GEOLOGICA CARPATHICA, 48, 5, BRATISLAVA, OCTOBER 1997

335–340

MICROMORPHOLOGICAL STUDY OF CALCRETES IN SOILS

OF THE ŽITNÝ OSTROV REGION, SOUTHWESTERN SLOVAKIA

SALEM YASEEN MEJEED

Department of Geochemistry, Faculty of Sciences, Comenius University, 842 15 Bratislava, Slovak Republic

(Manuscript received May 14, 1996; accepted in revised form December 12, 1996)

Abstract:

 Calcretes have developed in soils of the Žitný ostrov region in response to the mutual interactions between

the different prevailing geological and environmental conditions. The evapotranspirative climatic conditions and
shallow hydrocarbonatic groundwater play a very significant role in this aspect. Different micromorphological char-
acteristics have been described. The observed microstructures are of massive, intergrain micro-aggregate types, with
occasional occurrence of the pellicular type also, in addition to the vughy, intergrain channel, and fissure microstruc-
tures. The porosity of these calcretes is dominated by vughs, channels, and planes. There is also a dominance of monic
and porphyric related distribution patterns, and rarely of the chitonic type. Different pedogenic features are of special
interest, they represent mainly void coating and infilling, in addition to pedofeatures of pseudomorphic, textural and
impregnative types, nodules, intercalary crystals, and recrystallization.

Key words:

 micromorphology, soils, calcretes.

Introduction

Micromorphology is a valuable tool for studying pedogene-
sis. The carbonates in soils may initially be subdivided into
two main groups: primary from the parent rocks, and second-
ary, developed in the soil itself. The secondary carbonates
have been variously referred to as carbonate neoformations
(Dobrovolsky 1961), authigenic carbonates (Gile 1965), and
pedogenic carbonates (Blokhuis et al. 1968/1969; Sehgal &
Stoops 1972). In the course of this study the terms inherited
and pedogenic were adopted after Bullock et al. (1985) to de-
scribe the primary and secondary carbonates respectively.
Soils developed on sedimentary rocks and continental (detri-
tal) sediments may contain carbonates of both categories, and
they are, not always easy to distinguish. This is the case in the
Žitný ostrov region, as it was outlined by Hraško et al. (1972)
and Čurlík (1985). Numerous pedogenic carbonate features
have been reported and their genesis in relation to environ-
mental conditions in soils discussed (Dobrovolsky 1961; Gile
1961; Blokhuis et al. 1968/1969; Sehgal & Stoops 1972;
Mermut & Arnaud 1981; Mermut & Dasog 1986; Rabenhorst
& Wilding 1986a,b,c; Rabenhorst et al. 1991; West et al.
1988a,b; Drees & Wilding 1987). According to Dobrovolsky
(1961) there is considerable evidence to show that certain
types of carbonate neoformations are formed by supergene
processes which can be related to definite landscapes. This is
true in the case study of the superaqual landscape of Žitný os-
trov, where accumulation of pedogenic carbonates is ascribed
to a set of geochemical processes of a carbonatic nature
which range from neutral to gley processes (Mejeed 1993).

No detailed systematic micromorphological description

of calcretes from soils of Žitný ostrov region has been done
up to now. The objective of this work is to throw light on
this problem, with special stress on the pedogenic features
developed in these soils.

Materials and methods

A detailed description of the geographical location of the

study area, prevailing environmental conditions, and general
characteristics of the studied soil profiles have been given in
the preceeding contribution (Mejeed 1996).

Calcrete samples have been collected mostly by wet siev-

ing of the soil samples. They are mainly of nodular and pe-
dotubular forms (Čurlík & Mejeed 1996). Thin sections were
prepared by impregnating these calcrete forms with resin ma-
terial. The impregnated blocks were sliced and polished thin
sections were prepared. Later, a detailed micromorphological
description has been carried out according to the Handbook
of Bullock et al. (1985).

Results and discussion

Micromorphological description was done on calcretes

mainly of nodular and pedotubular forms (Pl. I: Figs. A–C).
Micromorphology of the studied samples revealed the pres-
ence of many types of microstructures ranging from massive
(Pl. II: Fig. A) to intergrain micro-aggregate (Pl. II: Figs. B,
C) structures, which are equivalent to the mudstone to wack-
stone and packstone fabrics, respectively, of Dunham (1962),
with more abundance of the intergrain micro-aggregate type.
Occasionally, the pellicular type is also represented (Pl. III:
Fig. A). This occurs in the case of the simple microstructures
which are related to the coarse and fine material. In addition,
other microstructures which are based on void patterns are
also represented. These occur frequently as vughy structure
(Pl. III: Fig. B), intergrain channel structure (Pl. III: Fig. C),
and sometimes as the fissure type (Pl. IV: Fig. A).

Accordingly, the porosity of the studied samples is domi-

nated by vughs, channels, and planes. According to Bullock

336                                                                                                 MEJEED

et al. (1985). The vughs are spherical to elongated, irregular
and not normally interconnected to voids of comparable
size. Channels are elongated, cylindrical or arched, with
regular conformation, usually smoothed wall and a uniform
cross section over much of their length. Planes are designat-
ed according to the ratio of the principal axes.

The coarse/fine (c/f) limit is set at 10 µm. This implies

that materials less than 10 µm are considered fine compo-
nents, and materials greater than 10 µm are coarse compo-
nents. In this work the c/f ratio, and at the same time the

abundance of the coarse materials were only relatively esti-
mated. Hence, generally for those samples showing massive
microstructure, there is only dominance of the fine material
and lack of the coarse material (Pl. II: Fig. A). In samples

Plate I: Fig. A:

 Pedotubular calcrete (rhizolith structure of root-

casts type). Scale in mm. Fig. B: Pedotubular calcrete (rhizolith
structure of root-tubles type). Scale in mm. Fig. C: Irregular nod-
ule containing some root moulds. Scale in cm.

Plate II: Fig. A:

 Massive microstructure with monic related distribu-

tion pattern. Micritic groundmass is impregnated by homogeneous
patches of iron oxides. Dark patches mostly represent manganese ox-
ides. XPL. Fig. B: Intergrain micro-aggregate microstructure with
open porphyric related distribution pattern. The coarse fraction is
randomely distributed and consists mainly of mica (biotite and mus-
covite), quartz, and feldspars. XPL. Fig. C: Intergrain micro-aggre-
gate microstructure with close porphyric related distribution pattern.
The coarse fraction is randomly distributed and consists mainly of
mica (biotite and muscovite), quartz, feldspars, carbonates, and rock
fragments. XPL.

MICROMORPHOLOGICAL STUDY OF CALCRETES IN SOILS OF THE ŽITNÝ OSTROV REGION                     337

showing intergrain micro-aggregate structure, this ratio may
range from 1 : 2 (Pl. II: Fig. B) to 1 : 1 (Pl. II: Fig. C). Qualita-
tively the coarse components consist substantially of quartz,

mica (biotite and muscovite), feldspars (orthoclase, micro-
cline, perthitic feldspars, plagioclase), detrital carbonates
(inherited), as well as some chlorite, amphiboles, and rock
fragments (Pl. II: Figs. B, C; Pl. III: Fig. A). These minerals
are randomly distributed within the fine groundmass and
mostly have subhedral to anhedral shapes. In addition to sin-
gle mineral grains, inorganic residues of biological origin are

Plate III: Fig. A:

 Intergrain micro-aggregate to pellicular micro-

structure with close porphyric to chitonic related distribution
pattern.The coarse mineral grains are randomely distributed. Patch-
es of microsparite are distributed within the micritic groundmass.
PPL. Fig. B: Vughy to massive microstructure. Pedofeatures are mi-
crocalcitic coating of voids, dense complete and incomplete infill-
ing of voids. Micritic groundmass is inhomogeneously impregnated
with iron oxides. XPL. Fig. C: Intergrain channel microstructure.
Channels after roots show loose discontinuous infilling with soil
material (textural pedofeature). The channel on the right side shows
compound pedofeature resulting from association of infilling with
microcalcitic coating (crystalline pedofeature). XPL.

Plate IV: Fig. A:

 Fissure microstructure with a pedofeature of dense

complete drusy calcite infilling of planar void. Microsparitic dense
complete infilling of some vughs is also visible.The micritic ground-
mass is inhomogeneously impregnated with iron oxides. XPL. Fig. B:
Dense complete drusy calcite infilling of channel. XPL. Fig. C:
Rhombic calcite crystal embedded in micrite giving typical open por-
phyric related distribution pattern. XPL.

338                                                                                                 MEJEED

occasionally represented among the coarse fraction. These
are mollusc shells consisting mainly of calcium carbonate
(aragonite or calcite) and are thus characterized by high in-
terference colours. The fine materials consist of carbonate
mud (micrite), occasionally moderately impregnated with
iron oxides, and sometimes with manganese oxides (Pl. II:
Fig. A; Pl. III: Fig. B; Pl. IV: Fig. A; Pl. V: Fig A). This
composition of the fine groundmass is reflected in its b-fab-
ric which usually shows a strongly crystallitic fabric charac-
terized by the presence of small birefringent crystallites
(calcite).

The represented related distribution patterns (RDP) are in

line with the observed microstructures in the studied samples.
Here there is a dominance of monic (Pl. II: Fig. A), and por-
phyric, both open (Pl. II: Fig. B) and closed (Pl. II: Fig. C) re-
lated distribution patterns. Occasionally the chitonic type was
also observed (Pl. III: Fig. A). The porphyric RDP is charac-
terized by coarse materials embedded in the dense ground-
mass of fine material. The chitonic RDP is best described by
the presence of fine materials as thin incomplete coatings on
coarse materials.

The concept of pedofeatures was introduced by Brewer &

Sleeman (1960 in Bullock et al. 1985) for all units resulting
from past or present pedological processes.

Many types of pedofeatures were observed in this study es-

pecially those of crystalline type. They occur mainly as mi-
crocalcitic void coatings of typic and crescent type (Pl. III:
Fig. B) and are usually developed on vugh and channel walls.
Microsparitic and drusy calcite infilling of vughs (Pl. III:
Fig. B), channels (Pl. IV: Fig. B), and planes (Pl. IV: Fig. A)
represent the further noted crystalline pedogenic features.
They occur as both dense complete and dense incomplete in-
filling. Another type of void infilling which was also ob-
served is the loose discontinuous infilling of channels by soil
material akin to the groundmass material (Pl. III: Fig. C).
This type of pedofeature represents a textural type according
to the classification of Bullock et al. (1985).

The other interesting crystalline pedofeatures belong to the

pseudomorphic type according to Bullock et al. (1985). They
are usually composed only of crystalline material pseudomor-
phosing, partially or completely, plant tissues, faunal remains
or rock or soil fabric. In the case study, calcite is completely
pseudomorphosing plant roots and preserving their cellular
structure (Pl. V: Figs. B, C). This is equivalent to the “petrifi-
cation” process (i.e. calcification) of Klappa (1980). It results
from the filling of the actual root zone by pedogenic carbon-
ate during the life of the plant and hence the cellular structure
of the root can be seen clearly in the resulting carbonate.
Sometimes, an envelope of micritic calcite (tubule) is formed
around the root.

The amorphous pedofeature of impregnative type (Bullock

et al. 1985) represents another prominent pedofeature in this
study. It is formed by amorphous material (iron oxides or less
common manganese oxides) impregnating the nodules fab-
ric. These nodules are moderately impregnated as the origi-
nal material (carbonates) which has been impregnated by the
iron oxides is common and clearly identifiable. Impregnation
could be of homogeneous central type, with iron oxides uni-
formly impregnating the central part of the nodules leaving

free-iron oxides peripheral rims (Pl. V: Fig. A), or some-
times exists as homogeneous patches of iron oxides (Pl. II:
Fig. A), or inhomogeneous impregnation (Pl. III: Fig. B).
These pedofeatures are ascribed to the oxidizing carbonatic
processes that prevail in some parts of Žitný ostrov, which
are also reflected in the morphology of soil profiles where
special horizons have been developed of oxidizing (bright
brown mottling) phenomena.

The above mentioned pedofeatures represent simple ped-

ofeatures composed of a single component or characterized

Plate V: Fig. A:

 Part of nodule with massive microstructure and

almost homogeneous central impregnation. XPL. Fig. B: Petrified
root and consequent preservation of its cellular structure. PPL.
Fig. C:

 Petrified root. PPL.

MICROMORPHOLOGICAL STUDY OF CALCRETES IN SOILS OF THE ŽITNÝ OSTROV REGION                     339

by an internal fabric. Frequently, pedofeatures are a com-
posite mixture of two or even more fabric units, in which
case they are termed “compound” (Bullock et al. 1985).
They are important because they often indicate a change in
environment. Such compound pedofeatures were occasion-
ally observable in this study. For example, an association
between channel-microcalcitic coating (crystalline pedofea-
ture) and channel infilling with soil material (textural ped-
ofeature) results in a further compound (Pl. III: Fig. C).

Other observed pedofeatures are those which are not related

to voids, grains and aggregates (Bullock et al. 1985 ) such as
nodules and crystals. We should stress that the micromorpho-
logical description in this study was done on nodules already
separated from soil samples, and hence they should represent
one of the main pedofeatures developed in the soils of Žitný
ostrov. Observed nodules or glaebules (Pl. I: Fig. C) mostly
have an undifferentiated internal fabric (structureless), of
course are calcareous and sometimes impregnated with Fe and
Mn oxides, distinct because they could be easily removed from
the soil mass, and are mostly irregular in shape. Wright &
Tucker (1991) have considered diffusion of carbonate to cer-
tain sites as a critical factor in nodule formation, which is usu-
ally followed by precipitation and displacive growth, for most
nodules contain very little of the original matrix.

Another characteristic pedofeature is the presence of in-

tercalary rhombic calcite crystals with well developed sur-
faces, i.e. euhedral (Pl. IV: Fig. C). They are embedded in
the carbonatic fine groundmass and frequently show typical
porphyric related distribution pattern. These isolated crys-
tals are considered to be crystalline pedofeatures by Bullock
et al. (1985). Such rhombic calcites are common in recent
subaerial carbonates and have been described from soils and
weathering profiles (Folk 1971, 1974; Chafetz & Butler
1980; Wright 1982). Folk & Land (1975) considered that in
very dilute waters, without competing cations such as Na

+

and K

+

, calcite forms rhombohedra 2–10 µm in size. The

rhombic form of the calcite crystals has been attributed to
formation in the fresh water realm under conditions of slow
precipitation (Folk 1971, 1974).

A recrystallization process may probably proceed in the

studied soils, where micrite is recrystallized to microsparitic
calcite which is usually distributed as patches within the mi-
critic groundmass (Pl. III: Fig. A). However, this observation
is limited to very few samples. The literature on modern and
ancient calcretes contain many references to recrystallization
(Retallack & Wright 1990). Sehgal & Stoops (1972) pro-
posed that the zones of coarse, granular calcite in the soils of
Punjab are formed by recrystallization of microcrystalline
calcite.

Conclusions

From the previous discussion it can be concluded that the

following forms of pedogenic carbonates are forming in the
soils of Žitný ostrov:

— Accumulation in the fine groundmass (micritic).
— Void coating, commonly associated with vughs and

channels.

— Void infilling associated with vughs, channels, and

planes.

— Intercalary rhombic calcite crystals.
— As products of recrystallization of micrite resulting in

microsparitic calcite within micrite.

— Pseudomorphosing of plant root (calcification).
Among these forms the micritic ones are the most abundant

followed by void coating and infilling, while other forms are
much less frequent.

Such carbonate forms are ascribed to the epigenetic geo-

chemical carbonatic processes prevailing in the area. These
processes are mainly neutral, but they become oxidizing at
some places as is evident from iron oxide-impregnations, and
elsewhere they become gley leading to remobilization of iron
oxides as is illustrated by some iron oxide-impregnated nod-
ules with iron oxide-free peripheries. These processes are also
shown by the soil morphology, where different horizons have
been developed, calcic, oxidizing (bright brown mottling), and
gley/reducing (grey mottling). Thus, both micromorphological
and morphological evidences indicate the influence of ground-
water on soil cover in Žitný ostrov which results in consequent
formation of calcretes and other types of duricrusts (ferricretes
and manganocretes).

Acknowledgement: 

This work is a part of a Ph.D. thesis sub-

mitted in the Department of Geochemistry, Faculty of Natural
Sciences, Comenius University, Bratislava. The work was
supported through the project of soil monitoring carried out in
the Soil Fertility Research Institute, Bratislava. Thanks are
due to Doc. Ján Čurlík, Soil Fertility Research Institute, Brat-
islava, the supervisor of the work for his help and discussion.
My sincere gratitudes are due to Prof. Georges Stoops, Uni-
versity Gent, Belgium for reading and improving the manu-
script.

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