The Geology of the Bergamasc Alps Lombardia Italy

In the Bergamasc Alps we have observed one major unconformity between the Basement rock and the overlying Permian. The total absence of any recognisable Palaeozoic sedimentary rocks accentuates this unconformity, and moreover this enormous hiatus makes the dating of any Palaeozoic event impossible. However, by comparison with the Central Alps and Kärnten, we learned that the Asturian orogenitic phase precedes the deposition of the first volcanic sediments. In analogy with the Aar and Gotthard masses we presumed the intrusion of the less metamorphic ortho rocks of the Basement, the granodiorite and the gneiss chiaro, to be of Upper Carboniferous age. The close resemblance of the chemical composition and differentiation of the Permian volcanic rocks and the Upper Carboniferous intrusive rocks induces us to assemble this period of magmatic activity into one period of Permo-Carboniferous age. In long NE—SW striking anticlinal zones these intrusives have penetrated into the old paraschists, causing some contact metamorphism. In the Lugano region where the volcanoes are better preserved and the differentiation of the lavas is more complete, we have seen 1) that the last feature of magmatic activity had been the pressing out of the granophyr, an acid igneous rock, in a very large dome-like structure. The chemical composition of this granophyr is so much like that of the gneiss chiaro or the granites of the Val Rossiga that there can be little doubt that they all belong to the same magmatic source. Also, the intrusive rocks of the Err-Bernina, Lower East-Alpine thrustsheets and their Permian porphyries have a similar chemical composition and must be closely related to our intrusive and volcanic rocks. Hence the whole region of what Later became the Alpine geosyncline was in Permo-Carboniferous time the scene of extensive intrusion and extrusion of igneous rocks. In Permian time the topographical surface was above sea level in the Lugano region where erosion was active and the volcanoes were formed in a mountainous country, but it was mostly covered by shallow water further east. In the later stages of this period considerable tangential forces shaped long anticlines, pressed out the granophyr magma to the surface and formed the very deep central Permian trough and the Camonica uplift of the Bergamasc Alps (see Plate XLIII). Other structural features are indicated, but only these two latter structures, the Camonica uplift and the Permian trough, are clearly visible, and they may be the result of faulting instead of folding. The shape of the Permian trough with its steep flanks and flat bottom would indicate perhaps a fault trough rather than a syncline. This trough is flanked in the NW by the Averara ridge, which, however, is a more pronounced uplift in the Middle Triassic than in the Permian. Whether the Brinzio-Maroggio anticline of the Lugano district, along which the volcanoes are arranged, must also be regarded as a Permo-Carboniferous structure can not he ascertained. Both the Lower Permian (Collio) and the Upper Permian (Verrucano) increase in thickness in eastern direction (compare fig. 16 and 17). In the Lugano region the Verrucano is only preserved in the small outcrops of the San Martino conglomerate at both sides of the Lugano lake. East of the Como lake it has a thickness of less than 50 m, but increases gradually to sonic 800 m in the eastern Bergamasc Alps. The Collio has a similar development of its thickness but is in the west a pure volcanic formation and is first observed round the Valsassina core as a sedimentary rock, further west only irregular patches of volcanic rocks have been deposited. In the East Alpine thrustsheets the Verrucano is generally present but not in great thicknesses, except in the Campo sheet. The Permian in the Lower East Alpine sheets (Bernina sheet) consists of porphyries only. The western limit of the Permian is again observed in the Helvetian thrustsheets, where the most western Axen sheet does not contain any Permian, whilst the more eastern Glarner and Mürtschen sheets contain thick Verrucano masses. The same wedging out of the Permian towards the west is observed along the Tavetscher zone between the Gotthard and Aar massives. The Triassic of the Lombardic Alps is its most interesting and best developped formation. The Werfenian of Lugano consists of a simple coarse sandstone, and the upper dolomitic member is encountered for the first time in the Valsassina. Through the whole Bergamasc Alps the Werfenian is rather sandy but becomes more and more shaly and calcareous towards the east, apparently we pass from a purely continental region in the west to a marine facies in the east. The same tendency was found in the Upper Permian where the Bellerophon horizon of South Tirol sets in above the Verrucano from the Brenta group eastwards. The development of the Middle Triassic as Anisian and Ladinian in distinct facies, in the Bergamasc Alps increasing in thickness in eastern direction, connects with the development of these stages on the Mt. Giorgio, where the Salvatore dolomite is already split in two by the Bituminous Horizon on the boundary between the two stages. The Middle Triassic from Lugano, with its Salvatore dolomite where Anisian and Ladinian can hardly he distinguished, slowly develops in the Bergamasc facies of Ladinian Esino dolomite-limestone and Anisian Gracilisschists and Trinodosus horizon. We have seen that the northerly facies of the Ladinian contains mostly Buchensteiner and Wengener, in the southerly facies the Esino occupies the whole Ladinian. Over the Averara ridge both stages are much thinner and incomplete, and the Anisian increases in thickness towards the Val Camonica, whereas the Ladinian decreases. Here we find also the distinct Wengener splinter shale basin. On the westerly border of the Camonica ridge many facies changes take place. FABER (lit. 21) pointed out that the wedging out of the Collio, the facies change from cavernous dolomite to Elto dolomite of the Upper Werfenian, and the rapid transition from Wengener shales to Esino dolomite all occur on approximately the same line, the one above the other. In Southern Tirol the Middle Triassic has much the same development, the total thickness depending mostly on the presence of thick reef limestone (dolomite), e.g. the Schlern dolomite or Marmolata limestone. One pecularity is, however, very striking in the region between the Pale San Martino and the Adamello and that is the disappearance of the Raibler as a distinct lithological horizon. The merging of Carnian and Ladinian dolomites sets in in the Val Camonica, in the Brenta group only occasionally some Upper Raibler mals are observed and the Raibler appears again north of the Pale San Martino. At the same time the Lower Ladinian facies of Buchensteiner and Wengener is also absent. Elsewhere the Raibler, although very variable, has very much the same shallow water facies, with occasional tuffogenous intercalations. Is is much thinner in the Lugano region. The Upper Triassic and Rhaetic are very different in the regions of Tirol, Bergamasc Alps and Lugano. In the east the two formations are developped as one dolomitic mass, the Dachstein dolomite; in the Bergamasc Alps we find a thick Norian Hauptdolomite and a complete series of well developped Rhaetic series, whereas in the Lugano region the Rhaetic is either absent or represented by the Upper member, the Conchodon dolomite. At the same time the Liassic rests here uncomformably on the Rhaetic or Norian with the typical transgressive Hierlatz facies. The Liassic siliciferous limestones are very much the same from west to east, somewhat thicker in the west, specially in the large complex from the Mt. Generoso to the Como Lake. The comparison of the three regions, Lugano, Bergamasc Alps and South Tirol has been summarized in a tabel. The boundaries between these geographical units are not constant though. The boundary between Tirol and Bergamasc Alps lies during the Norian-Rhetic in the Brenta group and in the Carnian-Ladinian and in the Permian west of Val Camonica. The Collio reappears even in a thick complex east of the Camonica ridge in the Val Trompia. The boundary between the Lugano region and the Bergamasc Alps is even less fixed, it lies somewhere between the Generoso and the Alta Brianza Lecco region, but can not be determined much further as the Liassic limestones cover all the older formations between these two points. The Averara ridge, altough very pronounced in the Permian, Lower and Middle Triassic is not a facies boundary, at both sides the facies is very similar. It has always been known that the Lombardic Trias facies is very much alike that of the East Alpine thrustsheets. Both in the Helvetian and in the Pennine zones of the Alpine sedimentation basin the Triassic is very poorly developped, and can in no way be compared to that of the Southern and Eastern Alps. When we consider the conformity between the Lombardic and eastern Alps facies somewhat closer, we observe a great similarity between the Lugano region and the Lower East Alpine unit. Both have porphyries in the Permian and no Verrucano, in both the boundary between Ladinian and Anisian is very vague. The whole Triassic in the Err-Bernina sheets is much reduced as compared to the Triassic of Campo- and Silvretta thrustsheets. The Rhetic is much completer in the Err-Bernina than in the Lugano region, but both are again characterized by thick siliciferous Lias limestones, which is transgressive with a Hierlatz limestone facies on the Rhetic and Norian in both tectonical units. The Middle East-Alpine thrustsheet, the Camposheet and its accessory units, is characterized in the Münster valley by a thick Verrucano series of some 600 m. with pebbles of quartzporphyry and granite. Porphyry sheets are lacking in this serie. The Triassic of the Camposheet as a whole is much thicker than that of the Lower East-Alpine sheets, but the Anisian is not very thick yet, much less than in the Upper East-Alpine sheets, and the Werfenian is hardly represented. Lugano Bergamasc Alps South Tirol Liassic Siliciferous limestone 100—1000 m. Transgressive Hierlatz facies Siliciferous limestone 500—1000 m. Limestone 300—400 m. Rhetic. Absent, or only Upper member Conchodon dolomite Complete from Alta Brianza to Brenta group 550—800 m. Daehstein dolomite ;' 1000 1400 m Norian Hauptdolomite 250 m. 1200 m. Hauptdolomite Carnian Series of shales, marl, dolomite 100—350 m. Thick series of shales, marl, dolom. and sandstones 250—700 m. Western facies Eastern facies Schlern dolomite porphyries, tufs etc. from Pale S. Martino 150 m shale Sst. dolomite Northern facies Southern facies Northern facies Southern facies Marmolata St. Cassian limestone Wengener Esino limestone Wengener sst. and sh. Buchensteiner chert, limestone 600—1200 Esino dolomite, limestone Wengener, splinter shale facies Ladinian Salvatore dolomite 300—600 m Salvatore dot. Bituminous horizon Mendola dolomite Buchensteiner or Beitzi sch. Anisian Trinodosus hor. 50—150 m. Gracilis Schists from W—E 150—450 m., Nodulous limest. Mendola dolomite Gracilis schists marls, dolom. """"Werfenian 50 m. sandstone Cavernous dolomite 200—450 m. shale, marl, sst. Servino Campiler sch. 250 m. Gastropod, list. Seiser sch. 80 m. Permian absent or porphyries, tufs etc. basal congl. Verrucano from WE 50—800 m. Collio, porph. vole. sst. tufs etc. central Collio shale basin 0—2000 m. Basal conglomerate Bellerophon hor. 0—250 m. Ciavflpnn Sst 100 9.CV\ m (Vermomnn} Bozener porphyries Basal conglomerate (Collio) The Ladinian is present as Wettersteindolomite (250—600m) without the typical Partnach facies of the Upper East-Alpine thrustsheets. The Raibler is some 400 m thick, dolomites, shales, shaly limestones, rauhwacke and gypsum, porphyrites etc. The Norian is very thick, 500—2000m, and developed as typical Hauptdolomite, whereas the Rhetic is present in the facies of the Kössener schists, black and reddish shaly limestones and shales, which can be compared to the Lombardic facies of the Scisti neri. The agreement with our western Bergamasc Alps is striking. Exeptionally thick Norian, Esinodolomite, thin Anisian, and thick Verrucano are the characteristics of the region between the Valsassina and the Val Seriana. The Werfenian is much completer in Lombardia, and the Collio of the central trough is absent in the Camposheet but in general the similarity is not less striking than that of the Err-Bernina sheet with the Lugano-Grigna region. The Averara ridge although not the boundary between the two facies, can possibly be correlated with the geoanticlinal ridge between the Lower and Upper East-Alpine sheets. The Upper East-Alpine thrustsheets, (Lechtal, Silvretta) show a great similarity with the eastern Bergamasc Triassic. The Werfenian has an Upper Rauhwacke member, the Anisian shows the nodulous limestone (Reiflinger Knollenkalk), the Gracilis limestone, the brachiopod limestone etc. in exactly the same facies. The Ladinian is not identical to such a degree as the lower members of the Triassic, but the Arlberg Limestone and dolomite can be very well compared to the Esino limestone and the Partnachschichten to the Wengener shales (splinter-shales!). The Carnian again is very similar, rauhwacke, marls, gypsum, shales and sandstones, black limestones are present in both units. In the Lechtal sheet the Norian Dachstein limestone and the Rhetic Dachstein corraline limestone are only separated by the „Kössenerschichten”, corraline limestone and shales of the Lower Rhetic. The Norian is reduced in comparison with that of the Camposheet. The Carnian of the Ducan region is exeptionally thick, some 900 m, with an upper 300 m of Upper-Carnian dolomites 1). Such development of the southerly part of the Upper East Alpine thrustsheet can already be regarded as a transition to the Camonica facies where nearly the whole 700m thick Raibler is developped as dolomites. Striking as the agreement of the development of the sedimentary sequence in Lombardia and in the east Alpine thrustsheets may be, great differences can also be noted. First of all the Permian of the Bergamasc Alps with its central Permian trough with 1500—2000 m of Lower Permian Carona shales and volcanic rocks can not be found back in the Eastern Alpine thrustsheets. In the second place the typical development of the Lower Ladinian in Buchensteiner and Wengener facies is restricted to the Southern Alps and Tirol. Finally the „Flecken mergel”, (mottled marls), and Allgäuschiefer of the Liassic of the eastern Alpine facies are not represented in Lombardia. On the other hand the abyssal facies of Upper Liassic, Dogger and Malm in Radiolarite and Aptici limestone and marl is present in both stratigraphical units. That great differences exist between two regions, which in their original position in the geosyncline are widely separated although in the same basin, is quite logical. Lombardia is the southwesterly extension of a large basin, of which the East-Alpine thrustsheets occupy the centre and the north easterly end. Moreover the basin must have widened out considerably in NE direction. That the troughs and ridges opened fan-like in this direction from Lombardia follows from the fact that the E—W distance from L. Maggiore to the Val Camonica is less than the combined breadth of the East Alpine thrustsheets. Moreover we must not forget that even in the small width of the Bergamasc Alps already considerable facies change from North to South could be demonstrated, both in the Ladinian and in the Anisian. The main differences are found, as mentioned above, in the Permian and in the Lower Liassic, particularly in the Middle and Upper East-Alpine sheets. The development of the Permian in the Bergamasc Alps is due to late Variscian movements which apparently are not parallel to the Alpine geosyncline, and therefore need not continue in similar facies in the direction of the Alpine geosyncline. The Liassic Allgäuschiefer of the East-Alpine facies can be regarded as a transition between the penninic Bündnerschiefer facies and the Lombardic silieiferous limestone facies. The Cretaceous of the East Alpine basins can in no way be compared to the Lombardic Majolica and Scaglia. This is due to the fact that in Upper Cretaceous time the Alpine orogeny attacked this northern part of the Alpine geosyncline, whereas Lombardia remained mostly undisturbed. The dividing line between the southern and eastern Alps originated with the folding of the East-Alpine sheets, and became accentuated when the Pennine sheets were folded in the Oligocene, and became still more pronounced when the uplift of the central folded system occurred in the post Oligocene Insubric phase. In the tectonical part we have shown that the youngest Tertiary tectonical direction is purely W—E. The Orobic thrustfault and its accessories cut off obliquely the older ENE—WSW structures as for instance the Orobic anticline. This latter direction is mainly pronounced in the anticlinal structures, e.g. the Brinzio-Marroggio anticline, the Orobic anticline, the Cabianca-Trabuchello anticline, and the Cedegolo anticline, but also in some faults as the Clusone and Bondione faults. The great thrustmovements, the Grigna thrustsheets, the thrusting against the Valtorta and the Valcanale faults, further the Timogno and Ardesio thrusts, and the eastern thrusts of the Pzo Camino and the Palline Borno-Lozzio masses is all bound to the E—W strike or the N—S compression. The Insubric line, the boundary between the Southern Alps and the Central Alps, i.e. the division line between Pennine root zone and the Orobic zone, has also a W—E strike from the Lago Maggiore to Dinaro. Therefore also this major tectonical line probably originated only in a later period of the folding process. This conclusion is in complete accordance with the views of the general conception of the Alpine orogeny, which places the origin of the Insubric line in the post Oligocene, older Insubric phase. In this phase the roots of the Pennine thrustsheets were tilted in a vertical position. The Insubric phase, the tilting of the root zones is naturally a time of uplift, the Central Alps rose above their fore- and hinterland. This is also the origin of the several fault steps we could discern in the Bergamasc Alps. In the Younger Insubric phase (Pliocene) when the final compression took place, all the Bergamasc thrustsheets were formed, they were sheared of their substratum from a higher step and pushed over the lower step. The N—S faulting has a intermediate position, it is younger than the old anticlinal folding and older than the final thrust, and is probably connected with the older Insubric phase when the uplifting of the steps occurred. The stratigraphic comparison has made it clear that the southern, the central and the eastern Alpine basins were portions of one geosyncline, separated from another probably by ridges, geanticlines, but still forming together one continuous unit. This connection was ruptured by the first severe Upper-Cretaceous Alpine orogenesis, the origin of the east-Alpine thrustsheets. At that moment an oblique line cut a southern minor portion from the rest. This rupture line later became the Insubric line. By its present position we can still follow its course in the original basin, because the southern Alps are only little changed in aspect compared to the more central parts. West of the Lago Maggiore it followed the ridge dividing the southern basins from the central Pennine ones, then, north of this lake it curves round to an E—W strike thus cutting obliquely through the basin structures. It retains this diagonal coarse untill it had crossed or just reached the very important Camonica geanticline, it then swung back to its original direction parallel to this ridge along the so called Judicaria line. Finally it resumes its E—W strike as the Pusteria line and limites southern Tirol to the North, separating this region from the East Alpine thrustsheets. This early boundary line is not quite identical with the Insubric line, because the latter cuts occasionally with a very sharp angle through the root zones of the Pennine thrustsheets, but the two lines are sufficiently alike to identifie them for our purpose. The remarkable wavy course of the Pusteria-Insubric line is thus due to the fact that the N—S compressional direction necessitated an E—W strike but the existing inhomogenities of the region indicated a NE—SW strike, between those two influences the result alternated. The ENE—WSW anticlinal structures being older than the original Insubric line, belong therefore to a prae-Cretaceous or Cretaceous phase, a phase which also accounts for the totally different facies of the Cretaceous in East-Alpine and Lombardic sedimentary-basins. If this is true some erosion on the crests of the Cretaceous structures may have taken place before the much later, probably Pliocene, finial compression took place. RASSMUS 1) has thoroughly treated the Cretaceous folding phase of the Lombardic Alps. The Scaglia of the foothills, in which unfortunately no fossils of stratigraphic value have been found, belongs probably to the Cenomanian-Turonian and is a typical regressive facies with which the Alpine sedimentary cycle closes. In the thick Santonian gravels, which were deposited in the Po plain, the material is derived from Liassic and Jurassic rocks, but also of Triassic rocks and even of Permian porphyrites. This conglomerate can he regarded as a equivalent of the Gosau Schists of the northern Alps. The folding phase preceding the erosion can be put therefore in one of the subhercynic phases of Stille. Undoubtedly the final thrusting has therefore been preceded by erosion, and we may presume that some of the thrusting has the character of „reliefüberschietrangen” as advocated by AMPFERER 2). In general, however, our thrustsheets are of too small dimensions to allow the determination of the characteristics of this particular way of thrusting. This phenomenon may to a certain extent account for the fact that the Grigna thrustsheets pass over the faulted and folded underground with plane thrustplanes without being affected in the least by these structures. I can not find much evidence in favour of such theory, though, because most of the structural features of the underground are of equally recent datum as the thrusting movement, or only very slightly older. The Valtorta fault for instance is certainly older than the thrusting, both because the thrustplanes pass over the fault and because the Norian and Raibler of the southern limb have been pressed against it. But it is not as old as the Orobic anticline, although it is fairly parallel to this structure, because it certainly belongs to the phase of uplifting of the central Alps, the older Insubric phase, and therefore not to the Cretaceous phase of folding. Still, even between the Older and Younger Insubric phases some erosion may have taken place, that is between the Miocene and the Lower Pliocene, and the height differences along this fault may have been removed to some extent. The same is true for the Clusone fault in connection with the Presolana sheet and the Pilo fault in connection with the Lozio overthrust. Let us summarize the results of our deductions in a short tabel. Extensive denudation removing all palaeozoic sedimentary rocks. Asturian folding followed by extensive intrusion of acid magmas in long stretched NE—SW zones. Permian. Erosion continues in the west. Magmatic intrusion is followed by widespread volcanic action. In the east deposition of large subaquatic volcanic sediments. Saalic compression, origin of central Permian Collio trough, Camonica uplift and extrusion of granophyr, Erosion in the western region continues, in the east deposition of Verrucano conglomerates. Triassic. Continuous sedimentation in the south-east Alpine basin of Triassic rocks. Older Kimmeric phase uplift of the Arzo anticline followed by erosion and transgressive Hierlatz facies in the Lugano-Lower East Alpine region. In the Lugano region the movement started already in the Rhaetic. Sedimentation of Liassic and of abyssal Dogger and Malm and bathyal Lower Cretaceous. Austrian or Subhercynic folding („Juvavische phase” of R. Staub) origin of long ENE—WSW anticlines. Only the first beginnings of the strong Cretaceous orogenesis of the East-Alpine sheets has effected Lombardia, later in this phase the eastern Alps were cut off along a diagonal line partly following the anticlinal ridges and were severely compressed in thrustsheets. The major Pennine (Oligocene) phase of the folding of the Pennine sheets and further compression of the east Alpine sheets did not reach the southern alps. Insubric phases, lste phase. The central Alps were raised to considerable height, the roots were tilted in vertical position and the ,,steps"""" of the Lombardic Alps were formed. Origin of BNE—WSW faults (Valtorta, Clusone, Valcanale faults). N—S striking fault systems (Val Vedra fault trough, Manina fault troughs). Intrusion of Adamello tonalite. 2nd. phase. N"""" to S compression, the lower limbs of the ENB—WSW faults were pressed against the fault, origin of Timogno and Ardesio thrusts. Origin of Tertiary dikes. 3rd. phase. Possibly some erosion. Strong N to S compression. Origin of Orobic thrust and accessory thrusts, (Jrigna thrustsheets, Arera thrust, Palline-Borno and Lozzio oventhrusts and the Presolana and ('amino thrustsheets. Often renewed activity along existing faults (Clusone fault). The age of these Insubric phases can be judged by the fact that the Miocene inolasse has been folded, and that (the horizontal Pliocene has been deposited in fjords eroded in a strongly dissected landscape.