Early post-rift confined turbidite systems in a supra-detachment basin: Implications for the early to middle Miocene basin evolution and hydrocarbon exploration of the Pannonian Basin

Abstract

Understanding the interplay of sedimentation and deformation style is essential to develop an adequate model of the evolutionary history of an extensional sedimentary basin. Due to their volume and temporal extent, late post-rift deposits are often in the focus of research; however, early post-rift deposits are less studied. The aim of the study is to reconstruct the tectono-sedimentary evolution of a syn -rift to early post-rift succession in the hanging wall of a low-angle detachment zone and to recognize the effects of this deformation style on turbidity current pathways and depocenters. The small Őri sub-basin of the western part of the Miocene Pannonian basin is used as a study area where the oldest early post-rift turbidites of ~15–12.8 Ma occurred. These sediments were deposited in a supradetachment setting, where inherited and actively forming structures controlled the sediment pathways and accommodation of both the distally and locally sourced marine turbidites. Seismic data, well logs and core information were integrated to identify and analyze unconformities, structural elements, and turbidite deposits. Syn-rift, early post-rift and late post-rift system tracts were identified. The syn -rift/post-rift unconformity is a result of combined eustatic sea level drop and the change in deformation intensity. The main low-angle detachment fault, orthogonal hanging wall transfer faults and related folds together controlled the complex synformal shape of the basin, which was related to major corrugations of the footwall detachment. Four turbidite phases were distinguished based on upward increasing sand content, areal extent, reservoir quality, and degree of confinement as the turbidity currents reached the basin margins. (1) Small fans deflected by transfer faults developed, followed by (2) an axial turbidite system that reached an anticline and was deflected by it. (3) Lateral confinement by the anticline increased significantly and thick sandstone bodies with abrupt pinch-outs formed, while the influence of smaller faults diminished. (4) The axial turbidite systems covered the basin floor and their interaction with basin margins created continuous sand bodies of confined character. The sand fairways and types of confinement of a supra-detachment basin differ from high-angle normal fault systems. The exhumed metamorphic rocks provided the sediment, and the extension-parallel antiformal and synformal corrugations acted as lateral confinement. The gentle back-tilt of the detachment hanging wall block exerted a frontal confinement for the extensive turbidite system at the distal part of the basin, but the gentle NE-ward dip over the detachment still permitted long run-out distances. • Combination of eustasy and tectonics leads to syn-rift-early post-rift unconformity. • Corrugation fold of a detachment creates an intricate topography for sediment flows. • Deformation activity decreases during deposition of early post-rift turbidites. • Lateral, frontal confinement of turbidity currents controls reservoirs quality. • Differences between sand fairways in high- and low-angle extensional systems.