Chapter 1 : Stratigraphy - Wikipedia The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. The direction of the remnant magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence. Magnetostratigraphy The Earth generates a magnetic field that encompasses the entire planet. It exists because currents in the electrically-conducting fluid outer-core produce dynamo-effect. At a given spot, the orientation of the magnetic field is described by: The "dip" angle between field lines and the horizontal. The "strike" angle between field lines and true north. The orientation of the field changes over time, including: In a given region, variations in inclination and declination create a distinct pattern that can be used to correlate sediments in which they are detectable. The changes represent a "random walk" - i. Can only be applied on a regional scale because of relative motion of continents. For unknown reasons, at unpredictable intervals, the north and south poles trade places. Remember, these were instrumental in establishing the reality of sea-floor spreading. Global in scale - therefore excellent marker beds Capable of being obscured by vagaries of deposition and diagenesis, necessitating the use of other sources of stratigraphic data. This can happen in three ways: Many minerals that solidify from magma E. C for hematite, deg. Liesegang banding in sandstone Chemical remanent magnetization CRM: In this case, the field may overprint or obfuscate the intrinsic field of the framework clasts of the rock - a source of analytic difficulty. The primary remanent magnetic polarity of rocks can be either: Magnetic north coincides roughly with geographic north. Magnetic north coincides roughly with geographic south. The record of these reversals is preserved in ocean floor basalt, which both: Because these reversals occur at irregular intervals, periods of time in which they occur have unique, bar-code-like fingerprints. In the Magnetic Polarity Time Scale an imaginary composite stratigraphic column in which deposition is constant right, unique patterns of reversals are clear. In principle, any stratigraphic section that preserved remanent magnetism containing a series of reversals ought to be matchable to this scale. This is the heart of magnetostratigraphy - the correlation of stratigraphic units by the matching of their unique remanent magnetic patterns. The methods strengths include: Unique patterns of geomagnetic reversals are isochronous global marker beds. They are present as long as minerals containing remanent magnetism are present in the rock, totally independent of facies. This makes it possible to correlate marine to continental units. Knowing the paleomagnetic polarity of a sample can, therefore, give an independent means of constraining its age. Nevertheless, radiometric dating of Quaternary volcanics is our primary method of calibrating the chronology of the Magnetic Polarity Time Scale. But reality intrudes to impose limitations on their use: Whereas a lithostratigraphic marker bed, biozone, or chemostratigraphic anomaly might be recognizable from a single outcrop, magnetostratigraphy absolutely requires a stratigraphic section in which a pattern of reversals is visible. Real sections contain changes in depositional rate or disconformitites that obscure the magnetostratigraphic pattern. These must be identified before matching to the Magnetic Polarity Time Scale is possible. Unless the top of the section is recent and the first reversal down is the most recent one, some other stratigraphic method is required to identify the correct region of the Magnetic Polarity Time Scale. Good times and bad: Some intervals of geologic time are not useful because they contain few reversals, including: Long intervals of the Cretaceous right The Upper Carboniferous and Permian And yet, decades of effort have enabled the development of an ideal Magnetic Polarity Time Scale that extends to the Carboniferous, serving as the basis for correlation of real-world sections. Global magnetostratigraphy of the Triassic - Jurassic boundary from Donohoo-Hurley, et al. In the early days of magnetostratigraphy - the s intervals of the same polarity were called epochs and named after luminaries in the field of magnetism E. Brunhes, Matuyama, Gauss, etc. Short intervals of polarity change within epochs were called events and named after the localities where they were found. Olduvai As identifiable intervals increased, a numbering system was substituted for the names of great men. Eventually, magnetostratigraphers switched to a more rational system in which: The top of each normal interval is the top of a "chron". Chrons are numbered but have a letter C prefix to distinguish from the earlier numbering system. Normal and reverse sections of each chron are designated with Page 1
suffixed N and R. Like all other stratigraphic units, magnetostratigraphic chrons are rock units with type localities. I inflict the old system because you are likely to encounter it in the literature. Nuts and Bolts of Sampling: Samples typically collected using a drill that recovers a small core. Such samples must have their orientation recorded! In order to reduce uncertainty, at least three samples, and preferably more are taken from a given locality. Their mean paleomagnetic declination is obtained. The closer together individual samples plot, the more reliable the signal is considered. In the lab Demagnetization: Remember Chemical remnant magnetization CRM? This process tends to overprint the original Natural remanent magnetism NRM of the sample with a weaker and more recent one. To eliminate this overprinting, the sample must ironically be demagnetized. This can be done by: Alternating field AF demagnetization: When this is complete, hopefully only the original NRM remains. Two methods are available: Employs a magnetometer that spins the sample in a range of directions and measures the induced current in an adjacent electrical coil to identify the orientation and strength of the field. This can only be used on physically robust specimens. Beginning in the s, cryogenic magnetometers became the tool of choice. These use an array of cryogenically cooled superconducting sensors to measure the magnetic field of the sample. Page 2
Chapter 2 : Geologic TimeScale Foundation - Stratigraphic Information Magnetic Stratigraphy is the most comprehensive book written in the English language on the subject of magnetic polarity stratigraphy and time scales. This volume presents the entirety of the known geomagneticrecord, which now extends back about million years. Lithostratigraphy Chalk layers in Cyprus, showing sedimentary layering Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type lithology. This variation can occur vertically as layering bedding, or laterally, and reflects changes in environments of deposition known as facies change. These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called the law of superposition, states: Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy. Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals particularly carbonates, grain size, thickness of sediment layers varves and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates. Biostratigraphy Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. It provides strong evidence for the formation speciation and extinction of species. The geologic time scale was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating, which gave it and the stratigraphy it was based on an absolute time framework, leading to the development of chronostratigraphy. One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon -bearing reservoir rocks, seals, and traps of petroleum geology. Chronostratigraphy Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock strata. The branch is concerned with deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created the rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth. A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity. Magnetostratigraphy Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout a section. Upon burial, that orientation is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava. Oriented paleomagnetic core samples are collected in the field; mudstones, siltstones, and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. If the data indicate that the North Magnetic Pole were near the South Rotational Pole, the strata would exhibit reversed polarity. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale. This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for the estimation of sediment-accumulation rates. Page 3
Chapter 3 : Historical Geology: Stratigraphy, part 5 Read the latest chapters of International Geophysics at blog.quintoapp.com, Elsevier's leading platform of peer-reviewed scholarly literature. Technique[ edit ] When measurable magnetic properties of rocks vary stratigraphically they may be the basis for related but different kinds of stratigraphic units known collectively as magnetostratigraphic units magnetozones. The direction of the remnant magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called "magnetostratigraphic polarity units" or chrons. If the data indicate that the North Magnetic Pole was near the Geographic South Pole, the strata exhibit reversed polarity. Sampling procedures[ edit ] Oriented paleomagnetic samples are collected in the field using a rock core drill, or as hand samples chunks broken off the rock face. To average out sampling errors, a minimum of three samples is taken from each sample site. In sedimentary layers, the preferred lithologies are mudstones, claystones, and very fine-grained siltstones because the magnetic grains are finer and more likely to orient with the ambient field during deposition. The NRM is then stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component. Magnetic orientations of all samples from a site are then compared and their average magnetic polarity is determined with directional statistics, most commonly Fisher statistics or bootstrapping. The latitudes of the Virtual Geomagnetic Poles from those sites determined to be statistically significant are plotted against the stratigraphic level at which they were collected. These data are then abstracted to the standard black and white magnetostratigraphic columns in which black indicates normal polarity and white is reversed polarity. Correlation and ages[ edit ] Geomagnetic polarity in late Cenozoic normal polarity black reverse polarity white Because the polarity of a stratum can only be normal or reversed, variations in the rate at which the sediment accumulated can cause the thickness of a given polarity zone to vary from one area to another. This presents the problem of how to correlate zones of like polarities between different stratigraphic sections. To avoid confusion at least one isotopic age needs to be collected from each section. In sediments, this is often obtained from layers of volcanic ash. Failing that, one can tie a polarity to a biostratigraphic event that has been correlated elsewhere with isotopic ages. These ages provide relatively precise dates for features in the rocks such as fossils, changes in sedimentary rock composition, changes in depositional environment, etc. They also constrain the ages of cross-cutting features such as faults, dikes, and unconformities. Sediment accumulation rates[ edit ] Perhaps the most powerful application of these data is to determine the rate at which the sediment accumulated. This is accomplished by plotting the age of each reversal in millions of years ago vs. This provides the rate in meters per million years which is usually rewritten in terms of millimeters per year which is the same as kilometers per million years. Knowing the depth of a hydrocarbon source rock beneath the basin-filling strata allows calculation of the age at which the source rock passed through the generation window and hydrocarbon migration began. Because the ages of cross-cutting trapping structures can usually be determined from magnetostratigraphic data, a comparison of these ages will assist reservoir geologists in their determination of whether or not a play is likely in a given trap. Evidence to strengthen this interpretation can often be found by looking for subtle changes in the composition of the rocks in the section. Changes in sandstone composition are often used for this type of interpretation. Page 4
Chapter 4 : Magnetostratigraphy - encyclopedia article - Citizendium Magnetic stratigraphy The magnetic polarity pattern of the Coaledo Formation is shown in Fig. 5. Almost the entire lower member, and the lower third of the middle member, was normal in polarity, except for a short reversed magnetozone in the upper part of the lower member. Magnetostratigraphic polarity units A. Nature of Magnetostratigraphic Polarity Units When measurable magnetic properties of rocks vary stratigraphically they may be the bases for related but different kinds of stratigraphic units known collectively as "magnetostratigraphic units" "magnetozones". Such reversals of the polarity have taken place many times during geologic history. The direction of the remanent magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called "magnetostratigraphic polarity units". A magnetostratigraphic polarity unit is present only where this property can be identified in the rocks. Conversely, if it points to the present magnetic South Pole, the rock is said to have"reversed magnetization", or "reversed polarity". Magnetostratigraphic polarity units are, therefore, either normal or reversed. A problem arises because the north paleomagnetic pole is believed to have crossed the geographic equator in Paleozoic time, so that for some lower Paleozoic and older rocks it is unclear which is the direction of the North Pole and which the South Pole. Polarity must in these cases be defined with respect to the apparent polar wander path APWP for the crustal plate where it is found. If the direction of magnetization of a rock unit indicates a paleomagnetic pole that falls on the APWP that terminates at the present North Pole, the rock unit has normal polarity; if the magnetization is directed degrees from this, it has reversed polarity. Magnetostratigraphic polarity units have been established in two ways: It has been shown that the two kinds of investigation are correlative and record the same causative process. The first type may be handled by using normal stratigraphic procedures. Units of the second type, currently identified by "anomaly numbers", are deduced from a remotely obtained record of the overall variations of the geomagnetic field from unseen rocks on or below the sea floor. Marine magnetic anomalies are, thus, not true conventional stratigraphic units. However, they are useful units in the reconstruction of continental plate motions and in the interpretation of the geologic history of the ocean basins. The relation of magnetostratigraphic polarity units to other kinds of stratigraphic units is discussed in Chapter The element of stratigraphy that deals with the magnetic characteristics of rock bodies. The organization of rock bodies into units based on differences in magnetic character. A body of rocks unified by similar magnetic characteristics which allow it to be differentiated from adjacent rock bodies. A body of rocks characterized by its magnetic polarity that allows it to be differentiated from adjacent rock bodies. Magnetostratigraphic polarity-reversal horizons and polarity-transition zones. Magnetostratigraphic polarity-reversal horizons are surfaces or thin transition intervals across which the magnetic polarity reverses. Where the polarity change takes place through a substantial interval of strata, of the order of 1 m in thickness, the term "magnetostratigraphic polarity transition-zone" should be used. Magnetostratigraphic polarity-reversal horizons and polarity-transition zones provide the boundaries for magnetostratigraphic polarity units. Kinds of magnetostratigraphic polarity units The basic formal unit in magnetostratigraphic polarity classification is the magnetostratigraphic polarity zone, or simply polarity zone. Polarity zones may be subdivided into polarity subzones and grouped into polarity superzones. Magnetostratigraphic polarity zones may consist of bodies of strata unified by: Procedures for establishing magneto-stratigraphic polarity units. Standards of reference and stratotypes for polarity units require special treatment. The standard of reference for the definition and recognition of a magnetostratigraphic polarity unit for land-based units is a designated stratotype in a continuous sequence of strata that shows its polarity pattern throughout and clearly defines its upper and lower limits by means of boundary stratotypes. These are marked with artificial permanent markers to facilitate restudy. The standard of reference of marine-based units is a designated profile along a designated traverse with all instrumental and guidance conditions specified. This pattern of polarity reversals from the ocean floor should be dated by extrapolation and interpolation from isotopic and paleontologic information. Procedures for Extending Magnetostratigraphic Polarity Units A magnetostratigraphic polarity unit and its Page 5
boundaries may be extended away from its type locality or stratotype only as far as the magnetic properties and stratigraphic position of the unit can be identified. Naming of Magnetostratigraphic Polarity Units. The formal name of a magnetostratigraphic polarity unit is formed from the name of an appropriate geographic feature combined with a term indicating its rank and direction of polarity, e. Jaramillo Normal Polarity Zone. The currently well-established names derived from the names of distinguished contributors to the science of geomagnetism for example, Brunhes, Gauss, Matuyama should not be replaced. Numbered or lettered units may be used informally, but this is not recommended as a general practice. However, the classic linear magnetic anomalies of the ocean floor are excepted, because of their historical importance and dominance in the literature. The time interval represented by a magnetostratigraphic polarity unit is called a chron superchron or subchron if necessary. Chronozone is the term used to refer to the rocks formed anywhere during a particular magnetostratigraphic polarity chron Table 2. Revision of Magnetostratigraphic Polarity Units. Chapter 5 : Magnetostratigraphy - Wikipedia Get this from a library! Magnetic stratigraphy of the type Montediablan stage (late Miocene), Black Hawk Ranch, Contra Costa County, California: implications for regional correlations. Chapter 6 : GEOL - Sedimentation and Stratigraphy magnetic stratigraphy [magâ ²ned ik stré â ²tig ré  fä ] (geophysics) paleomagnetic stratigraphy. Page 6