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Sedimentary Rocks

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by

Alexis Albertie

on 16 September 2014

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Transcript of Sedimentary Rocks

Sedimentary Rocks
Sedimentary Rocks
Rocks that forms at or near the surface of the Earth
Sedimentary rocks can be formed by the cementing together of clasts
Clasts are produced by
The physical or chemical weathering of preexisting rock
Growth of shell masses or cementing together of shell fragments
Accumulation and alteration of organic matter from dead plankton or plants
Precipitation of minerals from water solutions
Sedimentary Structures
Bedding & Stratification
Ripple Marks, Dunes, & Cross Bedding
Turbidity Currents & Graded Beds
Bed-Surface Markings
Bedding & Stratification
Bed
Strata
The arrangement of sediment into a sequence of beds is bedding, or stratification
Bedding Formation
Stratification Formation
Sedimentary Basins
Thick accumulations of sediment form in special regions
Subsidence
Categories of Basins
Rift Basins
Passive-margin Basins
Intracontinental Basins
Foreland Basins
Transgression and Regression
Sea-level changes
control the succession of sediments that are seen in a sedimentary basin
Transgression
Regression
Transgression and Regression lead to the formation of broad blankets of sediment
Diagenesis
Is an explanation physical, chemical,and biological processes that
transform sediment into sedimentary rocks
alter characteristics of sedimentary rock after the rock has formed
Lithification is an aspect of diagenesis
In sedimentary basins, sedimentary rocks can become deeply buried
Diagenesis, under the conditions of higher pressures and temperatures, can cause chemical reactions
can produce new minerals
can cause cement dissolve or precipitate
As temperatures and pressures increase, metamorphism begins
Transitions between diagenesis and metamorphism occur between temperatures of 150℃ and 300℃
Depositional Environments
The conditions in which sediment was deposited
Geologists examine grain size, composition, sorting, bed-surface marks, cross bedding, and fossils to identify a depositional environment
Examples of different depositional environments:
Terrestrial (Nonmarine) Sedimentary Environments
Coastal & Marine Environments
Terrestrial (Nonmarine) Sedimentary Environments
Develop inland
Far enough away from the shoreline that they are not affected by ocean tides and waves
Examples:
Glacial Environments
Mountain Stream Environments
Alluvial-fan Environments
Sand-dune Environments
River (Fluvial) Environments
Lake Environments

Coastal & Marine Environments
Occur along the seashore
Marine environments start at the high-tide line and extend offshore, to include the deep ocean floor
Examples:
Marine Delta Deposits
Coastal Beach Sands
Shallow-marine Clastic Deposits
Shallow-water Carbonate Environments
Deep-marine Deposits
Weathering
Sediments are produced by the weathering of preexisting rock
Definition: processes that break up and corrode solid rock
Types of Weathering
Physical Weathering (PW)
Chemical Weathering (CW)
Physical weathering speeds up chemical weathering, and vice-versa
Because CW reactions take place at the surface of a material, when PW occurs, breaking a large rock, the surface area increases
CW speeds up PW by dissolving away grains that hold a rock together, making rock weaker to disintegrate more easily
Physical Weathering
Breaks intact rock into unconnected clasts (grains or chunks)
Types
Jointing
Joints: natural cracks that from in rocks due to decreased pressure and temperature
Frost Wedging
Salt Wedging
Root Wedging
Thermal Expansion
Animal Attack
Chemical Weathering
Chemical reactions that alter or destroy minerals when rock comes in contact with water solutions and/or air
Types
Dissolution
Hydrolysis
Oxidation
Hydration
Dissolution
Soil
Consists of rock or sediment that has been modified by physical and chemical interaction with organic material, rain water, and organisms over time
How Does Soil Form?
1st: CW and PW produce loose debris, new minerals, and ions in solution
2nd: rainwater filters through the debris and carries dissolved ions and clay flakes downward
3rd: Microbes, fungi, plants, and animals interact with sediment, such as absorbing nutrients and leaving behind organic waste/remains
Soil Horizons
Soil Profiles
Soil-Forming
Factors
Climate
Substrate Composition
Slope Steepness
Wetness
Time
Vegetation Type
Soil Classification
Soil Erosion
Agriculture, overgrazing, and clear-cutting have led to the destruction of soil
Crops remove nutrients from the soil
When natural plant covers disappears, the surface of the soil becomes exposed to wind and water
Definition: the removal of soil by running water or by wind
By: Alexis Albertie &
Chi-chi A. Nwankwo

Classes of Sedimentary Rocks
Clastic
Biochemical
Organic
Chemical
Clastic
Cemented-clasts, solid fragments and grains broken off of preexisting rocks
Detritus
Loose clasts that have been stuck together in a solid mass
Formation of Sedimentary Rocks
Formation takes place in five steps
1. Weathering - detritus is formed by disintegration of bedrock
2. Erosion - combination of processes that separate rock from its substrate
3. Transportation -gravity, wind, water, or ice carry sediment
4. Deposition - process by which sediment settles out of the transporting medium
5. Lithification - transformation of loose sediment into solid rock
Classifying Clastics
Clast

Size
- diameter of fragments or grains making up a rock
Clast

Composition
- make up of clasts in sedimentary rocks
Angularity & sphericity
- indicates the degree to which clasts have smooth or angular corners and edges
Sorting
- the degree to which the clasts in a rock are all the same size or include a variety of sizes
Character of cement
- Not all sedimentary rocks have the same kind of cement
Biochemical
Solid material in dead organisms shell survives and then lithifies.
This material is what biochemical sedimentary rocks are comprised of.
Limestone - solid shells made of calcium carbonate may accumulate rock and sand create the biochemical version
Chert - made from cryptocrystalline quartz; shells of silica secreting plankton accumulate on the sea floor, dissolve, and form silica rich gel.
Organic Sedimentary Rocks
Lithified organic-rich sediment
Coal is 50-90% carbon which occurs in large, complex organic molecules.
Plant remains have been buried deep and long enough for the material to compact and lose hydrogen, water, ammonia, and carbon dioxide
Chemical Sedimentary Rock
Rock primarily formed by the precipitation of minerals
Evaporites - salt deposits formed because of evaporation
80% of the water evaporates gypsum forms
90% of water evaporates halites precipitates
Travertine
Chemical limestone
Composed of crystalline calcium carbonate
Formed by chemical precipitation of groundwater that has seeped out at the ground surface in hot or cold springs or on cave walls
Dolostone
Carbonate rock that contains the mineral dolomite
Forms from a chemical reaction between solid calcite and magnesium-bearing groundwater
Most may originate as limestone but change into dolostone as dolomite crystals replace the calcite.
Chert
"Replacement Chert"
Formed when cryptocrystalline quartz gradually replaced calcite crystals within limestone
Can come in many colors like black, white, red, brown, green, and gray because of impurities it may contain
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