Septarian Nodule Fragment (Turtle Stone / Mud Crack Septarium)
Inorganic sedimentary rock structure (Septarian nodule/concretion showing calcitic shrinkage cracking) • Pseudofossil / Diagenetic Concretion (often misidentified as a body fossil)
Geological Period
Upper Cretaceous (typically Pierre Shale Formation or equivalent)
Estimated Age
70 to 100 million years
Preservation Type
Mineral precipitation / Cementation in desiccated sedimentary nodule
Condition Assessment
Fair (Incomplete fragment of a larger septarian nodule with exposed cleavage plane showing internal mineralization).
Taxonomic Classification
Non-biological inorganic structure (Sedimentary Concretion)
Morphological Description
Angular fragment displaying internal septa (veins) filled with crystalline calcite and chalcedony/quartz, surrounded by tan-brown claystone/siltstone mud matrix fragments. Shows characteristic angular blocky shrinkage patterns typical of septarian cracking.
Rock Matrix
Ferruginous claystone/siltstone matrix with white calcite crystal deposits and bluish-grey silica/microcrystalline quartz linings.
Formation & Location
Pierre Shale (Western Interior Seaway, USA) or similar Cretaceous shale deposits in Utah, Madagascar, and Morocco.
Size & Dimensions
Approximately 8 cm x 4 cm x 3 cm (small hand-sized fragment).
Value & Rarity
Estimated Market Value
$5 - $15 USD
Auction Estimate
$10 - $20 USD
Rarity Assessment
Common (Widely distributed in marine shale formations globally)
Scientific Significance
Provides insights into ancient marine basin geochemistry, compaction, and fluid movement during early diagenesis in muddy shelf environments.
Preparation Recommendations
No chemical preparation needed. Can be polished on cut faces to enhance color contrast between the brown mudstone matrix and white calcite veins.
Notable Features
Distinctive dark grey and white calcite/quartz mineral vein filling the central fracture void against the yellowish-brown iron-stained matrix blocks.