ABSTRACT

An exceptionally well-developed and well-preserved upper Visean succession rich in sponge-bryozoan-crinoid mounds in the foreland Carboniferous Bechar Basin (northwestern Algerian Sahara) is described and discussed. The succession is composed of recurrent facies mosaics forming individual superposed members. An ideal member is made up of two distinct fades assemblages. The lower assemblage forms the actual mounds, and is composed of sponge bafflestone-wackestone at the base, overlain by massive sponge-fenestellid bafflestone-wackestone, and capped by massive crinoid wackestone with bedded flanks of lithoclastic wackestone. In contrast to massiveness of the mounds, the upper facies assemblage is composed of well-bedded crinoid packstone-grainstone and ooid grainstone, with local rugose coral and algal-foraminiferal banks. On the basis of benthic assemblages and nature of substrate, seven bathymetric zones are defined, and indicate that each member of the bioherm-rich formations is a shallowing-upward parasequence controlled by an asymmetrical transgressive-regressive cycle. The local curve of relative sea level for the late Visean shows 13 cycles interpreted as fourth-order eustatic sea-level 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 developed in a rapidly subsiding foreland basin. Lateral distribution of facies on the ramp depended strongly on local tectonic setting, whereas vertical development was controlled by sea-level fluctuation. The deep-water, mud-rich, sponge-bryozoan-crinoid mounds developed in 100-150 m water depths during phases of sea-level highstand, whereas deposition of shallow-water facies occurred on top of the mounds, in less than 70-80 m water depths during regressive phases.

The Bechar mounds share similarities with the classical Lower Carboniferous Waulsortian mounds, but they differ in two aspects: the abundance of large sponges, which is unique, and their vertical zonation, wherein basal facies consist of sponge-dominated assemblages.

The proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbon reservoirs in the Bechar Basin, because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migration, and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles.