Cartography Cornisello Lakes

In the map we wanted to exemplify the transition between the traditional Quaternary survey systems and the innovative ones of the CARG project, by printing the two versions, both using the "lithomorphogenetic" criterion and distinguishing "informal stratigraphic units", to be subsequently merged into B.S.U. or Allostratigraphic Units.

The use of a legend derived from a simplification of the geomorphological one made it possible to draw up the first map (lithomorphogenetic map), which, associated with the information needed to describe an outcrop of surface formations (nature and type of boundary surface, alteration, soils, outcrop conditions, lithology of the sedimentary body, granulometry, texture, direct and indirect elements for dating, relationships between underlying and overlying units, numerical dating, etc.), led to the drawing up of the second map (lithomorphogenetic map).), led to the drafting of the second document, in line with the requirements of the CARG Project.

The Quaternary deposits were divided into eight units (highlighted by various colours) belonging to different basins (identified with Arabic numerals). In view of the 1:50,000 scale cartography they will undoubtedly have to be "downgraded" to subunits or members within higher-ranking units, or even merged between discontinuity surfaces of much greater significance.

With this in mind, only two units have been distinguished: the "Postglacial Unit" (Holocene) and that comprising the deposits of the last great glacial expansion and the late glacial phases, proposed as the "Adige Unit" (Castiglioni, oral communication).

In view of the experimental nature of the "Cornisello Lakes" section, all these passages have been deliberately kept separate to demonstrate the logicalprocedure used in the various phases of the drafting of the papers.

Geographical and geomorphological framework (by A. Carton and C. Baroni)

Section no. 042130 of the General Topographic Map of the Autonomous Province of Trento at a scale of 1:10,000, "Cornisello Lakes", includes the heads (or parts of them) of Val Stavel, Val Piana (head of Val di Bon and Val Caldura), Val Nambrone and Val Nardis. Most of the area is occupied by the wide basin of Amola and Cornisello, which are inserted to the hydrographic right in the middle Val Nambrone at altitudes between 1500 and 3500 m. The ridge line Cima d'Amola, Cima Scarpacò, Cima di Bon, Cima Giner, Monte Caldura, traces the watershed between the Adige (Noce) and Po (Sarca - Mincio) basins.

In the area there are numerous glaciers, or parts of them, some of which are still quite large. Their presence has necessarily influenced the morphological characteristics of the area and conditioned the sedimentation of Quaternary deposits (Baroni & Carton, 1991 b, 1992, 1996; Penk & Brueckner, 1909). The most important are: Presanella Glacier, Western Nardis Glacier, Amola Glacier and Southern Cornisello Glacier. As is known from the literature, the maximum Holocene expansion occurred in the Little Ice Age (16th-19th century A.D.). It can currently be reconstructed on the ground thanks to a series of characteristic dikes, almost always well expressed, and is documented by numerous historical testimonies. In the 20th century, disregarding brief periods of advancement, all glaciers suffered a strong retreat (Nardis -890 m, in the period 1905-1984; Cornisello -790 m, in the period 1921-1985; Presanella -810 m, in the period 1920-1987; Amola -580 m, in the period 1948-1987). The short positive pulsations, placed in the general retreat, to which the sedimentation of new glacial deposits is often associated, were not recorded at the same time in all glaciers, but were distributed in different periods: 1927-1928, 1976-1979, 1983-1986 for the Western Nardis Glacier; 1934-1937 for the Southern Cornisello Glacier; 1976-1986 for the Presanella Glacier. An evident slowdown in the general retreat can be seen since the 1950s.

Val Cornisello and Val d'Amola, two hanging valleys entirely above 2000 m, connect with the main valley axis of the Sarca di Nambrone by means of two outlet steps, placed in succession, and oriented N-S. Holocene and late-glacial glacial deposits, almost always modelled into lateral and frontal moraine banks, rest directly on the rocky substratum. Complex and characteristic moraine apparatuses develop on the margins of the main present-day glaciers. Proglacial deposits are modelled into small alluvial plains and in places into conoids, located at the positions reached by the glacial fronts. Some conoids are exclusively related to the activity of surface running water.

A large part of the slopes of the valley furrows are banded by debris slopes, also with large blocks, often shaped into cones. Some of these have been generated not only by gravity, but also by runoff, avalanches and debris flow, giving rise to those characteristic mixed forms typical of the Alpine environment, known as murkegel. Landslide phenomena are rare and of little significance, almost exclusively linked to small collapses of portions of the rock face. The presence of rock glaciers, some of which are still active, should be noted.

In several cases, the morphology of the slopes and some ridges is influenced by structural reasons (prevalent fractures), which have directed and favoured selective erosion, generating rectilinear valleys, gullies and serrations in the profiles of the ridges.

The Presanella Group, to which the area of the 'Laghi di Cornisello' section belongs, has been considered by various authors in its various aspects. However, there are no specific contributions referring to the Quaternary deposits and morphology of this region, with the exception of the work by Castiglioni (1961), which analyses glacial deposits in particular. Some works or maps of a strictly geological nature provide fragmentary information on debris covers, more or less detailed depending on the author's sensitivity to Quaternary deposits (Andreatta et al., 1953; Fenoglio, 1939; Salomon, 1908-1910).

The vast bibliography on glaciology Marson (1906, 1912), Laeng (1913), Merciai (1923;1928-1939), Monterin (1927), Morandini (1941-1950; 1954), Vanni (1948), Marchetti (1953-1977; 1978-1992), C.G.I. - CNR (1962) focuses exclusively on glacial bodies, recording their volumetric variations, but ignoring glacial deposits. The works of Segre (1948) and Malaroda (1948), although referring to areas adjacent to that of the 'Laghi di Cornisello' section, consider glacial deposits, cataloguing and dating them; they also propose a succession of events that can be compared with what occurred in the area under description.

The only complete work on Quaternary deposits (Castiglioni, 1961) takes into consideration all deposits belonging to late-Glacial and Holocene 'episodes', mapped and dated using the snowline, and also attempts to reconstruct the extent of the glacial cover divided into the various traditional Alpine stages.

Structural geological framework (by S. Martin)

The area of the 'Laghi di Cornisello' section is located in the eastern sector of the Tertiary Adamello batolite, in the central-eastern portion of the Southern Alps. The batholith consists of a system of plutons of variable age between the Eocene and Upper Oligocene intruded in the Variscan (Hercynian) basement and in the Permian and Triassic overburden sequences, at the intersection of the Periadriatic Lineament and the Giudicarie Line. The present-day structuring of the Southern Alps is essentially attributable to neogenic (neo-Alpine) deformation phases that produced conspicuous shortening on the order of tens of kilometres (Schonborn, 1992, Picotti et al., 1995) and the development of a broad chain of folds and overthrusts with a Po Valley vergence.

The Variscan basement of medium-low metamorphic grade (Southalpine) intruded by the Adamello is in tectonic contact with units of high-grade Variscan basement (Austroalpine) outcropping to the north along the Tonale Line. The latter is an important sub-vertical ductile-fragile shear zone of Oligo-Miocene age that is part of the tectonic system known as the Periadriatic Lineament, extending from Val d'Aosta to Slovenia.

Distributed in the vicinity of the lineament are the Biella, Traversella, Bregaglia, Jorio, Vedrette di Ries, Rensen and Karawanken plutons, the Adamello Batolite, various lode systems and local volcanic coverings (Exner, 1976; Dal Piaz & Venturelli, 1985), i.e. the major Tertiary intrusive and effusive manifestations described since the time of Salomon (1897) under the term 'Periadriatic Magmatism'. They developed at an advanced stage of the Alpine continental collision, shortly after the thermal peak of the Lepontine regional metamorphism described by Frey et al. (1974) and Trommsdorff & Nievergelt (1985).

The northern and north-eastern margins of the Adamello Batolite and the southern margins of the Bregaglia and Karawanken plutons, as well as other minor bodies, were intensely deformed by the Periadriatic Lineament, which was already active in the Upper Oligocene (around 30 Ma) and remained active as a right-trending lineage until the Middle Upper Tortonian (Castellarin et al, 1993), when the Giudicarie Fault began to act as a ramp for Miocene and post-Miocene SE- and S-vergent overthrusts.

The Adamello batholith is composed of a series of intrusions, each with its own differentiation history (Callegari & Dal Piaz G.B., 1973; Zattin et al., 1995). The lithological, chemical and isotopic heterogeneity of the magmatic complex suggested the existence of fractionation processes at different structural levels, accompanied by different degrees of crustal assimilation (CALLEGARI, 1985). The batolite was subdivided by Bianchi, Callegari and Jobstaibitzer (1970) into various magmatic groups: Presanella Group, Adamello s.s., Corno Alto etc.. Each group was further subdivided on the basis of the various lithotypes, analysed and ordered according to a precise intrusive sequence derived from the analysis of intrusion relations between the various plutons and the study of xenoliths (Callegari & Dal Piaz G.B., 1973). The authors finally proposed the following intrusion sequence starting from the oldest magmatic complex: the gabbrite-granodioritic complex of Re di Castello, the biotite quartzodiorite of the Vette Centrali, the gabbrite-tonalitic complex of western Adamello, the micaceous quartzodiorite of Val d'Avio, the tonalite of central Presanella, the quartzodiorite of lower Val di Genova, the tonalite of central Presanella, and the tonalite of north-eastern Presanella. This sequence was later confirmed by Rb/Sr radiometric dating performed on biotite and white mica (Del Moro et alii 1985; Martin et alii instampa).

The early intrusions, represented by the Re di Castello gabbros, granodiorites, diorites and trondhjemites from Sostino and Corno Alto, are aligned in a NNE-SSW direction in the central-eastern sector of the batholith; the ultrabasic bodies are located exclusively at the southern margin of the Re di Castello complex (ULMER et al., 1985; MACERA et al., 1985; BLUNDY & SPARKS, 1992). The basic and ultrabasic intrusions were fed by mantle springs contaminated by fluids from ancient subduction zones, while the dioritic and trondhjemitic plutons by magmas contaminated by lower crustal melt. The tonalitic plutons, intruded later, derived from the melting of deep mafic bodies and upper crust (Dal Piaz & Venturelli, 1985; Kagami et al., 1991; Macera et al., 1995).

The Presanella complex, to which the Cornisello area belongs, is the northernmost of the batholiths and comprises the Presanella and Nambrone plutons. The former is formed by hornblende tonalites and medium to coarse-grained biotite, with a massive structure in the inner parts, foliated to cataclastic in the marginal zones; the Nambrone pluton is mainly formed by leucotonalites (BIANCHI et al., 1970), with a moderately to strongly oriented structure with local cataclastic effects (Montresor & Rigatti, 1995). There are no clear-cut boundaries between the lithotypes of the Presanella complex, but rather more or less gradual transitions. The intrusion of this magmatic complex occurred prior to the age of 33-28 Ma (Table 1), where the more recent age obtained in the marginal foliated tonalites suggests a possible rejuvenation by tectonic activity following the intrusion (Del Moro et al., 1985). In fact, the tonalites of the northern (Val di Sole), southern (Val di Genova) and eastern (Val Meledrio) margins are characterised by intense ductile-fragile deformation concentrated in bands a few km wide (Dal Piaz G.B., 1953). In the first case the foliation has an approximately ENE-WSW trend and is parallel to the edge of the Pluto and the Tonale line, in the second case it is instead oriented E-W and forms an angle of 30°-40° with the Giudicarie Line.

Tab.1 - Radiometric ages of the Adamello batholith

* data interpreted as cooling ages rejuvenated by the intrusion of Presanella plutons; ** cooling ages of foliated tonalites

The Adamello plutons are intruded within various varicoloured basement units known as Scisti di Rendena in Val Rendena and Val Meledrio (Salomon, 1910), Scisti di Edolo in Val di Sole and Val Camonica, Scisti delle Tre valli bresciane in Val Brembana and Val Trompia (Origoni-Giobbi & Gregnanin, 1985). The intrusions developed contact aureoles with associations of andalusite, cordierite, biotite, garnet, sometimes K-feldspar, sillimanite and spinel depending on the depth of intrusion, fluid activity and temperature reached during the intrusion. Where the basement is not metamorphosed by contact from Tertiary intrusions, it is possible to recognise a pre-alpine metamorphic imprint in facies from green schist to amphibolite to staurolite (Scisti di Rendena, Bianchi & Dal Piaz G.B., 1950), sometimes with fibrolitic sillimanite (Scisti di Edolo) and a local metamorphic retrocession in green schist facies. The micaschists, probably derived from pelitic-arenitic sediments, sometimes carbonaceous, of pre-Ordovician age (470-430 Ma, Boriani & Giobbi-Origoni, 1982) include intercalations of metabasites, porphyroids, metavulcanites and orthogneisses and show remarkable analogies with the medium-low grade variclastic basement of the eastern Alps. Intruded within the basement are granitoid complexes, apophyses and strands of the Lower Permian age (e.g. Granodiorite of Caderzone, Dos del Sabion magmatic complex) with the development of contact metamorphic aureoles; there are also Ag-Pb-Zn-F hydrothermal mineralisations and widespread phenomena of retrometamorphism and metasomatism attributed to a Triassic age thermal event (De Capitani et al.,1994).

The regional schistosity in the basement near the eastern and southern plutons has a predominantly NNE-SSW direction, but it rotates at the basement wedge between the Re di Castello tonalite and the central Adamello tonalite assuming a NNW-SSE direction. The regional trend of the axes of the Alpine kilometric folds that predate the Adamello intrusion (Brack, 1985) is around NNE. This is attributed to the late (neogenic?) rotation of the axes, originally oriented NE-SW, in a counterclockwise direction.