Silica cement source and porosity preservation for Ohio's Lower Silurian sandstones

The Lower Silurian sandstone sequence of Ohio is comprised of sandstone, siltstone, and shale, commonly recognized as three more-or-less distinct informal subunits. The lowermost is called white Clinton, the intermediate is red Clinton, and the uppermost is stray Clinton. These subunits have been correlated by others, respectively, as the Cabot Head Shale, the Grimsby Sandstone, and the Thorold Sandstone of western New York. The sandstones and siltstones are generally tight and highly cemented by pervasive secondary silica. Original depositional clay was probably mixed-layer illite-smectite which, as indicated by x-ray diffraction, was transformed to the present discrete illite and very minor iron-chlorite. Remnant expandable illite-smectite is present as less than 5% total clay. The released silica of this transformation probably provided much, if not most, of the silica cement. The remnant mixed-layer illite-smectite occurs within the sandstones where apparently it was preserved by the protective framework of detrital sand grains. Other probable minor sources of silica cement were pressure solution of original quartz grains and dissolution of allogenic feldspar, as exhibited in this section and by scanning electron microscopy. Porosity increase toward the middle of some sandstone intervals suggests migration of silica from transformed overlying and underlying shales. Porosity was preserved, in part, by collophane grain coatings inhibiting authigenic silica growth, as exhibited by petrography and scanning electron microscopy, and by clay overgrowths, as emonstrated in earlier study.