Sedimentary Rocks

How sedimentary rocks form: Weathering: Physical and chemical breakdown of existing rocks creates sediment particles of various sizes - from boulders to clay. Erosion and Transport: Water, wind, ice, and gravity move sediment. During transport, particles are sorted by size and rounded by abrasion. Different transport agents create different deposits. Deposition: Sediment settles when transport energy decreases. Different environments (rivers, lakes, oceans, deserts, glaciers) create different sedimentary deposits. Lithification: Sediment becomes rock through: - Compaction: Weight of overlying material compresses sediment - Cementation: Minerals precipitate between grains, binding them together - Recrystallization: Some minerals recrystallize, creating stronger bonds Time: Lithification takes thousands to millions of years. Recent sediments are unconsolidated; older sediments are rock. Preservation: Not all sediments become rocks. Many are eroded before lithification. Only a fraction of Earth's history is preserved in sedimentary rocks.

Rocks formed from rock fragments: Classification by Grain Size: - Conglomerate: Rounded gravel (>2 mm). Poorly sorted, contains various sizes. - Breccia: Angular gravel (>2 mm). Fragments are sharp, not rounded. - Sandstone: Sand-sized grains (0.0625-2 mm). Can be well-sorted or poorly sorted. - Siltstone: Silt-sized grains (0.004-0.0625 mm). Feels gritty. - Shale: Clay-sized grains (<0.004 mm). Feels smooth, splits into thin layers. Grain Composition: Can be composed of any mineral or rock fragments. Quartz is most common (resistant to weathering). Feldspar, rock fragments, and other minerals also common. Sorting: How uniform grain sizes are: - Well-sorted: Most grains similar size (beach sand) - Poorly sorted: Wide range of sizes (glacial till) Rounding: How rounded grains are: - Well-rounded: Smooth, rounded edges (transported far) - Angular: Sharp edges (not transported far) Cement: Material binding grains together: - Silica: Very hard, resistant - Calcite: Reacts with acid - Iron oxide: Red, brown colors - Clay: Soft, less durable Rockhounding Value: Sandstones can contain interesting minerals. Shales often contain fossils. Conglomerates can contain interesting pebbles.

Rocks formed from chemical precipitation: Limestone: Composed of calcite (CaCO₃). Forms in marine environments from shell accumulations or chemical precipitation. Reacts with acid (effervesces). Can contain fossils. Important building stone and industrial material. Dolomite: Similar to limestone but contains magnesium (CaMg(CO₃)₂). Forms from alteration of limestone or direct precipitation. Less reactive with acid than limestone. Chert/Flint: Composed of microcrystalline quartz (SiO₂). Very hard, breaks with conchoidal fracture. Forms from silica precipitation or replacement of other materials. Can be colorful (agate, jasper are forms of chert). Evaporites: Formed from evaporation of water: - Halite: Rock salt (NaCl) - Gypsum: Forms alabaster and selenite - Anhydrite: Gypsum without water Coal: Formed from accumulation of plant material. Classified by carbon content: - Peat: Low carbon, not yet coal - Lignite: Brown coal, low carbon - Bituminous: Soft coal, medium carbon - Anthracite: Hard coal, high carbon Iron Formations: Banded iron formations (BIFs) are ancient chemical sediments rich in iron. Important iron ores. Formed when Earth's atmosphere had little oxygen. Rockhounding Value: Limestones often contain beautiful fossils. Chert can be colorful and interesting. Evaporites can form beautiful crystals.

Features that reveal depositional environment: Bedding (Stratification): Layers in sedimentary rocks. Each layer represents a depositional event. Thickness and composition vary. Cross-Bedding: Layers at angles to main bedding. Formed by currents (water or wind). Common in sandstones. Indicates direction of current. Ripple Marks: Wave-like patterns on bedding surfaces. Formed by water or wind currents. Can be symmetrical (waves) or asymmetrical (currents). Mud Cracks: Polygonal cracks in mud that dried. Indicates alternating wet and dry conditions. Common in some shales and mudstones. Fossils: Preserved remains or traces of ancient life. Very common in sedimentary rocks. Provide information about past life and environments. Graded Bedding: Grain size changes within a layer (coarse at bottom, fine at top). Formed by settling from suspension. Concretions: Rounded masses of mineral material. Form after deposition. Can contain fossils or interesting minerals. Nodules: Similar to concretions but different composition. Chert nodules in limestone are common. Geodes: Hollow, crystal-lined cavities. Form in sedimentary rocks, especially limestones. Highly valued by collectors. Rockhounding Value: Structures help identify rocks and environments. Fossils are major collecting interest. Concretions and geodes can contain interesting materials.

Where different sedimentary rocks form: Marine Environments: - Deep Marine: Fine muds, some limestones. Often dark, fine-grained. - Shallow Marine: Sandstones, limestones, shales. Rich in fossils. - Reefs: Limestone from coral and other organisms. Can be very fossiliferous. Continental Environments: - Rivers: Sandstones, conglomerates. Often cross-bedded. - Lakes: Fine-grained rocks, sometimes with varves (annual layers). - Deserts: Well-sorted sandstones (dunes), some evaporites. - Glaciers: Poorly sorted deposits (till), various rock types mixed together. Transitional Environments: - Deltas: Where rivers meet oceans. Mix of sand, silt, and clay. - Beaches: Well-sorted sandstones. - Tidal Flats: Mudstones, some evaporites. Recognizing Environments: Sedimentary structures, fossils, and rock types help identify depositional environments. This reveals past geography and climate. Rockhounding Implications: Different environments produce different rocks and contain different materials. Marine rocks often have fossils. Some environments produce specific minerals.

Preserved evidence of past life: Types of Preservation: - Body Fossils: Actual remains (shells, bones, teeth) - Trace Fossils: Evidence of activity (tracks, burrows, coprolites) - Molds and Casts: Impressions or filled impressions - Petrified: Original material replaced by minerals - Carbonization: Organic material compressed to carbon film Common Fossil Types: - Invertebrates: Shells, corals, trilobites, brachiopods, mollusks - Vertebrates: Bones, teeth, sometimes complete skeletons - Plants: Leaves, wood, pollen, sometimes complete plants - Microfossils: Tiny fossils requiring magnification Best Rocks for Fossils: - Limestone and other marine rocks (abundant marine fossils) - Shale (often preserves delicate fossils) - Sandstone (can preserve large fossils) - Some coal (plant fossils) Collecting: Many rockhounds specialize in fossils. Important to: - Know local regulations (some areas protect fossils) - Collect responsibly (don't damage sites) - Document locations - Learn identification Scientific Value: Fossils provide information about past life, environments, and Earth's history. Important for understanding evolution and past climates. Rockhounding Value: Fossils are highly collectible. Well-preserved specimens are beautiful and scientifically valuable. Many areas are famous for specific fossil types.