Tectonic origin of carbonate depositional sequences in a strike-slip setting (Aptian, northern Iberia)

Abstract

The Lower-Upper Aptian transitional series of the Aulesti area in the northeastern Basque-Cantabrian region (northern Iberia) shows a depositional sequence arrangement conditioned by local synsedimentary folds and faults. Three sequence boundaries (Sb1, Sb2, Sb3), dated approximately between the deshayesi and nutfieldiensis Zones, are characterized by erosional surfaces with both palaeokarst and palaeovalley reliefs up to 150 m deep in the downthrow blocks of normal faults; one sequence boundary (Sb2) is also a clear angular unconformity with fossilized folds and faults created by strike-slip action. Facies analyses of the two depositional sequences (S1 and S2), reveal general deepening of the sea upward through the sedimentary column. Only the sequence boundary Sb3 could be eustatic, according to the Haq et al. (1987) curve. However, all sequence boundaries are primarily attributed to local tectonic pulses: sudden relaxations of transtensional stresses and associated tectonic uplifts occur after phases of transtensional stress accumulation associated with relative sea-level rises. The intraplate stress tectonic mechanism of Cloetingh et al. (1985) is thought to offer the best explanation for the origin of the sequential arrangement seen.