Neogene Shale Tectonics in Offshore Tarakan Basin, North East Borneo (Kalimantan): Insights from 3D Seismic Interpretation 

Abstract

<p>The geometry and evolution of continental margins can be influenced by gravity-driven, thin-skinned deformation above mobile shale. The response of the mobile shale and its overburden to gravitational collapse is complex due to: (i) spatial and temporal variations in the timing and magnitude of extension within prograding deltaic systems driving deformation; and (ii) the behavior of the weak, basal shale layer. This complexity, together with the difficulties in seismically imaging mobile shales and their strongly deformed overburden, mean we have a relatively poor understanding of the distribution and evolution of syn-sedimentary extension in large, supra-shale deltaic systems.</p><p>In this study we use a 3D pre-stack time migration (PSTM) seismic dataset located on the shelf-edge to upper slope of the Offshore Tarakan, North East Borneo. We combine our seismic-stratigraphic analysis and a detailed seismic interpretation with published well data, producing six age-constrained structural and thickness maps that document the Neogene tectonic evolutions of this shale-rich delta system. Our study reveals that the Tarakan delta system, including its underlying basal mobile shales, is deformed by a range of NE-trending shale structures, and NE-SW-striking, basinward- (i.e., eastwards) and counter-regional (i.e., westwards) dipping shale-detached (i.e., basement-decoupled) extensional faults. The extensional faults typically have a listric geometry, merging towards the top of an interval inferred to be within the mobile shales. Lateral throw distributions of each extensional listric faults appear to decrease southwestward. Hangingwall rollover-related deformation is accommodated by planar crestal faults. In relatively distal locations we document broad, shale-cored anticlines. We also observe mud volcanos and diapirs that are located above and along the shale-detached normal faults and shale-cored anticlines, respectively. Isochrone maps document complex thickness patterns through time, reflecting the complex interplay between mobile shale flow, supra-shale extension, and sea-level variations. Taken together, we identify three main tectonic stages: (i) Middle Miocene - fault nucleation, growth, and local linkage in the proximal domain, and formation of a shale-cored anticline more distally; (ii) Upper Miocene-Pliocene – lateral propagation and eventual retreat of the extensional faults, and mud diapirism; and (iii) Pleistocene-Holocene – extensional faults reactivation, decay and death, accompanied by mud volcanism.</p><p>We suggest that the extensional faults in the Tarakan delta system formed in response to Neogene tilting and gliding of supra-shale sequence in response to the uplift of Borneo. Updip extension was accommodated by and kinematically linked to, downdip contraction, and the formation of shale-cored anticlines. We speculate that mud volcanoes and shale diapirs formed above these extensional and contractional structures in response to mobile shale ascending fault- and fold-related fracture. Our careful analysis of the supra-shale faults and underlying shale structures can provide insights into the three-dimensional kinematic evolution of other mobile-shale provinces in deltaic systems, such as those characterizing North West Borneo, the Niger Delta, and the Ceduna shelf margin in Southern Australia.</p>