SEDIMENTARY ROCKS What are sediments? They are “loose” rock particles.

Formation of Sediments 1) Weathering of preexisting rocks (like igneous rocks). 2) Accumulation of biological matter (shells, plant fragments). 3) Chemical precipitation of minerals from water.

Sediment Classification by Particle Size Boulder - >256 mm Cobble - 64 to 256 mm Pebble - 2 to 64 mm Sand - 1/16 to 2 mm Silt - 1/256 to 1/16 mm Clay - <1/256 mm

Turning Sediments into Sedimentary Rocks (4 Steps) 1) Transportation- The movement of sediment away from its source rock by water, wind, or ice Rounding of particles occurs due to abrasion during transport. Rounding increases as transport distance increases. Size sorting occurs by transport agents, especially running water. Sediment size decreases as transport distance increases.

2) Deposition- The coming to rest of transported rock material, or the accumulation of chemical or biologic sediments (typically in water). Environment of deposition- the location where deposition occurs. Different environments have different energies, and different amounts of rounding and sorting. Fossils can be key indicators of environment of deposition.

Examples of Environments of Deposition: RiverDesert Dunes DeltaBeach

3) Burial- Sediment must be buried by additional sediments in order to become preserved as a sedimentary rock. 4) Lithification- Lithification is the processes of converting loose sediment into sedimentary rock. Lithification is really a combination of compaction (by burial) and cementation (precipitation of cement by fluids). The common cements are quartz, calcite, and the iron oxide hematite.

Warmup 10/22 What are the four main steps needed to turn sediment into sedimentary rock?

Classification of Sedimentary Rocks by Origin Detrital (aka clastic): Sediment grains come from pre-existing rocks, and are cemented (the most common sedimentary rock type). They are classified by grain size. Breccia- A coarse- grained clastic rock composed of angular rock fragments cemented together.

Conglomerate- A coarse-grained clastic rock made of rounded gravel cemented together. Detrital Continued.

Sandstone- A medium-grained clastic rock. Types of sandstone, depending on the mineral composition: Quartz sandstone - contains >90% quartz grains. Arkose - contains mostly feldspar and quartz grains mixed together. Graywacke - contains sand grains surrounded by dark, fine-grained (often clay-rich) matrix (the "dirty" sandstones). Detrital Continued.

Siltstone- A fine- grained clastic rock. Siltstones have a gritty feel, and the individual grains can be seen with hand lens. Detrital Continued.

Shale- A fine-grained sedimentary rock containing silt and clay- sized grains. Shale spits into thin layers, a behavior known as fissile. Mudstone- Mudstone is made of silt and clay-sized grains. Mudstones tend to be blocky instead of fissile. Claystone- Claystones contain predominantly clay-sized grains and are non-fissile. Detrital Continued.

Warmup Classify each of these clastic sedimentary rocks:

Chemical (and biochemical) sedimentary rocks- Chemical sedimentary rocks form by the precipitation of minerals from water (this process may or may not involve the actions of biological organisms). In contrast to the clastic textures of the rocks discussed above, chemical sedimentary rocks have crystalline textures. Limestone- Limestone is composed mainly of calcite (CaCO 3 ). The can be inorganic chemical, or biochemical (fossils). Chemical alteration of limestone in Mg- rich water can produce dolomite, CaMg(CO 3 ) 2.

Chert- Chert is hard, compact, very fine- grained, and formed almost entirely of quartz (SiO 2 ). Conchoidal fracture! Chert can occur as nodules within other sedimentary rocks, especially in limestone. Evaporates- Evaporites are rocks that grow upward from salty lakes and seas due to water evaporation. Common examples are rocks made of gypsum and halite.

Organic sedimentary rocks- Fossil fuels are sedimentary rocks with a biochemical origin. Coal- Coal is a sedimentary rock formed from the compaction of partially decayed plant material (requires stagnant water and rapid burial). Oil and natural gas- Subsurface “cooking” can change organic solids into oil and natural gas. These fossil fuels can rise upward and accumulate in porous overlying rocks.

Warmup! The two main classes of sedimentary rocks are: A) ___________________ (also called ____________) and B)_____________________________________ The rocks in each class can either be inorganic or biological.

Sedimentary Structures- Features that provide clues about the environment of deposition.

1) Bedding (called laminations if millimeter-sized). Horizontal layers within an outcrop of rock. The most common sedimentary structure!

2) Ripple Marks Small ridges formed on the surface of sediment layer by moving water or wind. Symmetrical ripples  Beach ripples. Asymmetrical ripples  Water or wind current ripples.

12 3) Cross-Bedding Series of thin, inclined layers. Represent preservation of migrating wind or water- generated sand ridges. You are looking at the side of a bed rather than the top of the bed.

12 4) Graded-Bedding A change in grain size from the bottom to the top of a bed (fining- upward). Formed by underwater landslides called turbidity currents (initiated by earthquakes).

5) Mud Cracks Polygonal cracks formed in drying mud. Form in sediments that are sometimes exposed to air (dry lake beds and tidal flats).

14 6) Fossils Any evidence of past life preserved in rock. Hard parts (shells, bones) are most easily preserved.

WARMUP! Copy these hints for the sedimentary rock lab. Oolitic limestone. Dolostone- 2 samples! Coquina limestone.Rock gypsum. Limestone. Chert Conglomerate. Siltstone. Shale.Breccia. Quartz sandstone.Coal. Graywacke sandstone. Arkose sandstone.

WARMUP What are the 6 sedimentary structures we discussed?