Sequence architecture and sedimentary characteristics of a Middle Jurassic incised valley, western Sichuan depression, China
Sequence architecture and sedimentary characteristics of a Middle Jurassic incised valley, western Sichuan depression, China
Jun-Long Liu 0 1 2 3
Wei Yin 0 1 2 3
You-Liang Ji 0 1 2 3
Tian-Yun Wang 0 1 2 3
Fu-Xiang Huang 0 1 2 3
Hai-Yue Yu 0 1 2 3
Wen-Shu Li 0 1 2 3
0 Xinxing Geophysical Department, Bureau of Geophysical Prospecting, CNPC , Zhuozhou 072751, Hebei , China
1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum , Beijing 102249 , China
2 & You-Liang Ji
3 Daqing Oil Company , CNPC, Daqing 163000, Heilongjiang , China
The Middle Jurassic Shaximiao Formation encompasses tens of meters of thick lowstand meandering valley (LMV) strata in the western Sichuan foreland basin. Ancient LMVs newly discovered in this area were further studied based on sequence stratigraphy and seismic sedimentology. The aim of the present study was to investigate the sedimentary characteristics, sequence architecture, and the controls on LMV deposition in this tectonically active basin using field survey data, seismic sections, seismic amplitude imaging, core description, and comprehensive application of drilling data. The results show the following: (1) Three regional sequence boundaries and two flooding surfaces were recognized, and the Shaximiao Formation was divided into two-third-order sequences and four systems tracts. (2) Three sedimentary facies associations were identified: incised valley-fill, tributary channel, and overbank facies. Incised valleys are 5-17 km wide, 20-60 m deep and traceable for 120 km along their axes. (3) In the downstream segment, the role of tectonism gradually diminishes, and periodic base-level changes control the form and evolution of the incised valleys. Three types of LMVs-A1, A2, and A3-developed with changes in base level (lake level); of these types, the base level of the A3 LMV was likely the lowest.
Sequence architecture; Sedimentary characteristic; Lowstand meandering valley depression; Shaximiao Formation
Incised valleys have been widely discussed in terms of
traditional depositional models
(Vail et al. 1977;
Posamentier and Vail 1988; Van Wagoner et al. 1988; Zaitlin
et al. 1994; Willis 1997; Veiga et al. 2002; Catuneanu
2006; Catuneanu et al. 2009)
; however, detailed analysis of
Edited by Jie Hao
Sinopec Exploration & Production Research Institute,
Beijing 100083, China State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China
the sequence architecture and sedimentary characteristics
of incised valleys based on seismic data has rarely been
attempted in the past. In addition, previous studies of the
form and evolution of incised valleys have primarily
focused on Quaternary valleys
(Reynaud et al. 1999;
Nordfjord et al. 2006; Bettis III et al. 2008; Green 2009;
Lin et al. 2010; Paquet et al. 2010; Le´vy et al. 2012;
KeenZebert et al. 2013)
rather than on ancient valleys.
The Jurassic Shaximiao (J2s) Formation was deposited
during a stable tectonic period in the western Sichuan
foreland basin. According to previous studies, its
sedimentary environment is characterized mainly by fluvial–
(Li et al. 2007, 2012; Wang et al. 2007, 2010;
Zhu 2009; An 2011; Li 2011; Yang et al. 2011; Qian et al.
. Through integrating previous interpretations with
applications of sequence stratigraphy and seismic
sedimentology, researchers identified lowstand meandering
valleys (LMVs) in this area
(Zhu 2009; Li 2011; Qian et al.
. Previous studies have described the distribution
patterns of the sedimentary environments and depositional
facies of the Shaximiao Formation in this area; these
studies have contributed to exploration of local gas fields.
Nonetheless, because of the lack of data (i.e.,
three-dimensional (3D) seismic data did not cover the entire study
area in early studies), the following problems remain. (1)
Horizontal 3D seismic slices reveal the abnormality of
infilled incised valleys, which were approximately 10 km
in width during the early depositional periods of both
members of the formation; this finding may contradict the
interpretation that the Shaximiao Formation was deposited
by classical meandering rivers. (2) The base-level changes
that control the sediment infilling of incised meandering
valleys have not yet been extensively investigated in a
rejuvenated foreland basin. Therefore, further
investigations are needed to identify the sequence architecture and
sedimentary characteristics of these LMVs.
Using an integrated dataset from seismic profiles,
outcrops, well logs, and cores, this study aims: (1) to examine
the sequence architecture of the LMVs; (2) to identify the
sedimentary characteristics of the LMVs; and (3) to study
the spatial and temporal evolution of the LMVs. This
research not only deepens insight into ancient incised
valleys in 3D by studying the nature of the LMVs, but also
provides insight for gas exploration and exploitation in the
study area, as J2s sandbodies are important tight sandstone
reservoirs in the subsurface
(Luo and Chen 2004; Zhu
2009; Wang et al. 2010; An 2011; Li et al. 2012; Qian et al.
2 Geological setting
2.1 Geographic location
The western Sichuan depression is located in the western
part of the Sichuan Basin, surrounded by the Longmen
Mountain Thrust Belt (LMTB) and the Longquan
Mountain foreland Uplift Belt (LMFUB)
(Fig. 1; Yang et al.
2005, 2016; Jiang et al. 2007; Li et al. 2009, 2011; Luo and
Li 2009; Huang et al. 2010; An et al. 2011; Liu et al. 2011)
The study area is located in the middle of the western
Sichuan depression. This area, which has a total area of
1.1 9 104 km2, is confined by Dujiangyan City to the west,
Huluxi City to the east, An’xian City to the north and
Chengdu City to the south.
2.2 Tectonic evolution of the foreland basin
The western Sichuan depression transformed from a
peripheral foreland basin in the Late Triassic into a
rejuvenated foreland basin in the Middle Jurassic; this
transformation is attributed to the subduction of the Yangtze
(Fig. 2; Wang and Meng 2008; Zheng et al. 2008;
Zou et al. 2013; Yu et al. 2016; Ye et al. 2017)
. During the
depositional period of the Shaximiao Formation, the
LMTB was relatively static, whereas the Micang–Daba
Mountains Tectonic Zone (MDMTZ; Fig. 1) was more
active because of the ‘‘scissor-type’’ subduction of the
(Deng 2007; Jin et al. 2008; Liu et al. 2010;
Yuan et al. 2012; Bian et al. 2015)
. As a result, the
foredeep depression was transferred from the northern section
of the Longmen Mountains to the Micang–Daba Mountain
(Chen et al. 2008; Luo et al. 2011)
. The study area,
which is in the middle part of the western Sichuan
depression, is located far from the MDMTZ and was
tectonically stable during the depositional period of the
Shaximiao Formation; consequently, its thickness changes
laterally only slightly in the study area (Fig. 2).
2.3 Sedimentary environment and stratigraphic characteristics
The Ganzi–Aba Indosinian orogenic belt and the Qinling
orogenic belt formed at the end of the Late Triassic; as a
result, the climate of the Sichuan Basin was transformed
from warm and humid to hot and arid, thereby facilitating
the deposition of Jurassic red beds in the study area
et al. 2008; An et al. 2011)
. Controlled by tectonic activity,
large amounts of terrestrial debris were transported from
the mountains at the basin margin, and a succession of
fluvial and lacustrine red clastic rocks was deposited under
semiarid to arid climatic conditions
(Li et al. 2009; Zhu
et al. 2010)
. During this period, the paleocurrent direction
was mainly from northeast to southwest (Figs. 1, 3). The
net sandstone percentage (NSP) and mean grain size
(MGS) data were obtained from boreholes in the upstream
area, A, B and C, and downstream, A0, B0 and C0. The
statistics of these data (Fig. 3) indicate that both the NSP
and MGS values decrease in the downstream direction.
The Shaximiao Formation (Fig. 4), the target
stratigraphic interval, overlies the Qianfoya Formation and
underlies the Suining Formation
(Chen et al. 2011)
Shaximiao Formation (Fig. 4) is subdivided vertically into
the Lower Shaximiao (J2s1) Member and the Upper
Shaximiao (J2s2) Member by the top of the Conchostraca
shale or the bottom of the Jiaxiangzhai Sandstone. Twelve
sub-members can be further divided.
The Shaximiao Formation is composed of several
interbedded layers of gray to grayish-purple, thick-bedded,
blocky, fine- to medium-grained subarkose, arkose, purple
siltstone, and mudstone; each of these components exhibits
(Fig. 4; Wang et al. 2001; Lin 2009)
The J2s1 Member consists of two to four sets of
superimposed gray to grayish-purple, thick-bedded, fine- to
medium-grained subarkose, arkose, purple siltstone, and
mudstone; each layer typically exhibits a thickness of
100–300 m and occasionally 400–500 m. The J2s1 Member
WJ2 in Fig. 8 is shown in the northwest of the study area. P1–7 are
typical outcrops shown in Figs. 5, 7, and 9, including Guangyuan–
Jiange (section P1), Dayi (section P2), Pengzhou (section P3),
Chongzhou (section P4), E’mei (section P5), Longquanyi (section
P6), and Yuelanshan (section P7). The cored wells WX2, CD620, ZJ10, and DSh2 represented in Fig. 10 are shown in the east of the study area. City names (capital and lower case letters) are given for reference
(400–2000 m); its thickness increases from west to east and
from south to north, with the thickest succession found at
the Micang–Daba Mountain front. The lower part of this
member contains gray mudstones that are typically
interlayered with abundant beds of thin marls and clam fossils,
whereas the upper part contains variegated mudstones with
The ancient J2s lake has been studied sedimentologically
and paleoclimatologically (Wang et al. 2001; Wang and Xu
2001; Cao 2007; Wang et al. 2008; Qian et al. 2012; Zhang
0 100 km
0 100 km
Daba Mountains; therefore, it was not uplifted by thrusting and
steadily subsided during the depositional period of the Middle
Jurassic Shaximiao Formation. Abbreviations include Kunlun Block (KLB), Qiantang Block (QTB), and Yangtze Block (YZB)
2013). Two kinds of paleolakes have been identified in the
study area: perennial and non-perennial lakes
Wang et al. 2008; An 2011; Qian et al. 2012)
nonperennial J2s lakes, which were influenced by seasonal
climatic changes, developed on the floodplain
An 2011; Qian et al. 2012)
. When floods occurred, the
floodplain was partially inundated, and the non-perennial
lakes were interconnected. When the floods receded, the
water evaporated because of the hot weather, and these
non-perennial lakes were separated again. The
Sandstone (Jiaxiangzhai ST.), Guankou Sandstone (Guankou ST.) and
Conchostraca shale are marked beside the lithologic section
3.1 Data and materials
To study the sequence architecture and sedimentary
characteristics of the LMVs, we used outcrop data, seismic
datasets, well drilling data, and cores. In the Shaximiao
Formation, seven representative outcrop profiles that are
well exposed in the western Sichuan depression were
selected. These profiles included the Guangyuan–Jiange
(section P1 in Fig. 1), Dayi (section P2 in Fig. 1),
Pengzhou (section P3 in Fig. 1), Chongzhou (section P4 in
Fig. 1), E’mei (section P5 in Fig. 1), Longquanyi (section
P6 in Fig. 1), and Yuelanshan (section P7 in Fig. 1) sites.
The Guangyuan–Jiange and Dayi profiles display the most
typical valley infills of the Shaximiao Formation. Three
merged 3D seismic volume datasets covered the
Zhongjiang-Huilong (southeast of the study area in Fig. 1),
Mianyang (northeast of the study area in Fig. 1), and
Chengdu (west of the study area in Fig. 1) areas, of which
the LMVs in the first area are the most developed. The 3D
seismic data used in this study cover a total areal extent of
approximately 8500 km2, which is 80% of the study area.
We also used drilling data, including logging and core
analysis data from 99 exploratory wells that together cover
every tectonic section of the western Sichuan depression;
among these wells, 19 were cored.
3.2 Research methods
3.2.1 Depositional facies and sequence analysis of outcrop
To investigate the sedimentary characteristics and
sequence architecture of the LMVs, we analyzed the
representative outcrop profiles, which displayed the size and
distribution patterns of meandering valleys. The sequence
architecture, architectural elements, vertical stacking
patterns, and intersecting orders of the LMVs were analyzed.
Paleocurrent directions on outcrop profiles were identified
based on analysis of tens of samples.
To provide a general overview of the sequence
stratigraphy, two outcrops (sections P1 and P7 in Fig. 1) located
at the basin margin were correlated with three boreholes in
the center of the basin (BB’ in Fig. 1). The tectonic setting
of the study area was an extensive foreland basin, and
baselevel (lake-level) rise and fall strongly controlled the form
and evolution of the LMVs; therefore, Catuneanu’s
depositional sequence scheme has been applied in this area. In
addition, lithofacies were identified from the outcrops.
3.2.2 Detailed description of 3D geometry based
on combined borehole-seismic data
The China Petroleum and Chemical Corporation
(SINOPEC) acquired 3D seismic datasets from several regions of
the interior part of the western Sichuan depression, where
outcrops are rare. In this study, these raw data were used to
reveal the 3D geometry of the LMVs of the Shaximiao
The main technology used in this study comprised
synthetic seismograms, seismic profiles, and the
rootmean-square (RMS) amplitude attribute generated using
the Landmark software. Synthetic seismograms were used
to link borehole and seismic data. Based on these data, we
interpreted the seismic profiles to describe the LMVs of the
Shaximiao Formation, including three sequence
boundaries, two flooding surfaces and several parasequence
boundaries. Attributes such as RMS amplitudes, average
intensities of reflection, and arc lengths were then obtained
from the 3D seismic volume datasets. Among these
attributes, the RMS amplitude showed a clear response to the
J2s LMVs; therefore, this attribute was investigated
extensively in the study. Moreover, a
sequence-stratigraphic framework was established based on combined
borehole and seismic data, and seismic and depositional
facies in the established framework were analyzed. All of
these investigations contribute to detailed analysis of the
lateral and vertical distribution patterns of meandering
3.2.3 Core facies and vertical section analysis
Core data, which like outcrop data provide crucial
firsthand information about meandering valleys, are
conventionally used to analyze depositional facies based on
recognition of the presented lithology and sedimentary
structures. In this study, cores from 19 typical wells were
observed and analyzed. Vertical sedimentary sections of
LMVs were constructed and used as references to identify
sedimentary microfacies and to investigate sedimentary
patterns and evolution in similar areas.
4.1 Sequence stratigraphy of the Shaximiao
Based on Catuneanu’s depositional sequence
, three regional sequence
boundaries (SB) and two flooding surfaces (FS) were
recognized, and the Shaximiao Formation was divided into
two-third-order sequences (SQ1 and SQ2) with four
systems tracts and several parasequences (Table 1, Fig. 5).
4.1.1 Sequence-stratigraphic interfaces
SB1 is a regional unconformity (third order). The Guankou
Sandstone, which is a clear marker of SB1, is
approximately 65 m thick at the Daba Mountain front (Fig. 5a) and
tapers to the west and south. SB2 is a large-scale erosional
discontinuity (third order) caused by environmental
changes. The Jiaxiangzhai Sandstone is the typical marker
for SB2; it is approximately 75 m thick at the Daba
Mountain front (Fig. 5c) and tapers to the west and south.
SB3 is a regional unconformity (second order) related to
basin-scale tectonic events. At the E’mei outcrop (Fig. 5d),
SB3 corresponds to a basal erosional surface. In addition,
truncation reflectors in the seismic profiles, which are also
important evidence of sequence boundaries, were identified
at the top of sequences SQ1 and SQ2 (Fig. 6).
FS1 and FS2 correspond to two flooding events in the
study area. FS1 was the maximum flooding surface (MFS)
during the Middle Jurassic, which corresponds to the
Conchostraca shale. The Conchostraca shale is embedded
in the upper part of SQ1 and is a widely developed and
easily recognizable MFS in the Sichuan Basin (Fig. 4).
This shale (Fig. 5b) divides SQ1 into transgressive
(lowstand) and highstand systems tracts, which present an
asymmetric ‘‘dual structural’’ pattern. The characteristics
of these sequences and systems tracts are described below.
4.1.2 Sequence architecture
220.127.116.11 Sequence 1 (SQ1)
SQ1 comprises the J2s1 Member (Table 1; Figs. 5, 6). SQ1
thickens progressively from northeast to southwest
(Figs. 5, 6). Its thickness is generally less than 150 m in the
southwest and reaches 500 m in the northeast. SQ1 pinches
out near Dujiangyan and Dayi at the Longmen Mountain
front where it is less than 100 m thick and composed
mainly of alluvial conglomerates.
During the SQ1 depositional period, the Sichuan Basin
transformed into a rejuvenated foreland basin, the Micang–
Daba Mountains were intensively uplifted, and a
corresponding piedmont depression formed in the western part
of the western Sichuan depression. Under arid climatic
conditions, predominantly fluvial systems developed in the
study area during SQ1
(An et al. 2011)
The highstand systems tract (HST) above the MFS
rarely developed, or even failed to develop, because of
erosion after deposition of the J2s1 Member. The top of the
Conchostraca shale was in direct contact with the bottom of
the overlying Jiaxiangzhai Sandstone. Therefore, SQ1
generally consists of a fining-upward lowstand and
transgressive systems tract (LST ? TST). The basal sandstone
is the well-known Guankou Sandstone, which has many
conglomeratic lag deposits at its base. Trough
cross-bedding, tabular cross-bedding, and parallel bedding
successively dominated upward, and wave-ripple bedding was
evident at the top of this unit, which indicates fluvial–
deltaic channel facies.
18.104.22.168 Sequence 2 (SQ2)
SQ2 comprises the J2s2 Member (Table 1; Figs. 5, 6). SQ2
thickens progressively from northeast to southwest
Sequence unit boundary SB3
the Qianfoya Formation and the Shaximiao Formation. b Maximum
flooding surface of the Shaximiao Formation. c Sequence boundary
between the J2s1 and J2s2 members. d Sequence boundary between the
Shaximiao Formation and the Suining Formation
(Figs. 5, 6). Its thickness reaches at least 1300 m, possibly
more than 1500 m, around Guangyuan–Jiange in the
northeast, and declines to 400 m around E’mei-Ya’an in
SQ2 was deposited in a similar tectonic setting and
sedimentary environment to SQ1, and arid climatic conditions
dominated the rejuvenated foreland basin during this
(Li et al. 2009; Zhu et al. 2010)
. Vertically, SQ2
consists of coarsening upward sedimentary facies
associations that transition from lacustrine beach bars to sandy
braided-river deposits in the study area. This coarsening
upward trend of facies associations within SQ2 is
demonstrated further near De Yang; the internal SQ2 transition
found east of the vertical section described above,
progresses upward from deltaic facies to meandering-river
deposits and then to braided-river facies. Additional
evidence from the center of the basin reveals upward change
from predominantly lacustrine to predominantly fluvial
facies. Horizontally, this sedimentary system consists of
fluvial overbank, deltaic, and lacustrine facies from the
basin margin to its center.
In the study area, the LMVs developed mainly during
the early stage of the LSTs and/or TSTs, the late stage of
Stra. 1G80R2/0AP18I0Demp., Lith. SeSqutrean.ce FAascsieos. TLyMpVe
the HSTs and/or FSSTs, and the transitional period from
FSST to LST. During these periods, the base level was
consistently low because of the arid climatic conditions,
and the sediment supply rate was greater than the basement
(Zhu 2009; Li 2011; Qian et al. 2012)
these conditions led to the formation of various scales of
4.2 Sedimentary characteristics of the LMVs
Based on sequence-stratigraphic analysis of the Shaximiao
Formation, we analyzed the lithofacies and sedimentary
facies of individual valleys. A simple subdivision into
gravel, sand, and fine-grained lithofacies is a useful initial
step for description and classification
(Miall 1985, 1996)
In this study, nine dominant lithofacies, Gh, Gw, St, Sh, Sp,
Sw, Sm, Fl, and Fm, were considered in accordance with
Miall’s classification of fluvial lithofacies (Table 2). Gh
indicates clast-supported gravel lithofacies with horizontal
beddings and normal grading (Fig. 7a–b). Gw indicates
well-sorted and well-rounded gravel lithofacies. St, Sh, Sp,
Sw and Sm indicate lithofacies of fine- to coarse-grained
sandstones that are trough cross-bedded, horizontally
laminated, planar cross-bedded, wavy bedded, and
structureless (massive), respectively (Fig. 7a–f). Fm and Fl
indicate lithofacies of mudstone or siltstone that are
smallripple-laminated and structureless, respectively (Fig. 7g–i).
In previous studies, described infilled sequences and
internal sedimentary facies of incised valleys have differed
because of their different locations
(Li and Zhang 1996;
Ferguson and Davis Jr. 2003; Rossetti and Santos Jr. 2004;
Lin et al. 2005a, b; Breda et al. 2007; Yang et al. 2010;
Ielpi 2012; Eoff 2014)
. However, certain interpretations
have been agreed upon. First, channel, overbank, estuarine,
and deltaic facies associations are the main facies
associations represented in the infill deposits
(Sakai et al. 2006;
Wilson et al. 2007; Abrahim et al. 2008)
finingupward trends dominate in infill sedimentary sequences,
such as in transitions from lower fluvial facies to upper
deltaic facies. Based on these previous studies, we
classified three types of sedimentary facies associations
(Table 3), incised valley-fill (IVF), tributary-channel
(TCH), and overbank (OF) facies associations, by
integrating facies markers recognized in outcrops, cores,
seismic profiles, and well logs. IVF was subdivided into
abandoned channel (CH) and point bar (PB), and OF was
subdivided into alluvial flat (AF) and floodplain (FP).
Among these facies associations, IVF is most dominant in
4.2.1 Incised valley-fill
In a narrow sense, channels are perennial and correspond to
the lower part of fluvial systems that are constantly
submerged underwater. In this study, the channel is a broad
concept and includes the major channel as well as the
marginal point-bar infills.
22.214.171.124 Abandoned channel
Channel facies commonly develop at the base of
depositional sequences in individual valleys, and fine upward into
PB facies associations or the base of the next depositional
sequence. For example, in the cores from well WJ2 in the
Chengdu depression (Fig. 8), five channel depositional
cycles are subdivided upward. These five hierarchies of
channels were successively stacked vertically and became
thinner upward with less complete cycles. Above the
erosion surface at 3316.8 m, conglomerate units could be
subdivided into two belts, the lower of which consists of
fine, highly mature conglomerate, whereas the upper belt of
quartz conglomerate exhibits low maturity.
In terms of lithology, sandy CHs (SCHs) are
characterized by conglomeratic lag deposits, conglomeratic
sandstone, and medium- to coarse-grained sandstone at the
base (Figs. 7a–f, 8); the cumulative grain-size distribution
curves of such deposits present a three-segment pattern
(Fig. 10d) with relatively small amounts of suspended
sediments. Dominant sedimentary structures are basal
boundary surfaces and boulder clay (Figs. 7a–b, 10).
Channel facies comprise one of the main facies
associations in individual valleys. The CH classification can be
subdivided into SCH and muddy channels (MCH) based on
the last type of infill deposited. SCHs are more common
than MCHs because MCHs are less likely to be preserved
after millions of years.
Channels may migrate laterally and stack vertically
because of autocyclic processes, as in the case of the five
channel cycles in well WX 2, which were attributed to
gradually decreasing accommodation and frequent major
channel diversions (Fig. 8). The divisions above the
erosion surface at 3316.8 mare presumed to have occurred for
the following two reasons. (1) The previously accumulated
sediments had high structural maturity because of
secondary transportation from distant provenance. Overlying
quartz conglomerates scoured the previously deposited
pebbles because of valley progradation or flooding
episodes. (2) Lower and upper conglomerates were deposited
by the same flooding episodes and separated by differential
126.96.36.199 Point bar
PBs formed through lateral migration and accretion and
subsequently developed on the convex sides of the fluvial
system; these deposits overlie channel facies and pass
Fig. 7 Representative outcrop photographs showing the lithofacies of
the Shaximiao Formation. All outcrop positions are labeled in Fig. 1.
a Channel (CH) facies with a basal bounding surface, above which is
a thick sandy CH unit (Gh, Sp), and below which is brown floodplain
mudstone to siltstone (Fl). Section P7 in Fig. 1. Magnification of the
erosional belt indicated in the rectangular area shows b lag deposits
(Gh) of sandy CH, with conglomerate and stratified pebbly sandstone.
Section P7 in Fig. 1. c Cross-bedded unit that consists of CH and LA
elements (Sh, Sp). Section P6 in Fig. 1. d Point-bar facies with
largescale parallel bedding (Sh, Fm). Section P2 in Fig. 1. e Point-bar
upward into the FP facies or the base of the river’s next
sedimentary sequence (Fig. 8). PB facies are lithologically
finer than SCH facies; furthermore, PB facies are
characterized by medium- to fine-grained sandstone, with
cumulative grain-size distribution curves that sort well into
two section types (Fig. 10d). Moreover, large-scale
inclined bedding with mica pieces is more prevalent in PBs
than in SCHs (Fig. 10c) because of their different
flowregime conditions. Other bedding types with a
highstrength flow, such as large-scale trough cross-bedding,
wedge bedding, and parallel bedding, are also commonly
observed in PB facies (Figs. 7c–f, 10).
facies with large-scale wedge cross-bedding (Sp), representing a
highstrength flow. Section P5 in Fig. 1. f Point-bar facies with trough
cross-bedding (St). Section P7 in Fig. 1. g Alluvial-flat facies, brown
to red-brown mudstone interbedded with siltstone and fine-grained
sandstone (Sw, Fm). Section P5 in Fig. 1. h Alluvial-flat facies,
brown mudstone interbedded with siltstone and fine-grained
sandstone (Sw, Fm). Section P3 in Fig. 1. i Alluvial-flat facies (Fm); note
the iron concretions, which indicate the warm and arid climatic
conditions. Section P4 in Fig. 1
In outcrop, PB facies are typically characterized by
lateral accretion units. For example, representative
Shaximiao Formation fluvial facies associations exposed in the
Guangyuan–Jiange profile (Fig. 9a; see section P1 in
Fig. 1) are divided into three-fourth-order channel
architectural elements (Fig. 9b). PBs include third-order
architectural elements (such as lateral accretions, downstream
accretions, and sandy bedforms) and are recognized in the
1-IVF and 3-IVF units (Fig. 9b) based on evident internal
lateral accretion bedding. After analyzing the internal
configuration of the 1-IVF unit, we have recognized three
fluvial systems from the base upward; sedimentary
structures are dominated by large-scale planar and
crossbedding (Fig. 9c), which indicate that unloading in the
fluvial system was dominated by traction currents. The
multiple stages of PB facies (i.e., LA or DA elements) are
observed based on internal evidence. At region d in Fig. 9,
one PB unit with recognizable lateral accretion bedding
was truncated early by channel-fill facies (Fig. 9d); at
region e, PBs with recognizable lateral accretion bedding
were truncated by an overlying channel (Fig. 9e).
Similar to SCHs, the PB facies association is one of the
main types of sandstone body in individual valleys. PBs
can be differentiated from SCHs based on sorting and
lateral accretion bedding
(Fustic et al. 2012)
macroforms, LA deposits, and sandy bedforms are the most
common architectural elements in PB facies
In general, PB sandstones are not recognizable in seismic
profiles or well logs, whereas outcrop and core data are
reliable for investigating these units.
The Guangyuan–Jiange profile (Fig. 9) is oriented NW–
SE, which is perpendicular to the paleocurrent direction
(P1 in Fig. 1); therefore, depositional units with lateral
accretion can be interpreted as PBs. Additionally, the
depositional evolution of the incised valleys can be
interpreted as follows. During the development of the 1-IVF
unit, valleys formed in early stages with relatively high
accommodation. The main channel migrated
northwestward, and the resulting lateral accretion units constitute the
dominant sedimentary bodies in the most notable outcrop.
During the development of the 2-IVF unit, SCH was
deposited from its concave-up erosion surface upward
because of the relatively low accommodation; the main
channel migrated southwestward and truncated the fluvial
systems of the 1-IVF unit. During the development of the
3-IVF unit, the major channels changed route again and
migrated northwestward based on the internal
northwestinclined lateral accretion bedding; these units can be
interpreted as PBs. Therefore, as interpreted from the
outcrop, there were multiple stages of channel
4.2.2 Tributary channel
TCH deposits developed at the same time as coeval incised
valleys, but their geometric parameters (width and depth)
are much smaller than those of the corresponding incised
valleys. TCH fills primarily consist of the Gw, Sh, Sp, Sw
and Sm lithofacies (Fig. 7a–f). Planar cross-beds and
several erosional structures, such as basal bounding surfaces,
lag deposits and muddy clasts, were observed in the core.
Tributary channels and major channels within LMVs have
similar sedimentary characteristics but can be distinguished
based on the following aspects: major channels developed
within long, wide, confined areas, which result in
lowfrequency lateral accretion but high-frequency vertical
accretion. In contrast, tributary channels are distributed
horizontally across much broader, flat regions, which
results in high-frequency lateral migration and
low-frequency vertical accretion.
Stra. G18R0/2A01P80I Demp., Lith.
Core facies and lithofacies
Fig. 8 Channel facies analysis of core data from well WJ2. The
vertical core section was analyzed based on core description, and five
SCH depositional cycles were subdivided; see text for detailed
descriptions. Note the two erosional surfaces at which the contrast in
lithology is apparent. a–f Representative images for the vertical
section; the intervals shown in (a)–(c) developed within the same
channel-fill, and the intervals (d)–(e) developed in another shared
channel-fill. a Point-bar facies with parallel-bedded sandstone,
4.2.3 Overbank facies
OF facies are found overlying channel facies in
complete sedimentary sequences and can be subdivided into AF
and FP deposits based on sedimentary environment and
characteristics (Fig. 10). In the study area, OF deposits are
characterized by brown to red-brown mudstone
interbedded with siltstone and fine-grained sandstone with rare
sedimentary structures (Fig. 7g–i); these characteristics
Lateral accretion beds
3,311.2 m in depth. b Sandy channel facies with fining-upward
cycles, 3,315.5 m in depth. c Erosional belt conglomerate unit,
3,315.80 m in depth. d Point-bar facies with cross-bedded sandstone,
3,317 m in depth. e Sandy channel facies with fining-upward cycles,
3,317.7 m in depth. f Conglomeratic lag deposits with low maturity,
3,318 m in depth. For scale, the diameter of the coin is approximately 3.5 cm
indicate overflow deposition in a shallow-water
AF deposits are lithologically dominated by thin- to
medium-bedded siltstone and fine-grained sandstone with
rare sedimentary structures. FP facies are found at the very
top of fluvial sedimentary sequences, but are scarcely
preserved in a single valley influenced by low
accommodation and the resulting rechanneling and cross-cutting. FP
facies are lithologically dominated by fine-grained
sediments, such as shale and argillaceous siltstone rich in
B Fig. 9c1C-PB A2
Fig. 9 Analysis of outcrop architectural elements and depositional
facies of the Shaximiao Formation LMVs. a Representative
photographs facing southwest to show the cross-cutting relationships of
different stages of channel-fill. See the location labeled in Fig. 1,
section P1. b Architectural element analysis and interpretation of an
outcrop profile. Note three-fourth-order bounding surfaces outlined by
A to C, two-third-order bounding surfaces recognized within the
1-IVF unit, three identified channel-fills, and 1-IVF subdivided into
three sedimentary units. Three rectangular areas showing detailed
organic matter. Sedimentary structures in FP facies are
characterized by parallel and wavy laminations associated
with low-strength flow.
The AF is the basal flat region of a valley. Water on the AF
will evaporate or penetrate into the subsurface during the
normal season in this region, but the AF will be submerged
when flooding occurs.
The FP represents relatively low regions between
channels in valleys, and its sedimentation is characterized
by fine-grained deposition. Water may remain on the FP
even after a flood has receded. The processes, rates, and
patterns of FP reworking in the reaches in the LMVs have
been defined, and these findings contribute toward
understanding the mechanisms by which sediments are
deposited, modified, and preserved in rivers
(Keen-Zebert et al.
elements are outlined and magnified in (c–e). c Point-bar facies with
large-scale tabular cross-bedding. d Sandy channel with concave-up
erosional surface cut by overlying point-bar lateral accretion units.
e Typical lateral accretion units showing point-bar facies cut by
overlying sandy channel. See text for detailed descriptions of
architecture elements. The red lines are labeled as 6-ordered sequence
boundaries, the yellow lines are labeled as lateral accretions, and the
blue lines are labeled as cross beddings
Two primary issues remain to be investigated and
discussed. (1) Further investigations should be conducted
regarding the stacking patterns of LMVs in different stages
and the sequence architectures associated with base-level
fluctuation. (2) Whether the LMVs evaluated in this study
developed widely in the foreland basin during tectonically
stable periods, as well as which model should be used to
interpret the evolutionary processes of the valleys, remain
to be determined.
Fluvial incision may be driven by one or more of the
following: autogenic forcing, such as fluvial vertical
accretion, and/or allogenic forcing, such as tectonism,
climatic change, variation in sediment supply, and increase in
discharge. In this study, the subsidence rate in the study
area during deposition of the J2s Formation was relatively
c f (d)
saCnodasrtsoene saMneddsituomne sanFdisnteone Siltstone Mudstone Boculaldyer bPeadradlilnegl bWedadvinyg sMhieceat seCcotrioen
Fig. 10 Single-well sections for interpretation of the facies within the
LMVs based on core and grading analysis. Core facies interpretations
for four wells from facies-association scale to facies scale; well
positions are marked in Fig. 1; a = WX2; b = CD620; c = ZJ10;
and d = DSh2. Three types of LMV, A1, A2, and A3, are labeled in
the vertical section. Note that the core photographs are labeled a, b, c,
etc., to show different sedimentary structures in the core. a Three
facies associations were recognized based on electrofacies analysis;
note that four stages of channel-fill were identified in the core
section. Photographs a, b, and c show large-scale parallel bedding and
cross-bedding, interpreted as point-bar facies; d shows a basal
boundary surface, interpreted as sandy channel facies; e shows
variegated mudstone, interpreted as floodplain facies. b Three facies
associations were recognized based on electrofacies analysis. Note
that two stages of channel-fill were identified in the core section.
Photographs a and b show parallel bedding and wavy laminations; c and d
consistent at approximately 90 m/Ma
(Zhu 2009; Cheng
, and the sediment supply rate can be regarded as a
(Li et al. 2009; Zhu 2009; An 2011)
variations in the preserved sedimentary sequence can be
attributed to base-level changes and/or autogenic factors.
5.1 Base-level control on the sequence architecture
The base level is analogous to a potential energy surface
that describes the directions in which sedimentation,
show boulder clays; and e shows a basal bounding surface, interpreted
as sandy channel facies. c Three facies associations were recognized
based on electrofacies analysis. Note that four stages of channel-fill
were identified in the core section. Photographs a, b, c, d, and e show
large-scale parallel bedding and cross-bedding, interpreted as
pointbar facies. d Three facies associations were recognized based on
electrofacies analysis. Note that three stages of channel-fill were
identified in the core section. Photographs a and b show large-scale
parallel bedding and cross-bedding with boulder clay at the bottom,
interpreted as sandy channel facies; c shows a burrow structure; and d
shows brown mudstone, interpreted as floodplain facies; e and f are
cumulative grain-size distribution curves, e shows a bi-segment
pattern, interpreted as point-bar facies, and f shows a three-segment
pattern, interpreted as sandy channel facies
sediment bypass, and erosion in a stratigraphic system are
likely to occur
(Shanley and McCabe 1994)
. Tectonic uplift
and climatic changes can drive the base level; however,
their influence varies from upstream to downstream. The
mid- to downstream segments of rivers are primarily
controlled by lake- or sea-level (analogous to the base level)
(Hampson et al. 2012; Alqahtani et al. 2015)
whereas the upstream segments of rivers may be more
directly controlled by tectonism
(Shanley and McCabe
1994; Mattheus et al. 2007; Gonza´lez-Bonorino et al. 2010;
Alqahtani et al. 2015)
The seismically imaged valleys correspond to mid- to
downstream parts of the J2s incised valleys (Fig. 1), which
were closer to the perennial lake
(Fig. 2a; An 2011; Li
et al. 2012)
. Therefore, relative lake-level fluctuations
could have exerted significant control on the form and
evolution of the incised valleys.
During deposition of the studied strata, the climate was
becoming more arid over time
(Cao 2007; Wang et al.
2008; Qian et al. 2012)
, which resulted in evaporation and
lake-level fall. Three types of LMVs (Fig. 11), A1, A2, and
A3, developed along with base-level (lake-level) changes,
among which the base level for A3 LMV was likely the
lowest. This division is based on sequence stratigraphy and
the LMV sedimentary characteristics of the Shaximiao
Formation in the western Sichuan depression, including
sequence boundaries and the internal
sequence-stratigraphic characteristics of systems tracts.
These three types of LMV, A1, A2, and A3, developed
in different stages of sequence infilling and show
inheritance from each other in time and space (Table 4, Fig. 11).
Periodic base-level (lake-level) fluctuations controlled the
progradation and retrogradation of the valleys
McCabe 1994; Olsen 1995)
. The widths and depths of the
incised valleys were controlled mainly by the rate and
magnitude of local base-level fall and delayed
(Alqahtani et al. 2015)
Fig. 12 Seismic facies and well-correlation framework showing the c
sedimentary facies and geometry of IVF in the J2s13 sub-member (A1),
J2s21 sub-member (A2), and J2s22 sub-member (A3). All of the wells
crossed are formation top flattened. See the text for details
These three types of LMV with varying sequence
architectures are genetically inherited vertically. LMVs
increase in horizontal sinuosity from type A1 to type A3
(Fig. 12), but maximum width and depth are achieved
during periods of type A3. The overall vertical sedimentary
section is lithologically characterized by fining-upward
cycles. This phenomenon strongly indicates the regularity
of valley evolution in terms of overall morphology and
vertical succession with base-level fluctuations in a
foreland basin setting during tectonically stable periods.
5.2 Sedimentary evolution model
In the present paper, LMV sedimentary evolution models
are divided into three stages based on analysis of the
sequence architecture and sedimentary characteristics of
LMVs in the Shaximiao Formation. We have also
considered the influence of fifth-order base-level cycles on LMV
development. Three stages are noted as follows (Fig. 13):
an incision-dominated valley evolution pattern in the early
stage; an inherited incision-dominated valley evolution
Qian et al. (2012)
Fig. 11 Lake-level changes and sequence architectures of LMVs in
the middle part of the western Sichuan depression. a J2s13 (A1 in
Fig. 12), J2s21 (A2 in Fig. 13), and J2s22 (A3 in Fig. 14) LMVs are
labeled and related to lake-level changes. The curve data are from
Cao (2007) and Qian et al. (2012). b Typical vertical section showing
different sedimentary infill cycles corresponding to the respective
sequence architectures of LMVs. Full lithological meanings of the
abbreviations are given in Table 2. See text for more detailed
d e arrsaevel
GR Demp., Lith.
• At the early stage of FSST
• The main facies are short distance
transported alluvial fan and long
distance transported meandering river
• At the late stage of FSST
• The dominated sedimentary dynamics
mechanism is incision
• Narrow-shallow in feature
• Incised valley type
30m 0 30m
30m 0 30m
Fig. 13 Sequence-sedimentary infill evolution model of the
Shaximiao Formation LMVs in the western Sichuan depression. LST
lowstand systems tract; TST transgressive systems tract; HST
highstand systems tract; FSST falling-stage systems tract. Influenced
by fifth-order base-level changes, four LMVs evolutionary stages are
summarized in (a) to (d), and among which (b) to (d) are the main
phases during which the LMVs developed. b Incision-dominated
early evolutionary stage. c Inherited incision-dominated middle
evolutionary stage. d Filling-dominated late evolutionary stage
30m 0 30m
30m 0 30m
• At the main stage of LST
• The dominated sedimentary dynamics
mechanism is incision
• The fluvial beds are wider and deeper
than the A1 LMV, Wide-deep in feature
• Inherited incised valley type
• At the early stage of TST
• The dominated sedimentary
dynamics mechanism is not incision
• Inherited filling valley type
pattern in middle stage; and a filling-dominated valley
evolution pattern in late stage.
5.2.1 Incision-dominated early evolutionary stage
The western Sichuan depression in the Middle Jurassic was
characterized by widely developed fluvial systems in the
early FSST (Fig. 13a), in which LMVs formed in the early
incision-dominated stage because of the arid climate and
continuously decreasing base level (Fig. 13b). The valleys
in this early evolutionary stage were small in depth and
width; therefore, the dominant mechanism of the
sedimentary dynamics was incision, which is characteristic of
type A1 valleys in terms of sequence architecture.
5.2.2 Inherited incision-dominated middle evolutionary
As the base level consistently decreased, valleys entered
their peak development periods, i.e., the inherited
incisiondominated middle evolutionary stage (Fig. 13c). In this
stage, valleys inherited the sequence patterns of the earlier
incised valleys, and their main mechanism of sedimentary
dynamics was incision. However, relative to the previous
stage, the valleys may have undergone more intense
downcutting erosion and lateral migration to produce much
deeper and wider morphology, which is characteristic of
type A2 in terms of sequence architecture. The
corresponding seismic facies are clearly marked by seismic
events, and can also be distinguished on attribute maps.
Therefore, this type of valley has great significance for
gasfield exploration and exploitation.
5.2.3 Filling-dominated late evolutionary stage
With gradual rise in base level, valleys entered the
fillingdominated late evolutionary stage (Fig. 13d) as they aged.
Valleys in this stage may become infilled with sediments
unloaded by rivers with weak ability to suspend and
The western Sichuan depression was tectonically stable as
a rejuvenated foreland basin when the Middle Jurassic
Shaximiao Formation was deposited. The basement
subsidence rate in this depression was lower than the sediment
supply rate. Fluvial and lacustrine red clastic sediments
developed under semiarid to arid climatic conditions. For
the first time, ancient LMVs were discovered in this area
and studied using sequence-stratigraphic analysis and
seismic sedimentology methods. The results of this study
show that the incised valleys were 5–17 km wide, 20–60 m
deep, and traceable for 120 km along their axes. These
characteristics make these valleys among the longest
seismically imaged incised valleys in the world.
Three sequence boundaries and two flooding surfaces
were recognized and correlated using an integrated
approach with field survey data and borehole logs to enable
subdivision of the Shaximiao Formation into
two-thirdorder sequences and four systems tracts. Based on core
descriptions, seismic facies analysis, and comprehensive
application of the drilling data, we categorized three
sedimentary facies associations: IVF, TCH, and OF. IVF is
subdivided into CH and PB, whereas OF is subdivided into
AF and FP. IVF is the most dominant facies in the LMVs.
The role of tectonism gradually diminishes in the
downstream direction, and periodic base-level changes
control the form and evolution of the incised valleys. The
LMV sedimentary evolution model was divided into three
stages based on the influence of base-level changes:
incision-dominated valley evolution in the early stage,
inherited incision-dominated valley evolution in the middle
stage, and filling-dominated valley evolution in late stage.
The valleys developed in the middle stage were wide and
deep in morphology, and thick in terms of infilled
sandbodies, relative to the other two stages. This result is of
great significance to gas-field exploration and exploitation.
Acknowledgements This project was supported by the Natural Science Foundation of China (Grant No. 41672098) and the National
Science and Technology Major Project (Grant No. 2016ZX05002
006). We thank SINOPEC for providing data and generous technical
support. We also thank Halliburton for providing the Landmark
software used to interpret the seismic data. The authors are grateful to
the reviewers for commenting on the original draft and improving the
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