Facies architecture and diagenesis of Belgian Late Frasnian carbonate mounds

Abstract

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 fr...

Potyphyllum, a new phillipsastreid genus of rugose corals in the Upper Frasnian of Belgium with precisions about the age of the Petit-Mont Member

“…A similar palaeogeographic distribution has been pointed out by Coen-Aubert (2015) Another important conclusion of this work concerns the red lenses of the Petit-Mont Member which occupy a variable position in the Upper Frasnian Champ Broquet Formation of the Dinant Synclinorium. Though their lithofacies and microfacies are rather similar according to the detailed descriptions of diverse outcrops given among others by Boulvain (1993Boulvain ( , 2001), their ages are very different as it can be deduced from the identification of conodonts and massive rugose corals (Figs 3-4 (Roemer, 1852) and F. kaisini (Tsien, 1978). Some of these species are sometimes observed at the top of the Petit-Mont Member in the Philippeville Massif and the Renlies Anticline.…”

Early Middle Frasnian platform reef strata in the Moravian Karst interpreted as recording the atmospheric dust changes: the key to understanding perturbations in the punctata conodont zone

“…In addition, these sedimentary forms are mostly related to time spans with reduced shallow water reef development, ramp morphology and bacterial-sponge invasions on the shelf, and eustatically they signify transgressive system tracts (TST) with recovery from a significant lowstand or catastrophic damage and reshaping of the reef systems. A conspicuous re-emergence of stromatactis was documented mainly in Belgian E-MF (Boulvain 2001, where the cm-dm sized stromatactis were correlated with the lower third of the punctata conodont zone, in the lower part of the Arche mound member. The initiation by the E-MF sea level rise was inferred from sequential architecture of the formations.…”

Magnetic susceptibility evolution and sedimentary environments on carbonate platform sediments and atolls, comparison of the Frasnian from Belgium and Alberta, Canada

“…2) (Boulvain et al, 1999). In the more distal part (SW of Belgium, southern border of the Dinant Synclinorium), the Middle Frasnian succession (Moulin Liénaux and Grand Breux Formations) is characterized by three carbonate mound and atoll levels (Arche, La Boverie and Lion members) separated by argillaceous intervals (Boulvain, 2001) (Fig. 2B).…”