Module 9A: Sedimentary Rocks And Processes

• a rock is classified as a sedimentary rock if: it is composed of sediments - either detrital or biochemical - that undergo lithification by compaction and cementation or; it is composed of minerals that form in situ and accumulate and bind into a chemical sedimentary rock
• In page 4, From left-to-right: coquina, a biochemical clastic rock composed primarily of carbonate shell fragments; sandstone; conglomerate; fossiliferous mudstone; and halite, an evaporite
• Sedimentary rocks are important because sedimentary rocks serve as record of the earth's geological history and serve as a host for many mineral and energy resources.
• Sedimentary Processes refers to all processes that shape a sedimentary rock - from the sediment source to its formation and modification
• Erosion is the process by which soil, and mineral and rock particles are worn away (weathering) and moved elsewhere by gravity, or by moving transport agents - wind, water of ice (transportation)
• Weathering and transport requires an agent or medium –which could be water, wind, ice, gravity, or biological organisms.
• Weathering is the process that breakdown rocks into sediments. Weathering has two types –physical weathering and chemical weathering. The weathering action of organisms, biological weathering, can be a combination of both physical and chemical weathering.
• In Physical (Mechanical) Weathering, rocks are broken down into pieces due to physical stresses, without undergoing compositional change. It promotes further weathering by increasing the surface area available for physical forces and chemical reactions to act on. Prevails in high altitudes, dry regions, cold climates.
• Frost wedging occurs due to stresses induced by the expansion of ice when water trapped in existing joints and fractures freezes. It forms talus deposits downslope of bare rock surfaces.
• Exfoliation due to unloading occurs when overlying material is removed such that previously buried rock masses, which were subjected under compressive stresses may expand due to the removal of loading pressure. The expansion results into exfoliation(Aka sheeting or onion weathering), wherein material is shed superficially as scales or layers. Common in batholiths.
• Abrasion is the mechanical wearing down, scraping, or grinding away of a rock surface by friction, resulting from the action of running water or waves, wind and ice. Entrained particles enhance the effect of abrasion, especially in the case of wind (sandblasting). Tafoni are(rounded pits and cavities) in granite formed by wind abrasion and chemical weathering.
• Weathering by impact may cause a rock to break, as in rockfalls and landslides.
• The kinetic energy of flowing water alone, without suspended sediments, may be enough to erode rocks and sediments in a stream bed, as in evorsion, although the effect is enhanced by abrasion.
• The impact of ocean waves may break rocks, especially when cavitation occurs.
• Cavitation as a mode of coastal erosion occurs when air bubbles in an incoming wave are forced into cracks in the cliff being eroded.
• Varying pressure decompresses some vapor pockets, which subsequently implode.
• Heating and cooling (Insolation Weathering) is the repeated expansion and contraction due to rapid temperature changes which may cause the formation of joints and fractures. It's thought to cause exfoliation in rocks.
• in, Wetting and drying fractures may also form from repeated expansion and contraction of materials due to wetting and drying. Common in clay-bearing rocks and soils.
• Chemical Weathering - rocks are broken down into pieces by chemical reactions. Attacks less stable minerals by leaching or alteration to more stable minerals. Prevails in warm and humid climates. The diagram above is also known as the Bowen’s Reaction Series. Flipping the diagram shows the common rock-forming minerals in increasing stability at surface; this diagram is know as the Goldich Stability Series

  
• Dissolution - breakdown of minerals into ions. Dominant process in evaporites and limestones.

  
• Hydrolysis- common for silicate minerals, where H+ and OH- are introduced in the mineral causing the original structure to break down. Commonly result in the formation of clay minerals.
• Hydration and Dehydration - addition of water molecules in mineral structures

  
• Oxidation - alteration of minerals by the exchange of electrons with other elements or compounds. It acts with hydrolysis to decompose ferromagnesian minerals to form hematite and limonite.
• Biological weathering - living organisms may weather rocks by mechanical or physical means.
• Some examples of biological weathering include root wedging (Physical weathering), bioturbation (physical weathering) and weathering due to lichens (chemical and physical weathering).
• Transportation - relocation of weathering products from its source to its depositional site via the five agents of erosion.
• Transportation via gravity - slope failures transport rocks and sediments downslope (mass wasting)
• Transportation via water - sediments may be transported via water currents by: solution, suspension, saltation, and traction.

  
• Stream competence - maximum size of particles a stream can transport.

  
• Stream capacity - measure of the total sediment a stream can carry,

  
• Transportation via wind - similar with water flows, wind may carry sediments by suspension, saltation, and traction. Transport mechanism depends on the grain characteristics and wind velocity.
• Transportation via ice - glacial sediments (till) may be derived by plucking and abrasion along the rock-glacier interface or by deposition from mass wasting and superglacial streams. Glaciers may carry sediments as an internal load, or as a surface load.

  
• Transportation and sediment characteristics - relation between sediment characteristics and sediment transport distance from source can be made.
• Transport distance and grain size - longer transport distance allows more opportunity for abrasion to act, further reducing the grain size of sediments.
• Stream gradient - the grade measured by the ratio of drop in elevation of a stream
• Tributaries - a stream or river that flows into a larger stream or main stem (or parent) river
• Bed load – sediment transported by fluid flows via traction
• turbidity current - a rapid, downhill flow of water caused by increased density due to high amounts of sediment. Can be caused by earthquakes, collapsing slopes, and other geological disturbances.
• An ideal turbidite sequence, also known as a Bouma sequence. The normal grading of turbidite sequences record the decreasing energy of turbidite flows.

  
• A longer transport distance allows more opportunity for abrasion to act, increasing the roundness of sediments. On the other hand, the sphericity of grains are not affect much by transport distance.

  Longer transport distance = increased grain roundness = decreased grain angularity.