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Abstract

The Red Mountain Formation is an unconformity-bounded unit including all Silurian strata in the Appalachian Valley and Ridge province of Alabama and Georgia. It is also host to the well-known Birmingham iron-ore field. The formation is divided by paraconformities into six members that generally coincide with depositional sequences. From biostratigraphy, the lower four members are established as Llandoverian: Taylor Ridge (Rhuddanian), Duck Springs (? early Aeronian), Birmingham (middle Aeronian-early Telychian), and Ruffner (late Telychian). Upper members are: Rocky Row (Wenlockian) and Sparks Gap (Pridolian). Facies indicate deposition on a storm-dominated shelf with coarse-grained, cross-bedded sandstone in the shoreface passing seaward into hummocky cross-bedded sandstone and shale on the inner shelf, and interbedded shale and graded sandstone or limestone (storm beds) on the outer shelf. Unless truncated by erosion, all Llandoverian sequences consist of thin retrogradational facies successions in transgressive systems tracts and thick progradational successions in highstand systems tracts. Accommodation on the shelf was provided by a combination of flexural subsidence, driven by tectonic loading of the Appalachian orogen during waning stages of the Taconic orogeny and glacial eustasy. It is possible to recognize similar lithofacies and sequences at least as far north as New York State.

Coarse-grained, ferruginous, cross-bedded sandstones (ironstones) occur as sharp-based shoreface facies associated with sequence boundaries; at the base of the transgressive systems tract, or making up the lowstand systems tract or falling stage systems tract. Such shoreface deposits are commonly highly condensed with ooids of hematite-chamosite and skeletal debris coated and replaced by hematite. Mineralization was evidently favored by periods of sediment starvation and reworking on wave (transgressive systems tract) and tidal (lowstand systems tract) ravinement surfaces. However, the richest ores in the Birmingham district (Big and Irondale seams) developed in a falling stage systems tract apparently as a consequence of meteoric diagenesis during forced regression.

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