Transtensional tectonism and its effects on the distribution of sandbodies in the Paleogene Baiyun Sag, Pearl River Mouth Basin, China

The Baiyun Sag, situated at the north continental slope of the South China Sea, is a main sub-unit in the Southern Depression Belt of the Pearl River Mouth Basin. In this Sag, the middle Eocene Wenchang and upper Eocene–lower Oligocene Enping Formations had developed in the evolution stage of continental faulted basin. Seismic stratigraphic sequences and fault structures revealed that the Baiyun Sag was short of long-reaching boundary faults, and that it was a rifted basin greatly influenced by basement faults rather than a typical half-graben. Different from the sags in Northern Depression Belt of the Pearl River Mouth Basin which controlled by large-scale NEE-strike faults, the Baiyun Sag had been controlled by two groups of NWW-strike en echelon fault belts with approximate opposite dips, which developed in the southwest and northeast of this Sag respectively and had played the roles of boundary faults. These en echelon faults, together with narrow synclines, partial flower structures and fluid diapirs, indicated the left-lateral transtensional activities, which had resulted in subsidence center departing to main faults and stretching S-shaped. Moreover, the en echelon faults had constructed many composite transfer zones of relay ramps, and controlled the distribution of sandbodies. The en echelon fault belts are located in accordance with Nw-striking Mesozoic basement faults. Hence the left-lateral transtensional activities were responsible for the Western Pacific Plate subducting and strike slip reactivation of the basement faults. Significantly, NW-striking basement faults had forcefully determined the development of not only the Baiyun Sag but also the Xingning Sag.