liquefaction-induced structures in quaternary alluvial gravels ......liquefaction-induced structures...
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Engineering Geology 76
Liquefaction-induced structures in Quaternary alluvial gravels
and gravelly sediments, NE Brazil
Francisco H.R. Bezerraa,*, Vanildo P. da Fonsecaa, Claudio Vita-Finzib,
Francisco P. Lima-Filhoa, Allaoua Saadic
aDepartamento de Geologia, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN 59072-970, BrazilbDepartment of Mineralogy, Natural History Museum, Cromwell Rd, London SW7 5BD, UK
cDepartamento de Geografia, Instituto de Geociências, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627,
Belo Horizonte, MG 31270-901, Brazil
Accepted 2 July 2004
Available online 11 September 2004
Abstract
We have identified numerous well-preserved elutriation and fluidization structures probably induced by liquefaction in
Quaternary gravels and gravelly sediments of braided fluvial channel deposits in the Rio Grande do Norte and Ceará states,
northeastern Brazil. They show evidence of upward-directed water escape after sediment deposition and before sediment
compaction. Among the several types of structures observed, the most frequent are pillars, pockets and dikes. These structures
range in width from a few centimeters to as much as 4 m, and in height from 60 cm to 4 m. Dikes, pillars and pockets are
systematically associated. Clastic dikes vented large quantities of sand to the upper layers or the surface; pebbles and cobbles
from the host rock sank into the dikes and formed pillars and pockets. Pockets form the root part; pillars form the intermediate
part and dike, the upper part of the composite structure. The morphology of the structures in sectional and plan views indicates a
3D geometry composed of a tabular dike and pillar that present a downward continuous transition to a bowl-shaped pocket. This
bstratigraphyQ of liquefaction features is different from that usually presented in the current literature.Field data suggest that both the location and the geometry of the features were controlled by sedimentary properties rather
than joints and small faults. The size and abundance of these features suggest that they were formed by great events rather than
localized mechanisms. Field evidence also indicates that these features are the product of fluidization and elutriation and may
have been induced by liquefaction processes associated with seismic shaking. A nonseismic origin related to elutriation
processes, however, cannot be ruled out for some of the features.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Liquefaction; Quaternary; Gravel; Neotectonics; Brazil
0013-7952/$ - s
doi:10.1016/j.en
* Correspon
E-mail addr
(2005) 191–208
ee front matter D 2004 Elsevier B.V. All rights reserved.
ggeo.2004.07.007
ding author.
ess: [email protected] (F.H.R. Bezerra).
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208192
1. Introduction
Liquefaction is a process whereby a granular
material from a solid state is induced to behave like
a liquid as a consequence of a sudden increase in
pore-water pressure in the sediment matrix (Youd,
1973). It is one of the most common soft-sediment
deformation mechanisms associated with seismic
shaking and generally occurs shortly after sediment
deposition and before sediment compaction. During
liquefaction, the loosely packed grain framework is
broken down and grains become temporarily sus-
pended in the pore fluid or are lifted so that the grain
framework is destroyed (Lowe, 1975; Allen, 1984;
Owen, 1987). Liquefaction can lead to the settlement
or tilting of buildings, ground cracking, dam insta-
bility, the failure of road embankments and many
other kinds of damage bearing on public safety (Youd,
1973; Youd and Perkins, 1978).
Fig. 1. Simplified geological map and main site location of liquefied featu
(major geological units modified from DNPM, 1983, 1998; maximum int
Samambaia; AB, Afonso Bezerra; J, Jundiaı́. Inset: location of study area
Although there are abundant field studies on
liquefaction in sands, few cases of liquefaction in
gravels and gravelly sediments are reported, and from
these reports, field data are generally inadequate for
firm conclusions to be drawn on the basis of the
sedimentological nature of the feature.
Likewise, whereas liquefaction of sandy sedi-
ments has been widely documented in the laboratory,
experimental assessment of liquefaction in gravel is
difficult because large clasts in gravels and gravelly
sediments hamper conventional field sampling as
well as testing of the samples (Yegian et al., 1994).
The possibility of liquefaction potential in gravel is
thus important in geotechnical investigations (Evans
et al., 1992; Amini and Sama, 1999). Because of the
many parameters affecting liquefaction, a large
number of field cases are required for the formula-
tion of a valid liquefaction model. Until the deficit is
rectified, one-way forward is to compare new field
res. Sites quoted in text are denoted by capital letters and numbers
ensities modified after Ferreira et al., 1990). Faults cited in text: S,
in the South American continent.
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208 193
investigations with well-known cases presented in
the literature.
Our main objective is to contribute to this
discussion by analyzing features in gravels and
gravelly sediments and describing the process of what
occurred within the deformed gravels. The study is
divided into four parts. We first describe the alluvial
sediments, where our sites were identified, and review
their tectonic setting. Second, we describe the features
in detail. Third, we compare the field data with other
field examples of seismic and nonseismic-induced
features in an attempt to understand the spatial
relationship and formation of dikes, pillars and
pockets in gravels. Finally, we use field criteria to
support our interpretation for a liquefaction origin,
and we discuss its implications for the Quaternary
sedimentary record. The study area is located in Rio
Grande do Norte and Ceará states, northeastern Brazil,
where there are numerous exposures of fluvial
deposits having similar features that can be observed
(Fig. 1).
2. Geological setting
2.1. Quaternary alluvial deposits
The term bQuaternary alluvial depositQ is used inthe present study to designate all the alluvial rocks
observed in the study area. They overlie Precambrian
crystalline rocks, Cretaceous sandstones and con-
glomerates (Açu Formation), Cretaceous limestones
(Jandaı́ra Formation) and Pliocene sandstones (Bar-
reiras Formation; Fig. 1). Most stratigraphic studies
indicate that the alluvial deposits range in age from
Pleistocene to Holocene. Silva (1991) obtained a
Pleistocene age of 30,190F370 years BP and hasconfirmed the estimated age for the post-Barreiras
Formation deposits in the Assu delta. Furthermore,
some coastal deposits which interfinger with alluvial
deposits along the littoral zone yielded ages as old as
210,000 years (Barreto et al., 2002).
The alluvial deposits were mapped using remote
sensing imagery and maps such as DNPM (1983,
1998). No formal names were applied to these
Quaternary units, as there is a lack of consistency in
the current literature. The Quaternary alluvial deposits
are found within active river valleys which are
characterized by river gradients lower than 1% and
by ephemeral streams.
In the semiarid region of northeastern Brazil, the
alluvial sediments were transported by seasonal flash
floods to form braided deposits. Therefore, the most
abundant type of deposit is clast-supported conglom-
erate in a sand matrix. Some of these deposits
represent abandoned channels whose grain size
ranges from boulder to clay, with sand and pebbles
being the most common. Most of the pebbles are
well rounded and composed of quartz (85% or more)
and subordinate fragments of quartzite, granite,
gneiss, diabase and pegmatite. Mud- and silt-rich
sediments occur in some of the other flood plain
deposits as a product of fluvial deposition or the
weathering of feldspar. The braided deposits are
characterized by cyclic fining-upward layers which
present trough cross-beds (facies Gm, Gt, Gp, St and
Sp of Miall, 1978). These findings indicate that the
deposits are fluvial belts, some of them more than
20-km wide. Minor lag deposits, longitudinal bars,
channel fills, linguoid bars and transverse bars were
also observed.
2.2. Tectonic setting
Two major and pervasive sets of neotectonic
faults were recognized across the area: a NE-
striking set and a NW-striking set. Their cross-
cutting relationships show that they locally form a
conjugate set and display both a strike-slip and a
dip-slip component of movement. These sets have
generated troughs filled by as much as 260 m of
Pliocene to Quaternary sediments. Radiocarbon
dating shows that some of these faults slipped in
the Holocene (Bezerra and Vita-Finzi, 2000).
Tectonic movements were detected along the Car-
naubais fault where rapid emergence by at least 4–5
m occurred on the SE block between 4080 and
2720 cal. year BP, and along the Jundiaı́ fault where
movement took place between 4860 and 4570 cal.
year BP (Bezerra and Vita-Finzi, 2000; Bezerra et
al., 2001; Fig. 1).
Northeastern Brazil has experienced several earth-
quake swarms with a recurrence period of approx-
imately 4 years, each lasting at least 6 months (Takeya
et al., 1989; Ferreira et al., 1998). The earthquake
swarms include events up to 5.2 mb (body wave
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magnitude) and MMI (modified Mercalli intensity
scale) VII, and the swarms occur along seismic faults
that reach the surface (Bezerra and Vita-Finzi, 2000).
The most striking example was observed along the
Samambaia fault (Fig. 1) where a seismic record of
more than 40,000 seismic events occurred from 1986
to 1989; 15 had mbN4.0 and one event had mbz5.1(Takeya et al., 1989).
Liquefaction occurred in the region on at least two
historical occasions. The first was during the Arati-
cum–Ceará State earthquake swarm in April and
March 1969, when soil collapse and earthquake-
induced landslides were recorded (Ferreira, 1983;
Fig. 1). Liquefaction also occurred during the
Itaparica–Bahia State earthquake of 22 March 1911
(MMI VII), 1000 km to the south of the study area.
This event was also followed by soil collapse in the
epicentral area and localized subsidence along the
coast (Ferreira, 1983).
Fig. 2. Liquefaction pockets indicated by high concentration of
pebbles and cobbles in vertical view: (A) base of a fluvial cycle
where a pocket developed; (B) pocket formed from the middle to
the lower part of the fluvial cycle. Note the high length/width ratio
of the feature. Both pockets form the base of pillars. Site T1 shown
in Fig. 1.
3. Documentation of elutriation and fluidization
structures
3.1. Size and abundance of the features
We conducted a systematic search in Quaternary
alluvial deposits in the region. The features occur in
several river valleys, usually in braided fluvial
channel belts composed of gravel and gravely sand.
In each of the valley investigated, the features occur at
several sites within a few kilometers of one another.
Examination of about 35 sites in quarries, road cuts
and trenches resulted in the discovery of more than
400 features. Each site, shown in Fig. 1, presents
dozens of features. They are abundant in the Assu,
Ceará-Mirim and Jaguaribe valleys, at depths between
1 and 5 m.
The dimensions of features vary across the study
area. Sites where dikes are the dominant features are
marked with solid circles, whereas sites where
pockets/pillars are the dominant features are marked
with squares. The size of a circle and a square
indicates the average length and width of the feature.
The major concentration of features is along the Assu
valley, where dikes up to 0.5 in width and pocket/
pillar up to 4 m in length were found, and near the
Ceara-Mirim estuary (Fig. 1).
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3.2. Morphology of features in sectional and plan
view
The features that we observed are described in
detail in this section. Beds present in our study area
have no apparent internal organization, but the field
evidence indicates an internal bstratigraphyQ that ischaracterized, from base to top, of pockets, pillars and
dikes. These features are overlain and underlain by
undeformed beds and are spatially and mechanically
related.
bPocketQ is a term that is used for a bowl-shapedstructure partly filled with pebbles and cobbles which
sometimes form fining-upward graded structures
(Postma, 1983). They correspond to the B-type pillars
of Lowe (1975). In northeastern Brazil, the basal part
of the features form pockets that have a coarse
granulometry, and represent the accumulation and
resedimentation of clasts after water expulsion.
Fig. 3. (See Site T2 Fig. 1 for location). (A) Schematic 3D view of poc
concentration and alignment of pebbles in panel B and the disorganized c
The 3D morphology of pockets indicates they are
bowl-shaped features that differ from pothole or cut-
and-fill structures because the margins of pockets are
steep and narrow. The fill material is composed
almost entirely of pebbles and cobbles, according to
the Wentworth size classification, and is ~2 to ~15
cm in length. In sectional view, pockets range from
~20 cm to ~1 m in height. Their walls are sharply
defined. Pockets are usually deposited in a gravel or
sand layer (Figs. 2 and 3). In plan view, pockets are
not well defined, but they tend to be circular to
elliptical in shape. In addition, the length/width ratio
of pockets is usually higher than in synsedimentary
structures, whereas pothole and cut-and-fill structures
usually form small, shallow and rounded depressions
filled by clasts (Fig. 3). Some pockets present the
kind of bpear dropQ-shaped disturbance described byScott and Price (1988) in Plio-Quaternary sediments
in southwestern Turkey. A few of the pockets in the
ket, (B) section and (C) plan view indicating a bowl shape. Note
luster in panel C. Arrows show realignment of pebbles.
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Assu valley are cuspate and display pebbles and
cobbles which apparently sank into the liquefied sand
underneath (Fig. 4). Their final shape is identical to
that of the detrital wedges described by Estévez et al.
(1993) in Miocene sediments of southeastern Spain,
which are also inferred to have formed during
liquefaction.
bFluidization pillarQ or simply bpillarQ is a geo-metric term for vertical or steeply inclined structures
(Lowe and LoPiccolo, 1974; Lowe, 1975). The term
Fig. 4. Cuspate pillars in vertical section. Scale given by ham
was first applied by Wentworth (1966) to vertical
zones of massive sand between the upturned margins
of dish structures. More recently, the term pillar has
been applied to the steep orientation of pebbles along
the margins of fluidization channels, caused by the
rotation and resedimentation of clasts (Postma, 1983).
In the study area, pillars form the middle part of the
features and present an internal organization. Pillars
are the most common types of structure in the study
area especially along the Jaguaribe and Assu valleys.
mer ~20-cm long (location of Site T1 shown in Fig. 1).
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Fig. 5. Liquefaction pillar in gravelly sediments; the arrows mark
alignment of displaced pebbles: superposition of pillars in gravelly
sediments. Note that the larger pillar (centre of the picture) is
superposed on another smaller pillar (lower-left corner). Scale given
by GPS receiver ~13-cm long (modified from Bezerra and Vita-
Finzi, 2000). Site T5 is shown in Fig. 1.
Fig. 6. Pillars in gravelly sediment (a) capped by mud-bearing
sandstone (b) and soil (c); (d) represents road pavement. Note tha
both layers above the liquefied gravel are undisturbed. Scale given
by hammer ~20-cm long. See Site T4 in Fig. 1.
F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208 197
t
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Fig. 7. (See Fig. 1 for location of Site T2). (A) Schematic 3D view and (B) sectional view of pillar. Note pocket at base of pillar; (C) structures
continue to the opposite trench wall. The information in panels B and C indicates a tabular pillar. Arrows show realignment of pebbles.
Fig. 8. Vertical section showing host gravelly sediment (a) affected by small pillars (b) that occur alongside a sand dike (c). They are capped by a
mud–sand layer (d). Note that scattered clasts occur in the dike; a cone-shaped form is observed at the top of the dike (modified from Bezerra
and Vita-Finzi, 2000; location of Site T5 shown in Fig. 1).
F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208198
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208 199
In our study area, the 3D morphology of pillars
indicates that they are vertical columns of pebbles and
cobbles in sectional view, which are tabular in plan
view. Pillars die out upwards or slightly bend to
horizontal-bedding position on the top. Going down-
ward, they normally narrow, form funnel-shaped
structures and show a continuous transition to pock-
ets. In sectional view, pillar sidewalls are smooth to
sharply defined. Elongated pebbles and cobbles in
pillars usually show realignment parallel to these
sidewalls. Pillar average height is 1.0 m but a few
exceed 2.0 m; pillar width ranges from 20 to 50 cm
(Figs. 5, 6 and 7). Pillars having vertical heights
exceeding 2 m are usually spaced more than 5 m
Fig. 9. Vertical section showing detail of sand dike cutting across grav
gravelly layers. Note the sharp contrast between intrusion and the host
adjoining location ~30-cm wide. The gravitational settling and reorientat
host rock, (b) sand dike, and (c) trench spoil (location of Site T5 in Fig
apart; whereas pillars or pockets ~20-cm long are
usually spaced less than 3 m apart. In some cases,
there is a textural-upwards zonation, represented by
pebbles and cobbles at the base, which changes
gradually upward to finer sediments composed of
gravely and coarse sand. In plan view, the tabular and
organized shape of this middle zone grades up to a
subzone characterized by pebbles and cobbles show-
ing no orientation (Fig. 7). Pillars are never associated
with faults. In the Assu and Ceará-Mirim valleys,
pillars are in places a few centimeters apart (Fig. 5).
The morphology of the pillars described in this
study accords with some features described in the
literature. They are similar to those described by
elly layers: (A) sand dike about 45-cm wide cutting across two
material. Scale given by notebook ~10-cm wide; (B) sand dike in
ion of larger pebbles are evident in the lower part of the dike: (a)
. 1).
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208200
Postma (1983) and Mather and Westhead (1993) in
Pliocene conglomeratic deposits in Spain.
bDikeQ is a geometric term used for a vertical orsteeply inclined intrusion of sand or soft sediment
(Lowe, 1975). In the study area, pillar structures
change gradually upward to dikes. Dikes that occur
above pillars are more common along the Assu and
Ceará-Mirim valleys.
These dikes are composed chiefly of loose,
unsorted sand transported from underlying gravel
layers and usually contain pebbles and cobbles from
the host sediments. The coarse sand grades upward to
a medium to fine sand granulometry. Several dikes
have an overlying cap of thin sand and silt.
Field data indicate that the 3D morphology of dikes
presents a tabular form in plan view. In sectional view,
Fig. 10. (See Fig. 1 for location of Site T3). (A) Schematic 3D view, (B) s
top of pillar. Plan view indicates tabular shape. Arrows show realignmen
the dikes are roughly planar bodies, oblique to the
sedimentary bedding, ~1- to 50-cm wide and ~2 m in
height, and cut across the gravelly beds (Figs. 8, 9 and
10). From Figs. 8 and 9, it can be observed that there
is a fining-upward grain size in the dikes. In these
cases, there is a textural-upwards zonation, repre-
sented by pebbles and cobbles at the base which fines
upwards to gravelly and coarse sands. The dikes of
Figs. 8, 9 and 10, for example, are overlain by an
undisturbed sand layer more than 2-m thick. In plan
view, the sand derived from pocket/pillar area has an
elongated shape that presents a sharp contact with the
gravel layers (Fig. 10).
The combination of the sectional and the plan
views indicate that the sand–water mixture from the
pocket/pillar part of the feature intruded overlying
ection and (C) plan view of base of dike. Note sand concentration at
t of pebbles.
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208 201
layers (Fig. 11). The contact between the injected sand
and the upper layers is sharp (Fig. 10).
3.3. Texture and grain size of the features and host
rock
The host deposits investigated are clast-supported
braided fluvial deposits composed chiefly of gravel
and sand. Several fluvial cycles were observed at each
site. Each of these cycles is usually a fining-upward
succession. The top part of each succession is marked
generally by a silty sand layer which may present a
pedogenetic zone.
The fluidization/elutriation features present a fin-
ing-upward grading. The lower part, which corre-
sponds to pockets, presents the coarsest grain size of
the feature.
We analyzed the grain size distribution of dikes
and host rock matrix at four sites. From Fig. 12, it can
be concluded that the dikes and the host rock matrix at
Fig. 11. (A and B) Top part of fluvial cycles intruded by sand vented
these four sites share similar grain size distributions.
The dikes consist of silty sand very similar in color,
texture and grain size to the host rock from which it
was derived. The maximum diameter of the matrix
particles is ~4 mm. However, the silt–mud content
(less than 0.062 mm) in the dikes varies from ~26% to
31%; whereas it varies from 7% to 26% in the host
rock matrix. The grain size lower than 0.062 mm is
partly composed of mud probably derived from in situ
feldspar weathering.
3.4. Spatial relationship between features
The spatial relationship between pockets, pillars
and dikes is illustrated using a schematic cross-
section taken from a road cut (Fig. 13) and a 3D
schematic view (Fig. 14). Three fining-upward
cycles of alluvial channel filling were identified.
The deposit is composed of conglomeratic and
unsorted coarse sand layers. Close inspection of the
from the pocket/pillar part of the features. See Site T1 in Fig. 1.
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Fig. 12. Grain-size curves for samples collected in host rock matrix and dike at four sites.
F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208202
levels of tops of features indicates that much of the
deposit was affected by at least six cycles, i.e.,
unconformities, spaced in time (Fig. 13). The layers
are laterally continuous, but locally pinch and swell
and undulate along the section. A swarm of more
than 40 closely spaced pillars disturb the sedimentary
layers. Several associated pockets occur in the
exposure. Dikes were identified above pillars and
pockets at six levels. The dike infillings consist
mainly of coarse sand and silt.
Fig. 13. Schematic cross-section of a typical succession long cut into three
various features. See Site T1 in Fig. 1.
Fig. 14 is a schematic 3D view of a complete
structure, which represents a combination of the
several features observed both in sectional and plan
views. Although the variations in the 3D schematic
view presented in Fig. 14 are relatively large from site
to site, the model can be used as a starting point for
discussion of the mechanisms involved in the lique-
faction process.
Fig. 14 shows the transitions generally present
between pockets, pillars and dikes. The composite
segments. Light horizontal lines show relative elevation attained by
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Fig. 14. Composite figure based on trenches, road cuts and quarries. (A) Schematic 3D view (exaggerated height scale), (B) plan view at base
(Y–YV) and intermediate (X–XV) height of feature in panel A. Key: Po, pocket; Pi, Pillar; D, dike.
F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208 203
structure cuts through interbedded sand and gravel
layers. Pillars are key features. When the lower part
of a pillar is completely filled by pebbles and
cobbles, pocket structures are commonly found at
depth and dikes form the upper part of pillars. The
pebbles and cobbles from the sidewalls that were
transported downwards mark the basal (pocket) and
middle part (pillar) of the structure. Also common
are small pockets that seem to be isolated from the
middle and upper parts of the structure, i.e., pillars
and dikes. The matrix of the sediments within and
close to the features (pockets, pillars and dikes) is
composed of medium to coarse sand (see grain size
data).
The morphology of the uppermost part of the
features is also important to interpretations of origin.
The upper part of the composite structure (Fig. 14),
formed by pillars and dikes, presents infilled planar
breaks that may bear some relation to the cohesion in
the host deposit. This morphology pattern occurs
where that portion of the host deposit near the ground
surface is weakly bonded by cohesion of sand–silt
grains which existed when the intrusion occurred.
Thus, these breaks strongly suggest that a sudden and
forceful application of pore-water pressure occurred
beneath the weakly cohesive materials.
The composite structure is associated with defor-
mation on the host sediments. The intercalated layers
of sand and gravel seem to have deformed plastically
near the sidewalls of the structure, as evidenced by the
observation that sand and gravel layers are usually
wavy as they approach pillars and pockets.
Several variations were observed of the host
deposits in the 3D schematic view depicted in Fig.
14. One of the most frequent is that of host deposits
being composed of thick layers of gravel, where no
sand layers are observed. The number of layers of
sand and gravel also varies at each site.
4. Discussion
4.1. A model for the formation of elutriation and
fluidization structures in northeastern Brazil
Previous field studies show that flow structures
associated with dikes project upwards and are related
to tensile fractures caused by seismic shaking or a
nontectonic source of energy. Obermeier (1996a,
1998), for example, interpreted many of the dikes
that affect Holocene gravelly sand sediments along
the Wabash River and the New Madrid seismic zone,
central USA, as burst-out structures generated by high
fluid pressure of seismically induced liquefaction.
Others are caused by lateral spreading and surface
oscillations. He observed that the liquefied gravelly
sand zone showed evidence of flow into dikes, which
cut up into the low-permeability cap above. Accord-
ing to Obermeier (1996a, 1998), flow structures
project upwards from the liquefied bed onto the
bottom of the dikes. A few clasts of sidewall material
had collapsed into the dikes, indicating that the cap
had sufficient cohesion to behave as a very weakly
lithified mass.
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Alternatively, there are cases where clastic dikes
have been interpreted as the product of nontectonic
processes. Rijsdijk et al. (1999), for example, inter-
preted clastic dikes in late Quaternary gravelly till
deposits on the east coast of Ireland as the infillings of
hydrofractures, which formed when fluid pressure in
the lower gravel layer exceeded the overburden
pressure and tensile shear strength of the capping till.
Both studies of tectonic (Obermeier, 1996a, 1998)
and nontectonic processes (Postma, 1983; Rijsdijk et
al., 1999) observed that high fluid pressure within a
confined gravel aquifer could result in tensile fractur-
ing of the overlying till. The water flowing through
the fractures fluidized the gravels and intruded them
into the overlying layers. In both cases, the infillings
of dikes consisted mostly of poorly sorted gravel,
clasts were aligned parallel to the dike walls and
funnel-shaped clusters of pebbles formed a fan at the
top of the dike.
The distinction between tectonic and nontectonic
processes in coarse-grained deposits, however, may be
based on the type of sedimentary deposit and the
geometry of the features. Nontectonic processes occur
in particular sedimentary deposits. In turbidite depos-
its, for example, they are favored by decreasing
permeability within the units (Lowe, 1975). In fan-
delta deposits, they occur mainly in unstable proximal
gravelly delta facies or in the fine-grained uppermost
distal delta deposits (Postma, 1983). In glacial and
subglacial deposits, they occur in diamitic sediments
confined by an impermeable cap, and the confining
pressure may lead to the forceful upward flow of
water and clasts through tensile cracks (Rijsdijk et al.,
1999).
In our study area, the features occur in braided
fluvial deposits and are marked by pebble alignment
that project downwards. We interpret this as the result
of elutriation; that is, resedimentation of clasts into the
dike channel after the water–sand mixture was vented
to the upper layers or surface. A good analogy with
these features described in northeastern Brazil may be
found in Charleston, coastal South Carolina (USA;
Obermeier, 1996a, 1998). The pockets described in
northeastern Brazil had probably an origin very much
like that of the craterlets that developed in Pleistocene
beach deposits caused by the earthquakes in Charles-
ton in 1886. In South Corolina, the lower part of the
bowl is filled with the coarsest, densest sediment, and
the underlying feeder dike is circular in plan view.
This morphology is very similar to the one described
in northeastern Brazil (Fig. 14B).
Grain-size behavior within the features is also
important to interpretations of origin. It is possible
that the sloping gravel-rich layers (e.g., Fig. 14) were
the result of liquefaction-induced fluidization in these
layers. Alternatively, it is possible that the sloping
beds were the result of elutriation of much sandy and
finer-grained sediments at depth, resulting in down-
dropping of already-present flat-lying gravel-rich beds
towards the nearly vertical core region. We prefer this
latter explanation because the grain sizes do not show
a significant decrease away from the vertical core
area.
The following alternative interpretation is consis-
tent with our field observations. A fine-grained
pedogenic layer observed in the top part of several
alluvial cycles may not have allowed the dissipation
of excess pore-water pressures as they were being
induced by an external event. At the same time,
because of much higher permeability, sand beds
became the feeding layers of sand dikes. The same
conduit from which the liquefied mixture was vented
was also the site of pebble collapse. During lique-
faction–fluidization, pebbles and cobbles sank after
sand and silt were remobilized and vented to the top
of the deposit. Pockets were formed at the base of
pillars by pebbles and cobbles being deposited in
bowl-shaped troughs. In most cases, the escaping
water was not forceful enough to eject pebbles and
cobbles because of the coarse texture of the liquefied
sediments and large weight of clasts.
Nevertheless, pillars, pockets and dikes do not
always occur together, and one or two of them may be
missing. It is possible that, in the layered sediments,
the permeability in a horizontal direction is locally
higher than in the vertical, so that the excess water
pressure would migrate and dissipate laterally along
the sedimentary layer.
The precise position and shape of the structures
could have been controlled by local sedimentary
properties. Nichols et al. (1994) have proposed that
the most probable controls on the formation of
liquefaction-induced features are the thickness of the
impermeable cap layer and the grain size of the
liquefied sediment. We observed that our structures
were preferentially formed where the pedogenic cap
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208 205
that would have prevented dissipation of high water
pressure is present. We also observed that the
thicker the liquefied layers, the longer the structures
and the greater the distance between liquefaction
structures.
4.2. Seismic liquefaction origin
Individual sites may have various interpretations
for origin, being either seismic or nonseismic. The
question is why these fluvial deposits can be
interpreted as having been liquefied at most of our
study sites. The vast majority of structures present
some characteristics similar to those generated by
earthquakes (e.g., Mather and Westhead, 1993; Yegian
et al., 1994; Tuttle and Schweig, 1995; Obermeier,
1996a,b). A few structures in the study area, however,
may be identical to structures generated by non-
tectonic processes.
The collective evidence and the regional occur-
rence of the features rule out localized factors, such as
karst structures, artesian flowing and syndepositional
deformation. Each is considered in the following.
Features in terraces which overlie carbonate rocks
that are similar to like those in the central part of the
Assu valley were evaluated as resulting from alluvial
collapse over caves. But in northeastern Brazil,
collapse does not provide a plausible explanation
because our features also occur in terraces that overlie
sandstones and crystalline rocks.
Artesian flowing can be ruled out as the origin of
the liquefaction features because conditions for
artesian flow (e.g., Mansur et al., 1956) very probably
were not present. First, the sand vented forcefully.
Second, there is no evidence of rhythmic sand boils in
the clastic dikes, which shows that the sand vented
infrequently. Third, the gravel and gravelly sediment
deposits are located in a region where there is no
artesian flow, and modern production wells in the area
have never been free flowing. A lack of artesian
conditions probably existed in Quaternary time, as
fine-grained caps in the region likely occurred only in
flood-plain deposits associated with the alluvial sedi-
ments. These deposits usually occur in patches
associated with meander belts. Thus, artesian flowing
is unlikely to have affected an area as vast as
northeastern Brazil. Finally, dikes from artesian sand
boils are circular in a fine-grained cap, whereas
earthquake-induced liquefaction develops tabular fis-
sures except where very minor liquefaction has
occurred (Obermeier, 1996a).
Moreover, the sand dikes cut across sedimentary
strata younger than the sand-dike source, which
excludes the possibility of syndepositional processes.
In addition, liquefaction can be either statically
induced or seismically induced. Static liquefaction
occurs only in very fine sands and silts except where
very significant artesian pressures are present (Ober-
meier, 1996a). In gravel-bearing deposits such as
those in the study area of northeastern Brazil, the fine
sand content was not high, thereby probably eliminat-
ing static liquefaction. Still, the elutriation process that
has produced some of our features may be a
combination of both gravity and overload. Liquefac-
tion-induced structures of static origin may be
generated on steep depositional slopes such as those
of fan deltas or under gravitational sliding on very low
slopes given suitable conditions (e.g., Lowe, 1975;
Postma, 1983). A few pillars in our study area which
present a pocket shape and consist of bowl-shaped
structures filled by clasts (usually pebbles) are similar
to those structures generated on steep depositional
slopes by gravitational sliding and are described by
Postma (1983) in the Almeria Basin, Spain. The
terrace slopes in northeastern Brazil are not negligible,
which cannot rule out the possibility of gravitational
sliding of material. In addition, at a few locations, the
cusp-shaped pillars like the flame structures and load
casts associated with passively deformed beds (e.g.,
Allen, 1984) could be sedimentary features caused by
overload.
Because the features in the study area are found at
multiple locations, sometimes as clusters, and because
these features are overlain and underlain by unde-
formed beds and occur near Quaternary and active
faults, they may be interpreted as the products of
seismic shaking. In addition, the tectonic setting in
northeastern Brazil has no long been considered
unlikely to result in liquefaction (Bezerra and Vita-
Finzi, 2000; Bezerra et al., 2001).
Recognition of earthquake-induced liquefaction
features usually requires an impermeable cap above
a body of sand that is loose, wet and mud-poor or
mud-free (Obermeier et al., 1990). In the study area,
the pedogenic layer observed in several deposits at the
top of a fluvial sequence could have acted as an
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F.H.R. Bezerra et al. / Engineering Geology 76 (2005) 191–208206
impermeable cap. This mechanism would perhaps
also account for liquefaction–fluidization in braided
fluvial channels and other coarse sediments.
Several investigations have concluded that the
liquefaction of gravel and gravelly sand requires a
much higher threshold magnitude than do deposits
chiefly composed of sorted sand (Tinsley et al., 1985)
because the high gravel content of a sedimentary
deposit increases the internal friction resistance
(Obermeier, 1996a). Liquefaction was generated in
gravel in modern-day earthquakes, including the 1988
Armenian earthquake (surface wave magnitude Ms=
6.8, Yegian et al., 1994), as well as the Borah Peak–
Idaho earthquake (Ms=7.3; Youd et al., 1985), but it
is less common than liquefaction generated in sorted
sand. Valera et al. (1994, in Obermeier, 1996b) stated
that the threshold moment magnitude (M) to produce
seismites in gravel is 7, whereas it is only about 5.5 in
sand deposits.
We cannot rule out, however, some kind of
nontectonic process that has produced elutriation in
the formation of some of the observed features. This
process might involve separation of the finer particles
as sand and silt from the coarse particles such as
pebbles, inducing the transport of the finer particles
upward and allowing the coarse ones to sink.
Finally, field studies in Brazil by others have
described liquefaction features in gravels similar to
those ascribed here to palaeoearthquakes (e.g., San-
t́anna et al., 1997 in southeastern Brazil), thereby
demonstrating that the structures and processes we
have described in northeastern Brazil may occur
elsewhere. Also reported have been clastic dikes,
pillars and convoluted folds in the Miocene con-
tinental deposits in Ceará State (Saadi and Torquato,
1992) and in Quaternary alluvial deposits in Rio
Grande do Norte State (Fonseca, 1996).
5. Conclusion
Analysis of field data indicates the widespread
occurrence of fluidization and elutriation in gravel and
gravelly sediments in northeastern Brazil. The field
evidence also points to high water pressure and strong
water escape in the sediments during liquefaction.
The internal stratigraphy of deformed deposits
consists, from base to top, of pockets, pillars and
dikes. The features present a fining-upward grain
size distribution. Dikes and pillars present a tabular
shape in plan view, and they show a continuous
transition downward to bowl-shaped pockets at the
base of the system. There is nothing to suggest that
the dike and pillar channels acted as faults. Any
brittle structures that occur postdate the liquefaction
features.
The collective occurrence of elutriation/fluidiza-
tion structures in a variety of lithological, sedimen-
tological and topographic conditions, as well as their
spatial and stratigraphic association and some sim-
ilarity to current earthquake-generated features,
suggests that the structures identified in the Quater-
nary record in northeastern Brazil are probably the
result of seismic shaking. The short seismic record
does not include events N5.2 mb. But the size of the
liquefaction-induced structures, in addition to the
coarse texture of the liquefied sedimentary deposits,
indicate paleoearthquakes with larger magnitudes,
perhaps as high as M~7. Some kind of nontectonic
elutriation process, however, cannot be ruled out at
some sites investigated in this report. Exact ages and
sizes of the features in the region are evidently
needed to refine analysis of the features under
review.
Acknowledgments
This work was supported by the Brazilian Grants
CNPq/CTPETRO 461450/01-1 and FINEP-ANP
65.00.0397.00 (MAP-AÇU). We thank Kris Vanneste,
Andrew D. Miall and G. Postma for valuable
suggestions. Stephen F. Obermeier deserves special
thanks for his numerous constructive suggestions
which greatly improved this work.
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Liquefaction-induced structures in Quaternary alluvial gravels and gravelly sediments, NE BrazilIntroductionGeological settingQuaternary alluvial depositsTectonic setting
Documentation of elutriation and fluidization structuresSize and abundance of the featuresMorphology of features in sectional and plan viewTexture and grain size of the features and host rockSpatial relationship between features
DiscussionA model for the formation of elutriation and fluidization structures in northeastern BrazilSeismic liquefaction origin
ConclusionAcknowledgmentsReferences