Jan Walstra, Michiel Dusar, and Marleen De Ceukelaire (2014)
Geological model of the Ypresian Clay.
Unpublished.
This study is framed in a programme, set up under coordination by Lie Sun Fan (Faninbel
bvba), to create depth models for the important clay layers in the subsurface of Belgium,
focussed on the clay layers of the Boom (Rupelian) and Kortrijk (Ypresian) Formations. The
results concerning the Boom Clay were reported separately (Walstra & Dusar 2013). A
characterisation of borehole breakouts in Ypresian clays with reference to geophysical well
logs has also been reported (De Ceukelaire et al. 2012). The present document only treats the
Ypresian Clay complex. It is tried to define and model the totality of the Ypresian aquiclude,
including the Kortrijk Formation and overlying clay units of the Tielt Formation, irrespective
of the lithostratigraphical subdivisions in use.
The model is based on high-quality borehole data and geophysical well logs from the archives
of the Geological Survey of Belgium (GSB) and Databank Ondergrond Vlaanderen (DOV)1,
complemented by Dutch data (DINOloket and NLOG of TNO-GDN)2. Interpretations of
boreholes were critically re-evaluated and modified when necessary. Models of interpretation
are discussed in the next chapter.
Compared to previous mapping projects by GSB (i.e. Tertiary Isohypse Maps by
Vancampenhout 2004 and Quasi-3D Model of the Lower-Rupelian and Tongrian by
Vancampenhout et al. 2008), additional new data were used and existing interpretations were
fine-tuned with the currently accepted lithostratigraphical subdivisions in Belgium. Because
different (bio-) lithostratigraphical subdivisions exist, these are first discussed in view of their
relevance for the objectives of this study, i.e. to define the Ypresian Clay complex as a single
comprehensive aquiclude. This proved to be a difficult and time-consuming exercise, not only
due to the palaeogeographical variability of the subunits, but also due to the variety of
stratigraphic terminology in use, the different interpretation criteria applied and the
inconsistent links between them.
From a technical perspective, the methodology used to realise the depth model is largely
similar to the one of the Quasi-3D model of the Lower Rupelian, except that the fault blocks
in the east of the study area were not treated separately. Furthermore, so-called “depth maps”
were produced – indicating the depth of the top and base of the clay unit below a variable
ground level instead of relative to sea-level.
In the first part of this report, an overview of the position of the modelled units within the
Belgian lithostratigraphical schemes is provided and discussed.3 The following chapters
present the data used and the cartographic methodology applied. The results are presented as a
number of maps and finally some problem areas are indicated where the model would benefit
from further fine-tuning.
Note that the term “Ypresian Clay” (“Ieperklei”) is an informal designation – throughout this
report this term covers both the Kortrijk Formation and the overlying clayey units of the Tielt
Formation (assigned to the Kortemark and Egemkapel Members, as will be demonstrated).
This term is more restrictive than the stratigraphic denomination Ypres (Ieper) Group which
encompasses also the overlying more sandy units included in the Tielt and Gentbrugge Formations.
Because of its relevance for defining the upper limit of the Ypresian Clay, this study is the first
attempt to systematically recognize and map the Egemkapel Member.
1 https://dov.vlaanderen.be/dovweb/html/index.html
2 http://www.dinoloket.nl; http://www.nlog.nl/nl/home/NLOGPortal.html
3 We discuss usage of lithostratigraphic terms. For references to the origins of the names and for a historical overview we refer to the publications cited in this text.
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