Sedimentary Rocks

What is a Sedimentary Rock? Sedimentary rock is formed by deposition and consolidation of sediment and organic material precipitation of minerals from solution

Where does sediment come from? Weathering is the process that reduces solid rock to rubble Two types Physical: solid rock to rubble without changing chemical composition Chemical: altering or dissolving minerals in a rock

Moving Rocks “Au Naturel” Direction of transport Erosion: transportation of sediment/ions streams, wind, glaciers super-strong hikers… Water is the most efficient natural transporter ~27 billion tons annually)

Grain Size and Distance It’s the size that counts… Sediment far removed from its place of origin Small and round Sediment close to source Larger and angular

Lithification Sediment is deposited in low spots Transportation continues—new sediment buries older sediment Buried sediment is compacted (loss of empty space) Cementation—filling in spaces

Compaction VS. Cementation

Types of Sed. Rocks Clastic Organic Chemical Bioclastic formed from compacted sediment Organic formed from lithified plant/animal remains Chemical formed from direct precipitation of ions from solution Bioclastic formed from broken shell fragments

Clastic Sed. Rocks →Increasing Grain Size → Shale siltstone conglomerate sandstone

Organic Sed. Rocks Coal Buried and compressed dead things From this To this …in a matter of eons

Chemical Sed. Rocks Water evaporates leaving behind mineral deposits (evaporite deposits) Death Valley, CA Great Salt Lake, UT

Bioclastic Sed. Rocks Seawater contains dissolved ions Critters use these ions to make their shells When they die, their shells fall to the ocean floor where lithification can take place coquina

Sedimentary Structures Features that develop in sediment during or shortly after deposition Most common types Bedding Cross-bedding Ripple marks Mud cracks

Bedding Layering in sedimentary rocks Sediment accumulates layer by layer

Cross-Bedding Basically, lithified sand dunes

Ripple Marks Small, parallel ridges and troughs Created by wave and wind action Modern ripples on a sand dune Ancient ripples preserved in sandstone→

Mud Cracks Polygonal cracks formed as mud dries and shrinks Modern mud cracks→ Ancient mud cracks in mudstone→