Typical earthquake-induced soft-sediment deformation structures in

Transcription

Journal of Palaeogeography
2012, 1(1): 71-89
DOI: 10.3724/SP.J.1261.2012.00007
Palaeoseismology
Typical earthquake-induced soft-sediment deformation
structures in the Mesoproterozoic Wumishan Formation,
Yongding River Valley, Beijing, China and interpreted
earthquake frequency
Su Dechen1, 2, *, Sun Aiping1, 2
1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
2. State Key Laboratory of Continental Tectonics and Dynamics, Beijing 100037, China
Abstract
The Mesoproterozoic Wumishan Formation, composed of dolomite is a widely
distributed stratigraphic unit in the Beijing area. It was formed over a long period of time in the
Yan-Liao aulacogen, a stable peritidal environment that was ideal for recording earthquakes
in the form of soft-sediment deformation structures (SSDS). Numerous examples occur in the
upper part of the Wumishan Formation, along the Yongding River Valley. In addition, brittle
structures include intrastratal fault and seismically cracked breccias. The soft-sediment deformation structures include liquefied features (diapirs, clastic dykes, convolute bedding),
compressional deformation features (accordion folds, plate-spine breccias, mound-and-sag
structures), and extensional plastic features (loop-bedding). Based on the regional geological
setting and previous research, movements along the main axial fault of the Yan-Liao
aulacogen are considered as the triggers for earthquakes since the Early Mesoproterozoic.
The number and distribution of the SSDS suggest the major earthquake frequency in the
Wumishan Formation of 20 to 32 thousand years.
Key words
soft-sediment deformation structures, Mesoproterozoic, Wumishan Formation,
Yongding River valley, China, earthquake frequency
1
Geological background
The Wumishan Formation is the most widely distributed stratigraphic unit in the Beijing area, making up
about 12% of the total area (Fig. 1). It is well exposed in
the Yongding River Valley, where it is about 1800 m thick.
It consists mainly of carbonate rocks (89%) and silicified
carbonate rocks or siliceous rock. Many well-preserved
sedimentary structures indicate deposition in a stable and
shallow peritidal environment.
* Corresponding author: Research Professor.
Email: [email protected].
Received: 2012-02-06 Accepted: 2012-03-29
Study of seismically induced soft-sediment deformation structures (SSDS) in the Wumishan Formation
began with a description of plate-spine breccias in an
outcrop near the Ming Tombs, in the Beijing area (Song,
1988). Somewhat later Liang et al. (2002) discovered a
seismic collapse mega-breccia in Laiyuan County, and
Zhang et al. (2007) found two layers containing earthquake- triggered mound-and-sag structures, hydraulic
shattering and liquefaction-induced dykes in the Yesanpo
Geological Park. Qiao et al. (2007a) discussed the relationship between these deformation features and the palaeogeographical framework of the Yan-Liao aulacogen,
and suggested that they reflect splitting of the North China
block from its larger precursor continent between 1600 and
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Fig. 1 Location map of the study area
A Simplified geological map, showing the distribution of the Mesoproterozoic Wumishan Formation and outcrop locations of SSDS
(After Beijing Geological Survey, 1991); B Satellite picture from Google Earth; C Geographic location of the study area
Liang et al. (2009) described sedimentary structures
induced by palaeoearthquake and tsunamis in a se-
quence at the Yesanpo Geological Park near Beijing,
and estimated the earthquake epicenter and intensity by
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Su Dechen et al. : Typical earthquake-induced soft-sediment deformation structures in the Mesoproterozoic
Wumishan Formation, Yongding River Valley, Bejing, China and interpreted earthquake frequency 73
comparing this succesion with seismic collapse megabreccias in Laiyuan County near Beijing. Ettensohn et
al., (2011) compared the SSDS of the Wumishan Formation with those of the Late Ordovician Lexington
Limestone in the US. They especially examined
sag-and-mound structures and so-called “lenticular
pods” in the Wumishan Formation and proposed that the
so-called “plate-spine breccias” be renamed “accordion
folds”, which, they suggest might be used as a new in①
dicator of seismic activity .
Our study of the SSDS in the Wumishan Formation
began in 2010, and we have now investigated all good
exposures of this unit in the Beijing area. Here we focus
on the SSDS from Zhuanghuwa Village to the Pearl Lake
along the Yongding River. The classification and interpretation of the various SSDS are based mainly on the
work by Liang et al.(1991,1994); Du and Han (2000),
Qiao et al., (2006), Montenat et al. (2007), Qiao and Li
(2009), van Loon (2009) and Owen (2011) .
A preliminary description of the SSDS in the study area was published in the Journal of Palaeogeography (in
Chinese) (Su and Sun, 2011). Since then, the authors
have carried out additional fieldwork and found more
SSDS. The present contribution is therefore an updated
version of the earlier publication.
The study area lies about 80 km west of Beijing City,
and extends from Zhuanghuwa Village to Pearl Lake
along the Yongding River. Most of the strata in this area,
which belong to the upper Wumishan Formation (Fig. 1),
strike 60o NW and dip 20o30o NE, although at Pearl
Lake the beds are almost horizontal. Because the
Yongding River Valley is deeply incised and strongly
scoured, some of the exposed sections have become
well-polished, resulting in perfectly observable SSDS.
Typical outcrops of SSDS are marked in Fig. 1 with red
dots.
The Zhuanghuwa section is located about 5 km from
National Road 109 (Site 1 in Fig. 1) at 115°4936  E longitude and 40°252 N latitude. The 20 m-thick section
consists chiefly of light grey, medium- to thick-bedded
carbonates interbedded with lesser amounts of dark,
thin-bedded, siliceous rock. Shallow water sedimentary
structures, such as ripple marks, hail marks, crossbedding
and stromatolites are common. Nearly 30 layers containing SSDS have been found in this section and the main
ones are marked in Fig. 2.
The Zhuwo Station crops out on the west bank of the
Yongding River (Site 2 in Fig.1) at 115°4758 E longitude and 40°33.4 N latitude. The typical SSDS there is
loop bedding.
Y606 Road is a very narrow country road connecting
the Zhuwo Village to two other small villages. The SSDS
outcrop is located about 1 km from Zhuwo Village along
this road, where extensional bedding, loop-bedding,
plate-spine breccias and mound-and-sag structures are
well exposed (Site 3 in Fig. 1). The coordinates are
115°478 E longitude and 40°226 N latitude.
The outcrop near Pearl Lake (Site 4 in Fig. 1) is another good place for observing SSDS in the Wumishan
Formation. Its coordinates are 115°4657 E longitude
and 40°30 N latitude. The main SSDS there include
convolute bedding, plate-spine breccias, loop bedding,
intrastratal faults, and seismically shattered breccias.
2
SSDS formed in a brittle state
The main SSDS in the study area that were formed by
brittle deformation are intrastratal faults and seismically
fragmented breccias.
2.1
Intrastratal faults
Intrastratal faults, which are typically regarded as direct evidence of earthquakes in the geological past, have
been discovered at several locations in the study area,
with displacements from 1.5 cm to nearly 1 m.
The largest intrastratal faults occur at site 4 in Fig. 1.
As illustrated in Fig. 3, one pair of intrastratal reverse
thrusts does not extend beyond Unit I and Unit II. The
displacement on the righthand fault is nearly 100 cm,
whereas that on the lefthand segment is about 30 cm,
forming a mini-graben that was filled with the sediments
from Unit III. Obviously, this pair of thrust faults was
formed after Unit II had been deposited. The lefthand
fault dips about 45, whereas the one on the right dips
about 15.
Another thrust fault occurs in the upper part of the
Zhuanghuwa section (Fig. 4), where it is restricted to a
single layer and has a displacement of about 50 cm. The
dip is 16 relative to the strata.
The smallest intrastratal fault was found in the uppermost part of the Zhuanghuwa section. It is a small normal
fault with a displacement of only 1.5 cm, but is well preserved (Fig. 5).
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①
When thin-layered, unconsolidated (or weakly consolidated) carbonate rocks are placed under compressional stress, they form accordion
folds. So we agree with the name of accordion fold. However, if the accordion folds break at the ends, we prefer to use the term of
“plate-spine breccias”. Please refer to 3.3 of this article for detail.
2.2
Seismically fragmented breccias
A special layer of in situ breccia occurs in the lower
part of the Zhuanghuwa section, where three outcrops of
the breccia can be seen in an area of less than 2 m2
(Fig.6). The breccia fragments are all angular, and vary
from several mm to over 10 cm in size. The laminations
on the fragments can be perfectly correlated. These
breccias are very similar to the autoclastic breccias reported by Montenat et al. (2007) in their Fig. 24C, and
are typically taken as evidence of ancient earthquakes
(Qiao et al., 2006; Qiao and Li, 2009).
Two layers of such breccia also occur in the Zhuanghuwa section (Site 1 in Fig. 1), and a similar layer is
found at the Pearl Lake site (Site 4 in Fig. 1).
One special layer containing mega-breccia occurs in
the Y606 outcrop. All of the breccia fragments are ~5 cm
in size, and can be easily correlated with one another.
Clearly, these rocks were already half-consolidated prior
to being shattered by an earthquake (Fig. 7). Another
similar layer of mega-breccia has been found in the Pearl
Lake section (Site 4 in Fig. 1).
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Fig. 2 Positions of the main SSDS in the Wumishan Formation in the Zhuanghuwa section, Yongding River Valley
The earthquakes intensities are estimated from the density of the SSDS at each level.
Fig. 3 A ground fissure formed by two syndepositional reverse faults in the Wumishan Formation at Zhuwo Village (Site 4 in Fig. 1)
This is a mini-graben produced by a pair of thrust faults that was filled with sediments from Unit III. The white line represents the
previous sedimentary surface. The GPS coordinates are given in the top-left corner.
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Fig.4 Intrastratal thrust fault in the upper part of the Zhuanghuwa section
The yellow ruler is 5 cm. This thrust fault is limited to a single layer.
Fig. 5 Intrastratal micro-fault in the Zhuanghuwa section
A A general view of the micro-fault. Note the ripple mark in the bottom layer, indicating the environment of formation;
B A close-up view of the micro-fault.
3
SSDS formed in a plastic or liquefied
state
SSDS have been found throughout the Wumishan
Formation in Beijing area. They are especially abundant
in the study area. Typical seismically-induced SSDS include deformation features formed in a liquefied state
(diapirs, clastic dykes and convolute bedding), compressional features (accordion folds, mound-and-sag structures), and extensional feature (loop bedding).
3.1
Diapirs and clastic dykes
Four layers with diapiric structures (C, I, S, X in Fig. 2)
and two layers with clastic dykes occur in the Zhuang-
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huwa section.
The diapirs range from 23 cm up to 40 cm in height
(Figs. 8, 9). All of them are composed of dark-colored
silicified carbonates, and intrude vertically upward (occasionally downward) into the overlying (or underlying)
light grey dolomite.
The largest diapir (40 cm high and 50 cm wide) occurs in the upper part of the Zhuanghuwa section (Fig.
8A, S in Fig. 2). The surrounding rocks show obvious
bending deformation. Diapirs up to 10 cm high occur at
3050 cm intervals in a single layer (Fig. 8B). They usually have dome shapes and are somewhat uniformly distributed.
The smaller diapirs are usually associated with obvious compressional deformation. One typical small diapir
shown in Fig. 9B is only 2 cm high and is composed of
black silicified rock. Its surrounding rocks are light grey
dolomite, which has been bent upward by intrusion.
The clastic dykes are typically vertical spindles, which
have also deformed the surrounding light gray dolomite,
particularly at both ends of the spindles (Fig. 9C).
The dark colored silicified rocks are involved in most
of the SSDS in the Wumishan Formation. Carbonate
rocks become cemented much faster than siliciclastic
rocks. When the earthquakes occurred, the silicified
rocks were still uncemented or unconsolidated. Pressure
caused by vibration related to the earthquakes forced the
unconsolidated, dark-colored, silicified rock to penetrate
into empty or weak areas in the light-colored dolomite to
form diapirs and clastic dykes.
3.2
Convolute bedding
Most dolomites of the Wumishan Formation are
composed of sand-size carbonate particles, and thus the
sedimentary structures in these rocks, such as cross bedding, ripple marks and hail marks, therefore resemble
those in normal siliciclastic sediments. In addition, when
an earthquake occurred, some of the SSDS that characterize siliciclastic sediments were also formed in the
Wumishan carbonate rocks. Convolute bedding is one
example.
Several layers containing convolute bedding have
been recognized in the Wumishan Formation and one
particularly well-developed example occurs in the upper
part of the Zhuanghuwa section in a thick layer of brown
dolarenite (Fig. 10).
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Fig. 6 Seismically fragmented breccias in the Wumishan Formation at Zhuanghuwa Village
Refer to the text for detailed explanations. A General plan view; B A close-up view of part of the outcrop in A; C A close-up view
of another part of outcrop A; D Cross-section view. Note also the “half soft breccia” indicated by the white arrow. Two large “soft
breccias” with round surfaces and flat bottoms are present in the same layer. They were not formed by erosion, but rather probably
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reflect seesaw shaking by an earthquake.
Fig. 7 Seismically shattered mega breccia at the Y606 outcrop (a, b, and c)
Note the three breccia clusters can be correlated, indicating that the rocks were consolidated before an earthquake shattered them.
3.3
Compression-induced SSDS: Accordion folds
and plate-spine breccias
There are numerous compressional SSDS in the Wumishan Formation, typically including accordion folds,
plate-spine breccias and mound-and-sag structures.
Plate-spine breccias in the Wumishan Formation were
first recognized in the Ming Tombs area by Song (1988).
He deduced that these breccias had been produced by an
earthquake or an earthquake-triggered tsunami (Song,
1988; Song and Einsele, 1996). More detailed descriptions of the plate-spine breccias were presented by Qiao
and Li (2009), who showed that such breccias typically
occur in very thin-layered strata as vertical or imbricated
in situ breccias. The bottoms of plate-spine breccia layers
are commonly undulating surfaces, whereas the top surface, if not eroded, typically have a pattern similar to that
of a lotus flower.
Ettensohn et al. (2011) compared the SSDS of the
Wumishan Formation, particularly the plate-spine
breccias, with those of the Lexington Limestone in the
US, and concluded that they represent earthquake-triggered deformation as previously suggested
(Song, 1988; Fairchild et al., 1997; Song and Einsele
1996). They also concluded that these imbricated and
edgewise breccias are actually tightly folded, dark,
laminae floating in a white micritc matrix, and they renamed these structures accordion folds (Ettensohn et al.,
2011).
Accordion folds and plate-spine breccias are well developed in the study area. They usually are associated
with other SSDS, especially with mound-and-sag structures in the same strata. In the Zhuanghuwa section, at
least three thick layers containing such deformation
structures have been found (D, P, V in Fig. 2).
3.3.1 Accordion folds and plate-spine breccias formed
by asymmetric compressional stress
When thin-layered, unconsolidated (or weakly consolidated) carbonate rocks are placed under asymmetrical
compressional stress, they form accordion folds that dip
in the direction of the stronger stress. If the accordion
folds break, they form imbricated plate-spine breccias.
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At the base of the Zhuanghuwa section, a 40 cm layer of dolomite must have become subjected to rather
strong asymmetric compressional stress during an
earthquake. Because the compressional stress from the
north was much stronger than that from the south, the
accordion folds and plate-spine breccias dip northward
at angles of 40 to 60 (Fig. 11). Also due to this strong
compressional stress, some small-scale diapirs were
formed.
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Fig. 8 Characteristics of diapir structure
A Liquefaction diapirs in the upper part of the Zhuanghuwa section A and in another outcrop 500 m away; B The diapirs consist
mainly of dark, silicified carbonate rock.
Fig. 9 Liquefaction diapirs and clastic dykes in the upper part of the Zhuanghuwa section
A General view of three diapirs and one dyke in dolomite; B A detailed view of a diapir and its surrounding rock. Note the fold
produced by the intrusion of the dark silicified diapir; C A detailed view of a clastic dyke. Note also the folding in the surrounding
rocks at both ends of the dyke.
Another good example of accordion folds and platespine breccias is found in the upper part of the Zhuanghuwa section in a thick layer of light brown dolorudite.
The breccia clasts are 1015 cm long and nearly 1 cm
thick. Most of the breccia clasts dip to the south at angles
of 3045 degrees (Fig. 12). This indicates that the compressional stress from the south was much stronger than
that from the north. Approximately 30% of the breccia
clasts are still connected at their ends. The directly underlying layer consists of pure dolarenite or dolorudite,
whereas the overlying rock is a light-colored micritic dolomite. The energy involved in the formation of the accordion folds and plate-spine breccia in this layer must have
been much stronger than that of the normal mm- scale
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accordion folds and plate-spine breccias (Fig. 12).
One interesting thing is that the deformed layer described here and the one shown in Fig. 11 occur in the
same location but formed at different times and reflect
different palaeo-stress fields. The earlier (lower) stress
field was south dominated, whereas the later (upper)
stress field was north-dominated. So the dipping of the
accordion folds and of the plate-spine breccia clasts
Fig. 10 Earthquake-triggered convolute bedding in the Zhuanghuwa section of the Wumishan Formation
The convolute bedding usually occurs in the upper part of individual layers, indicating vertical variations in cementation. The weakly
cemented upper part was most strongly affected by earthquake vibrations.
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Fig. 11 Accordion folds and plate-spine breccia formed by asymmetric compressional stress
This layer occurs at the bottom part of the section (C in Fig. 2). Accordion folds and plate-spine breccias are the main deformation
structures here, accompanied by some diapirs and liquefaction breccias. Once the accordion folds break at the end, they form
plate-spine breccias. The folds and plate-spine breccias dip to the north. Note the rapid decrease in fold strength from right to left,
indicating that the compressional stress from the north was much stronger than that from the south. Also note the small diapers
formed at the boundary between the accordion folds and the surrounding dolomite (④, ⑤ in this figure).
Fig. 12
Coarse plate-spine breccia and accordion folds formed by asymmetric compressional stress
in the upper part of the Zhuanghuwa section
Most breccia clasts dip to the right (south). The space between the breccia clasts is filled with dolomitic sand.
could possibly be used as a means to reconstruct the
stress fields.
3.3.2 Accordion folds and plate-spine breccias formed
by symmetric compressional stress
Symmetrical compressional stress fields are rather rare
and cannot persist for longer periods or over wide areas.
When the compressional stress field is symmetrical,
weakly indurated, thin-layered carbonate rocks are deformed into vertical accordion folds and plate-spine
breccias (Fig. 13). Along the strike direction, they probably dip in one direction.
In most cases, the top surface of the accordion folds
and plate-spine breccias are disturbed by later consolidation processes or cut off by later erosion. Only the lower
parts are left. A typical example of a cut-off plate-spine
breccia is shown in Fig. 13B.
In some cases, two or more different stacking patterns
of plate-spine breccias or accordion folds can be identi-
fied even within a 1 m-thick layer, showing the rapid
changes in the compressional stress field (Fig.14).
Another characteristic but complex example is present
in a large conglomerate clast of the Wumishan Formation
near the Zhuanghuwa section (Fig. 15). The clast is about
160 cm in its longest dimension, and contains at least three
SSDS units (A, B, C in Fig. 15). Unit A contains large accordion folds and plate-spine breccias with a height of 25
cm. Unit B contains a large fold and convolute bedding
with a height of 70 cm. Unit D contains the largest accordion folds and plate-spine breccias with heights of 40 to 45
cm. The breccias in Unit D are approximately 25 to 30 cm.
Unit C contains more normal features with heights of only
15 cm. Based on the ratio between event unit and normal
unit thickness, we can deduce that the frequency of
palaeoearthquake was very high and that the Zhuanghuwa
section was very close to the epicenter.
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3.4 Compressional SSDS: mound-and-sag structures
Mound-and-sag structures usually occur between two
normal sedimentary layers. Rossetti and Goes (2000)
provided a detailed illustration of how these features form,
based on examples from the Aptian Codo Formation in
Brazil.
Mound-and-sag structures were only recently discovered in the Wumishan Formation (Zhang et al., 2007),
where two types are now recognized, one formed in a
tidal-flat environment, the other in deep water. Qiao and
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Fig. 13 Vertical plate-spine breccias and accordion folds formed by symmetrical stress
A Some plate breccia clasts are vertical or dip slightly to the left. The spaces between the breccia clasts and accordion folds are
filled with greyish sand-sized particles of dolomite. The lower boundary between the breccias and the underlying rock is clear,
whereas the upper boundary is indistinct. There are even some traces of breccias in the overlying rocks, indicating that the breccias
were not completely consolidated when the overlying layer was deposited; B Vertical plate-spine breccias cropping out near Zhuwo
Station (Site 3 in Fig.1). The boundary between the breccia and the overlying layer is clear, probably indicating erosion. There is no
clear boundary between the breccia and the underlying layer, suggesting that the breccia was formed in situ.
Fig. 14 Stacking patterns of two different kinds of accordion folds, reflecting two successive earthquakes
The arrows represent the relative value of the compressional stress (nearby the Zhuanghuwa section).
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Fig. 15 Large-scale accordion folds, plate-spine breccias and seismo-unconformities in one clast near the Zhuanghuwa section
A General view of the clast; B Close-up of Unit D, showing its sharp upper boundary; C Close-up of the bottom-left part, showing complex structures, including a layer with loop bedding and a small-scale plate-spine breccia.
Li (2009) pointed out that onlapping of the sag onto the
top of the mound is an important identification characteristic.
There are three layers containing mound-and-sag stru-
ctures in the Zhunaghuwa section (A, F, G in Fig. 2). The
3-dimensional shape of the mounds is usually like a
brachyanticline with obvious onlapping at the top of the
mound (Fig. 16).
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Fig. 16 Mound-and-sag structures near the Zhuanghuwa section
A An outcrop showing the relationship between the mound-and-sag structures and plate-spine breccia. The mound is 110 cm long
and 30 cm high; B Simple mound-and-sag structure in the middle of the Zhuanghuwa section. The main component is dolomite.
The mound-and-sag structures usually occur in the
same layer as plate-spine breccias (Fig. 16A). In this
example, the top surface of the mound is mantled with a
layer of plate-spine breccias. There are also some platespine breccias in the inner part of the mound. Obvious
onlaps occur on both side of the mound. The close rela-
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tionship between the mound-and-sag structures and platespine breccias indicates that both were produced by compressional stress.
Some mound-and-sag structures are rather simple. In
the Zhuanghuwa section, one layer of mound-and-sag
structures is exposed on an eroded surface, showing that
the mounds have a circular cross-section (Fig. 16B).
3.5
Extensional SSDS: loop bedding
Loop bedding refers to segments of individual layers
separated by pinched zones in which the thickness approaches zero (Fig. 17A). This structure was originally
interpreted as the result of stretching of unlithified to pro-
gressively more lithified, laminated sediments of deep lacustrine facies in response to successive minor seismic
shocks (Rodriguez et al., 2000), but loop bedding can also
occur in sediments formed in epicontinental seas (Tian et
al., 2006; Qiao et al., 2007 a; Qiao and Li, 2009).
Excellent loop bedding structures have been found in
several outcrops in the upper part of the Wumishan Formation. A string of loop bedding features occurs in the
bottom part of a thick layer of dolomite in the Zhuanghuwa section (Fig. 17A). In this outcrop, the loop
bedding segments are about 25 cm thick and 2025 cm
long. Some platespine breccias occur adjacent to the bottom of these loop bedding structures.
Fig. 17 Characteristics of loop bedding structure
A Loop bedding in the Zhuanghuwa section; B Large loop bedding at the Zhuwo railway station (Site 2 in Fig. 1). Red arrows
represent the directions of shear stress. Black arrows indicate that the pressure originated from the weight.
The best and largest loop bedding structures occur at
the Zhuwo Railway Station (Site 2 in Fig.1, Fig. 17B).
These loop beddings are 810 cm thick and 5055cm
long. Extension of the layer is clearly seen where two
segments adjoin, and some segments are completely
separated.
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4
Discussion
4.1
Seismic setting
The abundance of so many types of SSDS in the study
area is surprising. In the 17-m section at Zhuanghuwa
alone, 29 layers with SSDS have been recognized. The
abundance of earthquake-related SSDS indicates that
earthquake must have occurred frequently during the accumulation of the Wumishan Formation.
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In the Mesoproterozoic (16001200 Ma), the study
area was located in what is now the Yan-Liao aulacogen,
which was very close to the syndepositional fault that
extends from northeast Lingyuan to southwest Laiyuan
(Fig. 18; Qiao et al., 2002). This syndepositional fault
initially became active in the Changzhougou Stage (1600
Ma) and continued to be active for at least 400 Ma. We
deduce that earthquakes generated by movement along
this syndepositional fault zone were responsible for the
SSDS described in this paper.
Fig. 18 Structural palaeogeographical map of the Yan-Liao aulacogen from the Gaoyuzhuang to the Wumishan Stage, showing an
epicontinental sea opened to North (after He et al., 2000; Qiao, 2002; Qiao et al., 2007b; Qiao and Gao, 2007)
1 Palaeocontinent; 2 the epicontinental sea; 3 Axial part of the Yan-Liao aulacogen; 4 Outer ocean; 5 Syndeposional fault;
6 Location of geological records of earthquakes. The red star indicates the study area.
4.2
Earthquake frequency
The identification of seismic records and the syn- or
post-depositional character of SSDS are two key aspects
in the estimation of the frequency of earthquakes. Not all
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earthquakes have a sedimentary record, as some earthquakes happen under conditions that do not allow them to
leave any trace, for instance, in the form of unconsolidated surficial sediments that are susceptible to shockinduced deformation. In addition, some earthquakes may
not be strong enough to affect sediments, or their epicenter may be too far away, so that the induced shock waves
are no longer strong enough. On the other hand, some
earthquakes can disturb or even destroy the records of
earlier seismic activity. Biological effects, diagenesis and
later tectonic and volcanic activities can also confuse or
destroy earlier earthquake records. Consequently, the
actual earthquake frequency in the geological past was, as
a rule, higher than that deduced from the sedimentary
records.
Field observations in North China have led to the
recognition of four major periods of the Mesoproterozic
earthquake activity (Qiao et al., 1994, 1997). Liu (2001)
calculated a seismic cycle of 65 Ma based on sedimentary
records in the Gaoyuzhuang Formation at Pingquan,
Hebei Province. Duan et al. (2002) identified 10 seismically active intervals in the Yan-Liao aulacogen from the
Mesoproterozoic to the Neoproterozoic. All these frequency estimates are much lower than expected on the
basis of knowledge of modern earthquake activity.
Calculation of the frequency of earthquakes during the
accumulation of the Wumishan Formation requires insight into the duration of the accumulation. Precise data
are, however, not available at present. The most commonly accepted time-span for the accumulation of the
Wumishan Formation is from ~1310 Ma to ~1207 Ma
(Wang et al., 1995), indicating a depositional period of
about 100 Ma.
The sedimentation rate of the Wumishan Formation in
Jixian (the depositional center) is calculated to have been
30 m per million years (Mei et al., 2001). If the sedimentation rate was the same in the Beijing area, the shortest
average duration time for the 17 m-thick section at
Zhuanghuwa is 0.57 Ma. However, the real thickness of
the Wumishan Formation in the Beijing area is 1800 m,
suggesting a depositional rate of 18 m per million years,
which would make the greatest duration time for the 17
m-thick section as 0.94 million years.
These calculations suggest that the accumulation of the
17-m section took place over a period between roughly
0.5 Ma to 1 Ma. Thus far, we have found at least 29 layers containing SSDS in this section. Based on these data,
the calculated frequency of earthquake resulting SSDS
would have been between 20,000 and 32,000 years.
However, as pointed out above, this estimate must be
significantly lower than the real frequency based on observation of modern earthquakes. We need further detailed field and laboratory studies before we can obtain a
more precise estimate.
Acknowledgements
The authors are strongly indebted to Prof. Qiao Xiufu
for his instructive comments and scientific remarks, and
for his guidance during several field trips. We are grateful
to Prof. Feng Zengzhao for his scientific comments and
constructive suggestions. We also thank Prof . Paul Robinson and Prof. A.J van Loon for their scientific suggestions and improvement of the English.
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Vol. 1 No. 1
Outcrop observation of soft-sediment deformation structures
of the Mesoproterozoic Wumishan Formation,
Yongding River Valley, Beijing, China
In September, 2011, guided by Prof. Su Dechen who is the author of the last paper of this issue, the editor-in-chief of
our journal, Prof. Feng Zengzhao observed the outcrop of soft-sediment deformation structures of the Mesoproterozoic
Wumishan Formation, Yongding River Valley, Beijing. The soft sediment deformation structures were formed by
palaeoearthquake. A spot reviewing for Prof. Su Dechen's manuscript was thus finished during the observation. The
specialist of seismites, Prof. Qiao Xiufu, the associate editor-in-chief, Dr. Zheng Xiujuan, editor Dr. Li Xinpo, and others also joined the outcrop observation. Through detailed observation, Prof. Feng Zengzhao proposed his revision
comments on the determination of some rock names and gave excellent evaluation to Prof. Su Dechen's manuscript.
The followings are the photographs of this observation.
The scholar putting his hand
on the rock is
Feng Zengzhao,
and the one next
to his left hand
is Su Dechen.
(Photographed
by
Zheng
Xiujuan)
The front row
are respectively
Zheng Xiujuan,
Feng Zengzhao
and Qiao Xiufu
(from left to
right).
(Photographed
by Su Dechen)
Feng Zengzhao
discussed with
Qiao Xiufu.
(Photographed
by Su Dechen)
From left fo right respectively are Su
Dechen, Wang Zhengyang, He Bizhu,
Guo Xianpu, Qiao Xiufu, Feng Zengzhao,
Ji Youliang, Zheng Xiujuan, Yu Enxiao,
Qin Gang.
(Photographed by Zhou Yong)

Typical earthquake-induced soft-sediment deformation structures in

Transcription

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