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

Jiahao Wang,Baojun Liu,Donghao Xu,Daqing Tang,Xiong Pang

doi:10.1007/s11001-013-9200-x

Abstract

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.

Highlights

The Baiyun Sag is situated at the northern continental slope of the South China Sea with water-depth from 300 to 4,000 m, and area of approximately 20,000 km2
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 NWWstrike 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
Based on comprehensive interpretation of seismic stratigraphic sequences and faults, this paper indicated that the middle Eocene to early Oligocene Baiyun Sag was formed under the stress field of left-lateral transtension rather than that of simple extension, and that abundant en echelon faults had constructed a lot of transfer zones of relay ramps to remarkably control sandbodies distributions

Summary

Introduction

The Baiyun Sag is situated at the northern continental slope of the South China Sea with water-depth from 300 to 4,000 m, and area of approximately 20,000 km. In the development of Wenchang Formation, the Panyu Low Uplift, Yunkai Low Uplift and Southern Uplift Belt were widely exposed to supply abundant sediments, the faults were strongly active, so these complexes of relay ramps had prominently controlled on sandbodies distribution. During T83–T84, four largescale deltas in the northwestern Baiyun Sag distributed in correspondence with four relay ramp complexes These deltas present seismic facies of sigmoidal progradational reflection, and incised valleys with concave shape and erosional bottom surface indicated the routes of sediment transportation (Fig. 9a). Three large fan deltas and turbidite fans in the southwestern Baiyun Sag, which present seismic facies of sphenoid progradational reflection and mound-shaped reflection respectively (Fig. 9b), distributed in correspondence to three relay ramp complexes

Discussion

Conclusion