Columnar conglomeratic sedimentary structures in the Montsant Formation. Serra de La Llena, Vilanova de Prades, Tarragona, Spain.

Abstract

Some conglomeratic bodies of the Montsant Fm display column-like structures at their basal contacts with a characteristic arrangement that is orthogonal to the stratification plains. The accumulation of gravel-rich materials led to large discharges produced a conglomeratic body, which was covered by lutites because of the sedimentation of mud-rich materials carried by the overflowing waters. Thereafter, the generation in the lutitic levels of a vegetated cover led to several types of vertical bioturbation produced by grass root development. Scattered in the lutitic materials were some isolated traces of tree roots. The recurrent mud-rich flood episodes led to the death of the trees. The illite-rich lutites favored the development of the tree-trunk casts. The destruction and disappearance of the wood through rotting produced vertical casts that were temporarily empty. Later, further discharges carrying large amounts of gravels made up of rounded clasts caused the cylindrical vertical holes to be infilled by these clasts. This, which resulted in the generation of column-like structures, because of intermittent progradation of large alluvial fans in this area during the Oligocene. The aim of the present work is to elucidate the genesis and depositional significance the of column-like conglomeratic structures.