|Growth and diagenesis of cryptalgal-bryozoan buildups within a Mid-Visean (Dinantian) cyclic sequence, Belgium|
Lauwers, S. (1992). Growth and diagenesis of cryptalgal-bryozoan buildups within a Mid-Visean (Dinantian) cyclic sequence, Belgium. Ann. Soc. géol. Belg. (1968) 115(1): 187-213
In: Annales de la Société Géologique de Belgique (1968). Vaillant-Carmanne: Liège. ISSN 0037-9395, more
Cryptalgal-bryozoan carbonate buildups are reported from the Middle Visean (Upper Dinantian) near Namur, Belgium. They occur in a 20 m-thick, upward-shallowing, carbonate rhythm - the thickest in a succession of such rhythms.The rhythm comprises three phases; from bottom to top: (1) packstones-grainstones with rich and diverse biota, indicating open, shallow marine conditions; grading to (2) wackestones, with a poorer and more restricted biota, passing upwards to cryptalgal boundstones; and(3) mudstones with highly restricted and reduced biota.Small- to medium- sized mounds (2-11 m thick) occur at the top of Phase 2 where they established on coral thickets. The buildups consist of a meshwork of bryozoans encrusted by finely laminated cryptalgal coatings and encrusting bryozoans. Brachiopods, sponges and vermetid gastropods are also present. Voids were filled by internal sediments and early marine cements. The growth of the buildups was probably interrupted by a short-lived sea-level drop, recorded laterally as what seems to be a karstic surface. Vadose internal sedimentation and minor leaching by meteoric water occurred in the ?exposed buildups. Marine conditions were then restored and, at the beginning of Phase 3 deposition, coral thickets (with cryptalgal coatings) colonized the top of the largest buildup. Most remaining pore space was filled by burial cements. Other minor diagenetic processes include neomorphism, dolomitization, precipitation of Ca-sulphates and silicification.