The Basque-Cantabrian basin is located in northern Spain in the westernmost part of the Pyrenees. It is a Mesozoic rift, inverted during the Tertiary. In this basin, a subsiding deep-water depocenter, called the Basque Trough formed during the Early Cretaceous, in response to the opening of the Bay of Biscay. In the Basque-Cantabrian basin, the Triassic salt-bearing red clays are exposed in several diapirs that display discordant contacts with the Mesozoic and the Tertiary successions, suggesting a long-lasting halokinetic growth at regional scale. The synthesis of previously published works, together with the analysis of the geological maps from the Spanish geological survey (IGME) as well as the building of new structural cross-sections, allows reviewing the history of halokinesis in the basin. At least four distinct areas may be defined according to the paleogeographical locations of the diapirs: the northern and southern margins of the Basque Trough, and the southern and eastern areas of the Basque-Cantabrian basin. In the northern margin of the Basque Trough, the Bakio and Gernika diapirs mainly recorded an Aptian-Albian growth history, although older and younger growth cannot be ruled out. These diapirs were growing in relatively deep-water environments and created some paleo-highs where isolated carbonate platforms developped. In the southern margin of the Basque Trough, the Villasana de Mena, Orduña, Murguía diapirs recorded an Early Cretaceous to Late Turonian growth evolution. These diapirs were growing in relatively shallow-water environments at the shelf of the southern margin. In the southern area of the Basque-Cantabrian basin, the Salinas de Rosío and Salinas de Añana diapirs recorded a Cretaceous salt growth in a shallow-marine to continental environment and the Tertiary reactivation during the inversion of the basin. The Salinas de Rosío diapir shows a salt glacier overlying the adjacent Tertiary Villarcayo Syncline that displays a mini-basin shape with a strong thinning of the Tertiary succession toward its margins. In the eastern area of the Basque-Cantabrian basin, five diapirs (Estella, Alloz, Salinas de Oro, Ollo and Anoz) are aligned along the Pamplona fault, that represent a Cretaceous transverse fault bounding the Basque Trough to the east. The Tertiary succession covers the older units masking the possible Cretaceous salt growth evolution. However, strong thinning of the Tertiary succession toward these diapirs together with the lateral facies changes highlights the Tertiary reactivation of these structures during the basin inversion. The compilation of all these data allows creating a geological chart that depicts the evolution of the salt structures through time and in the different areas of the Basque-Cantabrian basin.
Caprock assemblages associated with salt bodies typically consist of a vertically zoned sequence in ascending order: anhydrite directly above the salt body, a transitional gypsum zone, and occasionally a complex zone of limestone and/or dolomite. Caprock forms when the upper part of a rising diapir is exposed to a crossflow of NaCl-undersaturated water, causing halite to dissolve and the less soluble components, largely anhydrite (CaSO4) and to a lesser extent gypsum (CaSO4•2H2O), to accrete via underplating to the base of the previously formed caprock. If hydrocarbons are present, the CaSO4 minerals are replaced by carbonate minerals in a process mediated by sulfate-reducing bacteria. Previous descriptions of carbonate caprock recognize only two general fabric types: an upper variegated limestone and a lower banded zone that comprises carbonate and sulfate lithologies. Utilizing new facies mapping and petrographic analysis of outcropping caprock from three different salt basins, Paradox Basin and Gulf Coast Region, USA and Flinders Ranges, South Australia, we recognize a wider variety of fabrics and mineralogies. This variety is owed to the location of caprock at the salt-sediment interface, where it is highly prone to substantial chemical and mechanical alteration, resulting in diagenetic textural overprints of precursor caprock fabrics. We propose a new classification based on fabric types in order to facilitate a discussion and interpretation of caprock lithologies in an organized and effective manner. The development of a comprehensive classification is the first step toward deciphering the complex diagenetic processes involved in caprock formation. Understanding the genetic history of caprock fabrics will allow for better identification and prediction of the distribution of caprock mineralogies and fabrics. Our proposed new classification scheme is based on the recognition of four distinct megascopic fabrics: 1) Massive: homogeneous, with micro-to-coarsely crystalline subdivisions; 2) Layered: subdivisions based on thickness of laminae include micro-laminated, laminated, and banded; 3) Brecciated: subdivided into mosaic and disorganized, dictated by clast orientation and volume variations in inter-clast matrix or cement; and 4) Porphyritic: comprising two distinct crystal sizes. These fabrics are not restricted to any one mineralogy (i.e. are found within anhydrite, gypsum and carbonate caprock) and commonly comprise more than one fabric type (e.g. the brecciated clasts of a carbonate caprock can display a layered or massive fabric). To address this issue, the dominant mineralogy and subordinate fabric types is attached to the overall fabric name as prefix-type modifiers (e.g. massive brecciated dolomite caprock). Silicification, carbonate and silica pseudomorphs after gypsum and anhydrite, localized isoclinal folds, secondary dissolution and recrystallization porosity, as well as dead oil are common features found within all fabric types.