13/4: 483-496

GEOLOGICA BELGICA (2010) 13/4: 483-496

Frasnian reefs, mounds and atolls from Belgium: sedimentology and magnetic susceptibility – A FIELDTRIP GUIDEBOOK

Frédéric BOULVAIN & Anne-Christine DA SILVA

(17 figures, 2 tables)

Département de Géologie, Pétrologie sédimentaire, Bât. B20, Université de Liège, Sart Tilman, B-4000 Liège, Belgium, e-mails : [email protected], [email protected]

1. Introduction and the “northern belt” -the shallower- crops out in the northern part of the Dinant Synclinorium, the Namur During the Frasnian, a 5000 km2 carbonate platform Synclinorium and the Vesdre area. developed in Belgium, showing environments ranging from restricted shallow-water lagoons and supratidal areas to a relatively deep external ramp with carbonate mounds. 2.1. The southern belt This carbonate platform is especially instructive because of a combination of extraordinary outcrops (“marble” The southern belt is characterized by carbonate mound quarries with large sawn sections) and a long history of sedimentation with associated flank and off-mound facies. paleontological study which has led to a refined Since the classical studies by Mailleux (1913), three levels stratigraphic framework (Boulvain et al., 1999). Carbonate of carbonate mounds were known. These are in ascending mounds have been the subject of intense investigation order the Arche, Lion and Petit-Mont Members, belonging carried out by generations of geologists (e.g. Lecompte, respectively to the Moulin Liénaux, Grands Breux and 1959; Tsien, 1975; Boulvain, 2007) but a few of these Neuville Formations (Fig. 2). The famous Arche and Lion studies focused on the shallow-water part of the platform buildups are located in the vicinity of Frasnes, historical (Da Silva & Boulvain, 2004). stratotype of the Frasnian. Recently, Boulvain et al. (2005) gave information about a set of outcrops located some distance from Frasnes: the La Boverie quarry, close to 2. Geological setting Rochefort, and the Moulin Bayot sections, close to Southern Belgium belongs to the northern part of the Vodelée. At both locations, it was possible to study the Rhenohercynian fold and thrust belt. Frasnian carbonates whole middle Frasnian succession, starting near the base and shales are exposed along the borders of the Dinant, of the Arche Member and ending within the Lion Member. Vesdre and Namur Synclinoria and in the Philippeville Moreover, at both locations, an additional buildup was Anticline (Fig. 1). The platform can be divided in three recognized between the Arche and Lion Members. The main depositional areas characterized by a different facies presence of this additional buildup along all the south side association, carbonate production rate and sedimentary of the Dinant Synclinorium is now supported by its evolution. occurrence in boreholes drilled in the Nord quarry at During the Middle Frasnian, the most distal part of the Frasnes. The name of La Boverie Member was introduced platform (“southern belt”) is located along the southern by Boulvain & Coen-Aubert (2006), as a subdivision of border of the Dinant Synclinorium, the “intermediate the Moulin Liénaux Formation, for the carbonate deposits belt” corresponds mainly to the Philippeville Anticline lying between the Arche and Bieumont Members.

3°30' 2°30' Long. E . NCL SY N RES SD BRABANT MASSIF VE

50°30' Lat. N NCL. MUR SY NA T RIUM O INO VEL L STA YNC S IF ANT SS

4 MA e

X s DIN

u e M Dinant 3 La Boverie 20 km PHILIPPEVILLE 2 1 ANTICLINORIUM Fault Givet Famennian & Carboniferous Y Middle Devonian Brussels & Frasnian 50° Lower Devonian Figure 1. Geological SERPONT Lower Palaeozoic map of southern ROCROI MASSIF Platform outcrops MASSIF Atoll outcrops Belgium with location of stops. 484 F. Bo u l v a i n & A.C. Da Si l v a

S Dinant Synclinorium SE Dinant Synclinorium N Dinant Synclinorium SE Philippeville NW Philippeville S Namur Synclinorium Anticlinorium Anticlinorium

MATAGNE Petit-Mont VALISETTES AISEMONT

E LUSTIN NEUVILL GRANDS PHILIPPEVILLE Boussu BREUX ES PRESL LLE OUX Lion Bieumont A FO LE R T DE L PON Ermitage Boverie MOULIN LITHOLOGY LIENAUX FACIES shale Arche Châlon open marine argillaceous limestone NISMES nodular shale mound bedded limestone FORMATION reef Member dolomite NES 100m FROMELEN biostrome Formation boundar y lagoon ~1 km

Figure 2. Cross-section in the Belgian Frasnian sedimentary bassin, prior to Variscan tectonism (this section corresponds to the line X-Y on Fig. 1).

2.2. The intermediate belt 3. Facies and microfacies In the Philippeville Anticline, the carbonate mound- Data comes from the detailed study of more than 5000 bearing levels were replaced by shales and argillaceous thin sections from 20 outcrops from the Dinant, Namur limestones (Pont de la Folle Formation) followed by and Vesdre Synclinoria and from the Philippeville bedded limestone consisting of open-marine facies and Anticline. In the following descriptions, microfacies are biostromes (Philippeville Formation). ordered from the most distal to the most proximal, or from the deeper to the shallower. However, this order is not 2.3. The northern belt always effective, due to lateral variations, especially in the more proximal parts of the platform. Microfacies are Along the northern border of the Dinant Synclinorium grouped in 4 main facies belts : carbonate mounds and (“northern belt”), the Middle Frasnian consists of bedded flank deposits (M), external platform or ramp (E), limestones, exhibiting a distinct proximal aspect with biostromes (B) and internal platform (I) (Fig. 3). Tables 1 biostromes alternating with lagoonal facies followed by & 2 compile detailed sedimentological characteristics and palaeosoils and lagoonal deposits. bathymetrical interpretations for the different During the Upper Frasnian, a general northern shift or microfacies. retrogradation of the platform is observed: the southern belt extends into the Philippeville Anticline with 3.1. Carbonate atolls and mounds (M, southern and spectacular development of Petit Mont Member mounds intermediate belts, Figs 4 & 5) and the intermediate belt shifts to the Northern border of The analogy between closely related facies in the Dinant Synclinorium. This belt is characterized by stratigraphically distinct buildups was highlighted by shales with two carbonate levels dominated by rugose Boulvain et al. (2001) who employed the same facies corals or oncoids (Aisemont Formation) (Boulvain, 2001) designation, i.e. a number following a specific letter for (Fig. 2). the member name (for example: A2 and L2, corresponding

Distal 7 Proximal lagoon 6 Figure 3. General (facies I) 5 4 organization of biostrome nds Frasnian sedimenta- ou (facies B) m tion domains. 4-7 re- e M) nat s fer to Figs 4 to 7 external platform bo ar facie or ramp (facies E) C ( (sedimentary models). Fi r s t IGCP 580 m e e t i n g f i e l d t r i p g u i d e b o o k 485 100-150m 80-100m 60-80m 30-60m 30-60m 0-30m 0m 5-10m 80-100m 60-80m 60-80m BATHY- METRY on In ter p r E tati Aphotic, below SWAZ Aphotic, below SWAZ Subphotic, close to SWAZ Euphotic, close to FWWAZ Euphotic, close to FWWAZ In the FWWAZ Intertidal Subtidal Below SWAZ Close to SWAZ Close to SWAZ En er gY low Very low Very Low, episodically moderate Moderate Moderate High Low Low Low Low, episodically moderate Low, episodically moderate ↑ ↑ ↑ ↓ ↑ ↑ ↓ ↓ ↓ ON / P RESERVATI tra n s po rt P reservation Transport ↓ Transport P reservation ↓ Transport P reservation ↓ Transport P reservation ~ ~ Transport P reservation ~ Transport P reservation ↓ Transport P reservation ~ ↓ Transport P reservation ↓ Transport P reservation ↑ Transport P reservation ↑ Transport P reservation ↑ Transport

and and and

and and and bryozoans, A N D on OU S and stromatoporoids on OU S BI O TA and stromatoporoids and iron bacteria oc ht a u t Sponges Sponges, corals, crinoids iron bacteria Corals, crinoids, brachiopods, bryozoan Corals, stromatoporoids, dasycladales, cyanobacteria Corals, cyanobacteria stromatoporoids Dendroid stromatoporoids cyanobacteria Dendroid stromatoporoids paleosiphonocladales Paleosiphonocladales calcispheres Corals, brachiopods, ostracods bryozoans Corals, brachiopods, ostracods bryozoans Stromatoporoids corals, brachiopods all oc ht / T e x t u r E / C o l r / S tr uc t u re Red mudstone or wackestone Red or pinkish mudstone, floatstone pinkish or greenish, Grey, floatstone, (rudstone) Grey grainstone, rudstone bindstone bafflestone Grey, rudstone, m-thick Grey, beds grainstone- laminar, Grey, wackestone wackestone- dm-thick, Grey, mudstone dm-thick, bedded Dark grey, packstone dm-thick, bedded Dark grey, packstone-grainstone dm-thick, bedded Dark grey, packstone-grainstone a c i E s / Carbonate mound facies (M) Lateral facies (M) F on ass oc iati M1. Stromatactis, sponges M2. Stromatactis, corals and crinoids M3. Stromatactis, corals and stromatoporoids M4. Corals, peloids and dasycladales M5. Microbial limestone M6. Dendroid stromatoporoids M7. Loferites M8. Bioturbated limestone M9. Microbioclastic packstone M10. Bioclastic packstone, grainstone Peloids and M11. intraclastic packstone and grainstone

Table 1. Main characteristics of the carbonate mound facies. to nearly equivalent facies in the Arche and Lion members). wackestone with stromatactis (facies M1), which becomes In this more synthetic fieldtrip guidebook, the facies progressively enriched in crinoids and corals (M2); grey numbers are simply preceded by “M” for “mound”. or pinkish limestone with stromatactis, corals, and Eight facies were recognised in the buildups (Table 1), stromatoporoids (M3); grey limestone with corals, peloids each characterized by a specific range of textures and and dasycladales (M4); grey, microbial limestone (M5); assemblage of organisms (Boulvain, 2007): spiculitic grey limestone with dendroid stromatoporoids (M6); grey, 486 F. Bo u l v a i n & A.C. Da Si l v a nder SWAZ, external U nder SWAZ, deposits slope U nder SWAZ, deposits Under or in SWAZ, biostromes biostromes In SWAZ, SWAZ Subtidal, restricted Subtidal to intertidal, channels Intertidal, local emersion features Intertidal, local emersion features Supratidal, emerged on In ter p r E tati En er gY Low Low , episodically moderate Episodical High Mainly low, episodical agitation Low Moderate Low Low to moderate Low ↓ ↓ ↑ ↓ ↑ ↑ ↑ ↑ ↑ ON / P RESERVATI tra n s po rt P reservation ↑ Transport P reservation ↑ Transport P reservation ↓ Transport P reservation ↕ Transport P reservation ↑ Transport P reservation ↓ Transport P reservation ~ ↓↑ Transport P reservation ↓ Transport P reservation ↓ Transport P reservation ↓ Transport ,

and A N D on OU S on OU S BI O TA oc ht a u t Laminar stromatoporoids, ostracods and brachiopods Low domical stromatoporoids crinoids Dendroid stromatoporoids, ostracods , clotted matrix , dasycladales Amphipora peloids Umbella , clasts, crinoids, O stracods, dasycladales Mainly peloids Dasycladales all oc ht C rinoids, ostracods C lasts, crinoids, ostracods / T e x t u r E / C o l r / S tr uc t u re Dark grey dm beds, packstone to wackestone Dark dm beds plurim. beds, Light grey, coverstone to rudstone, plurim. beds, Grey, rudstone pluridm to Light grey, plurim. beds, floatstone to bindstone metric beds Light grey, floatstone to wackestone metric beds, G rey, heterogeneous, subnodular Decimetric to metric dark grey beds Dark dm beds, with undulated lamination P luridm beds, light grey, with pink staining a c i E s External platform or ramp facies (E) Biostromal facies (B) Internal platform facies (I) F E1. C rinoidal packstone E2. Lithoclastic packstone to grainstone B1. Laminar stromatoporoids B2. Low domical stromatoporoids B3. Dendroid stromatoporoids , I1. Amphipora dasycladales and peloids Umbella packstone I2 . I3. Mudstone I4. Laminated limestone I5. P aleosols

Table 2. Main characteristics of the external, biostromal and internal facies. laminar fenestral limestone, (M7); grey, bioturbated microbioclastic, often argillaceous packstones with limestone (M8). ostracodes, trilobites and cricoconarids (M9); bioclastic packstones, grainstones and rudstones with intraclasts Laterally to the buildup facies, thin-bedded bioclastic (M10) and packstones, grainstones and rudstones with and intraclastic facies are observed, most elements of peloids and intraclasts (M11). which underwent a certain transport. Frequent sorting and rounding of their elements characterize these facies. They Sedimentological evidence suggests that facies M1 are ordered according to their content and grain-size: and M2 correspond to iron bacteria-sponge-dominated Fi r s t IGCP 580 m e e t i n g f i e l d t r i p g u i d e b o o k 487

Figure 4. Sedimentary Petit-Mont Mb. model of Late Frasnian Petit- Mont Member mounds in tst4 the Philippeville Anticline. Valisettes Fm. ~100 m lst3 5 S hst3 Neuville Fm.

Philippeville Fm.

M1 M2 M3 M4 M5 BUILDUP

shale argillaceous limestone nodular shale crinoidal limestone

communities, developing in a quiet aphotic and hypoxic mound influence as most of the bioclastic and intraclastic environment (Bourque & Boulvain, 1993; Boulvain et al. material is derived from these buildups. (Humblet & 2001). M3 developed between storm wave base and fair Boulvain, 2001). weather wave base, in an oligophotic environment. Facies The main differences between the Middle and Late M5 developed close to fair weather wave base. Facies M6 Frasnian mounds concern facies architecture and are a and the fenestral limestone M7 correspond to an consequence of different palaeoceanographic settings. environment with slightly restricted water circulation. The large flattened Middle Frasnian Arche and Lion Facies M8 developed at subtidal depths in a quiet, lagoonal buildups show limited vertical differentiation, large-scale environment. progradation features, extensive exportation of material Microbioclastic packstones (M9) are characterized by towards off-reef environment and development of inner an open-marine facies with brachiopods, bryozoans and lagoonal facies. They grew offshore from a well-developed crinoids, whereas bioclastic rudstones (M10) and carbonate platform with a healthy carbonate factory. intraclastic packstones or grainstones (M11) show a clear Middle Frasnian sea level fluctuations were relatively

~100 m Matagne Petit-Mont hst3+lst3 7 tst2+hst2 lst1

Lion Mb. hst1 tst1

Arche Mb.

Nismes Fm. BUILDUP M4 M8 L: lithoclastic grainstones and rudstones M3 M7 B: bioclastic grainstones and rudstones M2 M6 b: argillaceous limestone with microbioclasts S: shale M1 M5

Figure 5. Sedimentary models of Middle to Late Frasnian mounds along the Southern border of the Dinant Synclinorium. 488 F. Bo u l v a i n & A.C. Da Si l v a

N Section in a biostrome Figure 6. Facies Exportation Exportation model of Middle to the lagoon to the ext. platf. Frasnian biostromes. Explanation of symbols, see Fig. 7.

B3 B2 B2/B1 ~10m Proximal no horizontal scale Distal B1

mild, and sedimentation was able to keep-up with sea- 3.2. Carbonate platform (E, B, I, intermediate and level rise (Boulvain, 2007). Reef initiation occurs during northern belts, Figs 6 & 7) a transgression with the development of mud or skeletal The ideal shallowing-upward facies succession starts with mound facies (M2-3). During the subsequent lowstand, open-marine deposits corresponding to crinoidal reef growth was restricted to a downslope position, packstones (E1). They are followed by biostromes with resulting in the development of a circular reef margin laminar stromatoporoids (B1), overturned and broken (atoll crown). During the following transgressive stage, massive stromatoporoids (B2) and dendroid algal and microbial mound facies with stromatoporoids, stromatoporoids (B3). Then, biostromes are overlain by corals and peloids were deposited (M4-6). The occurrence subtidal lagoonal facies with Amphipora, of relatively restricted facies (M7-8) inside this crown is paleosiphonocladales and peloids (I1), followed by possibly the result of a balance between sea-level rise and mudstone (I3) and laminated peloidal facies (I4) in the reef growth. During the regression which causes the intertidal zone. The subtidal and intertidal zones were cut emersion of the top of the reef and the displacement of the by channels filled by Umbella and intraclasts (I2). The facies downslope, some flank deposits are developed supratidal zone was characterized by paleosols (I5) (Da corresponding to the dismantling of the top of the mounds Silva & Boulvain, 2004) (Table 2). (M10-11). An important sedimentological observation concerning At the opposite extreme, during the Late Frasnian, platform evolution (intermediate and southern belts) is the severe eustatic rises, together with rising oceanic hypoxic apparent division seen in all the sections between an upper conditions were responsible for frequent collapses of the and a lower unit (Da Silva & Boulvain, 2002) (Figs 10 & carbonate factory, drowning of the Middle Frasnian 13). The lower unit is dominated in the intermediate belt carbonate platform, and development of buildups with by ramp facies with some biostromal interruptions, and in relatively limited lateral extension, high vertical facies the northern belt by biostromes with lagoonal interruptions. differentiation, low potential for material exportation and The upper unit (lagoon) consists of an alternation of high content in microaerophilic iron bacteria (Boulvain, biostromes and lagoonal facies in the intermediate belt 2001). In this context, the main part of the Petit-Mont and of lagoonal facies (with paleosol) in the northern mounds developped as a catch-up sequence (M1-3) during belt. a highstand while the shallowest algal-microbial facies Within these sedimentological units, facies are stacked (M4-5) were the consequence of a sea-level drop. into metre-scale cycles, showing mainly shallowing-

Section in the internal platform

Supratidal zone

Intertidal zone I5 I4 Subtidal zone N I3 I2 ~10m Proximal no horizontal scale I1 Distal

Brecciated limestone (paleosols) Dessiccation features Laminated carbonates Peloids Figure 7. Facies Low domical Umbella Paleosiphonocladales Laminar stromatoporoids stromatoporoids model of the Middle Frasnian internal Amphipora Encrusting organisms platform. Explana- dendroid stromatoporoids (algae or stromatoporoids) Stachyodes tion of symbols for Figs 6 & 7. Fi r s t IGCP 580 m e e t i n g f i e l d t r i p g u i d e b o o k 489

Figure 8. Drowning of the Hautmont mound. Vodelée, Petit-Mont Member.

M5 M1-2

Hautmont 20

N B C B 15 C 10 C D

A 5 0m D

30 m

Figure 9. Logs of sections in the Hautmont quarry. Vodelée, Petit-Mont Member. 490 F. Bo u l v a i n & A.C. Da Si l v a upward trends. Such cyclicity is common in Devonian plastic deformation of the sediment, presence of overturned shallow-water carbonates. Different kinds of cycles coral colonies (very spectacular in the lower central panel however, are identified here. of the quarry), formation of zebra structures by lateral In the biostromal unit from the intermediate belt, compression, scarcity of hardgrounds and of sediment sedimentation is mainly acyclic with the stacking of 10 borings, and the irregular character of some synsedimentary cm-thick crinoidal beds, probably due to the deeper fractures indicate an absence of early lithification. It environment being less sensitive to minor relative sea- appears that the sediment was initially sufficiently ductile level variations. to permit synsedimentary deformation, yet sufficiently In the lagoonal unit, the cycles are characterized by coherent to have maintained open cavities (stromatactis) biostromes followed by lagoonal deposits and capped by and significant relief. It is likely that the sediment had a intertidal laminites. In the northern belt, the biostromal gel-like consistency, probably related to the presence of unit shows one or few metres-thick cycles, with crinoid beds (the colonisation stage) followed by massive significant quantities of organic matter. biostromes and lagoonal deposits and capped by intertidal laminites. The lagoonal unit is characterized by restricted Magnetic subtidal and intertidal facies covered by, or transformed Microfacies Villers susceptibility into paleosols. These cycles are not always complete.

4. Stops 100

4.1. Stop 1: Hautmont quarry, Vodelée, Petit-Mont Member This stop is dedicated to a very spectacular Late Frasnian carbonate mound (Petit-Mont Member): the Hautmont mound near Vodelée. This active quarry is located on the SE end of the Philippeville Anticline. The central part of 80 the mound is in nearly horizontal position. The top of the mound is well accessible and all stages of mound drowning are visible (Figs 8 & 9). The upper central part of the mound shows a core of grey microbial, coral stromatoporoid limestone (M5). This facies forms massive limestone with stylolites. Decimetre- to metre-scale growth cavities cemented by granular spar are abundant. Breccia is locally 60 present. The fauna is dominated by subspherical coral colonies (Hankaxis, Phillipsastrea, Alveolites), l a g o n u i t Thamnopora, brachiopods and subordinate dendroid stromatoporoids (Amphipora). Renalcis is locally abundant. Thrombolitic structures and microbial mats are present. Within thrombolites, Renalcis is often associated with Palaeomicrocodium. 40 4.2. Stop 2: Beauchâteau quarry, Senzeilles, Petit-Mont

Member b i o s t r m a l u n This abandoned marble quarry, located near the village of Senzeille in the SW part of the Philippeville Anticline, is the most spectacular outcrop of a Late Frasnian carbonate mound in Belgium. The mound is standing in subhorizontal 20 position and large sawn sections expose facies ranging from the middle part of the mound (M3) to its top (M4 and 5). The upper central panel shows interfingering between grey massive microbial facies and pink bedded bioclastic flank sediments. The left part of the quarry shows crinoid- rich argillaceous flank sediments. Contradictory inferences about the initial mechanical state of carbonate mound mud appear to derive from field 0 B1 B3 I2 I4 m³/kg -8 -7 E1 B2 I1 I3 0 5.10 1.10 observations. The persistence of dips as high as 35° on the external biostr. lagoon flanks of several mounds, the presence of lithoclasts in the grey limestone (M5) and the sharp distinct character of Figure 10. Log of the Villers-le-Gambon section with microfacies some fractures indicate early lithification. Conversely, and magnetic susceptibility. Fi r s t IGCP 580 m e e t i n g f i e l d t r i p g u i d e b o o k 491

4.3. Stop 3: Villers-le-Gambon section, Philippeville of the section starts along the main road with oolitic Formation hematitic beds alternating with dark shales (Lower Frasnian, top of Presles Formation, Fig. 11). Carbonate This section along a disused railway (Fig. 10) is located in production starts with crinoidal beds (boundary with the the S of the Philippeville Anticline. It is one of the rare Lustin Formation) and development of biostromes. These section exposing almost completely the Philippeville biostromes are dominated by massive or laminar Formation. The lower 50 m are dominated by dark stromatoporoids. The succession continues in the Tailfer argillaceous crinoidal limestones (E1) with some quarry with a splendid sawn section of a lamellar biostromal beds and chert intercalations. The upper 50m stromatoporoids biostrome (Fig. 12). This is followed by are characterized by biostromal facies with well-developed an alternation of lagoonal deposits and well-developed massive stromatoporoids alternating with fenestral paleosols (Fig. 13). mudstones (I1) or branching stromatoporoids (I3) rudstones. Biostrome edification by lamellar stromatoporoids (Facies B1) corresponds to the following ecological 4.4. Stop 4: Tailfer quarry, Lustin Formation sequence (Fig.12): lamellar stromatoporoids rudstone with crinoids, brachiopods and packstone matrix Located along the Meuse river, the Tailfer section is part characterises the colonisation phase (B1/3, Fig.12B), and of the northern flank of a major anticline, close to the developped during relatively high energy events. Then, Northern border of the Dinant Synclinorium. The first part two other lamellar stromatoporoids facies with tabulate

N S

1 5 2 3 4 N 3 15 11 1 1 2 3 8 10 Presles Lustin Formation 2

16 26 28 29 31 32 33 34-35

4 36-37 39 40 41 38 42 - 43- 44 45-46 47 48 49 52-51 53 60 61-65 66-71 91 101 115 126 S

132 142

103 102 99 5 2m 55 57 59 72-737577

Figure 11. Tailfer section. Presles and Lustin Formations. 492 F. Bo u l v a i n & A.C. Da Si l v a

A D Figure 12. Lamellar stromatoporoids Lagoo nal facies biostrome, Tailfer quarry, Lustin For-

S t r o m . mation.

Erosive L a m i n r b i o s t r m e surface Lagoonal facies

Facies B1/3 Erosive surface Lagoonal facies microfacies B1/1 C microfacies B1/2 Internal sediment

in place lamellar stromatoporoids overturned lamellar stromatoporoids rugose coral Thamnopora brachiopods 5cm

Sequence 5 m Sequence 3 A m C 48 70

46 68 46 66 44 64 42

40 62

38 60 36 58 34 56 MS (m³/kg) 32 B -8 -7 -7 -7 E I 0 5.10 1.10 1.5.10 2.10 30 Microfacies MS (m³/kg) E B I m Microfacies -8 -8 -8 -8 -1.10 0 2.10 4.10 6.10 94 Sequence 10

93 m Sequence 4 56 92 91 D 55 54 B 90 Figure 14. MS 53 89 evolution with se- 52 MS (m³/kg) 88 MS (m³/kg) quence evolution, -8 -7 -7 -7 E B I 0 2.10-8 4.10-8 6.10-8 8.10-8 B I 0 5.10 1.10 1.5.10 2.10 close up from Fig. Microfacies Microfacies 13. Fi r s t IGCP 580 m e e t i n g f i e l d t r i p g u i d e b o o k 493

Tailfer Microfacies Magnetic susceptibility Platform facies model m

90 E1 B1-3 I1 I2 I3-4 I5 E a r l y R h e n

80 6x10-8

-8 ( m ³ / k g )

J a m i e 5x10 70 4x10-8 M S 3x10-8 -8 60 2x10 1x10-8 Belgian platform 0 l a g o n u i t E1 B1-3 I1 I2 I3-4 I5 50 Distal Proximal Facies

Figure 15. Mean MS evolution with microfacies on the carbonate 40 platform from Belgium. H a s i . l

-8 MS (m³/kg) 30 4.10 b i o s t r m a l u n 3,5.10-8 3.10-8 2,5.10-8 20 2.10-8 1,5.10-8 1.10-8 10 0.5.10-8 0

u n c t a Barse Aywaille Tailfer Villers P 0 m³/kg B1 B3 I2 I4 0 1E-07 2E-07 3E-07 Figure 16. Mean MS evolution for platform sections, from more E1 B2 I1 I3 I5 proximal zone of the platform (Barse), to the more distal zone ext. Biostr. Lagoon (Villers).

Figure 13. Log, microfacies and magnetic susceptibility of the change and MS signature. Detailed results were published Tailfer section, Lustin Formation. Northern border of the Dinant in Da Silva & Boulvain (2002, 2006, 2009a & b) Synclinorium. MS evolution seems to be related to different parameters. We will illustrate the relationship between MS and fourth (a), third (b) order sequences, with corals (B1/2) and mud (B1/1), alternate in dm-scale units microfacies (c) and with the position of the section in the (Fig.12C-D), probably in relation with short term higher basin. energy events. Muddy microfacies settled in quiet to very quiet water (immediately below the storm wave zone) a) The first trend is a correlation between the cycles while reef bioclasts-rich microfacies corresponded to identified on the MS curve and the fourth order sequences. higher energy periods. Water energy however remained Each regressive trend corresponds to a MS peak on the relatively weak (in comparison with microfacies B1/3), as MS evolution curve. On Fig. 14, the detail of some of indicated by good preservation of fossils. these trends at the scale of fourth order sequences is presented and the link between MS and sequence evolution 5. Magnetic susceptibility and platform is obvious. Concerning the sequence 3 (Fig. 14), the first sediments trend is a short transgressive event, with biostromal facies (facies B3) grading to external crinoidal deposits (facies The trends in the MS signature are similar for all described E1). This transgressive phase corresponds to decreasing stratigraphic sections. We will use the Tailfer section as a MS values. The second trend is an aggrading biostrome, reference to describe the relationship between facies mainly built by lamellar stromatoporoids (facies B1) that 494 F. Bo u l v a i n & A.C. Da Si l v a - 7 1 ? 1 1 , 5 . 0 9 - 7 3 5 ( m ³ / k g ) 1 . 0 e v n t e v n t b a s e d - 8 7 e v n t 8 6 B a r s e M S 5 . 1 0 M S M S 0 M S e v n t s M S o s i t v e C o r e l a t i n s o n P C o n s t a N e g a t i v 0 m 4 0 2 0 - 7 1 3 1 1 5 3 . 1 0 1 7 1 - 7 9 1 6 1 3 5 1 2 7 2 . 1 0 / k g ) 1 4 1 0 3 4 S u s c e p t i b l y 8 6 2 - 7 ( m 1 . 0 0 M a g n e t i c a i l f e r T 0 5 0 4 0 2 0 1 0 9 0 8 0 7 0 6 0 m - 7 2 . 1 0 - 7 9 1 ? 1 1 0 ? 1 3 ? 1 , 5 . 0 1 2 ? - 7 5 3 ( m ³ / k g ) 7 4 1 . 0 - 8 6 2 8 1 M S 5 . 1 0 0 y w a i l e A 0 6 0 2 0 4 0 m 1 0 8 0 - 7 1 , 5 . 0 - 7 1 3 ? 8 1 ? ( m ³ / k g ) 9 - 8 1 . 0 5 1 7 6 7 1 2 ? 5 . 1 0 1 0 ? 6 8 5 4 M S 0 V i l e r s

8 0 6 0 4 0 2 0

1 0 t i n u l a n o o g a L t i n u l a m o r t s o i B

Figure 17. MS curves and correlations for 4 sections from the carbonate platform of Belgium. Fi r s t IGCP 580 m e e t i n g f i e l d t r i p g u i d e b o o k 495 corresponds to low (around 0 m³/kg) and almost constant BOULVAIN, F., 2007. Frasnian carbonate mounds from MS values. A regressive trend caps the sequence, with Belgium: sedimentology and palaeoceanography. In high domical stromatoporoids (facies B2) followed by Álvaro, J. Aretz, M. Boulvain, F. Munnecke, A. Vachard laminar limestones (facies I4) from the intertidal zone; D. & Vennin, E. (eds) Palaeozoic Reefs and this regressive trend corresponds to increasing MS values. Bioaccumulations: Climatic and Evolutionary Controls. Within sequence 4, the transgressive phase is almost Geological Society, London, Special Publications, 275: absent and is followed by an important aggradational 125-142. phase (biostromes with laminar stromatoporoids, facies B1) and a regressive phase (high domical stromatoporoids, BOULVAIN, F. BULTYNCK, P. COEN, M. COEN- facies B2 followed by mudstone, facies 7). The AUBERT, M. LACROIX, D. LALOUX, M. CASIER, corresponding MS evolution is almost constant during the J-G. DEJONGHE, L. DUMOULIN, V. GHYSEL, P. aggrading phase and is followed by increasing values GODEFROID, J. HELSEN, S. MOURAVIEFF, N. during the regressive phase. The sequences 5 and 10 (Fig. SARTENAER, P. TOURNEUR, F. & VANGUESTAINE, 14) also show transgressive trends followed by regressive M., 1999. Les Formations du Frasnien de la Belgique. trends with the MS curve respectively decreasing and Memoirs of the Geological Survey of Belgium, 44: 125 increasing. pp. b) The second trend is a subdivision of the curve in BOULVAIN, F. & COEN-AUBERT, M., 2006. A fourth two distinct parts (Fig. 13). The biostromal unit presents level of Frasnian carbonate mounds along the south side very low MS values (mean of 2x10-8 m³/kg). Just above of the Dinant Synclinorium (Belgium). Bulletin de the boundary between the two units, the values are l’Institut royal des Sciences naturelles de Belgique, increasing strongly. The upper portion of the curve is Sciences de la Terre, 76: 31-51. characterized by higher values (to means of 6.62x10-8 m³/kg), and corresponds to the lagoonal unit. BOULVAIN, F. DEMANY B. & COEN-AUBERT, M., 2005. Frasnian carbonate buildups of southern Belgium: c) Fig. 15 presents the relationship between magnetic the Arche and Lion members interpreted as atolls. susceptibility and microfacies. On the horizontal axis, the Geologica Belgica, 8: 69-91. microfacies succession is represented, with increasing proximality from the right to the left. The relationship BOULVAIN, F., DE RIDDER, C., MAMET, B., PRÉAT, between magnetic susceptibility and microfacies is A. & GILLAN, D., 2001. Iron microbial communities in obvious, with increasing MS related to proximality. Belgian Frasnian carbonate mounds. Facies, 44: 47-60. Effectively, the external and biostromal microfacies show MS values around 2x10-8 m³/kg, while the lagoonal facies BOURQUE, P.A. & BOULVAIN, F., 1993. A model for shows values around 6.7x10-8 m³/kg, with in details the origin and petrogenesis of the red stromatactis subtidal facies 5.5x10-8 m³/kg, intertidal facies 8.5x10-8 limestone of Paleozoic carbonate mounds. Journal of m³/kg and supratidal facies 5x10-8 m³/kg. It appears that Sedimentary Petrology, 63: 607-619. paleosols always have lower MS values than intertidal CRICK, R.E., ELLWOOD, B.B., EL HASSANI, A., deposits. This can be explained by the fact that these FEIST, R. & HLADIL, J., 1997. Magnetosusceptibility paleosols correspond mainly to subtidal deposits affected event and cyclostratigraphy (MSEC) of the Eifelian– by pedogenesis. So the MS signal of the subtidal deposits Givetian GSSP and associated boundary sequences in seems to be preserved during emersion and pedogenesis. North Africa and Europe. Episodes, 20: 167–175. MS is related both to microfacies and to third order sequential evolution. DA SILVA, A-C. & BOULVAIN, F., 2002. Sedimentology, magnetic susceptibility and isotopes of a Middle Frasnian d) If we compare the MS values of the biostromal unit carbonate platform: Tailfer section, Belgium. Facies, 46: of the different sections (Fig. 16), the mean values are the 89-102. highest in Barse which is the most proximal section and decrease distally. For the Villers section, the values are a DA SILVA, A-C. & BOULVAIN, F., 2004. From palaeosols little bit higher, maybe because of a lower sedimentation to carbonate mounds: facies and environments of the rate and local condensed intervals. Middle Frasnian platform in Belgium. Geological Correlations are made on basis of magnetic Quarterly, 48: 253-266. susceptibility peaks (events with the same pattern) which DA SILVA, A-C. & BOULVAIN, F., 2006. Upper are considered isochronous (Crick et al., 1997). Third Devonian carbonate platform correlations and sea level order to fourth order correlations are proposed on the variations recorded in magnetic susceptibility. basis of MS peaks (Fig. 17). Palaeogeography, Palaeoclimatology, Palaeoecology, 240: 373-388. 6. References DA SILVA, A.C., MABILLE, C. & BOULVAIN, F., BOULVAIN, F., 2001. Facies architecture and diagenesis 2009a. Influence of sedimentary setting on the use of of Belgian Late Frasnian carbonate mounds (Petit-Mont magnetic susceptibility: examples from the Devonian of Member). Sedimentary Geology, 145: 269-294. Belgium. Sedimentology, 56: 1292-1306. 496 F. Bo u l v a i n & A.C. Da Si l v a

DA SILVA, A.C., POTMA, K., WEISSENBERGER, J.A.W., WHALEN, M.T., MABILLE, C. & BOULVAIN, F., 2009b. Magnetic susceptibility evolution and sedimentary environments on carbonate platform sediments and atolls, comparison of the Frasnian from Belgium and from Alberta. Sedimentary Geology, 214: 3-18. HUMBLET, M. & BOULVAIN, F., 2001. Sedimentology of the Bieumont Member: influence of the Lion Member carbonate mounds (Frasnian, Belgium) on their sedimentary environment. Geologica Belgica, 3: 97-118. LECOMPTE, M., 1959. Certain data on the genesis and ecologic character of Frasnian reefs of the Ardennes (transl. by P.F. Moore). International Geological Review, 1: 1-23. MAILLIEUX E., 1913. Nouvelles observations sur le Frasnien et en particulier sur les paléorécifs de la plaine des Fagnes. Bulletins de la Société belge de Géologie, XXVII: 67-104. TSIEN, H. H., 1975. Introduction to the Devonian reef development in Belgium. 2nd Symposium international sur les Coraux et récifs coralliens fossiles, Paris, livret- guide exc. C: 3-43.

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