Stratigraphy relative dating

This principle is a powerful tool for determining the age of sedimentary rocks. Index fossils are ones that only occur within limited intervals of geologic time. Much geological research has been done to determine the extent of geologic time through which particular index fossils occurred. By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale.

Relative Dating - Example 2

They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones. In the 20th century, radiometric methods of absolute age determination were developed. These methods allow the ages of certain types of rocks and minerals to be quantified in terms of years. By the s absolute dating methods had been used to determine the ages of many rocks from all the continents and ocean floors.

Repeatedly, the absolute age determinations confirmed what geologists already knew, for example that the Cambrian period occurred before-is older than-the Ordovician period. The absolute dating methods proved that the relative dating methods had been correct, and now geologists can say not only state the sequence of geologic time, they can also estimate fairly accurately how many years ago each division in the sequence occurred.

Another essential concept in stratigraphy is the unconformity. An unconformity is a surface upon which no new sediments were deposited for a long geologic interval. During this interval, erosion may have occurred before more deposits of sediments covered the surface.


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An unconformity marks a "gap in geologic time" because the rocks below and above it come from widely separated geologic times. There are no sedimentary strata to record what happened during the intervening interval. Instead, there is just an unconformity, a buried erosional or non-depositional surface. Unconformities separate chapters in the geologic history of a given region. For instance, an orogenic episode a long geologic episode of mountain building may finally come to end and the eroded mountains may be buried beneath a new sequence of sediments.

A major unconformity would mark the change from the building up of mountains to the wearing down of those same mountains and the subsequent blanketing of the area with sediments.

Relative dating

There are several specific types of unconformities. The three major, specific types of unconformities are included here. The key to identifying each specific type of unconformity is recognizing what the unconformity is on top of.


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  6. The possibilities for what is in the rocks immediately beneath the unconformity are 1 layers of sedimentary or volcanic rock strata that have been tilted or folded prior to development of the unconformity; 2 a stratum is parallel to the unconformity and parallel to the stratum above the unconformity; or 3 plutonic or metamorphic rocks, which originated much deep in the earth's crust rather than at its surface.

    An angular unconformity is an unconformity beneath which the strata were tilted or folded before deposition of the younger layers of sediment above the unconformity. After being tilted or folded, the older layers of sediment were eroded. Then younger layers of sediment were deposited on them. The angular unconformity is the contact between the younger layers of sediment and the older, tilted strata beneath. A nonconformity is an unconformity with sedimentary or volcanic strata on top and, beneath it, either plutonic rock such as granite or metamorphic rock such as schist.

    Because granitic and metamorphic rocks form deep in the earth's crust, a significant amount of time is required for uplift and erosion to expose them. Nonconformities mark major chapter breaks in the geologic history of an area. In the example below, the contact between the conglomerate and the granite beneath it appears likely to be a nonconformity.

    However, it is possible that the granite may have intruded as a magma within the crust, beneath conglomerate, after the conglomerate formed. If so, the granite is younger and the boundary between the granite and the conglomerate is an intrusive contact rather than a nonconformity. To determine the nature of the contact - whether it is an intrusive contact or a nonconformity - further evidence from field investigations would be needed.

    Evidence such as angular pieces of conglomerate surrounded by the granitic intrusion, and contact metamorphism of the conglomerate adjacent to the granite, would indicate that the granite is younger and intruded the older conglomerate. Evidence such as rounded pebbles of the granite within the conglomerate would indicate that the granite is older and underwent erosion prior to the conglomerate forming, and the contact is a nonconformity.

    Stratigraphic Dating

    The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat facies change in sedimentary strata , and that not all fossils may be found globally at the same time. The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous. Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin.

    Sediment will continue to be transported to an area and it will eventually be deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source. Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location.

    In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material. The lateral variation in sediment within a stratum is known as sedimentary facies. If sufficient sedimentary material is available, it will be deposited up to the limits of the sedimentary basin.

    Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type. Melt inclusions are small parcels or "blobs" of molten rock that are trapped within crystals that grow in the magmas that form igneous rocks. In many respects they are analogous to fluid inclusions. Melt inclusions are generally small — most are less than micrometres across a micrometre is one thousandth of a millimeter, or about 0.

    Nevertheless, they can provide an abundance of useful information. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of useful information from melt inclusions.

    Basics--Stratigraphy & Relative Ages

    Two of the most common uses of melt inclusions are to study the compositions of magmas present early in the history of specific magma systems. This is because inclusions can act like "fossils" — trapping and preserving these early melts before they are modified by later igneous processes. In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements such as H 2 O, CO 2 , S and Cl that drive explosive volcanic eruptions.

    Sorby was the first to document microscopic melt inclusions in crystals. The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques. Scientists from the former Soviet Union lead the study of melt inclusions in the decades after World War II Sobolev and Kostyuk, , and developed methods for heating melt inclusions under a microscope, so changes could be directly observed.

    Although they are small, melt inclusions may contain a number of different constituents, including glass which represents magma that has been quenched by rapid cooling , small crystals and a separate vapour-rich bubble. They occur in most of the crystals found in igneous rocks and are common in the minerals quartz , feldspar , olivine and pyroxene.

    The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both volcanic and plutonic rocks. The law of included fragments is a method of relative dating in geology. Essentially, this law states that clasts in a rock are older than the rock itself. Another example is a derived fossil , which is a fossil that has been eroded from an older bed and redeposited into a younger one.

    This is a restatement of Charles Lyell 's original principle of inclusions and components from his to multi-volume Principles of Geology , which states that, with sedimentary rocks , if inclusions or clasts are found in a formation , then the inclusions must be older than the formation that contains them. These foreign bodies are picked up as magma or lava flows , and are incorporated, later to cool in the matrix.

    Additional Topics

    As a result, xenoliths are older than the rock which contains them Relative dating is used to determine the order of events on Solar System objects other than Earth; for decades, planetary scientists have used it to decipher the development of bodies in the Solar System , particularly in the vast majority of cases for which we have no surface samples.

    Many of the same principles are applied. For example, if a valley is formed inside an impact crater , the valley must be younger than the crater. Craters are very useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.

    Relative dating methods in archaeology are similar to some of those applied in geology. The principles of typology can be compared to the biostratigraphic approach in geology.