--> Abstract: Recognizing Ancient Carbonate Eolianites, by D. Loope, F. E. Abegg, and P. M. Harris; #90937 (1998).

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Abstract: Recognizing Ancient Carbonate Eolianites

LOOPE, DAVID, Geosciences, University of Nebraska, Lincoln, NE; F. E. ABEGG, Chevron USA Production Company, Midland, TX; P.M. HARRIS, Chevron Petroleum Technology Company, La Habra, CA

Carbonate eolianites have been described from Mississippian, Pennsylvanian, Permian, Jurassic, and Quaternary strata, but many examples may have been overlooked because few carbonate sedimentologists are familiar with their diagnostic features. Climbing translatent stratification, produced by the migration and climb of wind ripples, is the only commonly occurring and completely trustworthy criterion for the recognition of ancient eolianites. Inverse grading and rare ripple foreset laminae are best observed in climbing translatent strata that are >5 mm thick and where exposures cut stratification at a very low angle. Most climbing translatent strata, however, are <5 mm thick and consist of very fine sand-medium sand couplets that lack obvious grading and ripple foreset laminae. Such coarse-fine alternations have been misinterpreted by some workers as tidal laminations.

Additional criteria for recognition (Table 1) are rare or nondiagnostic, thus multiples lines of evidence are required to support an eolian interpretation. Air flows lack the competence of aqueous flows, therefore eolianites typically lack clasts larger than 3-4 mm in diameter; coarser clasts may be found as basal lags. Lack of large-scale cross stratification should also not be used to disprove an eolian origin; not only are some sets of eolian cross-strata quite thin (<15 cm), but eolianites can be composed solely of low-angle climbing translatent strata.

Subaerial exposure features can occur in both eolian and subtidal strata. Calcretes occur within and below many eolianites, as indicated by scattered rhizoliths, alveolar texture, and vadose pendant cements. Depleted d13C whole-rock values in eolian strata largely result from concentration of 12C in such vadose features.

Diagenesis is not diagnostic of eolian deposition, but is important in porosity evolution. Early meteoric diagenesis, as observed in Holocene islands, results in moderate to poor cementation. Meteoric cementation is typically blocky low-magnesium calcite, with common meniscus cements in the vadose zone. Cement volumes are greatest at the water table, with the vadose and upper phreatic zones containing similar volumes of cement. Cements are more evenly distributed in the phreatic zone, but interparticle porosity is rarely completely occluded. Pleistocene eolianites are more cemented at the surface and at the water table, but not necessarily more cemented in the vadose zone in between. Greater amounts of recrystillization and dissolution produce more moldic and locally vuggy porosity. Many Paleozoic carbonate eolianites lack porosity, as limited meteoric cementation allowed compaction to destroy interparticle primary porosity.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah