Seds - General Flashcards
Stratigraphy
o Sequence stratigraphy, a branch of sedimentary stratigraphy, deals with the order, or sequence, in which depositionally related stratal successions (time-rock) units were laid down in the available space or accommodation. The chronostratigraphy of sedimentary rocks tracks changes their character through geologic time.
o Depositional units or sequences are sedimentary packages bound by unconformities or sequence boundaries
o Sequence boundaries are formed from sea level fall
o Help to define the chronstratigraphical order of sedimentary basins
Uses of sedimentary study?
- Always a sedimentary rock which produces hydrocarbons
- The search for water and aquifers is dependent on the sedimentology and specifically the pore space (porosity)
- BIF’s: Band Ironstone Formation – source of iron ore
- Mining for either deep coal or open cast (growth in India and China)
- Search for placer deposits such as diamonds and precious stones
- Aggregates – limestone sandstone, cement etc……..
What are clastics?
The sediment is commonly referred to as clastic, because it consists of individual clasts (grains, pebbles, boulders etc).
Sedimentary rocks derived from this sediment are referred to as clastic rocks (in decreasing grain size: conglomerates, gravels, sandstones, siltstones, mudstones, clays). Limestones and evaporites are different
Erosion rates
Not surprising globally the largest sediment supply is associated with the World’s largest rivers draining large actively uplifting Mountain belts.
• Higher in Asia due to altitude
o Himalayas
o Orogenic belt
• Lower in Africa even though it has large rivers because
o Craton
o Low elevation
• Glacial periods have higher erosion rates
o Glaciers are powerful eroded
o Not much vegetation so sediment is not rooted and can be transported
Denudation budget: How much sediment can be eroded away
• Denudation budgets are critical for long-term planning and appreciating the erosion from mountain belts – both for Present-day and through out the Phanerozoic
Changes in transportation of seds
• Pre-carboniferous o Braided rivers • Post-carboniferous o Braided and meandering rivers Due to carboniferous explosion and vegetation slowing the movement of rivers • Affects the sediment routing systems
Mass wasting vs Slumping
Mass wasting: downslope movement of soil or rock under gravity (slow – creep)
Landslide: rapid downward and outward movement of soil or rock
Thermohaline circulation
• The movement of surface currents pushed by the wind is fairly intuitive. In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature differences (cold at the poles and warm at the equator). This movement of water and circulation also transports fine-grained sediment on a global scale. Contourite sediments make up the largest group of sediments that are distributed by thermohaline circulation in the World’s Oceans. Contourites are reworked turbidite sediments and form a finer-grained fraction
Biogenic sediments
A sedimentary rock formed of skeletal fragments of calcite or aragonite and commonly known as a limestone. However, the direct precipitation of calcium carbonate from warm supersaturated, shallow highly agitated marine waters can leads to ooids and oolitic limestone.
Organic sedimentary rocks
Form from the accumulation and lithification of organic debris, such as leaves, roots, and other plant or animal material. Good examples include coal, oil shales and shale gas. Bioclastic limestone, and skeletal limestone are also technically organic sedimentary rocks but are usually grouped with the other limestones as being biogenically precipitated.
Chemogenic sediments
Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include evaporites such as gypsum, halite, baryte, sylvite
Siliciclastic Sediments
- Range from mudrocks to conglomerates and breccias
- Composed of grains (clasts) from pre-existing rocks
- Pre-existing clasts which make it up are released through erosion and transported through wind, glacial, river currents, tidal currents, debris flows and turbidity currents
- Composition of siliciclastic rocks determined largely by the climate and the geology of the source area
Skewness:
• A measure of the symmetry of the distribution of grain size and sorting
• +0.3 = strongly fine skewed to -0.3 = strongly coarse skewed
• Reflects the depositional environment
o Beach sands have a negative skew because finer components are carried off by wave action
o River sands are positively skewed as the silt and clay is not removed by currents but is trapped between larger grains
• In general sediment becomes more negatively skewed and finer grained along its transport path
Quartz Arenite, description + location
Quartz arenite:
• >95% Qtz, well rounded, well sorted, texturally mature, monocrystalline qtz dominate
• Extended periods of reworking
• Warm humid climate - removal of unstable components
Arkose, description + location
• >25% Feldspar, mostly K-feldspar, usually fresh, detrital mica, poorly to well sorted, angular to subangular grains o Transport dependant o Derived from granitic source areas o Semi-arid and glacial favour arkoses o Particularly common for deltaic Ssts
Litharenites, description + location
Litharenites (Subgreywackes):rock fragements in excess of feldspar, 20-25% of all Ssts, texturally and compostionally immature
o Short transport distances
o Nature of lithics may reflect uplift history
o ORS - Devonian
• Many fluvial and deltaic Ssts
