Frédéric Boulvain & Anne-Christine da Silva
Pétrologie sédimentaire, B20, Université de Liège, Sart Tilman, B-4000 Liège
The aim of this fieldtrip is to enlighten the sedimentology of the Frasnian reefs and mounds from Belgium. Palaeoenvironments ranging from supratidal (paleosoils, restricted lagoons) to open marine (carbonate mounds) will be illustrated by selected high-quality outcrops. Detailed architecture of platform and carbonate mounds will be discussed and related to sea-level variations. The use of magnetic susceptibility for high precision stratigraphic correlation and for bathymetric reconstruction will be illustrated by several examples.
GENERAL STRATIGRAPHY AND LOCATION OF STOPS
Geological setting of the Frasnian in Belgium.
a. Geological map of Belgium, with the outcrops (yellow spots).
b. N-S section in the Belgian sedimentation basin before Variscan structuration. The framed area represents the middle part of the Frasnian
STOP 1: Villers-le-Gambon section, Philippeville Formation
Villers-le-Gambon section: location of observations and sampling.
Log, facies and magnetic susceptibility of the Villers-le-Gambon section (Philippeville Formation).
STOP 2: Tailfer section, Lustin Formation
The Tailfer outcrop.
Tailfer section: location of observations and sampling.
Log, facies and magnetic susceptibility of the Tailfer section (Lustin Formation).
STOP 3: Barse section, Lustin Formation
Barse section: location of observations and sampling.
Log, facies and magnetic susceptibility of the Barse section (Lustin Formation).
STOP 4: Huccorgne section, Bovesse and Huccorgne Formations
Huccorgne section (vicinal): location of observations and sampling.
Huccorgne section (quarry): location of observations and sampling.
Log and magnetic susceptibility of the Huccorgne section (Bovesse and Huccorgne Formations).
GENERAL STRATIGRAPHY AND LOCATION OF STOPS
Simplified geological map of southern Belgium with location of stops.
N-S transect in the Dinant Synclinorium, before Variscan deformation. Conodonts after Gouwy & Bultynck (2000)
STOP 1: Les Wayons quarry, Merlemont, Petit-Mont Member
This abandoned marble quarry, now used as a training area for speleology, exposes the lower part of a Late Frasnian mound. Stratification is nearly vertical and the base of the mound is visible in the NW part of the quarry. Some 20 m of red stromatactis limestone (Pm1, see sedimentological model below) forms the lower part of the mound. Red colour is related to high amounts of microaerophilic iron bacteria in the sediment and stromatactis are derived from collapse of sponges whose spicules are very abundant in this facies.
Location of Les Wayons quarry (stop 1) near Merlemont (Philippeville Anticlinorium).
Sedimentological model of Late Frasnian carbonate mounds (Petit-Mont Member).
Examples of facies from Petit-Mont Member mounds. A: red limestone with stromatactis (Pm1); B: pink limestone with corals, brachiopods, stromatoporoids, stromatactis (Pm3); C: grey microbial limestone with corals, stromatoporoids (Pm5).
Hematite-rich microaerophilic iron bacteria, responsible for the red color of facies Pm1, Pm2 and Pm3. Filaments have a diameter of 2µm and are close to the Sphaerotilus-Leptothrix group of bacteria. Coccoids could be related to Siderocapsa. Rochefontaine quarry, Merlemont, Petit-Mont Member, Frasnian, Belgium.
STOP 2: Hautmont quarry, Vodelée, Petit-Mont Member
This stop is dedicated to a very spectacular Late Frasnian carbonate mound (Petit-Mont Member). The top of the mound is well accessible and all stages of mound drowning are visible.
Location of Hautmont quarry (stop 2, arrowed) near Vodelée (Philippeville Anticlinorium).
Transition from mound core to upper flank facies in Hautmont (stop 2).
Logs of sections A to D in the Hautmont quarry (stop 2). Explanation of symbols, see stop3; explanation of facies, cf. sedimentological model (see stop 1).
STOP 3: Arche quarry, Frasnes, Arche Member
This abandoned marble quarry exposes a very complete section of an Arche Member mound, from pink coverstones with stromatactis, corals and crinoids (A2) with some beautiful Receptaculites and zebra to grey microbial limestone (A5) forming the upper part of the quarry. Less than 50 m north of the main quarry, downhill, it is possible to have access to the base of the mound, characterized by a transition from shale with abundant rugose corals to limestone with sponges, corals, crinoids and iron-bacteria.
Location of stops 3 (Arche quarry), 4 (Lion quarry) and 5 (Nord quarry) near Frasnes (southern border of Dinant Synclinorium).
Schematic geological map of the Frasnes area, with location of Arche (Arche Member), Nord and Lion (Lion Member) quarries.
Accumulation of disphyllids (rugose corals) in argillaceous sediments, forming the substrate of the Arche mound. Arche Member, Frasnian, Frasnes, Belgium (stop 3).
Log of the Arche section (stop 3), facies and explanation of symbols.
STOP 4: Lion quarry, Lion Member, Frasnes
Location, see stop 3.
This abandoned quarry exposes a very nice section in a Lion Member mound, from prograding bioclastic fore mound facies (SW) to loferitic back mound facies (NE). A small wire-cut section close to the entrance of the quarry shows lenses of grey limestone with dendroid stromatoporoids (L6) and microbial bafflestones (L5). A metre-thick neptunian dyke with parietal encrustations of iron-bacteria cuts this unit. The main access trenches to the quarry expose a very interesting section in the Boussu-en-Fagne Member shale, deposited during the drowning of the mound.
NE-SW section in the Lion quarry with sampling lines and facies.
Large-scale progradation structures in the fore-mound part of the Lion quarry (stop 4). Regional dipping of formations is 35°N (towards the left of the picture). Lion Member, Frasnian, Frasnes, Belgium.
Contact between the last limestone beds of the Lion mound and the Boussu-en-Fagne Shale in the access trench to the quarry (stop 4). Lion Member, Frasnian, Frasnes, Belgium
Composite sedimentological model of Middle Frasnian carbonate mounds from Belgium.
STOP 5: Nord quarry, Lion Member, Frasnes
Location, see stop 3.
This quarry shows a complete section of a Lion Member mound. Of particular interest is the transition from the Arche to the Lion Members.
General view, geometrical data and location of sampled sections in the Nord quarry, Frasnes.
Logs of section A-D from the Nord quarry in Frasnes.
STOP 6: Beauchâteau quarry, Petit-Mont Member, Senzeilles
This abandoned marble quarry is the most spectacular outcrop of a Late Frasnian carbonate mound in Belgium. The mound is standing in subhorizontal position and large sawn sections expose facies ranging from the middle part of the mound (Pm3) to its top (Pm 4 and 5). The upper central panel shows interfingering between grey massive microbial facies and pink bedded bioclastic flank sediments. Dipping of mound flanks is partly the result of differential compaction: sedimentary slopes rarely exceed 30°. The left part of the quarry shows crinoid-rich argillaceous flank sediments.
Location of Beauchâteau quarry (stop 6).
The Beauchâteau quarry, exposing the upper half of a Petit-Mont Member mound (stop 6). The buildup is standing in subhorizontal position. Petit-Mont Member, Frasnian, Senzeilles, Belgium.
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Date de dernière mise à jour : 18-5-2009
Pétrologie sédimentaire, B20, Université de Liège, B-4000 Liège