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Sedimentary Rock Classification

Sedimentary rocks record Earth's surface history and often contain fossils, making them crucial for understanding past environments and finding resources. Clastic Sedimentary Rocks Formed from rock and mineral fragments clasts , classified by grain size: | Rock | Grain Size | Description | |------|------------|-------------| | Conglomerate | 2mm | Rounded gravel/pebbles | | Breccia | 2mm | Angular fragments | | Sandstone | 0.0625-2mm | Sand-sized grains | | Siltstone | 0.004-0.0625mm | Gritty feel, thin layers | | Shale/Mudstone | <0.004mm | Clay-sized, fissile shale or massive mudstone | Sandstone Varieties - Quartz sandstone : 90% quartz, well-sorted, mature - Arkose : 25% feldspar, derived from granite - Graywacke : Mixed composition, poorly sorted, immature Depositional Environments Grain size indicates energy: - Conglomerate → High energy rivers, beaches - Sandstone → Moderate energy beaches, dunes, rivers - Shale → Low energy deep lakes, ocean floors Chemical Sedimentary Rocks Precipitated from solution: - Limestone : CaCO₃, may contain fossils, fizzes in HCl - Dolostone : CaMg CO₃ ₂, fizzes weakly in HCl - Chert : SiO₂, very hard, conchoidal fracture - Rock salt halite : NaCl, evaporite, salty taste - Gypsum : CaSO₄·2H₂O, soft H=2 , evaporite Biogenic/Organic Rocks Formed from organic material: - Coal : Compacted plant material peat → lignite → bituminous → anthracite - Coquina : Loosely cemented shell fragments - Chalk : Microscopic calcareous shells coccolithophores Sedimentary Structures Features that record depositional conditions: - Bedding/Stratification : Layering - Cross-bedding : Angled layers dunes, river bars - Ripple marks : Wave or current action - Mud cracks : Drying of wet sediments - Fossils : Preserved organisms

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • How rocks are classified — U.S. National Park Service. Rock-group classification using composition, texture, and geologic processes.
  • Collecting rocks — U.S. Geological Survey. Field context, specimen observation, and responsible rock-collecting guidance.

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