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Diagenesis is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation (lithification).
Table of contents
- Stress-induced chemical waves in sediment burial diagenesis
- Introduction to diagenesis
- How Sediment Turns to Rock
Marine waters with high salinities or fresh waters with high magnesium to calcite ratios may form dolomite s.
Stress-induced chemical waves in sediment burial diagenesis
The carbonate cements. Those that are of immediate interest to the carbonate geologist are strontium, manganese and iron because they provide data that can be used to interpret cement origin. Strontium substitutes for calcium in the aragonite lattice, whereas when the waters in contact with the cement are reducing, manganese and iron substitute for calcium in the calcite lattice. Manganese and iron are especially important because they occur in ancient limestone s and dolomite s.
Trace element composition of carbonate s is studied by staining of Rock slabs or thin sections, chemical analysis, cathodoluminescence, and EDAX or microprobe see Scholle, , p. Stable isotope composition of carbonate s can be used, like trace elements, to unravel carbonate diagenetic history. For instance, the two most common isotope s of oxygen, and , have different compositional ratios depending on the diagenetic environment. Similarly the common isotope s of carbon, 13C and 12C, show much the same relationship. Lighter carbon is associated with fresh water and deeper burial.
Some of the light carbon may be derived from organic carbon associated with migrating hydrocarbons. In cases where oxygen and carbon isotope s have been systematically analyzed through a cement, they lighten from the base of a crystal to its margins. The setting and timing of diagenesis is interpreted by the petrography and geochemistry of cement crystals along with their distribution within the Rock left figure.
For instance, cement crusts form in both marine and fresh-water phreatic environments. The marine crusts are commonly isopachous fringes of aragonite fibers or magnesium calcite blades. Unfortunately the latter may be confused with fringes of calcite blades that form in fresh waters.
Irregular crusts are associated with poor permeabilities or low rates of calcium carbonate precipitation, and isopachous crusts are associated with good permeabilities. Cements precipitated from fresh waters in the vadose zone typically have a patchy distribution, reflecting the occurrence of both air and water in the pore space.
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Where pores are incompletely filled, the cement occurs only at grain contacts and crystals have curved meniscus faces. Late-stage, deep burial cement crystals are also unique. They commonly are large crystals filling more than one interparticle pore. They can be confused with early-formed epitaxial overgrowths on echinoderm plates in a Rock that contains echinoderm debris.
Introduction to diagenesis
The diagenesis of carbonate s can take place in many settings: the marine environment during deposition of the sediment, near the sediment surface where fresh waters penetrate the sediments, or in brines of the deeper subsurface figure above. The diagenetic setting can be reasonably interpreted with detailed study.
In this way the porosity evolution of a carbonate sequence can be unraveled to more accurately predict reservoir porosity trends. Marine cements form in a broad spectrum of environments, extending from the deep sea to beaches left figure. Deep sea cements, commonly mammilated or isopachous layers of magnesium calcite and aragonite, produce hardgrounds where there is good bottom current movement. The flanks of carbonate platforms and the margins of submarine channels and canyons are such sites. Other deep-sea cementation may also occur in areas of negligible sedimentation and may be associated with volcanics or with elevated salinities in enclosed basin s.
Cementation is common in shallow water where the reef margin is one of the better-documented examples above figure. Reef boundstones are commonly tight, thus dissolution by fresh waters or fracturing are important processes in creating reservoir quality porosity. Reef cementation usually occurs as bladed or pelleted micritic magnesian calcite or fibrous aragonite cements.
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The fabric of the cement is controlled by access to circulating water. Voids with greater permeability may be filled by isopachous or equant cements whereas the more confined voids show irregular fibrous crystals.
The zone of active cementation in reefs appears to be confined to the outer few feet, where circulation of waters being pumped through the reef is at its greatest. Evidence of this localization of cementation has been recognized in the reefs from Belize, which are cemented only on their seaward margin.
Eniwetok atoll left figure provides a good example of changing cementation patterns through time. The initial cements in reef debris are magnesium calcite and aragonite marine cements. The marine cements were followed by a fresh water calcite cement which in turn may have marine sediment perched on it.
The cement sequence is a response to the different waters which came into contact with the sediments during a sea level lowering and reflooding. Syndepositional cementation of carbonate sediments also occurs on the shallow platform or shelf immediately behind a break in slope. Cementation occurs during the formation of oolites , grapestones, and hardened pellets. For instance, the oolites which occur in high energy zones are formed by the precipitation of calcium carbonate around a nucleus.
Grapestones, found to the lee of oolite shoals in a quieter water setting, are formed by the precipitation of carbonate cement at points of contact between sand-sized grains and by partial disaggregation of cemented crusts by storms. Further evidence of cementation is the hardening of pellets on open shelf seabottoms, whereas in protected water leeward of the hardened pellet zone on the Great Bahama Bank, lime mud pellets remain uncemented.
How Sediment Turns to Rock
Submarine cemented crusts that are common to platform settings are formed during deposition. In the Arabian Gulf, extensive cementation in crusts causes the crust to expand and override itself forming compressional ridges or submarine tepees figure above.
http://blacksmithsurgical.com/t3-assets/picture/shock-the-rare-book.php These mark the margins of saucer-like expansion megapolygons that form during the cementation and expansion of these submarine surfaces. Submarine crusts or hardgrounds can be recognized in the Rock record by the presence of multiple generations of borings which cut through both sediment and cement.
The diatom flora initially indicated an in situ succession of later middle Miocene age 1,2 , which was supported by the occurrence of benthic foraminifera 3. However, subsequent diatom analyses record the presence of flora with mixed ages mostly Miocene but also Pliocene and Pleistocene throughout both of the two lithological units present in the cores. These flora are taken to indicate repeated reworking, which most recently occurred no earlier than the late Pleistocene 4. These contrasting views have since been re-affirmed 5,6.
Our studies of sediment geochemistry show self-consistent patterns in organic carbon, organic nitrogen and sulphur diagenesis which suggest a Recent, in situ origin, at least for the upper unit. Brady, H. Science , — Webb, P. RISP Tech. Kellogg, T. Nature , — Hudson, J. Acta 31 , — Scott, A. Soil Sci. Krom, M. Wrenn, J. When you read an eBook on VitalSource Bookshelf, enjoy such features as: Access online or offline, on mobile or desktop devices Bookmarks, highlights and notes sync across all your devices Smart study tools such as note sharing and subscription, review mode, and Microsoft OneNote integration Search and navigate content across your entire Bookshelf library Interactive notebook and read-aloud functionality Look up additional information online by highlighting a word or phrase.
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