Formation of Northern Louisiana Ironstones

R. Dehon, L. N. Glawe, P. A. Washington, L. M. Young, and E. A. Morehead
Department of Geosciences, The University of Louisiana at Monroe, Monroe, LA 71209

A belt of Middle Eocene ironstones of Cook Mountain timeequivalent formations extends from northern Texas across northern Louisiana into central Mississippi and central Alabama. Northern Louisiana ironstone occurs as nearly pure goethite lenses, goethite in sedimentary boxworks, concretions cored with “green sands”, and as goethite-rich sandstones. Ironstone lenses constitute in situ alteration of green sand to goethite as well as transportation and precipitation of goethite in pores of underlying sands above the water table. The green sand was identified by previous workers first as glauconite and later as chamosite; however, the green sand of the Kilpatrick Iron Ore District of north central Louisiana is identified by x-ray diffraction as the 7/\, 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 Formation. 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.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana