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

To understand the mechanisms of formation and preservation of alteration rinds on the clasts in older sedimentary sequences, pebble-size clasts from the Lower Cretaceous Potomac Fm. were collected from the Puddledock quarry (Vulcan Materials Co.). Sandy gravel in the quarry contains siliceous metavolcanic clasts (rounded pebbles) with conspicuous alteration rinds. Rinds and cores of the clasts were subjected to mineralogical analysis by X-ray diffraction, elemental mapping by micro XRF, and optical petrographic analysis.

Irrespective of the size and composition of the clasts, the alteration rinds are generally uniform in thickness (~4 to 9 mm). In most cases, the clasts have a single outer band of alteration, but some have multiple bands. The alteration rinds typically have an outer light-gray to white band and surround a darker core. Some clasts also have a thin (~2 to 4 mm) outer coating of crystalline silica.

The cores of the clasts contain quartz (~54.5%), feldspar (~22.8%), kaolinite (~23%), hematite (~1.5%) and traces of anatase (~2.4%), by weight. The rinds (white bands) contain comparatively more quartz (~74%) and feldspar (~16.7%), and lesser amounts of kaolinite (~9.4%), hematite (~0.3%), and anatase (~1%). The enrichment of quartz and depletion of kaolinite, hematite and anatase in the rinds appear to reflect dissolution of feldspar and removal of clay, probably due to continued hydrolysis. Optical petrographic analysis and SEM-EDS revealed hydrous aluminum silicates that fill fractures and fissures in the cores but are depleted in the rinds of most specimens.

Concentrations of transition and post-transition metals (i.e. Fe, Al and Ti), along with sulfur, in cores of the clasts exceed those in the rinds. Alkalis do not show any trend in most clasts, but a few show an increased concentration in the rind, perhaps due to incomplete kaolinization.

Results indicate that the alteration rinds formed primarily by (1) partial removal of transition and alkali metals by hydrolysis due to chemical weathering during transport and/or after deposition, accompanied by (2) preferential removal of fracture-fill clay minerals in the rinds, followed by (3) preservation in a reducing environment and precipitation of crystalline silica from pore water during burial diagenesis.