Remarks to discussion text “An update of the lithostratigraphy of the Ieper Group”, published 9
March 2015 on Proposals and discussions, edited by Marleen De Ceukelaire, Tim Lanckacker, Peter
Stassen, Hervé Van Baelen and Noël Vandenberhge.
Rik Houthuys, 8 April 2015
This is a well-documented text where I welcome the use of geophysical logs by including them as
interpreted reference figures in the text.
Just out of curiosity, was this text discussed with Etienne Steurbaut? The chronostratigraphic
framework that shook the established lithological framework goes back to his and Nolf’s 1986
publication on the revision of the Ypresian stratigraphy, and his more recent work.
Here are my major comments:
1. This text was clearly written with a general depositional – sequence stratigraphic model in
mind. But this model is not included in the text, while it would help the user to accept the
new proposed litho units.
2. I think it’s not a good idea to group members into formations with the dominant criterion of
grain size. The nature of the boundary planes and the depositional buildup are equally
important. E.g.: the Kortemark Silt grades from the Aalbeke Clay, while the Egemkapel Clay
has an erosive contact with the Kortemark member, yet both members are grouped in the
Tielt Formation. As the Egemkapel Member is bounded above and below by erosive surfaces
and appears to have a wide areal extent, it may be considered to raise its rank to a
formation.
3. I suggest to use more local names, and to avoid stratotypes in outlier areas (Hyon, MontPanisel). They may be chronostratigraphically well positioned, but the lithostratigraphic
position with respect to the main outcrop area is less clear.
4. It is strongly recommended, when defining new lithological units or introducing significant
changes, to wait until the real sequence stratigraphic and paleogeographic relationships have
been made clear, and to undertake action to settle the relationships. This remark applies to
the fine-sandy upper part of the Ieper Group.
5. In addition to remark no. 4, some descriptions are not sufficient to help the user to identify
formations and members in the field (e.g. Hyon Formation: the Egem Member does not
contain siliceous concretions; Gentbrugge Formation: how can this in the field be singled out
from the Mont-Panisel Member?).
6. I suggest not to define members based on one observation.
More on my remark n° 4: this concerns the complete upper part (now Tielt – Hyon – Gentbrugge
formations):
Mons-en-Pévèle Member: the upper part of the Mons-en-Pévèle Member in the area south of
Brussels, but also as far as Postel, cannot be distinguished from the Egem Member in WestVlaanderen, as it contains the same lithology and sedimentary structures. The depositional
environment was definitely the same. The only distinction seems to be that in Egem, the base is
erosive, while in the Mons-en-Pévèle Member area, there are some repeated intercalations of Egemlike sand, especially near the top. The separation goes back the biochronological results by Steurbaut
& Nolf.
Hyon Formation: It is strongly recommended to keep the Egem Member out of the Hyon Formation
and if necessary, make it into a new formation.
Gentbrugge, Hyon and Vlierzele Formation: in the area between Zenne and Dender (and probably
also in southern Oost-Vlaanderen), I see no way to decide between the three formations. I
demonstrated for the Oudenberg-Kesterberg area that a deposit with definite Vlierzele Sand
characteristics is intercalated there in Panisel-like deposits (see my 2014 paper in Geologica Belgica).
Why not stick to the old interpretation of one deposit (the former lower Panisel Formation, or the
Anderlecht Clayey Sands)? There are some well recognizable characteristics to speak for continuing
to keep these deposits in one formation: the sands and clayey sands contain an elevated content of
relatively coarse glauconite (order of 20-25%) and often, siliceous concretions (veldsteen) are
present. See also the next point.
On the separation of the Vlierzele Sands in a formation belonging to the Zenne Group:
The position of this deposit is far from settled! Maybe it would be wise to let chronostratigraphic
data from the subcrop area predominate in deciding on its stratigraphic position.
The matter of the erosive lower boundary is not settled. The incision detected in the North Sea by
De Batist and colleagues is at the Vlierzele stratigraphic position. Mostaert (1985) assumed the base
of Vlierzele is erosive in the Pittem Clay in the Brugge area. Kaasschieter (1961) assumed the lower
contact of the Vlierzele Sands at Vlierzele was gradual; he assumed the deposit beneath were the
Anderlecht Clayey Sands. My observations in the type area confirm the lower contact is gradual, but
in the hydrogeological report that was made for the large Vlierzele sandpit, the deposit below was
identified with the Merelbeke Clay. This was also followed by Steurbaut & Nolf (1986).
Vlierzele-like sands are found all over southern Oost-Vlaanderen, and they are definitely intercalated
in more clayey sands. Even in the Vlierzele type area itself, sporadically, more clayey sands, even
containing “veldsteen”, are found on top of typically Vlierzele Sand (see fig. 5.14 of the Dikkelvenne
sandpit in my 1990 publication, layer “j”). The same is now confirmed for the Oudenberg-Kesterberg
area in my 2014 paper: typically Vlierzele Sand occurs as lenses or wedges in “lower Panisel” clayey
sand.
Maybe the top of Vlierzele contains a second sequence, to be correlated to the North Sea incisions?
Or the incisions should be correlated to the Aalter Formation?
The exact relationships necessary await new observations! So long, it is strongly recommended not
to change the litostratigraphic names.
Some small remarks
The coordinate system used is not specified(sometimes Belgian, then French). The decimal separator
should be the point.
In fig. 1 and 2, it would be helpful if the grains size of e.g. the black coloured bar in the grain size log
is specified. It is not clear whether the interpretation at the right refers to the present discussion text
or to the source text of the figures. It is not made very clear why the interpretation of the members
should deviate from the interpretations made previously by the other authors in the figures.
It is not a good idea to define a Mons-en-Pévèle formation, which is surrounded by members of the
Kortrijk Formation. Due to this position, I would keep it a member.
Is it Mont-Panisel Member or Panisel Sand Member?