Origine, processus de formation et évolution des mardelles du Nord-Est du Bassin Parisien (France)

Abstract

Thousands of sub-circular closed depressions or “mardelles” are observed on Keuper marls of the North-eastern Paris Basin. Their anthropogenic or geological origin is highly debated by archaeologists and naturalists since the first half of the 19th century. During rescue archaeological excavations along the high speed train line ‘LGV-Est Européenne’ seven closed sub-circular depressions were investigated to resolve the question of their origin, their formation processes and their evolution of their sedimentary filling. To that goal, a new field methodological approach combining geoarchaeology, archaeobotany and archaeology was carried out by the optimization of mechanical excavator. The realisation of long and deep trenches allowed direct observations of both substratum and sedimentary filling. This study evidences the natural origin of closed depressions by the play of the following processes:
 (1) Initiation of closed depressions is realised by ancient fractures affecting the Triassic marls;
 (2) Deepening of closed depressions is realised by alteration of superficial marls and residual clays passage down several wells formed through existing faults;
 (3) When the closed depression exists, the modifications of the shape of the closed depression's edges, is due to degradation of marl blocks;
 (4) The following sedimentary filling traduces environmental conditions influenced by forest evolution and land use changes since Antiquity.
 Direct observations at the scale of closed depression complete spatial information from a Lidar survey along the LGV-Est line. The analysis of this survey shows alignments of closed depressions with orientations similar to those of the drainage system, the origin of which is probably linked to the fracturing of the Keuper marls. Finally, even if a process of natural formation is envisaged, a role of human activity in the initiation of the process could be envisaged in particular by the action on the evolution of the land use (plant cover) and thus in the marls karstogenesis.