Petrology of Eolian Carbonates, Upper Hermosa Formation (Pennsylvanian), Southeastern Utah

Michael A. Boubin, David B. Loope

Wind-deposited calcarenite, or eolian carbonates, are present in the upper member of the Hermosa Formation (Missourian-Virgilian), southeastern Utah. This interval is a 250 m-thick carbonate-dominated unit composed of repetitively interbedded fluvial, eolian, and marine strata. Sharp contacts exist between subwave-base wackestones and overlying eolian carbonates. Skeletal and nonskeletal allochems are found in the eolian deposits. Cyclic marine/nonmarine units of the Hermosa are comparable in age and character to upper Paleozoic cyclothems of the Mid-Continent and were deposited during a period of rapid changes in sea level.

Thinly bedded red chert, chert nodules, and red-chert-replaced fossils are found in the marine carbonates. Bedded chert and nodules contain fossils partially or completely silicified. Many replaced fossils have thin rims of red quartzine and calcite cores. Evidence suggests that decomposition of fossil material and replacement (to pyrite) occurs in the phreatic zone, and subsequent oxidation and silicification occurs in the vadose zone of the mixed meteoric/marine coastal system. Red chert grains are a significant component (30-60%) of the coarse fraction of the bimodal eolian units. Red chert grains are concentrated at the base of eolian units and are most abundant where this base overlies wackestones and packstones rich in replaced fossils and nodules.

Field and petrographic evidence suggests that the eolian carbonates are the product of regression, exposure, and deflation of shallow marine carbonates. The coarse red chert in the eolian units indicates that selective silicification in the marine carbonates occurred early.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.