Silicified Stromatoporoid or Colonial Coral / Pseudofossil Epikarst Geode Structure
Indeterminate (cf. Stromatocerium sp. or Tetradium sp.) • Invertebrate body fossil / Mold & Cast (or biogenic chert nodule with solution cavities)
Geological Period
Middle to Upper Ordovician
Estimated Age
450 - 460 million years
Preservation Type
Silicification (chert replacement) with extensive secondary dissolution, weathering, and epikarst cavity formation.
Condition Assessment
Fair to Poor (fragmentary, weathered, heavily dissolved, and broken from a larger mass).
Taxonomic Classification
Phylum Porifera (Class Stromatoporoidea) or Phylum Cnidaria (Order Tabulata); exact organism indeterminate due to replacement and weathering
Morphological Description
Irregular, highly porous, jagged chunks of silicified fossiliferous material featuring cavernous voids, rounded solution tubes, internal chambers, and coarse, weathered bone-like or sponge-like texture with faint concentric layering and white-to-tan chalcedonic quartz coatings.
Rock Matrix
Ordovician fine-grained limestone (argillaceous to bioclastic limestone of the Stones River or Nashville Group), stained with iron oxides (limonite/hematite).
Formation & Location
Stones River Group or Nashville Group (e.g., Carters Limestone or Bigby-Cannon Limestone), Marshall County, Inner Nashville Basin, Tennessee, USA.
Size & Dimensions
Individual fragments measure approximately 8 to 15 cm in length, typical of broken siliceous chert nodules or fossil heads within Ordovician bedrock.
Value & Rarity
Estimated Market Value
$10 - $30 USD (primarily of local interest, educational value, or landscape/aquarium stone appeal)
Auction Estimate
$10 - $25 USD
Rarity Assessment
Common (silicified Ordovician fossil fragments and weathered chert heads are abundant throughout the Nashville Basin of Tennessee)
Scientific Significance
Illustrates secondary silicification of Ordovician marine reef-building organisms and subsequent karstification processes in the Nashville Basin, providing local paleoenvironmental and hydrogeological context.
Preparation Recommendations
Clean with soft brush and water to remove loose soil; light acid etch (dilute HCl) can expose remaining carbonate if desired, though dry storage and gentle mechanical cleaning are sufficient.
Notable Features
Distinctive subterranean dissolution cavities, iron-stained chalcedony patina, and weathered bone-like/pumice-like porous structure created by ground-water action in deep bedrock.
Notes
Three similar fossils broken from a much larger limestone boulder. These fossils found in the bottom of a deeply-excavated pond in Marshall County, Tennessee; Inner Nashville Basin.