Carbonate Eolianites—Depositional Models and Diagenesis
F.E. (Rick) Abegg, David B. Loope, Paul M. (Mitch) Harris, 2001. "Carbonate Eolianites—Depositional Models and Diagenesis", 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 contain sedimentary structures that are similar to their siliciclastic counterparts (see Loope and Abegg, this volume), but they differ in several fundamental ways. Larger carbonate grains can be transported by saltation owing to platy grain shapes, which reduce their threshold of motion, or to intraparticle and micro porosity, which reduces their bulk density. Whereas siliciclastics may form vast inland ergs, Quaternary carbonate eolianites accumulate almost exclusively as coastal dunes. Ancient carbonate eolianites, however, may have formed farther inland, from deflation of carbonate sediments. Coastal dunes can form at any time during a transgression-regression cycle, whereas inland siliciclastic ergs form preferentially during lowstands. Siliciclastic eolian strata typically lack significant early cementation, whereas meteoric cementation and well-lilhified calcrete paleosols may improve the preservation potential of carbonate eolianites. The preservation potential of coastal eolian dunes is reduced, however, because of high rates of coastal erosion.
Models for Quaternary carbonate eolianites differ depending on the shelf depositional profile and climate. Deposition of carbonate coastal dunes on steep-rimmed platforms may take place at any time during a sea-level cycle except during lowstand. Eolianite deposition ceases during sea-level lowstands when the shoreline drops below the platform edge and meteoric waters quickly cement the stranded sediments. Because a beach source for coastal dunes is absent, karst surfaces and paleosols represent lowstands on the platform top. Deposition of carbonate coastal dunes may take place any time during a sea-level cycle on a ramp, because the carbonate factory operates continuously. Climate is important because it influences how rapidly dunes are cemented and their potential to migrate far inland. Humid climates are marked by significant vegetation and meteoric cementation, which restrict carbonate eolianite deposition to a zone immediately adjacent to the shoreline. Arid climates are marked by minor vegetation and meteoric cementation, which allows coastal dunes to migrate up to 85 km inland (Williams and Walkden, this volume).
Resolving the sequence stratigraphy of eolian-subtidal successions is difficult because of the non-unique relationship between the formation of carbonate eolianites and relative sea level. In addition to the sedimentary characteristics of the eolianites, pronounced basinward shifts in lithofacies and strongly developed paleosols mark features that may indicate sequence boundaries. A basinward shift in lithofacies indicates a relative sea-level fall and is an important criterion for identification of a sequence boundary. Strongly developed paleosols indicate cessation of eolian deposition and dune stabilization by vegetation. Such paleosols may indicate a sequence boundary capping prograding coastal dunes or waning of a deflationary source that may not be a sequence boundary.
Most pre-Pleistocene carbonate eolianites are nonporous, whereas Quaternary eolianites exhibit interparticle porosity enhanced by meteoric dissolution. The absence of significant porosity in most pre-Pleistocene eolianites is attributed to physical or chemical compaction during burial and cementation. Physical compaction is aided by a paucity of early cementation in carbonate eolianites. Factors limiting cementation include arid climate, short residence time of vadose waters, case hardening, and the dominantly calcite grain mineralogy of many pre-Quaternary examples. Significant chemical compaction occurs during burial in known Paleozoic and Mesozoic examples, totally obliterating any remaining porosity.
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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.