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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Unpublished.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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