Sediments and Sedimentary Rocks PDF

Flat bedding Flat lamination Wavy bedding Lenticular bedding SEDIMENTS AND SEDIMENTARY ROCKS Sedimentary structures  Sedimentary structures structures within beds or on bedding planes, produced by sedimentary and biogenic processes. provide information on the...

Flat bedding Flat lamination Wavy bedding Lenticular bedding SEDIMENTS AND SEDIMENTARY ROCKS Sedimentary structures  Sedimentary structures structures within beds or on bedding planes, produced by sedimentary and biogenic processes. provide information on the environment of deposition.  Sedimentary structures Sedimentary structures can be – erosional – syn-depositional – post-depositional – biogenic  Erosional structures structures that form before the overlying sediment is deposited. e.g. erosional surfaces or scour channels.  Channel Channel was eroded before the sandstone infilled it.  Syn-depositional structures ripple marks cross-lamination cross-bedding mudcracks graded bedding  Ripple marks Flowing water or wind piles sand into ripples and these can become preserved on upper bedding planes as ripple marks. Ripple marks can be – symmetrical when formed by an oscillatory current – asymmetrical when formed by a unidirectional current Ripples show cross-lamination in cross-section.  lithified Symmetrical wave ripples Modern  Asymmetrical current ripples Fluvial Aeolian  Dunes Dunes are features of a larger scale than ripples. are produced by water and wind action (subaqueous dunes, aeolian dunes) show cross-bedding in cross-section.  Modern Aeolian sand dune Aeolian dune x-bedding – Navajo Sandstone  Cross-bedding Forms by the migration of the slip faces of dunes where sand grains (moving in wind or water) avalanche down Characterized by inclined laminations/beds (foresets) bounded by the upper and lower surfaces of a bed. Foresets are oriented in the direction of migration of the dune  Mudcracks Form by shrinkage of fine-grained sediment due to desiccation. are indicators of environments with repeated flooding and desiccation e.g. shallow lakes, tidal flats, fluvial floodplains  Lithified mud cracks Modern mud cracks Graded bedding  Post-depositional structures form after deposition of sediment, but before lithification are mainly a result of deformation, e.g. by slumping or rapid sediment dewatering also known as soft-sediment deformation features Slump folds  Load structures loading of sand layers into underlying mud layers form irregular lobes called load casts loading is due to density contrasts between sand and underlying water-saturated mud Load structures  Biogenic sedimentary structures are formed by the activity of organisms – vertebrate tracks – tracks, trails, or burrows left behind in sediments by invertebrates These phenomena are collectively known as trace fossils and their study is referred to as ichnology. Vertebrate footprints  SEDIMENTS AND SEDIMENTARY ROCKS Depositional environments  Depositional environments The type of sedimentary rock deposited and the sedimentary structures produced are factors of the depositional environment. A depositional environment is a geographic and/or geomorphic area where deposition of sediments is possible.  Depositional environments Continental Marine Fluvial Continental shelf Desert Continental slope Lake Deep-sea Glacial Coastal Delta Tidal flat Beach  Depositional environments Each environment has distinctive physical, chemical, and biological characteristics. Sedimentary rocks commonly exhibit a set of textures and structures that may be characteristic of a depositional environment.  Sedimentary facies A sedimentary facies refers to the sum of lithological, physical, and biological characteristics of a sedimentary unit. It is a distinctive rock type that broadly corresponds to a particular depositional environment or mode of origin. e.g. fluvial channel facies, overbank facies, shelf facies, turbidite facies  Sedimentary facies A facies in a sedimentary succession can change vertically due to a change of the depositional environment with time. A facies can change laterally due to a change from one depositional environment to another.  Facies models Facies models are schematic 3D representations of specific depositional environments that serve as norms for interpretation and prediction. Rhythmic bedding Massive bedding MASS MOVEMENTS oh s**t!  Mass movements also known as mass wasting downslope movement of materials (bedrock, unconsolidated sediment, soil) under the influence of gravity.  Factors controlling mass wasting Angle of slope Presence of water Presence of plants Natural and anthropogenic triggers  Angle of slope Downslope movement takes place due to the component of gravity acting parallel to the slope (downslope or driving force).  Angle of slope There are forces acting in the opposite direction to downslope movements (resisting force or shear strength) due to friction.  Angle of slope Movement will only take place when driving force > resisting force.  Angle of slope Angle of repose: – the maximum angle of a slope on which a pile of unconsolidated materials does not move. – depends on the size and sorting of the particles, their angularity, and the presence of water. Angle of repose increases with an increase in grain size and angularity of grains.  Angle of slope Over steepening of a slope results in slope instability; mass movement may then occur.  Angle of slope Much mass movement is the result of shear failure. Failure occurs along planes of weakness in sloping surfaces (e.g. fault planes, bedding planes, joints).  Angle of slope The orientation of layering relative to the orientation of the slope is also important for slope stability.  The role of water Small amounts of water can increase shear strength through surface tension. Kelly Mooney Photography/Corbis  The role of water Abundant water decreases shear strength and the grain-water mixture behaves like a fluid.  The role of water Water affects slope stability as it adds weight – increases the driving force on the material.  The role of water Water may change dry clay layers in a sedimentary sequence into soft layers of wet clay that are prone to slippage.  The role of water Water-saturated clay and sand are vulnerable to liquefaction. – Material loses cohesiveness and shear strength due to fluidization (quick clay, quicksand). – Liquefaction can be triggered by heavy rains and earthquakes. Alaska, 1964 Earthquake: landslide caused by liquefaction  The role of plants Roots hold the soil together; vegetation thus stabilizes slopes. Removal of vegetation on slopes (by logging, clearing, fire, drought) may result in slopes becoming unstable.  Triggering mechanisms for mass movements Natural triggers – earthquakes – volcanic eruptions – heavy or lengthy rainfall, snow melt Anthropogenic triggers – slope modification (road cuts) – deforestation – mining  Types of mass movements Mass movement of materials downhill takes four forms: – fall – slide and slump – flow – creep Sediments deposited by mass wasting are called colluvium.  Rock/debris falls Free-fall of rocks and debris, commonly from a cliff. Commonly triggered by undermining of cliffs. Debris collects at the base of the cliff, forming talus. https://www.youtube.com/watch?v=QdsxtC tX5nk  Slides rapid mass movement of rock and soil down along a nonvertical slope along a shear plane (bedding or fracture plane, layer of wet clay). – rockslide: movement of blocks of bedrock – landslide: movement of a mass of soil and rock fragments. Vaiont Dam disaster, 1963 Associated mining disasters  Slump Movement of semi-coherent blocks of rock or unconsolidated debris downhill along concave glide planes.  Scarp Older impact crater Slump blocks Toe  Flows A flow is a water-soaked mass of loose rock, soil and sediment that behaves as a viscous fluid. There are three main types of flows: Mudflow Lahar Debris flow  Flows - Mudflows Mudflow: Slurry of mud and water.  Flows - Lahar Lahar: mudflow on the flank of a volcano. prerequisites for a lahar to form: steep volcano slopes thick deposits of volcaniclastic sediment melting of snowcaps during eruption heavy rains Mt. St. Helens, 1980 Mt. Pinatubo, Philippines, 1991  Mt. Pinatubo, Philippines, 1991 The eruption of Mt Pinatubo caused a lahar 15m high which traveled at 70 km/h before it buried the town of Armero (48km from the volcano).  https://www.youtube.com/watch?v=kznwnpNTB6k  Flows - Debris flows Debris flows are moving masses of rock and soil more than 50% of the material is larger than sand size.  Creep Creep is the slowest form of downslope mass movement. It invokes only the upper few meters of regolith. Creep is a result of alternate wetting and drying of swelling clays frequent freezing and thawing of water within the regolith. When water freezes it expands, lifting rock and soil perpendicular to the slope, but on melting the materials are dropped vertically.  Solifluction Solifluction is a type of creep that occurs in polar regions where the ground is permanently frozen (permafrost). In summer the thin soil covering permafrost thaws and becomes water-saturated. Soil slips over the permafrost, and accumulates as solifluction lobes.  Slope and sheet wash Slope and sheet wash: Non-channelized surface runoff of rainwater during events of high precipitation.  Mass wasting on the ocean floor Slumps and slides also take place in the oceans mainly along the continental slopes. mainly triggered by earthquakes.  https://www.youtube.com/watch?v=Zb4T8a1K5tw  Mass wasting on the ocean floor Submarine slumps are made up mainly of semicoherent blocks. Submarine debris flows consist of a well mixed slurry of gravel, sand and mud. Turbidity currents are underwater mud and debris flows and consist of suspended sediment (mainly sand and mud) that travel as a dense, turbid layer moving at speeds >100 km/h and traveling hundreds of km. Turbidity currents deposit sediment as turbidites in submarine fans. Turbidity Currents  STREAMS AND STREAM SYSTEMS  Streams A stream (≈ river) is a channeled flow of water (of any size). Water is derived from surface runoff (rainwater, meltwater) and groundwater. Streams transport and deposit sedimentary material. Running water is the most important agent for the shaping of landscapes by erosion.  Drainage basin The total area drained by a stream and its tributaries constitutes its drainage basin (catchment). Drainage Basin of the Mississippi River A drainage map showing the Mpate, Mfolozi, Mzinene, Hluhluwe, Nyalazi and Mkhuze rivers drain into the St Lucia estuary system, (adapted from the Department of Forestry, Fisheries and the Environment, 2020; DWS, 2022).  Drainage basin Individual catchments are separated by high ground known as a watershed or divide. Drainage divides separate adjacent drainage basins  29 Fig Map of southern Africa, showing the six river basins with their main rivers and tributaries, plus the approximate extent of the seven shared aquifer systems that South Africa shares with neighbouring countries. Basin data taken from Ashton et al. (2008a); aquifer data taken from Struckmeier et al. (2006)  Limpopo Drainage Basin 2nd largest basin that drains in the Indian Ocean The Limpopo Basin spreads across four countries, namely Botswana, Mozambique, South Africa, and Zimbabwe, and the river flows into the Indian Ocean at the Mozambique Channel  River system A river system can be divided into 3 subsystems: 1. The collecting system A network of tributaries in the headwater region, which collect and funnel water to the main stream. 2. The transporting system The main trunk stream which functions as a channelway for water to move from the collecting area toward the sea/lake. 3. The dispersing system A network of distributaries at the mouth of the river, where water is dispersed into an ocean/lake.  Order in stream systems There is a hierarchical system that divides streams up according to their order in a tributary network. A stream of order 1 has no tributaries; a stream of order 2 has tributaries of order 1 and so on.  Order in stream systems As the order of streams increase, the following changes are systematic: I. The number of tributaries decreases downstream. II. The length of tributaries increases downstream. III. The gradient (slope) of tributaries decreases downstream.  Drainage patterns Streams and their tributaries can have distinctive drainage patterns. Drainage patterns reflect topography, composition of the bedrock and geologic structure of the terrain.  Dendritic drainage the most common drainage pattern resembles a branching tree and is typical of terrain floored by uniform rock types. Not just restricted to Earth  Rectangular drainage Stream erosion is influenced by fractures, fault and joint systems in bedrock. Fracture systems commonly form a rectangular grid which may be followed by streams. Jeff Foott/DRK  Trellis drainage develops across a landscape of parallel valleys and ridges  Radial drainage Drainage radiates from a central high point (such as a mountain peak or volcano).  Drainage patterns Streams can also be classified according to the shape of their drainage pattern. An antecedent stream has maintained its course across an area of crust that was raised across its path by younger folding or faulting. Antecedant Stream  Superimposed Stream A superimposed stream is a stream that was let down from overlying strata onto buried bedrock having a lithology or structure unlike that of the covering strata. The stream pattern reflects the geology of the previous covering strata and is not controlled by the rocks on which it now flows. Superimposed Stream  Order in stream systems As the order of streams increase, the following changes are systematic: 1. The number of tributaries decreases downstream. 2. The length of tributaries becomes longer down stream. 3. The gradient (slope) of tributaries decreases down stream. 4. Stream channels become wider and deeper downstream. 5. The size of the valley is proportional to the size of the stream.  Drainage patterns Streams can also be classified according to the shape of their drainage pattern. The pattern of a consequent stream is determined by the direction of slope of the land. They are often found on massive or gently sloping rocks and commonly have dendritic patterns. A subsequent stream occupies belts of weak rock or other geologic structures. An antecedent stream has maintained its course across an area of crust that was raised across its path by younger folding or faulting. A superimposed stream is a stream that was letdown from overlying strata onto buried bedrock having a lithology or structure unlike that of the covering strata. Their pattern reflects the geology of the previous covering strata and is not controlled by the rocks on which they now flow. STREAMS AND STREAM SYSTEMS Flow of water in natural streams  Flow of water The flow of water in a stream is influenced by a number of factors: 1. Stream gradient 2. Stream discharge 3. Stream velocity 4. Channel shape 5. Base level  Stream gradient The gradients of all streams are steep at their source and taper to gentle slopes at their mouth. Streams exhibit a concave-upward longitudinal profile. Longitudinal Stream Profile  Stream discharge Discharge is the volume of water that flows past a given point in a given time (measured in m3/sec). Discharge tends to increase downstream (tropical and temperate climates). Discharge may vary seasonally: Permanent (perennial) streams continual groundwater seepage Intermittent (ephemeral) streams supply of groundwater seasonally depleted  Stream velocity In a channel the fastest flow is where there is the least friction (normally above its deepest part). The line of maximum depth and strongest currents in the channel is the thalweg. If a channel bends the zone of maximum velocity moves to the outside of the bend, while minimum velocity occurs on the inside. Regions of maximum velocity in a stream  Stream velocity The velocity of the flow is proportional to the stream gradient. The capacity of a stream to transport sediment increases greatly with its velocity.  Channel shape Stream cross-sectional area increases with increasing discharge. The cross-sectional shape of a stream channel influences the velocity of flow. ▪ A semi-circular channel has the least surface area per unit volume of water and, therefore, the least friction. ▪ Wide, shallow channels have higher friction. Smooth, semi-circular channel yields highest velocity Wide, shallow channel increases friction Rough channel also slows river at base  Base level A stream profile is controlled by its base level. ▪ The lowest level to which a stream can erode its channel The sea is the ultimate base level. Lakes and dams or stream junctions act as local base levels. Effects of Building a Dam Dam Forms New Local Base Level Deposition Upstream and Erosion Downstream STREAMS AND STREAM SYSTEMS Sediment transport  Stream load Stream load: the amount of material transported by a stream at a given time. it is generally less than its capacity. Capacity: the maximum quantity of sediment a stream can carry past a given point in a given period of time. is proportional to the discharge of the stream. Competence: the maximum sediment size the stream can carry is determined by the velocity of flow.  Sediment transport Sediment deposited by streams is called alluvium or alluvial deposit. Sediment is transported as: bed load suspended load dissolved load  Sediment transport The mode of transport of a sedimentary particle depends on its settling velocity. the velocity at which a particle sinks to the bottom If stream turbulence is greater than a particles settling velocity the particle will stay in suspension (if less the particle will sink).  Bed load Gravel and sand grains have such high settling velocities that turbulence cannot keep them in suspension for long. transport as bed load at the bottom of the channel. The bed load constitutes 5-50 % of the total load of a stream. Coarse particles are rolled, dragged and skipped along the bottom (traction) Finer particles may travel by saltation (the grains bounce along the bottom).  Bed load Bed load moves only if the current velocity is high enough. The movement of bed load results in the abrasion of the sides and bottom of the channel. The grain size distribution of bed load sediment depends on the flow velocity distribution in the stream.  Suspended load The suspended load is the dominant sediment load of a river. Fine silt and clay have low settling velocities and constitute most of the suspended load. The suspended load moves with the velocity of the water, while the bed load travels more slowly along the bottom.  Dissolved load Dissolved load: consists of ions carried in solution (Ca, Na, K, Cl, HCO3, SO4). derived mainly by groundwater seepage and derived from the chemical weathering of rock and soil. Flow velocity is irrelevant to a rivers capacity to carry dissolved material. Hjülstrom diagram – relationship between grain size and current velocity for sediment movement  Placer Deposits A placer deposit is a deposit of heavy minerals concentrated mechanically: behind hard rock bars. in bedrock holes. below waterfalls. inside of a river bed.  Placer Deposits Most placer gold occurs as grains the size of silt particles (“gold-dust”). Large particles of placer gold are called nuggets. Many heavy, durable minerals other than gold also form placers (e.g. platinum, diamond, zircon).  Sediment transport The grain size of sediment decreases downstream. Coarse bed load is gradually reduced in size by abrasion. When a stream reaches the sea, its bed load may consist mainly of sediment no coarser than sand.  Flow turbulence At low velocities water moves with laminar flow, but this is rare in natural streams except in a thin layer along the bottom and banks of the channel. If velocity increases, or the channel walls become roughened, laminar flow breaks up into chaotic eddies and swirls called turbulent flow. Turbulence in a stream is greatest where velocity changes are most abrupt, i.e., just above the channel bottom. Turbulent flow keeps sedimentary particles in suspension. Laminar flow Turbulent flow  Laminar to turbulent transition Laminar flow Turbulent flow ONERA  Sediment transport and bedforms The flow of water over sediment shapes the sediment into distinct geometric features termed bedforms (e.g. ripples and dunes of sand). Current ripples Reineck&Singh,1980 Subaqueous dunes STREAMS AND STREAM SYSTEMS Stream types, their morphology and deposits  Alluvial fans Alluvial fans: cone-shaped deposits of coarse-grained alluvial sediment. form along mountain fronts where high-gradient streams enter adjacent lowlands sudden drop in stream velocity causes rapid sedimentation. dominated by gravel and sand Michael Collier  Braided vs meandering stream The two main stream types are the braided stream and the meandering stream. They differ due to different sediment loads and discharge. They are distinguished by their sinuosity:  the ratio of channel length to the straight line distance downstream.  meandering streams have relatively high sinuosities (4 or more).  Braided streams form a series of interwoven converging and diverging channels separated by tear shaped bars of sand and gravel. sediment transport is dominated by bed load because of high flow velocities. typical of the piedmont (piemonte, foot of mountains) where gradients are steep and the sediment load is large. also common in glacial and desert regions. Tom Bean  Meandering streams have a large suspended load, but subordinate coarse bedload. the thalweg comes close to the bank on the outside of meander bends (cut bank) causing erosion. curved sand bars or mud banks accumulate on the inner side of bends (point bars). the ends of meander loops may get so close that the river cuts across results in the formation of an abandoned meander (oxbow lake) Meandering River Over Time  Point Bar Peter Kresan Cut Banks  Streams on Mars? 100 km Streams on Mars? Ancient stream channel on Mars (3 km wide) Floodplains wide, flat plains bordering streams that are periodically inundated by flood waters  Floodplain Gravel and sand is mainly deposited within the channel (channel deposits). Silt and clay is deposited by settling out from flood waters (overbank deposits).  Floodplain During floods coarse sediment is deposited mainly along river banks Successive floods build up ridges along the banks (natural levees). Sometimes the floodplain is below river level, is poorly drained, and is the site of marshes and swamps (back swamp). Formation of natural levees  Floodplain Sediment-charged waters can breach the levees during floods, forming thin sediment layers (splay deposits).  Floods A flood occurs when a stream’s discharge exceeds the capacity of the channel. As discharge increases during a flood, so does velocity, enabling a stream to carry a greater load and larger particles. The interval between floods depends on the climate of the region and the size of the channel. Japan- July 2018 South Sudan-2018 Bujumbura, Burundi The main bridge over the Shabelle River in Belet Weyne, Hirshabelle State, Somalia East African Heavy rains on the outskirts of Dar es Salaam caused a landslide on the banks of the fast-flowing river Floods 2024 A flooded section is seen at Entebbe Airport in Uganda Cows graze in a flooded paddock in Kisumu, Kenya Durban 1987  TOTI-2019 N2- CHATSWORTH Durban Umlazi H, L section where several RDP houses collapsed- 2019 Floods  A general view of the destruction at Umdloti beach North of Durban, South Africa, Thursday, April 14, 2022 Toyota South Africa Motors Prospecton plant in Durban 2022 DBN Shipping containers are strewn beside the N2 Highway in A collapsed bridge on the Griffiths Mxenge Highway in Durban Durban, South Africa, Wednesday, April 13, 2022 in Durban, South Africa, Wednesday, April 13, 2022  Cape town July 2024 Damage to the road at the Nels River (Oude Muragie) Retreat the streets of Bloekombos The Anchor Bay Cape Town taxi rank Oops too late  Flood recurrence interval A flood-frequency curve is produced by plotting the occurrence of past floods of different sizes on a probability graph. The measure of how often a flood of a given magnitude is likely to occur is called the recurrence interval. A flood having a recurrence interval of 10 years is called a “10-year flood.” Annual Flood Frequency Curve DELTAS  Deltas When a river enters a standing body of water (lake, sea) its flow velocity drops rapidly and it deposits its sediment load as sedimentary layers that form a delta. The coarsest material is dropped first (closest to the mouth) with progressively finer material being deposited further away from the mouth. Deltas produce thick accumulations of sediment that prograde seaward Prograde = progressive building out of sediments away from the sediment source.  Deltas As a river approaches the sea and lowers its slope, it branches into distributaries. At the mouths of the distributary channels the coarsest sediment is deposited as distributary mouths bars; these grow seaward.  Deltas The coarse sediment builds up a platform composed of foreset beds, inclined down current. Foreset beds are covered by horizontal topset beds that represent channel and interchannel deposits. Seaward of the foreset beds are horizontal bottomset beds of fine-grained sediment.  Deltas Delta growth is influenced by the balance between: the rate of sediment input by the river the rate of erosion by marine processes (waves and tides). Deltas are subdivided into river-, tide-, and wave-dominated, depending on which processes affect delta sedimentation most.  Deltas Nile (river/wave-dominated) Ganges-Brahmaputra (tide-dominated) Mississippi (river-dominated) Nile Delta NASA Ganges Delta NASA LANDSAT Image of Mississippi River Delta Wave- vs tide-dominated deltas Huang He (Yellow River) NASA DESERTS AND WIND ACTION  Atmospheric Circulation The earth absorbs more solar energy near the equator than near the poles. This uneven solar heating causes the atmosphere to circulate by convection. Each hemisphere consists of three large paths of moving air called cells. – Hadley Cells – Ferrel Cells – Polar Cells  Atmospheric Circulation Air rises near the equator and near 60° latitude north and south. These areas have high precipitation. Air sinks near 30° latitude north and south. These areas have low precipitation (horse latitudes).  Atmospheric Circulation Earth’s rotation causes the Coriolis effect: – Moving air is deflected in a clockwise direction around a low pressure in the Southern Hemisphere. – Air moves from high to low pressure and deflects clockwise along this path The vertical cells of circulating air, combined with the Coriolis effect, produce the prevailing surface winds.  Atmospheric Circulation Each hemisphere has 3 regions of prevailing surface winds: – The trade winds (or easterlies) blow from east to west in the region between the equator and 30° latitude. – The westerlies blow from west to east in the region between 30 and 60° latitude. – The polar easterlies blow from east to west in the region between 60° latitude and the poles.  DESERTS Deserts are arid regions where – rainfall is