Interplay between bathymetry, subsidence, and sedimentation in the configuration of halokinetic sequences at the deepwater Bakio Diapir (Basque-Cantabrian Basin, Pyrenees)

The Bakio Diapir is one of the few exposed deepwater passive diapirs, with both synkinematic carbonate and siliciclastic strata. It is located at the northern margin of the Basque-Cantabrian Basin. This basin developed between the Iberian and Eurasian plates during the latest Jurassic-Cretaceous opening of the Bay of Biscay and was later inverted during the Pyrenean orogeny (Late Cretaceous -Santonian- to middle Miocene) forming the Basque Pyrenees.

 This work evaluates growth strata adjacent to this diapir aiming to discuss the application of halokinetic-sequence concepts, mainly developed in shallow-water to subaerial environments, to deepwater depositional settings. We present a 3D analysis of this outstanding salt structure by integrating detailed geological maps, high-resolution bathymetry, seismic, and well data. The resulting reconstruction enables us to trace its evolution from its formation as a salt wall developed above the overlap of two basement-involved faults until its squeezing during the Pyrenean compression. But more significantly, it allows us to evaluate the factors controlling the configuration of halokinetic sequences in deepwater environments. The main results of our study show that:

 A) The geometry of the halokinetic sequences is defined, regardless of setting, by the thickness of the roof edges. Thus, thick diapir roofs generate wedge HS and tapered CHS, and thin diapir roofs form hook HS and tabular CHS.

B) The thickness of the diapir roof is often controlled by the ratio between salt-rise and local sediment-accumulation rates but also, in carbonate environments, by the water depth of the diapir roof and the environmental conditions that can promote aggradation of a carbonate buildup on top of the diapir. Thus, thick diapir roofs and tapered CHS can form even though the ratio was high in this case due to slow, marly deposition in the minibasins.

 C) The diapir roof thickness is also controlled in shallow-water carbonate settings by the accommodation space available over the top of the diapir, which itself is determined by: a) sea-level fluctuations; and b) the interplay between the uplift of diapir top and the regional/ local tectonic subsidence of the diapir base.

D) High and steep scarps over the edges of diapirs, and thus abundant debrites, are not exclusive to hook HS and tabular CHS. They can be also present in wedge HS and tapered CHS that formed from the aggradation of a thick carbonate buildup on top of a diapir.