Phosphate-Enriched Fossiliferous Phosphorite Nodule / Fossil Conglomerate
Indeterminate marine skeletal fragments within phosphatic / iron-stained matrix nodule • Composite fossiliferous nodule containing invertebrate fossil fragments (brachiopods, bryozoans, crinoid debris)
Geological Period
Middle to Upper Ordovician (Caradoc / Sandbian-Katian)
Estimated Age
450 - 460 million years
Preservation Type
Phosphatization, iron oxide replacement, and physical abrasion. Skeletal elements are partially replaced or phosphatized within a dense, phosphatic matrix.
Condition Assessment
Fair to Good; outer shell exhibits significant stream tumbling, rounding, and weathering, but internal fossil texture and phosphatic nodular structure remain clearly visible.
Taxonomic Classification
Phylum: Brachiopoda / Echinodermata / Bryozoa (Indeterminate mixed marine invertebrate hash)
Morphological Description
A well-rounded, stream-abraded phosphatic nodule composed of aggregated dark brown to reddish-brown micro-grained matrix binding sub-angular to rounded fossil fragments and small chert/phosphatic clasts. Surface displays bumpy texture with prominent iron/manganese oxide patina and visible fossil shell debris embedded throughout.
Rock Matrix
Phosphatic grainstone / micro-conglomerate with limonite/hematite staining, derived from Ordovician phosphatic limestone/shale beds.
Formation & Location
Bigby-Cannon Limestone or Hermitage Formation, Nashville Group, Marshall County, Tennessee, USA (Inner Nashville Basin)
Size & Dimensions
Approximately 8 - 12 cm length, typical for stream-rolled phosphatic lag nodules in the Central Basin of Tennessee.
Value & Rarity
Estimated Market Value
$10 - $25 USD
Auction Estimate
$15 - $35 USD
Rarity Assessment
Common in the Ordovician phosphatic horizons of the Nashville Basin, Tennessee.
Scientific Significance
Represents an ancient shallow marine condensed sequence/lag deposit where sea-level fluctuations and currents concentrated phosphatic hardgrounds and skeletal remains in the Ordovician epicontinental sea of North America.
Preparation Recommendations
No heavy mechanical cleaning required; gently wash with a soft brush and water to remove loose mud/sand. Avoid acid or chemical solvents which could dissolve carbonate fragments or alter iron oxides. Store in a dry environment.
Notable Features
Distinctive smooth, stream-tumbled egg/nodule shape showing dynamic transport history in modern streams (Flat Creek), combined with a dark, iron-rich and phosphatic mineralization characteristic of Ordovician hardground lag deposits.
Notes
Found in streambed of Flat Creek, Marshall County, Tennessee; Inner Nashville Basin. Several of these rounded fossils were found on various sand/gravelbars tops. Locations in these higher locations in the streambed indicate that these nodules are rolled along on the bottom in high flows but dropped on the top of gravelbars when they get in the upper part of the flow and can drop out.