Philippe Muchez

Diagenetic control on secondary porosity in flood plain deposits: an example of the Lower Triassic of northeastern Belgium

Abstract The Lower Triassic sandstones of northeastern Belgium were deposited in channels and by sheet floods. Introduction of clays, formation of iron oxides and alteration of feldspars in these sediments occurred at near-surface conditions. Subsequent calcite precipitation and the replacement of feldspars and quartz occurred at low temperatures (21°–26°C) in meteoric waters with a δ 18 O ≈ − 5.8‰ SMOW. Characteristic is the bleaching of the originally red-coloured sediments. Red coloration was caused by iron oxide pigments in clay coatings around quartz grains. The bleaching is the result of the reduction of these iron oxides, most likely by organic acids. The oxidation-reduction reaction produced carbon dioxide and protons, which altered the feldspars. The underlying Upper Carboniferous strata are a potential source for organic acids released by thermocatalytic degradation of its kerogen. The bleaching took place before a period of important authigenic quartz growth, which occurred after compaction. Feldspars, quartz and calcites have been replaced by dolomite and ankerite. These carbonates could have formed from meteoric waters at relatively low temperatures (e.g. 50°C) as well as from brines at higher temperatures (e.g. 120°C). Following dolomite/ankerite cementation, an intensive dissolution of feldspar took place. The secondary pores have been filled with kaolinite, barite and illite. Primary and secondary porosity is now significantly lower in clay-rich sandstones than in pure sandstones. Early diagenetic replacive calcites have a negative effect on late diagenetic porosity creation. Feldspar grains, which have been replaced by calcites were either slightly or not affected by late diagenetic dissolution.