Numerical Modeling as a Means to Enhance Genetic Sedimentary Basin Interpretation : A Case Study of the Southern Cantabrian Basin (NW Spain)

Abstract

The objective of this work was to quantify factors controlling the development of the Southern Cantabrian Basin (NW Spain). A sedimentological and sequence stratigraphic model was established for the syn-orogenic Carboniferous basin fill, and subsequently ­analyzed and calibrated by a combined ­approach of reverse basin and stratigraphic forward modeling. The basin fill comprises Cambrian to Carboniferous deposits that were deformed and displaced along thrust planes by thin-skinned tectonics within the Variscan fold-and-thrust belt. In order to carry out stratigraphic ­modeling, structural balancing was employed to determine initial distances between the recorded stratigraphic columns. Surface data only were available, therefore a family of structures was determined for the Southern Cantabrian Zone to construct admissible cross-sections. Within the thrust sheets, minimum shortening rates range from 19% to 54%, depending on the extent to which individual tectonic units were deformed. Minimum shortening rates range between 44% and 64%, including displacement along the thrust planes. The Carboniferous deposits reflect the deve-lopment from a simple underfilled foreland basin to a segmented and subsequently filled foreland basin system. Within the field area, distal siliciclastic foredeep deposits successively onlap and terminate the Valdeteja carbonate platform that developed in the position of the forebulge. The cratonward margin of the platform and the transition to the basinal back-bulge deposits are well exposed. The margin exhibits excellent stratification patterns that were able to be traced within aerial photographs due to the overly subvertical dip of the sedimentary package. Sequence stratigraphic analysis was based on stratigraphic columns measured during field work, thin section analysis, biostratigraphic data and aerial photographs. It was used to reach a higher time-resolution within the carbonate platform and to correlate the stages of platform development with the progressively ­approaching, predominantly siliciclastic, ­deposits of the San Emiliano Formation. Platform termination proceeded from the west and south to the east by increasing subsidence rates and terrigenous input. The change in ­depositional environment from a shallow-water platform to a mixed carbonate-­siliciclastic basinal deposition reflects the response of the lithosphere to the progressively approaching Variscan Orogen. Reverse basin modeling was applied to two restored, north-south oriented transects and also to a third, west-east oriented transect connec-ting them. Subsidence rates were obtained for the complete Paleozoic basin fill. These data cover the development of the basin from an early Paleozoic extensional to a late Paleozoic compressional setting that resulted in the creation of the Variscan Orogen. Due to the proximal position of the transects, pre-Carboniferous data show uniformly low subsidence rates with a maximum of 80m/Ma. Thermo-tectonic subsidence rates of 740m/Ma at maximum created the necessary space to accommodate the siliciclastics shed ­during the orogenic foredeep stage during the Carboniferous. Subsidence rates within the internal parts of the forebulge depozone reach a maximum of 110m/Ma, whereas ­subsidence in the outer parts of the forebulge reaches 270m/Ma. Spatial and temporal distribution patterns of subsidence values at the time of platform termination point to a complex non-linear setting in front of the Variscan Orogen that could be indicative of an orogenic recess or an oblique collision. Stratigraphic forward modeling highlighted the importance of geometrical constraints as one of the controlling factors for platform progradation into the basin. During aggradation, the carbonate repose angle increased to up to 45°. Carbonate production rates for the platform of the Valdeteja Formation reached a maximum of 260m/Ma. Furthermore, the Carboniferous eustatic sea-level curve was calibrated by the model.