EPS 101 Spring
2003– Adrian Brearley - Lectures 15-17 EPS 101 Spring
2003– Adrian Brearley - Lectures 15-17
Key concepts
Weathering – physical and chemical weathering. Physical weathering fractures rocks, chemical weathering converts minerals and rocks into new solids, solutions and sometimes precipitates.
Erosion – processes which remove weathered particles, e.g. water, wind, ice.
Transportation – movement of products of weathering downhill or donwstream by currents of water, air or motion of glaciers.
Deposition – settling of weathered particles from water or wind when currents drop or melting occurs at edges of glaciers. Form layers of sediments in water (oceans, lakes, rivers etc) or chemical or biochemical sediments (precipitates).
Burial – accumulation of layers of sediments buries older layers of sediments. Compaction of layers occurs as they are buried.
Diagenesis – diagenetic processes are physical and chemical changes that occur when sediment undergoes the change to a solid rock. Processes include the effects of heat, pressure and chemical reactions. Includes all types of postdepositional changes to sediment or sedimentary rock, excluding erosion or metamorphism.
Types of sediments and sedimentary particles
Clastic particles – all solid fragments formed from by weathering of preexisting rocks. Includes boulders, pebbles, sand, silt and clay.
Clastic sediments – accumulations of clastic particles, sometimes called siliciclastic sediments because they consist largely of weathering products of silicate minerals.
Chemical sediments – sediments formed by the precipitation of dissolved ions and molecules from water, usually seawater.
Biochemical sediments – sediments formed from the accumulation of the mineral remains of organisms or minerals precipitated as result of biological processes, e.g whole or fragmented shells.
Bioclastic particles – fragments of previously deposited and broken up bioclastic calcium carbonate sediments.
Sedimentary processes
Currents – movement of fluids such as air and water that are capable of transporting sediments. Fast currents of water can carry a wide range of a particle sizes in suspension, whilst slower moving currents can only carry smaller particles.
Suspension – currents lift and transport particles within the body of moving fluid (water or air). The particles are considered to be suspended in the fluid and are carried long with the flow.
Sorting – process of separating sedimentary particles into different size ranges depending on the ability of a current to carry different sized particles.
Particle rounding – transportation of sedimentary particles in water or air causes rounding of particles due to abrasion of sharp edges – a type of physical weathering. Rounding is a function the initial size of the particles and the distance transported.
Grain size reduction – transportation in fluid also causes a progressive decreases in the grain size of particles and is caused by collision of suspended particles with other particles and by collision with bedrock as the sediment is moved along in the fluid.
Sedimentary environments
Geographic locations characterized by particular combination of geological processes an environmental conditions, e.g. a lake, estuary, delta, deep ocean, etc.
Continental sedimentary environments.
Alluvial – sedimentary environment associated with a river, i.e. river channel, channel margins or borders, flood plain, etc. Activity of organisms can be important.
Desert – sediments formed by combination of wind action and river action (usually intermittent). Little organic activity.
Lake environment – in inland bodies of fresh or seawater – environment controlled by shallow waves and moderate currents.
Glacial – controlled by dynamics of moving ice sheets in cold climates.
Shoreline environments –
Deltaic environments – sedimentary environments where rivers enter lakes or oceans.
Tidal flat environment – environment where extensive areas are exposed at low tide – dominated by tidal currents.
Beach environments – controlled by strong waves approaching and breaking on the shore. Strips and sand and gravel laid down by wave action.
Marine environments.
Continental shelf – environments in shallow waters of continental shores. Sedimentation controlled by relatively gentle currents. Clastic or chemical sediments.
Continental margin – environment transitional between continental shelf and deep sea environments. Deep water environment where sediments are deposited by turbidity currents.
Organic reefs – carbonate structures formed by carbonate secreting organisms built up on continental shelves or oceanic volcanic islands.
Deep sea environments – floors of deep oceans, far from continents. Quiet waters disturbed only occasionally by deep currents. Includes deep trenches in oceans along subduction zones and abyssal plains built up of turbidity currents travelling far from continental margins.
Sedimentary structures
Bedding or stratification – parallel layers of sediments of different grain sizes or compositions indicative of successive depositional surfaces that were formed during sedimentation.
Cross-bedding – sets of bedded materials deposited by wind or water at angles up to 35º. Grains deposited on downcurrent slopes of sand dunes on land or sandbars in water.
Graded bedding – beds progress from coarse grains at the base to fine-grains at the top. Formed by deposition from a waning current.
Ripples – very small dunes of sand or silt with long dimension at right angles to current. Form low narrow ridges, centimeter or two high separated by wider troughs.
Bioturbation structures – bedding in sedimentary rocks can be broken or disturbed by the activity of burrowing organisms in the sediments. Roughly cylindrical tubes (centimeters in diameter) cut across bedding. Organisms ingest sediment and extract organic material, redepositing disturbed sediment.