PRESERVATION OF MIDDLE PENNSYLVANIAN RADIOLARIANS IN EARLY DIAGENETIC CARBONATE CONCRETIONS

Carbonate concretions occur in the Excello, Little Osage (Turner Mine Shale of Illinois) and Oakley shales of the Midcontinent and Illinois basins. These shales are laminated, black, organic- and nonskeletal phosphate-rich units deposited during sea level highstand in an anoxic environment. They represent a condensed interval with concentrations of pelagic fossils, including fish teeth, conodonts, foraminifers and radiolarians. Concretions formed by growth of authigenic carbonate cementing primary pore space of detrital and organic materials at or very close to the sediment-water interface. Porosities of greater than 70% may have been present in the unconsolidated sediment at the time of concretion formation. The authigenic carbonate cements, along with pyrite, occurred early; probably around the time of bacterial sulfate reduction. This early cementation prevented later diagenetic water from altering the sediments, allowing for exceptional preservation of fossils. Cementation occurred before compaction, as indicated by preservation of radiolarians, unbroken conodont elements (both rarely found in the surrounding shale) and shale draping around the concretions. Living radiolarian tests consist of biogenic opal, which is unstable at elevated P and T. The transformation of opal to quartz follows the series: globular opal → opal CT → low temperature tridymite → low temperature quartz. Preservation by casts of coarse- and fine-grained quartz are very common. Some casts consist of both silica and quartz. Preservation by pyrite as both casts and internal/external molds, occurs where silica is completely dissolved; possibly occurring during cementation. In some concretions, radiolarians are preserved as phosphatic internal molds/casts and as radial fibrous cements. This may have occurred during and shortly after sediment deposition, but before cementation. Extremely well preserved hydrocarbon casts probably occurred as organic material was mobilized after cementation.