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2.5.6 Fallais Formation - FLL


Author: Verniers, 1976 ms, 1983a.

Description: Unit of light green, olive-greenish grey, or light grey chloritic mudslate and mudstone, with rare siltstone and fine sandstone beds from distal turbiditic origin, mostly without laminated hemipelagite; in the Mehaigne area it can be subdivided into six members according to the frequency of siltstone and sandstone beds (except for member D and the volcanic Pitet layer): from top to bottom:

Member E: light or olive greenish grey, or light grey mudslate, mudstone, siltstone and fine sandstone, with non-calcareous chloritic pelite in the e-divisions; Tde sequences mainly between 8 and 11 cm; T(b)cde absent or rare (0-7%), generally about 75% thicker than the main Tde sequences, with the c-divisions between 11 and 20 cm thick; upper boundary placed below the first of frequently present obliquely stratified fine sandstone beds (a few per meter); lower boundary not observed but probably the top of the volcano-sedimentary layer of Pitet.

Volcano-sedimentary layer of Pitet (PTT): 20 m massive fining upward (very) coarse pure crystal tuff with slate and crystal lenses, passing gradually upwards into (medium) coarse tuff, heterogeneous with crystal and glass lenses; gradual transition into at least 5.5 m of (very) fine ash tuff or cinerite showing fine lamination, faint oblique stratification and compact sedimentation at the top; abrupt lower boundary observed with the member D.

Member D: medium to dark grey mudslate, mudstone and siltstone, quartzic, non-calcareous pelite; Tde sequences thin to medium thick (average of 12 cm); T(b)cde sequences frequent and generally thicker than Tde sequences; with thin c-divisions; lower boundary with member C not observed due to an observational gap of 30 m.

Member C: light greenish to olive-green mudslate, mudstone, siltstone and fine sandstone with non-calcareous chloritic matrix in the e-divisions; thin Tde sequences (on average 6-9 cm), T(b)cde sequences absent or rarely present (0-11%), generally about 60% thicker than the average Tde sequence; with c-divisions mostly between 0.5 and 9 cm and rarely (about 10%) between 11 and 16 cm; lower boundary with member B unobserved due to an observational gap of 7 m.

Member B: greenish mudslate, mudstone, siltstone and fine sandstone; in the e-division non-calcareous chloritic pelite; thin Tde sequences (about 10 cm); T(b)cde sequences frequent and much thicker (20-50 cm) than Tde sequences; lower boundary with member A not observed.

Member A: same lithology as Mbr C; lower boundary supposedly with the Latinne Fm via fault contact.

Stratotype: Mehaigne valley all around the village Fallais; the type sections of the members are: Mbr A in section IG-19, Fallais village centre, Mbr B and Mbr C in section KG-2. KG-3 and KG-4 in the sunken road north of the Ferme de Chantraine, Dreye; Mbr D in outcrop IF-9 in the hamlet Les Falihottes, Pitet; the volcano-sedimentary layer of Pitet in the abandoned quarry in the Butte St-Sauveur, Pitet and in the Bois Cornet, Pitet; Mbr E in section KF-2 in the sunken road 250-350 m south-east of the church of Dreye.

Area: Mehaigne and Burdinale valleys (type area); Orneau valley: a volcano-sedimentary rock is present at 20 m below the top of the formation. In the Thisnes valley (Monstreux; Verniers unpublished and Diependaele, 1997 ms) a volcano-sedimentary rocks (“porphyroid of Monstreux”) is present at about 8 to 10 m below the Corroy Fm and from 11 to 22 m below the same formation a purple shale member is observed, the only purple-coloured bed in the Silurian of the Brabant Massif. In the Sennette valley a local member B with thin bedded sandstone layers, showing oblique stratification and undulating bedding planes is separating two green shale members (A and C) with rarer sandstone (Verniers, unpublished).

Thickness: Mehaigne area: estimated for the formation: >626 m (Mbr E: 135 m, Volcano-sedimentary layer of Pitet: 31 m, Mbr D: >22 m, Mbr C: 140 m, Mbr B: >28 m, Mbr A: 270 m, Verniers, 1983a); Orneau valley: difficult to estimate: 300-400 m (Delcambre & Pingot, in press a); Thisnes valley (Monstreux): difficult to estimate: >450 m (Verniers, unpublished); Sennette valley: >375 m (cut off at the base by a fault; Verniers, unpublished).

Age: Based on acritarchs Telychian, upper Llandovery (Martin, 1969a); chitinozoans from the Mehaigne area indicate the longicollis global biozone for members A, B and C, calibrated with a range post-griestoniensis to pre-insectus graptolite biozones. The margaritana global biozone is observed in members D, Pitet and E. According to the latest calibration by Mullins (1998) these three members would correspond to the insectus graptolite biozone, topmost Telychian, uppermost Llandovery (reinterpreted after Verniers, 1981, 1982).