SEDIMENTARY PETROLOGY OF THE FORT PILLOW SANDSTONE, WILCOX GROUP (EOCENE), CROWLEY'S RIDGE, ARKANSAS

Pervasive quartz cement in the Fort Pillow Sand along the northeastern margin of Crowley’s Ridge is anomalous for sand units in the Wilcox Group, most of which are completely unconsolidated. Exposures of pervasively cemented sandstone occur between Lake Erierson State Park and Highway 412 in Green County. Within this belt, unconsolidated sands are absent. Sandstone units range to seven meters in thickness and are lenticular. Cross-stratification is prominent and plant rootlets occur in the sandstone at most localities. Sand grains are well rounded and are in the very fine to medium-sand size range. Sorting is excellent at specific localities though the size range from locality to locality is substantial. Detrital quartz composes over 95 percent of the grain constituents. Feldspar, muscovite, tourmaline, magnetite and chert rock fragments are present but are extremely rare. Quartz cement occurs as syntaxial overgrowths, as equant microcrystals, and as calcedonic druse. Overgrowths proceeded the precipitation of quartz microcrystals. Chalcedonic cement formed last. Porosity, determined petrographically, forms from one to four percent of the rock. The remainder of the intergranular areas is filled with quartz. The Wilcox Group on Crowley’s Ridge was never subjected to deep burial and elevated temperatures associated with deep burial. Most of the Wilcox sands, lateral to the cemented Fort Pillow, are unconsolidated and completely free of cement. Elevated temperatures (above 60 C) normally associated with quartz cementation must be attributed to mechanisms other than deep burial or a regionally high geothermal gradient. Magnetic and gravity surveys of the New Madrid Seismic zone in the upper Mississippi Embayment have revealed several strong anomalies interpreted to be plutons. The western margin of the magnetically-defined Paragould pluton, is directly beneath the silicified facies of the Fort Pillow Sandstone and may have provided a source of heat and of silica for the cement.