Difference between revisions of "Calcareous microfossils"
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==Calcareous foraminifera== | ==Calcareous foraminifera== | ||
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| + | [[file:applied-paleontology_fig17-1.png|left|thumb|{{figure number|1}}See text for explanation.]] | ||
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Calcareous foraminifera are a group of unicellular organisms (protists) that secrete a rigid calcite or aragonite shell (or test). Fossils of these forms are found in sediments of brackish to marine origin from Silurian to Holocene in age. Most are benthic (bottom dwelling), but a significant group in the late Mesozoic and Cenozoic are planktonic (floating) forms. | Calcareous foraminifera are a group of unicellular organisms (protists) that secrete a rigid calcite or aragonite shell (or test). Fossils of these forms are found in sediments of brackish to marine origin from Silurian to Holocene in age. Most are benthic (bottom dwelling), but a significant group in the late Mesozoic and Cenozoic are planktonic (floating) forms. | ||
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Because many species have limited and well-known environmental ranges, they are excellent paleobathymetric and paleoenvironmental indicators, especially in younger Phanerozoic rocks. | Because many species have limited and well-known environmental ranges, they are excellent paleobathymetric and paleoenvironmental indicators, especially in younger Phanerozoic rocks. | ||
| − | + | [[:file:applied-paleontology_fig17-1.png|Figure 1]] shows some typical calcareous foraminifera. | |
| − | [[file:applied-paleontology_fig17- | + | ==Ostracodes== |
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| + | [[file:applied-paleontology_fig17-2.png|thumb|{{figure number|2}}See text for explanation.]] | ||
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Ostracodes are microscopic crustaceans whose fossils are found in Cambrian to Holocene rocks. They occur in most marine and nonmarine depositional environments and are generally excellent environmental indicators. The paleontologic application of ostracodes is limited because (1) they are rare in many sections and (2) many species are endemic to local basins, so their age and environmental ranges are poorly understood. Ostracodes typically have rapid evolutionary rates and are useful biostratigraphic tools in some situations: | Ostracodes are microscopic crustaceans whose fossils are found in Cambrian to Holocene rocks. They occur in most marine and nonmarine depositional environments and are generally excellent environmental indicators. The paleontologic application of ostracodes is limited because (1) they are rare in many sections and (2) many species are endemic to local basins, so their age and environmental ranges are poorly understood. Ostracodes typically have rapid evolutionary rates and are useful biostratigraphic tools in some situations: | ||
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Ostracodes may also indicate thermal maturation of source rocks (see “Thermal Maturation”). | Ostracodes may also indicate thermal maturation of source rocks (see “Thermal Maturation”). | ||
| − | + | [[:file:applied-paleontology_fig17-2.png|Figure 2]] shows some typical ostracodes. | |
| − | [[file:applied-paleontology_fig17- | + | ==Calcareous nannofossils== |
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| + | [[file:applied-paleontology_fig17-3.png|left|thumb|{{figure number|3}}See text for explanation.]] | ||
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The term ''calcareous nannofossils'' includes both fossil coccoliths and nannoliths. '''Coccoliths''' are minute ( | The term ''calcareous nannofossils'' includes both fossil coccoliths and nannoliths. '''Coccoliths''' are minute ( | ||
Calcareous nannofossils are an excellent biostratigraphic tool because of their rapid evolution and geographic dispersal (i.e., their entire life cycle is in the photic zone of the ocean) as well as their varied and distinct morphologies. The oldest known calcareous nannofossils are Late Triassic; they are a crucial microfossil group in calibrating the Jurassic-Holocene marine record. Relatively little has been published about the paleogeographic distributions of calcareous nannofossils; less is known about their exact paleoenvironmental preferences, although they have been shown occasionally to penetrate into shallow marine environments. Their main industrial application is their calibration to published time scales and sequence stratigraphic records, especially the association of high abundance with condensed marine sections. | Calcareous nannofossils are an excellent biostratigraphic tool because of their rapid evolution and geographic dispersal (i.e., their entire life cycle is in the photic zone of the ocean) as well as their varied and distinct morphologies. The oldest known calcareous nannofossils are Late Triassic; they are a crucial microfossil group in calibrating the Jurassic-Holocene marine record. Relatively little has been published about the paleogeographic distributions of calcareous nannofossils; less is known about their exact paleoenvironmental preferences, although they have been shown occasionally to penetrate into shallow marine environments. Their main industrial application is their calibration to published time scales and sequence stratigraphic records, especially the association of high abundance with condensed marine sections. | ||
| − | + | [[:file:applied-paleontology_fig17-3.png|Figure 3]] shows some typical calcareous nannofossils. | |
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| − | [[file:applied-paleontology_fig17-3.png| | ||
==See also== | ==See also== | ||
Revision as of 16:31, 21 January 2014
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Predicting the occurrence of oil and gas traps |
| Chapter | Applied paleontology |
| Author | Robert L. Fleisher, H. Richard Lane |
| Link | Web page |
| Store | AAPG Store |
Calcareous microfossils have shells composed of calcite or aragonite. These organisms are present in most marine and in some nonmarine environments. At great oceanic depths characterized by low temperature and high hydrostatic pressure, however, calcareous remains are largely or completely dissolved. The depth below which this occurs, which varies in different oceanographic settings, is termed the carbonate compensation depth (CCD).
There are three principal types of calcareous microfossils: calcareous foraminifera, ostra-codes, and calcareous nannofossils.
Calcareous foraminifera
Calcareous foraminifera are a group of unicellular organisms (protists) that secrete a rigid calcite or aragonite shell (or test). Fossils of these forms are found in sediments of brackish to marine origin from Silurian to Holocene in age. Most are benthic (bottom dwelling), but a significant group in the late Mesozoic and Cenozoic are planktonic (floating) forms.
Some stratigraphically important foraminifera developed complex internal structures and, frequently, large test size. Studied primarily in thin section, these include the fusulinids (Pennsylvanian to Permian) and several groups of so-called larger foraminifera (Triassic to Holocene). They occur primarily in carbonate or fine-grained clastic rocks and are excellent time markers.
Because many species have limited and well-known environmental ranges, they are excellent paleobathymetric and paleoenvironmental indicators, especially in younger Phanerozoic rocks.
Figure 1 shows some typical calcareous foraminifera.
Ostracodes
Ostracodes are microscopic crustaceans whose fossils are found in Cambrian to Holocene rocks. They occur in most marine and nonmarine depositional environments and are generally excellent environmental indicators. The paleontologic application of ostracodes is limited because (1) they are rare in many sections and (2) many species are endemic to local basins, so their age and environmental ranges are poorly understood. Ostracodes typically have rapid evolutionary rates and are useful biostratigraphic tools in some situations:
- In Paleozoic sequences
- In marine environments where wide-ranging species are present
- For local stratigraphy in basins of limited extent
- In lacustrine environments, where they are frequently one of the few microfossils present
Ostracodes may also indicate thermal maturation of source rocks (see “Thermal Maturation”).
Figure 2 shows some typical ostracodes.
Calcareous nannofossils
The term calcareous nannofossils includes both fossil coccoliths and nannoliths. Coccoliths are minute (
Calcareous nannofossils are an excellent biostratigraphic tool because of their rapid evolution and geographic dispersal (i.e., their entire life cycle is in the photic zone of the ocean) as well as their varied and distinct morphologies. The oldest known calcareous nannofossils are Late Triassic; they are a crucial microfossil group in calibrating the Jurassic-Holocene marine record. Relatively little has been published about the paleogeographic distributions of calcareous nannofossils; less is known about their exact paleoenvironmental preferences, although they have been shown occasionally to penetrate into shallow marine environments. Their main industrial application is their calibration to published time scales and sequence stratigraphic records, especially the association of high abundance with condensed marine sections.
Figure 3 shows some typical calcareous nannofossils.
See also
- Paleontology and microfossils
- Microfossils in exploration
- Agglutinated microfossils
- Siliceous microfossils
- Phosphatic microfossils
- Organic-walled microfossils
External links
| find literature about Calcareous microfossils |
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