Integrated numerical modelling of a polyhistory basin, Southern Cantabrian Basin (Palaeozoic, NW-Spain)

Abstract

In order to model highly deformed sedimentary basins, structural balancing must be carried out prior to 2D reverse basin and 2D stratigraphic forward modelling. This study investigates the Southern Cantabrian Basin, which is the Variscan foreland fold-and-thrust belt of NW Spain. Structural balancing offers approximations of predeformational, spatial relationships between measured cross-sections. It provides information about minimal basin shortening and Late Devonian basin geometry. Three different structural domains can be distinguished along the synthetic, 18km long Bernesga Transect. The style of structural deformation defines the amount of shortening calculated for each domain. The Pedrosa domain has the highest values of up to 65%, followed by the northern Bodon (41%) and the southern Alba (25%) domains. Thrusting and folding each cause approximately the same amount of shortening. At basin scale, the amount of total shortening of the deformed basin infill is 54% at minimum, excluding small-scale faulting and folding. 2D numerical reverse basin modelling is used to estimate the basin architecture development and to establish thermo-tectonic subsidence rates as initial numerical input for the subsequent 2D stratigraphic forward modelling. Six major subsidence trends within the total subsidence rates and its components (thermo-tectonic, flexural-induced and compaction-induced) can be distinguished between 560Ma and 313Ma. These trends reflect a complex evolution from a rift stage, to a post-rift stage (passive continental margin) and finally to a foreland basin, governed by the approach of the Variscan Orogen in the Lower Carboniferous. The rift stage is characterised by decreasing thermo-tectonic subsidence rates until the Middle Cambrian. The time that follows (Late Cambrian until Late Ordovician) is marked by periods of tectonic quiescence and activity, until the passive continental margin established in the Silurian. The post-rift stage is represented by two second-order encroachment subcycles with durations of 20Ma and 41Ma respectively. They result from subsidence patterns and basinward shifts of regional onlap. Maximum marine flooding was reached during Early Llandoverian. The Variscan foredeep stage is characterised by a highly subsiding depocenter, which moves from S to N in time (present day coordinates). This movement reflects the propagation of the Variscan Orogen, with velocities ranging between 4 and 12km/Ma. 2D stratigraphic forward modelling visualises the predicted depositional history along the synthetic Bernesga Transect and quantifies the physical factors determining deposition. From the latest Neoproterozoic to the Early Ordovician the Southern Cantabrian Basin is dominated by a considerable flux of siliciclastic sediments (2600 to 4600m^2/ka). Following a substantial drop in siliciclastic input by more than 65% at the end of Silurian, significant carbonate production started in the Early Devonian. The Devonian is marked by alternating periods of considerable siliciclastic flux rates (2500 to 4600m^2/ka) and prolific carbonate production. Three Devonian carbonate factories (Abelgas, Santa Lucia and Portilla formations) display decompacted carbonate production rates from 90 to 780m/Ma, each depending on (i) amount of siliciclastic input, (ii) accommodation space available and (iii) palaeogeographic position of the depositional area. The production and destruction of accommodation space is governed by differential thermo-tectonic and flexural-induced subsidence, along with fluctuating eustatic sea-level and sediment flux. Due to increased siliciclastic influx and diminishing accommodation space, the decompacted carbonate production rates of the Portilla Fm. dropped by 81% in comparison to the Santa Lucia Fm.