Eolian Stratification and Beach-to-Dune Transition in a Holocene Carbonate Eolianite Complex, Isla CancÚN, Quintana Roo, Mexico
Robert G. Loucks, William C. Ward, 2001. "Eolian Stratification and Beach-to-Dune Transition in a Holocene Carbonate Eolianite Complex, Isla CancÚN, Quintana Roo, Mexico", Modern and Ancient Carbonate Eolianites: Sedimentology, Sequence Stratigraphy, and Diagenesis, F. E. (Rick) Abegg, David B. Loope, Paul M. (Mitch) Harris
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A Holocene carbonate beach to eolian-dune sequence is a significant geomorphic feature along the Isla Cancun barrier island. The dunes are low, coalescing, parabolic dunes forming a ridge parallel to the present shoreline. The dunes comprise the “Bianca eolianite” and the slightly older “Cancun eolianite.” The Blanca eolianite is similar in age to the modern beach system. The more lithified Cancun eolianite, which forms excellent outcrop sections, contains 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 and Blanca eolianites include animal burrows and rhizoliths.
The general lateral transition between the modern beach system and the adjacent Blanca eolian dune system appears to have a fair-weather equilibrium profile with no abrupt break between the back beach and toe of dune. The quantity of coarse shell material and animal burrows decreases significantly into the dunes, where eolian processes dominate over marine processes. Sediment sorting and plant-root traces increase dramatically in the area where eolian processes become dominant. During strong storms and hurricanes, marine processes may reach well into the eolian realm, removing the beach sediment in front of dune deposits and allowing the swash or surf to erode the dunes directly. During rare, exceptionally severe hurricanes, the dune complex can be truncated, as is evident in the Cancun eolianite. This interaction between marine and eolian processes forms a time-transient boundary (interfingering) in the beach-dune systems. Marine-produced features that punctuate eolian deposits are: (1) dune-front conglomerates produced during dune-front erosion and (2) intradune marine storm deposits laid down during dune truncation.
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Carbonate eolianites had long been considered to be limited to the Quaternary, but a number of Mesozoic and Paleozoic examples have been documented in the past 15 years. Thus, an increased awareness of carbonate eolianites is required to properly interpret the rock record and to assess their spatial and temporal distribution. The papers of this volume will help geologists to: (1) recognize carbonate eolianites and understand their preservation potential—recognitional criteria for most carbonate environments are common knowledge, but this is less true for carbonate eolianites; (2) understand their sedimentologic and diagenetic variability—diagenesis of carbonate eolianites has important economic considerations. Whereas Quaternary eolian limestones are commonly porous, Paleozoic and Mesozoic examples are typically tight owing to compaction; (3) understand the Psilionichnus (marginal marine) and Scoyenia (nonmarine) Ichnofacies—carbonate eolianites are not devoid of trace fossils; (4) interpret them in a sequence stratigraphic framework—interpretations of relative sea level during eolian deposition can be difficult, as differences between transgressive, regressive, and deflationsourced eolianites are subtle. Thus, the placement of sequence boundaries within interbedded eolian and subtidal carbonate successions is not entirely straightforward.