A PETROGRAPHIC COMPARISON OF THE SILURIAN-AGE OOLITIC IRONSTONES OF THE KEEFER FORMATION AND BIRMINGHAM MEMBER OF THE RED MOUNTAIN FORMATION

This study compares the petrographic differences observed between the Keefer Fm. (middle member Clinton Group) in West Virginia and the Birmingham member of the Red Mountain Fm. in Alabama. The Keefer Fm. is found in New York and extends to Tennessee while the Birmingham member extends from northern Alabama to Tennessee. The two units look nearly identical in hand sample. The environment of deposition for both units is shallow marine; sub-tidal with clear to slightly turbid water and a hard to firm substrate with normal salinity. Both units are moderately indurated, hematite-stained sandstones with interbeds of oolitic ironstone. Ooids are visible in both samples with the aid of a hand lens. Both sequences are intertongued with a series of sandstones and shales that mark the transgressive and regressive sequences that occurred during the Silurian. In thin section, the Keefer Fm. oolitic ironstone can be described as an oosparite under the Folk (1962) carbonate classification scheme. A representative sample is composed of 63 (modal) % ooids, 21% calcite spar, and 15% intraclast of sub-rounded quartz. The ooids have been entirely replaced by hematite cement and are ellipsoidal in shape. Mollusk and echinoderm bioclasts (replaced by calcite spar) were minor components ( < 1%). The Birmingham oolitic ironstone contained less ooids and is almost entirely composed of hematite cement and bioclasts. The ooids were originally formed from iron-rich clay, possibly bertherine or chamosite. The iron in the ooids has been oxidized to produce a deep red hematite cement and the ooids have the same ellipsoidal shape as found in the Keefer Fm. oolitic ironstone. The bioclasts are echinoderms, bryozoans, trilobites, and brachiopods. In the Keefer Fm., the bioclast void spaces have been filled but not replaced by hematite cement. It is possible that the hematite cement that filled these void spaces came from the ooids. In the Birmingham, the bioclast have been replaced by calcite spar.