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).

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.

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


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).


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.


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).

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.


t

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).



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).


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.


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.

Fig. 12. Grain-size curves for samples collected in host rock matrix and dike at four sites.


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

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.


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.


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


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


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

liquefaction-induced structures in quaternary alluvial gravels ......liquefaction-induced structures...

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