Recognizing Eolianites in thin Section. Review and Case Study—the Lower Cretaceous Chambotte Formation, SalÈVe Chain, Southeastern France
Pascal Kindler, Eric Davaud, 2001. "Recognizing Eolianites in thin Section. Review and Case Study—the Lower Cretaceous Chambotte Formation, SalÈVe Chain, Southeastern France", 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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Until now, carbonate eolianites have been identified only in upper Paleozoic, Jurassic, and upper Cenozoic strata. In this paper, we fust examine the relevance of the compositional, textural, and early diagenetic criteria commonly used for recognizing eolianites in thin section. Compositional criteria include grain diversity and the occurrence of broken and abraded ooids. Textural criteria comprise sorting and grain size, rounding and sphericity, packing and grain contact types, and the occurrence of thin, inversely graded laminae. Early diagenetic fabrics are represented by spar-filled micrite envelopes, spiny ooids, internal sediment, and peculiar sparry cements, all of which characterize the vadose meteoric realm. This review is based on both existing literature and petrological data collected from the Bahamas islands. Among these criteria, only thin, inversely graded laminae are an unambiguous indicator of eolian deposition. These small structures are relatively rare, however, in bioclastic and/or middle Pleistocene eolianites from the Bahamas. This is probably due to the absence of quatz, the good sorting of the parent material, and sediment disruption by animals, plants, or physical processes such as rainfall and waves. Our Bahamian data further suggest that grain diversity, sorting, and grain morphology can be considered as suspect criteria for recognizing eolian from marine grainstones in thin section. The occurrence of abraded and broken ooids, the lack of large (> 2mm) grains, relatively tight packing, and the presence of concavo-convex grain contacts in an open fabric appear to be more reliable. Diagenetic fabrics typical of the fresh-water vadose zone are certainly not an indicator of eolian deposition but can indeed reinforce an eolian interpretation suggested by other lines of evidence.
Application of these criteria to some key beds in the Chambotte Formation (Lower Cretaceous of the Jura realm, France), devoid of sedimentary structures at outcrop scale and up to now considered as marine deposits, indicate an eolian origin. These peloidal and bioclastic grainstones positively meet all the criteria listed above, are characterized by a negative shift in whole-rock δ13C, and occur immediately above well-identified beach deposits. These beds thus represent the first carbonate eolianites of Cretaceous age. This new interpretation requires revisions of regional sequence and cyclostratigraphic schemes for (his time interval.
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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.