Siliceous microfossils

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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
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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 silicoflagellates.


Figure 1 Typical 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.

Figure 1 The illustration below shows some typical radiolarians.


Figure 2 Typical diatoms.

Diatoms are photosynthesizing protists that occur in both marine and nonmarine environments. Marine diatoms range from Upper Jurassic or Lower Cretaceous to Holocene and are particularly useful for age and environmental determinations in the upper Cenozoic. Nonmarine diatoms range from Eocene to Holocene and also are useful in the upper 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, porcellanite, 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).

Figure 2 shows some typical diatoms.


3}Typical 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 upper 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.

Figure 3 shows some typical silicoflagellates.

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Siliceous microfossils
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