Extensional deformation of a shale‐dominated delta: Tarakan Basin, offshore Indonesia

Abstract

Deformation on shale-rich continental margins is commonly associated with thin-skinned extension above mobile shales. Normal faulting and shale mobilization are widespread on such margins, being associated with and controlled by progradation and gravitational failure of deltaic sedimentary wedges. However, due to uncertainties in seismically imaging mobile shales, our understanding of problems like how base mobile-shale controls deformation, and the shape, size, and distribution of shale structures remain poorly understood. We here use 3D seismic reflection data from the platform region of the Tarakan Basin, offshore eastern Indonesia to investigate the temporal and spatial evolution of thin-skinned deformation of the Neogene sedimentary section.Our detailed seismic interpretation reveals up to 74 km long, concave-and convex-into-the-basin normal faults, dipping both basinward (eastwards) and locally landward (westwards), which detach downwards on a basal mobile shale (Early-Middle Miocene). The base of the mobile shale unit dips gently (< 17 o ) seaward, although older (Eocene-Early Miocene), rift-related normal faults originate local structural highs deforming the base of mobile shales. Our isochore (thickness map) analysis shows that supra-shale normal faulting commenced in the Middle Miocene and was accompanied by the formation of hanging-wall rollover folds and associated crestal grabens, with the subsequent along-and across strike migration of the deformation related to the nucleation, lateral linkage, and reactivation of individual fault systems. Updip growth normal faulting was also accompanied by the downslope flow of mobile shale, accompanied by parallel and perpendicular variations of the differential loading in the delta system, and local contraction and mobile shale-upbuilding, resulting in the growth of large, margin-parallel shale anticlines further downdip. The growth faults and anticlines are locally overlain by up to 5 km tall of mud pipes and volcanoes. We suggest that variations in the rate of sedimentary loading, mobile shale flow, fault growth, and gravitational failure of the delta system above a seaward-dipping, but locally rugose base mobile-shale surface, controlled Neogene deformation in the Tarakan Basin. We also demonstrate how variations in the trend and dip of the base mobile-shale surface influences the position, timing of formation, and evolution of supra-shale normal faults and their associated depocenters along shale-rich, deltaic margins.