Hallaar Member

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

Abbreviation KlHa
Parent unit Kasterlee Formation
Child units Olen Gravel Bed
Lithological description In its type area around the hills of Heist-op-den-Berg and Beerzel up to Olen more the north, a disperse gravel of flattened and partly weathered flint pebbles, the Olen Gravel Bed, is present at the base of the Hallaar Member, together with coarse quartz grains and white weathered silex, described in detail in Verhaegen et al. (2014). The Hallaar Member has transitional characteristics between the Diest Formation and Kasterlee Formation. Glauconite content is significantly lower than in the Diest Formation but concentrations of >10% (glauconite/quartz ratio of 0.20–0.68) are still present (Verhaegen et al., 2020). The grain size distribution is bimodal. The coarser mode is similar to the modal grain size of the underlying Diest Sand and can be attributed to reworking of sediment from the Diest Formation. Another finer modal grain size is also present (< 200 µm) which is more typical of the Kasterlee Formation. The sediment has a large fine fraction, with 50% of grains <85 µm. The Hallaar Member has a similar content of dioctahedral 2:1 Al-rich layer silicates (11–15%, 2:1 Al-clay/quartz ratio of 0.22–0.23) to the Diest Formation, yet higher than the overlying Beerzel Member and the sandy parts of the Heist-op-den-Berg Member. Feldspar content (4–5%, feldspar/quartz ratio of 0.06–0.09) is significantly higher compared to the Diest Formation and similar to the overlying Beerzel and Heist-op-den-Berg members. In outcrops or cores, the Hallaar Member may be recognized by its brownish-green weathered color if oxidized above the groundwater table, significant glauconite and clay content, and a mottled appearance due to the presence of green-greyish clay-enriched patches of sediment (Verhaegen et al., 2020). The Hallaar Member, with its reworking of the underlying Diest Formation sand, appears to be present across most of the occurrence area of the Kasterlee Formation, based on CPT profiles and core observations. On CPT’s there is a drop in qc value from the Diest Formation to the Hallaar Member (Vandenberghe et al., 2020). North of Olen no basal gravel is present. In this area, the Hallaar Member is distinguished based on its transitional characteristics between the Diest and Kasterlee formations.
Age No age data are available for the Hallaar Member, yet it is underlain by the late Tortonian to Messinian Campine Diest Sand and overlain by the Heist-op-den-Berg Member in which dinoflagellate cyst biozone DN10 of late Tortonian to Messinian Miocene age was identified.
Thickness The Hallaar Member comprises the lower couple of meters of the Kasterlee Formation, in which reworking of the underlying Diest Formation is apparent. In the type section, the thickness is approximately 3 m and this appears rather consistent throughout the occurrence area to the north, with a thickness of 2-3 m.
Area of occurrence The Hallaar Member appears as a transitional basal unit of the Kasterlee Formation throughout most of its occurrence area. Only in the northwest such a transitional unit cannot always be observed and the Diest Formation is directly overlain by the Lichtaart Member.
Type locality The proposed type section of the lower three members of the Kasterlee Formation, including the Hallaar Member, is the sunken lane atop the hill of Heist-op-den-Berg (DOV TO-20140919 and TO-20190617). The type section is described in detail in Verhaegen et al. (2014) and Verhaegen et al. (2020), and was originally proposed by Fobe (1995).
Alternative names Formerly part of the at the time not yet subdivided Kasterlee Formation sensu De Meuter and Laga (1976) and Laga et al. (2001).
Authors Verhaegen, J. & Vandenberghe, N.
Date 01/09/2023
Cite as Verhaegen, J. & Vandenberghe, N., 2023. The Hallaar Member, 01/09/2023. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Hallaar-Member
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