Siliceous microfossils
| Exploring for Oil and Gas Traps | |
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| 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 |
Siliceous microfossils are protists with shells constructed of opaline (amorphous) silica. There is no intense dissolution of siliceous remains in the deep ocean. Sediments deposited below the carbonate compensation depth are commonly enriched in silica by removal of the carbonate, sometimes to the point of forming siliceous oozes. With subsequent remobilization of the silica, deep-sea cherts may be formed. Siliceous microfossils are subject to burial diagenesis and become rare at great well depths except when recrystallized, preserved in nodules or concretions, or replaced by pyrite or calcite.
There are three major groups of siliceous microfossils: radiolarians, diatoms, and silicofla-gellates.
Radiolarians
Radiolarians are planktonic protists that occur primarily in open marine, deep-water settings. They are useful time indicators and are found in rocks of Cambrian to Holocene age. They may be the only common microfossils in abyssal environments, commonly forming radiolarian oozes. Radiolarian chert, the product of silica diagenesis, is fairly widespread in the geologic record. Radiolarians are common in some marine source rocks.
The illustration below shows some typical radiolarians.
Diatoms
Diatoms are photosynthesizing protists that occur in both marine and nonmarine environments. Marine diatoms range from Late Jurassic or Early Cretaceous to Holocene and are particularly useful for age and environmental determinations in the late Cenozoic. Nonmarine diatoms range from Eocene to Holocene and also are useful in the late Cenozoic. These microfossils can be a major rock-forming group, forming sedimentary rock (diatomites) consisting primarily of diatoms. Diatomaceous sediments, when altered by burial diagenesis, are converted to siliceous shale, porcelanite, and chert. Such rocks can serve as sources and fractured reservoirs for hydrocarbons (e.g., Monterey Formation of California). The changes in rock properties associated with silica diagenesis permit seismic definition of silica-phase transformation zones in the subsurface (e.g., bottom-simulating reflector).
The illustration below shows some typical diatoms.
Silicoflagellates
Silicoflagellates are another group of planktonic photosynthesizing marine protists; they commonly occur with diatoms. Silicoflagellates range in age from Cretaceous to Holocene. Although not as common as diatoms, they are useful time indicators, particularly in the late Cenozoic. As a group, they were much more abundant during the early and middle Cenozoic than today. They have been used to estimate marine paleotemperatures in the late Tertiary and Quaternary.
The illustration below shows some typical silicoflagellates
See also
- Paleontology and microfossils
- Microfossils in exploration
- Calcareous microfossils
- Agglutinated microfossils
- Phosphatic microfossils
- Organic-walled microfossils
External links
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