DOLOMITES - Annexes 2-8 - Provincia di Udine

NOMINATION OF THE DOLOMITES FOR INSCRIPTION ON THE WORLD NATURAL HERITAGE LIST UNESCO
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Volcanism during the post-Hercynian changed as indicated by the geochemical evolution of magmas
from rhyodacite lavas in basal units, to rhyolite lavas at the top. The change however, is not gradual
and the presence of andesite lavas, in both the initial phases of the magmatic activity in the rhyodacites
and the summit of the rhyolites indicates that both basic magmas and evolved magmas erupted
during this volcanic episode.
Geochemical differences in volcanics are used by researchers to divide the vulcanites of the southern
Alps volcanic system into two main groups that differ by composition and morphology. The volcanic
sequence of the lower sections is comprised of andesite, dacite and rhyolite lavas that are gray-green
or purple, as well as the upper section of rhyolitic ignimbrites.
The upper rhyolitic ignimbrites are the result of successive pyroclastic flows that expanded during
eruptions, for many kilometers in length and for hundreds of meters in thickness, in tectonic depressions
within the Metamorphic Basement. Both lavas and ignimbrites accumulated in subaerial conditions.
Layers of tuff, sandstones and conglomerates are common between the flows and reveal the
erosive action of the small streams that furrowed this arid volcanic region. In some cases these sediments
are hundreds of meters thick and have been distinguished as distinctive geological formations
(the Collio Formation and the Tregiovo Formation), where fossil have even been found such as the
small proterosaurus reptile, Tridentinosaurus antiquus.
the transgression of the Upper Permian
255 to 260 million years ago (Upper Permian) a sea-level transgression submerged progressively from
east to west a large part of the actual Alpine area. Mountainous reliefs were present to the west of the
Southern Alps, and sloped down towards a piedmont belt of alluvial fans (central-west Lombardy).
This terrain was replaced in Trentino, in Alto Adige/Südtirol/South Tyrol and in Carnia by a vast
arid plane furrowed with sinuous rivers and dotted with isolated, small, ephemerals lakes (Massari et
alii, 1998). Following the termination of extensive volcanic activity in the Alpine area, temperature
variations and strong winds reshaped this arid plateau, eroding its surface.
Sands and gravels accumulated in depressions and lithified to form sedimentary complexes such as
Verrucano Lombardo in the Giudicarie – Dolomiti di Brent area, and the Val Gardena Sandstone
(AVG) (“Grödner Sandstein” by Richthofen, 1860) of the central-eastern Dolomites. Today the
thickness of these rock formations varies greatly, ranging from a few meters in the far west, to more
than 500 meters in the easternmost sections of Comelico. The thickness of these accumulated sands
and gravels can vary greatly even over small areas, indicating that they were deposited in uneven topography
over a bed of vulcanites and metamorphic basement rock. The AVG are made up of conglomerates,
coarse to fine sandstones, red with occasional gray siltstone and pelites that collectively
form decimeter to meters thick stratosets both massive and stratified. Its sandstones are often crossbedded,
either with festoons or planar. At most, the coarsest lithotypes (clast-supported conglomerates;
paraconglomerates in arenaceous or, rarely, pelite matrix; microconglomerates) characterize
the lower parts of the formation, whereas upper units are dominated by finer lithotypes (siltites and
pelites alternating with smaller sandstones, with intrusions of gray dolomite with limited microfauna
that are a prelude to the Bellerophon Formation above). In general, the Val Gardena Sandstone form
a large, fining-upward sequence, within an overall transgressive trend.
At the top, the AVG unit grades into the Bellerophon Formation, with which it has a complex transition
of lateral interfingerings. The conventional boundary between the two formations can be identified
by the last (uppermost) red siltites and pelites, although characteristic Bellerophon lithotypes,
such as gray dolomites and marls may appear several meters below this limit. The Upper Permian
(Tatarian) designation, of the AVG is based on its tetrapod ichnofauna (dominated by Rhyncosauroides
e Pachypes) and on the palynoflora.
A large part of the analytical data were acquired in the historical section of Bletterbach-Butterloch/Rio
delle Foglie near Redagno/Radein (Bolzano/Bozen), known for the footprints of tetrapods