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

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.

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.

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.