The chitinozoan biostratigraphy of the Silurian of the Ronquières–Monstreux area (Brabant Massif, Belgium)

Abstract

A major outcrop area in the Silurian of the Brabant Massif (part of the Anglo–Brabant Deformation Belt) lies along the Sennette valley, from the hamlet Fauquez to the area around Ronquières, and along the Samme and the Thisnes valleys, around the city of Nivelles and its hamlets Monstreux and Bornival. Recent lithostratigraphical mapping in the study area has shown the presence of nine Silurian formations (from bottom to top the Brutia, Bois Grand-Père, Fallais, Corroy, Petit Roeulx, Steenkerque, Froide Fontaine, Vichenet and Ronquières formations, the latter in its type locality). One sample from presumably the Huet Formation, one of the three Upper Ordovician formations present in the Monstreux area is also studied. The presumed presence of Upper Ordovician in an area poor in outcrops in the north of the Monstreux area is confirmed by the chitinozoans in that one sample. Fifty one samples from seven Silurian formations were dissolved to extract chitinozoans and 37 proved to be fossiliferous. Two formations remain unstudied. The chitinozoan assemblages are poorly to moderately preserved due to the high anchizonal metamorphism in the outcrop area. The assemblages are diverse and can be abundant, with a content of between 0.1 and 23 chitinozoans per gram of rock. They can be correlated with the global Silurian chitinozoan biozonation (the Angochitina longicollis, Margachitina margaritana, Cingulochitina cingulata biozones are recognised), with the Welsh chitinozoan biozones for the Wenlock in the Builth district where the graptolite biozonation was described at first, for the Ludlow with the chitinozoans biostratigraphy in the chronostratigraphical type sections of the Ludlow series at Ludlow, Shropshire, and with the local Silurian chitinozoans biozonation in the Brabant Massif. The presumed Late Ordovician age of these three poorly outcropping and previously poorly dated formations in the Monstreux area is confirmed with the chitinozoans. Four of the nine Silurian formations in the study area were previously dated with graptolites (Brutia and Ronquières Formations), with acritarchs (Fallais Formation) or with chitinozoans (Steenkerque Formation). The ages of the latter two formations was confirmed or refined with the chitinozoans. Three other formations (Corroy, Petit Roeulx and the combined Froide Fontaine and Vichenet formations) are dated for the first time in the study area. The Ronquières Formation was dated previously with graptolites as lower Ludlow, which is confirmed by the chitinozoans and refined to the lower middle part of the Gorstian. A very rapid sedimentation of this thick turbiditic formation is deduced. Only one Silurian formation (Bois Grand-Père Formation) remains undated in the study areas. A new species is described: Conochitina pumilio sp. nov.