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Surface Processes & Sediments

Surface Processes & Sediments. Associate Professor John Worden DEC University of Southern Qld. Surface Processes & Sediments. Rock-forming Processes: Igneous , Sedimentary , or Metamorphic . Next examine Surface Processes which occur at 1A, 25 o C.

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Surface Processes & Sediments

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  1. Surface Processes & Sediments Associate Professor John Worden DEC University of Southern Qld

  2. Surface Processes & Sediments • Rock-forming Processes: • Igneous, Sedimentary, or Metamorphic. • Next examine Surface Processes which occur at 1A, 25o C. • Colliding, rotating, grinding & twisting plates generate tectonics which thrust up Mountain Ranges. • As steadily as Mountains form, Erosion wears them down. • Erosive agents - Wind, Water and Ice. • Transport rock debris to depositional sites; • Accumulates as Sediment and, • Compacted, & Lithified to Sedimentary Rocks. • Time is recorded by Sedimentary Rocks.

  3. Surface Processes & Sediments • Principles & Terms • Sediments characterised by layers termed ‘Strata’. • Discrete recognisable layers termed ‘Beds’, and these define ‘Bedding’. • Depositional processes produce ‘Stratification, a record of changing environmental conditions throughout time. • Law of Original Horizontality: • States that sediments are deposited in Strata that are horizontal. • Principle of Stratigraphic Superposition: • States that in any sequence of sedimentary strata, “The oldest rock is at the bottom, and the youngest rockon the top, of the sequence”.

  4. Surface Processes & Sediments • Conformable sequences consist of strata uniformly following each other without interruption. However, breaks common. • Sedimentary record incomplete due to ‘Unconformities’- periods of non-deposition or erosion. • Three types of Unconformities- • Non-conformity = Change in major classes of rock types, • Angular unconformity =Period of tectonism between rock types, • Disconformity = Irregularerosion surface separating two sedimentary layers. • Unconformities are time gaps in the geological record.

  5. Surface Processes & Sediments • Weathering: • The disintegration & decomposition of rock to produce ‘Regolith’. • Two types of Weathering: Physical and Chemical. • Physical weathering reduces large masses to much smaller units thereby increasing surface areas for chemical attack. • Chemical & Biological weathering convert mineral components to new components stable at 1A and 25o C. • TheWeathering zone extends down as far as water & oxygen penetrate, and is due to: • Unloading of rock formations as exposed; • Penetration of soluble salts & later crystallisation; • Water freezing & expansion as ‘frost wedging’; • Plants wedging rocks apart along cracks, and • Repeated bushfires causing unequal heating, expansion, and fracturing.

  6. Surface Processes & Sediments • Soils: • Equate to accumulated ‘Regolith’, which is composed of decomposed products of rocks & minerals + organic matter. • Forms a succession of weathered zones termed ‘Horizons’. • Not strata, but comprise a soil profile, which usually consists of: • An organic-rich surface layer; • Upper A horizon ( dark grey /black, depleted in clay, & enriched in organic M); • Middle B horizon ( brown/ red, blocky, enriched in clay, & Fe-Al hydroxides); • Possible K horizon ( impregnated with Ca Carbonate); • Underlying C horizon ( yellow/ brown, sl oxidised). • Development dependent on : • Climate, Vegetation cover, Soil organisms, Regolith composition, Topography, & Time.

  7. Surface Processes & Sediments • Erosion is the downslope movement of regolith/debris under the pull of gravity. • Termed ‘Mass Wasting’. • Influenced by composition, texture of regolith, amount of water/air mixed with debris, & steepness of slope. All influence velocity. • Two general types: “Slope Failure” & “Sediment Flow”. • Slope failure is downslope movement of relatively coherent masses of rock or regolith slumping. • Sediment flow is the downslope movement of“Regolith”/ debris mixed with water + air. • Landslides are examples of mass wasting. • Once at bottom of slope, other agents transportdebris to lower elevations in the Landscape.

  8. Surface Processes & Sediments • Approx 30% of precipitation collects as ‘runoff’ in streams, which move downslope (stream flow). • This transports debris particles & dissolved substances along channels. • Factors controlling channel flow include: • Average width & depth; • Channel gradient; • Average velocity of water flow; • Discharge volume; and • Total sedimentload.

  9. Surface Processes & Sediments • Sediment Loadtransport moves in three ways: • Coarse particles move along stream bed = ‘Bed load’, • Fine particles move suspended in water = ‘Suspended Load’, & • Material in solution = ‘Dissolved load’. • Bed load: • Moves by “Saltation”- Sand -sized grains propelled into suspension, pulled down by gravity & returned to stream bed. Progress is by short intermittent jumps along arcuate paths. • Suspended Load: • Silt and Clay particles moving in suspension, • Derived from fine-grained regolith, • Particles settle when velocity of turbulent flow declines. • Dissolved Load: • Bicarbonate, Ca, sulfate, Na, K, Mg, and Chloride ions.

  10. Surface Processes & Sediments • As particles move downstream, particle size decreases due to abrasion and impacts. Gravel in headwaters, sand & silt at stream mouth. • Grains are progressively ‘rounded’ tending toward spherical shape. Degree of “rounding” indicates transport distance. • Grains are also ‘sorted’ by size or hydraulic equivalency. • Therefore, detrital sediments demonstrate: • Transport distance by degree of ‘rounding’; • Source rock type by sediment mineralogy; • Type of transport agency by grain surface texture; • Efficiency of transport by degree of ‘sorting’.

  11. Surface Processes & Sediments • Water also exists as ice where T << 0o C. • Precipitation falls as snow down to snowline ( lower limit of perennial snow). • Snow compacts, recrystallises to ice, and moves downslope as ‘Glacier’. • Valley glaciers at high latitudes move from high to low altitude forming ‘Fjords’, where they reach sea level. • Ice sheet flows internally and by basal sliding in response to gravity. • Glacier scrapes bedrock removing soils & regolith, plucks rocks, and grooves bedrock. Bedrock displays striations, etc. • Debris is transported by ice to depositional site: • Glacier terminus as terminal moraine. • As lateral moraines, icebergs, etc

  12. Surface Processes & Sediments • Wind also transports sediment: • Dominant in dry climate areas where no vegetation to retain regolith. • Transported particles are mostly sand and finer grain sizes. • Sand particles moved as high as 1 metre above surface by “saltation”. • Form ‘Dunes’ of different types when encounter obstructions. • Sand grains move by saltation up windward slope of dune(~ 12o); • Grains fall over slip face of dune ( ~ 33o) and come to rest. • Dune migrates downwind. • Process of grain migration forms ‘Cross bedding’. • Silt and clay particles carried in suspension until: • Wind velocity decreases and particles settle to surface. • Form deposits of ‘Loess’; uniformly blanket surface; • Lack stratification, but can contain fossils, etc.

  13. Surface Processes & Sediments • Deposition: • Sediment deposited when stream/ wind velocity decreases or ice melts. • Turbulent flow ceases & coarse sediment deposits first, progressively followed by fines. Hence sediment sorted by relative grain size. • Depositional sites occur from stream floodplain to Oceans. • Meandering streams ( with large loop-like bends) in fine- grained alluvium & gentle gradients. • Erosion of outer bank of meander & deposition on inner bank. • Produces ‘Point bars’ • As stream erodes fine-grained silts, may cut off loop. • Forms ‘Oxbow lake’. • Braided streams form when debris load very high. • Stream divides into many channels & reunites often.

  14. Surface Processes & Sediments • Braided streams form ‘Braid bars’ consisting of coarse debris. • Such bars and channels are constantly re-worked by the stream. • Deposition at river mouths may form ‘Deltas’, or be reworked by longshore currents into ‘spits’, ‘bars’,’barrier islands’, etc. • In all situations deposited sediment carries imprint of transporting processes in it’s texture, rounding, & degree of sorting. • Ice transport produces ill-sorted sediments. • Diverse grain size range; • Minimal degree of rounding; • Cosmopolitan range of sediment compositions; & • Diverse types of resultant landform features.

  15. Surface Processes & Sediments • Lithification: • Sediment is ‘lithified’ as it is buried by later sediments. • Formation water is expelled due to load pressure. • Compaction occurs as void space is reduced. • Grains frequently ‘cemented’ together by later mineral deposition in void space. Cementing agents include Calcite, Silica, Fe oxides, etc. • Resultant rock becomes ‘indurated’ or hardened & strengthened with loss of ‘porosity’. • Sediment generation is related to tectonics: • Thickest sequences form near Mountain Belts. • These are typically ‘immature’ sediments. • Little tectonics means mature sediments.

  16. Surface Processes & Sediments • Sedimentary Rock Practical: • Important factors when describing & identifying rocks are: • Texture- shape of fragments and their mutual arrangement; • Rounding- degree of reduction towards spherical shape; • Sorting- tendency towards ‘unimodal’ or same grain size; • Porosity- percentage of void space in rock; • Cement- nature and type of cement; • Fissility- ability to split along bedding planes; • Friability- tendency to crumble; • Mineral Composition- of fragments, grains, & matrix; • Structure- bedding, cross bedding, joints, etc

  17. Surface Processes & Sediments • Sedimentary Rocks: • Classified into two major groups- Detrital or Clastic & Non-clastic. • Clastic rocks subdivided on grain size basis into Coarse/ Medium/ Fine; • Coarse-grained further sub-divided on shape of fragments/grains into: • Conglomerates- with rounded grains; • Breccias- with angular grains or fragments. • Medium-grained sub-divided on basis of degree of sorting & rounding of grains into: • Sandstones / Arenites- well sorted, & well rounded; • Greywacke - poorly sorted, angular fragments. • Fine-grained divided into: • Shale- bedding visible & frequently fissile; • Mudstone- bedding absent, non-fissile, often ‘blocky’.

  18. Surface Processes & Sediments • Non-clastic or Chemical Sedimentary rocks: • Include Carbonates, Siliceous, Carbonaceous, Ferruginous & Evaporites. • Carbonates are usually of two types; • Limestones- made up of calcite derived from shell fragments of organisms, direct precipitation from sea water or fresh water, and possibly “oolites”. • Dolomites- generally converted limestones that have been re-crystallised by solutions to Ca-Mg carbonate. • Siliceous rocks form from siliceous skeletons of marine organisms which are later ‘lithified’ to “cherts”. • Carbonaceous rocks form from vegetation: • Various types of Coal, Peat, & Organic-rich Shale. • Evaporites are direct precipitates from seawater: • Include Rock Salt (NaCl), Gypsum, and Anhydrite. • Rarer salts such Potash, etc

  19. Surface Processes & Sediments • Depositional Environments: • Sediments are products of Provenance (Source region), Transport history, & Depositional Environment. • A hole in the ground is a necessary pre-requisite (Need Subsidence!). • Depositional Environments have gradational boundaries. • Sediments occur in distinct “Packages” in a vertical section sense. • In conformable sequences, only adjacent environment “packages” can occur immediately overlying or underlying particular sediments. • Results in superimposed sedimentary “cycles”. • Examples: • Fluvial/Alluvial; Deltaic; Coastal; Turbidite; etc. • Consider only Fluvial & Deltaic Environments. • Point Bar deposits.

  20. Surface Processes & Sediments • Point Bar Sediments: • Meandering stream deposits. • Record ‘fining upwards’ cycles of grain size in sediments, ie • With each flood, deposition commences with gravel, grits , & coarse sands. • As current slackens, sands become progressively finer-grained. • Sluggish current enables deposition of silt and clay layers. • Next flood repeats cycle. • As river meanders over floodplain, deposits “packages” of gravelsandclayfining upwards cycles. • Deltaic sediments in contrast, “coarsen upwards”. • Deeper water in front of delta is more distal from river. • Results in deposition of fine-grained silts and clays. • As river advances, introduces fine sands, later coarsersands and grits. = “Coarsening Upwards” cycles.

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