Selective and reversible carbonate—silica replacements in Lower Cretaceous carbonate‐bearing turbidites of the Eastern Alps

Abstract

ABSTRACTIn the diagenetic history of calcareous sandstones, silicacementation and silicification may be followed by carbonatecementation and replacement and vice versa, and the change‐over from one to the other may occur more than once. This is well illustrated by calcareous and siliciclastic turbidites of the Gault Formation (Aptian to Albian) of the Eastern Alps which have been interpreted as deep‐sea trench plain and deep‐sea fan deposits.In these turbidites silicification selectively affects ooids and a few other biogenic carbonate fragments rich in organic matter (algae and bryozoans) which form a small fraction of the bulk sediment. The type and sequence of diagenetic changes are largely controlled by host‐rock composition and may vary vertically within individual beds as a result of compositional grading. In the carbonate turbidites, silicification follows widespread calcite cementation. The process is slow, resulting in relatively coarsely crystalline replacement quartz. In ooids with quartz nuclei, rim‐quartz forms mostly monocrystalline ‘overgrowths’ by outward replacement of the concentrically laminated carbonate cortex. This type of silicification is often incomplete leaving parts of the ooid cortices unaffected. In quartz arenites and sublitharenites silicification precedes calcite cementation. There the process is rapid, forming microcrystalline quartz. Even if the ooid nucleus consists of quartz, a syntaxial ‘overgrowth’ does not normally form. The replacement quartz is almost always polycrystalline. Late‐stage diagenetic calcite and dolomite which develop euhedral crystal shapes and cut across grain boundaries may replace the earlier secondary rim‐quartz of the ooids as well as other minerals.Possible sources of the silica are pressure‐solution of quartz, dissolution of opaline silica of radiolarian tests and of sponge spicules, and feldspar in the host bed. In a number of examples an increase in the degree of silicification can be observed towards the lower bedding planes of individual turbidites requiring an additional external source of silica which seems to necessitate cross‐formational flow of pore solutions. Silicification in both the carbonate and the siliciclastic turbidites probably took place at about the same time; in the carbonate turbidites it was preceded, however, by calcite cementation, which significantly reduced porosity and permeability before silicification took place.The greater degree of alteration experienced by the Gault turbidites of the Falknis and Tasna Nappes, which are more internal structural units of the Alps (compared to the Flysch Zone of the Eastern Alps), is reflected by the growth of quartz ‘beards’ and spikes from the ooids in the direction perpendicular to maximum stress. This is the only case observed where the rim quartz of the ooids grows beyond the original grain boundaries.