Tectono-sedimentary signature of the second rift phase in multiphase rifts: A case study in the Lufeng Depression (38–33.9 Ma), Pearl River Mouth Basin, south China sea

Highlights

• Tectono-depositional patterns of the second rift phase (RP2) cycle are described in the Lufeng Depression.

• Axially-dominated deltaic systems are facilitated in the rapidly reactivated fault areas in the early stage of the RP2.

• An increase of dip-slope sourced systems is developed in low-displacement faulted areas in the early stage of the RP2.

• Large-sized shallow-water deltaic systems are promoted during the late stage of the RP2.

• A two-stage tectono-sedimentary motif of the RP2 in the multiphase rifts is proposed.

Abstract

Compared to first rift phase (RP1) or single rift basins, the relationship between fault behaviours and depositional architecture during the second rift phase (RP2) remains insufficiently understood in multiphase rifts. This study attempts to provide implications for tectono-sedimentary signatures during the RP2 in multiphase rift evolution, according to an integrated analysis of 3D seismic reflections, wire logs and cores from the Lufeng Depression in the Pearl River Mouth Basin, South China Sea. The four recognised third-order sequences, ESQ1-4, define a two-stage tectono-sedimentary evolution, including an early (ESQ1-2) and a late stage (ESQ3-4) of the RP2 (38–33.8 Ma) in the Lufeng Depression. Fan-deltaic, braided deltaic sandstone-prone and lacustrine mud-prone deposits are interpreted from the basin margin towards to the central areas. During the early stage of the RP2, two relatively steep dipping E-W striking faults in the northern area were very active, undergoing rapid reactivation with a high displacement, while other faults were less active, had low displacements with respect to the extension direction and possibly represent pre-existing fault geometries. Two tectono-depositional signatures are identified during the early stage of the RP2: 1) facilitation of newly captured axially-dominated deltaic systems in the rapidly reactivated fault propagated areas, as a more uniform distribution of connected and elongated depocentres establishes an axial-supply drainage catchment; 2) sustained previous drainages from the end of the RP1 showed an increase of dip-slope sourced systems in low-displacement faulted-controlled areas, i.e. these areas lack fault-controlled topographic re-organisation but show establishment of a hanging dip-slope. The late stage of the RP2 is characterised by a low-accommodation basin as faulting activities totally waned, and thus large-sized shallow-water deltaic systems and long-distance transported sandstones are promoted. This study highlights the distinctive stratigraphic architectures and associated sediment dispersal patterns induced by progressive fault activity during the RP2 cycle. Furthermore, the proposed depositional signatures may be used as a reference when studying less data-rich multiphase rifts elsewhere.

Introduction

Numerous rift basins worldwide are observed to undergo two or more distinct phases of extension, e.g. East African Rift, the East Greenland rift system, the northern North Atlantic, Gulf of Thailand, Dampier Sub-basin in Western Australia, Bohaibay Basin in Eastern China, South China Sea; these basins are referred to as multiphase rift basins (Bell et al., 2014; Whipp et al., 2014; Duffy et al., 2015; Henstra et al., 2017, 2019; Deng et al., 2017a; Jin et al., 2018; Phoosongsee and Morley, 2019; McHarg et al., 2019; Ma et al., 2019). Natural and physical examples reveal distinctive fault behaviours between the first rift phase (RP1) and second rift phase (RP2) in multiphase rift basins (Chattopadhyay and Chakra, 2013; Henstra et al., 2015; Deng et al., 2017b; Ma et al., 2019; Phoosongsee and Morley, 2019). The transition of fault growth and reactivation in separate rift phases would inevitably lead to changes in stratigraphic configuration, facies association, drainage catchments and sediment partitioning pathways. Currently, the relationship between tectonic and depositional response in multiphase rifts has attracted attention from numerous geologists worldwide (Pereira and Alves, 2012; Henstra et al., 2017, 2019; Gawthorpe et al., 2018; Phoosongsee and Morley, 2019).

Over the past decades, numerous generic fault-drainage dispersal models have been proposed for the RP1 or single rift basins (Davies et al., 2000; Gawthorpe and Leeder, 2000; McLeod et al., 2002). Typically, the RP1 or single rift cycle comprises a progressive evolution, including an isolate fault, fault interaction and linkage, through-going faulting during climax stages, followed by a final fault-death stage (Prosser, 1993; Ravnås and Steel, 1998; Gawthorpe and Leeder, 2000; Cowie et al., 2000; McLeod et al., 2002; Morley, 2002). In these proposed genetic models, the transverse-sourced systems are commonly dominant during the fault interaction and linkage stage, whereas the axial-drainage supply is likely facilitated during the end of through-going faulting and increases towards to the fault-death stage (Gawthorpe and Leeder, 2000; McLeod et al., 2002; Lin et al., 2001, 2004; Ge et al., 2018).

Compared to the RP1 or single rift, rift-related fault array activities and basin configurations are rather complex in the RP2. The faults of the RP2 are determined by a range of factors such as the geometry (e.g. orientation and dip) of first-phase faults, the orientation of the first-phase faults relative to the second-phase extension direction (Henza et al., 2010, 2011; Whipp et al., 2014; Duffy et al., 2015), and sometimes lithosphere conditions in a large-scale view (Bell et al., 2014; Claringbould et al., 2017). The second-phase faults are either formed: (1) by selectively reactivating some of the first-phase faults; or (2) as new faults that either propagate from the first-phase faults or develop as isolated faults in previously unruptured areas (Henza et al., 2010, 2011). This complexity of faulting patterns promotes the diversity of stratigraphic and depositional systems during the RP2 in multiphase rift basins.

Few tests of tectono-sedimentary interactions during the RP2 in multiphase rifts have been conducted. Henstra et al. (2017) documented that the immediate localisation of strain and the selective reactivation of a few faults during the RP2 would restrict fault-transverse sediment supply more than axial sediment supply. Until now, our knowledge of fault behaviours and depositional signatures during the RP2 cycle remains insufficient (Teixeira et al., 2018; Henstra et al., 2019). This knowledge gap may be caused by: (1) limited subsurface data, such as low-quality or small 3D/2D seismic surveys; and (2) poorly exposed outcrops and few well-based observations (Pereira and Alves, 2012; Bell et al., 2014; Henstra et al., 2017; Gawthorpe et al., 2018).

The Pearl River Mouth Basin (PRMB) is one of the known petroliferous basins in the South China Sea (SCS; Ru and Pigott, 1986; Gong and Li, 2004). Located inside the PRMB, the Lufeng Depression is a two-phase rift basin, including first rift phase (RP1; 47.8–38 Ma) and second rift phase (RP2; 38–33.9 Ma) phases (Zhou et al., 1995; Shi et al., 2009). The RP2 related succession, the Enping Formation, has generated widespread interest for both hydrocarbon and scientific significance after an oil breakthrough in the Lufeng Depression (Luo et al., 2011; Shi, 2015; Ge et al., 2017). Acquired high-quality 3D seismic reflection, wells and core data in the Lufeng Depression offer us a precise opportunity to probe into a tectonic-depositional evaluation of the RP2. The goals of this article primarily include (1) demonstrating the seismic geomorphologic and sediment routing systems responsible for the distribution of the Eocene Enping Formation; and (2) illustrating the temporal and spatial stratigraphic and depositional variability in response to fault patterns during the RP2.