Rheomorphic structures in a high-grade ignimbrite: the Nuraxi tuff, Sulcis volcanic district (SW Sardinia, Italy)

The observed pattern of structures of the ignimbrite plateau accords well with a conceptual model in which syn-depositional structures are formed during non-particulate, basal laminar flow due to agglutination of juvenile particles within the lower part of the pyroclastic density current. The flow was controlled by the substantial transfer of momentum from the high-speed, particulate upper part of the current to the top of the non-particulate part of the current and the onset of a complex relationship between welding, flow and sedimentation processes. It is proposed in particular that elongated gas pockets played a major role in guiding the laminar shear within the non-particulate part of the current in flat, horizontal areas, eventually promoting brittle failure of the deforming tuff mass along shear planes.