4.2.11. Montfort Formation - MFT

Authors: Davreux, 1833; Mourlon, 1875; Thorez et al., 1977.

 

Description:  The Monfort Fm (former names: Montfort assise, Montfort “psammites” or paving sandstone of Monfort) is made up of micaceous, dark grey bluish, sometimes nearly black by weathering, thick (up to 5 m) sandstone beds, practically without any shaly intercalations.  Locally, at the top, the rocks take a maroon (“lie de vin” in french) color, especially in the Vesdre Nappe.  Plant debris are abundant.  The Monfort Fm represents a sand barrier complex.  It was subdived by Thorez et al. (1977) into three members, respectively from bottom to top :

1.     the Bon-Mariage Mbr : forebarrier environment, with rhythmic deposits of a typical tidal-flat environment, with subtidal, intertidal and occasionally supratidal environments;

2.     the La Gombe Mbr : sand barrier environment itself, virtually barren of limestone or calcareous sediment; the rocks are higly sandy, arkosic, and organized in at least one meter thick beds, exhibiting a systematic reverse grading; clay material occurs occasionnaly interlayered within the sandy beds;

3.     and the Barse Member : back-barrier environment with typical lagoon-evaporitic (sabkha) sedimentary deposits.  The carbonate material is always a “primary” dolomite, without any fossil content.

 

Stratotype: Ourthe valley, railway trench, 1700 m south of the Esneux railway station.

 

Area: Part of the Namur Synclinorium, Dinant Synclinorium, Vesdre Nappe, Theux Window.

 

Thickness: 170 m in the Dinant Synclinorium of which 150 m in the Ourthe valley; ±350 m for both the Monfort and Evieux Fm in the Vesdre Nappe.

 

Age: Middle (Fa2b) to Upper (Fa2c) Famennian.  Late Sc. velifer, Polygnathus styriacus, Palmatolepis perlobata postera, early Palmatolepis gracilis expansa conodont zones.