45 Questions
What are the primary processes through which sedimentary rocks form?
Deposition of weathered material by water, wind, or ice; direct inorganic chemical precipitation from water; precipitation by organic processes
What is the main focus of Sedimentary Petrology?
Studying the composition, characteristics, and origins of sedimentary rock
What are the main processes involved in the formation of sedimentary rocks?
Weathering, erosion, transport, deposition, diagenesis
What are the outcomes of weathering on source rocks?
Physical and chemical breakdown, formation of secondary minerals such as clay minerals and iron oxides
What are the major components of sandstone?
Detrital minerals, lithic fragments, biogenic particles, and matrix
What is the most common mineral in sandstone, typically composing 65-100% of its composition?
Quartz
What is the size range of sand grains that define sandstone?
0.0625 mm to 2mm
What type of matrix supports the clasts in paraconglomerate?
Disorganized matrix
What are resistant lithologies less susceptible to?
Breakdown
What affects mineral abundance in sandstone?
Availability, stability, climate, and relief
What is the texture of laminated pebbly mudrocks?
Delicately laminated mudrocks with framework grains
What does diamictite consist of?
Poorly sorted, mud to gravel-sized sediment
What further divides paraconglomerates?
Inferred origin, matrix size, and organization
What are sand grains formed by?
Weathering and erosion, and within the depositional environment
What are rock fragments useful in for sandstone studies?
Provenance studies
What can the disorganized matrix in paraconglomerates be?
Tillite or tilloid
Which type of weathering involves changes to the minerals that make up a rock through processes such as dissolution, hydration, and oxidation?
Chemical weathering
What are the agents of physical weathering that include freeze-thaw action, temperature change, salt dome intrusion, root wedging, wetting and drying, organic activity, and unloading?
Rain and wind
In which type of weathering does the physical breaking up of rock into smaller pieces occur in cold climates, high altitudes, and dry regions?
Mechanical weathering
What involves the removal of material by mobile agents such as water, wind, ice, or man?
Erosion
What is used to distinguish between laminar and turbulent motion in fluid flow?
Reynolds number
What are the different modes of transport for sediment, including rolling, saltation, and suspension?
Air and water
What flows have almost no mixing and thoroughly mix heterogeneous fluids?
Laminar and turbulent flows
What includes volcaniclastic sediments, terrigenous clastic material, carbonates, evaporites, and other sediments and sedimentary rocks?
Sediment origin and classification
What makes up a significant portion of the sedimentary rocks in the stratigraphic record?
Limestones, sandstones, and conglomerates
Which factor controls weathering and includes source composition, climate, and topographic relief?
Climate, topography, and rock composition
What involves the physical breaking up of rock into pieces without involving chemical reactions?
Physical weathering
What involves changes to the minerals that make up a rock through processes such as dissolution, hydration, and oxidation?
Chemical weathering
What is the equation for the Wentworth Scale to measure grain size?
$\phi = -log_2(\text{Diameter in mm})$
Which type of sedimentary rock is categorized based on grain size?
Conglomerate
What distinguishes extraformational conglomerates from intraformational conglomerates?
Clasts derived from a distant source
What characterizes petromict conglomerates?
Clasts of many different compositions
What makes up about 5% of the stratigraphic record?
Terrigenous clastic sediments and sedimentary rocks
Which mineral is a major component of terrigenous clastic sediments and sandstone, averaging 10-15% and reaching 50% in arkoses?
Feldspar
What method is used for sandstone classification by examining a thin section on a petrographic microscope to yield the proportion of each component?
Dott’s scheme point counting
What type of sandstone has at least 95% quartz grains and is considered the most compositionally mature?
Quartz arenite sandstone
Which heavy mineral gives indications of provenance and source area events in terrigenous clastic sediments and sandstone?
Garnet
What type of climate favors the formation of arkoses, which are typically red or pink due to the presence of finely disseminated hematite?
Humid
What do quartz arenite sandstones typically consist of?
At least 95% quartz grains
What do mica and clay minerals in sandstones and coarse clastics reflect?
All of the above
What can be used to identify petrographic provinces within a formation and reflect the effects of weathering and transport on their assemblage?
Heavy minerals
What are other components of terrigenous clastic sediments and sandstone, aside from heavy minerals?
All of the above
What influences the color of arkoses, making them typically red or pink?
Feldspar content
What is the study of heavy minerals used to identify within a formation?
Petrographic provinces
What type of sandstone can be formed in situ by extreme chemical weathering or persistent wave or current reworking on stable cratons and passive margins?
Quartz arenite sandstone
Study Notes
Weathering, Erosion, and Sedimentation
- Physical weathering breaks solid rock into pieces without involving chemical reactions
- Agents of physical weathering include freeze-thaw action, temperature change, salt dome intrusion, root wedging, wetting and drying, organic activity, and unloading
- Mechanical weathering involves the physical breaking up of rock into smaller pieces, occurring in cold climates, high altitudes, and dry regions
- Examples of mechanical weathering include frost wedging, heating and cooling, wetting and drying, and the action of organisms
- Chemical weathering involves changes to the minerals that make up a rock through processes such as dissolution, hydration, and oxidation
- Factors controlling weathering include source composition, climate, and topographic relief
- Erosion involves the removal of material by mobile agents such as water, wind, ice, or man
- Transportation of sediment occurs through air, water, ice, and gravity, with different modes of transport such as rolling, saltation, and suspension
- Flows can be laminar or turbulent, with laminar flows having almost no mixing and turbulent flows thoroughly mixing heterogeneous fluids
- The Reynolds number is used to distinguish between laminar and turbulent motion in fluid flow
- Sediment origin and classification include volcaniclastic sediments, terrigenous clastic material, carbonates, evaporites, and other sediments and sedimentary rocks
- Limestones, sandstones, and conglomerates make up a significant portion of the sedimentary rocks in the stratigraphic record
Terrigenous Clastic Sediments and Sandstone: Key Components and Classification
- Feldspar is a major component of terrigenous clastic sediments and sandstone, averaging 10-15%, but reaching 50% in arkoses, and is less stable than quartz due to low chemical stability.
- Mica and clay minerals, including muscovite and biotite, are common in the matrix of sandstones and coarse clastics, reflecting source geology, climate, and weathering processes.
- Heavy minerals, such as apatite, epidote, garnet, rutile, staurolite, tourmaline, and zircon, give indications of provenance and source area events, with major changes in source-area geology being recorded in their assemblage.
- The study of heavy minerals can be used to identify petrographic provinces within a formation and reflect the effects of weathering and transport on their assemblage.
- Other components of terrigenous clastic sediments and sandstone include fossils, non-skeletal grains, skeletal phosphate, glauconite, chamosite, and organic matter.
- Dott’s scheme point counting is a method used for sandstone classification, involving the examination of a thin section on a petrographic microscope to yield the proportion of each component.
- Quartz arenite sandstones, with at least 95% quartz grains, are the most compositionally mature and are typically the products of extended periods of sediment reworking, influenced by climate and relief factors.
- Many quartz grains in quartz arenites are 2nd cycle and can be formed in situ by extreme chemical weathering or persistent wave or current reworking on stable cratons and passive margins.
- Arkoses, with over 25% feldspar, are typically red or pink due to the presence of finely disseminated hematite and are derived from granites and gneisses, influenced by climate, relief, and degree of transportation.
- Arkoses formation is favored by humid and glacial climates, where feldspars weather to clay, and rapid erosion in areas with high relief can also produce arkosic detritus.
Test your knowledge of physical and chemical weathering, erosion processes, sediment transportation, sediment origin and classification, and the components and classification of terrigenous clastic sediments and sandstone.
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