MICROCHEMISTRY OF ALTERATION RINDS ON CLASTS IN GRAVEL OF THE CRETACEOUS POTOMAC FORMATION, PRINCE GEORGE, VIRGINIA

Weathering rinds are common in various terrestrial environments and have been used for decades as relative age indicators in Quaternary sequences. However, little study has been conducted on the development and preservation of alteration rinds on metamorphic and igneous clasts in older sedimentary sequences. Metamorphic clasts with conspicuous alteration rinds sporadically occur in a lower Cretaceous sandy gravel of the Potomac Formation exposed in the Puddledock quarry (Vulcan Materials Co). Polished thin sections of clasts were prepared for petrographic and elemental mapping by FIB-SEM with EDS. Major elemental compositions of bulk samples were determined by XRF. The clasts vary in lithology and color, and surfaces range from smooth to irregular due to flaking. Variable major element compositions (wt%), SiO₂ (59.2-87.81), Al₂O₃ (5.76-15.42), Fe₂O₃ (1.54-19.214) and K₂O (.075-12.1), reflect a variety of metamorphic rock types. The alteration rinds are generally uniform in thickness, typically 4 to 9 mm, irrespective of the size and type of the gravels. Most of the clasts have a single outer band of alteration, but some have multiple bands. The latter alteration rinds typically have an outer light-gray to white band and an inner darker band. Some clasts have a thin (typically 2 to 4 mm) outer coating of crystalline silica. The outer rinds (white bands) appear to reflect strong dissolution, probably due to hydrolysis, and are enriched in silica. The inner darker bands are enriched in aluminum (Al), and correspond to the highest level of Chemical Index of Alteration [molar (Al₂O₃/Al₂O₃+Na₂O+K₂O+CaO)*100]. Both outer and inner bands are devoid of significant concentrations of iron (Fe), although cores of some clasts are rich in Fe. Preliminary results are not entirely consistent with typical physical and chemical characteristics of weathering rinds, suggesting that the mechanism of formation may be more complex, such as: (1) formation of the darker Al-rich inner bands and outer light gray to white bands under oxidizing conditions concurrent with transport from a proximal source and deposition in the gravel bed, (2) preservation of weathering rinds in a reducing environment facilitated by burial diagenesis, and (3) further alteration by burial diagenesis with precipitation of silica from pore water on clast surfaces.