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The AAPG/Datapages Combined Publications Database
Formation of Northern Louisiana Ironstones
A belt of Middle Eocene ironstones of the upper Claiborne Group extends from northern Texas across northern Louisiana into central Mississippi and central Alabama. Northern Louisiana ironstones occur as nearly pure goethite lenses, goethite in sedimentary boxworks, concretions cored with "greensands," and as goethite-rich sandstones. Ironstone lenses constitute in situ alteration of greensand to goethite. Iron leached from the greensand horizon is precipitation as goethite in pores of sands above the water table. The "greensand" was identified by previous workers first as glauconite and later as chamosite. The greensand of the Kilpatrick Iron Ore District of north central Louisiana is identified herein by x-ray diffraction as the 7Å, trioctahedral clay mineral, berthierine. Glauconite, chamosite, and berthierine originate under different environmental conditions. Glauconite forms by extremely slow accumulation on the outer continental slope, whereas berthierine forms in estuarine or prodeltaic environments. Thus the environment of formation of berthierine strongly correlates with the marginal marine to fluvio-deltaic facies environment of the Cook Mountain and Cockfield Formations. Therefore, we interpret the ironstone deposits of north-central Louisiana as being derived from oxidation of estuarine deposits that are portions of transgressive systems tracts.
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