DEPTH-RELATED ECOLOGICAL ZONATION OF A CARBONIFEROUS CARBONATE RAMP -UPPER VISEAN OF BECHAR BASIN, WESTERN ALGERIA

Following the demise of the stromatoporoid-coral reef community in Lat e Frasnian time, Lower Carboniferous carbonate shelf profiles possesse d a ramp geometry, with major organic buildups represented by mud-rich mounds. Microfacies petrography of the exceptionally well-preserved U pper Visean (Lower Carboniferous) carbonate ramp of the Bechar Basin, Algerian Sahara, may well contribute significantly to our understandin g of the paleoecological zonation of Carboniferous non-rimmed platform s, and of the still enigmatic mounds commonly referred to as Waulsorti an banks or mounds. Facies are grouped into two broad groups: (a) a mo und facies group which comprises sponge wackestone-bafflestone, sponge -fenestellid bafflestone-wackestone, crinoid wackestone-packstone, and bedded flanks of intraclastic wackestone-packstone, all four facies c omposing the actual mud-rich mounds, and (b) a supramound facies group composed of coral-microbial framestone, crinoid packstone-grainstone, algal-foraminiferal grainstone and oolite grainstone. Calcareous alga e are important bathymetric indicators and are used to delineate three bathymetric zones based on light penetration: the aphotic zone, which contains no calcareous algae; the dysphotic zone, where there is litt le ambient light, and which is characterized by the presence of red al gae (Fasciella, Ungdarella, Stacheia, Epistacheoides) and absence of g reen algae; and the euphotic zone, which receives the full spectrum of sunlight, and is characterized by the occurrence of both green algae (Koninckopora, Kamaenella, Kamaena, Palaeoberesella, Calcisphaera, Ant hracoporellopsis, Issinella, Exvotarisella) and red algae. Integration of algal zonation, distribution of the other biota, and recurrence of distinct assemblages, enable recognition of seven depth-related benth ic assemblages. Together with the physical properties of the facies, t he benthic assemblages were used to define seven bathymetric zones, fr om upper to lower ramp: (1) algal assemblage (upper ramp); (2) crinoid -ramose bryozoan assemblage (mid-ramp); and (3) productid brachiopod a ssemblage, (4) colonial rugose coral-microbial encruster assemblage, ( 5) crinoid-fenestellid assemblage, (6) sponge-fenestellid, and (7) spo nge assemblage (lower ramp). The vertical zonation of the mud-rich mou nds and associated facies differ from that reported from the classical Upper Tournaisian-Lower Visean Waulsortian mound-bearing successions.