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| Question | Answer |
| Principle of Original Horizontality | Sediments are deposited under the influence of gravity as nearly horizontal beds. (Folded or bedded strata indicates that the beds were deformed by tectonic forces after deposition) |
| Principle of Superposition | Each layer of an undeformed sedimentary sequence is younger than the one beneath it, and older than the one above it. |
| Principle of Lateral Continuity | Layers of sediment are continuous, extending in all directions until they thin to zero or terminate against the edges of the original basing of deposition. If there is a barrier, the layers of sediment are prevented from spreading out during deposition. |
| Principle of Cross-Cutting Relationships | If rock unit A cuts through (intrudes) rock unit B, unit B must have been there before A and is therefore older than the intrusion. |
| Principle of Faunal Succession | The layers of sedimentary rocks in any given location contain fossils in a definite sequence; the same sequence can be found in rocks elsewhere, and hence strata can be correlated between locations. |
| What are the fundamentals for stratigraphy? | 1. Sedimentary rocks must be described completely 2. Determine what features are depositional vs. deformational 3. Determine the relative age of geological features 4. Walther's Law 5. Beware of time gaps |
| Lithostratigraphy | Correlating lithologically similar rock units (define a formation, i.e. a mappable, lithologically distinctive rock unit) |
| Chronostratigraphy | Methods to establish relative or absolute age |
| Chronostratigraphy - relative ages | Biostratigraphy = fossil based methodology, i.e. faunal succession Sequence Stratigraphy = stratal-surface-based methodology, i.e. fixated on relative changes in sea level Cyclostratigraphy = hierarchical cycle-based methodology) |
| Chronostratigraphy - absolute ages | Magnetostratigraphy = based on palaeomagnetic data (polarity normal/reversals barcode) Geochronology of synsedimentary volcanic unitys = U-Pb, Ar-Ar, Re-Os dating techniques |
| Chemostratigraphy | Correlation of isotopic excursions |
| How are time gaps typically formed in sedimentary rocks? | Erosion or non-deposition |
| How are time gaps categorised? | Unconformities = 10^5-8 years Diastems/hiatal surfaces = 10^2-4 years |
| Unconformity | The surface between two beds that were laid down with a time gap between them |
| Angular Unconformity | A layer of beds on top of an older layer that has been deformed and eroded. After sediments have been deposited on an ocean floor, tectonic forces cause uplift, folding, and deformation. The top of the folds are eroded, leaving behind an uneven plain with exposed portions of several folded beds. Subsidence below sea level deposits new sediment layers on top of the previous land surface. |
| Disconformities | The lower sedimentary sequence is still horizontal (not deformed), has been eroded down and a new sequence deposited on top of it. |
| Nonconformities | Unconformities that separate igneous or metamorphic rocks from overlying sedimentary rocks. Often indicate a long period of erosion prior to deposition |
| How is the Geological Time Scale divided? | 1. Chronostratigraphy divisions - applied to rocks that were deposited during that time, e.g. eontherm, erathem, system, series, stage 2. Geochronological divisions - the duration of time for each chronostratigraphic unit, e.g. eon, era, period, epoch, age |
| What are the rules for defining a formation? | 1. Must be composed of lithologically similar layers, or a distinctive set of interbedded rock types 2. Have a distinct lower and upper boundary, i.e. different rock unit above and below it 3. Have a type locality/section; a particular place where it is well exposed and can be compared to other localities 4. The name of the formation can either be the geographic name of the locality, or based on the main lithology which typifies the unit |
| Transgressive | Landward migration of environments (retrogradational) |
| Regressive | Seaward/basinward migration of environments (progradational) |
| What criteria makes a fossil useful for biostratigraphy? | 1. Distinctive - easily identified 2. Widespread distribution 3. Abundant 4. Independent of facies 5. Evolved rapidly 6. Exited for a short time range (i.e. 10^5-6 years) The best organisms are pelagic, not benthic, e.g. ammonoids, graptoloids, conodonts, foraminifers |
| Biostratigraphic Zonation | Determining the vertical distribution (i.e. time) of fossils (i.e. their range) |
| Range/Biozone | A fossils actual stratigraphic distribution |
| First Occurrence | First appearance in a region |
| Last Occurrence | Fossil disappears in a region |
| Biostratigraphic Unit | A body of rock defined by a distinctive fossil content Only provide a relative age There are three types |
| Interval Biozone | Strata that occur between the lowest and/or highest occurrences of a single taxa |
| Assemblage Biozone | Strata that occur between the lowest and/or highest occurrences of more than one taxon (assemblage) |
| Abundance Biozone | Characterised by quantitively (statistically) defined maxim of relative abundance of one or more taxa |
| Biostratigraphy vs. Lithostratigraphy | Lithostratographic frameworks are comprised of distinct rock units whose geographic distribution record the migration of a depositional environment through time. Biostratigraphic frameworks are palaeontologically distinct packages of strata that define the palaeogeographic distribution of a suite of depositional environments at a given time. |
| Chemostratigraphy | Chemical fingerprinting of rock units to aid their correlation Isotopic Ratios used to construct stratigraphic profiles and/or plots of elemental ratios |
| What isotopic ratios are used in chemostratigraphy? | C isotopes (13C/12C) O isotopes (18O/16O) Strontium isotopes (87/88Sr) Sulphur isotopes (34 or 36S/33 or 32 S) Fe, Mn, Ca, Mo, N, Ni, B etc. |
| How does chemiostratigraphy work? | All sediments show changes in chemical composition, the processes that fractionate the isotope ratios can be recorded in the rock. These changes reflect minor fluctuations in variables such as sediment source, facies, palaeoenvironment, palaeoclimate and diagenesis. Because it is difficult to accurately measure isotope ratios, they are compared to an internationally agreed standard which is represented in delta form. |
| Define kinetic and vital effect fractionation | Kinetic = an element with two isotopes, the one with the more neutrons is heavier Vital Effects = organisms use C12 more than C13 |
| What are the benefits of chemostratigraphy? | 1. Applied to sediments of any age, lithology and depositional environment 2. Applied to any sample, even cuttings and outcrop samples 3. Applicable in the lab and at wellsite |
| What can chemostratigraphy tell us? | The fraction processes are often global in scale and secular, therefore they can reveal information about ocean mixing times. The changes in isotopic ratio species are observed worldwide, e.g. SPICE. Sometimes elemental ratios can help distinguish between rock units. They can also reveal information about the links between the evolution of the biosphere and the Earth System. This is because stable isotopes are fractionated by biological processes (i.e. the vital effects of organisms). |
| What is the late Cambrian SPICE event? | Marks a global oceanographic event 500 Ma which saw the formation of anoxic ocean conditions, triggering an extinction event and thus the burial of large amounts of organic matter. Photosynthetic plankton consumed the detritus, removing CO2 from the atmosphere and increasing the O2 concentrations. |
| Magnetostratigraphy | This technique uses preserved magnetic reversal patterns to determine the age of a succession of sedimentary and/or volcanic rocks. Relies on DRM (Detrital Remnant Magnetism) or SRM (Sedimentary Remnant Magnetism). By combining DRM data with geochronologically constrained episodes of magnetic reversals preserved in rocks we get the Palaeomagnetic Time Scale. |
| Cyclostratigraphy | This technique uses astronomical tuning of cyclical sedimentary sequences to determine timing of deposition (Milankovitch cycles). |
| Sequence Stratigraphy | Imposes the dimension of time on the relationships of rock units in space (area and depth). The seismic reflectors follow time surfaces instead of facies impedance boundaries. It attempts to combine the concepts of seismic stratigraphy, lithostratigraphy, and chronostratigraphy to construct and interpret depositional frameworks. |
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