Sedimentary Characteristics and Tectonic Setting of the Middle Triassic Pingxiang Basin, Guangxi
The middle-Triassic Pingxiang basin,located within the suture of Indochina and South China blocks, records the closure of the Paleo-Tethys and the collision of the Indochina and South China blocks. Based on the detailed analyses of the geological sections, sedimentary facies, and provenance of this basin were systematically studied in this paper, and then the tectonic setting of this basin was also discussed. Our new results indicate that the sediments of the Pingxiang Basin are dominated by deep-water basin facies, turbidite fan facies and delta facies deposits. A series of tectonic rises and deep valleys were developed during Middle-Triassic because of subduction and collision. The deep-water basin, which is composed of dark-grey mudstone with debris flow interlayers, is formed on the steep slope of the tectonic rises. The turbidite fan facies is characterized by flute cast, normal-graded bedding, climbing ripple lamination, convolute lamination, bidirectional cross bedding, lenticular bedding and parallel bedding. The delta facies, characterized by large tabular cross bedding, tidal creek deposits, thick lenticular sandbody and brick red mudstone, is similar to the Jamacia-type fan delta that is developed in a subduction-related tectonic setting, which probably occurs in a gentle slope of the tectonic rises. Sedimentation analysis indicates that the deposits are predominantly produced by debris flow, turbidity current, additionally, and bottom current. Paleocurrent flow suggests material is transported to north and south. Petrography features show that the sandstones, which have low compositional maturity and textural maturity, derived from recycled orogen or collisional orogen. Our result suggests that Pingxiang basin, considered as a synchronal product with arc-type rhyolite (the Beisi Formation), is an intensively modified remnant forearc basin that resulted from the closure of the Paleo-Tethys ocean. The sedimentation started in late-Permian and ended in the late stage of mid-Triassic, implying that the final collision between Indochina and South China blocks occurred in mid-Triassic.