DIAGENETIC PROCESSES INVOLVED IN THE FORMATION OF A STIGMARIA SAND CAST

Siliceous Carboniferous Stigmaria, roots of Sigillaria and Lepidodendron, were found in plowed fields located in eastern Ohio. Five petrified Stigmaria samples were carefully observed. Petrographic features strongly indicate that the root was preserved through a sand casting process after a mold was delicately formed, and the wood tissue was completely removed. The most common petrification process of gradual silica replacement is ruled out by the distinct crystal morphology of quartz grains, the abundant amount of pore space, and the presence of rock fragments and other silicate minerals such as muscovite. A few key observations significantly restrict the possible taphonomic processes for the fossil preservation. 1) The mold should be mechanically strong enough to withstand a normal strain factor of 2 with respect to the burial pressure. 2) The process of mold filling by sand and silt grains should be delicate enough to allow the formation of sorting and distinct lamination. 3) The clay content among the infilling sediments should be minimum and is not commonly observed under SEM. 4) The cement for the infilled sand is mainly silica in the form of quartz overgrowth. This required a silica-rich, alkaline pore fluid at a burial depth where the porosity was at least 10% or higher. 5) The silica cementation should only occur on the inside of the mold, but not on the outside of the mold, so that a cast of Stigmaria with a perfectly preserved external texture could be cleanly separated from the host rock. How could all of the above five conditions be satisfied in a sedimentary/diagenetic environment? This question is still unanswered.