Sedimentary and structural record of the Albian growth of the Bakio salt diapir (the Basque Country, northern Spain)

Abstract

AbstractHowever salt has a viscous rheology, overburden rocks adjacent to salt diapirs have a brittle rheology. Evidence of deformation within the overburden has been described from diapirs worldwide. Gravity‐driven deposits are also present along the flanks of several diapirs. The well‐known example from the La Popa Basin in northern Mexico shows that such deposits may be organized into halokinetic sequences. This leads to several questions: (i) How does diapir growth contribute to overburden deformation? (ii) Are halokinetic sequence models valid for other areas beyond the La Popa Basin. The Bakio diapir and its well‐exposed overburden in Basque Country, Spain provides key elements to address these questions. The Bakio diapir consists of Triassic red clays and gypsum and is flanked by synkinematic middle to upper Albian units that thin towards the diapir. The elongate diapir parallels the Gaztelugatxe normal fault to the NE: both strike NE–SW and probably formed together during the middle Albian, as synkinematic units onlap the fault scarp. The diapir is interpreted as a reactive diapir in response to middle Albian motion on the Gaztelugatxe fault. The rate of salt rise is estimated to be about 500 m Myr−1 during this passive stage. During Late Albian, the diapir evolved passively as the Gaztelugatxe fault became inactive. Synkinematic units thinning towards the diapir, major unconformities, slumps and other gravity‐driven deposits demonstrate that most deformation related to diapir growth occurred at the sea floor. Halokinetic sequences composed of alternating breccias and fine‐grained turbidites recorded cyclic episodes of diapir flank destabilization. This work provides insights into drape fold and halokinetic sequence models and offers a new simple method for estimating rates of diapir growth. This method may be useful for outcrop studies where biostratigraphical data are available and for other passive diapirs worldwide.