Recognition and Geologic Preservation of Ancient Carbonate Eolianites
David B. Loope, F.E. (Rick) Abegg, 2001. "Recognition and Geologic Preservation of Ancient Carbonate Eolianites", 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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Eolian dunes composed of calcium carbonate grains are today widespread along subtropical shorelines. The most definitive criterion for recognizing ancient dune deposits (regardless of original mineralogy) is inversely graded lamination produced by the migration and climb of wind ripples. Additional features useful for recognition of carbonate eolianites are rare (e.g., vertebrate trackways or raindrop imprints) or nondiagnostic, but provide supporting evidence. Eolianites typically lack clasts larger than 3 to 4 mm in diameter. Absence of large-scale cross stratification should not be used to disprove an eolian origin. Calcretes occur within and immediately below many eolianites, as indicated by scattered rhizoliths, alveolar texture, and vadose pendant cements. Depleted δ13C whole-rock values in eolian strata largely result from concentration of 12C in such vadose features.
Shorelines have high wind energy, and carbonate panicles have been produced in great abundance throughout the Phanerozoic, but the pre-Quaternary record of carbonate dunes appears to be meager. Poor preservation potential of topographically high dunes would contribute to the paucity of ancient carbonate eolianites. Preservation may be enhanced by cementation of dunes or if dunes climb during migration, burying strata below interdune surfaces. If subsidence is sufficient, dune strata will escape ravinement during subsequent transgressions.
Many upper Paleozoic carbonate eolianites may not have been sourced by beaches but by deflation. Broad exposure of subtidal deposits resulting from rapid regression across flat platforms or ramps would allow deflation of carbonate sediments. Deflation may have been more widespread in the geologic past, inasmuch as rhizoliths suggest that plants may not have adapted to mobile dune substrates until the Cretaceous.
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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.