate beach to eolian dune sequence is a significant geomorphic feature along the Isla Cancun barrier island. In some areas, the beach passes a short distance landward into an eolianite ridge up to 17 m high. The dunes may be described as coalescing, low, parabolic dune sand sheets forming a transverse ridge parallel to the present shoreline. Holocene sediments on Isla Cancun are made up of the Blanca eolianite which forms a veneer over the slightly older Cancun eolianite on parts of the island. Sediments are composed of well-sorted, oolitic fine- to medium-grained carbonate sand and are nonlithified to moderately well lithified.

Cancun eolianites contain three stratification types: 57% of the dune system is climbing translatent stratification, 36% is grainfall lamination, and 7% is sandflow cross-stratification. Biogenic structures in the Cancun eolianite include root molds and rhizoliths, plant-related eroded depressions, and crab burrows.

The lateral transition between the aggrading Blanca beach/dune system into adjacent eolian dunes appears to have a fairweather equilibrium profile with no abrupt break between the back beach and toe-of-dune. The quantity of coarser shell material and animal burrows decreases significantly into the dunes where eolian processes dominate over marine processes. Sediment sorting, as well as root burrowing, increases dramatically as eolian processes become dominant.

During strong storms and hurricanes, marine processes may reach well into the eolian realm. Strong storms can also remove the beach sediment in front of dune deposits, allowing the swash or surf to directly erode the dunes. During rare, exceptionally severe hurricanes the dune complex can be truncated. This interaction between marine and eolian processes forms a time-transient boundary (interfingering) in beach/dune systems. Marine-produced features that punctuate eolian deposits are: 1) storm-erosion deposits (dune-front erosion), and 2) marine-storm deposits (dune truncation).

Pleistocene/Holocene carbonate eolianites are known worldwide to form thick deposits (up to 100 m or more) and laterally extend kilometers in a strike direction and hundreds of meters in a dip direction. Early lithification stabilizes the eolianites and enhances preservation potential. During the Pleistocene and Holocene, carbonate eolianite deposits commonly formed thicker sequences than time-equivalent marine sand deposits. It is possible that ancient, cross-bedded, well-sorted ooid or peloidal eolianites have been misinterpreted as marine sands.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah