EPIGENESIS OF MOLLUSK-BEARING CARBONATE ROCKS OF THE CONESTOGA FORMATION (?ORDOVICIAN), KING OF PRUSSIA, PENNSYLVANIA

Rare siliceous internal molds of gastropods and nautiloid cephalopods were collected from the metasedimentary Conestoga Formation marble at King of Prussia (KOP), PA, by Florence Bascom, Louis Woolman, and J.E. Ives between 1890 and 1909. Preservation of such mollusks in the recrystallized carbonates of the Chester Valley, outside this locality, is unknown. Although the nautiloid internal molds are too poorly preserved for identification, their size and gross morphology are consistent with typical post-Cambrian forms.

In 1989, a similarly preserved sample with gastropod molds was collected at KOP by PennDOT geologist Theodore Gill during bridge construction. The sample was found in terra rossa derived from marble excavated during construction. Recently, rock cores were obtained next to the bridge, and oriented samples were taken from an outcrop beneath the bridge. Siliceous rock similar to the fossil-bearing material was collected from the cut slope beneath the bridge.

Petrographic analysis of the core material and examination of the hand samples indicates that both the siliceous fossil molds and the phyllitic, carbonate host rock (calcite, quartz, biotite, muscovite, pyrite) experienced the same epigenetic and/or metamorphic history. Initial deposition of the mollusk shells in shallow-marine carbonate sediment was followed by replacement of the sediment filling the shells with authigenic silica during diagenesis. Volume reduction of calcite and formation of stylolites during burial was followed by compressional, ductile deformation of the fossils and recrystallization of quartz and calcite. Later, coarse-grained calcite veins crosscut bedding. Lastly, 2mm-wide, brittle fractures formed, offsetting the fossils. Early Triassic exhumation promoted the development of solution cavities and was followed by reburial beneath Triassic rift sediments. Local flooding of solution cavities with hot siliceous hydrothermal fluids—possibly driven by a deep Mesozoic thermal event—saturated the walls of the cavities containing the fossil molds, caused crystallization of quartz on the walls of these cavities, and healed the small brittle fractures. Erosion of Triassic sediments and selective weathering of calcite spar to terra rossa left the quartz crystals, vein-filling quartz, and fossil molds intact.