Effect of Diapir Growth on Synkinematic Deepwater Sedimentation: The Bakio Diapir (Basque-Cantabrian Basin, Northern Spain)
The Bakio Diapir (Basque-Cantabrian Basin) is an outcropping salt structure flanked by turbidites. Its structural and sedimentological features make it an interesting exposed analog for deepwater plays in the Gulf of Mexico and other passive margins. The studied dataset includes hundreds of stations with structural data, stratigraphic sections, and a detailed geological map. The data were acquired by conventional field techniques, combined with more innovative ones, such as mapping on photorealistic digital terrain models, analysis of airborne LIDAR data, and multiview 3D reconstruction. Surface data have been complemented by subsurface 2D seismic data. The Bakio Diapir is a NNE-elongated salt structure developed during Lower Cretaceous rifting related to the Bay of Biscay opening and was subsequently reactivated during the Pyrenean contractional deformation (Upper Cretaceous-Miocene). The diapir (comprising Upper Triassic evaporites, mudstones and basic subvolcanic rocks) is flanked by synkinematic deepwater strata showing growth geometries characteristic of tapered composite halokinetic sequences. The deposits are made up of a lower carbonate unit and an upper siliciclastic unit bounded by a sharp contact, a facies change thought to be related to regional basin-scale processes. A comparison of the two exposed flanks of the diapir reveals remarkable differences in terms of dominant facies and trends. In the SE, the carbonate unit is fining-upwards and is dominated by a limestone-breccia facies, whereas in the NW it exhibits an overall coarsening-upwards trend dominated by a fine-grained facies. The siliciclastic unit is coarsening-upwards in the SE and fining-upwards in the NW. It is coarser-grained and more erosional in the NW than in the SE, with erosional channels and abundant debrites in the NW and deposits from unconfined flows and shallow channels in the SE. The observed differences in near-diapir facies on either flank are probably related to differences in salt deformation. Factors that may have contributed include: variable thickness of the diapir roof; the height and width of topographic relief over the two diapir flanks; the dip of the salt-sediment interface (outwardly dipping or flaring); and the amount and rate of deep salt evacuation beneath the two minibasins.