clayey-top facies

The information on this page is a summary description.
The full formal description is available here: clayey-top facies

Abbreviation DiCl
Parent unit Diest Formation
Child units
Lithological description The clayey-top-facies consist of a glauconite-rich, poorly-sorted clayey sand with a characteristic higher clay content compared to the coarse Kempen Diest Member below. The size distribution mode typically ranges between 170 µm and 250 µm. The clay content (dispersed, <2µm) typically ranges between 1% and 10%. The pelletal glauconite content ranges between 26% and 67% and is on average 32.9% (Adriaens, 2015). The mineralogical and clay mineralogical composition of the clayey-top-facies is very specific and differs from the rest of the Diest Formation. The clay mineralogy consists of a significant amount of expandable clay minerals: dioctahedral smectite but also, and more distinctly, trioctahedral Fe-rich vermiculite (see Adriaens & Vandenberghe, 2020 for details). This type of vermiculite is rare in the stratigraphic column, and is assumed to be related to a glauconitic soil environment. Most likely, it was not formed in-situ but originates from a remote source area where a soil developed on glauconitic sediment in very specific, yet poorly understood, conditions. Apart from the distinct vermiculite, the clayey-top-facies is also relatively enriched in siderite, apatite and sporadically vivianite and chlorite but the relatively high clay content and the presence of Fe-vermiculite are the distinctive criteria for the clayey-top-facies. This facies needs better descriptions of extent, correlations and depositional structures. Therefore, it is defined as a facies and not as a formal member inside the Diest Formation.
Age Dinoflagellate cyst biozone DN9 was identified in the clayey top facies attributing a late Tortonian to Messinian Miocene age to this facies (Louwye et al., 2007). Additional data are required.
Thickness The clayey-top-facies reaches its largest thickness of about 12 m in the ON-Dessel-2 and ON-Dessel-3 boreholes.
Area of occurrence The occurrence of the clayey-top-facies is confined to the central Campine area in and around the villages Mol and Dessel. The unit is not recognized in the outcrops more to the western (Kasterlee) and southern (Olen, Heist-op-den-berg) part of the Campine and is also absent in the Hageland area. The extension of the facies in the eastern part of the Campine is not investigated although the occurrence of organic-rich clay lenses at the top of the Diest Formation reported at several locations in Limburg (Helchteren kb25d62e-B272/ 62E261, Peer, Wijshagen kb18d48w-B181/ 48W 180) by Gulinck suggests a possible relation with the clayey-top-facies in the central Campine.
Type locality The name clayey-top-facies was first proposed by Wouters and Schiltz (2011) in their description of the ONDRAF/NIRAS boreholes in the central Campine in region Kasterlee – Mol – Dessel. For now, the occurrence of the facies is also confined to this area. The type localities are the ON-Dessel-2 and ON-Dessel-3 boreholes. At present, the facies has not been recognized in outcrops. The facies was recognized in following boreholes:
ON-Dessel-2 (031W0338/ kb17d31w-B299)
ON-Dessel-3 (031W0354/ ON Dessel-3)
Mol (031E0435/ B/1-96196)
Rees (017E0399/kb8d17e-B495)
Alternative names Diest D4 facies in Adriaens (2015)
Authors Adriaens, R. & Houthuys, R.
Date 01/09/2023
Cite as Adriaens, R. & Houthuys, R., 2023. The clayey-top facies, 01/09/2023. National Commission for Stratigraphy Belgium.
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