Phosphatic black pebbles and nodules on a Devonian carbonate shelf (Anti-Atlas, Morocco)

Abstract

In an upper Famennian shelf limestone succession of the eastern Anti-Atlas (Jebel Rheris), black pebbles and nodules occur in certain horizons, which are laterally traceable over several kilometres. Two types of these components can be distinguished: (1) quartz grains containing black pebbles and (2) less common black nodules, containing carbonate bioclasts. XRD studies reveal that type 1 black pebbles consist of quartz, calcite, and apatite (35.4–53.6 wt.%). The average content of total organic carbon is 0.12 wt.%. Phosphorous is an important constituent of type 2 black nodules, though it has not been found in the embedding rocks. Therefore the black colour is the result of phosphatization, which occurred during two different periods: black pebbles consist of sandstone, they must have been derived from Ordovician strata north of the Jebel Rheris, which were phosphatized during that period. Black nodules, in contrast, were formed during the Famennian, because they consist of the same facies as the surrounding rock; moreover a gradual transition from blackened to unstained areas has been observed. It is argued that a late Famennian transgression was responsible for the accumulation of phosphatic black pebbles, they were reworked by the progressing coastal erosion and shed onto the shelf. Thus type 1 black pebbles are indicative of a transgressive systems tract, they occur within retrogradational parasequences. The occurrence of black phosphatic nodules is not related to sea-level changes.