stratigraphy Flashcards
what is lithostratigraphy based on?
– Lithostratigraphy is based on rock boundaries
what is biostratigraphy based on?
– Biostratigraphy is based on fossil boundaries
what is chronostratigraphy based on?
– Chronostratigraphy has definite time boundaries using the stratigraphical time column.
this column is made by specific events including extinctions and from fossil collection
what is lithostratigraphy?
it is the description, comparison & correlation of layered rock units in space & time
it is the basis for all geological exploration
what does the word formation mean?
• The word formation has a strict meaning, it is the base unit
• The Formation is the basic unit, which:
– Is internally homogenous (mostly)
– Has clear boundaries
– Can be mapped out (at scale of 1:10,000 : scale traditionally used for mapping)
what do geological maps show/define?
maps define the lateral extent, as they appear at the surface, of lithostratigraphical units (i.e. formations)
time tools that are independent of rock type allow correlation by establishing time lines. these are created by using:
– Fossils (evolution/extinction)
– Events (global/regional footprint)
what is diachronism?
when the deposit may be lithologically identical, but of differnet ages in seperate places
what are most fossils and why?
- Most fossils are:
- Marine in origin
- Have hard parts
- Why?
- Marine organisms more likely to be buried
- Soft parts decay
- Rapid burial is best
- On land organisms are normally eroded before they have time to be fossilised
define biostratigraphy
is the method of subdividing the rock record into units based on fossils
what is biostratigraphy based on?
• Based on ‘Principle of Faunal and Floral Succession’:
– Life evolves, becomes extinct & leaves a record of existence as a series of fossils in order of appearance
– Some organisms (guide fossils) are better suited than others for global correlation
what six points make a good guide fossil?
Essential:
• Free moving
• Widespread
• Rapidly evolving
Desirable:
• Abundant
• Readily preserved
• Easily recognised
define chronostratigraphy.
- Considers stratigraphy as time passed using the stratigraphical column that we have been told to learn. This column has been internationally agreed.
- Chronostratigraphical units (e.g. Systems) have isochronous boundaries, defined by biozones or events
- Radiometric dating of these provides an absolute timescale
what is this called?
when?
info?
trilobites (trilo = three lobes)
• Cambrian–Carboniferous
- Large number of species & habitats
- Hard exoskeleton with jointed limbs
what is this called?
when?
info?
grapolites
• Cambrian–Devonian in age
- Floating, branched colonial organisms
- Many different types: good guide fossils
what is this?
when?
info?
Brachiopods
• Cambrian–Recent: dominant in Palaeozoic
- Great diversity in fossil record
- Bivalved shell, mostly attached to seafloor
what is this?
when?
info?
Corals
• Cambrian–Recent
- Major extinction end Permian: Palaeozoic types differ from Mesozoic
- Can be compound (colonial) or solitary
what is this?
when?
info?
Bivalves
• Ordovician-Recent: dominant Mesozoic–Recent
- Two valves or shells: differ from Brachiopods
- Infaunal (equivalve): equal valves: buried into sand as it has two equal valves
- Epifaunal (inequivalve): often unequal valves: a curved valve and a flat edge it can’t bury itself into the sand
what is this?
when?
info?
ammonites
• Devonian–Cretaceous
- Chambered shells
- Rapid evolution; great diversity
- Superb guide fossils
what is this?
when?
info?
gastropods
- Ordovician-Recent
- most common in Cenozoic
- land, freshwater, sea
- Poor guide fossils overall as they have had a very slow evolution
Conical shells are more typical
what is this?
when?
info?
Echinoderms
• Cambrian–Recent
- Shell made of calcite crystals
- Based on five-ray symmetry
- Stalked crinoids (usually Palaeozoic)
- Sea urchins (usually Mesozoic)
what is stratigraphy?
it is the interpretation of rock units as sequences of events in earth history
stratigraphy is the interpretation of rock units as sequences of events in earth history. How do we establish the sequence?
we use:
- sequncing tools (superpositon, way up, etc)
- time tools ( litho-, bio, and event stratigraphy)
stratigraphy is the interpretation of rock units as sequences of events in earth history. how do we interpret the sequence?
we use the primary tool actualism (uniformitarianism)
actualism allows us to interpret rocks as products of…
an environment
define facies.
facies: the sum total of all of the characteristics of a rock body that allows us to determine its depositional enveironment.
typical characteristics of facies include:
lithology
fossil content
sedimentary structures
a facies is not usually a single…
rock type
facies names reflect, with increasing confidence of interpretation:
product, e.g sandstone facies
process, e,g, turbidite facies
environment, e.g. submarine fan facies
the priciple characteristics of sedimentary facies are:
- geometry of the rock body
- lithological characteristics
- sedimentary structures
- fossil content
taken together, these aid in the identification of the environment
ammonites give indication to what environment?
fully marine
corals give indication to what environment?
warm, tropical waters
not a question - for a given time interval, the facies produced by environments will giveway to each other in gradual transition
mechanics of facies change
variations occur by __________ and __________.
explains these
mechanics of facies change
variation occurs by sedimentological processes and external controls
sedimentological processes
- depend on sediment suply
- autocyclic variation
external controls
- depend on, e.g. sea level rise, tectonism
- allocyclic variation
define Walther’s principle
walther’s principle
that vertical sequences are products of lateral facies variation in time.
as environments shift position through time, the continuosly built up facies will succeed each other in vertical relationship
facies analysis allows us to…
facies analysis allows us to reconstruct ancinent evironments.
- palaeogeography
- palaeoclimatology
- palaeoecology
- palaeo sea levels
define absolute dates
measured in year, however it is difficult to obtain and use in the field.
define radiometric dating
radiometric dating uses the half life of raioactive isotopes to determine absolute ages (e.g. Ur - Pb, K - Ar) - used for dating igneous/ matamorphic rocks.
define relative dates
rlative dates depend on position of one layer relative to another: stable and easier to apply in field geology than absolute dating.
define relative dating
relative dating uses a relative chronology (using superposition, fossil content)
define stratigraphy
stratigraphy
the study of rock succesions, and the interpretation of these sequences f events in the history of the earth
what are tie lines?
tie line math up rock types.
units of the same rock are not necessarly of the same age
what are time lines
time lines match up using fossils, events etc, independent of the rock type.
matches product of the same time interval
units of the same age are not necessarily of the same rock type
define uniformitarianism
uniformitarianism is defined as the present is the key to the past
Only by investigating present-day processes can we hope to interpret the products of processes acting in the past.
Includes the uniformity of processes and the uniformity of rates.
what is the difference between uniformitarianism and actualism
**uniformitarianism **
unifomity of process
uniformity of rate
actualism
uniformity of process
no uniformity of rate (geological processes can operate gradually or rapidly)
sumarise actualism
no powers are to be employed that are not natural to the globe, no action is to be admitted except those which we understand and can observe.
what is the principle of superposition?
superposition
in any undisturbed sedimentary rock sequence, the layer at the base of the pile was formed first
define original horizontality
original horizontality
layers of rock were originally deposited horizontally
define original lateral continuity
original lateral continuity
layers of rocks extend laterally until physically constrained in some way.
what is the younging direction?
the younging direction is the direction from the oldest rocks to te youngest.
in certain circumstances, rocks maybe overturned and thus the rocks at the bottom are actually the youngest.
give six examples of way up criteria in terms of sedimentological, lavas, and fossils.
• Sedimentological
– Graded bedding (coarsest at base)
– Ripple marks (sharp ridges, round troughs)
– Cross-bedding (truncated at top)
• Lavas
– Gas bubbles (top), cooled base
– Rounded tops to pillows
• Fossils
– Sessile organisms (corals), burrows
-geopetals
how do you use ripples for way up criteria
the pointed side points upwards.
when using pillow lavas as way up criteria, which way is up?
the curved side points upwards.
faults formed _______ than the rocks they cut.
faults formed after the rocks they cut.
whats the difference between dykes and sills?
dykes are vertical
sills are horizonatal
what do unconformities represent?
unconformities represent time gaps in the rock record: time unrecorded by rocks
name the four types of unconformities.
angular unconformity
disconformity
nonconformity
paraconformity
describe the steps in an angular unconforimity
- deposition
- uplift & tilting
- erosion
- redeposition
describe the steps in a disconformity.
- deposition
- uplift
- erosion
- redepositon
describe the steps in a nonconformity.
- deposition
- uplift
- erosion
- redeposition
the rock record is the product of ____________
the rock record is the product of environmental change
actualism represents the key to understanding environmental change through …
actualism represents the key to environmental change through comparison of geological product with present-day processes.
the present is the key to the past and the past is the key to…
the present is the key to the past and the past is the key to the predicting the future
what are the four key variables that control the formation and preservation of the rock record?
tectonics
climate/ atomsphere
sea level
biosphere
what is the major controller of the formation and preservation of the rock record?
plate tectonics
as plate tectonics can make space for the others to do what they do and equally destroy the work done by them.
other than the four key variables what else has an effect on the rock record?
extra terrestrial
can have an effect on:
sea level
biosphere
climate
e.g. a meteorite collision could cause a mass extinction and send ash up into the air, thus effecting the biosphere and climate/ atmosphere.
plate tectonics lead to the formation of _________ and the deformation/ destruction of the _____________
plate tectonics lead to the formation of sedimentary basins and the deformation/destruction of the rock record
what does ths map show?
this map shows the precambrian cratons and the different orogenic belts welded on to it. these belts show a opening and closing of a sea in terms of the wilson cycle and the end closing of the ocean creates a mountain belt.
the different mountain belts show different oceans being closed at different times.
climate/atmosphere controls…
climate/ atmosphere controls:
- major sea level change
- biosphere change
content of sedimentary basins
sea level controls…
sea level controls:
- bioshpere change
- extent of deposition in sedimentary basins
- contents of sedimentary basins
as the earths temperature rises, sea level __________ due to ___________
as the earths temperature rises, sea level rises due to expansion
the biosphere controls…
the biosphere controls:
- atmosphereis change
- the content of sedimentary basins (carbonates, biogenic minerals)
- the modification of the land surface (surface run-off, soils)
what is the closed system in terms of the four key variables?
climate - sea level - biosphere forms a closed system
the ultimate control is plate tectonics
extra-terrestrial influences are also felt at this scale
plate tectonics create sedimentary basins.
what two basins would you find at a divergent margin
what four basins would you find at convergent margins?
• At Divergent Margins
– Rift valleys
– Passive margins (trailing shelves)
• At Convergent Margins
– Trenches
– Fore-arcs
– Back-arcs
what is the wilson cycle? and what was it developed from?
the wilson cycle describes the birth and death of oceans.
it was developed from trilobite distributions
separated fossils in the uk indicate a ‘pre-atlantic’ ocean called ‘Iapetus’
describe the wilson cycle.
the supercontinental cycle is the reslt of successive ________ ______
the supercontinental cycle is the result of successive wilson cycles.
the major change to the atmosphere through time has been increased oxygen through photosynthesis.
this led to the archaen-proterozic transition.
what is this transition?
it changed from a reducing to oxidising environment.
what is meant by icehouse and greenhouse.
the icehouse greenhouse cycles are controlled by:
icehouse - high-latitude polar ice
greenhouse - no polar ice
ice house - greenhouse cycle are controlled by:
- continental accumulation
- open oceans
- greenhouse gases
- milankovitch cycles
sea level changes through time.
describe the large scale and small scale changes.
large scale changes
- eustatic fall and highs
- associated with climate change
- also ocean-volume changes
small scale changes
- mon-eustatic changes
- regional uplift/ sediment supply
what are the causes of eustacy
what is another term for extinctions and radiations and what causes them?
• Extinctions (clear ecospace)
– Extra-terrestrial impacts
– Volcanism/climate/sea level
• Radiations (fill ecospace)
– Fill now open ecospace
– Developed from adaptive advantages
record of the rocks = record of________
record of the rocks = record of earth histrory
the record of the precambrian is very small in the UK
around how long was the precambrian?
what eons was it made up of?
around 4000ma
made up of the archean and proterozoic.
when where plate tectonics born?
plate tectonics were born in the precambrian
what are the precambrian cratons?
precambrian cratons: crystalline/ metamorphic ‘ heart’ of all continents.
where would you find precambrian rock in the UK?
there is a tiny amount of precambrian rock in the north west coast of scotland.
orogenic belts are formed by…
orogenic belts are formed by wilson cycles
what are terranes?
terranes are distinct crustal units bounded by faults.
describe the precambrian climate.
- snowball earth
- precambrian ‘icehouse’
- cryogenian glacial sediments seen in scotland.
describe the precambrian life.
the cambrian explosion of life denotes …
precambrian life is restricted to stromatalites and loose cells.
the cambrian explosion of life denoted the end of the precambrian.
what terrane is south of the Iapetus suture (line representing the closure of the Iapetus ocean)
the avalon terrane is south of the Iapetus suture.
name the three terranes from north scotland to the iapetus surture
lewisian and torridonian
moine
dalradian
