K-feldspar overgrowths

K-feldspar occurs as overgrowths and fracturehealing of detrital K-feldspar, and as discrete K-feldspar crystals (Fig. 9A) disseminated in both shelf and turbidite arenites in trace amounts. K-feldspar overgrowths and discrete crystals are covered and engulfed by, and therefore pre-date, calcite cement (Fig. 9A). The diagenetic K-feldspar shows a near stoichiometric KAlSijOg end-member composition. 

Together with calcite, other diagenetic features of the Namorado Sandstone include dolomite, opal, kaolinite, K-feldspar overgrowths, chlorite, albite, anatase, barite, pyrite and pseudomatrix generated by squeezing of clay clasts. The whole diagenetic evolution is detailed by Carvalho (1990), Abreu et al. (1992) and Carvalho et al. (1995). 

Figure 5 Highly microporous aggregate of authigenic albite replacing a K-feldspar (evidenced by the adularia overgrowth, K). Frio Formation, Oligocene, South Texas. SEM image (source Milliken, 1989) (reproduced by permission of SEPM (Society for Sedimentary Geology) fromi. Sedim. Petrol, 1989, 59, 364).

Fig. 4. Post-quartz late calcite (c) localized on K-feldspar remnants (k). Arrow indicates euhedral terminations of quartz overgrowths. Back-scattered electron image. Scale bar lOOftm.

Fig. 7. Thin-section scale localization of authigenic ferroan dolomite. Back-scattered electron images. Scale bars 100 tm. (A) Ferroan dolomite (f) localized on non-ferroan detrital dolomite (d) that has been fractured, partially dissolved, and partially replaced by the authigenic overgrowth. Note that the outer zones of the ferroan dolomite (f ) are brighter, reflecting their greater Fe content. (B) Ferroan dolomite (ankerite) (f) localized in the vicinity of a partially dissolved and replaced K-feldspar (k). The arrow indicates euhedral termination on a quartz overgrowth.

Authigenic albite occurs as discrete euhedral crystals (10 pm), overgrowths on detrital feldspars, and most commonly as replacement of detrital K-feldspar and plagioclase in the proximal and middle domain. K-feldspar grains in these areas commonly show partial albitization, whereas the detrital plagioclase is totally replaced by albite. Albitized feldspar grains display the typical optical, textural and chemical characteristics outlined by... 

Morad (1986) and Morad et al. (1990). Authigenic K-feldspars occur as overgrowths around detrital orthoclase and microcline in the deeper portions of the Caioba area of the distal domain, where feldspar kaolinization/dissolution was less intense (av. < 1%). In places overgrowths surround secondary pores formed by the dissolution of host grains. 

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. 

Calcite cementation in bioclastic hybrid and lithic arenites of the Bismantova-Termina succession is pervasive along layers and concretionary horizons. Cementation in the hybrid shelf arenites was mostly precompactional and began with marine calcite rims, syntaxial overgrowths on echinoderms, K-feldspar and dolomite overgrowths, chloritic clay rims, framboidal pyrite and heulandite, followed by... 

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