Sedimentary practical's (pre-reading) Flashcards
What are poorly sorted sediments?
when the grains are of varying sizes
What are well sorted sediments?
when the grains are all about the same size
How does rounding occur?
when the sharp edges of rock is worn down by abrasion when transported
What does the speed of sediment rounding depend upon?
hardness and composition
What are ripples and what do they reflect?
Type of bedform
Reflect flow (direction and energy)
What 2 types of sediment ripple are there?
Symmetrical
Asymmetrical
How do symmetrical ripples form?
oscillatory flow (on a beach)
Forward and back current flow
Where do asymmetrical ripples form?
areas with unidirectional flow (rivers or beach’s with dominant current direction)
What is a facie?
distinctive sedimentary rock packet with distinctive sedimentary features
What are lithofacies?
facies based on sediment characteristics such as grain size and minerology
What are biofacies?
facies based on fossil content
What can form when facies interact?
facie association or succeed one another to form facie sequence
What is Walthers law?
states vertical succession of facies reflects lateral environment change
What are good examples of vertical stratigraphic succession?
Marine transgressions (sea level rise)
and
Marine regression (sea level fall)
What is another name for ‘rock layers’?
strata
What are Nicolaus Stenos 4 principles of stratigraphy?
Principle of superposition
Principle of original horizontal
Event that cuts across exiting strata is younger
Strata presumed to continue laterally from present end point
What is subsidence?
local and regional scale change in earths crust in the form of a downward shift
When do rift basins form?
When the continental crust is stretched
During early rifting earth surface subsides as its stretched and thinned
What is a good example of a rift basin?
the east African rift system
What sort of area will intracratonic basins form?
Interior of continents away from plate margins and mountain belts
What are intracratonic basins usually like?
round or oval in shape
Long geological history of low subsidence
What is one formation theory for intracratonic basins from rifting?
Form over areas of previous rifting
Rifting stops and hot stretched crust cools it contracts and shrinks (thermal sag)
What 2 things can intracratonic basins be filled with?
Continental sediment
Flooding from adjacent ocean
What can the flooding of a intracratonic basin form?
Epicontinental seas
Where do passive margin basins form?
form along the margin of continents that are not tectonic plate boundaries
What sediments are associated with passive margin basins?
Carbonate and clastic 10-20km thick
What is a good example of modern passive margin basin?
Gulf of Mexico margin along southern USA
What are foreland basins associated with?
Regions of compressional tectonics and form adjacent and parallel to mountain belts
How are foreland basins created?
Downward flexing of lithosphere in response to mountain belt
Where does sediment come from for foreland basins?
sediment eroded off adjacent mountain belt
What will the transition of sediment in a foreland basin be like?
from deep marine to continental being in excess of 10km
What is a good example of a foreland basin?
Persian gulf - adjacent to Zagros mountains of Iran
How are strike-slip faults classified?
by a horizontal sense of movement along the fault plane
What types of sediment can strike slip basins be filled with?
continental or marine sediments
What are some examples of modern strike-slip basins?
San Andreas Fault in California
How do you transition from an igneous to sedimentary rock?
Igneous- weathering/erosion- Sediment-lithication- Sedimentary rock
What is sediment?
collective term for loose fragments of rock or minerals that originate from weathering and erosion of pre-existing rock
OR
Precipitation out of solution
What is unconsolidated sediment?
Each grain is separate and not cemented together
What are the main transportation methods for sediment?
Wind
Ice
Water
Gravity
What is rounding of sediment?
when rock fragments bump and scrape each other removing corners and angular edges
What is sorting?
the process by which sediment grains are selected and separated according to size during transportation
What is glacial sediment sorting like?
Poorly sorted
What type of sediment will be well sorted?
windblown sediments of a sand dune
What is the scale for roundness?
Angular
Sub-angular
Sub-rounded
Rounded
What is the scale for sorting?
Very poorly sorted
Poorly sorted
Moderately sorted
Well sorted
Very well sorted
What is lithification?
when loose unconsolidated sediment changes into sedimentary rock
What 2 main processes form lithification?
compaction
cementation
What is compaction when forming sedimentary rock?
loose grains are packed together more tightly
What is cementation when forming sedimentary rock?
precipitation of a new mineral around sediment grain binds them into a firm coherent rock
What are biogenic sedimentary rocks derived from?
skeletal remains and soft organic matter of pre-existing organisms.
What are organic sedimentary rocks composed of?
organic carbon compounded from relicts of plant material
How are chemo-genic rocks formed?
direct precipitation of minerals from a saturated solution
What are volcanoclastic sedimentary rocks composed of?
Grains and fragments derived from volcanic activity
What is the make up of sedimentary rocks on earth?
70-85%= clastic
15-25%= biogenic
What are conglomerates and breccias?
rocks formed through cementation of rounded (conglomerate) or angular (breccia)
What is the grain size scale? (Just names: bit to small)
Pebble
Granule
Coarse sand
Medium sand
Fine sand
Silt
Clay
What is the grain size range for clastic sedimentary rocks?
conglomerate and breccia to fine grained mudrock
How are clastic sedimentary rocks classified?
size of individual rock fragments and/or grain size
How are sandstones characterised?
medium grained sedimentary rocks containing more than 50% sand size grains
What is the most common mineral in sandstone?
Quartz
What different types of sandstone can occur?
Quartz arenites
Arkose
Lithic sandstone
Greywacke
What % quartz is quartz arenites?
95
How are grains usually arranged within quartz arenite?
well sorted and rounded as a product of extensive sediment reworking so all gains beside quartz broken down
What are arkose?
A sandstone which contains at least 25% feldspar
What type of source rocks are arkose from?
Feldspar rich:
- Granites
- Gneiss
What are Litharenites?
25% of all different sandstone types
Rock fragments in excess of feldspar
What is the grain arrangement like in litharenites?
Poorly sorted
Sub-rounded to sub-angular
What does the immature sandstone composition of litharenites imply?
short to moderate transport distance
High rates deposition
What is an example of a high rate deposition area litharenites might be deposited?
deltaic deposition environment
What are greywackes?
sandstones with at least 15% fine grained muddy matrix between quartz grains
How did most greywackes probably form?
Turbidity currents - sediment gravity flow
What is limestone formed from?
skeletal fragments of calcite or aragonite
What are crinoids? (crinoidal limestone)
Sea animals that had long stems cup-like bodies and long filter arms abundant in the Palaeozoic
Where do ooids form from?
direct precipitation of carbonate from warm, supersaturated, shallow, highly agitated marine water creating sub spherical sand size grains
What is the mechanism of formation for ooids?
Small ‘seed’ fragment of sedimentary rock
Strong currents was seed around sea bed where chemically precipitate calcite is accumulated
What is a carbonate sediment composed of ooids called?
oolite
What is an ancient example of oolite
Middle Jurassic age (135 million years ago) bath stone used in many buildings in bath
How does dolomite differ from limestone?
the limestone has been replaced by dolomite in the process of dolomitization
What is coal?
organic sedimentary rock that results from compaction and burial of plant material that has not fully decayed.
What are the general characteristics of coal?
brown to black
soft to hard
low density
Why is coal economically important?
its key role in energy generation
What are oil shales/ black shales?
when organic matter mixes with mud and can be incorporated into mudrocks
How do evaporites form?
Directly from the precipitation of crystals from water following salt concentration from evaporation
What are the main types of evaporite mineral?
Gypsum
Anhydrite
Halite
What are crystals within evaporites like?
Crystalline texture
Crystal size varies depending on salt concentration
How do pyroclastic flows usually move?
Hug the ground
Downhill or laterally (under gravity)
What is the speed of pyroclastic flows dependent on?
Density of current rock and gas debris
Volcanic output rate
Slope gradient
What is the volcanic output rate?
total rate of magma emplacement and volcanic output for the Earth
What is pyroclastic surges?
pyroclastic flows which contain a much higher proportion of gas to rock
What is a pyroclastic fall?
lower density flow which allows them to flow over higher topographic features such as ridges and hills
What is a lahar?
A type of volcanic mudflow or debris flow composed of slurry of pyroclastic material, rocky debris and water
What is diagenesis?
All physical, chemical and biological changes that occur during burial but before metamorphism
What is lithification?
when sediment hardens into rock
What is the physical process that occurs with diagenesis?
compaction of components due to pressure increase on burial
What is the chemical process that occurs with diagenesis?
precipitation of of mineral cements from pore fluids
What can occur to sediment with continued burial?
dewatering and compaction leading to closer packing
How much can mudrock be compacted by burial?
90% (coarser grain 40-50%)
How old are precambrian rocks?
540 million years
What are geopetals useful for?
paleohorizontal level
As will have a void space (air) filled with mudrock showing the top
What is the difference between beds and Laminae?
Beds= thicker than 1cm
Laminae= thinner than 1cm
What are the different scales of bed? (largest to smallest)
Very thick = >100cm
Thick = 30-100cm
Medium = 10-30cm
Thin = 3-10 cm
Very thin = 1-3cm
What are the different scales of laminae? (largest to smallest)
Thick = 0.6-1cm
Medium = 0.3-0.6cm
Thin= 0.1-0.3cm
Very thin= <0.1cm
How do erosional structures tend to form?
the actions of erosive currents with the removal of sediment to form the structure
What are flute clasts also known as?
scour marks
What are flute clasts/ where do they occur?
occur on the underside of many beds deposited by sediment laden flows or storm currents
What do flute clasts look like?
heel shape - bulbous upstream end which flares downstream to merge with bedding
How do flutes form?
localised erosion of sand laden currents passing over cohesive muds
How are tool marks formed?
when an object i.e. a pebble is carried by a moving current and impacts/scrapes along a sediment surface
What are examples of discontinuous tool marks?
prod, bounce, skip and roll markings associated with turbulent flows
What are examples of more continuous tool marks?
Grooves and chevrons
Who was first to pioneer the use of ancient raindrops left in sediment?
Charles Lyell 1851
How are bedforms formed?
the interactions of sediment and wind or water on a bed surface
What is an example of a bedform?
ripples in the sand in a flowing stream
What are the characteristics of current ripples?
asymmetrical in cross section
Down current slope (lee) steeper than the up-current slope (stoss)
What are the different ways that ripples can be seen in plan view?
Straight
Undulatory (long waved laterally)
Catenary (short waved laterally)
Linguid (horse shoe like)
What are the dimensions of current ripples?
2-5cm in height
up to ~40cm wavelength
What are dunes/ how are they created?
Larger bedform that is formed due to increased current speed
What are the dimensions of dunes?
1m high
0.5m to 10m wave length
Why are bedforms found at the higher flowrates hardly preserved in the geological record?
as they are usually reworked as soon as flow speed decreases
How are wave ripples produced?
by the oscillatory motion of waves in shallow marine and intertidal settings
How are wave ripples different from current ripples?
they are symmetrical
How is hummocky cross stratification (HCS) produced?
when oscillatory wave generated flows produced by the passage of storms with large storm waves creating a large undulatory bedform
What form will HCS take?
hummocks (small mound) and swales (shallow channel)
What will the size of HCS be?
wavelength of 1-5m
height a few tens of cm
What is normal grading?
coarser sediments at the base which grade up to finer ones
What will normal grading be evidence if for deposition environment?
one in which transport energy has decreased over time
What is inverse grading?
when grain size increases up the bed
Where is inverse grading most common?
debris falls and aeolian ripples
What is bioturbation?
when there is a irregular disruption of sediment
What are trace fossils?
discrete organised markings in the sediment
What are stromatolites?
direct biogenic growth structures
What are some examples of trace fossils?
record the action of organism in the environment so: burrowing, walking and feeding
What is another name for trace fossils?
ichnology
What is each group of distinct trace fossil called?
ichnofacies
What is an example of a regular burrow network trace fossil?
Paleodictyon
What is an example of a resting/ crawling trace fossil?
Rusophycos and Cruziana
What is an example of a vertical U-tube with spreite trace fossil?
Diplocraterion
What is an example of a straight tube trace fossil?
Skolithos
What is an examples of a branching dwelling burrow trace fossil?
Thalassinoides
What is an examples of horizontal- subhorizontal U-tube with spreite trace fossil?
Rhizocorallium
What are the 5 commonly considered continental sedimentary environments?
Glacial
Alluvial fan
River
Lake
Hot desert
How much of land is covered by glacial ice?
10%
Where will a glacier get material/ sediment?
falling from adjacent cliffs or plucked from the underlying rocks
How do glacial striations form?
this is when large boulders carried by a glacier carve long scratches into the underlying rock
What is the deposited material from a glacier called?
Glacial till
How do alluvial fans form?
when a stream emerges from mountains onto flatter plains material is deposited in broad fan or cone shaped deposits
What will alluvial fans be like in semi-arid environments?
their radius will be between 2-15km
What sediments are related to alluvial fans?
conglomerates
breccias
sandstones
Lesser amount mudstone
What is the classification of a desert?
less than 250mm rain per year
Vegetation covering less than 15% land
What are ventifacts?
rocks whose surface has been faceted by wind
What is the characteristic sedimentary structure in desert sandstone?
large scale cross bedding and absence of finer grained sediments
What are ancient desert dune sandstones like?
highly porous so make excellent resevoirs for water and hydrocarbons
What is one of the characteristic features of lake sediments?
the presence of millimetre scale rhythmic laminations
What are ephemeral lakes?
a lake which might remain flooded for shirt time but might not be present if flooding has not occurred
What are ephemeral lakes characterised by?
evaporitic sediments
mud cracks
fossil fish
What are red beds?
this is when river sediment is deposited in a semi-arid climate which reacts with oxygen in surface/ ground waters to produce rust like iron oxides
What sedimentary structures are found in the delta front?
sands with crossbedding, ripples and bioturbation
What is the pro-delta characterised by?
organic rich, laminated, and bioturbated mudstones
What is deposition and delta migration like at the delta front?
rapid deposition resulting in seaward migration
How will wave action affect sediments on a beach?
wave action will winnow (blow/ remove) mud and silt to produce well sorted well rounded grains of sandstone
What is a lagoon?
semi-enclosed quiet shallow body of water between a barrier island or reef and the mainland
What sediment are lagoons typically dominated by?
Muds
some cross bedded sand (storm washover)
How can lime muds and limestone be found in lagoonal sediment?
associated with coral patch reefs
What are oolites?
distinctive carbonate sedimentary rock
Where do oolites form?
highly agitated intertidal environments
How do oolites form?
small fragment of sediment “seed” which are washed on the seabed accumulating layers of chemically precipitated calcite
What is the main source of sediment for the deep sea?
from the continental shelf via transportation by slumps and slides which cause turbidity currents
What is the Bouma sequence?
A sequence identified by Arnold Bouma for dividing deep water turbidity’s into intervals
How does the Bouma sequence look?
divided into 5 intervals A to E
A is the bottom E is the top
What are body fossils?
ones which represent the actual remains of a once living organism
What are some general examples of body fossils?
Shells
Bones
Teeth
Claws
What is a coprolite?
fossilised faeces
What are chemical fossils?
chemicals found in rocks that provide an organic signature for life
What are microfossils?
ones to small to be seen with the naked eye or a hand lens so a microscope must be used they are usually (1mm or less)
What are the 4 main areas of microfossils?
calcareous microfossils
siliceous microfossils
phosphatic microfossils
organic microfossils
What are some examples of calcareous microfossils?
foraminifera
coccolithis
What are some examples of siliceous microfossils?
diatoms
radiolarians
What are some examples of phosphatic microfossils?
conodonts
What are some examples of organic microfossils?
pollen and spores
What are fossil lagerstatten?
fossil sites with exceptional preservation sometimes including preserved soft tissue
What is the oil window?
the narrow range of temperatures in which oil forms
What are reservoir rocks?
rocks which contain an abundant amount of easily accessible oil and gas
What characteristics must a reservoir rock have?
sufficient pore space for oil and gas to reside
What are the most common reservoir rocks and why?
Sandstone and limestone due to permeability and porosity which allows oil to migrate and be stored
What are banded iron formations (BIF)?
these are formed due to alternating bands of grey iron oxide minerals and bright red chert
What are BIFs important in showing?
formed between 2.5-1.8 billion years which shows not enough dissolved iron in seawater after 1.8bn years ago
What are placer deposits?
found in rivers where the moving water has mechanically concentrated minerals formed by gravity separation
