Allogenic and authigenic collophane in Lower Silurian sandstones

Cores of Lower Silurian sandstones from the Appalachian basin of northeastern Ohio commonly contain both allogenic and authigenic collophane. The sandstones, equivalents of the Grimsby Sandstone of the Medina Group of western New York State, belong to a deltaic, nearshore, fluvial sequence and are characteristically very tight and well cemented by quartz. In addition to standard petrographic examination, the common occurrence of collophane was ascertained through the study utilizing electron microprobe and x-ray dot-mapping. Allogenic collophane, common as shell fragments and organic-rich clastic pellets, is rarely observed as detrital grains. Authigenic collophane occurs as replacements of clays (illite and Fe-chlorite) and iron oxide, as overgrowths and rims on etched detrital quartz grains, and also as precipitates and/or replacements on and about fossil shell, probably clay, and other nucleation sites. Core analysis porosity values compared with collophane occurrences show very high and close correlations of authigenic collophase abundance to greatest amounts of porosity. The presence of authigenic collophase thus appears to inhibit syntaxial quartz overgrowths in much the same manner as authigenic clays, preserving, at least in part, primary and/or secondary porosity.