Overgrowth cements quartz

Reed R. M. and Laubach S. E. (1996) The role of microfractures in the development of quartz overgrowth cements in sandstones new evidence from cathodoluminescence smdies. Geolog. Soc. Am. Annual Meeting, 28(7),... 

Franks and Forester (1984) have discussed this mechanism in detail for Gulf Coast sediments, with particular emphasis on pre- and post-secondary porosity mineral assemblages. For many localities they found strikingly similar mineral assemblages (Table 8.1). Early carbonate cements had precipitation temperatures in the range of 40° to 75°C. Quartz overgrowths were observed to precipitate... 

Opaline/fine-grained massive Quartzitic No detrital component, massive, comprising opaline, chalcedonic or cryptocrystalline silica Grain supported fabric cemented by overgrowths on detrital quartz... 

GS-fabric Grain supported fabric - skeletal grains (i.e. grains >30 pm diameter) constitute a self-supporting framework. Subdivided by cement type (a) optically continuous quartz overgrowths (b) chalcedonic overgrowths (c) microquartz/cryptocrystalline/opaline silica in-fill... 

Quartz cement occurs as syntaxial overgrowths on detrital quartz grains (Fig. 14B) heals fractures in detrital quartz, and forms discrete prismatic crystals (Fig. 14A). It is more common in the coarse-... 

Fig. 14. (A) Scanning electron micrograph of prismatic quartz outgrowths covering and engulfing chlorite rims (B) optical photomicrograph of a sandstone extensively cemented by quartz overgrowths which contain bitumen inclusions crossed polarizers.

Authigenic K-feldspar is present in the Gurten samples as thin overgrowths on detrital K-feldspar grains. Quartz cement was found only in the SEM in two samples from Altishofen. Sulphates (barite. 

Chaunoy sandstones of the same depositional facies are cemented by both quartz and dolomite (Fig. 3B). Quartz cement forms approximately equal thickness overgrowths whereas dolomite cements tend to fill pores (Fig. 3B) (Cade et al., 1994). These two different cement morphologies haye profoundly different effects upon the pore network. 

Calcite cement typically pre-dates quartz cementation, as indicated by the lack of quartz overgrowths within calcite-cemented zones (Table 1). As abundant fluid inclusion evidence (see review in Walderhaug, 1994) strongly indicates that quartz cementation typically becomes significant at temperatures of 70-80°C, this is strong confirmation that most calcite cementation was completed at temperatures below around 0 C. 

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. 

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