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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 462

Last Page: 462

Title: Geochemistry, Mineralogy, and Petrography of "Hooker" Ironstone Unit, Middle Ordovician Shellmound Formation, Northeast Georgia: ABSTRACT

Author(s): Annabelle Foos

Article Type: Meeting abstract


The "Hooker" ironstone poses a unique opportunity for the study of the origin of oolitic ironstones in general because both unaltered chamosite facies and altered hematite facies, along with the transitional facies, are preserved in one unit. This unit is composed of the following three facies: (1) a gray chamosite mudstone containing chamosite ooids, (2) a brown chamosite mudstone containing altered chamosite ooids, and (3) a carbonate-cemented hematitic grainstone containing hematite ooids. The vertical association of these facies suggests that the hematitic grainstone is an alteration product of the gray chamosite mudstone and that the brown chamosite mudstone represents the transition zone between the two.

The mineral assemblage in the gray chamosite mudstone (chamosite, pyrite, and ankerite) suggests that the chamosite ooids formed by early diagenesis in the zone of sulfate reduction below the sediment water interface. The gray chamositic mudstone grades upward into the brown chamositic mudstone. The minerals of this facies (chamosite, pyrite, and hematite) represent a disequilibrium assemblage. The altered ooids are composed of fine-grained mixtures of chamosite and hematite. The brown chamosite mudstone resulted from the partial oxidation of the gray chamosite mudstone due to exposure to the aerobic zone of active bioturbation. There is a sharp boundary between the brown chamositic mudstone and the hematite grainstone. The ooids and interclast in the hematitic grainstone were derived from the subaqueous erosion of the brown chamositic mudstone. Exposure of altered chamosite ooids to oxygen-rich marine water would accelerate the oxidation of the ooids producing hematite ooids.

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