Ichnology of Holocene Carbonate Eolianites of the Bahamas
H. Allen Curran, Brian White, 2001. "Ichnology of Holocene Carbonate Eolianites of the Bahamas", 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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Wind-deposited, terrestrial carbonate grainstones, formed concurrently with sea-level transgression, constitute the greater part of the Holocene rock record throughout the Bahama Archipelago and are particularly common along windward coastal reaches, Eolianite exposures of the Rice Bay Formation on North Point and along the Hanna Bay cliffs on San Salvador are characterized by well-preserved physical sedimentary structures, most notably large-scale cross-stratification and distinctive, millimeter-scale, inversely graded lamination couplets, along with other mesoscale structures. These carbonate eolianites also typically contain a diverse assemblage or ichnocoenosis of plant and animal trace fossils. Structures formed by plant roots are nearly ubiquitous in Bahamian Quaternary limestone facies and commonly occur in these Holocene eolianites. In addition, the above-ground parts of plants and trailing roots may form distinctive trace fossils along the bedding planes of eolianites. Animal trace fossils, including Skolithos linearis formed by tube-dwelling insects and/or arachnids, small, irregular burrows formed by insects or insect larvae, large cluster burrows formed by digger wasps, large stellate burrows probably formed by burrowing bees, and small burrows likely representing ant nests, also characterize these eolianites. Indeed, the dunal ichnocoenosis commonly exhibits a higher level of diversity than that found in adjacent shallow subtidal and intertidal-supratidal environments, and the burrowing activity of several tracemakers of the dunal ichnocoenosis can produce ichnofabrics distinctive to carbonate eolianites. In outcrops of limited exposure or in core samples, the occurrence of individual trace fossils generated by invertebrates or of an ichnofabric should not be used as evidence to rule out an eolian environment.
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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.