DIAGENESIS OF PELOIDAL FOSSIL IN OHIO ORDOVICIAN LIMESTONE

Ordovician red limestone samples from southern Ohio are studied on its diagenesis of hematite replacement. The limestone is a packstone with obviously layered accumulation of hematitic peloid smaller than 1 mm in diameter. The layers are about 5 mm to 10 mm in thickness and are separated by calcite-rich layers of 5 mm thick. Most of the peloids are elliptical in shape and have poor linear alignment. Thin section observation shows the peloid has various internal structures. Some are completely filled with hematite. Some have hematite shell but the interior are completely filled with calcite. Yet some have hematite, which outlined the framework structure of fossil but calcite filled the rest of the space. Careful observation by SEM revealed the detail texture of the peloids and the relation between the peloids and the matrix. Weak acid leaching indicates that most of the carbonate is calcite. Isolated dolomite rhombohedra of 0.1 mm size are commonly scattered in calcite. Elliptical peloid has hematitic shell separates calcite inside and outside. It is not uncommon to see the cleavages of calcite are consistent in orientation across the hematitic shell. Hematite crystal in the shell of peloid is platy in shape with flame-like edge. The crystal size is about 50µ and crystals are packed subparallel to make a dense layer of shell. Hematite crystal that substituted the internal fossil structure shows a different morphology. Equant shape crystals of 30µ size stacked up like a wall. In some cases, the grain size could go down to 5µ or smaller.

Based on the microscopic observation, a part of the diagenetic history of the hematite-rich limestone could be compiled together. The peloid shape fossils were deposited in alternating layer with other fossils in a deposition of fine laminated carbonate sediments. Iron bearing solution brought in hematite, which replaced the carbonaceous hard skeleton of the fossil during shallow burial. Dolomite rhombohedrons crystallized inside the peloids based on the availability of small among of Mg. Micritic calcite recrystallized in deeper burial, that also filled the rest of the open space in the peloids.