Early post-rift sequence stratigraphy of a Mid-Tertiary rift basin in Taiwan: Insights into a siliciclastic fill-up wedge

In order to scrutinize the development, process and control of an early post-rift siliciclastic fill-up wedge, sequence stratigraphy was applied to the Oligo-Miocene coastal-shelf strata of a mid-Tertiary rift basin in Taiwan. Four sequences at the million-year/3rd-order scale, constrained by microfossil biohorizons and radioactive dates, were identified and correlated regionally. The lower sequence belongs to the fault-bounded Oligocene syn-rift wedge. The middle two comprise the lower Miocene early post-rift wedge, onlapping onto the Mesozoic basement highs. The upper sequence is the lower Miocene late post-rift drape on the continental margin. The middle two sequences record a process of active topographic transformation through successive coastal-shelf progradations. At first, the remnant rift topographic low received drastic progradations from both the inner and outer highs. During the later progradations principally from the inner high, the topographic low was filled up for the successive drape. This wedge formation was characteristically short-lived and high in sedimentation rate, when accommodation space and sediment supply both increased. The initial accommodation space of the remnant rift deep shelf was augmented by rapid thermal subsidence and long-term eustatic sea level rise. The sediment supply, in spite of a decrease in provenance exposure due to onlapping, was promoted by periodic exposures due to 3rd-order eustatic sea level falls, and additionally by a warm and humid climate. These falls are age concordant with the sequence boundaries and thus evidently important for the sequence boundary formations and sediment dispersal.