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Sedimentary facies distribution and genesis of a recent
carbonate-rich saline lake: Gallocanta Lake,
Iberian Chain, NE Spain
A. Perez a,*, A. Luzon a, A.C. Roc a, A.R. Soria a, M.J. Mayayo b, J.A. Sanchez c
aArea of Stratigraphy, Department of Earth Sciences, Zaragoza University, 50009, Zaragoza, SpainbArea of Mineralogy, Department of Earth Sciences, Zaragoza University, 50009 Zaragoza, SpaincArea of Geodynamic, Department of Earth Sciences, Zaragoza University, 50009, Zaragoza, Spain
Accepted 1 August 2001
Abstract
The study focuses on the Holocene sedimentary infill of the Gallocanta lacustrine basin in the Iberian Chain, NE Spain. The
Gallocanta lake is a saline wetland with a maximum length of 7.5 km and a maximum width of 2.85 km. The water depth varies
significantly, from a maximum depth of 2 m to completely dry. In the central areas (central subenvironment) sapropels and salts
develop, with halite, gypsum, anhydrite, dolomite, aragonite, calcite, magnesite, and lesser amounts of quartz and clay minerals.
Cyanobacteria filaments are related to the aragonite and dolomite crystals. The marginal subenvironment either has a gradual or
a sharp change from that of the central subenvironment. An inner area with desiccated light grey lutites is present in this
marginal subenvironment. In SEM and X-ray diffraction analyses, quartz, clay minerals, aragonite, calcite and small quantities
of dolomite, gypsum, anhydrite and halite can be identified. This inner area is surrounded by an external fringe composed of
light brown lutites and a high concentration of Salicornia meadwod and microbial mats. This zone is only occasionally
submerged and contains sandy and conglomerate islets. Active palustrine areas are flood zones, where grey lutites with a
significant quantity of vegetation, such as reeds, are common. In general, this entire sector is being modified by human action.
Five sedimentary facies have been defined, and this has enabled the identification of three distinct stages in the general
evolution of the basin. The first stage is an alluvial period, developing during an arid climate. After this, a more humid stage
facilitated the installation of a shallow carbonate-rich lake. A reduction of the water level, probably due to a more arid stage,
induces a salinity increase of the lacustrine brine and the change to the third stage, which corresponds to the present conditions.
The water level experienced frequent oscillations, and alternations between humid conditions with a high production of organic
matter, which favors carbonate formation, and water level dropdowns, even to total dryness, with saline sedimentation. D 2002
Elsevier Science B.V. All rights reserved.
Keywords: Lake sediment; Evaporite; Carbonate; Bacteria; Holocene; NE Spain
1. Introduction
In north-east Spain (Fig. 1) there are over 100
small closed depressions where highly mineralised
lakes form, locally called ‘‘saladas’’. Most of these
0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0037-0738 (01 )00217 -2
* Corresponding author. Fax: +34-761088.
E-mail addresses: [email protected] (A. Perez),
[email protected] (M.J. Mayayo), [email protected]
(J.A. Sanchez).
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Sedimentary Geology 148 (2002) 185–202
lakes contain evaporites derived from Cl–SO4–Na–
(Mg) and Na–Mg–Cl–(SO4) surface brines. These
are chlorides (halite), sulphates (gypsum, anhydrite,
mirabilite–thenardite, hexahydrite, and bloedite) and
small quantities of carbonates (calcite, dolomite, ara-
gonite and magnesite) (Dantın, 1942; Ibanez, 1975;
Pueyo, 1979; Comin et al., 1990; Schutt, 1998; Luzon
et al., 1999; Perez et al., 1999). These saline lakes are
located on mudstone, sandstone, carbonate and gyp-
sum deposits of Triassic age in the Iberian Chain
(Buntsandstein, Muschelkalk and Keuper facies) and
Tertiary age (upper Oligocene–lower Miocene) in the
Ebro Basin (Bujaraloz Member of the Alcubierre
Formation, and Zaragoza Formation). The climate of
these areas is markedly arid, with an average annual
rainfall of less than 350 mm in the Ebro Basin located
at 300 m upper the sea and 488 mm in the Iberian
Chain located at 1000 m upper the sea. Extreme
temperatures (absolute values varying from � 15 to
42 �C) and dry north-westerly winds are common.
The evaporation is 850 mm/year.
The sediments generated in existing saline lakes
have been studied by numerous authors due to their
economic and environmental interest. They are
ephemeral saline lakes, in areas where the evaporation
rate is greater than the annual rainfall, and they are
therefore similar to playa lakes. This type of lake can
be found in many places around the world and they
have a great variety of local names. In the north of
Africa there are about 1000 playas where they are
known as ‘‘dayas’’ (Mitchell and Willimott, 1974;
Babikir, 1986) or ‘‘Sebkhas’’ (Perthuisot, 1977). In
the Southern High Plains of Texas and New Mexico
there are approximately 30,000 small ephemeral
lakes, that are in most cases the remains of Pleistocene
lakes (Morrison, 1968; Gustauson et al., 1980; Wood
et al., 1992; Brown and Sharp, 1992; Wood and
Sanford, 1995). In Southern and Western Australia
there are hundreds of silt–clay ponds known as
‘‘dongas’’ (Bowler, 1986). They can also be seen in
the Atacama Desert and the Andes, in northern Chile,
in the fringes of the Indus Plain in Pakistan, where
they are called ‘‘chor’’, and in South Africa, Bot-
swana, Zimbabwe and Zambia (Goudie and Thomas,
1985).
Basic summaries of carbonate and evaporitic lacus-
trine sedimentation include the reviews by Kelts and
Hsu (1978), Dean (1981), Dean and Fouch (1983),
Fig. 1. Situation of Gallocanta lake and geological framework of the NE of Spain. Main saline lakes areas in the Tertiary fill of the Ebro basin
are mentioned.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202186
Eugster and Kelts (1983), Platt and Wright (1991),
Renaut and Last (1994) and Gierlowski-Kordesch and
Kelts (1994). The reconstruction of seasonal produc-
tion, temperature changes, and pa-laeohydrology of
these lakes is based primarily on the combination of
sedimentological and geochemical studies (Kelts and
Talbot, 1990; Talbot, 1990; Camoin et al., 1997). The
texture of microbial sediments has been analysed by
Cryo-scanning electron microscopy and the processes
and types of bacterial carbonate genesis have been
revealed in numerous studies (Castanier, 1987; Adol-
phe et al., 1989; Chafetz and Buczynski, 1992; Folk,
1993b; Defarge et al., 1996; Braithwaite and Zedef,
1996; McGenity and Sellwood, 1999; Castanier et al.,
1999a). The important role of bacteria has recently
been revealed as a mediator in the precipitation and
diagenesis of dolomite in anoxic conditions in a
current lagoonal medium (Vasconcelos et al., 1995;
Vasconcelos and McKenzie, 1997), in the formation
of aragonite (Riccioni et al., 1996; Coshell et al.,
1998), of magnesite (Pontoizeau et al., 1997) as well
as in the nucleation of salts ooid (Castanier et al.,
1999b).
These observations can be applied to our study
area, the Gallocanta lake, since sedimentary pro-
cesses are currently mainly controlled by the cyano-
bacteria action. The principal objective of this paper
is the characterisation and mapping of the sedimen-
tary facies developed in the lake, with particular
emphasis on sediments with microbial textures. The
lateral and vertical relations that these facies hold
within them allow to depict the evolution of this
lacustrine area.
2. The Gallocanta lake
The Gallocanta lake is the largest continental saline
wetland in the northeast of the Iberian peninsula. It
has a surface area of 14.14 km2, most of which
belongs to the province of Zaragoza and only its
southeast fringe is within the province of Teruel. Its
co-ordinates are 40�500N, 2�110W, and it is situated at
an altitude of 990 m, in a large endorheic depression
which extends from Cubel towards the region of the
Jiloca river. The depression parallels the orientation of
the structural features of the region. The climate is
sub-arid Mediterranean, and is also highly continental.
The temperature varies from � 15 �C in January to
29.5 �C in July. The prevailing winds come from of
the NW and can reach speeds of 100 km/h. The mean
annual rainfall is 488 mm.
The lake is situated in the central sector of the
Iberian Range, and more specifically where the
Aragonese and Castilian branches meet. It is situated
on carbonated and evaporitic Triassic deposits, and
is bounded to the north and northwest by the
predominantly quartzite Palaeozoic lineaments of
the Santa Cruz mountain range (Fig. 2), and to the
south and southwest by carbonate Mesozoic out-
crops of the Lower Jurassic and Upper Cretaceous.
There is also substantial development of coarse
detrital Pliocene and Holocene deposits bordering
the lake.
The lake is elongated, with a maximum length of
7.5 km and an average width of 2.85 km. Within this
area three quite morphologically distinct sectors can
be identified. In the extreme NW is ‘‘El Lagunazo de
Gallocanta’’, which is almost circular in shape and has
a diameter of 1.2 km. It is separated from the main
part of the lake by a conglomerate ridge locally
known as ‘‘Los Picos’’, although this has been cut,
possibly by human action, to join the different sectors
of the lake. ‘‘El Lagunazo grande/central’’ is the main
body of the lake. It is elongated in a NW–SE
direction, approximately 5 km long and 3 km wide,
and several springs (‘‘Los ojos de la laguna’’) are
located in its western part, around which is an
extensive marshy zone, much affected by human
action. In the eastern fringe is ‘‘Los lagunazos de
Tornos’’, which consists of a marshy area 2.9 km long
by 4.2 km wide.
The lake does not exceed 2 m in depth and dries
out in summer. The composition and hydrochemical
characterisation of its waters (Comin et al., 1990)
shows that it is a hypersaline lake of the type Na–
Mg–Cl–(SO4), which during dry periods exhibits the
precipitation of various salts such as halite, bischofite,
epsomite, hexahydrite and mirabilite. The waters
come from subterranean flows, although torrential
flows, which carry surface waters, also exist. Schutt
(1998) carried out a geochemical study on samples
taken from 90 cm short core obtained from the north-
eastern sector of the lake. She established three super-
posed sedimentary units characterised by different
mineralogical associations, which this author inter-
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202 187
prets as related to climatic changes from sub-arid
conditions to sub-humid and then back to sub-arid,
respectively. Previously, Calvo et al. (1978) and
Gonzalez-Lopez et al. (1983) revealed that primary
dolomite was precipitated in this lake.
More recently, Burjachs et al. (1996) carried out a
palynological study. Using a 110-cm sequence of
sediment extracted from the centre of the lake, which
was dated using 14C, 210Pb and 137Cs, the climatic
history and lake level changes for the last 150 years
was established. The base of the core (93–95 cm)
gave an age of 12.230 ± 70 years BP, and the interval
between 58 and 60 cm gave age of 840 ± 70 years BP.
The origin of the basin, which contains the lake, is
very controversial. Traditionally (Hernandez Pacheco
and Aranguren, 1926; Dantın, 1941), it is believed to
have a tectonic origin; this basin has been interpreted
as a graben that originated from the activity of tension
faults. Presently this interpretation is being questioned
by Gracıa et al. (1999) who consider that it could be a
large polje, which, during its Quaternary evolution,
produced the present Gallocanta lake. This interpreta-
tion agrees with that formulated by Sanchez et al.
(1998) on the origin of the shallow lakes in the Ebro
Basin (region of Monegros), in which the combined
action of subterranean flows and wind processes are
thought to be the principal causes of these depres-
sions.
3. Materials and methods
To carry out the cartography of the facies and the
precise delimitation of the sub-environments, aerial
photographs at scale 1:33,000 (1957 flight) and scale
1:18,000 (1978 flight) have been used, in addition to
Fig. 2. Location of sampling stations in Gallocanta lake. The correlation sketch of Fig. 8 is drawn from A–B transect.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202188
satellite images and aerial photos taken in July 1998.
The field research was carried out between September
and November 1998 and from July to September 1999
as the studied sector of the lake was almost dry during
this period of the year. In fact, there were only thin
layers of water in the northwestern sector (region of
Los Aguanares) and the central sector (Ojos de la
Laguna), which correspond, respectively, to in-flow
areas of water from the ravine emerging from Santed
and to the presence of springs.
The sediment samples that were studied originated
from 31 sedimentary cores, between 1 and 2.33 m
long, taken from different points around the lake (Fig.
2). The sampling was carried out using 4-cm diameter
PVC tubes, between 1 and 3 m in length, which were
driven into the ground manually with a 1.8-kg mallet.
The tubes were connected to a vacuum pump to aid
driving and sediment retention. For extrusion of the
tube sampler, a pulley system and a platform were
used. In addition, a modified Livingstone piston corer
(5 cm in diameter) was also used. The degree of
sediment compaction was monitored in-situ by noting
the difference between the sampling depth and the
total length of sediment recovered. After sectioning
the tubes in the laboratory and making a photographic
record, a visual description of each one of the samples
was made, identifying the lithology, colour, grain size,
macroscopic plants remains, organic content and sedi-
mentary structures. Subsequently, samples were taken
every centimeter.
X-ray diffraction and SEM analysis were used in
order to determine the mineralogical composition. For
the X-ray analysis, a Philips 1729 X-ray generator was
used, by the powder and orientated aggregate method.
The SEM studies were performed using a JEOL JSM
6400 scanning electron microscope coupled with an
energy dispersive X-ray for elemental identification.
For the SEM analysis the samples were coated with
gold according to the standard procedures for using
SCD 004 equipment. Photomicrographs and quantita-
tive analyses of chemical elements were undertaken.
The carbonate content was analysed in a GEOSERVI-
CES calcimeter, which calculates the total percentage
of carbonate and the calcite/dolomite ratio. C, N and S
analyses were done in a PERTINEERKIL-ELMER 24
microanalyser. The pH, Eh, conductivity, Na and K
values, were calculated using 0.25-g solutions of finely
powdered specimen in 100 cm3 of distilled water,
which were shaken for 120 min, using a pH meter
3071 and conductivity meter 4071 JENWAY and a
meter NAK-I.
4. Characterisation of the sedimentary facies
4.1. Facies description
Based on the study carried out on the samples, we
have defined five sedimentary facies in the Gallocanta
lake (Fig. 3). They are numbered I to V from bottom
Fig. 3. Characterisation of the sedimentary facies defined in Gallocanta lake.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202 189
to top. The overall thickness of the deposits varies
between 70 and 145 cm. An average compaction of
25% have been calculated for muddy facies, with only
9% for sandy facies.
The sediments contain authigenic minerals (gyp-
sum, dolomite, calcite, aragonite, magnesite, hexahy-
drite, bloedite, halite, anhydrite, thenardite, bassanite
and traces of smectite), diatomites, ostracods, charo-
phytes, organic material and detrital silicates (quartz
and very small quantities of feldspars and clay min-
erals: illite, kaolinite and chlorite).
Facies I consists of lutites, sands and orange-ochre
gravels with reddish patches. The gravels are com-
posed of angular or sub-angular siliceous granules,
from 1 mm to 3 cm in diameter. The sands are fine- to
coarse-grained, with occasional millimetric quartzite
particles. This facies, which has a maximum known
thickness of 45 cm, is bedded in various decimetric
thick sequences, with gravels at the base, sands in the
middle and lutites at the top. In the samples from the
central sector, the lutites display 1-cm thick lamina-
tions of different colours (from ochre to brown). The
lutites consist of quartz and dolomite (both between
70% and 20%), the remainder composed of clay
minerals and traces of calcite and halite.
Facies II consists of orange-ochre lutites with
occasional sand-size siliceous particles. Red or grey
patches can be seen, which are more abundant
towards the upper part of the facies. The thickness
of this facies varies between 8 and 27 cm. X-ray
diffraction analysis shows it consists solely of quartz
(50–75%) and dolomite (50–25%).
Facies III is the thickest among all collected
samples. Its thickness varies between 20 and 75 cm,
which are reached in the central area of the lake. It
consists of marls, clayey marls, and grey and whitish
grey marley clays, massive or with vertical bioturba-
tion (Fig. 4C). The bioturbation, which are centimetric
in length and millimetric in diameter, was due to
roots. Its contains dolomite (25–50%), quartz and
clay minerals (5–50%), and in smaller proportions,
calcite, halite, gypsum, anhydrite (2–25%) and traces
of magnesite and bassanite. Dolomite is present
throughout the whole section, but dominates in the
lower part. The calcite muds, however, are only
present in the upper part of the section. There are
occasional levels consisting almost entirely of gyp-
sum. Samples contain large quantities of charophytes
remains and sporadic remains of ostracods. The vol-
atile elements are composed of: 2.9% TOC, 0.8% N
and 0.8% S.
The sampling carried out at the far edges of the
lake identified grey patches of gravels and sands,
which predominate towards the base of the marley
sequence. The sands are mainly siliceous, fine- to
medium-grained, and the gravels consist of white
quartzite and grey limestone granules, which are
round and 1 cm in diameter. These lithologies are
much more developed in the southern area of the lake,
where they reach thicknesses of more than 30 cm and
constitute a cyclic sequence with gravels at the bottom
and top, and sands in the middle. A maximum thick-
ness of 6 cm can be observed in the rest of the lake.
Facies IV consists of grey marls with black and
brown patches, which are more abundant towards the
upper section (Fig. 4B). This facies can be seen in all
of the samples although it is thinner than facies III. Its
thickness varies between 10 and 30 cm and the lower
part exhibits a gradual transition with facies III. The
mineralogical content is composed of clay minerals
and halite in similar proportions, between 25% and
50% of each one, as well as quartz, calcite, aragonite,
gypsum, and traces of dolomite, magnesite, bassanite,
and anhydrite. In SEM analysis framboidal pyrite has
been observed (Fig. 6C). It contains 3.3% TOC, 1.1%
N and 4.1% S.
Facies V consists of black muds, which are either
laminated, an alternation of dark and light layers, or
massive (Fig. 4A). The total thickness of the section
varies between 20 and 30 cm. The mineralogical
composition is very similar to the previous facies
and is characterised by halite, gypsum, clay minerals,
quartz, calcite, dolomite, aragonite, and traces of
hexahydrite, bloedite, magnesite, and feldspars.
The black layers have thicknesses between 0.5 and
4 cm and contain clay minerals (66%), halite (15%),
quartz, calcite, aragonite and, magnesite (19%) and
traces of dolomite. The SEM reveal a generalised
development of layers of cyanobacteria which contain
authigenic and detrital minerals, pollen grains, and
diatoms (Figs. 5 and 6A,B). It is worth pointing out
the spectacular growth of salts in cubic and hopper
crystals, as well as the development of aragonite.
Occasionally, within this black mass a millimetric
lamination is observed, which was created by accu-
mulations of diatoms or grey marls.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202190
Fig. 4. Photographs of sediments. From the top to the base, A: facies V; B: facies IV; C: facies III.
A.Perez
etal./Sedimentary
Geology148(2002)185–202
191
The light layers are ochre, with a thickness varying
between 1 ml and 1 cm. They predominantly consist
of gypsum and/or halite, which can exceed 70% of the
total composition, about 10% each of quartz, calcite,
and magnesite, and also some traces of detrital feld-
spars. In these layers, algal structures can hardly be
recognised, although in some places (central areas of
the lake) around 2-cm-thick halite crusts (Fig. 6D,E,F)
can be observed (cubic and hopper crystals), appear-
ing intermingled with filamentous algae. Different
specimens from both layers have been analysed quan-
titatively by SEM and the results are consistent with
the composition stated previously. It is necessary to
note the high content of titanium in some particles,
Fig. 5. SEM images of facies V black mud. A and B: microbial filaments covering clay minerals and carbonate particles. C and D: aragonite
aggregates with filaments of cyanobacteria. E and F: algae and diatoms between carbonated particles.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202192
which indicates the presence of rutile grains. In some
areas, the previously mentioned laminations are
observed, 2 to 3 cm thick, within a basal lamina of
dark grey marls, an intermediate section of black
muds, and a lighter upper section of salts.
4.2. Interpretation
Facies I and II are interpreted as alluvial deposits,
with streams originated from the surrounding relief.
These subenvironment correspond to mudflats where
Fig. 6. SEM micrographs of facies Vand IV. (A and B) Filaments of cyanobacteria covering clay minerals and carbonate particles of black muds
from facies V. (F) Framboidal pyrite between clay minerals of facies IV. (C, D and E) Halite crystals and carbonate particles in light laminae of
facies V.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202 193
occasionally tractive flows deposit layers of sand and
gravel. These materials were subject to subaerial con-
ditions, as demonstrated by the reddish and ochre tones.
SEM analysis showed that the halite in facies I is a
cemented form of detrital grains. In this context, the
high content of Mg2 + in the brines favour the early
diagenetic transformations of calcitic micrite to dolo-
micrite (Salvany andOrtı, 1994). Facies II characterises
more distal environments than facies I and is the result
of edaphic processes acting upon the sediment. The
color of the lutites in this case suggest that the area was
subject to frequent fluctuations of the phreatic level.
The sediments of the facies III and IV are specifi-
cally lacustrine. The materials have been deposited by
floculation under a permanent or semi-permanent layer
of water and their sedimentation is directly or indirectly
related with the arrival of fine terrigenous particles
from surrounding areas. The flora and fauna (ostracods
and charophytes) are typical of shallow carbonate-rich
lakes (generally less than 2 m deep) with high levels of
oxygen and light. In these lakes the origin of the
carbonates is mainly bio-induced. The marls show
deposition in zones far away from the influence of
coarse terrigenous particles. In such areas the bioturba-
tion of the sediment, generated by organic activity,
distorts the lamination produced by the floculation
process. In contrast, the action of bioturbation by roots
in the central areas (facies III) is correlative with high
concentrations of gypsum and halite around the edges.
These relationships show that during the evolution of
the lake, there have been variations in water level. The
presence of bassanite (CaSO4�1/2H2O), a highly unsta-
ble calcium–sulphate salt, produced by dehydration of
gypsum, has been reported by Akpokodje (1984) in
ground from arid areas of Australia and byMees (1998)
regarding saline lake deposits in north Mali. Such
studies indicate the arid and warm environmental
conditions which have been experienced by the studied
area, since gypsum begins to dehydrate at 50 �C. In thesouthern sectors, gravel lenses form small deltaic
systems at the opening of stream channels.
The distinctive characteristic of the facies V is the
presence of laminations. Laminations in lacustrine
deposits with light and dark bands are frequently
associated with the existence of stratified waters and
the development of a hypolimnion, a characteristic of
deep lakes. Only anaerobic bacteria or aerobic–anae-
robic facultative bacteria live beneath the lower hypo-
limnion and the laminations are preserved because of
the lack of bioturbation (Ludlam, 1976; Freytet, 1984).
However, Freytet (1984) recognized finely laminated
levels without bioturbation in old lake deposits, which
occasionally have indications of immersion. In this
case the interpretation of a deposit in lakes with deep
hypolimnion with rapid water level variations is not
very appropriate, so the author considers that the
deposits are generated in shallow lakes.
Vasconcelos and Mckencie (1997) identified lami-
nated facies in the present lagoon of Vermelha, in Rio
de Janeiro. It is a small lake, with a surface area of 2.4
km2 and shallow waters (less than 2 m deep), where
sulphate-reducing bacteria of the Desulfovibrio group
grow. The sediments analysed by these authors are
mainly carbonate-rich muds, with alternating light and
dark layers. In shallow lakes, the lighter layers gen-
erally develop during hot, humid periods (with greater
water dilution), when there is a significant abundance
of living organisms and the biotically induced precip-
itation of carbonates. The dark layers represent very
dry periods during which the layer of water body is
shallower and there is a lower water oxygenation.
Such conditions, in combination with the high organic
production in the lacustrine area, favour the waters
eutrophication and a change to anoxic conditions at
the water–sediment interface. This induces bacterial
sulphate reduction of previously deposited materials
and the genesis of a black layer of mud, which is very
rich in organic material.
The interpretations of Vasconcelos and Mckencie
(1997) may explain the genesis of the dark bands in
facies V of the Gallocanta lake. However, owing to
current observations and to the significant presence of
salts, the interpretation of more humid conditions for
the light layers does not apply to Gallocanta lake
deposits. As a matter of fact, it has been observed that
the black muds of the Gallocanta lake are forming
under a very thin water sheet, no more than 30 cm
thick. Besides extremely dry conditions, a crust of
saline efflorescences of gypsum, halite, hexahydrite
and bloedite has formed on the lacustrine surface.
Presently, the main water supply comes from the
highly mineralised groundwater flows and the phre-
atic level stays a few centimetres below the surface.
During stormy periods the stream channels discharge,
introduce detrital particles and slightly dilute the
superficial water body. In addition, such conditions
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202194
favour the gathering of aquatic birds up, whose bio-
logical activity produces a great volume of excrement
which is incorporated into the sediment. This fact,
combined with the significant development of micro-
bial mats means that, during the periods of low water
level, highly anoxic conditions prevail and bacteria
sulphate-reducing favours the precipitation of authi-
genic minerals. During extremely dry periods the
phreatic level drops down, leaving the sediment sur-
face uncovered with water for long periods of time.
The intense evaporation during these periods causes
capillary rise of subterraneous water flows, which
generates salinity increase, thus giving rise to precip-
itation of previously mentioned salts. Furthermore, as
surface deposits and organic material are in contact
with the air, they are oxidised, the sediment turns to
grey or brown tones while becoming enriched with
carbonate or evaporitic particles. In this way, the black
layers of facies V correspond to the dampest con-
ditions that currently prevail (with a shallow layer of
anoxic water). The light layer corresponds to dry
periods, with a high insolation. The phreatic level
drops down, and capillary ascension of the brines
occurs and salts deposition takes place.
5. Mapping of lacustrine subenvironments
The mapping of the facies (Fig. 7) corresponds to
periods when the water layer is at a low level, that is
to say when the lake is practically dry. However, at
high level some observations have also been done.
The present distribution of facies in the Gallocanta
lake allows three sedimentary subenvironments to be
identified, each with well-defined characteristics: cen-
tral lacustrine area, the marginal lacustrine area, and
the palustrine area. Amongst the central and marginal
lacustrine areas there are some points where there is a
Fig. 7. Map of sedimentary subenvironments and alluvial systems in the Gallocanta basin.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202 195
sharp change, and some with a more gradual transi-
tion. Exists an area of intermediate characteristics,
which we have named central/marginal lacustrine
area. The subenvironments distribution within the lake
is asymmetric, such that the marginal lacustrine facies
are more developed in the SW margin, whereas the
palustrine facies are dominant in the SE sector and in
the springs area of ‘‘Los ojos de la Laguna’’.
5.1. Central lacustrine subenvironment
This area is located to the immediate NWand SE of
the ‘‘Los picos de la laguna’’ sector. In the area of
‘‘Lagunazo’’ it occupies an area of 1� 0.5 km and
4.2� 1.2 km in ‘‘Laguna Grande’’. During conditions
of low water level the central sector is covered by a thin
layer of water, which is only a few centimetres deep,
and in extreme conditions it can be totally dry, although
the phreatic level always stays just below ground level.
This sector is characterised by the development of
black muds and the growth of halite hopper crystals
intermingled with filamentous cyanobacteria, as a
consequence of the rise of phreatic level. In the
samples taken, and especially in the upper part of
them, the black laminated lutites predominate with the
typical characteristics of facies V, which are here
exceptionally well developed. Through SEM it is
possible to observe aragonite crystals on fibres or
aggregates covered with cyanobacteria filaments and
unicellular bacteria (Figs. 5C,D and 6A,B). NaCl
crystals occur in cubic or hopper form. The filaments
may also cover other carbonate and clay particles (Fig.
5A,B). Occasionally, remains of pollen and diatoms
are seen (Fig. 5E,F), the latter in accumulations up to
100 mm thick. Underneath facies V, grey marls have
been identified with abundant black patches and little
or no development of cyanobacteria. These materials
correspond with facies IV and frequently contain of
pyrite framboids (Fig. 6F). In the basal samples from
this sector only grey marls with charophytes have
been identified. They correspond to facies III. A few
layers of facies II, made of ochre and brown lutites,
are also encountered.
5.2. Marginal lacustrine subenvironment
This surrounds the central subenvironment and
occupies most of the Gallocanta lake. During con-
ditions of low water level it is emergent, whereas in
the winter it may be covered by a thin layer of water
only a few centimetres deep.
During dry periods this subenvironment is charac-
terised by the presence of grey or brown marly sedi-
ments on the surface, which are affected by the
development of desiccation cracks and by root or
worm bioturbation (of little significance). Under con-
ditions of maximum aridity, the surface is partially
covered by a crust of saline efflorescence which
disappears with the first autumn rain. During winter,
given the larger quantity of water, it is common to
observe discoloured marls on the surface with the
typical characteristics of facies IV, or the early devel-
opment of black organic mud of facies V. Underneath,
a generalised development of grey marls can be
observed, which display brown patches on the surface
in the external sectors. These materials present the
typical characteristics of facies III. In this subenviron-
ment it is possible to distinguish an internal area and
an external one.
The internal marginal subenvironment occupies a
0.2- to 0.5-km-wide band, around the central area. It
has a very low topographic gradient and shows a
marked asymmetrical distribution of the facies, so that
it is more developed in the S and NW areas (‘‘Lagu-
nazo de Gallocanta’’) of the lake. In this subenviron-
ment, grey or brown marls and lutites predominate.
They often have desiccation cracks on the surface and
occasionally stromatolitic laminations with halite
crystals. In the NW area there are sands and isolated
quartzite blocks. The granulometric analysis carried
out on these sands reveals that they have been trans-
ported in suspension. In conditions of low water level
these materials may have brown tones on the surface
due to oxidation, as a result of aeration and very little
bioturbation. When they are covered by a layer of
water and under reducing conditions, grey or black
colouring predominates (facies IV). The sediments are
composed of brown lutites, grey marls, and light
brown coarse sands with dispersed quartzite pebbles.
The marl of facies III, which is the first lacustrine
facies, is the thickest one, and sometimes constitutes
almost all the sedimentary column in this sector.
The external marginal subenvironment occupies a
0.7-km-wide band along the edge of the lake; it
consists of an irregular area. It is rarely inundated
and the brown marls and lutites predominate. The
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202196
topographic gradient in this subenvironment is greater
than that of the internal marginal. At low level water,
there is a characteristic development of Salicornia
ramosissima between which the development of
microbial mats can be identified, which gives way
to stromatolitic levels frequently affected by desicca-
tion cracks. In the area SE of ‘‘Lagunazo de Gallo-
canta’’ S. ramosissima do not grow, because of the
presence of beach deposits of calcareous and quartz-
itic sands and conglomerates. The location of the
beach deposits is consistent with the direction of the
prevailing winds of the region. In the external mar-
ginal sector, sediments are composed of grey or brown
sands and lutites of facies III and IV. These facies lie
on brown lutites and gravels, typical of facies II and I,
respectively. In conditions of intermediate lake level
that are frequently reached during winter, this sector
represents the limit of the shoreline.
5.3. Palustrine subenvironment
This is an asymmetric sector with a variable area. It
occupies a 2.9-km-wide and 4.2-km-long area. Palus-
trine facies are concentrated in two main regions. The
first one is in the NW of the lake and is characterised
by the existence of numerous springs (‘‘Los ojos de la
laguna’’). The second one is in the SE of the lake. In
this subenvironment, dark grey marl with gastropods,
reeds, and rushes can be observed. Most of the marly
area is now being cultivated land.
Towards the SE area, between the central and
marginal sectors, sediments with mixed characteristics
are identified; this area has been named the marginal–
central sector. In contrast, in the south fringe there is an
area named the lacustrine/palustrine area in which sand
bars can be identified. This area may have been covered
by water during maximum inundation conditions, in
relation to humid periods during the last 100 years,
when the lake depth could have reached 2 m.
6. Evolution of the Gallocanta basin
The facies sequence observed in the Gallocanta
lake, together with the characteristics and the distribu-
tion of the present subenvironments, allow to tenta-
tively redraw the evolution of the area. Sedimentation
was at the beginning typically alluvial and turned to an
essentially lacustrine one. Furthermore, the evolution
of the strictly lacustrine area can be subdivided into at
least two stages (Fig. 8).
6.1. Alluvial stage
The lowermost sediments reached in sampling
demonstrate that the initial sedimentation was mainly
detrital deposition. The detritus originated from
upland Mesozoic and Palaeozoic areas adjacent to
the current Gallocanta lake. These were typical
mud-flat environments, which received tractive flows
carrying sand and gravel. As a result of a decrease in
the flow capacity, probably in relation to a reduction
of the topographic gradient, the detrital materials were
deposited in this zone. The correlation sketch (Fig. 8)
demonstrates a reduced terrigenous supply from near
the source area to more distal zones. Facies I demon-
strates that the materials were frequently subjected to
aerial exposure, and that the region evolved under an
arid–subarid climate. After Salvany and Ortı (1994),
in such conditions the cementation of terrigenous
materials by salts is favoured. The reduction in coarse
terrigenous supplies, the development of edaphic
processes, and frequent fluctuations of the phreatic
level favoured the development of the facies II lutites.
Hence, the succession of facies I and II demonstrates
the retrogradation of the alluvial environments with
time. Facies I is organised in fining-upward sequen-
ces, which implies changes in the energetic conditions
of the flow during its development.
6.2. Lacustrine stage
After the period of alluvial sedimentation repre-
sented by facies I and II, a lacustrine period settle
(facies III to V), and is still lasting to date. The lake
has experienced frequent variations in the water depth.
In the past, there have been times when the water
layer has been rather deep and times in which it has
been totally or almost totally dry. Facies III, inter-
preted as deposits generated in a carbonate-rich lake,
indicates that the beginning of the lacustrine sedimen-
tation took place under humid conditions. The water
level was higher than today, and the salinity was
moderate. During these conditions, it was essentially
a shallow carbonate-rich lake, with ostracods and
charophytes. In this phase, the water depth could have
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202 197
reached 4 m. The sedimentation during this period
was predominantly marley, and organisms were abun-
dant. The bioturbation of the sediment gave it a
uniform appearance. At the periphery of the lake,
notably in southern sectors, an essentially detrital
sedimentation took place. Under subaquatic condi-
tions, small deltaic systems developed. The character-
istics of facies III and the vertical sequence indicate
that during this period water level fluctuations oc-
curred and that the terrigenous supplies flowed into
the lake predominantly during its initial development.
Midway up the sequence, a section of bioturbated
marls can be seen (probably due to plant colonisa-
tion), which are interbedded with gypsum lamina in
the central area and high gypsum content levels near
the marginal areas. Such a feature displays a drop
dower of water level, the colonisation of the bottom of
the lake by vegetation while the brine concentration
favoured salt (gypsum) precipitation. Such occurs
repeatedly, and the presence of bassanite between
these deposits suggests periods of aridity and very
intense insolation. After this period of decrease of
main water layer, an increase occurred during which
the upper part of facies III was deposited.
In more recent times, the lacustrine level falls due
to a more arid period, and this favoured the change to
Fig. 8. Facies correlation sketch along A–B transect (see Fig. 2).
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202198
more saline conditions that were not very favourable
for the growth of the previously mentioned organisms.
During these conditions, diatoms were common and
the lacustrine bottom was colonised by microbial
mats. The eutrophication of the waters, favoured by
intense organic activity, provides an environment
suitable for the conservation of organic material
generated not only by the activity of the lacustrine
organisms, but also by the numerous aquatic birds that
temporarily occupied Gallocanta lake. Under those
conditions, which correspond with those of the central
lacustrine subenvironment, facies V was generated. In
this special environment, the organic material can be
recycled by sulphate-reducing bacteria whose activity
should favour the precipitation of Mg–calcite and
Ca–dolomite. Such phenomena are also favoured by
the high concentration of sulphate ions, and calcium
and magnesium ions. The precipitation of carbonated
minerals by bacterial mediation in nature and in
laboratory experiments have been reported during
the last 100 years (Nadson, 1903, 1928; Bavendam,
1932; Krumbein, 1974, 1979; Chafetz and Folk, 1984;
Adolphe et al., 1989; Chafetz and Buckzynski, 1992;
Folk, 1993a,b; Defarge et al., 1996; Chafetz, 1994).
Castanier (1987) and Chafetz (1994) obtain precip-
itation of calcite, aragonite, and high-Mg calcite in the
laboratory, and the investigations of Vasconcelos et al.
(1995) and Vasconcelos and McKencie (1997) con-
firm the relation between dolomite precipitation and
bacterial activity. During drier periods the phreatic
level drops down, leaving the sediment surface
exposed. The capillary fringe is occupied by highly
mineralised interstitial waters, with a high concentra-
tion of SO42� , Ca2 + and Mg2 + ions. This is because
the main groundwater supplies pass through the
gypsum of Keuper facies. Upon evaporation, these
waters generate salt layers (gypsum and halite, also
bloedite, hexahydrite, and magnesite). The surface
deposits oxidise when in contact with air, acquiring
light brown or grey tones. These characteristics cur-
rently develop in the internal marginal subenviron-
ment. These combinations of processes have gen-
erated the laminations that are observed in the surface
deposits of the Gallocanta lake.
If the water layer is lowered further, the superficial
sediment can become totally exposed, and under
conditions of high oxygenation, can acquire light
ochre or grey tones and undergo processes of bio-
turbation—typical characteristics of facies IV. It may
also dry up completely, and undergo partial or even
total brecciation. These characteristics are present in
the external marginal subenvironment.
The general evolution of this area can be charac-
terised by three main periods (Fig. 9). During the first
Fig. 9. Evolutive stages interpreted for the Gallocanta lake.
A. Perez et al. / Sedimentary Geology 148 (2002) 185–202 199
one, a sedimentation typical of distal alluvial areas
occurs. The materials originated from the surrounding
Mesozoic and Palaeozoic outcrops. With time, these
areas undergo a retrogradation towards the source
area. During this first period, the climatic conditions
should have been notably arid. The alluvial facies
possess playa-lake characteristics with development
of salts and smectites. After this time, during a more
humid period, a carbonate-rich lacustrine area devel-
ops. Within this phase, generally more humid, a more
arid period occurs in which the lake might have even
totally dried up. Afterwards, the water level drop
down is probably in relation to a more arid stage.
The basin changes to the present conditions (third
stage), in which a carbonated–evaporitic sedimenta-
tion is produced with high production of organic
matter linked to the occupation of the zone by
migratory aquatic birds and microbial mats. During
this final stage, the water layer has undergone frequent
variations and lake-bed surface has been sometimes
subaerially exposed. As it is indicated on the correla-
tion sketch, the lacustrine depo-center has been dis-
placed towards the NE in recent times.
7. Conclusions
The sedimentological study of the Gallocanta lake
has identified five different facies, two correspond to
distal subenvironments of alluvial fans and the rest to
lacustrine deposits.
The alluvial facies are found at the base of the
sequence which fills the Gallocanta basin. These facies
were deposited on mud flats, onto which tractive flows
occasionally arrive.
The present distribution of lacustrine facies enables
to define three sedimentary subenvironments: central
lacustrine, marginal lacustrine, and palustrine. The
distribution of the subenvironments is asymmetrical,
so that the marginal facies are better developed in the
SW margin, whilst the palustrine facies are predom-
inant in the SE sector. The central lacustrine subenvir-
onment is characterised by the development of black
organic muds and the growth of halite, calcite, ara-
gonite, and dolomite crystals intermingled with fila-
mentous cyanobacteria. The marginal subenvironment
contains grey or brown marls affected by desiccation
cracking and bioturbation. During extremely arid
conditions, this subenvironment appears partially cov-
ered by a salt crust, whilst during humid periods this
crust disappears and there is an early development of
black organic muds. The marginal subenvironment is
divided into an internal zone of low topographic
gradient, and an external zone that is only occasion-
ally covered by water. The external zone is also
characterised by the development of Salicornia veg-
etation, microbial mats, and stromatolitic construc-
tions, often affected by desiccation cracks. The active
palustrine areas consist in dark grey marls with gastro-
pods, reeds, and rushes.
The general evolution of the basin is characterised
by the development of three different stages. The first
one is an alluvial stage, which developed under
particularly arid conditions. The second stage is of
carbonate shallow lake and the third stage corresponds
to an evaporitic–carbonated sedimentation with a
high production of organic matter. The genesis of
the carbonated deposits in the last stage is directly
related to the development of cyanobacteria, which
facilitates the precipitation of aragonite, calcite and
dolomite. The genesis of salts results from periods of
high concentration of the lacustrine brine.
Acknowledgements
This research was supported by the Government of
Aragon, project no. P122/97. Our very special thanks
to Dr Angel Corrochano and Dr Pedro Barba of
Salamanca University and to Drs Joaquın Villena,
Angel Gonzalez, Arsenio Munoz, and Concha Arenas
for their assistance in sampling and helpful comments
during the investigation phases, and to Dr. Mariano
Laguna (University of Zaragoza) for the geochemical
analysis. Reviewers Sabine Castanier and Henry
Chafetz offered many useful suggestions.
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