The evolution of the Condroz-Brabant Basin from Middle Ordovician to Llandovery: Lithostratigraphical and Chitinozoan Biostratigraphical Approach

A recent model sketches the Cambrian to Middle Devonian basin evolution of Belgium, East Anglia and 
the surrounding areas, all situated at the southeastern edge of the Avalonian microcontinent (Verniers et 
al., 2002). This model divides the Lower Palaeozoic succession of Belgium into three megasequences, each 
related to a different geodynamic context. The present study evaluates the model for the Middle Ordovician 
to Llandovery interval, starting at the basis of the lithostratigraphy and the chitinozoan biostratigraphical 
study of the sediments of the Condroz Inlier and the Brabant Massif. The interval studied corresponds to 
megasequence 2 and the lower part of megasequence 3.
The lithostratigraphy of the Condroz Inlier is poorly known and mainly based on studies from the first 
half of the past century. Integrated field observations of the lithofacies and a biostratigraphical study with 
chitinozoans allow a more detailed characterisation of the lithostratigraphy and more precise dating. This 
leads to a revision and formalisation of the lithostratigraphical units of the Condroz Inlier, with the introduction of two new formations and five new members. Furthermore, three new chitinozoan species are 
described.
Additionally, the present study revises the chitinozoan biostratigraphy of the Upper Ordovician of the 
Brabant Massif. Combination with data from the Condroz Inlier and the literature leads to the construction of a chitinozoan biozonation for the Middle Ordovician to Llandovery of Belgium. This is a contribution to the establishment of a chitinozoan biozonation for the Ordovician of Avalonia, which is still under 
construction. 
The present study also considers the palaeobiogeographical affinities of the chitinozoans of the 
study area. During the late Floian to early Darriwilian, the chitinozoan fauna resembles that of northern 
Gondwana, whereas during the late Sandbian to Hirnantian interval, it displays many similarities with the 
Baltoscandian fauna. Several not yet recorded species occur in the latest Darriwilian to early Sandbian interval of the Condroz Inlier; this may be explained by Avalonian chitinozoan endemism during this interval.
The chitinozoan biozonation, integrated with biostratigraphical data from the literature and completed 
with lithostratigraphical data, allows correlation of the sections within the Condroz Inlier and of those of 
the Brabant Massif. This correlation confirms the facies differences between the Brabant Massif and the 
Condroz Inlier, but also reveals lateral facies changes within the Condroz Inlier. The importance of these 
facies differences is evaluated by the assessment of the depositional environment from lithofacies data and 
literature data on depth-dependent species. The evolution of the depositional environment and comparison with sea-level reconstructions allow evaluation of the cause of the facies changes through time, and 
provides an answer to the question whether they are eustatic or tectonic. A model for the basin evolution 
of the Condroz-Brabant Basin is constructed.
Prior to the mid to late Llanvirn (late Darriwilian), the depositional environment of the Condroz Inlier did 
not differ much from that of the Brabant Massif, and deposition occurred on the shelf edge to slope. From 
the late Llanvirn onwards, differentiation of the Condroz-Brabant Basin occurs: the Condroz Inlier records 
shelf to upper slope deposition, whereas the sediments of the Brabant Massif have predominantly a slope 
signature. 
We endorse the position of the megasequence boundaries in the Lower Palaeozoic succession, as recognised by Verniers et al. (2002): the boundaries correspond to basin-wide unconformities. However, the 
previously inferred depositional settings need modification. The Brabant Massif succession comprises predominantly slope sediments during the Middle Ordovician to Llandovery interval, whereas a shelf setting 
was assumed in earlier publications. An interval dominated by outer shelf deposits occurs in the Ashgill of 
the Brabant Massif and we consider it as the lower sequence of megasequence 3. The studied interval in 
the Condroz Inlier incorporates not exclusively shelf sediments, as previously supposed.
In the western Condroz Inlier, a turnover from outer shelf settings over middle shelf back to outer shelf 
environments is noted during late Onnian (mid Katian) to Rawtheyan (late Katian) times; subsequently, a 
previously unknown emersion phase is detected during the Hirnantian-mid Aeronian. This situation differs 
clearly from that in the eastern Condroz Inlier, where the same interval shows an evolution from mid shelf 
over outer shelf environments (mid to late Katian) and inner shelf deposits (probably Hirnantian) to outer 
shelf to slope settings during the Llandovery. The facies evolution in the eastern Condroz Inlier fits well the 
eustatic sea-level reconstructions for these times, but for the western Condroz Inlier, an important uplift 
phase can be proposed. The uplift may be related to the Ardennian Deformation Phase, but thermal uplift 
is also possible.
A hiatus occurs at the Caradoc-Ashgill boundary in the central Condroz Inlier and is correlated with an 
anomalous interval of middle shelf deposits in the outer shelf to slope succession of the Brabant Massif. The 
present study does not confirm the observation in the Condroz Inlier of an angular unconformity at this 
contact, previously interpreted as the echo of the Ardennian Deformation Phase. Chitinozoan biostratigraphy allows correlation with the Caradoc-Ashgill unconformity of the Welsh Basin and with a pronounced 
lowstand interval in Baltoscandia (the Solvang Lowstand) and in Gondwana. Via carbon istopes studies 
from the literature, the lowstand is correlated with the Laurentian late Mohawkian to early Cincinnatian 
lowstand, following an important positive d13C excursion, which is proposed to be related to the onset of 
glaciation in Gondwana. If confirmed, the proposed correlation allows for the first time global recognition 
of this important event. 
Finally, the obtained basin evolutionary model is compared to other depositional areas on eastern 
Avalonia, confirming most previously obtained correlations, but also necessitating some modifications. 
Importantly, our results show that for the Caradoc-Ashgill unconformity, eustatic movement has to be 
taken in consideration.