GLAUCONITE, FRACTURED GRAINS, AND HEMATITE: SHALLOW-BURIAL DIAGENESIS IN MISSISSIPPI

The Cretaceous Eutaw and Paleocene Wilcox Formations form much of the characteristic red hillsides of east-central Mississippi. Amongst the silt and clay, well-indurated sandstones are uncommon and poorly preserved. Where found they usually occur as thin discontinuous lenses and laminae. The paragenetic sequence of events in these rocks is complicated with extensive dissolution of framework grains and many episodes of cementation.

Diagenesis of the shallow marine facies Eutaw Formation is dominated by reaction of glauconite. Iron oxides and hydroxides are the most common cements in the rocks. Meniscus and pendant textures are found, attesting to the shallow depth of diagenesis of the unit. In any one outcrop glauconite can occur from completely fresh, through a full range of oxidation and alteration, to being almost fully dissolved. Muscovite is common and shows the same range of alteration as does glauconite. There is evidence of recrystallization or neoformation of clays in muddy samples. Local opal-rich layers preserve unaltered glauconite. Fragments of diatom frustules in the opal demonstrate the biologic source for at least some of the silicon.

Glauconite in the Wilcox Formation is more thoroughly removed than in the Eutaw, producing an unusual swiss cheese-like texture in thin section. Spectacular hematite rims > 100 µm thick coat and cement framework grains. Reflected light petrography shows evidence of multiple generations of cement formation in what had once been centimeter-size voids. Iron oxides and opal occur together as a pore-filling mosaic, with the opal forming last in the center of the pores. Hematite is found outlining and differentiating polycrystalline quartz domain boundaries, and iron-precipitating microorganisms, probably fungi, are observed in thin sections of these rocks.

Many of the quartz framework grains in the Wilcox are highly fractured. Not all grains in any one thin section are fractured, and there is no relationship between framework grain size and likelihood of fracture. In the same thin section it is possible to find fractures filled with botryoidal-texture hematite, as well as thick hematite grain coats cross-cut and displaced by fractures. The timing and origin of the fracturing is unclear.