Tectonic controls on eocene deltaic architecture, jaca basin, Spanish pyrenees

Abstract

The Jaca Basin lies to the south of the Pyrenean mountain chain, in Spain, and was formed by the Late Cretaceous and Tertiary convergence between the Iberian and European tectonic plates. During the Bartonian (Middle Eocene), sediment flux from the uplifting Pyrenees was deposited in this basin as the Belsué-Atarés Fm. deltaic system. At the same time, southward propagation of deformation from the orogen created a number of emergent thrusts and thrust-related anticlines along the margins of the basin and within the basin itself. The effect that the growth of these kilometre-scale structures had on the coeval marine depositional systems is the focus of this work. Although the effects that uplifting intrabasinal structures have on fluvial systems and the effects that basin margin structures have on marine systems are well covered in the literature, the influence of intrabasinal compressive structures on coeval marine sedimentation has been largely neglected. By undertaking detailed fades, palaeocurrent and compositional analysis of the Belsué-Atarés Fm. deltaics across the Jaca Basin, it has been found that local tectonics had the strongest control on the marine sedimentation. The structurally defined basin margins largely acted as barriers to external depositional systems, causing large parts of the basin to be dominated by marl deposition. However, a total of four structurally controlled low points through the northern and southern basin margins allowed the entry of large volumes of Pyrenean axial zone sediments, beginning at 41.5 Ma. These were composed of silts, sands and pebbles, and formed the axial deltaic system. Once in the basin, a total of ten, kilometre-scale, growing thrust-related anticlines acted as barriers to the progradation of the axial system, causing facies associations to vertically aggrade behind each structure. At 37.5 Ma, after 4 Myr of vertical aggradation, a basin-wide fall in relative sea-level allowed the facies associations to rapidly prograde, breaching the crests of each of the barrier anticlines. The principal controls on the distribution of facies associations through time (sequence development) in the Jaca Basin were therefore local tectonic ones, with relative sea-level being secondary. This finding calls into question the work of the few existing studies into marine intrabasinal growth structures, which tended to use passive margin sequence stratigraphic concepts i.e. assume that relative sea-level was the primary control on sequences. The development of new techniques, such as numerical modelling, is needed before these types of complex geological situations can be fully understood. The results of this work will be of great relevance to basin dynamics and fold kinematics studies, and for hydrocarbon exploration in thrust-top basin settings.