Knowledge reviews Flashcards
What is sedimentology?
Study of sediments, sedimentary rocks, and the processes that lead to their formation, transport, deposition, and transformation at, or near, planetary surfaces.
What is stratigraphy?
The study of rocks to determine the order and timing of events in Earth history: it provides the time frame that allows us to interpret sedimentary rocks in terms of dynamic evolving environments.
Hvad er diagenese?
Alt det, der sker med et sediment
Hvad er lithifikation?
Processen, hvor sediment bliver til sten
Which of the three (component, matrix, cement) is always present in sedimentary rocks?
Cement
How are terrigenous rock classified?
Siliciclastic; terrigenous
Derived from the erosion of rocks on land
Classified based on grain size:
- Pelites
– Claystone (mud)
— Clay
– Siltstone (silt)
— Very fine silt > fine silt > medium silt > coarse silt
- Arenites
– Sandstone (sand)
— Very fine sand > fine sand > medium sand > coarse sand > very coarse sand - Rudites
– Conglomerate/breccia (gravel)
— Granule > pebble > cobble > boulder
What is an allochem?
Components
Main allochemes:
- Bioclasts (skeletal material)
- Ooids
- Peloids
- Intraclasts
What is the difference between an ooid and a peloid?
Ooid
< 2 mm
Smooth and regular laminae formed as concentric coatings around a nucleus
Peloid
< 1 mm
Commonly without internal structure
Mudstone, wackestone, …?
Packstone, grainstone, boundstone
What is the difference between weathering and physical erosion?
Erosion = displacement of solids through wind, water, and ice; eroded materials are displaced.
Weathering = decomposition of the rocks, soil, and minerals through direct contact with the atmosphere.
What is the Goldich stability series?
A method of predicting the relative stability or weathering rate of common igneous minerals on the Earth’s surface.
Minerals that form at higher T and p = less stable on the surface
- Mafic: olivine, augite, hornblende…
- Felsic: Ca plagioclase, Na-Ca plagioclase…
Minerals that form at lower T and p = more stable on the surface
- Quartz, muscovite…
SLIDE 8, PPT 3
What is the link between clay minerals and climate?
Different clay minerals are formed under specific climatic and weathering regimes = clay can be used to figure out previous climates in a specific place.
Kaolinite: warm & humid, acid weathering conditions from the alteration of feldspar-rich rocks.
Smectites: drier climate, weathering of igneous rocks under alkaline conditions.
Can you explain the chemistry of seawater carbonates (in a nutshell)?
Carbon dioxide (CO2) dissolves in the ocean and reacts with water to form carbonic acid—which in turn generates bicarbonate, carbonate, and hydrogen ions.
See Henry’s law
See the carbonate reactions (here with Ω)
See the equality reaction (same slide as above)
In general:
- More CO2 = more acidic water = dissolves carbonates.
- Opløselighed af CO2 FALDER med STIGENDE T°.
What are the main evaporitic minerals and in which order do they precipitate?
1/2 vand = Calcit (CaCO3)
1/3 vand = Gips (CaSO4)
1/10 vand = Halit (NaCl)
1/20 vand = K- og Mg-salte
When is a rock a sedimentary rock (not volcanic or metamorphic)?
Depositional signs
Biological component
What is diagenesis?
Going from loose sediment to a sedimentary rock
Physical and chemical changes
How is pressure solution working?
3 serial processes:
- dissolution at grain contacts
- diffusion along the interfacial water film
- precipitation on pore walls.
What is syntaxial overgrowth?
a crystallographically oriented overgrowth of a mineral on a substrate of the same chemical composition.
en krystallografisk orienteret overvækst af et mineral på et substrat af samme kemiske sammensætning.
How are carbonate cement named?
Micrite: carb. mud
Sparite: carb. cement
A non-turbulent fluid is a ___?
laminar flow
What is the Froude number of a fluid?
In flowing water, there is a velocity above which it will not be possible for waves to move upstream.
Visualized by (Fr) –> ratio of inertial to gravitational forces in the flow
If Fr > 1: rapid (supercritical) flow
If Fr < 1: tranquil (subcritical) flow
When does a hydraulic jump occur?
At a sharp transition between the two stages (rapid and tranquil flow)
What is shown in the Hjulström-Sundborg diagram?
Used to determine whether a river will erode, transport, or deposit sediment
Hvordan forskellige mængder energi i flows kan transportere og erodere forskellige størrelser sedimenter.
Øverst: Erosion
Venstre/Midten: Transport
Højre: Deposition
What differentiates a mudflow from a turbidity flow?
Mudflow: cohesive strength, conserves the density
Turbidity flow: constantly changes the density through erosion, deposition, and entrainment (the process of making something part of a liquid or flow of something and carrying it along).
What are the main types of sedimentary structures?
Induced by physical processes
- Erosional (before deposition)
- Depositional (during deposition)
- Deformational (after deposition)
Biologically induced
- Bioturbation
- Binding
Chemically induced
- (like teepee-structure where crystals grow and crack)
What is the Hjulström-Sundborg diagram?
(Probably at the exam!)
Erosion vs. deposition
The behaviour of grains when subjected to shear by a current is important for understanding sedimentary structures.
x-akse = grain size
y-akse = flow velocity
The bigger the grain, the higher the chance for deposition or erosion (and less chance of transport as bedload)
What is the difference between lower and upper flow regime?
Lower flow regime = a tranquil flow
- Ripples and dunes
Upper flow regime = a rapid flow
- Plane bed and antidunes migrating upstreams
2D vs. 3D dunes: what is the control?
Control = current velocity
Two types of cross-bedding:
Planar (2D) and trough (3D)
Planar = skrå streger i rækker m. fladt lag mellem rækker, der står på hinanden
Trough = halvcirkler i halvcirkler ved siden af halvcirkler m. flade rækker oven på. Her vil vandretningen altid være, så strømmen rammer den mest buede del.
The curvature will always go ‘out’ towards the sides, and so we know the direction of the water
Describe hummocky cross-stratification.
HCS
Hummocky is when it goes down (happy smile - think of a hammock)
Product of strong and complex wave activity, mainly in areas below fair-weather wave base.
Typically interpreted as produced by !storm activity!
Typically found in silt or fine sandstones
Describe the classical Bouma sequence.
Describes a classic set of sedimentary structures in turbidite beds deposited by turbidity currents at the bottoms of lakes, oceans and rivers.
Ie. the ideal vertical succession of structures deposited by low-density
Divided into 5 distinct layers labelled A through E, with A being at the bottom and E being at the top - with the layers overall getting finer-grained from bottom to top.
What are the differences with a slump structure?
Sedimentary folding affecting several beds.
Deposited flat –> instability (like earthquake) –> sediment starts to slide
Usually occurs in marine environments
Landslides involve rock and debris moving downslope along a planar surface, whereas slumping usually occurs along a curved interface and as a single large unit.
How are stromatolites produced?
Cyanobacteria
Produces a slime (Exo-Polymeric Substance) that can trap and bind surrounding sedimentary materials and/or favor precipitation of early diagenetic carbonates ➙ Microbial mats
Made out of dolomite; favoured the precipitation of dolomite
What is the difference with thrombolites, oncoids?
Thrombolites = also microbial mats; microbial structures - we just don’t see the structure
Oncoids = spherical or less well- rounded structures, commonly up to 2-3 cm in diameter but sometimes larger. The binding starts over a core (something that was there before).
Lamination similar to stromatolites.
Common ground: All are microbialite
What are the main reef-building organisms?
Today = corals
Cretaceous = rudists (bivalves)
Jurassic = lithiotids (bivalves)
Before = worms (serpulids)
What is an ichnofacies?
A type of biofacies that describe the presence of trace fossils and may also be present is some basinal sequences.
What are the main depositional environments?
Continental environments
- Fluvial – Alluvial
- Desert
- Glacial
- Lacustrine
Transitional environments
- Marine deltaic
- Estuarine – Lagoonal
- Littoral
Marine environments
- Shallow siliciclastic seas
- Shallow-marine carbonate - Pelagic
- Deep clastic
Volcanic environment
What is the difference between cold and temperate glacier?
Cold glacier = moves due to internal shearing; glacier ice is frozen to the bedrock
Temperate glacier = moves due to a layer of water between glacier and bedrock
How can we recognize dropstones in ancient sedimentary sequences?
Isolated fragments of rock found within finer-grained water-deposited sedimentary rocks or pyroclastic beds.
Size: small pebbles to boulders
Evidence of them NOT transported by normal water currents but rather DROPPED vertically through air or water column
Why is the Sahara an arid place?
Evaporation exceed precipitation; related to global wind pattern and water vapor
23°N is in the climatic belt where deserts form
Located in the horse latitudes under the subtropical ridge. Air from the upper troposphere usually descends, warming and drying the lower troposphere and preventing cloud formation.
Name the different types of dunes and explain their genesis?
Barchan dunes
- crescent
- with the ends pointing away from where the wind blows
- uniform winds
- sparse sand supply
Parabolic dunes
- the opposite of barchan dunes (with ends pointing towards the wind)
Transverse dunes
- linear
- perpendicular to direction of wind
- abundant sand supply
Linear dunes
- linear
- oblique thanks to two prevailing wind directions
- sparse sand supply
Longitudinal dunes
- linear
- same direction as the wind thanks to three prevailing wind directions
Star dunes
- multiple arms
- 4 prevailing wind directions (variable winds)
- can become very big
- abundant sand supply
What are the geomorphologic differences between meandering and braided river system?
In general:
- braided rivers occur in proximal areas
- meandering rivers occur further downstream
Braided:
- deposition on mid-channel bars
- coarser sediment
- variable flow
- sparse vegetation
Meandering:
- deposition on point bar (inner side)
- erosion on cut bank (outer side)
- levees form when flood waters rapidly deposit sediment close to the bank
- crevasse splays are created when levee is breached
- fine sediment
- relatively constant flow
- dense vegetation
What are the controls on river types?
The principal physical controls on river type:
- Sediment load
- Slope
- Seasonality of runoff - Baseline
How are point bars created?
Point bar = inside curve, deposition
Cutoff bank = outside curve, erosion
Water on outside flows faster than inside = difference in velocity –> corkscrew pattern of flow –> erosion on outside bank and deposition on inside –> river moves laterally
Explain the concept and use of channel amalgamation / non-amalgamation?
Important: How much sand is there compared to mud?
Braided = amalgamated (sand-deposits are interconnected)
Meandering = non-amalgamated
What are calcretes nodules?
A hardened layer in or on a soil
Formed on calcareous materials as a result of climatic fluctuations in arid and semiarid regions
What separates the shoreface from the offshore zones from a sedimentological point-of-view?
Fair Weather Wave Base
Shoreface = mean low water to fair weather wave base
Offshore-transition = upper offshore, moves only in storms = fair weather wave base to storm wave base
Offshore = lower offshore, moves only in really big storms = beneath storm wave base
Describe a tempestite sequence.
Sharp, possibly erosive base, overlain by structureless coarse sediment
As the storm wanes, hummocky–swaley cross-stratification forms in finer sands
Overlain by fine sand and silt that shows horizontal and wave-ripple lamination
At the top of the bed the sediment grades into mud.
What are the 6 parameters required to fully characterize the Seawater carbonate system?
What is the difference between photozoan and heterozoan organisms?
Photozoan = warm-water corals
- symbiosis with zooxanthella
- makes photosynthesis
- shallow water
- algae, corals, nummulites, rudists
Heterozoan = cold-water corals
- no symbiosis with zooxanthella
- cannot make photosynthesis
- shallow to deep water
- echinoderms, brachiopods
What are the main sedimentological differences between a carbonate ramp and a rimmed shelf?
Ramp = ramp that continues down in a straight line (like a ramp…)
Rimmed shelf = shelf with a rim/barrier of reefs or carbonate sand before it goes down at a high angle
What is the difference between density flows and turbiditic currents?
Density flow
- fluid with high density = more number of molecules per unit volume = more viscous or heavier = more energy is required to move the fluid = low velocity.
- grain-to-grain support
Turbiditic current
- current of fast-moving, sediment-laden water moving down a slope
- turbulent support
Why are pelagic sediments so boring?
Pelagic sediments are boring because of how they are deposited in layers on layers
What is the carbonate compensation depth?
Just below 4km (but above 4,5 km)
The depth in the oceans below which the rate of supply of calcite lags behind the rate of dissolution so that NO calcite is preserved
Over the CCD = carbonate
Under the CCD = no carbonate
How does CCD affect pelagic sediments?
Below the CCD, only carbonate-free sediments accumulate = dominated by siliceous ooze
What separates the shoreface from the offshore zones?
Offshore transition
Between the fair-weather and storm wave bases
Sands are deposited and reworked by storms
Periods between storm events = deposition of mud from suspension
Storm deposits are therefore separated by layers of mud
What is the difference between a delta and an estuary?
Delta = on a prograding (water moving away) coast
Estuary = transgessive (water moving towards land) coast
Name the different types of delta and estuary?
Three (four) types of deltas:
- The original
- River-dominated
- Wave-dominated
- Tide-dominated
Two types of estuaries:
- Tide-dominated
- Wave-dominated
What is a beach berm?
Marks the division between the foreshore and back- shore area
Aka. a ridge of sand between the actual beach and the sea
Behind it is the backshore (oftentimes wet)
What characterizes lagoonal facies?
May be overlain by fluvial, shallow marine, continental or delta facies.
