Solution and recrystallisation processes and associated landforms

in gypsum outcrops of Sicily

Francesco Ferrarese a, Tommaso Macaluso b, Giuliana Madonia b,Adelina Palmeri b, Ugo Sauro a,*

aDipartimento di Geografia, Universita di Padova, Via del Santo 26, Padua 35123, ItalybDipartimento di Geologia, Universita di Palermo, Corso Tukory 131, Palermo 90134, Italy

Received 20 July 2001; received in revised form 20 February 2002; accepted 14 March 2002

Abstract

Four small areas of Messinian (Upper Miocene) age gypsum, outcropping in western Sicily, are described. Messinian age

evaporites are found in Sicily over a 1000-km2 area. Here, gypsum outcrops extensively as a consequence of soil erosion induced

by human impact. Geomorphological maps show how the rocky surfaces are characterized by a wide range of forms. There are

large, medium, small, and microsized forms, which can be identified as belonging to different morphotypes. The morphotypes can

be classified into two main categories: those that originated by solution and those that originated through recrystallisation.Four

areas, illustrated by geomorphological maps, were specifically chosen to describe a type of medium-sized form: dome-like hills.

These medium-sized forms are covered by a mosaic of smaller forms, related to both the previous categories: different types of

karren and of ‘‘expansion’’ forms. The types of karren can be explained as the results of the solution process under different hydro-

dynamical behaviour; the dome-like hills and other related ‘‘expansion’’ forms are more difficult to understand. These

‘‘expansion’’ forms can be explained by the same process that leads to the development of gypsum tumuli. The outcrops of gypsum

lacking soil cover and influenced by alternating seasonal water conditions of surplus and deficit are affected by both solution and

recrystallisation processes. During the wet season, the water soaks into the rockymass, filling all the fissures and pores of the outer

rocky layer from a few centimetres to some metres below the surface. During the dry season, there is a capillary upward motion of

the water solution. Near the surface, gypsum precipitates from the oversaturated solution, increasing the crystal size or forming

new crystals. In this way, during the dry season, there is a pressure increase in the outer gypsum layers, which is responsible for the

development of a ‘‘gypsum weathering crust’’ and characterised by many different forms such as gypsum tumuli, pressure ridges,

pressure humps, and other related small forms. The crust may also lead to the development of mega-tumuli and dome-like hills.

From the morphostructural point of view, the dome-like hills do not seem to be controlled by the strike, dip, or fissuring of the

gypsum beds. Their evolution seems to be linked to the fact that on most of the dome surfaces, the weathering crust is evolving

through a nearly isotropic field of stresses, resulting in volume increase in the outer gypsum layer.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Evaporites; Gypsum karst; Weathering; Climatic geomorphology; Holocene; Sicily

1. Introduction

During the last two decades, knowledge about

gypsum karst has grown, thanks to a wide range of

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0169 -555X(02 )00159 -9

* Corresponding author.

E-mail address: ugo.sauro@unipd.it (U. Sauro).

www.elsevier.com/locate/geomorph

Geomorphology 49 (2002) 25–43

research (Agnesi et al., 1986; Agnesi and Macaluso,

1989; Ford and Williams, 1989; Calaforra et al., 1993;

Macaluso and Sauro, 1996, 1998a,b; Calaforra, 1996,

1998; Klimchouk et al., 1996–1997; Calaforra and

Pulido-Bosch, 1997, 1999a,b; Ferrini, 1999; Gutierrez

et al., 2001; Macaluso et al., 2001). Nevertheless,

some of the basic processes that affect gypsum envi-

ronments have yet to be understood in detail. The

analysis of the landforms and the evolution models

inferred from field observations are the starting points

for detailed studies, which will aim to verify and

improve the models, leading to a better understanding

of the processes.

The goal of this paper is to describe some typical

landforms of gypsum karst in Sicily and to discuss

models of the dynamics of the gypsum outcrops. In

particular, some mega-tumuli and dome-like hills will

be illustrated with the support of large scale geo-

morphological maps. The surfaces of these medium-

sized forms comprise a mosaic of small forms, some

of which are linked to the same processes, leading to

the development of the dome-like landforms. Maca-

luso and Sauro (1998a) have already presented a

preliminary description of these types of forms.

An overview of the basic processes and the most

common forms of gypsum karst, found particularly in

the Mediterranean region, is the first necessary step in

introducing further discussion. In addition to normal

slope processes, the two main processes taking place

on gypsum outcrops are solution and recrystallisation

processes.

The typical erosional process on gypsum is the

ionic solution of the rocky mass. In the Santa Ninfa

plateau (Sicily), the mean content of gypsum dis-

solved in the spring waters is nearly 2.5 g/l, and the

estimated average thickness of rock eroded by sol-

ution is of 382 mm/ka (Agnesi et al., 1989). The time

necessary for a solution crossing a gypsum bed to

reach saturation is very short—about 18 min (Cala-

forra, 1998). The solution rate is about one order of

magnitude greater than that of limestone.

According to this process, most of the surface

forms are explainable as solutional forms. The

gypsum presents a karst-type relief, similar to that

developed on limestone. The solution of gypsum

produces landforms that are different in size and

characteristics. Most forms are the result of the

space and time distribution of the chemical solution

in relation to different hydrodynamic and hydro-

logical processes. These processes are controlled by

geological, geomorphological, climatic, and biolog-

ical factors.

The best known medium-sized form of gypsum

karst is the doline, which, in the gypsum karst of

Sicily, shows mostly intermediate characteristics

between a typical solution doline on limestone and a

blind valley (Agnesi et al., 1989; Sauro, 1995). It is

the expression of the transition from a surface hydro-

graphic network to an underground one, marked by a

well-defined conduit system. So most of the gypsum

dolines of Sicily are point recharge-type depressions

as described by Ford and Williams (1989).

A classification of karren developed on gypsum

was illustrated by Macaluso and Sauro (1996). The

different types of solution forms are described in

relation to order of magnitude, the morphometric

characteristics, the lithology, the processes, and some

environmental variables. In the cited paper, it is noted

that in the gypsum karst of Sicily, fractures only rarely

evolve as fissures and grikes, and pits and shafts are

rare; so epikarst is missing or limited. Grikes and

corridors separating gypsum tables or pinnacles are

present only on some gypsum pavements, outcrop-

ping from highly permeable covers, such as alluvial

sands and gravels. Runnels are not very common,

especially on selenitic (macro-crystalline) gypsum

(Fig. 1). Besides this, the biological processes of

pioneer vegetation colonisation and soil formation

do not facilitate the solution process and sometimes

they play a protective action on the rocky surface;

such certain lichen colonies do.

The conclusion of Macaluso and Sauro (1998a) is

that: ‘‘From the strong analogy between most of the

solution forms on limestone and on gypsum, in

particular between the hydrodynamically controlled

forms, it is possible to infer that the differences

between the physico-chemical process of limestone

corrosion, in which three phases are engaged (rock,

water and carbon dioxide), and the two phases of the

simple solution of gypsum and salt (rock and water),

are inconsequential from the point of view of the style

of the elementary forms. Most of the differences

between the morphological evolution of the limestone

environments and that of the gypsum environments

are probably linked to the different role played by

pedogenetical and biological processes’’. In other

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4326

words, the appearance of the same types of forms, like

rills, runnels, and so forth, in limestone (see Perna and

Sauro, 1978), in gypsum, and in salt demonstrates that

the evolutionary patterns of such forms are not influ-

enced by the differences of the solution processes, but

are controlled mainly by the hydraulic behaviour of

the runoff.

From the analysis of the landforms, it is also

evident that, beside, the solution process, recrystalli-

sation also occurs. The clearest results of this process

are gypsum tumuli (Pulido-Bosh, 1986) produced

from the swelling of the outer gypsum layer and

described and interpreted in some recent papers (Cal-

aforra, 1996, 1998; Calaforra and Pulido-Bosch,

Fig. 1. Solution runnels on a macro-crystalline gypsum outcrop in the area of Calafimi (Tp).

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 27

1997, 1999a; Macaluso and Sauro, 1998a). In partic-

ular, in Calaforra (1998) and Calaforra and Pulido-

Bosch (1999b), the different hypotheses for the proc-

ess leading to the development of these forms are

discussed. Recrystallisation is considered to be the

only reasonable explanation for the increase of vol-

ume of the outer gypsum layer.

Gypsum tumuli are dome-like bulges ranging from

some decimetres to about 15 m in diameter and a few

centimetres to several decimetres in height. These

structures consist of a circular shell of gypsum arched

upwards above a cavity (Fig. 2). Macaluso and Sauro

(1998a) have observed how tumuli occur only on

gypsum outcrops directly exposed to the outer envi-

ronment without regolith and soil cover. They

describe other forms as pressure ridges, polygons,

pressure pans, pancakes, pressure humps, and steps,

interpreting them as peculiar morphostructures and as

the expression of an increase in volume of the outer

rocky mass. Pressure ridges are rises between pan and

pancake depressions, formed by two gypsum lips bent

upwards and often separated by a fissure. Their

heights range from a few millimetres to some deci-

metres according to lithological control, the morphol-

ogy of the surface, and the evolutionary stage of the

form. Sometimes silt and clay deposits are pinched

between the lips and inside the clefts between two

polygons.

Polygons are made by a system of rocky plaques

delimited by pressure ridges. The size of the polygons

is variable—from a few centimetres in balatine and

alabastrine gypsum to several metres in macro-crys-

talline gypsum. Pressure pans and pancakes are closed

or nearly closed basins, each having developed inside

a polygon with the margins bent upwards. Both forms

may be considered inverted forms of the gypsum

tumuli, or ‘‘reversed tumuli’’. While in pressure pans

the borders are single ridges, in pancakes, the borders

of the two contiguous polygons are separated, giving

rise to two small subparallel banks. Their heights

range from a few millimetres to some decimetres.

Pressure humps are nearly pyramidal or conical pro-

tuberances, from a few decimetres to more than 1 m

high. They develop in correspondence with a junction

point of the borders of three or four polygons. Steps

are similar to pans and/or pancakes but develop on

steep slopes, resulting in stair-like features.

Macaluso and Sauro (1998a) have also illustrated

some larger forms called mega-tumuli and dome-like

hills, and described them as the dynamic outcome of a

Fig. 2. A large partially collapsed gypsum tumulus in the area of S. Angelo Muxaro, S. Elisabetta. The tumulus has developed in macro-

crystalline gypsum and its shell does not correspond to a bed.

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4328

‘‘gypsum crust’’ resulting from recrystallisation and

the consequent swelling of the outer gypsum layer.

2. Geological outline of the studied area

In western Sicily, Messinian age (Upper Miocene)

gypsum outcrops cover an area larger than 1000 km2

(Fig. 3), forming most of the evaporitic sequence of

western Sicily. The evaporitic sequence, locally some

hundreds metres thick, consists of diatomites, evap-

oritic limestones, gypsum, and salt with clay and

intercalations of marls and carbonates. It lies uncon-

formably on the Terravecchia Fm. (Upper Tortonian–

Lower Messinian), a silico-clastic rock unit, and is

covered unconformably by the Trubi Fm. (Lower

Pliocene), made up of pelagic calcilutites and calcar-

enites (Catalano and Esteban, 1978; Catalano, 1979;

Decima, 1982; Catalano, 1986). In particular, two

main units are distinguishable: (a) lower evaporitic

unit, made up of diatomites, evaporitic limestones,

gypsum with marly intercalations, and salt (halite and

kainite); and (b) upper evaporitic unit, made up of

gypsum with clay and sand intercalations, bioclastic

limestones, and clayey sands (Catalano, 1986).

The sedimentation of the evaporitic sequence is the

consequence of the closure of the Mediterranean basin

that partially desiccated during the upper Miocene.

The mineral precipitation in the hypersaline basins,

occasionally fed by water coming from the Atlantic

Ocean, led to the accumulation of the evaporitic

sequence (Di Stefano and Catalano, 1978; Lo Cicero

et al., 1997). The Trubi Fm. marks the reestablishment

of the communication with the ocean. After the

deposition, the evaporitic bodies were folded, with

the development of E–W-, ENE–WSW-, and NW–

SE-oriented synclines and anticlines, translated south-

wards and uplifted (Ghisetti and Vezzani, 1983).

The lithology of the gypsum outcrops, nearly

homogeneous from the chemical point of view,

presents a large variety of different lithofacies such

as the selenitic (or macro-crystalline), alabastrine,

laminated balatino, and detritic lithofacies. There is

a wide range of crystal sizes, from tenths of milli-

metres to several decimetres, and of degrees of

porosity of the rock mass. It has been proved that

near the surface, for a thickness of several tens of

metres, very little anhydrite is present inside the

gypsum mass. The analyses carried out in some caves

of the Santa Ninfa plateau demonstrates that the

gypsum beds, even if characterised by frequent recrys-

tallisation and substitution phenomena, are not the

result of the hydration of anhydrite (Bertolani and

Rossi, 1989). Silty and clayey intercalations inside the

gypsum series are very scarce. In some facies, the

amount of kaolinite may be larger that that of chlorite

(Bertolani and Rossi, 1989).

At present, large surfaces of gypsum are without

soil cover as a consequence of soil erosion induced by

the different forms of human impact, and in particular

by forest clearing, fires, and by sheep and goat

grazing. In addition, the salinity of the soils does

not favour vegetation cover and, therefore, erosion

processes are more active.

3. The mapped landforms

Very large-scale geomorphological maps of the

sample areas are the result of a field survey firstly

based on the use of radio-and laser telemeters, optical

clinometers, and compasses. Selected topographic

profiles were constructed and plotted inside a GIS

computer application (Idrisi GIS package) to get the

digital elevation models, later verified in the field and

utilised as bases for the geomorphological mapping.

The four surveyed areas have been selected

between many similar landscapes in the evaporitic

rocks of western Sicily because they show typical

associations of a wide range of forms different for

size and genesis. The geomorphological maps (Figs.

4–7) illustrate three dome-like summits and a ridge

especially representative of assemblages of both

medium- and small-scale forms resulting through

swelling processes of the outer gypsum layer.

The forms represented on the maps have been

listed in the legend of Fig. 4. It is possible to

separate six main categories of forms: (1) features

that are expressions of the geological structure; (2)

features resulting mostly from the dynamics of the

gypsum crust; (3) features resulting from both the

crust dynamic and solution processes; (4) features

resulting mostly from solution processes; (5) fea-

tures resulting from solution and other weathering

processes; and (6) features resulting from human

activity.

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 29

Of the features that are expressions of the geo-

logical structure, only lines of fracture or of bedding

plane have been considered (1a). In fact, it is difficult

to distinguish between the two categories; the gypsum

often appears massive, without structures of these

types.

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4330

Of the features resulting mainly from the crust

dynamics, the following features have been distin-

guished: (2a) pressure ridges, (2b) polygons, (2c)

pressure pancakes, (2d) pressure humps, (2e) tumuli,

(2f) pressure steps, (2g) cavities under the crust, and

(2h) depressions formed by collapse of the crust. Not

all of these forms have been represented at the exact

scale. The polygonal pattern is often too dense to

allow an exact drawing. The pressure pans are often

too shallow to be clearly distinguished and only some

of the most evident pancakes inside the polygons are

indicated. Also only the most distinct pressure steps

have been identified.

The features resulting from both the crust dynamic

and the solution process are: (3a) irregular surfaces

characterized by closed basins, and (3b) open cavities

(swallow holes). Some pans inside the polygons

appear not only as the result of the crust deformation,

but also of solution processes, effective while water

exists inside the depression. Open cavities (swallow

holes) are rare cavities that occupy specific points of

the polygonal patterns, where the stress distribution

has favoured the opening of a fissure, later enlarged

by the solution by flowing water.

The features resulting mainly from solution pro-

cesses are: (4a) the karren, and (4b) the solution

levels. Even if there are different types of karren, it

is not possible to distinguish at this scale the different

solution forms. The solution forms are sometimes

medium-scale forms very similar to those of limestone

outcrops.

Of the features resulting from solution and other

weathering processes, a generic category of rounded

forms by solution and granular disintegration is indi-

cated (5a).

Of the features resulting from human activity, only

the (6a) box-like and (6b) grotto-type proto-historical

grave forms are present in the study area. The first

type was excavated on subhorizontal surfaces as a

hole, the second one as an opening on the nearly

vertical surfaces.

3.1. Cozzo Balatazza (Fig. 4)

Cozzo Balatazza is a dome-like summit bulging

from a slope. Its base is nearly circular with a main

diameter elongated for about 170 m in the direction

SE–NE, and a minor diameter of 160 m. The area is

of about 22,000 m2 and the volume is 580,000 m3.

The average height is 26 m. In the upper part, two

small cupolas separated by a saddle are easily recog-

nisable.

The best expressed polygon network is on the

south side. A certain number of open cavities (swal-

low holes) show a random distribution.

3.2. Cozzo di Rocca Entella (Fig. 5)

Cozzo di Rocca Entella is a small dome bulging up

from the base of a large slope. The average diameter

and height are, respectively, 90 and 23 m. The

maximum height is 42 m and the minimum is 17 m.

The area is of about 6500 m2 and the volume is

150,000 m3.

The best expressed polygonal network is on the

south side. On the steep northern slope, there are

typical pressure steps.

3.3. Cozzo Ogliastra (Fig. 6)

Cozzo Ogliastra is an elliptical dome-like summit

emerging from a slope, with its main axis elongated in

a SSW–NNE direction for about 100 m; the minor

axis is 75 m long. The minimum height above the

surrounding surface is 16 m, the maximum 22 m, the

average 14.3 m. The volume is of about 100,000 m3.

The top is nearly flat and marked by solution forms, in

particular by small closed basins. The polygons and

Fig. 3. Distribution and characteristic of the Messinian evaporites of Sicily. (a) Map of the Messinian-age gypsum outcrops in Sicily. Some of

the cited sites are indicated. (b) Sketch of the evaporitic sequence of western Sicily (modified from Catalano and Esteban, 1978; Catalano, 1979;

Decima, 1982). Legend: (1) marly limestones of Trubi Fm. (Lower Pliocene); (2) clayey sands ‘‘Arenazzolo’’ (Upper Messinian); (3) gypsum

with clay and sand intercalations (Upper Messinian); (4) bioclastic limestones (Upper Messinian); (5) intercalated marls (Upper Messinian); (6)

gypsum with marly intercalations (Lower Messinian); (7) salt (Lower Messinian); (8) evaporitic limestones (Lower Messinian); (9) diatomites

‘‘Tripoli’’ (Lower Messinian); (10) reef limestones (Lower Messinian); (11) conglomerates, marls, and sands of Terravecchia Fm. (Lower

Messinian–Upper Tortonian); (12) unconformity.

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 31

Fig. 4. Topography and geomorphological features of Cozzo Balatazza.

F.Ferra

reseet

al./Geomorphology49(2002)25–43

32

Fig. 5. Topography and geomorphological features of Cozzo di Entella (see the legend in Fig. 4).

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 33

Fig. 6. Topography and geomorphological features of Cozzo Ogliastra (see the legend in Fig. 4).

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4334

the pressure ridges are best developed on the southern

slope.

3.4. The ridge of S. Angelo-Muxarello (Fig. 7)

The ridge of S. Angelo-Muxarellois a complex

ridge, near the Case Salamone and not far from

Muxarello on the road to Sant’Angelo Muxaro. It is

a peculiar form consisting in a system of small domes

or mega-tumuli, aligned in E–W direction for about

240 m. The width is about 90 m. The height decreases

from west to east. With reference to the surrounding

surfaces, the highest dome presents a maximum

height of 32 m and a minimum of 17 m. The other

domes appear as humps on the ridge. Some small

domes present here seem to be different from other

larger dome-like hills for the presence of some cav-

ities below the surface. Because of the thickness of the

‘‘crust’’, these cavities are not accessible and so it is

difficult to understand the precise structure of the

forms. Anyway, these bulging forms could represent

intermediate morphostructures between large tumuli

and dome-like hills.

The surface occupied by the ridge is 15,000 m2 and

the volume is about 380,000 m3. The average height is

about 25 m.

The polygonal pattern is best expressed along the

axis of the ridge, on the acclivitous surfaces. The

southern slope has more examples of small closed

basins, rounded forms by solution and granular dis-

integration, and swallowing open cavities than the

northern one.

4. Some characteristics of the outer gypsum layer

Starting from the description of the forms, we will

try to understand the singular geomorphological envi-

ronment of the gypsum surface and of the outer

gypsum layer with special regard to the evolution of

the bulging forms, the mega-tumuli, and the gypsum

domes. Preliminary steps for describing a model are to

recognise the main characteristics of the rocks where

the processes take place, with reference to the forms

developed, and to consider the behaviour of the water

solution.

Different types of gypsum show a wide range of

grain size fabric and consequently very differentFig. 7. Topography and geomorphological features of the ridge of S.

Angelo-Muxarello (see the legend in Fig. 4).

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 35

primary porosity and permeability. There are very

fine-grained gypsum types such as balatine gypsum

and alabastrine gypsum and coarse-grained gypsum

types such as selenitic (macro-crystalline) gypsum,

with crystals ranging from millimetric to decimetric

sizes. There are also many types of detritic gypsum

containing clasts of different kinds and dimensions.

Some gypsum types are densely bedded; others are

massively bedded or without bedding planes. Also

fracture density is very different from place to place;

some fractures are of tectonic origin, while others are

the consequence of tensional relaxation in the context

of the slope processes. The tectonic setting varies as is

evidenced by the dip of the strata ranging from the

horizontal to the vertical.

While some types of forms are common in nearly

all the gypsum lithologies and structural and geo-

morphological settings, others forms are linked mostly

with specific lithological and structural conditions. In

particular, the polygons and the pressure ridges are

present on most bare gypsum surfaces and their size

seems to be partly related to the crystal size: very

small polygons are present on the balatine and detritic

sandy gypsum, a little larger on alabastrine gypsum,

and larger still on macro-crystalline gypsum. On the

macrocystalline gypsum, small tumuli develop in the

central part of some polygons. Single and multishelled

tumuli exist. The intercrystalline voids of a few tumuli

in selenitic gypsum are filled with silt and clay sedi-

ments, as in the pressure ridges and some pressure

humps. The bedding plane slopes in macro-crystalline

gypsum provide a very favourable environment for

the development of both large polygons and tumuli.

Mega-tumuli and dome-like hills have developed

mainly in massive macro-crystalline gypsum. In gen-

eral, the elementary swelling forms do not seem to be

dependent strictly on the lithological and structural

conditions, even if some types of forms are found at

their best in particular situations.

The behaviour of the water solution inside the

upper gypsum layer must be controlled by the litho-

logical and structural conditions and also by the

previous development of the secondary karst ‘‘poros-

ity’’ inside the rocky mass.

In comparison with the limestone karst, the gyp-

sum karst geosystem of Sicily shows a completely

different epikarst structure. From the analysis of the

dolines, of the caves, and of the escarpments of the

quarries, it is possible to recognise the main character-

istics of the epikarst. The porosity of the rocky mass is

often greater than that of limestones, but the network

of partly hierarchized small fissures is poorly devel-

oped; a few main conduits are present. Some of these

conduits start as swallow holes at the bottom of

dolines or at the end of blind valleys. These conduits

are fed mostly by the water flowing at the rock/air or

cover/rock interfaces, rather than by the aquifer hosted

in the epikarst. The water reservoir stored in the

epikarst could be relatively consistent in the macro-

crystalline gypsum, but does not seem to release water

downwards very much.

Some cavities such as clefts and interbedding

holes may be filled with silt and clay sediments

that are light grey in colour, derived from impur-

ities present within the gypsum mass, from soils,

from the precipitation of calcium carbonate, and

from residuals of the ‘‘Trubi’’ formation. Water

flows slowly inside these fillings, which also rep-

resent small water reservoirs.

The solution process of limestone is a reversible

chemical reaction: part of the mass dissolved at and

near the surface will precipitate as concretion and

flowstone in the fissures and in the caves. In gypsum

karst, the precipitation of gypsum is not so obvious as

in carbonate karst. In most of the middle latitude

gypsum caves, there are few gypsum spleothems,

which are caused mainly by evaporation processes.

The chemical deposits in many of these gypsum caves

are formed largely by calcium carbonate and are

related to the ionic combination of Ca2+, deriving

from the dissociation of gypsum, with HCO3� (hydro-

carbonic ion) present in the natural solutions (Forti

and Rabbi, 1981). In some places, on exposed gypsum

surfaces, it is possible to find stalagmites and flow-

stones of calcium carbonate as remnants of the floors

of caves that have completely dissolved except for

these fillings (e.g., in the areas of Santa Ninfa and

Serra Ciminna in Sicily).

From the analysis of the landform evolution, it is

easy to demonstrate that in gypsum epikarst, precip-

itation of CaSO4�2H2O also occurs. But the modalities

of these precipitation processes are different from

those known for the carbonate rocks. A preliminary

study and a proposal of an evolutionary model of the

outer gypsum layers were presented in Macaluso and

Sauro (1998a).

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4336

5. A dynamic model of the weathering crust

In the evaporite karst of Sicily, the migration and

precipitation of gypsum are linked to the soil water

cycle of the Mediterranean climatic regime character-

ised by a seasonal alternation of water surplus (wet

winter season) and water deficit conditions (dry sum-

mer season). Here, all the forms described find their

dynamic environment in the outer gypsum layer. This

layer is affected by an increase of volume, leading to

the expansion of the rocky mass, which changes its

structure, forming a ‘‘gypsum crust’’. The ‘‘gypsum

crust’’ is the outer gypsum layer where gypsum

reprecipitation occurs and expansion prevails, imped-

ing the opening of fissures and the development of

grikes (Macaluso and Sauro, 1996, 1996–1997).

The shells of the tumuli are good examples of this

dynamic gypsum layer formed by the alternation of

solution and reprecipitation processes (Calaforra,

1996, 1998; Calaforra and Pulido-Bosch, 1999b).

The inner limit of the tumulus crust is usually litho-

logical (i.e., a very thin clay stratum) or structural (i.e.,

bedding plane, pseudo-bedding plain parallel to the

topographical surface). These structural elements may

influence the downward migration of the gypsum

crust.

If the crust coincides with the shells of the tumuli,

its thickness ranges between a few centimetres and

some decimetres. But if we consider all the elements

of the swelling of the outer gypsum layer on some

slopes in Sicily, the crust thickness may reach some

metres (Macaluso and Sauro, 1998a,b). In fact, in

some quarries in the macro-crystalline gypsum, it is

possible to observe open fissures starting 1.5–3 m

below the surface.

An improved version of the model of the dynamics

of the crust illustrated by Macaluso and Sauro (1998a)

is outlined in Fig. 8a: during the wet season, the

rainfall starts overland flow and the water dissolves

gypsum on the surface. The silt transported by the

runoff is trapped in the pores and cavities; part of the

water solution manages to penetrate the rock. The

solution reaches saturation (with CaSO4) near the

surface or a few decimetres inside the rock, except

along the fissures where the saturation front may

reach downward (the B line in Fig. 8a). The solution

continues to penetrate both inside the pores and in the

silt and clay filling of the fissures and of other cavities

such as the tumuli and pressure ridges and humps (2),

reaching a water porosity front (C). The water may

also flow laterally (3 in Fig. 8a).

If there is a long period of water surplus, more and

more water manages to penetrate the rock and the

fissures; the porosity and the water capacity of the

rock increase and the solution flowing at the rock/

fillings interface of the fissures enlarges the cracks;

the speed of the water circulating in the rock increases

and the saturation front migrates downward.

During the dry season, the rock and the fillings of

the cracks lose water by evaporation (4). The over-

saturated inner solution moves by capillary action

towards the surface (5). In the D zone, gypsum

reprecipitates, feeding recrystallisation phenomena

and/or the development of new crystals. In the fissures

and the cavities filled with fine-grained materials, new

gypsum crystals may develop. The mass transfer from

the inner towards the outmost zone results in pressure

relaxation (6) in the C zone and in pressure increase

(7) in the D zone.

It is difficult to evaluate the velocity of water

circulation inside the pores of gypsum because there

is a wide range of primary porosity in relation to the

different lithofacies. The secondary porosity is influ-

enced not only by the solution processes but also by

the anisotropies generated by the stress fields induced

by the reprecipitation.

The time necessary for the water penetrating the

rock to reach saturation is relatively short (about 18

min). Calaforra (1998) considers two limiting cases

related to the thickness of the shells of the gypsum

tumuli: 2 and 30 cm; assuming that the inner surfaces

of these shells are the expression of the saturation

front reached by the solution, in the first case before

reaching saturation, the penetration velocity of the

porosity water front should be about 1 mm/min, in the

second 16 mm/min. Anyway, the inner surfaces of the

tumuli are often structurally controlled and since in

some dome-like hills the thickness of the crust reaches

some metres, the water should be able to penetrate and

to perform solution in a thicker layer.

In some human-made stone structures such as dry

walls and vaults, a rearrangement of the gypsum

crystals is evident due to mass transfer inside the

rocky volume. A very nice example of crystal growth

and rearrangement is represented by a dry wall on the

ridge of Contrada S. Elia, WNW of Sutera. The dry

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 37

wall stretches in a N–S direction and shows the stones

as closely bonded on the eastern side and loose on the

western side. An easy explanation of such an asym-

metry is that during the night, the wall becomes wet

by condensation and in the early morning, the sun

heats the east side and the water solution inside the

pores is drawn towards the warmed side; it then

becomes oversaturated and favours crystal growth

and rearrangement.

Thus, the main mechanism favouring the poros-

ity of water migration towards the surface of the

bare rock is the heat pump resulting from the input

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4338

of solar radiation, which is very effective during the

summer period. The moving water is an oversatu-

rated solution, leading to mass transfer and repre-

cipitation of the gypsum, by the formation of new

crystals and/or by growth of previous crystals. So

part of the dissolved gypsum at the outer surface or

in the upper decimetres of the rock reprecipitates in

the outer gypsum layer, inducing a volume increase

and tendency to expansion. This tendency operates

at different scales, leading to the development of

tumuli, polygons, other intermediate forms.

The role of the silty and clayey fillings of some

fissures between polygons and ridges does not

appear to be a determining factor for the develop-

ment of such forms. Polygons are also present in

balatine and alabastrine gypsum where the fillings

may be absent. Anyway, these structures could play

an important role in favouring a faster penetration

of the water inside the rocky mass and also locally

increasing the lateral pressure both for the clay

expansion and the growth of new gypsum crystals

inside the fillings.

In Fig. 8b, a different environment is outlined:

gypsum covered by a thick mantle of loose sedi-

ments hosting an aquifer. Here the rock is affected

by solution only, and so the evolution of fissures to

grikes and corridors, and the disjunction of tables

and pinnacles are possible. The resulting forms,

described in the past as a ‘‘gypsum paleokarst’’

(Ruggieri and Torre, 1987), are examples of this

type of evolution. Nice examples of complexes of

forms of this type can be seen on the Serra

Ciminna homoclinal ridge (Fig. 9) and near Siculi-

ana Marina.

6. The role of the weathering crust in the

development of the mega-tumuli and of dome-like

hills

The bare summits of many large hills in gypsum

present a dome-like form, similar to that of the

gypsum tumuli but of a different scale (Fig. 10).

The geomorphological maps (Figs. 4–7) illustrate

some examples of these types of summits, which are

mostly circular or elliptical in shape and show diam-

eters ranging between a few tens of metres and more

than 100 m and heights ranging between a few metres

and more than 30 m.

From the morphostructural point of view, these

forms do not seem to be controlled by the strike, dip,

or fissuring of the gypsum beds: forms of this type are

developed in gypsum outcrops with different dips,

from subvertical to subhorizontal. The evolution of

these forms seems to be linked to polygonal fissuring

occurring on most of the dome surfaces and to other

pressure structures resulting from the processes of

volume increase of the outer gypsum layer.

The genesis of the dome-like forms, which

resemble dome-like summits in crystalline rocks,

is the expression of the nearly homogeneous behav-

iour of the rock mass in comparison with the

erosional processes. The formation of a weathering

gypsum crust may favour the development of the

dome-like form through the creation of nearly

isotropic fields of radial stresses with reference to

the central part of the relief. This stress field

minimises the influence of preexisting structural

elements, such as bedding planes and fractures

(Macaluso and Sauro, 1998a).

Fig. 8. Sketch of the main processes occurring inside the outer gypsum layer in two different environments: (a) rocky surface directly exposed to

the atmosphere, and (b) rocky surface covered by a mantle of permeable loose sediments. The dynamics of the type I environment are strictly

linked with the typical Mediterranean climatic regime. During the winter season, the rainfall causes an overland flow and the water dissolves the

gypsum on the surface (A) while part of the water solution manages to penetrate inside the pores of the rock. The solution reaches saturation

(with CaSO4) on the surface or a few decimetres inside the rock (the B line may be considered a saturation front). The solution continues to

penetrate both inside the pores and in the fissures (2), reaching a front of porosity water (C). The water may also flow laterally (3). During the

dry season, the solution loses water by evaporation (4). The inner solution becomes oversaturated and moves by capillary action towards the

surface (5). In the D zone, there is precipitation of gypsum. In the fissures and cavities, the solution migrates upwards (6) and new gypsum

crystals may develop. The mass transfer from the inner towards the outmost zone results in pressure relaxation (7) in the C zone, where open

fissures may exist, and in pressure increase (8) in the D zone. In the type II environment, the runoff inside the sediment causes solution of the

underlying gypsum both at the interface and in the fissures and other discontinuities of the rock. In this way, covered solution pavements

develop, perceptible after erosion of the cover sediments, similar to some limestone pavements.

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 39

In some structural settings, the field stresses may

be influenced by discontinuities such as bedding

planes or systems of fractures or by pseudo-bedding

planes resulting through gravitational release. For

example, in large homoclinal slopes, like that of Serra

Balate, the crust coincides with one or more beds with

expansion, leading to the development of forms like

tumuli, polygons, and related forms (Macaluso et al.,

2001); here, no mega-tumuli develops. The steeper

slopes of some ‘‘half-domes’’ evolve through land-

Fig. 9. Points of solution pinnacles partially masked by a cover of permeable alluvial sediments on the Serra Ciminna plateau. Overland flow

and slope processes are exhuming these crypto-forms developed by solution at the interface rock-covering sediments.

Fig. 10. The ridge of Muxarello (near Case Salamone) is characterised by a succession of summits that may be considered intermediate forms

between mega-tumuli and small dome-like hills.

F. Ferrarese et al. / Geomorphology 49 (2002) 25–4340

sliding more than by solution and other weathering

processes. In the gypsum masses where a relative

isotropy of the stress field exists, the weathering crust,

as a new structural element, tends to favour the

development of dome-like summits, neutralising the

influence of the preexisting structural elements.

In relation to the existence of this weathering crust,

the epikarst in gypsum is not well developed. An

interesting aspect of this epikarst is that most of the

preexisting discontinuities are sealed near the surface,

but they tend to be open at a depth of a few metres.

This aspect, well observable in some quarries, is in

accordance with the model of volume increase of the

crust.

If the growth of morphostructures, ranging in size

between a few decimetres and some metres, such as

tumuli and polygons, is the expression of the dynam-

ics of an outer gypsum layer for a thickness between a

few centimetres and some decimetres, the evolution of

larger forms, ranging in size between a few deca-

metres and more than 1 hm, such as mega-tumuli and

dome-like summits, is the consequence of the swel-

ling of a layer between a metre and a few metres thick.

In the first case, the porosity of water cycles could be

linked to the alternation of a single heavy precipitation

event followed by some sunny days, in the second to

the alternating wet–dry season. From the morphos-

tructural point of view, except for the gypsum tumuli,

it is not possible to make a sharp distinction between

an outer thinner crust and a thicker one. Actually, field

observations evidence that gypsum may behave in a

relatively plastic way. This is also suggested by the

fact that in a gypsum tumulus, there is a unique cavity

delimited by a ‘‘shell’’ between a few centimetres and

2 dm thick, while in the crust of the dome-like hills,

there are many voids along the fractures and other

discontinuities, situated mostly between some deci-

metres and a few metres below the surface.

The lifetime of small forms of the weathering crust

like the pressure ridges and the tumuli may range

between a few years and several decades. The time

span for the formation of the dome-like summit could

be in the order of thousands of years. This last

estimation is based on the consideration that both

deforestation and soil erosion in this region occurred

during early protohistoric times and also by the

presence of tilted protohistoric graves on the flanks

of some domes.

Given the fact that the basic conditions for the

development of the weathering crust are: (a) lack of a

soil cover, which is very often due to human impact

since protohistoric times, and (b) the alternating of

water saturation and dryness conditions in the super-

ficial porosity zone of the rock, the weathering crust

may be considered both a morphoclimatic, and, at

least partially, a human-induced feature (Macaluso

and Sauro, 1998a).

7. Conclusions

The geomorphological environments of the gyp-

sum karst areas of Sicily are complex. The different

landscapes, the many types of landform, and their

wide range of sizes are the result of the combination

and interference of different processes: tectonic evo-

lution, solution, and the precipitation processes and

other weathering phenomena controlled by the cli-

matic regime, the role of vegetation, the impact of the

different types of soil use, and so on. The analysis of

the forms shows that an important morphogenetic

role is played not only by the solution process, but

also by the swelling of the outer rocky volume,

leading to the development of a weathering crust.

Many physical–chemical processes could explain the

change in volume and the development of the weath-

ering crust such as: (a) increase in porosity as a

consequence of tensional relaxation; (b) phenomena

of thermal dilatation and contraction of the crystals;

(c) plastic deformation linked to a rearrangement of

crystalline structure; (d) expansion and contraction of

the clay trapped between the crystals and in the

fissures; and (e) solution, recrystallisation, and crys-

tals growth in connection with the runoff and cycles

of the porosity water.

The most typical forms, such as the polygons and

the gypsum tumuli, are not explainable without taking

recrystallisation phenomena into consideration. In

effect, in some polygons, like those on alabastrine

gypsum, there is very little silt and clay fill in the

fissures. Polygons are found at their best on the south

exposed slopes, which are subject to a greater degree

of solar radiation. If the clay plays an important role in

the development of the pressure ridges between the

polygons, these forms would be best found on the

north-facing slopes, which are wetter and more

F. Ferrarese et al. / Geomorphology 49 (2002) 25–43 41

favourable to clay expansion. Hence, in general, it is

most likely that the main process leading to the

expansion of the weathering crust results from the

precipitation of gypsum near to the topographical

surface.

Also larger forms such as the mega-tumuli and

dome-like hills are not comprehensible on the basis of

simple diapiric processes because their structures do

not fit within these processes. The presence of a

weathering crust expressed by the bulging forms, by

the sealing of the fissures, and by the minimisation of

the influence of the preexisting structural elements,

such as bedding planes and fractures, is the only

reasonable explanation for these forms.

The proposed model gives only a preliminary

pattern to stimulate further discussion and research.

Certain aspects need to be investigated in detail to

improve this model. Since the rate of evolution of

some forms is probably perceptible during the human

life span and is easily monitored, these areas con-

stitute ideal natural laboratories to study the processes.

The establishment of some field laboratories will be

an essential step to enhance future research on this

geomorphological environment.

Acknowledgements

The paper is the result of the cooperation of all the

authors. In particular, T. Macaluso and U. Sauro have

written and edited all the general parts and the

discussion of the model. F. Ferrarese, G. Madonia, and

A. Palmeri undertook the field survey, the geo-

morphological maps, and the illustration of the sample

areas. The authors thank their colleagues and, in

particular, A. Klimchouk, J.M. Calaforra, and G.

Benito who, through discussion and criticism, have

contributed to the development of their ideas. The

research was carried out within the following research

programs: MURST 1997—Risposta dei processi

geomorfologici alle variazioni ambientali; Analisi

degli ambienti, dei paesaggi, delle risorse e della

morfodinamica in aree carsiche italiane e del Medi-

terraneo (60%); and COFIN 2000—Ricostruzione

dell’evoluzione climatica e ambientale ad alta risolu-

zione (1–10 anni) da concrezioni di grotta lungo una

traversa N–S in Italia con particolare riferimento

all’intervallo Tardiglaciale-Attuale.

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