ORIGIN OF SEDIMENTARY IRONSTONES IN THE LIGHT OF SEQUENCE STRATIGRAPHY OF (SILURIAN), BIRMINGHAM IRON ORES, ALABAMA, USA

Although chamosite ooids have been recovered from modern sediments, there is presently no adequate modern analog for the Phanerozoic oolitic ironstones. Thus, theories of origin have been difficult to constrain. The Birmingham ironstones occur in two stratigraphic settings within the Red Mountain Formation, either as thin condensed beds on marine flooding surfaces, or as thicker crossbedded deposits within incised channels immediately below the flooding surface. Ironstones were deposited on the shoreface and always introduce a pronounced basinward facies shift that defines a sequence boundary. During marine transgressions, clastics were held at the coast and thin, condensed, ironstones formed on the shoreface. They were preserved in lowstand channels or distributed as a thin veneer over the flooding surface. Ironstones consist of abraded and replaced skeletal debris, relict coarse clastics and hematite-coated ooids (originally chamosite). During highstand, thicker, shallowing-upward successions of shales and sandstones were deposited in deeper water on a storm-dominated shelf. Even on the shelf thin iron coatings are common on sand grains, and less common on silts and clays. Iron was likely delivered to the basin as insoluble grain coatings. It was then leached, especially from muds, and reprecipitated to form chamosite ooids, probably through a series of bacterially-mediated processes. Hematite is a pseudomorph after chamosite and formed during later oxidation, either penecontemporaneously or during diagenesis. From studies of the Silurian-age, Birmingham ironstones it is evident that facies relations and sequence stratigraphy place strict constraints on the depositional environment.