ABSTRACT

Nine distinct facies, differentiated on the basis of field studies, petrographic modal analyses, and mineralogy, comprise the carbonate members of the Swope Formation (Middle Creek and Bethany Falls Limestones) and the Dennis Formation (Canville and Winterset Limestones). These facies are 1. Osagia biomicrite; 2. Cryptozoon biomicrite; 3. brachechino biomicrite; 4. fusulinid biomicrite; 5. argillaceous, pyritic biomicrite; 6. micrite; 7. algal biosparite; 8. Cryptozoon biolithite; and 9. mixed oolitic-intraclastic facies.

The majority of the carbonates are calcitic, and recrystallization of microcrystalline calcite to coarser grained mosaics is widespread. Dolomite occurs as scattered grains in matrices of microcrystalline calcite, as replacements of allochems and burrow fillings, and as cavity fillings. Completely dolomitized rock units are rare. Four suites of clay minerals include various combinations of detrital illite, mixed-layered 10A expanding-non-expanding clays, chlorite, free -expanding clays, clay-size muscovite, and authigenic kaolinite that fills voided ooid molds.

Overlap of facies illustrates a transgression of 70 to 110 miles during deposition of the Middle Creek Limestone. The types, temporal sequence, and geographic position of facies within the Bethany Falls Limestone are remarkably similar to those of the Winterset Limestone. The boundary between lower and upper portions of each member is marked by a shaly interval and an abrupt change in carbonate facies array. These shale seams mark the change from transgression to regression in each cycle and represent excellent time-line markers. The sea bottom on which these carbonates accumulated sloped southwestward and was locally interrupted by shoals and deeper embayments. Paleocurrents flowed north-eastward in late Bethany Falls time.

Depth of water was the major control over facies arrangement, and changes in water depth were responsible for facies migration. To a lesser degree, variable amounts of terrigenous mud contributed to the differentiation of facies. Contrary to some earlier ideas, fusulinid-bearing sediment appears to have accumulated in water no deeper than that in which contemporaneous facies formed.