Formation and preservation of colluvial sedimentary breccias during early extension: processes and facies organization

Abstract

In this study, we focused on the formation and preservation processes of colluvial sedimentary breccias within an extensional context. The breccias studied in this work (from Crete and the Pyrenees) are characterized by poorly sorted polygenic deposits of highly angular carbonate clasts whose size ranges from pebbles to blocks measuring several millimeters to several meters. This study shows that the colluvial sedimentary breccias were formed during extensional tectonics and are spatially associated with large-throw normal fault. They are related to the creation of a substantial topography in the footwall of the normal fault followed by its collapse on the hanging wall, leading to fast sediment accumulation and preservation. These breccias are organized, with pebble- to cobble-sized clasts near the slope of the fault while the mega-clasts, such as boulders and blocks, are preserved away from the slope. Such a clast size organization is indicative of a dry rockfall process and is opposite to that of alluvial fan systems dominated by gravity process involving water. This study also revealed that the breccias from the NE of the Pyrenees, encountered on both flanks of the Bas Agly syncline and previously attributed to the Lower Cretaceous, are related to a first phase of extension during the Upper Jurassic.

Keywords

• Pyrenees
• Sedimentary breccia
• Rockfall
• Rockslide
• Colluvial fan
• Extension
• Crete
• Pyrenees