The Subsurface structure and stratigraphic architecture of ... · The Subsurface structure and stratigraphic architecture of rift-related units ... This contribution reports the basin

Accepted Manuscript

The Subsurface structure and stratigraphic architecture of rift-related units in

the Lishu Depression of the Songliao Basin, China

Hongyu Wang, Tailiang Fan, Yue Wu

PII: S1367-9120(14)00552-5

DOI: http://dx.doi.org/10.1016/j.jseaes.2014.11.026

Reference: JAES 2187

To appear in: Journal of Asian Earth Sciences

Received Date: 12 July 2014

Revised Date: 6 November 2014

Accepted Date: 14 November 2014

Please cite this article as: Wang, H., Fan, T., Wu, Y., The Subsurface structure and stratigraphic architecture of rift-

related units in the Lishu Depression of the Songliao Basin, China, Journal of Asian Earth Sciences (2014), doi:

http://dx.doi.org/10.1016/j.jseaes.2014.11.026

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The Subsurface structure and stratigraphic architecture of rift-related units

in the Lishu Depression of the Songliao Basin, China

Hongyu Wang 1,2 Tailiang Fan 1 Yue Wu1

1. China University of Geosciences, Beijing, 100083;

2.National Experimental Teaching Center of Geological Resources Exploration, Beijing,

100083;

Wang Hongyu, E-mail: [email protected], T:+861082323082

Fan Tailiang, E-mail: [email protected], T: +861082321559

Yue Wu, E-mail: [email protected]

Abstract: This contribution reports the basin configuration feature, stratigraphy and

sedimentary architecture of the Lishu Depression in the Songliao Basin, China. The

activity rate, distribution and style of local faulting demonstrate the timing and extent of

regional rifting. Distinct episodes of compressional tectonic activity caused uplift and

exposure of strata evident as the traditional syn- and post-rift stages of basin evolution.

These episodes led to the sequential denudation of the Upper Jurassic Huoshiling

Formation, Lower Cretaceous Yingcheng and Denglouku Formations, and corresponding

regional unconformities. Acting in tandem with regional compression, activity along the

major boundary faults influenced the evolving basin configuration, as well as seismic

sequences and sedimentary patterns. Seismic, well log and drill core data described here

show subdivision sections of the Lishu Depression strata according to discrete phases of

the traditional syn-rift stage of deposition. We refer to these sub-stages as the initial rifting,

the intensive rifting and the recession phases. The basin configuration shifted from a

graben / half-graben configuration during the initial rifting phase, to a dustpan-shaped

half-graben pattern during the subsequent phase of intensive rifting, and finally into a

gentle sedimentary basin during the final recession phase. The early seismic sequence

divides into a lowstand systems tract (LST), transgressive systems tract (TST) and

highstand systems tract (HST). Evidence of the LST within the seismic sequence becomes

less apparent with the intensive rifting phase, while the HST occupied an increasing

proportion of the section. The shallow water depositional fill formed during the final

recession phase consists only of TST and HST components. Depositional environment

then shifts from alluvial fan and shallow lacustrine systems to fan delta, braided delta –

lake, and finally to a braided fluvial setting. The vertical stacking pattern shifts from

retrogradational, to progradational, to aggradational. Identification of sub-structural units

and interpretation of their genetic relationships helps clarify basin evolution, and thus

serves larger-scale continental basin analysis.

Keywords: Songliao basin, basin configuration, rift-related unit, stratigraphic architecture,

the systems tract, sediment stacking pattern.

Introduction

Research concerning the tectonostratigraphic evolution of rift basins typically divides

basin development processes into pre-, syn- and post-rift stages (e.g. Bosworth, 1985;

Cohen,1990; Bois, 1993; Nøttvedt et al., 1995; Cloetingh et al., 1997; Gawthorpe &

Leeder, 2000; Martins-Neto, 2000; Arabi et al., 2003; Alves et al., 2003; Garcia et al.,

2008; Baudon et al., 2009). A single model is typically used to describe the unique

tectonic and sedimentary conditions operating during a given stage, even though each

stage may consist of several phases of activity. Previous studies addressing the

stratigraphic and sedimentary architecture of rift basins have focused on lithofacies

properties and sedimentary facies stacking patterns (Rosendahl, 1987; Lambiase, 1990;

Frostick & Reid, 1990; Papatheodorou & Fdrenttnos, 1993; Nøttvedt et al., 1995; Ravnås

et al., 1997; Leeder, 2011) as well as the effects of tectonism, sea-level change and

sediment supply rate on sedimentary architecture (Prosser, 1993; Howell & Flint, 1994;

Gawthorpe et al., 1997; Ter Voorde et al., 1997; Ravnås & Steel, 1998; Gupta et al.,

1999; Gawthorpe & Leeder, 2000; Yong et al., 2002; Gawthorpe et al., 2003). Few

studies have addressed stratal geometry and sedimentary sequence stacking pattern that

differentiate each phase of basin rifting. Systematic description of relationships between

basin configuration, sequence stratigraphy and other sedimentary characteristics suggest

that classic rift stages can be further subdivided into discrete phases according to unique

sedimentary conditions operating during these phases.

This paper describes the subsurface structure and stratigraphic architecture of strata

formed during rifting of the Lishu Depression in the Songliao Basin (Fig.1). The study

used 3-D seismic imaging and data from numerous wells in the study area. We interpret

fault activity, basin configuration, seismic stratigraphy and sedimentary features of

traditional syn- and post-rift stages, focusing particularly on unique phases of the syn-rift

stage. In Section 3, we describe major faults to introduce the overall basin structure.

Using this structural background and other information from seismic images, Section 4

interprets the basin’s syn- and post-rift evolution. Section 5 describes detailed

characteristics of the three discrete phases of the syn-rift stage, referred to as the rift

initiation phase, the intensive rifting phase and the rift recession phase. This study

provides a critical and high-resolution tectonostratigraphic perspective on the evolution of

a continental rift basin.

1 Geologic setting

Located in northeast China, the Songliao Basin is a relatively large continental

sedimentary basin that formed from Late Jurassic to Neogene time. The basin is divided

into six structural units: the northern plunge, the central downwarp, the northeastern uplift,

the southeastern uplift, the southwestern uplift and the western slope (Yu et al., 2001; Ren

et al., 2002; Ge et al., 2010; Feng et al., 2010) (Fig. 1). The Lishu Depression, located in

the area referred to as the southeastern uplift, abuts the Yang uplift to the north, the

Shenyang uplift to the west, and Gongzhuling uplift to the southeast. The Lishu

Depression includes an independent hydrocarbon generation center covering an area of

approximately 2350 km2. According to its present structural layout, the depression divides

into five secondary structural areas: the Sangshutai sag, the Central uplift, the Qikeshu

syncline, the Northern slope and the Southeastern slope (Fig. 1) (Yu et al., 2000).

The Songliao Basin experienced three major tectonic episodes that differentiated its

sediments into rift, depression and final structural inversion sequences (Fig. 2). Sediments

in the Lishu Depression reflect only rift and subsidence-related deposition (Figs. 2, 3), as

the depression does not include overlying structural inversion sequences associated with

the larger Songliao Basin (Song, 1997; Yu et al., 2000; Zhao, 2008). The rift-related

sequence in Lishu Depression, described in detail below, consists of the Upper Jurassic

Huoshiling Formation (J2h) overlain by the Lower Cretaceous Shahezi (K1sh), Yingcheng

(K1y) and Denglouku Formations (K1d). The depression-related sequence in Lishu

Depression consists of the Lower Cretaceous Quantou Formation (K1q) overlain by the

Upper Cretaceous Qingshankou (K2qn), Yaojia (K2y) and Nenjiang Formations (K2n).

The Sifangtai (K2s) and Mingshui Formations (K2m) of the Upper Cretaceous, Paleogene

and Neogene are missing in Lishu Depression.

The Lishu Depression shows the distributional and structural patterns of a typical rift

basin. The rift strata are distinguished by faulting in the western part of the depression and

thinning to the east (described below) (Fig.3). The overlap thinning extends

southeastward from the Sangshutai sag. Most rift strata near the Yang uplift have been

exposed and eroded (Yu, et al., 2000; Huang & Shen, 2007; Zhang & Ren, 2008; Zhao,

2008). Overlying post-rift (subsidence) strata are extensively distributed throughout the

Lishu Depression (Fig.3). Their lateral distribution exceeds that of the earlier rift-related

units and stratigraphically connects to coeval strata in the Songliao Basin. The lateral

thickness of the strata does not vary significantly in central parts of either the Changling

or Lishu Depression (Fig.3).

2 Data

Over the last 40 years, 2-D and 3-D seismic surveys have been specifically conducted to

investigate the deep structure and stratigraphy of the Lishu Depression. The dataset now

includes over 6000 km of 2-D seismic lines covering the entire Lishu Depression with a

2.0×4.0 km cell grid. The 3-D seismic data coverage area has reached 1500 km2 with an

inline and cross-line spacing of 25 m, covering most of the region (Fig. 1). By 2011,

around 190 exploratory wells had been drilled throughout the depression. Most of these

wells yielded gamma-ray, sonic, spontaneous potential, resistivity and density log data.

Cores or sidewall materials have been recovered from about 50 wells within rift-related

strata.

In addition to the 3-D seismic data covering 1500 km2 of the study area, this study

also refers to data from 2000 km of 2-D seismic lines (grid in Fig. 1) as well as well log

and drill core data. Calibrated sonic and density logs helped verify sequences interpreted

from seismic data. Stratigraphic features were interpreted to correlate with the

lithostratigraphic framework established for the Songliao Basin by Yu et al. (2000), Ren

(2002), Ge et al. (2010) and Feng et al. (2010). Gamma ray and spontaneous potential

logs and core material provided specific constraints on lithofacies and seismic facies.

3 Regional fault features

Regional seismic profiles show that the Sangshutai Fault is a major structural feature that

binds the Lishu Depression along its southwestern margin (Fig. 1) and extends to depth,

affecting the lowermost units of the section (Figs. 3, 4). This normal fault affects

Mesozoic basement units in the deepest part of the basin and extends upwards into the

Cretaceous Qingshankou Formation. The distribution of fault indicates that offset began

in the latest Jurassic and continued into the early Albian (Fig.5-A). Fault activity peaked

during the Barremian.

Stratigraphic deformation and contrasts in contacts on opposing sides of the fault

indicate that regional structures such as the Pijia, Xiaokuan, Qinjiatun, Qindong and

Jinshan Faults developed during discrete periods of tectonic activity, in spite of their

aerial extent (Fig.1). The Pijia, Xiaokuan and Qinjiatun Faults exhibit evidence of

left-lateral strike-slip movement (Yu, et al., 2000; Zhang & Ren, 2008). These structures

formed at the end of the Barremian and were reactivated at the end of the Aptian . The

Qindong and Jing-gang Faults only affect the Upper Jurassic Huoshiling Formation

(Fig.5-B, C). These small structures thus exerted only limited influence on sediment

distribution and stratigraphy of the basin.

4 Basin evolution

We interpret Lishu Depression structure and sedimentary features in terms of Late Jurassic

to Neogene rifting, post-rifting thermal subsidence and tectonic inversion phases that

affected the larger Songliao Basin (Yu et al., 2000; Zhao, 2008; Ge et al., 2010; Feng et al.,

2010; Wei et al., 2010). The entire rifting –thermal subsidence – uplifting process began at

the end of the Jurassic and occurred episodically up to the end of Barremian, Aptian,

Santonian and Maastrichtian.

Fault development features, regional unconformities, deposition rates and lithofacies

characteristics of Lishu Depression sediments indicate that rifting events consisted of

three sub-stages: a rift initiation phase, an intensive rifting phase and a final recession

phase. Below we describe each of the major stages, along with the three sub-stages

(phases) of rifting in terms of regional tectonics and their influence on basin configuration

and stratigraphic architecture.

4.1 Rifting stage

4.1.1 Initial-rifting phase

During the Late Jurassic, extensional tectonism thinned the crust of northeastern China

along a series of north-northeast-striking normal faults that developed in the Songliao

Basin (Watson et al., 1987; Yu et al., 2001; Ren et al., 2002; Johnson, 2004; Stepashko,

2006; Ge et al., 2010). Large-scale faults in the study area include the Sangshutai,

Jin-gang and Qindong Faults. These high angle faults formed numerous NNE striking

graben and half-graben structures (Figs.6, 7, 8-I, 9). Each graben includes sedimentary fill

consisting of interbedded clastic and volcanic rocks assigned to the Huoshiling Formation

(Fig.4). Deposition rates reached 250 m/Ma within the Sangshutai half-graben (Fig.5-A),

and exceeded 300 m/Ma in the Jin-Gang graben (Fig.5-C).

In the late Tithonian , regional uplift and tilting caused denudation of the Houshiling

Formation in most regions. A regional unconformity (T42 interface) formed between

Upper Jurassic and Lower Cretaceous units. On the seismic profiles of most regions, the

T42 seismic reflector occurs as an angular unconformity with onlap above the interface

(Fig.6).

4.1.2 Intensive-rifting phase

During the Early Cretaceous, regional about E-W extensional stress influenced the

Songliao Basin (Li et al., 1987; Stepashko, 2006; Feng et al., 2010). Continued

displacement along the Sangshutai Fault caused widening of the Sangshutai half-graben.

The cessation of activity along the Qindong and Jin-gang Faults also arrested the

development of small grabens formed during the initial rifting phase (Fig.8-II).

Extensional faulting exerted primary control on basin morphology and topography. From

Berriasian to Barremian time, the Lishu Depression was a wedge-shaped, half-graben

bound by the Sangshutai Fault along its western side, with overlapping layers extending

to the east (Figs.8-II, 9). Basin fill consisted of fan delta, braided delta, lacustrine and

fluvial deposits (Zhao, 2008; Yang et al., 2013). The Sangshutai Fault activity and rates of

deposition reached their maximum values during the Barremian (Fig.5-D). The Lishu

Depression also reached maximal rates of extension during this intensive rifting phase.

4.1.3 Recession rifting phase

At the end of the Barremian, northeastern China experienced a transpressional tectonic

movement (Yu et al., 2000; Ren, et al., 2002; Ge et al., 2010). The tectonic movement

uplifted strata, caused the denudation of the Yingcheng Formation and formed the

regional T4 disconformity (Fig.6). The tectonic event also formed several small-scale

strike-slip faults including the Xiaokuan, Qinjiatun and Pijia Faults (Figs.1, 4).

During the subsequent Aptian, extension along the Sangshutai Fault waned and no

longer exerted a strong influence on basin configuration (Figs.5-A, 9). Basin relief

diminished while the lateral extent of deposition broadened and the horizontal thickness

of units became more uniform across the basin. Rifting features within the basin were

buried. Sedimentary systems in most areas of the Lishu Depression reflect fluvial and

delta plain environments during this phase.

By the end of the Aptian , left-lateral transpressional strike-slip structures had formed

throughout the Songliao Basin (Li and Li, 1999; Huang and Shen, 2007; Gan et al., 2011).

This tectonic episode uplifted strata in the study area and contributed to the formation of a

regional unconformity, the T3 interface, between the Denglouku and Quantou Formations

(Yu et al, 2000; Ren et al., 2002). Stress fields reactivated the Pijia, Xiaokuan and

Qinjiatun Faults which became regional strike-slip systems with transpressional features

along their margins (Figs.1, 8-III).

4.2 Post-rift subsidence stage

From Albian to Santonian, the Songliao Basin entered the post-rift, thermal subsidence

stage. During this period, rifting ceased while deposition extended over a greater area and

occurred with more uniform thickness (Fig.8-IV). Smaller depressions developed into a

larger basin which filled with fluvial, deltaic and lacustrine sediments. At the end of the

Santonian, regional tectonism uplifted the Lishu Depression, precluding deposition of the

Sifangtai and Mingshui Formations.

4.3 Structural inversion stage

During the late Maastrichtian, the Songliao Basin entered the structural inversion stage

(Song, 1997; Yu et al., 2001; Fang et al., 2003; Stepashko, 2006; Ge et al., 2010) which

uplifted and folded the entire Lishu Depression section (Fig.3). As a consequence,

deposition did not occur in the Lishu Depression during the Paleogene and Neogene.

Instead, strata were severely folded into a large-scale, NNE-trending faulted-anticline.

Older, deeper faults were reactivated and connected with younger structures that cut

younger units in the depression. Older structures were also further folded (Figs.3, 4, 8-V).

The current structural framework of the Lishu Depression was thus finalized during the

late Maastrichtian (Yu et al., 2001).

5 Stratigraphic architecture of the rift sub-stratigraphic unit

The Lishu Depression consists of two megasequences (Fig.2). This paper focuses on the

rift-related megasequence formed from Kimmeridgian to Aptian time. Using seismic

stratigraphy and classic sequence division methods (Mitchum et al., 1977; Vail et al.,

1977; Vail, 1987; Vail, 1991), we identified five seismic sequences within this

tectonostratigraphic unit (Fig.10). The stratigraphic and sedimentary architecture show

changes in the seismic sequences that correspond to different rift phases (Fig.11).

5.1 Rift-initiation stratigraphic unit (SQ1)

The Huoshiling Formation records the initial rifting phase. This unit occurs at the base of

early, small-scale, NNE-trending graben or half-graben structures (Figs.7, 8-I, 9) that

range in thickness from 0 – 2500 m.

Seismic facies, well logs and drill core data

The bottom interface of SQ1 is a non-conformable contact with the Paleozoic

metamorphic basement (Figs.10, 12). The T5 seismic reflector from this surface

correspondingly shows high amplitudes and medium continuity. The T42 seismic

reflection interface forms the SQ1 top surface. In most regions of the depression, T42

shows features typical of an angular unconformity, including truncation below the

interface and overlap above the interface (Figs.6, 10, 12-A). Deposition of this sequence

is confined to grabens and half-grabens and shows apparent onlap and bi-directional onlap.

Seismic facies within the strata divide into two sets. Some wave groups show high

amplitude, medium to low continuity and chaotic to sub-parallel seismic reflection

patterns. Other wave groups show medium to high amplitude, low frequency, variable

continuity, and parallel to divergent seismic reflection patterns (Figs.6, 12-B).

Drill cores from this sequence exhibit interlayered volcanic material, coarse-grained

clastic rock and mudstone (Fig.13). Volcanic material is also evident from high amplitude

responses in Resistivity Deep-lateral (RLLD) and irregular Gamma-Ray (GR) well log

data. Most of the coarse-grained clastic rock layers show high amplitude responses with

box or bell motifs on GR and RLLD log curves.

Comparison of well log, drill core and seismic data demonstrates consistent

relationships between lithofacies and seismic facies. Areas of the section that include

volcanic material usually show high amplitude, medium to low continuity and chaotic to

sub-parallel seismic reflection configuration. Seismic data for areas of the section that

consist primarily of sedimentary rocks show medium amplitude, medium to good

continuity and sub-parallel to slightly divergent seismic reflection patterns.

Sedimentary facies and sequence stratigraphic characteristics

Both seismic and drill core data indicate a mixed sedimentary system that includes

alluvial fan, subaqueous fan, fan delta, lacustrine and volcanic depositional environments

(Fig12-C). Multilayered volcanic deposition occurred throughout the graben and a series

of fan-shaped sedimentary bodies developed along its faults. The alluvial fans include

variegated breccia and conglomeratic sandstone with abrupt upper and lower boundaries.

The fan delta sandstones have erosional contacts at their base and exhibit fining-upward

cycles wherein pebble sized clasts grade upward into cross-bedded sands at the top. The

subaqueous fans are characterized by interbedded conglomeratic sandstones and dark

mudstone layers.

This sequence shows signs of rapid sedimentation including abrupt shifts in

sedimentary facies and irregular distribution of volcanic horizons. The strata may also

have experienced episodes of post-depositional tectonic movement. Thus, subordinate

seismic sequence and system tracts cannot be easily subdivided according to classic

sequence stratigraphic criteria (Mitchum et al., 1977; Vail, 1987; Van Wagoner et al.,

1990). Lithostratigraphy offers a more consistent interpretation of stratigraphic features

for SQ1.

5.2 Intensive rifting units (SQ2, SQ3 and SQ4)

Intensification of movement on the Sangshutai Fault transformed the basin configuration

into a dustpan-shaped half-graben, with the Sangshutai Fault as its western boundary.

Overlapping deposition extended eastward (Figs.8-II, 9) to form the Shahezi (SQ2, SQ3)

and Yingcheng Formations (SQ4).

Seismic facies, well logs and drill core data

The upper boundary of the SQ2 sequence is a high amplitude reflector characterized

by baselap of the overlying reflections (Figs.10, 14-A). Its lower boundary coincides with

a high amplitude reflector that also shows marked baselap (T42). The lower part of the

sequence shows obvious mound and imbricate reflection patterns. The upper parts include

2-3 medium to high amplitude, sub-parallel continuous reflections. The sequence is only

visible in the Sangshutai Sag, which is adjacent to the Sangshutai Fault and has not been

accessed by wells..

The SQ3 sequence reaches thicknesses of more than 700 ms TWT in basinal areas and

is composed of three distinct seismic facies (Figs.10, 14-A). The lowermost facies

exhibits medium amplitude, typical mound and imbricate reflection patterns. In terms of

distribution, this facies is confined to the Sangshutai Sag area. The middle seismic facies

includes continuous, medium to high amplitude, sub-parallel to parallel reflections of

moderate frequency. This facies is more widely distributed. The uppermost seismic facies

contains continuous, moderate amplitude, sub-parallel reflections with low frequency.

About 50 exploratory wells were drilled into this sequence in the eastern region of the

Lishu Depression. Most wells displayed interbedded gray conglomeratic sandstone, fine-

to medium-grained sandstone and dark gray mudstone. GR and RLLD well log curves

usually show high amplitude responses with bell or finger motifs. The lithofacies

assemblage consists of cyclic sandstone- mudstone-sandstone interbeds.

The upper boundary of the SQ4 sequence is marked by erosional truncation in the

areas around the eastern margin of the basin. The base of the sequence is marked by a

moderate amplitude surface showing baselap. The sequence consists of high to very high

amplitude, sub-parallel continuous reflections throughout most of the basin. In

northeastern and southeastern regions of the basin, the upper facies includes distinctive

progradational reflection patterns (Figs.10, 14-A). More than 100 exploratory wells have

been drilled in this sequence. Well logs and drill cores indicate greater proportions of

interbedded sandstone and mudstone than those observed in SQ3. From the base of SQ4,

the depositional stacking pattern shifts from retrogradation to progradation, and then to

aggradation (Fig.15).

Sedimentary facies and sequence stratigraphic characteristics

Analysis of well logs, drill cores and seismic data indicate basin infilling by braided

delta, lacustrine, nearshore subaqueous fan and lowstand fan environments (Figs.14-B,

15). Sediments reflecting braided delta environments dominate the gentle slope areas of

the down-dropped block of the Sangshutai Fault. A series of near-shore subaqueous fans

developed along steeper fault scarps. In central areas of the basin, the basal areas of SQ3

and SQ4 show incision by lowstand fans. Basin infill show frequently alternating braided

delta and lacustrine facies.

Sequences SQ2, SQ3 and SQ4 show significant variation in their seismic stratigraphic

patterns. SQ2 and SQ3 include three distinctive sections: a low-stand systems tract (LST),

a transgressive systems tract (TST) and a high-stand systems tract (HST). The LST

reflection configuration is readily apparent due to its considerable thickness, especially in

the Sangshutai sag zone (Figs.10, 14, 16). Following development of the TST and HST,

deposition extends over a wider area and lateral thickness becomes more uniform.

The LST of sequence SQ4 did not accumulate with the same thickness as that of

earlier sequences and is thus difficult to discern in seismic profiles. The SQ4 TST

developed in thicker proportions with strata forming on gentle slopes. The HST is

relatively thick and exhibits progradational features (Figs.10, 14, 15).

5.3 Recession-rifting stratigraphic unit (SQ5)

Seismic facies, well log and drill core data

The lower boundary of the final recession-rifting sequence appears as the T4

reflector in seismic profiles. This boundary marks the contact between the Denglouku

(younger) and Yingcheng (older) Formations, which is a disconformable surface in most

areas and an angular unconformity along the basin margins (Figs.10, 14, 15). The T3

reflector marks the upper boundary of the recession sequence and corresponds to a large

regional unconformity occurring throughout the Songliao Basin (Yu et al, 2000; Ren et al.,

2002). Seismic reflection data shows continuous, sub-parallel contacts among different

reflectors. Layers are truncated in regions bordering the basin and seismic onlap, downlap

and toplap phenomena are no longer apparent within the basin.

Drill core material shows lithologies dominated by light gray conglomeratic

sandstone, medium- to coarse-grained sandstone and gray mudstone. Sandstone layers

usually keep an abrupt contact with gray mudstone layers. GR and RLLD log curves

reveal higher response amplitudes than those observed for SQ4 and SQ3, and show finger

or bell motifs. The T3 and T4 unconformities appear as abrupt changes in lithofacies and

log curve responses at the top and bottom of the section (Fig.15). The sandstone and

mudstone beds indicate aggradational conditions in the sequence.

Sedimentary facies and sequence stratigraphic characteristics

Well logs, drill core and seismic data indicate widespread fluvial and delta plain

depositional environments. Lacustrine deposition is confined to areas of the Sangshutai

Sag. SQ5’s aggradational features and shallow water environments contrast the prominent

progradational and retrogradational features in SQ3 and SQ4. Strata in this recession

sequence exhibit only minor lateral variations in thickness. Basin fill included shallow

water sediments that do not meet LST criteria (Patterson et al., 1995; Darmadi et al.,

2007). Subordinate seismic sequences and system tracts are difficult to identify.

6 Relationship between basin structure and stratigraphic architecture

Accommodation in marine rift basins is a function of tectonic subsidence, sediment

supply rate, sea level change and climatic conditions (Vail et al., 1977; Ravnås & Steel,

1998; Gawthorpe et al., 2003). Accommodation space in turn controls stratigraphic and

sedimentary architectures. The Lishu Depression is a relatively small-scale continental rift

basin, whose configuration and stratigraphic architecture are influenced primarily by

major fault displacement rates, regional tectonic events and sediment supply rates.

(Figs.11 ,17).

During the initial-rifting phase, rapid faulting formed a series of grabens and

half-grabens having relatively high relief. Sediment supply rates could not keep up with

displacement on faults and basin accommodation space increases. The igneous material

formed by episodic volcanism compensated for some of the difference between

accommodation space and sediment supply. Tectonic uplift at the end of Tithonian

finalized this sedimentary sequence. Mixed deposition of volcanic and clastic rocks

created somewhat chaotic seismic facies, making systems tracts difficult to identify.

Throughout the intensive-rift phase, the basin grew due to continuous movement on

the Sangshutai Fault. The basin assumed a wedge-shaped half-graben form with the

Sangshutai Fault as its western boundary, and overlapping layers towards the east. In the

early intensive-rift phase, rapid downthrow and rotation of Sangshutai Fault blocks

created abundant accommodation space. In the later intensive-rift phase, high sedimentary

accumulation rates (Fig.5-A) gradually reduced the accommodation space. The

Sangshutai Fault downthrow block defined most areas of basin, and transited from a high

dip angle to a gentler angle later in this phase (Fig.17). The eastern slope of the basin

resembled a passive continental margin setting at this point. Correspondingly, the seismic

sequence pattern is similar to that of a passive continental margin with a readily apparent

LST, TST and HST. As the basin surface features became more and more gentle, LST

features became less apparent and are difficult to recognize in seismic profiles (Figs.15,

16). Well data also show changes in sedimentary stacking patterns. From SQ2 to SQ4, the

retrogradation sequence occupies less and less space. The progradation sequence

meanwhile assumes a larger proportion of the section.

Following regional transpression at the end of the Barremian , the displacement rate

of Sangshutai Fault decreased. The sediment supply rate equaled the basin subsidence rate.

The basin surface leveled off by the Aptian and no longer showed sedimentary response

to the relief of previous rifting phases. The aggradation sequence formed, appearing in

seismic reflection data as continuous, sub-parallel contacts among different reflections.

These final units exhibit bimodal TST and HST sequence structure, without an LST

component.

7 Discussion

Above statements clearly reported the features of basin structure and stratigraphy

architecture and their relationship in the three rifting sub-stages for Lishu Depression. The

discussion part will be used as a supplement to illustrate the unique properties in the

rifting stage of Lishu Depression and point out some questionable aspects in this study.

1) Extension of the Lishu Depression was not a uniform or continuous event, but

rather consisted of several regional tectonic events. These events caused the uplift,

deformation and denudation of strata, evident from several regional erosional

unconformities. Unconformable surfaces form the boundaries of stratigraphic units that

represent sub-stages of larger-scale rifting event. Rift-related structural characteristics

tend to vary among different rift basins. The Northern Viking Graben (Ter Voorde et al.,

1997), Suez Rift of Egypt (Gupta et al., 1999) and northern of the North Sea (Nøttvedt et

al., 1995) for example show rotation of fault blocks with the growth of major listric

normal faults. The tilt of fault blocks in turn caused local region denudation of areas in the

fault’s footwall. Deposition in these basins was more uniform and does not show the same

punctuated phases as those observed in the Lishu Depression. Their unconformities are

also more localized and arise from different structural dynamics

2) In the Lishu Depression, the development of major faults exerted a primary

influence on basin geometry. The basin geometry and topography in turn influenced

stratigraphic and sedimentary architecture. In fact, regional tectonic subsidence, lake level

change and climatic conditions also are the important influence factors in basin evolution.

Regional tectonic subsidence may influence the amount of accommodation space

(Nøttvedt et al., 1995; Gawthorpe and Leeder, 2000). Climatic conditions usually have a

significant influence on the sediment source (Ter Voorde et al., 1997; Ravnås and Steel,

1998). Lake level change reveals an important influence on depositional stacking pattern

(Lambiase, 1990; Gawthorpe et al., 2003). The main purpose of this paper is to discuss

rifting sub-stage’s structural and stragiraphic features and analysis the relationship

between them. So these factors are thought as the stable background, not being discussed

here in detail.

3) Different slope areas corresponding to different fault segments show variation in

sequence stratigraphic style and sedimentary stacking pattern (Yong et al., 2002;

Gawthorpe et al., 2003). Most stratigraphic features described above occur on the slope

regions corresponding to the central part of Sangshutai Fault and occupy most of the basin.

Regions to the north and south of the Sangshtai Fault are not described and may exhibit a

different stratigraphic style and sedimentary pattern.

4) Classic sequence stratigraphic analysis derives from sedimentary sequence studies

of passive margin settings (Vail et al., 1977; Van Wagoner et al., 1990; Gawthorpe et al.,

1997). As a continental rift basin, the Lishu Depression exhibits classic triplets of LST,

TST and HST sequences only in the intensive rift sub-stratigraphic unit. The system tracts

in the initial rift sub-stratigraphic units are not clear as those found in other rift basins

(e.g. Prosser, 1993; Lambiase and Bosworth, 1995; Martins-Neto, 2000; Richardson and

Underhill, 2002; Renato et al., 2009). The recession rift units show a HST and TST, but

no clear LST. It also suggest that the stratigraphic division method of Classic Sequence

Stratigraphy has some limitations in continental rift basin. To setup high resolution

stratigraphic framework within initial rift and recession rift sub-stratigraphic units, more

methodologies and additional well log, drill core and high-resolution seismic data are

requisite (e.g. Henry et al., 1990; Cheng and You, 2001; Escalona & Mann, 2006; Neal

and Abreu, 2009; Reigenstein et al., 2011).

8 Conclusion

From the Late Jurassic Kimmeridgian to the Cretaceous Aptian , the Lishu Depression

experienced a punctuated and long-lived extensional event that consisted of three regional

compressional tectonic movements. These caused uplift, denudation, deformation and

corresponding development of unconformities within Lishu Depression sediments, and

established three phases of sedimentation during the rift period. Development of major

fault systems and changes in their rates of movement influenced basin configuration in its

transition from a graben / half-graben configuration during the initial rifting phase, to a

dustpan-shaped half-graben pattern during the subsequent phase of intensive rifting, and

finally to a gently-sloping sedimentary basin during the final recession phase.

During the initial-rifting phase, discrete graben and half-graben structures were filled

with volcanic material and clastic sediment. The seismic facies consist of medium to low

continuity and chaotic to sub-parallel seismic reflection patterns. Seismic data from the

initial rifting phase does not show clear systems tracts. The intensive-rifting phase was

characterized by increasing accommodation space and protracted depositional cycles.

Seismic data show obvious divisions among three different system tracts. Progressing

downward from the top of the section, LST features became less apparent, whereas HST

features occupy an increasing proportion of each sequence. Well logs and drill core data

from basal units also show clear progradational features. In the final recession phase of

rifting, deposition reflects shallow water aggradation with high frequency depositional

cycles affecting an extensive area of the basin. Stratigraphy exhibits a bimodal sequence

structure, consisting only of TST and HST components.

Acknowledgements

This work was sponsored by the University Science Fund of China (No.292013124). The

authors thank the Northeast Corp. of Sinopec for access to a large trove of seismic and

drilling data. We thank Sinopec geologists Yuming Zhang, Kongquan Chen, Chunman

Zhao. and Xiulin Wang, who offered many helpful suggestions concerning this study.

References

Alves, T.M., Manuppella, G., Gawthorpe, R.L., Hunt, D.W., Monteiro, J.H., 2003. The

depositional evolution of diapir- and fault-bounded rift basins: examples from the

Lusitanian Basin of West Iberia. Sedimentary Geology, 162, 273-303.

Arabi, E.H.E., Ferrandini, J., Essamoud, R., 2003. Triassic stratigraphy and structural

evolution of a rift basin; the Ec Cour Basin, High Atlas of Marrakech, Morocco.

Journal of African Earth Sciences, 36, 29-39.

Bois, C., 1993. Initiation and evolution of the Oligo-Miocene rift basins of southwestern

Europe: contribution of deep seismic reflection profiling. Tectonophysics, 226,

227-252.

Bosworth, W., 1985. Geometry of propagating continental rifts. Letters to Nature, 316,

625-627.

Baudon, C., Fabuel-Perez, I., Redfern, J., 2009. Structural style and evolution of a Late

Triassic rift basin in the central High Atlas, Morocco; controls on sediment deposition

(in Triassic basins of the Central and North Atlantic borderlands; models for

exploration ), Geological Journal, 44, 677-691.

Cheng, R.H., You, H.T., 2001. Tectono-Stratigraphy and Its Application in Oil and Gas

Exploration. World Geology, 20, 142-147 (in Chinese with English abstract).

Cloetingh, S., Fernández, M., Munoz, J.A., Sassi,W., Horváth, F., 1997. Structural

controls on sedimentary basin evolution: introduction. Tectonophysics, 282, xi-xviii.

Cohen, A.S., 1990. Tectono-stratigraphic model for sediment action in Lake Tanganyika,

Africa. Lacustrine basin exploration-case studies and modern analogs (Ed. by Katzb, J.),

AAPG memoir 50, 137-150.

Darmadi, Y., Willis, B.J., Dorobek, S.L., 2007. Three-dimensional seismic architecture of

fluvial sequences on the low-gradient Sunda Shelf, offshore Indonesia. Journal of

Sedimentary Research, 77, 225-238.

Escalona, A., Mann, P., 2006. Sequence-stratigraphic analysis of Eocene clastic foreland

basin deposits in central Lake Maracaibo using high-resolution well correlation and

3-D seismic data. AAPG, 90, 581-623.

Fang, L.M., Li, Y.X., Yin, J.Y., Chen, J.L., 2003. Characters of reverse tectonics and its

forming mechanism in late of fault depression in Songliao Basin. O.G.P., 38,

190-193(in Chinese with English abstract).

Feng, Z.Q., Jia, C.Z., Xie, X.N., Zhang, S., Feng, Z.H., Cross, T.A., 2010.

Tectonostratigraphic units and stratigraphic sequences of the nonmarine Songliao Basin,

northeast China. Basin Research, 22, 79-95.

Frostick, L.E., Reid, I., 1990. Structural control of sedimentation patterns and implication

for the economic potential of the East African Rift basins. Journal of African Earth

Sciences, 10, 307-318.

Gan, X.Q., Qin, Q.R., Su, P.D., Fan, Z.C., Song, W.Y., Cai, R., 2011. Influence of

left-lateral slip on structural form in Shiwu Oilfield and it’s petroleum geological

significance. Lithologic Reservoirs, 23, 40-43(in Chinese with English abstract).

Garcia, M., Ercilia, G., Anderson, J.B., Alonso, B., 2008. New insights on the post-rift

seismic stratigraphic architecture and sedimentary evolution of the Antarctic Peninsula

margin (Central Bransfield Basin). Marine Geology, 251, 167-182.

Gawthorpe, R.L., Hardy, S., Ritchie, B., 2003. Numerical modelling of depositional

sequences in half- graben rift basins. Sedimentology, 50, 169-185.

Gawthorpe, R.L., Leeder, M.R., 2000. Tectono-sedimentary evolution of active

extensional basins. Basin Research, 12, 195-218.

Gawthorpe, R.L., Sharp, I., Underhill, J.R., Gupta, S., 1997. Linked sequence stratigraphic

and structural evolution of propagating normal faults. Geology, 25, 795-798.

Ge, R.F., Zang, Q.L., Wang, L.S., Xie, G.A., Xu, S.Y., Chen, J., Wang, X.Y., 2010.

Tectonic Evolution of Songliao Basin and the Prominent Tectonic Regime Transition in

Eastern China. Geological Review, 56, 180-194. (in Chinese with English abstract)

Gupta, S., Underhill, J.R., Sharp, I.R., Gawthorpe, R.L., 1999. Role of fault interactions in

controlling synrift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez Rift,

Sinai, Egypt. Basin Research, 11, 167-189.

Henry, G., Clendenin, C.W., Stanistreet, I.G., Maiden, K.J., 1990. Multiple detachment

model for the early rifting stage of the Late Proterozoic Damara orogen in Namibia.

GEOLOGY, 18, 67-71.

Howell, J.A., Flint, S.S., 1994. A model for high resolution sequence stratigraphy within

extensional basins. In: High Resolution Sequence Stratigraphy: Innovations and

Applications (Ed. by J.A. Howell and J.F. Aitken), Geol. Soc. London Spec. Publ., 104,

129-137.

Huang, C.Y., Shen, W., 2007. Structural characteristics of deep fault depressions in the

southeastern uplift area of the Songliao Basin, China and their significance for

petroleum exploration. Geological Bulletin of China, 26, 841-848. (in Chinese with

English abstract)

Johnson, C.L., 2004. Polyphase evolution of the East Gobibasin: sedimentary and

structural records of Mesozoic-Cenozoic intraplate deformation in Mongolia. Basin

Research, 16, 79-99.

Lambiase, J.J., 1990. A model for tectonic control of lacustrine stratigraphic sequences in

continental rift basins, in Lacustrine basin exploration: case studies and modern

analogs (Ed. by B. J. Katz), A.A.P.G. Memoir 50, 265–276.

Lambiase, J.J., Bosworth, W., 1995. Structural controls on sedimentation in continental

rifts, in: Hydrocarbon habitat in rift basins (Ed. by Lambiase, J.J.): Geological Society,

London, Special Publication 80: 117-144.

Leeder, M.R., 2011. Tectonic sedimentology: sediment systems deciphering global to

local tectonics. Sedimentology, 58, 2-56.

Li, J.Z., Li, J., 1999. Genetic types and accumulation modes of structural belt in the

southeast up lift of Songliao Basin. P.G.O.D.D., 18, 7-9(in Chinese with English

abstract).

Li, S.T., Yang, S.G., Wu, C.L.,, Huang, J.F., Cheng, S.T., Xia, W.C., Zhao, G.R.,1987.

Late Mesozoic rifting in northeast China and northeast Asia fault basin system. Sci.

China Ser. D-Earth Sci., 30, 185-195.

Martins-Neto, M.A., 2000. Tectonics and sedimentation in a paleo/mesoproterozoic

rift-sag basin (Espinhaco basin, southeastern Brazil). Precambrian Research, 103,

147-173.

Mitchum, R.M., Vail, P.R., Sangree, J.B., 1977. Seismic stratigraphy and global changes

of sea level: Part 6, Stratigraphic interpretation of seismic reflection patterns in

depositional sequences (in Seismic stratigraphy; applications to hydrocarbon

exploration). AAPG Memoir, 26, 117-133.

Nøttvedt, A., Gabrielsen, R.H., Steel, R.J., 1995. Tectonostratigraphy and sedimentary architecture

of rift basins, with reference to the northern North Sea. Marine and Petroleum Geology,

12, 881-901.

Neal, J., Abreu, V., 2009. Sequence stratigraphy hierarchy and the accommodation

succession method. Geology, 37(9), 779-782.

Papatheodorou, G., Fdrenttnos, G., 1993. Sedimentation processes and basin-filling

depositional architectures in an active asymmetric graben: Strava graben, Gulf of

Corinth, Greece. Basin research, 5, 235-254.

Patterson, P.E., Sprague, A.R., Hill, R.E., 1995. Sequence stratigraphy and fluvial facies

architecture, Farrer and Tuscher formations (Campanian), Tusher Canyon, Utah (in

A.A.P.G. 1995 annual convention). Annual Meeting Expanded Abstracts –

A.A.P.G.(1995), 4, 74.

Ravnås, R., Bondevik, K., Helland-Hansen, W., Lømo, L., Ryseth, A., Steel, R.J., 1997.

Sedimentation history as an indicator of rift initiation and development: the late

Bajocian - Bathonian evolution of the Oseberg-Brage area, northern North Sea. Norsk

Geologisk Tidsskrift, 77, 205-232.

Ravnås, R., Steel, R.J., 1998. Architecture of Marine Rift-Basin Successions. AAPG Bull.,

82, 110-146.

Reigensten, H.M., Posamentter, H.W., Bhattacharya, J.P., 2011. Seismic geomorphology

and high-resolution seismic stratigraphy of inner-shelf fluvial, estuarine, deltaic, and

marine sequences, Gulf of Thailand. AAPG, 95, 1959-1990.

Renato, P.D.A., Liliane, J., Antonio, R.S., Ander, M.F., 2009. Evolution of a rift basin

dominated by subaerial deposits: The Guaritas Rift, Early Cambrian, Southern Brazil.

Sedimentary Geology, 217, 30-51.

Ren, J.Y., Kensaku, T., Li, S.T., Zhang, J.X., 2002. Late Mesozoic and Cenozoic rifting

and its dynamic setting in Eastern China and adjacent areas. Tectonophysics, 344,

175-205.

Richardson, N.J., Underhill, J.R., 2002. Controls on the structural architecture and

sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic.

Marine and Petroleum Geology, 19, 417-443.

Rosendahl, B.R., 1987. Architecture of continental rifts with special reference to East

Africa. Annual Review of Earth and Planetary Science Letters, 15, 445-503.

Prosser, S., 1993. Rift-related linked depositional systems and their seismic expression.

In: Tectonics and Seismic Sequence Stratigraphy (Ed. by G. D. Williams and A. Dobb),

Geological society Special Publication, London, 71, 35-66.

Song, T.G., 1997. Inversion styles in the Songliao Basin (northeast China) and estimation

of the degree of inversion. Tectonophysics, 283, 173-188.

Stepashko, A.A., 2006. The Cretaceous dynamics of the pacific plate and stages of

magmatic activity in Northeasern Asia. Geotectonics, 40, 225-235.

Ter Voorde, M., Ravnås, R., Færseth, R., Cloetingh, S., 1997. Tectonic modeling of the

Middle Jurassic synrift stratigraphic in the Oseberg-Brage area, northern Vikng Graben.

Basin research, 9, 133-150.

Vail, P.R., 1987. Seismic stratigraphy interpretation using sequence stratigraphy; Part 1,

Seismic stratigraphy interpretation procedure (in Atlas of seismic stratigraphy ). AAPG

Studies in Geology, 27, 1-10.

Vail, P.R., 1991. The Stratigraphic Signatures of Tectonics, Eustasy and Sedimentology -

an overview. in, Cycles and events in stratigraphy ( Ed. by Einsele, G., Ricken, W.,

Seilacher, A.): 2nd International meeting on cycles and events in stratigraphy,

Tubingen, 617-659.

Vail, P.R., Mitchum, R.M., Thompson, S., 1977. Seismic stratigraphy and global changes

of sea level, Part 3. Relative changes of sea level from coastal onlap, in Seismic

stratigraphy: Applications to hydrocarbon exploration (Ed. by Payton, C.E.), A.A.P.G.

Memoir 26, 63-81.

Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., Rahmanian, V.D., 1990.

Siliciclastic sequence stratigraphy in well logs, cores, and outcrops: Concepts for high

resolution correlation of time and facies, American Association of Petroleum Geologists

Methods in Exploration Series 7, 1-55.

Watson, M.P., Hayward, A.B., Parkinson, D.N., 1987. Plate tectonic history, basin

development and petroleum source rock deposition onshore China. Petrol.-Geol., 4,

205-225.

Wei, H.H., Liu, J.L., Meng, Q.R., 2010. Structural and sedimentary evolution of the

southern Songliao Basin, northeast China, and implications for hydrocarbon

prospectivity. A.A.P.G., 94, 533-566.

Yang, T.B., Wang, H.Y., Fan, T.L., Liu, Z.H., Hao, Y.J., Yang, F., 2013. Sedimentary

characteristics of Yingcheng Formation in Sujiatun area, Lishu Rift in Songliao Basin.

Oil & Gas Geology, 34, 349-356(in Chinese with English abstract).

Yong, M.J., Gawthorpe, R.L., Sharp, I.R., 2002. Architecture and evolution of syn-rift

clastic depositional systems towards the tip of a major fault segment, Suez Rift, Egypt.

Basin Research, 14, 1-23.

Yu, H.Z., Li, Y.W., Han, S.H., Yi, W.X., 2001. Tectonic evolution of Songliao Basin in

the palaeozoic. Geotectonica et Metallogenia, 25, 389-396 (in Chinese with English

abstract).

Yu, K., Yan, J.Z., Yang, Z.S., 2000. Structural framework evolution in Shiwu fault

depression and its relation to oil and gas. Nature Gas Ind. 20, 32-35 (in Chinese with

English abstract).

Zhang, Q.L., Ren, J.Y., 2008. Prediction of subtle traps based on tectono-stratigraphy:

taking the deep strata of Shiwu fault depression as an example. Offshore Oil. 28, 52-57

(in Chinese with English abstract) .

Zhao, Y.G., 2008. Classification and Identification of Sequences in Deep Strata of Shiwu

Fault Depression. Journal of Chongqing University of Science & Technology, 10,

10-12 (in Chinese with English abstract).

Figure 1 Index map showing the location, surface structural features, seismic and well

data coverage of the study area, as well as the transect location of the

cross-section shown in Fig. 3, and the transect locations for seismic profiles

cited in the text. For the sake of clarity, only a subset of wells is shown.

Figure 2 Stratigraphy, tectonic setting and seismic features of Lishu Depression

sediments.

Figure 3 Schematic two-dimensional representation of the Songliao Basin (Transect A-B

in Fig. 1), showing tectonostratigraphic characteristics of the Lishu Depression

(right).

Figure 4 Seismic profile showing faults and stratigraphy of the Lishu Depression

(Transect a-a' in Fig.1).

Figure 5 Histograms showing major normal fault activity (y-axis) and corresponding

maximum accumulation rates within the Lishu Depression. A. Sangshutai Fault;

B. Qindong Fault; C. Jin-gang Fault; D. the maximum accumulation rates for

different sedimentary units in the Lishu Depression. Estimates assume

continuous sedimentation for each depocenter. The depositional rates were

obtained according to the ratios of strata thickness and corresponding

accumulation time. In the Late Jurassic, the down-dropped region of the Jin-gang

Fault exhibits maximum depositional thickness. In the Early Cretaceous, the

Sangshutai Fault was continuously active. The depocenter shifted to the

Sangshutai Sag adjacent to the Sangshutai Fault.

Figure 6 Seismic profile of the Jin-gang graben and Qinjiatun half-graben, which evolved

in the Late Jurassic. Arrows indicate continuous erosional truncation features that

overlie both grabens (Transect b-b' in Fig.1).

Figure 7 Major faults and grabens that evolved during the initial Late Jurassic rifting

phase. The isolines indicate TWT thickness according to 3-D seismic images.

Figure 8 Regional cross-section showing the structural evolution of the Lishu Depression

(Transect c-c' in Fig.1). Note that the rift stage has been sub-divided into initial,

intensive and recession rifting phases.

Figure 9 Schematic diagram of how basin geometry evolved with different sub-stages

(phases) of rifting. F1: Shangshutai Fault; F2: Qindong Fault; F3: Jin-gang

Fault; “+” marks the basin depocenter.

Figure 10 Seismic profile (A) and sequence stratigraphic interpretation (B) of Transect

d-d’. For location see Fig.1. Note that T5, T42, T41, T4 and T3 reflectors all are

disconformities. Also note the obvious baselap reflection between SQ2 and

SQ3.

Figure 11 Division of tectonic stages and sequences within the Lishu Depression. The

lithofacies column also shows the sedimentary system cycles.

Figure 12 Characteristics of initial-rifting stratigraphic unit (SQ1) (Transect e-e' in Fig.1).

A. Seismic reflection features; B. Stratigraphic filling pattern of units remaining

after denudation caused by Late Jurassic tectonism; C. Depositional infilling of

original strata before the tectonic movement.

Figure 13 Lithology, log data and well-site seismic reflection profile of Sequence SQ1

(Huoshiling Formation) from Well Y204. Well data records unconformities at

the bottom and top of the sequence. Also note the igneous horizons in this unit.

Figure 14 Seismic stratigraphy patterns and depositional facies model of the intensive

rifting stratigraphic unit (Transect d-d' in Fig.1). For seismic profile see Fig.10.

Data from Well SW2 shown in Fig.15.

Figure 15 Lithology and well log data for SQ3, SQ4 and SQ5 sequences (Well SW2),

showing the sedimentary facies, system tract features and stratigraphic stacking

pattern. Well location is marked in Fig.1, Fig.10 and Fig 14. Note the abrupt log

curve responses and lithofacies features at disconformities.

Figure 16 Isopach map showing low-stand systems tract distribution characteristics of the

intensive-rifting stratigraphic unit. For sequences SQ2, SQ3 and SQ4, the aerial

extent of LSTs broadened, while the lateral thickness variation diminished.

Figure 17 Schematic diagram of sequence stratigraphic configuration, sediment stacking

pattern and key controlling factors of different sub-stages (phases) of rifting in

the Lishu Depression.

Highlights

1. Classic rift stages can be further subdivided into discrete phases on the basis of the variation of sedimentary conditions. 2. Lishu Depression has unique sequence stratigraphy architecture and sediment stacking patterns in each rifting sub-stage. 3. A critical and high-resolution tectonostratigraphic perspective on the evolution of a continental rift basin was provided.