Onset, growth, decline and decease of the Frasnian reefs and carbonate platform in the Frasnian of Belgium (original) (raw)
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Palaeogeography Palaeoclimatology Palaeoecology, 2012
The Upper Frasnian sequence of the Namur–Dinant Basin in southern Belgium consists of mixed siliclastic–carbonate succession of a ramp setting, where the sequence spans the rhenana–linguiformis conodont zones. Earlier studies investigated the chemostratigraphic variations during the Frasnian-Famennian event, but little has been yet known about the nature of the counterpart variations that immediately preceded that time interval. Despite the scarcity of well-preserved brachiopods, sixty–one calcitic shells were collected mainly from beds of the Neuville and Les Valisettes formations (Lower and Upper rhenana zones), to investigate biochemostratigraphic profiles of oxygen-, carbon-isotope and rare earth element (REE) variations of the time interval immediately before the Frasnian-Famennian boundary. The δ18O and δ13 C values of the well-preserved shells range from − 9.5 to − 5.6 ‰ VPDB (− 7.7 ± 1.1, n = 33) and from − 1.8 to 3.8 ‰ VPDB (1.1 ± 1.7, n = 33), respectively, which are within the documented global values. The C- and O-isotope profiles exhibit parallel shifts, particularly at the top of the Neuville Formation (top of the Lower rhenana Zone), which are associated with a sea-level rise and shrinkage in the brachiopod community. Also, the Th/U (0.9 ± 0.6, n = 16) and Ce/Ce* (2.2 ± 0.5, n = 16) ratios suggest deposition under reducing conditions consistent with sea transgression.
Facies architecture and diagenesis of Belgian Late Frasnian carbonate mounds
Late Frasnian Petit-Mont Member carbonate mounds occur in the southern part of the Dinant Synclinorium and in the Philippeville Anticline (SW Belgium). These mounds are 30 to 80 m thick and 100 to 250 m in diameter. They are embedded in shale, nodular shale and argillaceous limestone. Based on facies mapping of 14 buildups and related off-mound sediments, these mounds typically started from below the photic and storm wave base zones and builtup into shallow water environments. Above an argillaceous limestone substrate, the first carbonate mound facies consists of spiculitic wackestone with stromatactis (PM1), which becomes progressively enriched in crinoids and corals (PM2), then in peloids, stromatoporoids and cyanobacteria (PM3). PM4 consists of algal – coral – peloid wackestone and packstone with green algae and thick algal coatings. A core of algal and microbial bindstone (PM5) sporadically occurs within large mounds. The uppermost part of these mounds may show a recurrence of facies PM2 and PM1. PM1 to PM3 are coloured red by hematite derived from microaerophilic iron bacteria; PM4 and PM5 are grey. The transition from the aphotic to the cyanobacterial photic zone is recorded in the succession PM2 – PM3; the transition from the cyanobacterial to the green algal photic zone is recorded by PM3 – PM5. Storm wave base was reached within PM3 and fair-weather wave base within PM5. This paleobathymetric interpretation suggests a depth of 100 – 150 m during initial establishment of PM1. Three types of mounds can be distinguished on the basis of geometry and facies architecture: (1) ''Les Bulants''-type mounds display a continuous vertical facies succession (PM2 – 3 – 4 – 5) and low relief; (2) although exhibiting the same facies succession as ''Les Bulants'', ''Les Wayons''-type mounds show a distinct relief with steep flanks and bioclastic talus; (3) ''St.-Rémy'' mounds consist exclusively of PM1 and PM2, bioclastic flank deposits are not observed. From (1) to (3), these mound types represent successive deepening down a ramp. Biostratigraphic correlation on a regional scale provides good evidence that relative sea-level changes largely controlled lateral and vertical transitions of facies. Beyond that, hypoxic conditions are indicated by the sponge and iron-bacteria consortium in lower parts of the mounds. This is in agreement with the general assumption of stratified water masses during Late Frasnian, preceding the prominent Lower Kellwasser crisis. Cementation began with a radiaxial synsedimentary cement. A fringe of meteoric phreatic cement, initially nonluminescent, then with a bright orange luminescence, occurs in all mounds. It is contemporaneous with a nonluminescent pervasive cement of grainstones deposited in littoral areas. Differentiation between the (reducing) mounds and the (oxidizing) littoral area resulted from better aquifer circulation in sedimentary bodies close to the recharge area. Late burial cements occlude all the remaining porosity and are contemporaneous with the opening of the Variscan fracture system.
Geological Society, London, Special Publications, 2014
ABSTRACT Recent opening of the La Thure quarry in Western Belgium allowed the collection of new data from a poorly outcropping area of the Belgian Frasnian platform. The studied section covers an interval extending from the falsiovalis to hassi s.l. Zones. Sedimentological analyses allowed the reconstruction of depositional settings in the northwestern part of the Dinant Synclinor-ium after the demise of the extended Givetian carbonate platform. Two depositional models are distinguished: (a) siliciclastic drowned platform during the Early Frasnian; and (b) a fore-reef depositional setting belonging to a rimmed shelf during the Middle Frasnian. Moreover, interpreted depositional settings in the northwestern part of the Dinant Synclinorium allowed to constraint the direction of the main facies belts for the Belgian Frasnian platform. Combination of MS and geo-chemistry demonstrates the inherent-parallel link existing between variation in MS values and proxy for terrestrial input (such as Si and Al). This observation means that, despite the remagnetiza-tion occurring within the Belgian Frasnian lithologies, the main trends in the MS signal from the La Thure section still reflect some syn-sedimentary conditions. The increase in MS and clastic input proxies recorded in distal fore-reef deposit within the punctata Zone are likely to be considered as enhanced by rapid and strong shallowing events recognized in the 'puncata Event' interval. The Frasnian platform in Belgium shows important north–south and east–west facies variations (Boul-vain et al. 1999; Bultynck & Dejonghe 2001). The lasts decade's advances in biostratigraphy (Gouwy & Silva et al. 2011a, b) and magnetic susceptibility (Da Silva et al. 2009; Devleeschouwer et al. 2010) resulted in an improved understanding of the tim-ing and relationship between sea-level changes, water circulation, sediment supply and facies distri-bution of the Belgian Frasnian carbonate platform (synthesis in Da Silva & Boulvain 2012). Nonethe-less, the northwestern part of the platform did not receive as much attention as the southern and north-eastern part. As a consequence some weak points remained, hampering a full understanding of the facies belt's geometry within the Belgian Frasnian basin. Furthermore, the Lower Frasnian and the base of the Middle Frasnian in Belgium are dominated by shaly lithologies, often resulting in very poor out-cropping conditions. Consequently, very few data are available for this crucial period of time, charac-terized by the initiation of the Frasnian carbonate factory subsequent to the drowning and demise of the prominent Givetian rimmed shelf at the Give-tian –Frasnian boundary. This paper proposes a detailed sedimentological study of the outstanding and continuous Frasnian section recorded in the La Thure quarry and provides a sedimentological model covering the entire sequence of facies in the northwestern margin of the Dinant Synclinorium (DS). Lithostratigraphic and biostratigraphic data-sets from the literature have been combined with new sedimentological and geochemical analyses in order to gain a better understanding of the deposi-tional and environmental changes that occur within the Early and Middle Frasnian interval in the northwestern fringe of the Frasnian platform. More-over, geochemistry and magnetic susceptibility mea-surements helped to establish further correlation
Palaeogeography, Palaeoclimatology, Palaeoecology, 1996
The sedimentological evolution of the Frasnian-Famennian transitional strata in South China and southern Belgium has been investigated. A similar trend in the deepening and shallowing of the sedimentation environment occurs in the two palaeogeographically distinct areas. The stratigraphic succession has been subdivided into depositional sequences. A general deepening occurs in the Palmatolepis rhenana conodont zone. During the most rapid rise
The Givetian platform of the Ardennes Massif records several alternations between a siliciclastic-carbonate ramp and a carbonate shelf. Usually these depositional contexts are considered as a major disruption implying a perturbation of many ecological parameters. We established the impact of these variations on the biodiversity structure through the study of the trophic organisation. Thanks to a previous microfacies analysis, 550 levels of the Mont d'Haurs section in Givet and ten associated environments were precisely defined. Seven palaeotrophic levels are recognized from micropalaeontological data, including benthic, planktonic, heterotroph and autotroph organisms. The spatial and temporal distributions of these levels have been analysed through means of multivariate analyses. The statistic results show that the distribution of the palaeotrophic levels during periods characterized by a mixed ramp is not significantly different than during carbonate shelf influences. These environmental modifications do not affect the community-type. These results support recent studies performed on different benthic communities occurring in the Givetian of the Ardennes Massif. Indeed, trilobite and ostracod faunas of this period appear more affected by global environmental changes as the Kačák (uppermost Eifelian) and Taghanic (late middle Givetian) events. Moreover, along the proximal-distal transect on the platform, the reef constitutes the most singular environment. However, contrary with the Frasnian reefs of the Ardennes Massif, which consist of carbonate mud mounds laying on a deep mixed ramp, there is no trilobite community that appears restricted to Givetian reefs. Thus global bioevents during the Middle Devonian have a more important control on the biodiversity than the platform morphology. These results support the view that at wide scales (spatial and temporal), the biodiversity responds more positively to ecological disturbances.