Sedimentary Rocks: Formation and Weathering

• Sedimentary rocks form through processes like weathering, erosion, deposition, burial, and compaction.

Origins

• Sedimentary rocks are created when sediment, resulting from weathering and erosion, is deposited in layers, then compacted, cemented, and lithified.
• Dissolved minerals such as calcite and quartz act as cement, depositing within sediment layers as water moves through them and binding the sediment.
• Lithification is the hardening of sediment.
• Sedimentary rocks form near the surface, usually without significant heat or pressure, and can contain fossils.

Weathering, Erosion, and Deposition (WED)

• Weathering, erosion, and deposition are interrelated processes that alter the composition of rock material.
• Weathering is the process of breaking down rocks via wind, rain, and freezing.
• Erosion moves broken material.
• Deposition puts the material in a new spot

Weathering Types

• Weathering can be physical/mechanical or chemical.

Physical/Mechanical Weathering

• Physical/mechanical weathering involves breaking rock through the force of another substance.
• Examples include ice, water, wind, rapid heating/cooling, and plant growth.

Chemical

• Chemical weathering involves reactions between rock and another substance that dissolve or change the rock.
• Hydrolysis, breaking down the rock because of water, and oxidation, (O2 + H2O = rust) are examples of chemical weathering.

Erosion

• Erosion is about moving sediment, such as sand, silt, clay, and gravel, by wind, water, or ice.
• Uplift and increased elevation due to tectonic processes increase erosion.

Deposition

• Deposition occurs when Earth materials carried by wind, water, or ice settle out in a new location as energy levels decrease.
• Particle deposition depends on size, with larger, heavier particles depositing first and smaller, lighter particles depositing later.

Sediments

• Largest particles are Gravel (2 mm to 75 mm).
• Sand (0.05 mm to 2 mm) are the next biggest
• Silt (0.002 mm to 0.05 mm) follows sand
• Smallest are clay (less than 0.002 mm)

Composition of Sedimentary Rocks

• Sedimentary rocks are classified by formation type, with three different types.

Clastic Sedimentary

• Form when rock and mineral fragments are cemented together by dissolved minerals.

Chemical Sedimentary

• Form as minerals crystallize out of a solution

Organic Sedimentary

• The remains of once-living plants and animals.

Clastic Sedimentary Rocks

• Calcite and Quartz are usually the cement in clastic Sedimentary Rocks
• Texture is determined by size of the grains.
• Coarse-grained rocks includes conglomerate (round), and breccia (jagged).
• Medium-grained rocks includes sandstone.
• Fine-grained rocks includes siltstone (silt), and shale (clay/mud).

Chemical Sedimentary Rocks

• Rainwater picks up dissolved materials eventually making its way to bodies of water.
• Limestone and Rock salt are examples of chemical sedimentary rocks.

Karst Topography

• Karst topography develops in areas of carbonate rocks (chemical sedimentary rocks) like limestone and dolomite.
• Karst topography include features include caves and sinkholes formed when limestone slowly dissolves away by slightly acidic groundwater.
• Limestone are made of calcite and reacts to acid.
• Limestone and Karst topography is abundant in the Valley and Ridge province.

Organic Sedimentary Rocks

• Organic Sedimentary Rocks Are made from the fossils or decaying remains of organisms.
• Fossiliferous limestone (coral, and its ooze) is a type of organic sedimentary rock.
• Coal (decaying plant and animal matter) - Used for fuel.

Features Associated with Sedimentary Rocks

• Sedimentary rocks often display unique features, including:

Stratification

• Rock layers deposited in layers, one on top of the other.
• Layers are = Strata and can be seen in sed rock from a distance (Ex – Grand Canyon).

Mud Cracks

• Sediment is deposited in shallow bodies of water and then dries.

Ripple Marks

• A record of wave motion in sedimentary rock at the bottom of a lake or ocean.