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" Studies on the petrology and diagenesis of siliceous sediments: "
A. M. El-Shishtawy
R. E. Garrison
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Latin Dissertation
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English
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| Record Number
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1112384
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TLpq303837636
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| Main Entry
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A. M. El-Shishtawy
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R. E. Garrison
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| Title & Author
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Studies on the petrology and diagenesis of siliceous sediments:\ A. M. El-ShishtawyR. E. Garrison
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| College
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University of California, Santa Cruz
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| Date
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1990
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| student score
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1990
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| Degree
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Ph.D.
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| Page No
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359
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| Abstract
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Compositional and micro-textural analyses were carried out on samples of bedded and nodular cherts and porcelanites from the Miocene Monterey Formation of California (USA) and the Cretaceous and Eocene of Egypt. The purpose of this research was to better understand the diagenesis of the siliceous rocks in general and thinly-laminated cherts in particular, and to determine the processes of chertification along the boundaries between the chert and the host-rocks. Electron microscopic examination revealed characteristic textural differences between opal-CT cherts and porcelanites, as well as between quartz cherts and porcelanites. Quartz cherts and porcelanites have smoothly-surfaced, equant or irregular crystals (typically less than 5 microns in diameter), whereas opal-CT cherts and porcelanites consist of ultra-fine bundles of radiating needles (less than 1 micron in length). This study documented that the boundary between opal-CT and quartz is transitional through a zone (2-10 microns thick) of mixed compositions between the two silica phases. Another type of boundary examined in this study is that between chert and host carbonates; such boundaries are sharp between pure chalk and chert, but gradational between siliceous carbonates and chert. Silicified microfossils and replacement textures (i.e. replacement of opal-CT and carbonate by quartz) are most common along these boundaries. Thin, alternating laminae of opal-CT porcelanite, siliceous dolomite and quartz chert characterize parts of the Monterey Formation, but are not common in the Thebes nor the Duwi Formations. These laminations, produced in low oxygen environments, become greatly enhanced by diagenesis. TEM diffraction patterns across the boundaries between opal-CT and quartz laminae in laminated cherts show that gradual replacement of opal-CT by quartz greatly accentuates the laminations. Whereas the Monterey Formation cherts consist of either opal-CT or quartz, cherts of the Thebes and Duwi Formations of Egypt contain only quartz. Quartz replacing opal-CT, as well as sparse diatom fragments in some Monterey Formation cherts, support earlier suggestions that these cherts and porcelanites formed through a transformation of biogenic opal-A usd\tousd opal-CT usd\tousd quartz. Absence of opal-CT precursors and the relatively coarser grain-size of quartz crystals in Egyptian cherts, however, suggest that most of the quartz in these cherts may have transformed directly from opal-A without an intermediate opal-CT stage.
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Earth sciences
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Geology
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