Orchies Member

The information on this page is a summary description.
The full formal description is available here: Orchies Member

Abbreviation KoOr
Parent unit Kortrijk Formation
Child units Lower Orchies Member, Middle Orchies Member, Upper Orchies Member
Lithological description Compact and heavy stiff bluish‐grey clay occurring at the base of the Ieper Group only separated from the base itself by the underlying sandy Mont‐Héribu Member where this latter is present. The Orchies Member is overlain by more sandy or silty clay deposits of the Roubaix Member or Mons‐en‐Pévèle Formation. The thickness can be up to 40 m. A pebble layer has been reported occasionally at its base (Ya on the 1:40 000 geological maps). Whereas in the visual description of macroscopic samples, even from cores, it is very hard to see any further lithological subdivision of the Orchies Member, the geophysical log signatures (see reference boreholes in compendium) do show a systematic variability interpreted as grain‐size and mineralogical variations. The top of the very high gamma‐ray section at the base of the Orchies Member, about 10 to 15 m thick, is a correlatable surface (see also further Geophysical logs and Subdivisions). It corresponds to the top of the mistakenly named Mont‐Héribu Member (KoMh) unit in the correlation figures in Welkenhuysen and De Ceukelaire (2009 figs 12,14,16,18, 20, 22, 24) and approximately to the level 1 in the plates in Vandenberghe et al. 1991). The top of the Orchies clay Member/ base Roubaix Member or base Mons-en‐Pévèle Formation, is defined by the top of the very clay‐enriched sediment. This level could be identified with confidence in descriptions of a series of destructive boreholes by G. De Geyter (1990, Archives Belgian Geological Survey) as the transition from silty clay above to heavy clay below. (courtesy Marleen De Ceukelaire).
Area of occurrence The Orchies Member consistently occurs where the Ieper Group occurs in Belgium. In the Hainaut area thickness is between 10‐16m whilst in central Flanders and north Belgium thickness can be over 40 m. Towards the east in Brabant its thickness is reduced to a few meters. Originally the name was introduced by Gosselet (1874, p 611) to indicate the compact and stiff clays at the base of what is now known as the Ieper Group sediments; later, as a refinement of the lithostratigraphy, the sandy and silty Mont‐Héribu Member at its base was individualised as a separate unit and the name Orchies Member was reserved for the compact heavy clays above the Mont‐Héribu Member (Steurbaut, 1998). The later introduced name Saint‐Maur Member (Belgian stratotype area, Geets, 1988; Maréchal, 1993), used in the legend of the 1:50 000 mapping in Flanders is a synonym of the Orchies Member although it (Saint‐Maur unit) was generally used in a more restrictive way, because the lower part of the Orchies Member was erroneously assigned to the Mont Héribu Member; it is preferred to maintain the original name Orchies, a small locality to the southeast of Roubaix in Northern France. On maps 1:25 000 of the Brabant Wallon (Nivelles‐Genappe, Braine‐le‐Comte‐Féluy) the ‘Formation de Carnières’ is used for a unit ‘close to Orchies’. On the legend of the geological maps 1:40 000 the Orchies Member was included in the Yc clayey deposits and in the stratigraphic register (1929,1932) in the Y1a. The ‘argilite de Morlanwelz’ is a lateral equivalent of the Orchies Member (Steurbaut, 1991). The Wardrecques and Bailleul members are reported in King (1991). The lower part of the Wardrecques member belongs to the Orchies Member whilst the upper part and the Bailleul member corresponds to the Roubaix Member (King, 1991). This subdivision is not commonly used in the literature but the position of Wardrecques and Bailleul members is well documented in boreholes of the Moeskroen‐Kortrijk‐Marke‐Ooigem area by King (1991, fig. 11). In this area at least 5 glauconiferous beds occur, each less than 15 cm thick (King, 1991).
Type locality The Wahagnies clay pit (“Briquetterie de Libercourt”) in northern France, map sheet XXV‐5 (Carvin). Ortlieb & Chelloneix (1870, p25) had already used the name ‘Argile de Wahagnies’ to indicate the Orchies Member compact clays (Steurbaut, 1998). In the clay pit, the base is defined by the basal pebble bed below about 8 m of stiff clays. Coordinates: X = 649.250, Y = 310.600, Z = +50 m. The upper boundary, marking the limit with the overlying Roubaix Member, is proposed in the Kallo well 027E0148 (Gulinck, 1969) at 341m depth (Steurbaut, 1998, p 112) (see also below).
Alternative names
Authors Steurbaut, E., De Ceukelaire, M., Lanckacker, T., Matthijs, J., Stassen, P., Van Baelen, H. & Vandenberghe, N.
Date 09/01/2017
Cite as Steurbaut, E., De Ceukelaire, M., Lanckacker, T., Matthijs, J., Stassen, P., Van Baelen, H. & Vandenberghe, N., 2017. The Orchies Member, 09/01/2017. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Orchies-Member


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