BOURQUE, PIERRE-ANDRE, and ACHOUR MADI, Laval University, Quebec, Quebec, Canada; and B. L. MAMET, University of Montreal, Montreal, Quebec, Canada

ABSTRACT: Waulsortian-Type Sponge-Bryozoan Mound and Crinoid Bank Complexes, Bechar Basin, Northwestern Algeria: Architecture, Paleoenvironments and Sea-Level Changes

The 1500 m-thick Upper Visean (V3b-V3c) carbonate succession of the Bechar "reefal" province is made up of three main bioconstruction phases which form distinct parasequence sets corresponding to third order eustatic cycles. Siliciclastic influxes mark the end of each cycle which is composed of superposed shallowing upward parasequences, locally topped by beveled surfaces, testifying to smaller-scale sea-level fluctuations and occasional submarine erosion attributable to eustatic changes and/or tectonic movements. The architecture of individual parasequences follows the same general pattern. An ideal parasequence has a thickness of 70 to 140 m and results from a transgressive-regressive depositional event. The transgressive phase allowed colonization of the drowned sea floor by sponge , rapidly followed by a sponge-fenestrate bryozoan community. During sea-level highstand, sponge-bryozoan mounds, sometimes reaching heights over 100 m, flourished on the outer part of the carbonate shelf apparently devoid of reefs, except for low microbial-rugose coral thickets that grew upslope. During sea-level lowstand, crinoid-algal banks developed on the flanks and the top of the highest sponge-bryozoan mounds, while extensive oolite-coated grain shoals produced sand sheets that progressively buried the mounds. Oolite-coated grain production was inhibited by drowning of the shelf during the next transgression. While the architecture of the Bechar complexes shares some attributes with that of the classical Waulsortian mounds, particularly with respect to facies mosaic and vertical d pth zonation, it also presents significant differences.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.