Lecture Notes on Concretions

Definition and Formation

• Concretion: A hard, compact mass formed by mineral precipitation within the spaces between particles in sedimentary rock or soil.
• Typically found in sedimentary strata, forming early in the burial history of sediments.
• Concretionary cement often makes them harder and more resistant to weathering than the host rock.
• Formed by mineral precipitation around a nucleus, often organic (e.g., leaf, tooth, shell).

Distinction from Nodules

• Concretions: Formed from mineral precipitation around a nucleus.
• Nodules: Replacement bodies, different formation process.

Historical and Geological Curiosity

• Long regarded as geological curiosities.
• Misinterpreted as dinosaur eggs, pseudofossils, extraterrestrial debris, or artifacts.

Growth and Environmental Conditions

• Can grow by concentric or pervasive growth:
  • Concentric Growth: Successive layers of mineral around a core.
  • Pervasive Growth: Cementation occurs throughout the volume simultaneously.

Shapes and Sizes

• Varied shapes: Spheres, disks, tubes, grape-like or soap bubble-like aggregates.
• Range in size from microscopic to several meters in diameter.

Composition

• Commonly composed of a mineral present as a minor component of the host rock.
• Examples:
  • Sandstone/shale concretions: Often carbonate minerals like calcite.
  • Limestone concretions: Silica forms like chert or flint.
  • Black shale concretions: Pyrite.

Notable Concretion Types

Septarian Concretions

• Rich in carbonates, with angular cavities or cracks (septaria).
• Found in various mudstones, often marine shales.
• Formation debated; may involve dehydration, syneresis, or excess pore pressure.

Cannonball Concretions

• Large spherical concretions, resemble cannonballs.
• Found in North Dakota, Utah, Wyoming; formed by early cementation by calcite.

Elongate Concretions

• Form parallel to sedimentary strata.
• Provide information on past fluid flow and local permeability trends.

Additional Types and Features

• Hiatus Concretions: Stratigraphic history of exhumation, exposure, and reburial; encrusted and bored by marine organisms.
• Claystones/Fairy Stones: Disc concretions found in proglacial lake deposits; various forms and sizes.

Concretion Examples

• Moeraki Boulders: New Zealand, large septarian concretions.
• Martian Spherules: Found by Opportunity rover on Mars; hematite concretions.

Environmental and Geological Significance

• Concretions record information about early diagenesis and environmental conditions during their formation.
• Studied for their insights into past geological and environmental processes.

These notes summarize key points about the formation, types, and significance of concretions in geology, providing a comprehensive understanding of these intriguing geological features.