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2.1.14. Saint-Vaast Chalk Formation - SVA

Authors: F.L.Cornet & Briart (1870), and herein.

Description: White togreyishchalk, containing clay minerals but notmarly, difficult to break. The lower part ("Craie de Saint-Vaast inférieure" auct.) contains sponges and grey to black flints but only in the eastern part of the Mons Basin. At the base, a thin conglomerate with phosphatised chalk pebbles, phosphatised and glauconitised sponges and glauconitic grains.

The upper part (" Craie de Saint-Vaast supérieure" auct.) is not flinty but chalky with pyritic spheroids. The top is sometimes marked by a hardground. Sponges are more abundant than other fossils.

Stratotype:  The quarry at St. Vaast where the “Craie de Saint Vaast” was originally defined has been filled in. Still visible in the old quarry “des crayères” at Thieu.  

Area: MonsBasin. The upper part seems to be transgressive over the lower part of the Saint -Vaast Fm. or the Maisières Fm. or Devonian bedrock.

Thickness: 15 to 25 m on the southern and northern margins of the Basin, about 50 m on the eastern margin near the village of Trivières.

Age: The lower part of the St.-Vaast Fm. is of Coniacian age on the basis of: Micraster decipiens, Volviceramus involutus, the foraminifer Dicarinella gr. concavata. It also contains numerous specimens of Hyotissa semiplana.

The upper part of the St.-Vaast Fm. is of Santonian age on the basis of: Micraster coranguinum, Actinocamax verus, Gibbaster belgicus, Sphenoceramus digitatus, and foraminifers: Stensioeina exsculpta gracilis, Gavelinella clementiana, Bolivinoides strigillatus...

Remarks: - At this moment (2000) there are no good outcrops exposing the Saint-Vaast Chalk Fm.

- Reference: Godfriaux & Sigal (1969).