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

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