Overview
This lecture covers sedimentary rocks, their types, how they form, key processes, sedimentary environments, and their significance in interpreting Earth's history and resources.
Sedimentary Rock Basics
- Sedimentary rocks form from the accumulation, compaction, and cementation of sediments or by chemical precipitation.
- Main types: clastic (siliciclastic), biological (organic), and chemical sedimentary rocks.
- Sediments are loose mineral or rock particles produced by weathering, erosion, or biological processes.
Key Processes & Definitions
- Weathering: breakdown of rocks at Earth's surface (chemical, physical, or biological).
- Erosion: removal and transport of sediment by water, wind, ice, or gravity.
- Deposition: sediment comes to rest and accumulates.
- Lithification: sediments compact and cement into rock; involves compaction, cementation, and sometimes recrystallization.
- Cement fills spaces between sediment grains, binding them together.
Classification of Sedimentary Rocks
- Clastic rocks classified by grain size: gravel (>2mm, forms conglomerate/breccia), sand (0.0625-2mm, forms sandstone), mud (<0.0625mm, forms shale/mudstone).
- Conglomerates have rounded gravel; breccia has angular gravel.
- Organic rocks form from accumulated biological material: coal (plants), chalk (marine microorganisms).
- Chemical rocks form from precipitation of minerals: rock salt, gypsum, some limestones.
- Limestone can be biological (from shells) or chemical (precipitated calcium carbonate).
Sedimentary Structures & Fossils
- Sedimentary rocks display bedding (horizontal layers) and can preserve structures like ripple marks and mud cracks.
- Sedimentary rocks commonly contain fossils, which provide evidence of ancient life and environments.
Sedimentary Environments
- Continental: dominated by streams (sandstones, conglomerates), deserts (well-sorted sands), glaciers (unsorted sediments), swamps (coal).
- Marine: shallow (sandstones, limestones), deep (mudstones, shales); closer to continents = coarser grains.
- Transitional: beaches (sand, gravel), tidal flats (mud), lagoons, deltas.
Economic & Scientific Importance
- Sedimentary rocks and sediments host resources like fossil fuels (oil, gas, coal), minerals, and groundwater.
- Fossils in sedimentary rocks help reconstruct ancient ecosystems and date geological events.
- Sediment characteristics reveal past environments and landscape changes.
Key Terms & Definitions
- Sediment — loose mineral or rock particles from weathering, erosion, or precipitation.
- Weathering — breakdown of rocks at Earth's surface.
- Erosion — movement of sediment by natural forces.
- Deposition — process where sediment settles.
- Lithification — process turning sediment into rock.
- Cementation — minerals binding sediment grains together.
- Clastic rock — sedimentary rock formed from fragments.
- Organic/Biological rock — sedimentary rock from biological material.
- Chemical rock — formed by precipitation of minerals.
- Conglomerate — clastic rock with rounded gravel.
- Breccia — clastic rock with angular gravel.
- Fossil — preserved evidence of pre-existing life.
Action Items / Next Steps
- Review classification of sedimentary rocks by grain size and composition.
- Practice identifying sedimentary rocks using samples or images.
- Prepare for the next module on metamorphic rocks.