ABSTRACT

The Errigoiti Formation, located in the central domain of the Basque-Cantabrian basin, is an Albian to Santonian bathyal submarine volcanic system of alkali basaltic character. These volcanics occurred in an extensional geodynamic context generated by the drifting of the Iberian plate with respect to the European plate.

Stratigraphic analyses reveal the facies distribution and depositional architecture of a longitudinal cross section of the Errigoiti volcanic system. Four facies groups were distinguished: sheet lavas (SL), derived from low-viscosity lava flows at high magma discharge rate; pillow lavas (PL), derived from intermediate-viscosity lava flows at moderate discharge rate; pillow breccias (PB), derived either from in situ fragmentation of lavas (PB1) or rock-falling from pillow-lava slopes (PB2); and stratified volcaniclastites (VC), derived from volcaniclast resedimentation through gravity flows, either by early remobilization (VC1) or post-volcanic reworking (VC2). The facies groups occur in a predictable repetitive succession, essentially from base to top, an SL-PL-PB-VC stacking pattern. This is interpreted to reflect an upward waning in the magma discharge rate.

Five major volcanic sequences make up the Errigoiti Formation. Each comprises packages tens to hundreds of meters thick, deposited during short time spans of volcanic activity separated by long time intervals of volcanic inactivity. Consequently, boundaries between sequences are associated with marked hiatuses, reworked volcaniclasts, and/or turbidite-pelagic sedimentation.

Analysis of lateral and vertical facies evolution enabled us to propose a depositional model consisting of three depositional environments. From proximal to distal settings, these are: amalgamated pillow volcanoes, scattered pillow volcanoes, and volcanic fringe, basically responding to a proximal to distal PL/PB, PL/VC, and SL/VC evolution of facies associations. Contemporaneous fossiliferous turbidite-pelagic sequences that surround the volcanic succession allow paleobathymetry to be estimated on the basis of biofacies. Sites with the highest rate of volcanic emission correspond to an average depth of 900 m; sites with thinner volcanic pile accumulated at a minimum depth of 1100 m.

The facies sequence established in this work and the processes involved in its formation are comparable with the sequences and processes described in other submarine basic volcanic systems. Consequently, the depositional model for the Errigoiti Formation can perhaps serve as a reference model for this type of volcanic system.