Allochthonous salt advance recorded by the adjacent syn-kinematic sedimentation: Example from the Les Avellanes diapir (South Central Pyrenees)

Abstract

This work aims to present the Les Avellanes diapir as a field analog to inquire how the origin, advance, and emplacement of an allochthonous salt body in continental settings influence the local sedimentation in terms of facies distribution, sediment provenance, and stratigraphic relationships. At the frontal part of the South-Central Pyrenean fold-and-thrust belt (Spain), the Les Avellanes diapir is an outcropping salt structure made of Triassic evaporites, lutites and carbonates. At the diapir's western boundary, a structurally controlled sub-basin presents a well-preserved, early Oligocene in age, mixed clastic-evaporitic sedimentary sequence which recorded the lateral extrusion of the diapir and its emplacement as an allochthonous salt sheet. To define the events and processes recorded by the adjacent sedimentary sequences, and to unravel the diapir evolution and the nature of the diapir contact at the study area, we have combined sedimentary, petrologic, and stratigraphic data. Three stratigraphic sections have been built, from which 8 lithostratigraphic facies associations have been described, interpreted, and correlated across the sub-basin. The deformation within the diapir deposit is also described and interpreted together with the sedimentary rocks. A prograding alluvial to colluvial system is associated with the piercing of the salt, which was exposed at the surface towards the NE area of the sub-basin. The dissolution of the salt resulting in the formation of a caprock with stacks of stringers of intrasalt carbonates and dolerites layers. The ongoing uplifting at the NE caused the incision of the local drainage network, marked as a paleo-relief in the stratigraphic sequence, filled by syn-kinematic breccias derived by the erosion, transport, and sedimentation of the caprock. The headward erosion reached the salt underneath the caprock, triggering the lateral extrusion. Thus, salt flowed southwards, favored by the local topography, overriding the syn-kinematic breccia deposit. Foliation and other shear-related deformation structures are observed in a megabreccia made of caprock remnants which overlap the sedimentary, syn-kinematic breccias along the base of the salt sheet deposit. These structures were probably formed during the advance of the salt sheet. The data acquired and interpreted in this work allows for the conceptualization of the relative lateral movement of a salt sheet front as recorded by adjacent syn-kinematic sedimentation in continental settings. Salt supply and erosion rates are compared with topographic slope, sedimentation, and salt dissolution as major controlling parameters of the salt sheet advance. The resulting combinations are expressed by the progradation, aggradation, and retrogradation in terms of proximal over distal facies.