Facies variation in Waulsortian buildups, Part 1; A model from Belgium

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Abstract
A composite cross‐sectional model of a Mid‐Dinantian Waulsortian buildup and its lateral equivalents is assembled from 10 measured sections. Five of these are in the Furfooz reef southeast of Dinant and the others, well correlated, are in non‐reef facies in the Dinant and Namur synclinoria (ranging up to an original 100 km or more from the area of the buildups).This model provides the framework for a petrographic study (over 700 samples) in which 25 components, mainly allochems, were recorded. The distributions of 15 of the most interesting of these are figured and discussed. Major patterns, extending well beyond the limits of the Waulsortian facies, can be discerned. Four component assemblages (A‐D) are distinguished. Broadly arranged in strati graphic order (A at the bottom; D at the top), their constituents are. Echinoderm (mainly crinoid) debris, fenestellid bryozoans, and ostracods. As A plus sponge spicules (notably the large, hyalosteliid ones) and moravamminids. As B plus plurilocular foraminifera, small gastropods, intraclasts and peloids. As C plus micritization, cryptalgal coatings on grains, and calcareous algae. The simple A assemblage typifies the classic Waulsortian facies–and many of its lateral equivalents. The others have increasing compositional diversity, with B and C occurring in the Waulsortian buildup facies and assemblage D mainly limited to flank deposits.Many components of these assemblages (calcareous algae, micritization, plurilocular foraminifera, etc.) have distribution patterns showing marked diachronismdownslopeon the flank of the buildup. This is interpreted as representing a lowering of critical depth thresholds for individual components due to a major drop in sea level late in Tournaisian times. Different components shift downwards ‘in step’ at slightly different stratigraphic levels and this allows construction of a relative depth scale. Taking the lower limits of the calcareous algae as a reference level (0 m), other lower limits are, approximately: micritization and cryptalgal coating (‐90 m), plurilocular foraminifera (‐120 m), hyalosteliid sponges (–150 m). This relative scale is tentatively converted into a water depth scale using as criteria the distribution of the calcareous algae, the limit of micritization and cryptalgal coating, and evidence indicating that the upper parts of the buildup were in the shallows at the end of Tournaisian times when the buildup had a sea floor relief of about 220 m.Changes in the depositional environments throughout the formation of the buildup are then traced, from initiation of growth in subphotic depths, probably exceeding 300 m, through shallowing phases possibly leading to emergence.Acknowledgements.We gratefully acknowledge: E. Groessens of the Service Géologique de Belgique for much unpublished information on conodonts, particularly for the sections at Pont‐à‐Lesse and Furfooz 2, 5, and 6; R. Conil and M. Laloux for help with various aspects of the micropalaeontology; V. Leclère for sampling and initial study of the Pont‐à‐Lesse section; L. Hance for information on biostratigraphic problems in the Lower Viséan and for useful comments on the manuscript; and M. Hennebert and J. Miller for critically reading the manuscript.