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Sedimentology and diagenesis of the Dinantian succession in the Vinalmont borehole

The dolomites of the Grande Dolomie de Namur which make up the lower part of the Vinalmont borehole are considered to be eogenetic, i.e. relating to reflux of saline to hypersaline water or due to seawater circulation adjacent to the development of a mixing zone. Their depleted δ 18 O (-13.50‰ to -13.90‰) is interpreted in terms of recrystallisation due to interaction with high temperature fluids. Their δ 13 C-composition (-1.68%o to +1.50‰ PDB) roughly falls within the interval of the original Lower Carboniferous marine calcite, and thus has not been reset. Columnar calcites, which fill up many of the cavities within the upper part of the former formation, are interpreted as palaeo-speleothems of Upper Tournaisian/Lower Visean age. These dominantly non-luminescent columnar calcites with growth bands and microsparite intercalations of detrital origin display stable isotopic signatures supporting their meteoric origin (δ 18 O- and δ 13 C-values of -6.49‰ to -10.32%o PDB and -2.47‰ to -4.79‰ PDB respectively). In the Vinalmont borehole the Encrinite de Flemalle' and the Limestone of Avins' are missing. This is explained by non-deposition or by erosion in relation to the just cited karstification. Within the overlying Terwagne Formation mud- to wackestones and peloidal pack- to grainstones display many features indicative of restricted sedimentation conditions with episodic emergence as testified by the development of palaeosols and subaerial crusts. The stable isotope values of these horizons support soil-related processes (δ 18 O between -8.55%o and -6.55‰ PDB and δ 13 C between -5.95%o and -1.03‰ PDB). An evolution towards more open marine subtidal sedimentation conditions in the Vinalmont Limestones (dominantly pack- to grainstones) is apparent based on the varied fauna assemblage and sedimentary characteristics. The original marine stable oxygen signature has also been reset due to interaction with meteoric water (-9.45‰ and -9.70‰ PDB) but their δ 13 C (+0.27‰ to +1.07‰ PDB) reflect a clear rock buffering with preservation of the original marine Lower Carboniferous signal. The study of late diagenetic features is helped by the development of bed-parallel compactional and steeply inclined tectonic stylolites. Late diagenetic zebra-dolomite formation has been observed in the Grande Dolomie de Namur. They formed out of hot (mean T h of 115°C; δ 18 O of -9.1‰ to -9.4%o PDB) suprahydrostatic fluids circulating along steep faults. The slightly depleted δ 13 C-signal (varying between -2.08%o and -3.09‰ PDB) might also reflect high temperature precipitation conditions. The development of mono-crystalline non-luminescent columnar calcites in these dolomites, filling up large cavities is a second important late diagenetic process. They are interpreted to be the product of hydrothermal karstification (δ 18 O-values of -16.2%o to -16.7%o PDB and δ 13 C-values of +0.07‰ to -1.17‰ PDB). Furthermore several fracturation events followed by cementation have been recognised of which the later generations typically are characterised by bright yellow luminescent calcite. They are thought to relate to thermal sulphate reduction and thus could be time equivalent with the late diagenetic pyrite which preferentially crystallised along the stylolites.