Sedimentary Processes

• • Facies - all of the distinguishing characteristics of a rock that are produced by its depositional environment
  • Lithofacies - physical and chemical characteristics of a rock
  • Biofacies - the palaeontological characteristics of a rock
  • Facies association - a group of sedimentary facies that occur together typically representing one depositional environment
  • Palaeoenvironment - ancient environment that a rock represents
• Desert sequences

  • Evaporites and siltstones
  • Aeolian sandstone
  • Fluvial arkose
  • Wadi conglomerate
• Fluvial sequences

  • Clay/mudstones
  • Cross bedded sandstones
  • Conglomerate
• Deltaic Sequences

  • Coal
  • Seat earth
  • Cross bedded fluvial sandstones
  • Shale containing marine fossils
  • Marine limestone
• Clastic Shallow Sea

  • Accumulations of either sand or gravel running parallel to the coastline
  • Sand grains frequently agitated by wave action tending to be well sorted
  • Well rounded grains due to high attrition
  • Orthoquartzites common
  • Symmetrical ripple marks
• Shallow Carbonate Sea

  • Lack of clastic sediment - light in colour
  • No more than 2m deep
  • Agitated by waves
• Deep Carbonates

  • Chalk occurs in deeper water
• • Regolith - layer of unconsolidated material covering the bedrock of a planet
  • Diagenesis - group of physical and chemical processes that change sediment into rock at low pressures and temperatures near the Earth’s surface
  • Lithification - process changing unconsolidated sediment into rock.
• • Compaction - the process by which the weight of overlying sediment compresses sediment beneath, decreasing its volume and thickness.
  • Grains move closer together and the original thickness of the sediment is reduced.
  • Pressure dissolution - pressure is concentrated at contact points between grains causing minerals to dissolve more easily. They may recrystallise and be deposited nearby/removed.
  • Flat and platy minerals are aligned perpendicular to the load pressure.
  • Dewatering causes water to flow slowly out of and away from the sediment.
• Cementation - groundwater containing minerals in solution flows through pore spaces and can precipitate between them forming a cement
• • External energy: solar heating of the Earth’s surface drives the water cycle and influences weathering and erosional processes
• Erosion: the wearing away of rocks and fragments by movement (water, wind, ice, gravity)
• Physical weathering - occurs where physical forces cause the rock to disintegrate into smaller pieces
• Freeze thaw weathering - expansion of freezing water in fractures forcing rocks apart. Requires water, rocks with cracks, varying temperature, enough time for many freeze thaw cycles to occur
• Insolation - repeated expansion and contraction of minerals due to heating and cooling causes stresses which break rock. Action is the strongest on the outside of the rock.
• Salt crystal growth - saline solutions seep into cracks and pore spaces. Rock heats up, water is evaporated - salt crystals grow in the pore spaces and grains are forced apart - granular disintegration.
• Chemical weathering - the disintegration of rock by chemicals from the atmosphere and soil, forming new minerals. Minerals in igneous and metamorphic rocks form at much higher temperatures and pressure than exist at the Earth’s surface. Chemical weathering occurs as these minerals are exposed and chemically changed into new, more stable minerals.
• • Hydrolysis - hydrogen ions and water are reactive. Clay minerals plus K and Si ions in solution. Feldspar is broken down to potassium carbonate, kaolinite. Humid tropical climates - clay mineral formed is bauxite.
  • Hydrolysis is the most important type of chemical weathering because most rocks contain silicates. Minerals produced are stable at low temperatures.
• Carbonation - calcium carbonate reacts with acidic rain water and pore water in the soil to form carbonic acid, which is important in the weathering of limestone.
• Oxidation - the reaction that occurs between compounds and oxygen - the net result is the removal of one or more electrons from a compound causing structure to be less rigid and increasingly unstable. Most common oxides are iron and aluminum.
• • Biological weathering - where plants and animals cause the rock to break up
• • Erosion - wearing away of rocks and fragments by movement (water, wind, ice, gravity)
• Hydraulic action - force of moving turbulent water can disturb and remove loose deposits/ dislodge fragments of rock
• Corrosion - wearing away of bedrock and particles in the river by chemical action. Water dissolves soluble minerals and reacts with chemically active minerals.
• Abrasion - wearing away of the rock surfaces by mechanical effects: particles rubbing, grinding, scratching, smoothing effects
• Attrition - the wearing away of the particles involved in abrasion - they bang into each other and against the floor and sides of the river. Results in reduction in size, reduces angularity of fragments making it easier to transport them further. Forms pitting in frosted quartz.
• Factors influencing degree of rounding and size reduction - length of transport, physical properties of the mineral grains, chemical properties of the mineral grains
• Clastic sedimentary rocks are classified based on texture and composition
• Wentworth - grain size classification
• Sorting - the degree to which the grains are the same size
• Matrix - the background material of small grains in which larger grains occur
• Cement - minerals precipitating around grains, filling the pore space
• Desert sandstone - quartz with silica or iron cement - very well sorted and well rounded - medium or fine grained. Frosted quartz
• Greywacke - dark in colour, angular grains of quartz, fine-coarse, clay matrix - lithic fragments
• Arkose - often pink in colour - medium to coarse grained, quartz and feldspar and some biotite, subangular to subrounded, moderately to poorly sorted
• Orthoquartzite - greater than 90% quartz, limited amount of other grains, quartz cement, well sorted
• • Bedding - marked change in conditions, the rock type also changes - the bedding plane is then the boundary between the two rock types
• Graded bedding - particles settle out in order of size, largest first, smallest last.
• Desiccation features - wet sediment, high rates of evaporation, results in contraction of mud, formation of polygonal cracks, narrowest at the bottom, may be infilled by the sediment
• Cross bedding - produced by migrating ripples, sediment moves up the stoss slope by saltation, collects at the top and cascades down the LEE slope. Forms in high energy environments with a unidirectional flow.