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Sedimentary characteristics of large-scale lacustrine beach-bars

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					Sedimentology (2011) 58, 1087–1112                                                doi: 10.1111/j.1365-3091.2010.01196.x




Sedimentary characteristics of large-scale lacustrine
beach-bars and their formation in the Eocene Boxing Sag
of Bohai Bay Basin, East China
ZAIXING JIANG*, HUI LIU*, SHANWEN ZHANG , XIN SUà and ZHENGLO NG JIA NGà
*College of Energy, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China
(E-mail: jiangzx@cugb.edu.cn and lhpplu@yahoo.com.cn)
 Sinopec Shengli Oilfield Company, Dongying City, Shandong 257000, China
àCollege of Ocean Sciences, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China

Associate Editor – Daniel Ariztegui

               ABSTRACT

               The beach-bar reservoir play has become an important exploration target
               within the Bohai Bay Basin, especially in the Boxing Sag within the Dongying
               Depression, where a large-scale lacustrine beach-bar oil pool has been
               discovered recently. The sedimentary characteristics, distribution patterns
               and formation mechanisms of beach-bar sand bodies in the upper fourth
               member of the Eocene Shahejie Formation (Es4s) in the Boxing Sag were
               studied in detail based on seismic, well log data and core data. The Es4s in the
               Boxing Sag is composed of a third-order sequence consisting of three systems
               tracts, i.e. a lowstand systems tract, a transgressive systems tract and a
               highstand systems tract. Beach-bar sand bodies were deposited widely in the
               basin during the lowstand systems tract period. The sandy beach-bars are
               characterized by siltstones, fine-grained silty sandstones interbedded with thin
               mudstone units. The presence of well-developed sedimentary structures, such
               as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts,
               ripples, terrestrial plant debris and vertical burrows, suggests that beach-bars
               were deposited in a relatively shallow water environment under the influence
               of strong hydrodynamics. Laterally, the sandy beach facies occurred as a more
               continuous sheet-like body around the sandy bar in most parts of the sag.
               Stratigraphically, beach-bars were distributed mainly in the lowstand systems
               tract and they were less well-developed in the transgressive systems tract and
               highstand systems tract. Several factors were probably responsible for the
               occurrence of the large-scale beach-bars during the lowstand systems tract
               period, including: (i) a gentle palaeoslope and relatively weak structural
               activities; (ii) a shallow-water condition with a strong hydrodynamic
               environment; (iii) high-frequency oscillations of the lake level; and (iv) an
               abundant terrigenous clastic feeding system with multiple-point and linear
               sediment sources.
               Keywords Beach-bars, Bohai Bay Basin, Boxing Sag, Eocene, Lacustrine,
               Sedimentary characteristics.


INTRODUCTION                                                     lacustrine basin (Monroe, 1981; Reid & Frostick,
                                                                 1985; Zhu et al., 1994; Basilici, 1997; Mao et al.,
The term ‘beach-bar’ is a comprehensive term                     2004; Yuan, 2007). In the gentle-slope zone of a
usually used to describe the beach and the bar                   faulted lacustrine basin, thin interbedded sand
sand bodies in the shallow-water zone of the                     bodies such as beach-bars develop widely in
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shallow-littoral lakes (Otvos, 2000; Zhang et al.,              sedimentary characteristics and the formation
2005b; Lai & Li, 2007; Pan, 2007). Beach-bars can               mechanisms of the beach-bar sand bodies were
be divided into terrigenous clastic and bioclastic              investigated in great detail, on the basis of
(Chen et al., 2000; Jiang, 2003). Formation of                  detailed core descriptions, well log analyses,
beach-bar sand bodies is controlled mainly by                   seismic facies and sequence stratigraphic
palaeogeomorphology, waves, palaeowater depth,                  analyses.
palaeoshoreline and distribution of other associ-
ated sedimentary systems (Frey & Howard, 1988;
Osborne & Greenwood, 1992a,b; Otvos, 2000;                      GEOLOGICAL BACKGROUND
Houser & Greenwood, 2005; Zhang et al., 2005c,
2008; Zhou, 2008). Distribution of beach-bar sand               The Boxing Sag is located in the south-western
bodies is controlled primarily by the palaeogeo-                Dongying Depression of the Bohai Bay Basin in
morphology (Zhu et al., 1994; Zhang et al., 2005c,              east China (Fig. 1). It is 1320 km2 in area, and
               ´
2006; Guzman & Fisher, 2006; Castelle et al.,                   bordered by the Gaoqing Fault in the west, the
2007; Jiang et al., 2007a; Si & Zhang, 2008). In                Shicun Fault in the east, the Chunhua-Caoqiao
general, beach-bar sand bodies are characterized                Fault in the north and the Luxi Uplift in the
by thin-bedded sandstones with a restricted                     south. The Boxing-1 and Boxing-2 Faults are two
distribution.                                                   second-order faults in the centre of the Boxing
   Recently, the beach-bar reservoir play has be-               Sag. The whole sag is a west/east-oriented graben
come an important exploration target of the Bohai               defined by the Gaoqing and Shicun Faults, and a
Bay Basin, especially in the Boxing Sag in the                  half graben in a north/south direction controlled
Dongying Depression (Zhang et al., 2005c, 2008;                 by the Chunhua-Caoqiao Fault (Yuan & Jiang,
Jiang et al., 2007a). At the present time, four oil             2000; Si & Zhang, 2008; Fig. 2A and B).
fields aimed at beach-bar reservoirs within the low-               The Boxing Sag was developed on a northward-
stand systems tract (LST) of the upper fourth                   dipping subsiding, faulted block within the
member of the Eocene Shahejie Formation (Es4s)                  Dongying Depression (Fig. 2A and B). It com-
have been found, including Jinjia, Zhenglizhuang,               prises Cenozoic to Quaternary basin-filling strata,
Daluhu and Boxing oil fields (Fig. 1). These find-                locally up to 5000 m thick in the depocentre,
ings show that the beach-bar sand bodies in this                consisting of the Kongdian (Ek), Shahejie (Es),
area are a great prospect for exploration. The                  Dongying (Ed), Guantao (Ng), Minghuazhen (Nm)
hydrocarbons in the Es4s beach-bar reservoir were               and Pingyuan (Qp) Formations. The Es Formation
mainly migrated from the dark mudstone of the top               can be further subdivided into four members from
part of the Es4s and the overlying third member of              the base to top, including Es4, Es3, Es2 and Es1
the Eocene Shahejie Formation (Es3) (Li et al.,                 (Fig. 2C). The Es4 member, which is ca 0 to
2003; Sun et al., 2003; Li, 2004; Zhang et al.,                 >800 m thick and is the focus of this study,
2005b; Yuan, 2007). The overlying dark mudstone                 consists of a lower ‘red bed’ (Es4x; red interbed
with a huge thickness and high intra-formation                  of sandstone and shale) and an upper grey
overpressure is the favourable caprock for the                  mudstone interbedded with fine-grained and
beach-bar reservoirs underneath (Watts, 1987; Li                thin-bedded sandstones (Es4s). Es4 unconform-
et al., 2008). Due to the superior conditions of                ably overlies the Ek Formation and is overlain by
hydrocarbon source and sealing, the beach-bar                   the Es3 member, which comprises ca 1000 m
sand bodies are thus considered as the favoured                 brownish oil shales and dark-grey shales
exploration plays in this region.                               interbedded with thin sandstones.
   During deposition of the Es4s in the Boxing
Sag, especially within the LST, beach-bar sand
bodies were developed widely. These sand bodies                 METHODOLOGY
cover ca 50 to 70% of the Boxing Sag, ca 660
to 924 km2. Multiple beach-bar sand bodies are                  This study utilizes a comprehensive approach
found to have been superimposed vertically and                  that integrates seismic interpretation and wireline
amalgamated laterally. Individual sand bodies are               log analysis, with characterization and petro-
cumulatively thicker than 20 m, and extend                      graphic studies of core and cutting samples.
laterally for 5 to 25 km (average 10 km). There                 The basic data include nearly 1000 km2 three-
were few reports (Cui et al., 2005; Hou et al.,                 dimensional (3D) seismic data, wireline log data
2005) on the distribution and origin of such large-             from 75 wells, ca 400 m cores from 19 wells,
scale beach-bar sand bodies. In this study, the                 microfauna data from 9 wells, grain-size data
 Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists, Sedimentology, 58, 1087–1112
                                                                                                                                 A
                                                                                                                                                                                                                                                  B




                                                                                                                             Fig. 1. (A) Structural map of the Boxing Sag with well locations. It is 1320 km2 in area and bordered by the Gaoqing Fault in the west, the Shicun Fault in the
                                                                                                                             east, the Chunhua-Caoqiao Fault in the north and the Luxi Uplift in the south. Boxing-1 and Boxing-2 Faults are two second-order faults in the centre of the
                                                                                                                             Boxing Sag. At the present time, four oil fields aimed to beach-bar reservoirs within the LST of Es4s have been found, including Jinjia, Zhenglizhuang, Daluhu
                                                                                                                             and Boxing oil fields. (B) Tectonic setting of the Boxing Sag in the south-western corner of the Dongying Depression in the Jiyang Sub-basin ‘III’ (modified from
                                                                                                                             Jiang et al., 2007b). Other sub-basins in Bohai Bay Basin of North China are the Huanghua Sub-basin ‘II’, the Jizhong Sub-basin ‘I’, the Bozhong Sub-basin ‘IV’,
                                                                                                                             the Liaohe Sub-basin ‘V’ and the Dongpu Sub-basin ‘VI’.
                                                                                                                                                                                                                                                                                                Large-scale lacustrine beach-bars in North China




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                                                                                                                               A                                                                                                                         C
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                                                                                                                               B




                                                                                                                             Fig. 2. (A) Seismic section (A to A¢) across the Boxing Sag, exhibiting a typical half-graben geometry bounded by the Chunhua-Caoqiao Fault to the north,
                                                                                                                             based on 3D seismic data. (B) Interpretation of seismic section (A to A¢). (C) Stratigraphic column and sequence framework of the Boxing Sag. The age data are
                                                                                                                             derived from Jiang et al. (2007b) and Pan et al. (2003). See Fig. 1 for seismic section location.




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                                                        Large-scale lacustrine beach-bars in North China              1091

from 95 samples, and porosity and permeability                   above the FFS is mainly dark grey, whereas the
data from 75 samples. The data of 3D seismic,                    colour below the FFS is usually greyish-green or
wireline log, grain size, porosity and permeability              light grey, even purplish red; the MFS corre-
come from the Geoscience Institute of the Shengli                sponds to the condensed section consisting of oil
Oilfield, SINOPEC. The microfauna data are                        shales and dark mudstones with rich organic
provided by the laboratory of College of Ocean                   matter.
Sciences, China University of Geosciences. Depo-
sitional sequences and systems tracts in the study
                                                                 Stacking patterns of parasequences
area were identified first, mainly based on seismic
reflection and termination patterns these were                    The LST, the focus of this study, is composed of
further constrained by wireline log and core data                three parasequence sets (Fig. 5A). Parasequence
along key seismic profiles. Stratigraphic and                     set 1, comprising three parasequences, is coarsen-
depositional cross-sections along and across the                 ing-upward and progradational. It is characterized
depositional dip were constructed to delineate                   by light-grey mudstones intercalated with light-
the vertical and lateral distribution of the large               grey siltstones, argillaceous siltstones and calcar-
lake beach-bar facies. The formation mechanisms                  eous siltstones. Parasequence sets 2 and 3 are
of large beach bars were investigated through the                progradational successions. Parasequence set 2
reconstruction of palaeogeomorphology, hydro-                    (Parasequences 4, 5 and 6) is characterized by grey
dynamics, lake level and sediment source                         mudstones intercalated with siltstones and Para-
systems. A beach-bar distribution model was                      sequence set 3 (Parasequences 7, 8 and 9) is
established in the Boxing Sag.                                   characterized by dark grey mudstones intercalated
                                                                 with grey siltstones. The top of Parasequence set 3
                                                                 is characterized by oil shales interbedded with
SEQUENCE STRATIGRAPHIC ANALYSIS                                  siltstones and calcareous siltstones. The inter-
                                                                 beach mudstones in Parasequence set 1 are varie-
                                                                 gated and light grey, whereas the normal shallow
Sequence boundaries
                                                                 lake mudstones above Parasequence set 1 (major
Based on the analysis of drilling core, well log                 flooding surface) are dark grey. The colour of the
and seismic data, the Es4s in the Boxing Sag was                 normal shallow lake mudstones above Para-
interpreted as a third-order sequence, which                     sequence set 2 (major flooding surface) is also
includes an LST, a transgressive systems tract                   darker than the inter-beach mudstones in Para-
(TST) and a highstand systems tract (HST)                        sequence set 2. Meanwhile, the sandstone percent-
(Fig. 3). The duration of the Es4s is ca 3 Ma                    age decreases from Parasequence set 1 to
(Zhang et al., 2005a; Dai & Zhang, 2006). In                     Parasequence set 3. The variation trends of mud-
seismic profiles, the top sequence boundary is                    stone colour and sandstone content indicate that
marked by the T6 reflectance surface that sepa-                   the water depth got a little bit deeper and the lake
rates the Es3 and Es4 members; the bottom                        level rose slowly from Parasequence sets 1 to 3,
sequence boundary is delineated by the T7                        which is coincident with the lake-level changes in
reflectance, a boundary separating the Es4s and                   the Dongying Depression during this period that
Es4x (the lower fourth member of the Eocene                      were described by Cao (2005). Therefore, the
Shahejie Formation). T6 and T7 are unconformi-                   beach-bars were most likely to have been deposited
ties along the basin margins where onlap and                     in the late LST, the slowly rising process of the lake
erosional truncation terminations are interpreted                level as suggested by Plint & Nummedal (2000).
on seismic sections; they are conformable sur-                      The TST is characterized by a fining-upward
faces towards the central basin (Fig. 4). T6 and T7              retrogradational parasequence set, which is com-
are also marked by abrupt changes in physical                    posed of four parasequences (Parasequences 10,
properties indicated by apparent changes of wir-                 11, 12 and 13 in Fig. 5B). The main lithology is
eline log patterns (Fig. 3). The internal boundary               mudstones and oil shale, locally with thin-bed-
between LST and TST is the first flooding surface                  ded sandstones. It is interpreted as a deep-water
(FFS), and the boundary between TST and HST is                   deposit as a result of a rapid rise in lake level.
the maximum flooding surface (MFS). The seis-                        The HST consists of three aggradational para-
mic reflections of the FFS and MFS are not well-                  sequence sets (Parasequence sets 5 to 7 in Fig. 5C).
defined, but apparently they are marked in log                    Each parasequence set is characterized by moder-
patterns due to the abrupt changes in lithology                  ately deep lake mudstones followed by shallow-
(Fig. 3). For example, the colour of mudstone                    littoral mudstones intercalated with beach-bar
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Fig. 3. Sequence stratigraphic subdivision scheme for the Es4s sequence in the Boxing Sag. Es4s can be considered
as a third-order sequence, which comprises three systems tracts with seven parasequence sets. Beach-bar sand
bodies, as a reservoir, were deposited in the LST period generally. The age data are derived from Jiang et al. (2007b)
and Pan et al. (2003). The lake level is from Cao (2005).

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Fig. 4. Seismic section (south-east to north-west) showing sequence boundaries (T6 and T7) and systems tracts
boundaries [first flooding surface (FFS) and maximum flooding surface (MFS)] in Es4s sequence. T6 and T7 are
unconformities along the basin margins. T7 is identified as a surface of local erosional truncations with strong
reflection amplitude, whereas T6 is identified as a surfaces of onlap termination. The FFS is marked by the first onlap
point and its corresponding strong reflection. The MFS is marked by the distal onlap point with its correspondent
strong reflection. Reflection terminations are highlighted by arrows. See Fig. 1 for seismic section location.


sandstones. In general, the thickness of mudstone                bedding, wave-ripple bedding (Fig. 7B), dwelling
is greater than that of sandstone.                               burrows (Fig. 7C) and pascichnia (Fig. 7D), and
                                                                 ripples (Fig. 7E and F). Phytoclasts (Fig. 7G) can
                                                                 also be found in the cores. The sandy beach
SEDIMENTARY FACIES ANALYSIS                                      deposits have tooth-like and funnel-shaped
                                                                 curves in the SP log (Fig. 6A) and have dark
In this study, four major facies, beach-bar (includ-             belts alternating with light belts in the FMI
ing sandy beach-bar and gravelly beach), deltaic                 (Formation     Micro-resistivity   Imaging     log)
front, shallow-littoral lake mudstones and mod-                  (Fig. 9).
erately deep lake mudstones (Fig. 6), are identi-                  The sandy bar mainly comprises fine-grained
fied and differentiated on the basis of their                     sandstone, medium sandstone, siltstone and a
lithologies, sedimentary structures (Fig. 7), wire-              small amount of pebbly sandstone (Fig. 6B).
line log patterns and facies successions (Fig. 6).               A single sandstone layer of the sandy bar is
Oil has been found in sandy beach-bar, gravelly                  thicker than that of the sandy beach (2 to 6 m)
beach and deltaic front deposits. The general                    (Fig. 9, sandy bar facies). The sedimentary struc-
distribution of these facies associations in the                 tures of the sandy bar include parallel bedding
Es4s is illustrated in Fig. 8.                                   (Fig. 7H), wedge-shaped cross-bedding, micro-
                                                                 wavy bedding and parting lineation (Fig. 7I) with
                                                                 rare phytoclasts in cores. The sandy bar shows
Sandy beach-bar facies
                                                                 broad-amplitude funnel-shaped or box-shaped SP
Description                                                      log curves (Fig. 6B) and light belts intercalated
The sandy beach is characterized by siltstone,                   with dark belts in the FMI (Fig. 9).
silty fine-grained sandstone and fine-grained                        The beach-bar sandstone is well-sorted and
sandstone, intercalated with mudstone (Fig. 6A).                 well-rounded. The grain-size accumulation prob-
The single layer of the sandy beach is usually                   ability curve has three sub-populations, two
<2 m thick and frequently is interbedded with                    saltation sub-populations and one suspended
mudstone layers (Fig. 9). The sedimentary struc-                 sub-population (Fig. 9). The saltation sub-popu-
tures of the sandy beach include swash bedding                   lation has two segments and is well-developed,
(Fig. 7A), wavy bedding (Fig. 7B), microwavy                     whereas the suspended sub-population is less
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A                                                               B




                                                                C




Fig. 5. Stacking patterns of parasequences in the LST (A), TST (B) and HST (C) of the Es4s sequence in the Boxing
Sag. The LST is composed of three coarsening-upward progradational sets, the TST has only one retrogradational
parasequence set and the HST has three aggradational sets. Two types of mudstones are present in the LST; one is the
inter-beach mudstone and the other is the normal shallow lake mudstone. The colour of the normal shallow lake
mudstones is darker than the inter-beach mudstones. The variation in trends of mudstone colour and sandstone
content indicate that the water depth got a little bit deeper and the lake level rose slowly from Parasequence set 1 to
Parasequence set 3 in the LST. See Fig. 1 for well locations.


developed, or even absent. The porosity of bar                      Interpretation
sandstone is 10 to 20% and the permeability is                      The well-sorted and well-rounded fine-grained
0Æ1 to 100 mD, whereas the porosity of beach                        sediments (including siltstone, silty fine-grained
sandstone is usually 5 to 15% and the permeabil-                    sandstone, fine-grained sandstone, etc.) indicate
ity is 0Æ01 to 10 mD (Fig. 9).                                      a relatively long transportation distance. The
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                    A                                    B                           C




                    D                             E                              F




Fig. 6. Typical facies successions of four facies of the Es4s sequence in the Boxing Sag. Beach-bars facies include
sandy beach (A), sandy bar (B) and gravelly beach (C). The fluvial-dominated deltaic facies includes an under water
distributary channel and mouth bar, i.e. deltaic front (D). Shallow-littoral lake mudstones include inter-beach
mudstones and normal shallow lake mudstones (E) in the lake-level rising. Oil shale and mudstone facies deposited
in a moderately deep lake (F). See Fig. 1 for well locations.

presence of well-developed swash bedding                         accumulation probability curve (Fig. 9) suggest
(Fig. 7A), wave-ripple bedding (Fig. 7B) and                     that the formation of the beach-bar was controlled
ripples (Fig. 7E and F) suggests that this facies                by multiple currents, including offshore and
was deposited by lake wave swash/agitation.                      wind driven currents. Moreover, the two seg-
The common appearance of parallel bedding                        ments of saltation sub-population indicate that
(Fig. 7H), ellipsoidal muddy pebbles (Fig. 7A),                  beach-bar was deposited on the swash backwash
parting lineation (Fig. 7I) and vertical dwelling                zone. Consequently, this facies is interpreted as a
burrows (Fig. 7C) in sandstones is interpreted as                sandy beach-bar deposited in a shallow-littoral
having been deposited in relatively strong hydro-                lake environment.
dynamics as suggested by studies from Fraser &
Hester (1977), Clemmensen (1978), Jiang (2003)
                                                                 Gravelly beach facies
and Tucker (2003). The existence of varie-
gated mudstone pebbles, terrestrial phytoclasts                  Description
(Fig. 7G) and ripples (Fig. 7E and F) on the                     The gravelly beach, characterized by conglomer-
stratification plane indicates that the depth of                  ate and pebbly sandstone (Fig. 6C), was distrib-
water was relatively shallow when the sandy                      uted on the south slope of the Boxing Sag, near
beach-bar deposited, as described by Rev (1999),                 the Luxi Uplift. The gravels, generally 2 to 30 mm
Jiang (2003) and Tucker (2003). The three sub-                   in grain size, are well-rounded (Fig. 7J). Observed
populations, two saltation sub-populations and                   sedimentary structures in this facies include
one suspended sub-population, in the grain-size                  massive bedding or graded bedding and fragments
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        A                                B                                       C


                                                                        WB




                                                                      WRB


        D                                E                                   F




        G                                H                                            I




        J                                K                                   L




        M                                N                                        O




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Fig. 7. Core photographs showing different sedimentary structures in different facies in the LST of the Es4s sequence
in the Boxing Sag. (A) Low-angle swash bedding in grey fine sandstones with greyish-green flat muddy pebbles. From
Well F143 at 3141Æ3 m. (B) Wavy bedding ‘WB’ and wave-ripple bedding ‘WRB’ in dark grey muddy siltstones. From
Well G893 at 3281Æ63 m. (C) Vertical dwelling burrows in dark grey silty fine sandstones. From Well G893 at
3206Æ1 m. (D) Pascichnia in muddy siltstones with reverse succession. From Well 143 at 3111Æ7 m. (E) Truncated
ripple mark on the stratification plane of dark grey siltstones. From Well B901 at 2462Æ4 m. (F) Ripples on the
stratification plane of grey sandstones. From Well B901 at 2434Æ1 m. (G) Terrestrial phytoclasts in the massive grey
white silty fine sandstones. The terrestrial phytoclasts are disorganized. From Well F143 at 3123Æ3 m. (H) Parallel
bedding in grey fine sandstones. From Well F134 at 2869Æ4 m. (I) Parting lineation on the stratification plane in fine
sandstones. From Well F137 at 3170Æ2 m. (J) Well-rounded pebbles in the gravelly beach. Note that the whitish
gypsum occurs among the pebbles. From Well J27 at 839Æ2 m. (K) Gastropod and bivalve fossils in the grey pebbled
sandstones. From Well J27 at 839Æ6 m. (L) Lenticular bedding. Dark grey mudstones intercalated with grey fine
sandstones. From Well H17 at 2467Æ4 m. (M) The gastropod fossils distributed on the stratification plane of
grey mudstones. From Well B104 at 2029 m. (N) Variegated silty mudstone in shallow-littoral lake. From Well B104
at 2159Æ5 m. (O) Black mudstone with horizontal bedding. From Well F119 at 3303Æ1 m. The diameter of the core is
10 cm. See Fig. 1 for well locations.




Fig. 8. Sedimentary facies map of the Es4s sequence in the Boxing Sag. The predominant facies were beach-bar
(including sandy beach-bar and gravelly beach), deltaic front, shallow-littoral lake mudstones and moderately deep
lake mudstones. The deltaic front was deposited on the west near the Qingcheng Bulge. The gravelly beach was
developed generally as a sheet, and the sandy bar as a narrow belt on the south slope of Boxing Sag. The sandy beach
occurred as a more continuous sheet-like body around the sandy bar distributed in the majority of the sag.


of gastropod and bivalve (Fig. 7K). In the SP log                foreland basin. The linear distribution of con-
curves, the gravelly beach presents tooth-like                   glomerates and existence of pebbly sandstones
box-shape or funnel-shape patterns (Fig. 6C).                    suggest that they were in situ deposits caused
                                                                 by swashing erosions upon the host rocks.
Interpretation                                                   The occurrence of reworked gastropod and
The well-rounded dark-red conglomerates (Fig. 7J)                bivalve shells (Fig. 7K) suggests that the deposi-
developed along the south basin margin experi-                   tional environment was extremely shallow.
enced a relatively short transportation distance                 Therefore, this facies is considered as the gravelly
and were then reworked by strong lake waves                      beach deposited in an extremely shallow envi-
and offshore currents, as suggested by Massari &                 ronment along the south basin margin near the
Parea (2006) in their study on the Messinian                     Luxi Uplift.
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                                                                                                                             Fig. 9. The comparison of sandy beach and sandy bar of Es4s in the Boxing Sag. The grain-size accumulation curve comprises three segments, with two
                                                                                                                             saltation sub-populations and one suspended sub-population. The porosity of bar sandstone is 10 to 20%, and permeability is 0Æ1 to 100 mD, whereas the
                                                                                                                             porosity of beach sandstone is usually 5 to 15%, and permeability is 0Æ01 to 10 mD. In the FMI (Formation Micro-resistivity Imaging log), sandy beach
                                                                                                                             sediments show the appearance of dark belts alternating with light belts, mingled with the shallow littoral lacustrine dark-coloured mudstone belts,
                                                                                                                             whereas the sandy bar sediments present an appearance of light belts intercalated with dark belts. The FMI log data come from Well G89-11, presented by
                                                                                                                             the Shengli oil field, SINOPEC.




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                                                                 mudstones with horizontal laminations (Fig. 7O)
Deltaic front facies
                                                                 were interpreted as normal shallow lake mud-
Description                                                      stones deposited in a relatively deeper water
Deposited in the western Boxing Sag near the                     environment during lake-level rising (Fig. 6E).
Qingcheng Bulge, the deltaic front facies is char-
acterized by fine-grained sandstone, silty sand-
                                                                 Moderately deep lake mudstones facies
stone and pebble sandstone, intercalated with
thin-bedded mudstone (Fig. 6D). The sand in the                  Description
deltaic front facies, generally 2 to 10 m thick, is              This facies is dominated by dark grey or black
light grey to grey and well-sorted and well-                     mudstones, oil shales and calcareous shales
rounded. The interbedded mudstones, generally                    intercalated with grey thin-bed sandstones,
<2 m, are light grey to dark grey and thinly                     locally with pebbly sandstones (Fig. 6F). In
laminated. Sedimentary structures include wavy                   comparison with the shallow-littoral lacustrine
bedding, wavy cross-bedding, flaser bedding and                   facies, this facies generally is thicker and better
lenticular bedding (Fig. 7L), as well as tabular                 laminated, with more organic matter and rare
cross-bedding and trough cross-bedding. In the                   bioturbations.
SP logs, this facies generally shows a funnel-
shape pattern (Fig. 6D).                                         Interpretation
                                                                 The dominance of dark-coloured, well-laminated
Interpretation                                                   mudstone and oil shale without bioturbation
According to the distribution and sedimentary                    suggests that they formed in a persistent stratified
characteristics, for example, lithology and sedi-                and anoxic water-column, with extremely rare
mentary structures, this facies is considered as a               interruption by low-density turbidity deposits.
fluvial-dominated delta depositing in the shallow                 This scenario may be analogous to that of modern
lake, as suggested in studies by Jiang (2003) and                Lake Tanganyika, Africa, where the lake water is
Yuan (2007). The grey sandstones with wavy                       strongly stratified and permanently anoxic below
bedding, flaser bedding, lenticular bedding                       depths of 150 to 250 m as reported by Cohen
(Fig. 7L), wavy cross-bedding, tabular cross-bed-                (1989, 1990) and Huc et al. (1990). The occur-
ding and trough cross-bedding are interpreted as                 rence of this facies further supports a
the microfacies of a distal bar, mouth bar and the               permanently quiescent, moderately deep lake
distributary channel of a deltaic front (Fig. 7D).               environment that allowed the accumulation of
                                                                 significant quantities of muddy sediments and
                                                                 organic matter.
Shallow-littoral lake mudstones facies
Description
This facies is characterized by greyish-green to                 PALAEO-DIP AND PALAEOWATER
light-grey mudstones (Fig. 6E). Two types of                     DEPTH ANALYSIS
mudstone are present. One is thin (2 m thick)
and generally incorporates terrestrial phytoclasts
                                                                 Palaeo-dip
and gastropod fossils (Fig. 7M). Its colour is
variegated to light grey (Fig. 7N). The other                    Methodology
mudstone is thick (ca 2 to 10 m thick). Horizontal               Palaeo-dip can be reconstructed by integrating
laminations are observed inside (Fig. 7O). Lateral               structure and sedimentary analysis as described
distributions of both mudstones are stable.                      in studies by Deng et al. (2001), Jiang (2003), Jiang
                                                                 et al. (2007b), Zhao et al. (2003) and Lu et al.
Interpretation                                                   (2005). In the present study, the palaeogeomor-
The dominance of mudstones in this facies                        phology of the LST in the Es4s was reconstructed
indicates a low-energy environment, as suggested                 by means of five steps: (i) interpreting horizons
by Rev (1999) and Jiang (2003). However, those                   (T7, T6, FFS and MFS) on seismic data and well
two types of mudstones were deposited in differ-                 logs and creating an isochron map (two-way
ent environments. The thin-bed variegated to                     travel time thickness) for the LST (between T7
light-grey mudstones with terrestrial phytoclasts                and FFS); (ii) calculating an isopach map (Fig. 10
were interpreted as inter-beach mudstones depos-                 A) (true vertical thickness) for the LST (average
ited in a low-energy environment between beach-                  interval velocity was calculated from offset well-
bars, whereas the thick-bed light grey to grey                   sonic logs and checkshot data); (iii) calculating
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1100    Z. Jiang et al.

  A




  B




  C




Fig. 10. (A) Contour line map of the true vertical thickness of the LST in the Es4s sequence. (B) Contour line map of
the original thickness map of the LST in the Es4s sequence. (C) Average topographic slope of the LST in the Es4s
sequence in the Boxing Sag. The basin topography was very gentle and the palaeo-slope was between 0Æ5° and 2°,
below 0Æ5° on the south slope and 0Æ5° to 1Æ5° on the north zone near the fault generally.

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                                                        Large-scale lacustrine beach-bars in North China              1101

the true stratigraphic thickness of the LST, based               key index for reconstructing the palaeowater
on the present-day strata dip interpreted on                     depth by Wang et al. (1996), Zhuang & Sven
seismic profiles and well log data with Eq. 1;                    (2001), Xu & Awramik (2002), Zhang et al. (2007)
(iv) reconstructing the original thickness of the                and Tang et al. (2008), because: (i) benthonic alga
LST (Fig. 10B) based on porosities and other rock                are sessile colonies and thus represent the
properties from well logs and cores with the                     original depositional location once being pre-
decompaction method used by Allen & Allen                        served; and (ii) different species of benthonic
(1990), Qi & Yang (2001) and Yang & Qi (2003);                   algae live at different depths, and thereby can
and (v) calculating the palaeo-dip of the LST                    indicate a reliable indicator of water depth
(Fig. 10C) depending on the original thickness                   collectively.
data using the method as suggested by studies
from Zhao et al. (2003). In this study, the recovery             Description
of the amount of strata denudation was not run in                Abundant parallel beddings, ellipsoidal muddy
reconstructing the original thickness, because the               clasts and parting lineation (Fig. 7D to F) are
LST was not eroded:                                              present in cores in the LST of the Es4s in
                                                                 the Boxing Sag. Variegated mudstone pebbles,
                     h ¼ h0 Â cos a                    ð1Þ       layered terrestrial phytoclasts (Fig. 7G and H) and
                                                                 the ripples (wave-generated, composite and mod-
where h is the true stratigraphic thickness, h¢ is               ified ripples; Fig. 7L and M) on the stratification
the true vertical thickness and a is the present-                plane can also be found normally in the cores.
day strata dip.                                                  Furthermore, fossils and trace fossils including
                                                                 micro-algae, benthonic algae (Fig. 11) and ostra-
Description                                                      coda in the LST of the Es4s had been found in
During the deposition of the LST in the Es4s, the                the cores of several wells. For example, a colony
slope of basin was between 0Æ5° and 2°, generally                of blue-green algae micelles was found at
below 1Æ5°. Furthermore, the south slope (being                  2447Æ4 m in Well G351, and a colony of red algae
below 0Æ5° generally) was gentler than the north                 was found at 3199Æ6 m in Well G893 (Fig. 11).
zone (being between 0Æ5° and 1Æ5° generally) near
the fault (Fig. 10C).                                            Interpretation
                                                                 Abundant parallel bedding, ellipsoidal muddy
Interpretation                                                   clasts and parting lineation (Fig. 7D to F) indi-
This basin topography was probably associated                    cated a relatively strong hydrodynamic condition,
with the tectonic activity and the basin structure               and the presence of variegated mudstone pebbles,
of the Boxing Sag. Firstly, a relatively slow and                layered terrestrial phytoclasts (Fig. 7G and H) and
low-magnitude subsidence of hangingwall in the                   the ripples (wave-generated, composite and mod-
initial stage of graben (or half-graben) basin                   ified ripples; Fig. 7L and M) on the stratification
formation made the topography gentle on the                      plane indicated that the depositional depth was
whole (Zhang et al., 2005c). Secondly, the Boxing                relatively shallow (<15 m) as described by Cao
Sag being a half graben in the north/south direc-                (2005) when the beach-bar was deposited. Com-
tion, with the main fault, i.e. the Chunhua-                     paring the available data of ostracoda, micro-algae
Caoqiao Fault in the north, resulted in the south                and benthonic algae with modern species, such as
slope being gentler than the north zone.                         those described in studies by Sprachta et al.
                                                                 (2001) about the Tikehau atoll in French Polyne-
Palaeowater depth                                                sia and Herzschuha et al. (2005) on the Qilian
                                                                 Mountains (North-west China) and other research
Methodology                                                      results, the depth of every fossil species was
Some sedimentary structures (Fraser & Hester,                    estimated individually (Table 1), then the overlap
1977; Clemmensen, 1978; Thompson, 1992; Pat-                     depth interval was defined as the palaeowater
ranabis-Deb & Chaudhuri, 2007; Helland-Hansen,                   depth.
2010), and certain fossils and trace fossils (being                By integrating the observed sedimentary struc-
depth dependent) (Clemmensen, 1978; Buatois &                    tures and palaeontology, the average palaeowater
  ´
Mangano, 1998; Smith & Jacobi, 2001; Li et al.,                  depth of the LST in several wells was estimated
2005; Scherer et al., 2007), are important indica-               and the palaeobathymetry map in the sag was
tors of the hydrodynamics and water depths. For                  reconstructed (Fig. 12). The palaeowater depth
example, benthonic algae were considered as a                    during the LST is shallow, <10 m for most of the
Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists, Sedimentology, 58, 1087–1112
1102       Z. Jiang et al.

  A                                                            B




       200 µm                                                        40 µm




  C                                                              D




       200 µm                                                        40 µm




Fig. 11. Microscopic characteristics of benthonic algae. (A) Red algae mat. From Well B901 at 2429 m. (B) Colony of
blue-green algae micelle. From Well G351 at 2447Æ4 m. (C) Cystocarp of red algae. From Well F143 at 3120Æ7 m. (D) Frond
of red algae. From Well G893 at 3199Æ6 m. See Fig. 1 for well locations. Fluorescence microscope: Nikon Eclipse
E600WPOL; Lamp: Nikon LHS-H100C-1; Imaging device: Nikon digital camera DXM1200 (Nikon, Tokyo, Japan).


time in the study area. On the south gentle slope,              Therefore, each parasequence set lasted ca
the water depth was between 2 and 8 m, whereas                  233 000 years and each parasequence lasted for
on the north zone near the Chunhua-Caoqiao                      ca 77 000 years (Fig. 3). One stage of beach-bar
Fault the water depth was generally between 8                   deposition essentially reflects one cycle of lake-
and 10 m. This scenario is similar to that of the               level change from deep to shallow, as described
Erhai Lake, a modern rift lake in the Yunnan                    by Dott & Mickelson (1995) at the two rivers of
Province of South-west China, where the water                   Wisconsin. The depth of the water was relatively
depth is usually <20 m, and on the gentle slope                 deep when the deposition of the beach-bar began.
the value is usually 2 to 4 m as described by                   It then decreased to only several or even zero
studies from Eisma et al. (2000), Zhu & Wang                    metres when deposition was complete (Fig. 13).
(1994).
  Based on the same data, the changing process of
the palaeowater depth was reconstructed for the                 DISTRIBUTION OF BEACH-BARS WITHIN
Parasequence set 3 in the LST in Well G351                      THE LOWSTAND SYSTEMS TRACT
(Fig. 13). Parasequence set 3 includes three para-
sequences, representing three stages of beach-bar               During deposition of the LST in the Es4s, beach-
deposition. The LST lasted ca 700 000 years.                    bars were developed widely in the Boxing Sag.
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                                                        Large-scale lacustrine beach-bars in North China              1103
Table 1. Palaeowater depths estimated from paleontological markers for the LST in the Es4s sequence in the Boxing
Sag. Some data are collated from Sprachta et al. (2001), Herzschuha et al. (2005) and Geological Institute of Shengli
Oilfield, SINOPEC.

                                                                                      Probable                   Probable
                                                                                      minimum                    maximum
Main class                       Subclass                                             depth (m)                  depth (m)
Microalgae                       Chareae                                               0                          2
                                 Green alga                                            0                          5
                                 Pediastrum/Leiosphaeridia                             0                          2
                                 Granodiscus/Dictyotidium                              4                         10
                                 Dinoflagellate                                        10                         20
Benthonic algae                  Homoneneae mat                                        8                         26
                                 Cyanophyta lamina                                    13                         26
                                 Red algae                                             5                         30
                                 Bryophyte                                             0                          1
Ostracoda                        More individuals; less species                        0                          3
                                 None                                                  3                          5
                                 High biodiversity and abundance                       7                         13
                                 Less individuals; more species                       13                         20
                                 Less individuals and species                         20                         30
                                 Australocypris: less                                  4                          7
                                 Australocypris: more                                  4                         10




Fig. 12. Palaeo-bathymetry map of the LST in the Es4s sequence in the Boxing Sag. The basin was quite shallow. The
palaeowater depth was generally <10 m; 2 to 8 m on the south slope and 8 to 10 m on the north zone near
the Chunhua-Caoqiao Fault.

The gravelly beaches were deposited on the south                 924 km2. Multiple beach-bar sand bodies are
slope, showing sheet-like depositional bodies                    superimposed vertically and amalgamated later-
along the Luxi Uplift. In contrast, sandy bars                   ally. Individual sand bodies are cumulatively
occurred as a belt, which generally was sur-                     thicker than 20 m, and extend laterally for 5 to
rounded by more consistent sandy beaches                         25 km (average 10 km). In Parasequence set 1,
(Fig. 8).                                                        sandstones and conglomerates with a cumulative
  Beach-bar sand bodies are distributed widely in                thickness of >10 m account for over 20% of
Parasequence sets 1, 2 and 3 of the LST in the                   lithological columns. In the zone where the
Boxing Sag. These sand bodies cover ca 50 to                     sandstones and conglomerates are well-depos-
70% of the Boxing Sag, approximately 660 to                      ited, their cumulative thickness is over 20 m,
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                                                                                                                             Fig. 13. The lake-level oscillations of Parasequence set 3 in the LST in the Boxing Sag, derived from Well G351. Parasequence set 3 comprises three
                                                                                                                             parasequences, with each parasequence representing a beach-bar development phase. One phase of beach-bar deposition essentially reflects one cycle of
                                                                                                                             lake-level change from deep to shallow. The water depth was estimated based on the sedimentary structures, lithology and palaeontology.




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                                                        Large-scale lacustrine beach-bars in North China              1105

accounting for 60% of the lithological column                    The formation of large-scale beach-bars in the
(Fig. 14). During this period, sandy beach-bar,                  LST in the Boxing Sag was related to the unique
gravelly beach, deltaic front and shallow-littoral               palaeogeomorphology, palaeowater depth, sedi-
lacustrine mudstones predominated over the                       ment source system and the lake level.
lake. Sandy beach-bars were developed in the
lake centre, covering ca 35% of the lake. The belt
                                                                 Palaeogeomorphology
sandy bars were surrounded by sheet beach sand.
The continuous gravelly beach was confined to                     Palaeogeomorphology is known to have an appar-
the south slope surrounding the Luxi Uplift,                     ent control on sedimentation as described by
covering ca 25% of the lake (Fig. 15). Para-                     Galloway (1998), Paul et al. (1998), Lemons
sequence sets 2 and 3 inherit the sedimentary                    (1999), Jiang et al. (2007b) and Zou et al. (2008).
characteristics of Parasequence set 1 (Figs 14 and               Based on the study of modern beach-bar deposi-
15). Nevertheless, there were some minor differ-                 tion in a faulted lake basin, Soreghan & Andrews
ences. From Parasequence sets 1 to 3, the cumu-                  (1986) proposed that beach-bars may be well-
lated thickness and grain size of sandstones and                 developed on gentle slopes of lacustrine basins. It
conglomerates decreased generally, the area of                   was believed that a gentle foreshore area is a
beach-bars became reduced and the gravelly                       favourable zone for the deposition of beach-bars,
beach facies retreated to the Luxi Uplift (Fig. 14).             as discussed in studies by Sun et al. (2003) and
  Although beach-bar sand bodies were devel-                     Yuan (2007). In the present study, the distribu-
oped in all systems tracts of the Es4s in the                    tion of beach-bars in the LST of the Es4s matched
Boxing Sag, their distribution was quite different               very well with the basin topography (Figs 8, 10
(Fig. 16). Relatively, the LST period was the most               and 14), being on the south slope and basin
favourable period for the formation of beach-bar                 centre, where the slope was below 1Æ5°, generally
sand bodies, during which multistorey beach-bar                  below 1Æ0° (Fig. 10C). Therefore, the favourable
sand bodies developed, generating progradational                 zone of the beach-bar system in a basin can
parasequence sets. During the TST and HST                        effectively be forecast to determine lithological
periods, beach-bar sand bodies, although being                   traps on the basis of research on the relationship
developed, were restricted; they are generally                   between palaeogeomorphology and beach-bar
thinner than those formed within the LST, form-                  formation.
ing retrogradational or aggradational parase-
quence sets. Figure 16 illustrates a sedimentary
                                                                 Hydrodynamic environment
section through Wells J22, F127, F140 and F148
in north/south direction, showing the vertical and               The hydrodynamic environment is a key factor
lateral distribution of beach-bars. In this lake                 controlling the formation of the beach-bar, as
basin, a beach-bar system developed extensively                  discussed in studies by Graham (2000), Hou
in the LST stage, which retreated to the narrow                  et al. (2005), Wu et al. (2006), Pan (2007) and
shoreline zone, i.e. on the southern flank during                 Wang et al. (2008b). The shallow-littoral zone of
the later TST and HST stages. Gravelly beach                     a lake has a strong and complex hydrodynamic
sediments were dominated by conglomerates on                     condition. The lake waves and the associated
the southern flank (for example, in Well J22), and                offshore currents may be powerful enough to
gradually changed into sandy beach-bar sedi-                     alter the shoreline and rework the sediments, as
ments northward. This case example with such                     suggested by studies from Keighley et al. (2003),
large-scale extensive beach-bars developed in the                Jiang (2003), Zhang et al. (2006) and Zhou
lacustrine basin is quite different from the pre-                (2008), and thus facilitates the formation of
vious understanding of beach-bar sand bodies,                    beach-bar sand bodies. Based on the analysis
which generally are limited in distribution, as                  above, the basin of the LST had a relatively
described by studies from Zhu et al. (1994), Gao                 strong hydrodynamic condition, and the average
et al. (2003), Sun et al. (2007) and Wang et al.                 palaeowater depth for the LST of the Es4s was
(2008b).                                                         usually <10 m; 2 to 8 m on the south slope and 8
                                                                 to 10 m on the north zone near the fault
                                                                 (Fig. 12). Therefore, during the LST period, the
DISCUSSION                                                       basin was basically in a shallow environment
                                                                 with strong hydrodynamics, which contributed
Compared with the TST and HST, the LST                           greatly to the deposition of the beach-bar sand
contains well-developed large-scale beach-bars.                  bodies.
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                                                                                                                             Fig. 14. Isopach maps of sandstone and conglomerate thickness and sandstone and conglomerate content maps for three parasequence sets of the LST in
                                                                                                                             the Es4s sequence in the Boxing Sag. Generally, sandstones and conglomerates with a cumulative thickness of >10 m account for over 20% of lithological
                                                                                                                             columns.




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                                                        Large-scale lacustrine beach-bars in North China              1107




Fig. 15. Sedimentary facies for Parasequence sets 1 to 3 in the LST. The main facies includes sandy beach-bar,
gravelly beach, deltaic front and shallow-littoral lake mudstones. Gravelly beach facies were developed on the south
slope, showing a sheet body adjacent to the Luxi Uplift. Sandy bar facies occurred as belts/strings surrounded by
sandy beach facies. From Parasequence sets 1 to 3, the area of beach-bars tended to be reduced, and the gravelly
beach retreated to the Luxi Uplift.

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                                                                                                                             Fig. 16. Sedimentary cross-section from Wells J22 to F148 along a north/south direction in the Boxing Sag. During the LST, multistorey beach-bar sand
                                                                                                                             bodies developed, generating progradational parasequence sets, whereas, during the TST and HST, beach-bar sand bodies were restricted in develop-
                                                                                                                             ment. See Fig. 1 for section location.




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                                                        Large-scale lacustrine beach-bars in North China              1109

                                                                 starved, and formed dark grey or black mudstones
Sediment source
                                                                 and oil shales intercalated with grey thin-bed
Lacustrine clastic beach-bars may be fed by                      turbidite sandstones in deep water in the TST
deltas, braided deltas and fan deltas as described               (Figs 3 and 5B). As the upper section of the HST
by Pan (2007), Zhang et al. (2007), Wang et al.                  had been eroded by sequence boundary T6, the
(2008b), Zhang et al. (2008) and Zhou (2008).                    lower section of the HST was preserved during
During deposition of the LST of the Es4s in the                  the period of slow lake-level rising (Fig. 3) and
Boxing Sag, the deltaic front occurred on the west               the sediment supply was also insufficient, result-
as a point source (Figs 14 and 15), partly control-              ing in less-developed beach-bars (Figs 3 and 5C).
ling the formation of the sandy beach-bars nearby,
i.e. near Well G89. Deltaic front sand bodies
could have been reworked by wave currents,                       CONCLUSIONS
generating multistorey beach-bar sand bodies.
However, on the southern flank, a large linear                    The Boxing Sag was characterized by a gentle
source was present along the Luxi Uplift with                    palaeoslope, and a shallow depositional environ-
abundant and persistent sediment supply.                         ment with a strong hydrodynamic setting, pro-
Repeated erosion, reworking and transportation                   viding favourable conditions for the buildup of
by waves and offshore currents resulted in the                   large-area beach-bars. The sandy beach-bars are
deposition of a gravelly beach attached to the                   characterized by siltstone in the Boxing Sag, silty
uplift and the deposition of offshore sandy beach-               sandstone and fine-grained sandstone, interbed-
bars (Figs 14 and 15). The linear sediment source                ded with some thin mudstone units. The pres-
system along the uplift provided abundant mate-                  ence of sedimentary structures, including swash
rials to build up a large-scale beach-bar system                 bedding, parting lineation, parallel bedding, rip-
over a large area in the Boxing Sag.                             ples, muddy rip-up clasts, terrestrial phytoclasts
                                                                 and vertical burrows, suggests that the beach-bars
                                                                 were deposited in a relatively shallow water
Lake-level oscillations
                                                                 environment with a strong reworking by waves.
Lacustrine shallow-littoral sand bodies commonly                 Laterally, the sandy bars generally occur as belt/
occur along the shoreline, where oscillations of the             string sand bodies surrounded by, more or less,
lake level can drive the migration of shorelines                 continuous sandy beaches. Stratigraphically, the
back and forth, thereby controlling the lateral                  beach-bars mainly occurred within the lowstand
distribution of the shallow-littoral sand bodies as              systems tract, being characterized by relatively
described by Yuretich et al. (1984), Cui et al.                  thick multistorey sand bodies. The occurrence of
(2005), Hou et al. (2005), Wu et al. (2006), Zhang               a large-scale beach-bar system may have been
et al. (2007) and Xiang et al. (2008). The high-                 caused by: (i) a relatively gentle palaeoslope
frequency cycles may be relevant to the Milankov-                (mostly 0Æ5° to 2°); (ii) a strong wave reworking;
itch cycles according to studies by Wang et al.                  (iii) high-frequency oscillations of the lake levels;
(2008a) and Zhang et al. (2008). In the present                  and (iv) abundant sediment supply. This study
study, the water depth changed rapidly (Fig. 13)                 provides a useful analogue for beach-bar reser-
and the shoreline moved frequently during each                   voirs in graben (or half-graben) lacustrine basins
parasequence set, which most probably represents                 in east China, where beach bar reservoir plays
an overfilled lake as suggested by Carroll & Bohacs               may have been developed in a similar tectono-
(1999, 2001) and Bohacs et al. (2000). Against a                 depositional setting.
background of gentle palaeogeomorphology, the
changes in the lake level led to a wide planar
movement of the shoreline. The repeated oscilla-                 ACKNOWLEDGEMENTS
tions of the lake levels caused the beach-bars to be
stacked and laterally distributed over a large area in           This work was co-funded by the National Natural
the Boxing Sag.                                                  Science Foundation of China (Grant No.
   Due to the four favourable conditions above, the              40672078), Yangtze River Scholar and Innovative
large-scale beach-bars were developed widely in                  Team Development Plan of the Ministry of Edu-
the LST in the Boxing Sag. However, beach-bars                   cation of China (Grant No. IRT0864) and College
were restricted during the TST and HST periods.                  Doctoral Program Foundation from the Ministry
In the TST, the lake level rose quickly (Fig. 3) and             of Education of China (Grant No. 200804910004).
sediment supply was rare. The lake basin was                     The authors thank the Geoscience Institute of the
Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists, Sedimentology, 58, 1087–1112
1110     Z. Jiang et al.

Shengli Oilfield, SINOPEC, for permission to                         slope of the Dongying subbasin, northern Shandong, China.
access their in-house database. Thanks are also                     Geol. Bull. China, 25(9–10), 1168–1174 (in Chinese with
                                                                    English abstract).
extended to Prof. D. Z. Chen of the Institute of                 Deng, H.W., Wang, H.L. and Wang, D.Z. (2001) Control of
Geology and Geophysics of the Chinese Academy                       paleo-morphology to stratigraphic sequence in continental
of Sciences and Dr H. B. Lu of the Shell E&P                        rift basins: take lower Tertiary of western slope in Bozhong
Company, for their constructive advice about the                    Depression an example. Oil Gas Geol., 22(4), 293–296 (in
framework of this study.                                            Chinese with English abstract).
                                                                 Dott, E.R. and Mickelson, D. (1995) Lake Michigan water
                                                                    levels and the development of Holocene beach-ridge
                                                                    complexes at Two Rivers, Wisconsin: stratigraphic, geo-
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  and Application. Blackwell Scientific Publications, Oxford.        mentation in Erhai Lake, Yunnan Province, China. J. Lake
Basilici, G. (1997) Sedimentary facies in an extensional and        Sci., 12(1), 25–37.
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                                                                 accepted 20 August 2010
  description technology of beach-bar sand in the gentle slope




  Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists, Sedimentology, 58, 1087–1112

				
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