Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys (Brabant Massif, Belgium) and their stratigraphic implications

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Abstract

ABSTRACT. Early and Middle Ordovician strata from the Senne-Sennette river valleys, at the southern border of the Brabant Massif, have been intensively sampled for an acritarch study. Previous research, together with unpublished and new observations, are reported. Three sections are studied in the Sennette valley: along the Bruxelles-Charleroi canal south of Asquempont, in the Virginal railway trench and at La Tourette along the bief 29 of the old canal. Two other locations are also considered: a section along the Coeurq river and another along the railway and in the quarry at Quenast, in the Senne valley. An early Tremadocian assemblage is observed in the three localities of the Sennette valley within strata which biostratigraphical content allows their lithostratigraphical assignment to the Chevlipont Formation. Between the latter formation and the Rigenée Formation, rocks coeval to the Abbaye de Villers and Tribotte Formations of the Thyle valley, yielded two distinct assemblages. The Frankea hamata-Striatotheca rarirrugulata Acritarch Zone of the English Lake District, indicating a late Arenig age, is observed in sediments reported to the Abbaye de Villers Formation from the Virginal railway section. The lithostratigraphical assignment is confirmed as the same Zone is recorded from coeval strata from the Thyle section. A second assemblage, characterized by the co-occurrence of species such as Arbusculidium filamentosum and Frankea sartbernardensis, is recognized in an unnamed member attributed to the Abbaye de Villers Formation at Asquempont and in strata attributed to the Abbaye de Villers Formation in the Quenast railway section. In these two localities of Asquempont and Quesnast, chitinozoans indicating respectively a middle and a late Arenig age are also described. According to acritarchs, the age of those strata would be late Arenig. Combined acritarch and chitinozoan results indicate a probable late Arenig age. Moreover, due to their acritarch content, the lithostratigraphical attribution of the Asquempont and Quenast strata is questioned. They are coeval with the Tribotte Formation or with a level at the boundary between the Tribotte and the Rigenée Formations of the Thyle river section. Other ‘pre-Rigenée’ localities could also belong to the A. filamentosum-F. sartbernardensis assemblage, based on the presence of F. sartbernardensis or A. filamentosum. One locality is situated at Asquempont and consists of the main part of the section studied by Martin & Rickards (1979) and another is located at La Tourette. Samples of the Rigenée Formation have not revealed the stratigraphical markers known in the type section of the Thyle valley despite the great number that were investigated in different localities from the Senne-Sennette valleys.

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A propos de : Michel VANGUESTAINE