WAULSORTIAN-TYPE BIOHERM DEVELOPMENT AND RESPONSE TO SEA-LEVEL FLUCTUATIONS - UPPER VISEAN OF BECHAR BASIN, WESTERN ALGERIA

An exceptionally well-developed and well-preserved upper Visean succes sion rich in sponge-bryozoan-crinoid mounds in the foreland Carbonifer ous Bechar Basin (northwestern Algerian Sahara) is described and discu ssed. The succession is composed of recurrent facies mosaics forming i ndividual superposed members. An ideal member is made up of two distin ct facies assemblages. The lower assemblage forms the actual mounds, a nd is composed of sponge bafflestone-wackestone at the base, overlain by massive sponge-fenestellid bafflestone-wackestone, and capped by ma ssive crinoid wackestone with bedded flanks of lithoclastic wackestone . In contrast to massiveness of the mounds, the upper facies assemblag e is composed of well-bedded crinoid packstone-grainstone and ooid gai nstone, with local rugose coral and algal-foraminiferal banks. On the basis of benthic assemblages and nature of substrate, seven bathymetri c zones are defined, and indicate that each member of the bioherm-rich formations is a shallowing-upward parasequence controlled by an asymm etrical transgressive-regressive cycle. The local curve of relative se a level for the late Visean shows 13 cycles interpreted as fourth-orde r eustatic sealevel changes, each cycle averaging half a million years in duration. Third-order cycles could not be identified. The platform model proposed is a distally steepened ramp, 15-20 km wide, that deve loped in a rapidly subsiding foreland basin. Lateral distribution of f acies on the ramp depended strongly on local tectonic setting, whereas vertical development was controlled by sea-level fluctuation. The dee p-water, mud-rich, sponge-bryozoan-crinoid mounds developed in 100-150 m water depths during phases of sea-level highstand, whereas depositi on of shallow-water facies occurred on top of the mounds, in less than 70-80 m water depths during regressive phases. The Bechar mounds shar e similarities with the classical Lower Carboniferous Waulsortian moun ds, but they differ in two aspects: the abundance of large sponges, wh ich is unique, and their vertical zonation, wherein basal facies consi st of sponge-dominated assemblages. The proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbo n reservoirs in the Bechar Basin, because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migratio n, and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles.