Stratigraphy Flashcards
What is the difference between chronostratigraphy and geochronology?
Chronostratigraphy = material stratigraphic unit
- Anything I can touch
- Physical
- “This rock is cretaceous”
Geochronology = geological time unit
- Time
- Non-physical
- “This rock was formed during the cretaceous”
What does the law of superposition say?
… at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed
What does the principle of original horizontality say?
Strata - either perpendicular to the horizon or inclined to the horizon - were at one time parallel to the horizon
What does the principle of lateral continuity say?
Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way
What does the principle of cross-cutting relationships say?
If a body or discontinuity cuts across a stratum, it must have formed after that stratum (ie. an intrusion disturbs older layers).
What is an angular unconformity (vinkeldiskordans)?
En tydelig vinkel mellem den nedre lagserie og den øvre
What is a disconformity (diskonformitet)?
Exists where the layers above and below an erosional boundary have the same orientation.
What is a nonconformity?
Develops where sediments are deposited on top of an eroded surface of igneous or metamorphic rocks
What is a paraconformity?
Strata on either side of the unconformity are parallel, there is little apparent erosion
What is an unconformity (inkonformitet)?
A surface of contact between two groups of unconformable strata.
Et lagplan, hvor der mangler lag fra et tidsrum
Hvordan indtegnes en inkonformitet i et stratigrafisk kort?
Som en hiatus; lodret skravering
Et tegn på tidsrum, hvorfra der mangler lag
Hvad er litostratigrafi?
The smallest lithostratigraphic unit is a bed (“lag”).
Beds are grouped into a Member (”led”).
Member are forming a Formation.
Formations can be packed into a Group (“gruppe”).
Hvad kan være et eksempel på formationer?
Formationsgrænser ved litologiske skift:
Formation A: vekslende ler og sand men overvejende sand
Formation B: vekslende ler og sand, men overvejende ler
Formation C: vekslende kalk og ler, men overvejende kalk
Hvad er progradation og retrogradation?
Progradation (= regression): “sea level fall”
Retrogradation (= transgression): “sea level rise”
What is biostratigraphy?
Branch of stratigraphy
Focuses on correlating ans assigning relative ages of rock strata by using fossils
Hvad erkendte George Cuvier?
At dyr opstår, lever og uddør i en irreversibel proces
= fossiler (makro og mikro) kan bruges til aldersbestemmelse fordi de repræsenterer et unikt tidsrum
-> grundlæggelse af biosøratigrafi
Hvad erkendte William Smith?
At lag der litologisk ligner hinanden kan skelnes ud fra deres fossilindhold.
Hvad gælder om biostratigrafiske enheder?
- Benævnes biozoner (biostratigrafiske zoner)
- Består af bjergarter
- Defineres ved deres indhold af fossiler
– makrofossiler (ammonitter, muslinger, m. fl.)
– mikrofossiler (foraminiferer, kokkolitter, dinoflagellater, m. fl.) - Kan strække sig på tværs af litostratigrafiske grænser
- Kan korreleres fra én geografisk lokalitet til en anden
– samtidighed men ikke nødvendigvis samme alder overalt –> IKKE = kronostratigrafisk korrelation - Er lokale til regionale (evt. globale)
Hvad er taxon-range biozone?
Udbredelseszone
Karakteriseret ved den total horisontale og vertikale udbredelse
(range, ultimative opståen til ultimative uddøen) af et element i et fossilt selskab (typisk en art – kunne også være en slægt eller familie)
Hvad er interval biozone?
Intervalzone
Fossilførende interval mellem to definerede biostratigrafiske grænser.
Zonen er defineret ved de biostratigrafiske ændringer ved zonens grænser – dvs. fossilindholdet indeni zonen er ikke afgørende.
Hvad er assemblage biozone?
Selskabszone
Zone defineret ud fra en bestemt samling/sammensætning af fossiler.
Alle fossiler behøver ikke at være tilstede overalt i zonen og nogle kan også forekomme udenfor zonen.
Tæt forbundet med aflejringsmiljø.
Hvad er abundance/acme biozone?
Hyppighedszone
Zone karakteriseret ved den maksimale hyppighed af en art, men ikke dens totale udbredelse.
God kronostratigrafisk indikator.
What is cyclostratigraphy?
A branch of stratigraphy that is concerned by cyclic alternations of rock properties
Cyclostratigraphy older than 55 mio. years is very unprecise
Hvad er Milankovitch-cyklicitet?
Langsigtede cykliske ændringer af Jordens årstider og solenergi modtaget på forskellige breddegrader.
Nævn de 3 Milankovitch-cykler.
Precession: Jordaksens retning (23.000 år, cirkel)
Effekt på årstiderne ift. polerne
Obliquity: Jordaksens hældning (41.000 år)
Mellem 22.1-24,5 grader (= ændrer modtagelse af solens stråling mere lokalt)
Eccentricity: Jordbanens ellipseform (100.000 år og 400.000 år)
Ændrer sig fra mere cirkulær til mere oval (= effekt på indstråling fra solen over længere perioder)
What is perihelion and aphelion in the eccentricity cycle?
Perihelion = planet is going too fast so it moves outward
Aphelion = planet is going too slow so it moves inward
Eccentricity summary
Only parameter that affects the annual global insolation received by Earth (just 0.25% change)
Varies between nearly 0 and 0.06 with main periods of 100 000 and 405 000 years
Its main influence is through the climatic precession cycle
Precession summary
Describes the precessional motion of Earth’s axis relative to the elliptical orbit and varies in a cyclic way with an average period of ~21 000 years.
Does not influence the annual global insolation received by Earth, but determines the latitudinal and seasonal distribution of solar insolation, with an opposite effect on both hemispheres.
Eccentricity determines the amplitude (strength) of the climatic precession cycle.
Obliquity summary
Varies between 22 and 24.5 degrees with dominant periods of ~41 000 years.
Like precession, obliquity affects the latitudinal and seasonal distribution of solar insolation, but in a fundamentally different way having the same effect on both hemispheres.
In contrast with precession, its main influence is on higher latitudes and in winter.
The summer monsoon’s effect on circulation
Weak summer monsoon = weak Nile runoff = oxygen-rich deep water = normal deep-ocean sediments
Strong summer monsoon = strong Nile runoff = low-salinity lid (with organic debris) + oxygen-depleted deep water = organic-rich black muds
What is magnetostratigraphy?
A branch of stratigraphy that relies on the frequent Earth’s magnetic polarity change to provide a relative dating.
Magnetiske anomalier viser et symmetrisk mønster på hver side af riften, der stemmer overens med polvendinger.
Normal configuration => Mag N = Geo N
Reverse configuration => Mag N = Geo S
Hvad betyder magnetisk deklination og inklination?
Deklination: Lokal afvigelse mellem magnetisk nordpol og geografisk nordpol. Varierer med tiden.
Inklination: Vinklen mellem Jordens magnetfelt og det vandrette plan. Ved magnetisk ækvator: 0 graders inklination.
Hvordan opstår marin-magnetiske anomalier omkring spredningszoner?
Når basalt langsomt størkner, og når det er så mafisk (og dermed indeholder meget Fe (ferromagnetisk) og Mg), vil det naturligt antage en størkningsposition i den retning, Jordens magnetfelt peger på det tidspunkt.
Derved kan vi se, når spredningsryggen spreder sig, at der dannes stribede, magnetiske bånd på hver side af ryggen. Striberne kommer af polvendinger tilbage i tid.
De bliver ældre og ældre, jo længere væk fra spredningsryggen, man kommer.
Hvad er en kron?
Magnetisk polaritetszone
What is chemostratigraphy?
A branch of stratigraphy that relies on chemical properties of rocks as a relative dating technic.
Figure out the chemical composition of the ocean before (difference between Stontium 88 and 86); then compare with e.g. fossils (dated the same way)
Hvad er “apparent polar wander”?
En observation, man havde gjort, inden man havde kontinentaldriften på plads. Da mente man, at det var polerne, der vandrede, fordi det var lettere at forestille sig frem for, at det var de kæmpe kontinenter, der bevægede sig.
What is chronostratigraphy?
Chronostratigraphy is the application of disciplines such as biostratigraphy, magnetostratigraphy, chemostratigraphy, cyclostratigraphy, sequence stratigraphy, and numerical dating to stratigraphic successions in order to interpret temporal correlations.
What is geochronology?
Denotes time relations in all rocks, specifically when they formed, whether stratified or non-stratified. It also denotes the time of processes in which rocks not only formed but also were eroded (unconformities) and deformed (structural and cross-cutting relationships). It is used to denote the timing of events throughout all of Earth’s history that are interpreted from the rock record (e.g., climatic, biotic, tectonic, and oceanographic).
