Facies patterns and depositional environments of Palaeozoic cephalopod limestones

Abstract

In the eastern Anti-Atlas (Morocco) a platform and basin topography was established during the late Devonian, probably as a result of early Variscan tensional tectonics. Cephalopod limestones were deposited on shallow pelagic platforms, platform slopes and shallow, slowly subsiding basins. On the platform a transition from land areas into nearshore quartzose brachiopod coquinas, crinoidal limestones, condensed cephalopod limestones and finally into nodular limestones is observed. The latter often become disintegrated into incipient debris flows which pass into nodular limestone/marl alternations of a shallow basin. Deeper basins with shale sedimentation lack cephalopod limestones. Similar facies types also occur in the late Devonian of the Montagne Noire (France), Rheinisches Schiefergebirge (West Germany), Moravian Karst (Czechoslovakia), Holy Cross Mountains (Poland) and in the early Carboniferous of the Cantabrian Mountains (Spain). Due to strong late Variscan compressional tectonics and limited outcrops, detailed facies patterns could not be mapped in these regions, but the same facies types as in the eastern Anti-Atlas suggest similar coast/platform, slope and shallow basin topographies. During cephalopod limestone deposition water depth on the platforms was in the order of several tens to about one hundred metres, as is inferred from repeated subaerial exposures and distinctive depositional and faunal/floral features. Water depth in the adjacent shallow basins might have reached several hundreds of metres. Cephalopod limestones represent a typical stage in the evolution of geosynclines, characterized by extremely low sedimentation rates (1–5 m m.y. −1). This stage is preceded by deposition of thick neritic clastics and/or carbonates and is succeeded by deposition of deep-water clastics or flysch.