sedimentary facies distribution.pdf

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.


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.


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.


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.


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.


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


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.


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


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


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).


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.


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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