ANOMALOUS SILCRETE IN EOCENE STRATA ON THE FLANKS OF CROWLEY'S RIDGE, MISSISSIPPI EMBAYMENT

Tertiary deposits of the Mississippi Embayment are composed primarily of clay, uncemented sand, silt, and gravel. In marked contrast, pervasively-cemented, Eocene sandstones and siltstones crop out on the flanks of Crowley's Ridge in northeast Arkansas. Low temperature silica precipitation is indicated by an apparent shallow burial history and monophase fluid inclusions in quartz overgrowth cement. The intense, siliceous induration of these sediments suggests they should be termed silcretes.

During cementation, silica and iron saturation levels fluctuated resulting in alternating precipitation of iron oxides and silica. The Eocene silcrete preserves multiple episodes of phase transformations from amorphous silica to megaquartz. Syntaxial quartz overgrowths in crystallographic continuity with the detrital grains account for approximately 20% of the rock. The lack of meniscus or stalactitic cement, and the uniform cement thickness, suggests that the overgrowths formed in a saturated environment, either below the water table or at the interface of the vadose and phreatic zones. Repeated growth zonations are indicative of changing conditions – burial depth, redox, temperature, and silica and iron saturation. Late-stage diagenesis includes progressive ordering of microcrystalline quartz into more stable megaquartz overgrowths.

A cementation model under near-surface conditions provides an adequate supply of fluids, iron, and silica. The initial silica cementing fluids concentrated in a static-flow regime with adequate residence time for the precipitation of chalcedony. Subsequent recrystallization of microcrystalline quartz to megacrystalline quartz resulted in concentration of iron and quartz in microlaminae.

Experiments have shown that ferric hydroxide rapidly accelerates coagulation and polymerization of silica gel (Krauskopf 1959). Thus, dissolved iron may have acted as a catalyst to promote the precipitation of amorphous silica, chalcedony, or the recrystallization of progressively ordered quartz overgrowth cements. The presence of resistant silcrete contributed to the anomalous preservation of Crowley's Ridge in the unconsolidated sediments of the Mississippi Embayment.