Silica filled cysts in the Chattanooga Shale (Late Devonian) are a potential source of "in situ" silt to sand-sized quartz grains. Although these quartz grains may appear detrital at first glance, and thus extrabasinal, they actually formed during early diagenesis. Radiolaria that were a significant part of the biogenic sediment contribution dissolved while being buried a few decimeters within the sediment, and as a result pore water silica concentrations rose to saturation and chalcedony precipitated in available pore spaces, such as algal cysts. The following four pictures illustrate these grains.
Schieber, J., 1996. Early diagenetic silica deposition in algal cysts and spores: A source of sand in black shales? Journal of Sedimentary Research, v. 66, p. 175-183. download PDF file (left click on link and click on "Save as"...)
Schieber, J., Krinsley, D., and Riciputi, L., 2000, Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature, v. 406, p. 981-985. download PDF file (left click on link and click on "Save as"...)
Silica Filled Cysts in the Chattanooga Shale
