High-Resolution Sequence Stratigraphic Setting of Mississippian Eolianites, Appalachian and Illinois Basins
L.B. Smith, A. Al-Tawil, J.F. Read, 2001. "High-Resolution Sequence Stratigraphic Setting of Mississippian Eolianites, Appalachian and Illinois Basins", 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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Carbonate eolianites are abundant in the lower Chesterian (Upper Mississippian) succession in the Appalachian and Illinois Basins. The eolianites are quartz-peloid grainstones composed of well-rounded, very fine to fine sand-size peloids, whole ooids, broken ooids (hat have been re-rounded, skeletal fragments, and generally finer-grained subangular frosted quartz, Eolian deposits are 1 to 8 m thick and tens of meters to many kilometers wide. They have wedge sets of planar and tangential, sharply defined, inverse-graded laminae with dips of up to 20 degrees.
A high-resolution sequence stratigraphic framework was generated using the available biostratigraphy and closely spaced stratigraphic sections and core, and tracing regional disconformities marked by breccia, calcrete, and soil horizons between the sections. Eolianite units backstep within the transgressive part of the Chesterian supersequence and are absent from the highstand part, which is dominated by siliciclastics and likely formed in a more humid setting. Within third-order and fourth-order sequences, the eolianites occux updip in disconformity-bounded parasequences. In the transgressive and early highstand systems tracts of sequences, eolianites overlie exposure surfaces and are preserved in the transgressive parts of the parasequences. In the late highstand parts of sequences, eolianites are preserved in the regressive parts of parasequences and are capped by sequence-bounding disconformities marked by breccia and calcrete.
The abundance of carbonate eolianites in the Upper Mississippian is likely due to seasonal semiarid climate and moderate-amplitude fourth-order eustatic sea-level chaoges. The reservoir potential of the eolianites is limited by tight packing and calcite cementation. However, recognition of the eolianites is critical to understanding the vertical and lateral distribution of reservoir facies within the sequence stratigraphic framework because they indicate subaerial conditions and commonly mark subtle sequence boundaries.
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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.