biostratigraphy
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| Biostratigraphy has a fairly simple premise… | Biostratigraphy has a fairly simple premise… | ||
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| It is less amusingly defined as… | It is less amusingly defined as… | ||
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| What “makes biostratigraphy work” is evolution. Species evolve into new forms and have a measurable period of existence before becoming extinct. Each species has a discreet inception and extinction in geologic time. Therefore, periods of geologic time may be characterised by the presence (or absence) of a species, the co-existence of several species or their abundance (see below). | What “makes biostratigraphy work” is evolution. Species evolve into new forms and have a measurable period of existence before becoming extinct. Each species has a discreet inception and extinction in geologic time. Therefore, periods of geologic time may be characterised by the presence (or absence) of a species, the co-existence of several species or their abundance (see below). | ||
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| + | //Evolution drives the appearance of new species and the subsequent extinction of those species. These are the | ||
| + | stratigraphic ranges of many types of Graptolites – an extinct group of Paleozoic colonial organisms.// | ||
| **Biozonation** | **Biozonation** | ||
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| A “biozone” (see figure below) is an interval of rock between two successive biostratigraphic events in the rock record. Ideally, such events should be relatively closely spaced (i.e. not too far apart in geologic time), easily recognisable and commonly found in rock sections around the world. | A “biozone” (see figure below) is an interval of rock between two successive biostratigraphic events in the rock record. Ideally, such events should be relatively closely spaced (i.e. not too far apart in geologic time), easily recognisable and commonly found in rock sections around the world. | ||
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| + | //A properly defined biozone is independent of lithology type and sedimentation rate.// | ||
| In the literature most formally-described biozones are defined by the successive first appearances of new species in the fossil record (i.e., the “base” of a fossil’s total stratigraphic range). Why exactly this is, is uncertain, although it probably seems logical given that the passage of geologic time moves “upwards” through a rock succession. | In the literature most formally-described biozones are defined by the successive first appearances of new species in the fossil record (i.e., the “base” of a fossil’s total stratigraphic range). Why exactly this is, is uncertain, although it probably seems logical given that the passage of geologic time moves “upwards” through a rock succession. | ||
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| **Are “Horizons” biozones?** In the literature there are sometimes references to “horizons” named after fossil species. Again, these tend to occur more frequently in Mesozoic sections and concern ammonites. A “horizon” in this sense is a single layer, along which a particular species has been recorded. Logically, it is a two-dimensional surface lacking the dimension of time and therefore not a biozone. However, it may prove useful for correlation purposes although of course there is no guarantee that a “horizon” observed in one place was laid down at the same time as the same “horizon”” in another. This is because the species will have logically existed for at least a definite time “duration” in its existence on Earth. Ammonites however, are generally comprised of short-lived species and in the general sense, two separate “horizons” may be correlatable within those limits of resolution. | **Are “Horizons” biozones?** In the literature there are sometimes references to “horizons” named after fossil species. Again, these tend to occur more frequently in Mesozoic sections and concern ammonites. A “horizon” in this sense is a single layer, along which a particular species has been recorded. Logically, it is a two-dimensional surface lacking the dimension of time and therefore not a biozone. However, it may prove useful for correlation purposes although of course there is no guarantee that a “horizon” observed in one place was laid down at the same time as the same “horizon”” in another. This is because the species will have logically existed for at least a definite time “duration” in its existence on Earth. Ammonites however, are generally comprised of short-lived species and in the general sense, two separate “horizons” may be correlatable within those limits of resolution. | ||
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| + | //Types of Biozones and characteristics of species ranges that help define them. “Highest Occurrence Zones” are often used in industry because using extinction events or “tops” to define biozones mitigates against some of the effects of sample contamination due to “caving”.// | ||
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| + | //This diagram illustrates some of the natural factors controlling distribution that can have a significant effect on how a species (and therefore its fossil remains) can be distributed in time and space in rocks during the lifetime **of the species itself**. Other factors after death will also come into play (such as dissolution or weathering) which determine whether the fossil remains are ultimately preserved or not. Consequently we can never be 100% certain if (for example) the highest occurrence of a fossil species in an individual rock section represents its final " | ||
biostratigraphy.1679300154.txt.gz · Last modified: 20/03/2023 08:15 by mike_gss