Cements micritic cement

Calcite precipitation under phreatic conditions can continue uninterrupted by an air-water interface (Morse Mackenzie, 1990). Thus, isopachous or drusy, poikilotopic and blocky spar cements are most often associated with precipitation in the phreatic zone (Jacka, 1970 Folk, 1974 Retallack, 1990 Bums Matter, 1995). Sparry cements can also form in the vadose zone as calcans or crystic nodules, but they are associated with soil zonation, highly dense micritic cements and nodules (Weider Yaalon, 1982). Because these cements are not associated with such features they are unlikely to represent calcans or crystic nodules. 

Barite occurs in trace amounts as patchy poikilo-topic, grain-replacive cement associated with partially dissolved carbonate cement. Anhydrite cement forms < 1 vol% in the fine- to medium-grained sandstones that contain micritic carbonate intraclasts (calcretes/dolocretes), occurring in the upper part of... 

Diagenesis is the conversion of sediments into rock by organic, physical and chemical processes. Six main processes have been identified for limestone [2.3] — microbial micritization, cementation, neomorphism, dissolution, compaction and dolomitization. 

Nodular Term synonymous with glaebular and refers to soft to highly indurated concretions of carbonate, or carbonate cemented host material. The margins may be gradational to sharp, and internally the nodules may be uniform, showing concentric laminae or septarian cracks or veins. The nodules can range in shape from spherical to elongate. The nodules typically consist of micrite or less commonly microsparite. Nodular calcrete develops primarily in siliciclastic host material... 

Figure 6.8 (A) Scanning electron microscopy image of diatom frustules within actively forming barrage tufas at Cwm Nash, Glamorgan. Scale bar 20 pm. (B) Detrital tufas from ancient barrage at Caerwys, North Wales. Micritic peloids within sparitic cement with much remaining pore space. Photographed under crossed nicols, width of image 2.5 mm.

In addition to dissolution, the destabilization of carbonate cements may result in recrystallization and replacement by other carbonates. Microcrystalline calcite and dolomite are sensitive to recrystallization at various burial depths. The recrystallization of dolomite has been reviewed by Mazzullo (1992). Burial recrystallization of micritic/microsparitic cements in sandstones may result in the formation of poikilotopic calcite (Saigal Bjorlykke, 1987). However, poikilotopic calcite is also a common primary cement in calcretes (e.g. Knox, 1977 Tan-don Narayan, 1981). Recrystallized calcite and dolomite are recognized as patchily distributed, coarsened crystals. In contrast, precipitational vari-... 

Micritic fabric, meniscus cements, circumgranular cracking, crystallaria, alveolar textures, grain dissolution... 

Calcite cements associated with these concretions are dominantly micritic and exhibit circumgranular cracking, alveolar, micrite-spar and meniscus microtextures. Radial spar microtexture is present locally, characterized by bladed radial spar formed around a micritic nucleus (Fig. 1OD). 

Type 2 tabular units lack original sedimentary structures and are often associated with reddened clays and clayey sands from overbank fine (OF), palaeosol (P) and interdune (ID) deposits (Table 1 Fig. 6). Micritic calcite is the main cement, and micrite-spar textures, grain dissolution, alveolar structures, circumgranular cracking and meniscus cement are common. Type 2 tabular units are subdivided by outcrop morphology into massive, platy, wavy bedded, fractured and laminar types. 

Fig. 13. Ternary diagram showing composition of micrite, spar and Sand Hill fault cements from the study area. The scale of the plot is at 99 mol% CaCOj. Data for fault cements from Mozley Goodwin (1995b).

Cements near the vadose end-member are associated with typical vadose features however, these features are less apparent than in type 2 tabular units, and sparry void filling cements are sometimes more abundant than micrite. As stated previously, micrite-spar cement textures could have initially formed in the vadose zone as pendant and meniscus envelopes around grains or groups of grains (Jacka, 1974 Reeves, 1976 Warren, 1983). Upon burial, these initial vadose cements would provide sites for further calcite precipitation and the unfilled voids could subsequently be filled with sparry calcite in the phreatic zone (Jacka, 1970 Funk, 1979). The vadose contribution to cementation may have been overlooked in the past because of this overprinting. 

In the Upper Namur Sandstone the poikilotopic calcite cement is absent in moderately to poorly sorted quartz arenites and feldspathic quartz aren-ites that contain abundant primary porosity and some secondary porosity. Only minor patches of siderite micrite and microspar are observed in these elastics, where euhedral quartz overgrowths are well developed, albeit not volumetrically significant (< 5-10%) (Fig. 14D). The detrital grains are dominated by tangential and long contacts, with rare sutured contacts. 

Secondary porosity contributes more than half (6.7%) to the total porosity (average.of 11%, excluding tight-sandstone zones) of the Catalina Sandstone. Of the available drill cores from wells 0-35 and K-18, 45.7 and 17.5%, respectively, are completely cemented by early ferroan calcite. The difference between the two wells may be due to the fact that the sandstone beds are thicker in well K-18. Where the early ferroan calcite is absent, mechanical compaction and quartz overgrowths have reduced the primary porosity further. In K-18, the framework grains are largely coated by a micritic-calcite rim, approximately 25 pm thick, which prevented silica cementation. Dissolution of the early ferroan calcite cement contributed most of the... 

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