Overview
This lecture explains sedimentary rocks, focusing on their formation, classification, types, and key characteristics for identifying them.
Sedimentary Rock Formation
- Sedimentary rocks form by the compaction and cementation of sediments.
- Compaction means squeezing sediments together; cementation means gluing or sticking sediments together.
- Sediments are small rock fragments created by weathering (wind, ice, water).
Sediment Sizes
- Sediment sizes are classified as clay, silt, sand, pebbles, cobbles, and boulders (smallest to largest).
- Sand-sized sediments are between 0.006 cm and 0.2 cm; boulders are larger than 25.6 cm.
- The sediment size determines the rock type.
Types of Sedimentary Rocks
- There are three main types: clastic, crystalline, and organic (bioclastic) sedimentary rocks.
Clastic Sedimentary Rocks
- Clastic rocks are made from compacted, cemented sediments called clasts.
- They are also called "inorganic" and "land-derived" rocks.
- Examples:
- Conglomerate: rounded, varied sediment sizes.
- Breccia: angular, varied sediment sizes.
- Sandstone: compacted sand.
- Siltstone: compacted silt.
- Shale: compacted clay.
Crystalline Sedimentary Rocks
- Crystalline rocks form from chemical processes, not compacted sediments.
- Types include evaporites (form by evaporation, e.g., rock salt) and precipitates (form when dissolved minerals crystallize).
- Examples: rock salt, rock gypsum, dolostone.
Organic (Bioclastic) Sedimentary Rocks
- Organic rocks form from compacted biological material.
- Examples: coal (compacted plant remains), coquina (compacted shells).
Fossils in Sedimentary Rocks
- Sedimentary rocks are the only rocks likely to contain fossils.
- Igneous and metamorphic rocks cannot preserve fossils due to extreme heat and pressure.
Key Terms & Definitions
- Compaction — squeezing sediments together.
- Cementation — gluing sediments together.
- Sediments — fragments of rocks resulting from weathering.
- Clastic — rock made from compacted fragments (clasts).
- Crystalline — rock formed from mineral crystals after chemical processes like evaporation or precipitation.
- Organic/Bioclastic — rock formed from compacted organic material.
Action Items / Next Steps
- Have your rocks flowchart ready to fill out sedimentary rock information.
- Review sediment size categories on page six of the earth science reference tables.