Forms of mixed origin in the karst environment

of the Venetian Prealps

Ugo Sauro Dipartimento di Geografia dell’Università di Padova, Via del Santo 26, 35123 Padova, Italy

ABSTRACT

Most of the landforms of the middle latitude karst and in particular of the alpine karst are of mixed or

polygenetic origin, which result not only from the karst solution, but also from the cooperation of different

processes.

Between the well known forms of complex origin there are some glaciokarstic features, as some closed

depressions, and limestone pavements. But also some classical karst forms, as the dolines of the middle

latitude karst, have been modified by non karst processes and in particular by different types of periglacial

actions. In the Venetian Prealps there are very nice examples of dolines evolved, during some phases of

Pleistocene, in periglacial conditions. A peculiar type of doline evolving nowadays in periglacial conditions

is represented by the cylindrical dolines of some high mountains plateaux in the Dolomites.

Other peculiar forms of mixed origin, more or less controlled by the lithological and structural setting of the

limestone rock unit, are present in the Venetian Prealps. Between these types of forms are recognizable: a)

the complex niches, b) the dry valleys, c) some limestone pavements evolved as peculiar rock glaciers.

All these types of forms result from a combination or alternation of karst and non karst processes often

influenced by the lithological and structural setting of the limestone sequence.

The analysis of these forms help to understand the evolution of the relief in the context of the geodynamic

and climatic history of the Venetian Prealps.

KEY WORDS: karst, periglacial, landforms of mixed origin, Venetian Prealps, Italy

Most of the landforms of the middle

latitude karst and in particular of the alpine

karst are of mixed or polygenetic origin

resulting not only from the karst solution, but

from the cooperation of different processes.

Between the well known forms of complex

origin there are some glaciokarstic features, as

the glaciokarstic depressions, the limestone

pavements in glaciated areas of the two main

types: Rundhocker karst and Schichttreppen

karst.

Between the classical karst forms, nearly all

the dolines of the middle latitude karst have

been modified by non karst processes and in

particular by different types of periglacial

actions, as the gelifraction, the solifluction, the

piping of the different types of materials

infilling the depression, the loess deposition

(Castiglioni et al., 1990), etc.

In the Venetian Prealps there are very nice

examples of dolines evolved in periglacial

conditions during the Pleistocene. In some

areas, as the Naole Plateau in the Monte Baldo

(Province of Verona) and the Faverghera

plateau in the Prealps of Belluno, both “open

dolines” (forms partly fossilized by deposits

filling the depression up to the rim), and real

dolines (closed depressions) are common

(Benvenuti, Sauro, 1977; Magaldi, Sauro,

1982; Sauro, 1978, 2004). The latter, that often

present flat bottoms and a wide difference

between the maximum and minimum depths,

have evolved through a stage of open doline,

following the reactivation of the swallowing

124 Ugo Sauro

system and a partial evacuation through it of

the filling deposits.

Also relicts of dolines, partly destroyed by

gelifraction, are common in some areas.

A peculiar type of doline evolving

nowadays in periglacial conditions is

represented by the cylindrical dolines of some

high mountains plateaux in the Dolomites (see:

Meneghel, Sauro, 2006).

Aim of this paper is not to analyze the

above mentioned forms, but to describe other

peculiar forms of mixed origin, more or less

controlled by the lithologic and structural

setting of the limestone rock unit. Photo. 1. A doline with a flat bottom and a wide

difference between the maximum and minimum depths in

the Naole area of Monte Baldo. At the end of Pleistocene

the doline was completely full by the fillings made up by

cryoclastic fragments and loess like sediments. Now the

fillings are up to 12 m in depth.

The types of complex forms

The chosen types of forms are: a) the dry

valleys, b) the complex niches, c) limestone

pavements evolved as peculiar rock glaciers.

All these types of forms are relatively

common in the Venetian Prealps. Most of the

examples presented here are of the upper part

of the Lessini Mountains (Sauro, 1973, 2002).

The dry valleys In the Venetian Prealps there are more

types of dry valleys, entrenched in different

limestone formations and originated by several

processes.

The larger forms are canyon like valleys

some hundreds meters deep, with rocky bluffs

and a riverbed reactivated only exceptionally

during very heavy rainfalls. In some valleys

the intervals of inactivity are also longer than

10 years. The evolution of these forms is the

result of a combination of slope, fluvial and

karst processes (Nicod, 1997). Certainly, these

valleys have been much more active during

some phases of the Pleistocene, when the

upper belt of the mountain was hosting local

glaciers and the karst swallowing systems were

partly filled by sediments and partly blocked

by scattered permafrost.

Some medium sized forms consist in trough

shaped dry valleys characterized by wide and

irregular bottoms hosting closed depressions.

In these forms there are no evidences of fluvial

activity. The forms may be interpreted as

inherited forms, starting from a fluvial network

firstly developed inside the less karstifiable

cenozoic sedimentary rock formations, and

later entrenched in the limestone formations of

cretaceous and jurassic age. The bottoms of

these valleys are interested by accelerated

solution because most of the water of the

slopes is conveying in the lower part of the

depression and in this way the valleys continue

to maintain their shapes by “accelerated” karst

processes.

There are also smaller dry valleys with both

trough and V shaped cross profiles. The most

typical are developed in Maiolica – also called

Biancone fr. - (a rock unit similar to the Chalk,

staying above the Rosso Ammonitico), where

those with V shaped cross profile result often

from a rapid downcutting along tectonic

structures, as fault lines. In the bottoms of both

types, sinkholes and fillings of small rocky

fragments may be found, originated by frost

shattering. These forms, very similar to the

Forms of mixed origin in the karst environment of the Venetian Prealps. 125

“chalk dry valleys” described by Stevens in

New Zealand (1957), by Pecsi in Hungary

(1964) and by Mégnien in France, are for some

aspects also resembling to the “Dellen”.

Photo 2. A dry valley in the Maiolica interested by

episodic sheetflooding in the upper part of the Lessini

Mountains.

Photo 3. A dry valley in the Maiolica with development

of small dolines in the bottom (upper part of the Lessini

Mountains).

The processes now operating in these forms

are: a) the solution process, especially

energetic below the valley bottom, where the

water originates a hidden flow both inside the

fillings and the fissured rock, and so there is a

kind of “linear accelerated corrosion”; b) the

fluvial process operating as flash floods, only

during very concentrated rainfall events.

Witnesses of such rare flooding

occurrences have described medium size

dolines, completely filled by water, modified

in their shape after the event. During the cold

phases of the Pleistocene these forms were

probably interested by solifluction phenomena

and by a superficial flow during the snow

melting, also favored by the scattered

permafrost blocking the karst systems.

The complex niches

In the upper part of the slopes of the dry

valleys of the Venetian Prealps hollows and

niches are often present. These forms are

clearly the result of nivation processes and are

therefore classifiable as nivation niches. Most

of these forms are facing to the south, because

the prevailing winterly winds blow from the

north and pile up the snow in thick covers in

these depressions (Sauro, 1974, 2002). The

evolution of such niches is the result of both

periglacial weathering and accelerated karst

solution. In fact here a larger amount of water

is available following the melting of the snow

transported by the wind. The most common

sizes of the niches are: widths of 100-150 m

and depths of 20-40 m.

A common type of niche is clearly located

in a peculiar morphostructural position, i.e.:

entrenched in the slopes of both the rounded

and the nearly tabular ridges, in the transition

zone between the Rosso Ammonitico

formation and the Calcari del Gruppo di San

Vigilio formation. Some of these niches are

hanging laterally above the bottom of a dry

valley, others constitute the dry valley head,

similar to a pocket valley (reculé karstique).

The Rosso Ammonitico is a thin rock unit,

that behaves as a rock very resistant to the

weathering processes, giving origin to

structural terraces or to tabular summits

contoured laterally by ledges on the slopes.

Beside this, Rosso Ammonitico is a

hydrogeological filter, which partly blocks the

underground water circulating dispersedly in

the Maiolica formation, concentrating it along

some main fractures and focalizing the

speleogenesis in the underlying Calcari del

Gruppo di San Vigilio formation. In fact, on

the erosional surfaces cutting these lithological

transitions there is the highest density of

collapse dolines and “roofless caves” of the

upper part of the Lessini plateau.

So, these niches may have been originated

as “open collapse dolines”, or evolved starting

from depressions corresponding to most

karstified rocky volumes, in some cases also

126 Ugo Sauro

interested by a kind of karst sapping favored

by the structural setting of the area. Once

formed a niche may evolve for the nivation

processes combined with the accelerated karst

solution induced by the snow melting water.

Photo 4. A niche of mixed origin (by karst processes,

nivation etc.) near Malga Folignano di Sopra (upper part

of the Lessini Mountains). The nearly flat ridge summit

is a substructural surface inherited by the previous

tabular form in the Rosso Ammonitico.

Fig. 1. Sketch of a complex niche in a typical

morphostructural setting and of the main processes

operating in its evolution.

The limestone pavements evolved as peculiar

rock glaciers

The Rosso Ammonitico Veronese

limestone formation is a very thin rock unit,

only 30 m in depth, but it outcrops on a surface

similar in extent to that of the much thicker

overlying Maiolica. This depends on the slow

denudation to which Rosso Ammonitico is

subjected in comparison with the marly

limestone above. In fact, Rosso Ammonitico

(R.A.) originates giant karren landscapes, of

the “rock city” type, with large monoliths,

often separated by grikes and corridors. In the

Photo 5. A group of blocks of Rosso Ammonitico

detached by the main outcrops, because interested in the

past by a slow downslope movement, a kind of a slow

rockslide probably favorite by the periglacial

environment of the late Pleistocene.

context of sub-structural slopes,

partly controlled by the

bedding planes dipping 6-12°,

it is possible sometimes to

recognize both groups of

blocks and “isles” of R.A.

detached by the main outcrops

because interested in the past

by a slow downslope

movement, a kind of a slow

rockslide (Sauro, 1973, 1976).

Nowadays these forms are

stable and so, on the base of

their characters, it is possible to formulate the

hypothesis that the activity episodes have

taken place during the cold phases of the

Pleistocene, in typical periglacial

environments. In such environments the ice

fillings the grikes and corridors and forming

inside some bedding planes and some

epikarstic features (Tyc, 1997) has favored the

movement, causing the gradual transformation

of some outcrops of Rosso Ammonitico in

special types of rock glaciers. In fact, the more

or less chaotic structure of some Rosso

Ammonitico “isles” in relation with the

Forms of mixed origin in the karst environment of the Venetian Prealps. 127

distance from the detachment niches

confirms the rock glacier character of these

forms.

Photo 6. A “isle” of blocks of Rosso Ammonitico

evolved as a peculiar rock glacier during the late

Pleistocene.

Conclusive Remarks In the middle latitude karst the operation to

distinguish the forms on the base of simple

genetic processes is often not correct. Most of

the forms are clearly influenced by the

lithological and tectonic characters of the

mountain building and result from the

cooperation several processes, i.e. are

polygenetic in their origin. The climatic and

environmental changes have modified the

morphogenetic role of such processes during

the time.

Acknowledgements

I dedicate this paper to the great friend

Marian Pulina. I am sure that he will like the

chosen subject, aiming to evidence the

complexity of morphogenesis in the karst area.

Marian was always curious to understand karst

in the different environments of the Earth.

I firstly met Marian in 1973, at the

International Speleological Congress of

Olomouc. Second time I met him in the karst

of Provence and of Lessini Mountains in 1975,

during the “Table ronde Franco-Italienne sur le

Karst”.

I have been surprised by his interest not

only for the karst but also for others

geomorphological aspects. I remember that

when we visited the Covolo di Camposilvano

in the Lessini, he observed some periglacial

features in the filling deposits of this large

karst window.

Later I met Marian several times during

karst meetings, fieldtrips. He also comes twice

in Padova staying for a month to learn

karstology to my students. We have made

together fieldtrips in the Venetian Prealps.

I am sure Marian is present and still alive,

even in a different dimension and thank him

for his friendship.

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