Current and Wind Induced Facies Patterns in a Devonian Atoll: Iberg Reef, Harz Mts., Germany

Abstract

The Middle to Upper Devonian Iberg Reef was an oceanic atoll within the Rhenish Trough of the Variscan Geosyncline. It existed for at least 15 million years before it was drowned shortly before the turn of the Frasnian-Famennian. The spatial distribution of facies and early diagenetic features within these Frasnian reef limestones confirms the reef's paleogeographical location between the equator and 30° south. The southeastern, windward side of the atoll was dominated by massive stromatoporoids and bulbous corals as well as by encrusting and dendroid stromatoporoids. Reefbuilders were rarely preserved in situ. Several generations of early fibrous cement form thick isopachous crusts on the limestone components, confirming early lithification. On the northwestern, leeward side, platy and branching stromatoporoids and corals predominated. They were mostly preserved in growth position. Fine-grained sediment forms a large portion of the limestone matrix. A leeward lagoon entrance was marked by large thickets of branching rugose corals and a mixture of back reef and fore reef faunal elements in the northwesternmost lagoonal limestones. A chain of stromatoporoid gravel cays formed inside the lagoon due to wave-refraction on the southern reef front. INTRODUCTION