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Apuntes sobre el transito Permico-Triasico en Lombardia central y oriental (Alpes Meridionales, Italia).

Remarks on the Permian-Triassic transition in Central and Eastern Lombardy (Southern Alps, Italy)

1. Introduction

This research aims at investigating the Permian-Triassic boundary (PTB) in central and eastern Lombardy, since previous knowledge has been limited by the lack of or the inadequate presence of paleontological records. Only after the Second World War did it attract the attention of geologists from Pavia and Ferrara, mainly look ing for a correlation with the famous "Tesero Horizon" at the base of the Lower Triassic Werfen Formation in the eastern Southern Alps, which has been the topic of a great number of studies since its stratigraphic introduction by Bosellini in 1964. In later times, further investigations by researchers on both the shores of Lake Como and to the east centred on the Permo-Triassic Verrucano Lombardo and Servino formations, and highlighted the importance of this chronostratigraphic interval essentially based on petrographic detrital modes. Our overview regards some selected sections from the literature and subjects them again to study in the light of wider research and correlation. Locations of the main visited PTB sections are reported in Fig.1. Furthermore, this work is accompanied by a general view of the late-Variscan volcanic and sedimentary evolution in order to understand better the key-significance of this geological period, heralding the Alpine Era.

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2. Stratigraphic framework

2.1. Permian

The South Alpine Permian can be clearly subdivided into two major well-differentiated tectonosedimentary cycles, separated by a marked unconformity and a gap of as-yet-uncertain duration (Italian IGCP 203 Group, 1986; Cassinis et al1988; Figs. 2 and 5).

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2.1.1. Lower Cycle

In Lombardy, the lower cycle (cycle 1), up to 1500 m-thick, is made up of acidic to intermediate volcanics and alluvial to lacustrine continental deposits (Collio Formation, Ponteranica and Dosso dei Galli Conglomerates, and other lithosomes), both infilling intramontane fault-bounded subsiding basins isolated from each other by metamorphic and igneous structural highs. The boundary paleofaults normally show SSW-NNE and W-E trends, and often coincide with important Alpine tectonic lineaments (such as the Val Trompia, Giudicarie and Insubric lines; Fig. 3). In addition, in the western and central sectors of the investigated region, conglomerates, sandstones and finer-grained siliciclastic products (known under the general name of "Basal Conglomerate") crop out underneath the above deposits, but in non-time-equivalent position, extending from the Late Carboniferous to Early Permian, and could be also ascribed to another independent cycle. During this period the climate was warm and moderately semi-arid (Dal Cin, 1972), with alternating wet and dry periods.

In the Orobic Anticline stricto sensu (Figs. 1B, 2 and 4.B), the Lower Permian sandstones of the Collio Formation and Ponteranica Conglomerate can be classified as volcanic arenites (average detrital modes: Q = 10 [+ or -] 5, F = 20 [+ or -] 7, L = 70 [+ or -] 8), and further petrographical data from the Basal Conglomerate are reported in a thorough and extensive work of Sciunnach (2001b), to which we refer for further details. In the Brescian Prealps (Fig. 1B) also, along and around the Maniva-Croce Domini road which crosses the local Paleozoic to Lower Triassic massif, a petrographical and mineralogical study of the Lower Permian Collio Formation (Cassinis et al, 1978; Figs. 2 and 4.A) records, in the varicoloured shales of the lower Pian delle Baste Member, a stable mineralogical association of illite-chlorite and some poor flakes of kaolinite; the alternating sandstones and siltstones prevalently consist, after Folk's systematics (1968), of arkose and lithic arkose, due to the presence of abundant feldspars (albiteoligoclase), locally rimmed by neogenic albite. Upwards, the thin detrital rocks (represented again by sandstones and siltstones), which rest conformably on the volcaniclastic mass-flow deposits of the so-called "M. Dasdana I Beds" (Breitkreuz et al, 2001), are characterized by immature sediments, where the feldspars prevail over the lithics. The mineralogical composition of the associated pelites consists, as in the underlying member, of dominant illite. In contrast, the siliciclastic deposits which mark the onset of the Dosso dei Galli Conglomerate Fm. are made up of lithic sandstones and conglomerates with mineralogical-petrographical features clearly different from the previous units. In fact, they include for the first time conspicuous metamorphic and volcanic rock-fragments, also of large size. As a consequence, this event appears clearly connected with a pronounced erosion of uplifted source areas, essentially represented by the Variscan crystalline basement.

Paleontological investigations of the macro- and microflora, and tetrapod footprints, indicate that the aforementioned older cycle/cycles began locally in the Westphalian but developed generally during Early Permian times (Geinitz, 1869; Venzo and Maglia, 1947; Jongmans, 1960; Remy and Remy, 1978; Ceoloni et al., 1987; Conti et al., 1991; Cassinis and Doubinger, 1992; Nicosia et al., 2001). Radiometric ages for a large number of igneous, extrusive and intrusive bodies also agree, generally, with the above dating (Cadel, 1986; De Capitani et al, 1988, 1994; Thoni et al, 1992; Schaltegger and Brack, 1999).

2.1.2. Upper Cycle

The Permian upper cycle (cycle 2), from few tens of metres to nearly 600 m-thick (Perotti and Siletto, 1996), consists in central and eastern Lombardy of the Verrucano Lombardo fluvial red clastics, which form a widespread blanket from Lake Como to the western Trentino, covering both the basins of the lower cycle/s and the surrounding highs. However, east of the Camonica Valley, the Verrucano Lombardo is represented in continuity by continental wine-red sandy-silty rocks which, on the whole, might be interpreted as a progressive facial change towards the well-known Val Gardena Sandstone of the Dolomites. Generally these sediments, compared with the products of cycle 1, appear more widely distributed even if less thick.

Compositional analyses of the Verrucano Lombardo (Fig. 4.B) document gradual attenuation of the relief, with dissection of the Lower Permian volcanic plateaux and, subordinately, unroofing of Variscan basement rocks and their local "aporphyric" siliciclastic cover (Cassinis, 1968a; Fontana and Zuffa, 1983; Sciunnach et al.,, 1996; Garzanti et al, 2003). According to Sciunnach et al. (1996), in the Orobic Alps the abrupt change from this lithic petrofacies P1 to the quartzo-lithic petrofacies P2, observed in the Inferno Valley (Valtellina, Fig. 1A for location), records a significant hiatus; in fact, at the base of this second petrological interval, the appearance of arenaceous rock fragments (very fine-grained quartzarenites lacking in volcanic lithics, similar to those typical of the "Basal Conglomerate"), and a much higher quartz content suggest wider dissection of the underlying volcanic plateaux, with erosion reaching into the Upper Carboniferous siliciclastics and the deeper-seated crystalline basement.

Further to the east, at the Passo San Marco area (Fig. 1A), feldspar increases in the upper part of the Verrucano Lombardo (Sciunnach et al, 1996; Garzanti et al., 2003). According to the same authors, this trend probably points to progressive local erosion of granitoid rocks, rather than climatic causes, since feldspar abundance was not observed in other sections of the Orobic Alps.

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Detrital modes of some selected samples collected in the same unit by Fontana and Zuffa (1983) are consistent with the petrofacies P2 (Valsassina) or P1+P2 (Pizzo della Nebbia and Val Sanguigno in the Valtellina-Brembana-Seriana central sector of the Orobic Alps; Fig. 1A for location). Sandstones much richer in feldspar (F>20) crop out, in the Brescian Prealps, around the Passo di Croce Domini area (Cassinis, 1968a; Sciunnach et al., 1996; Fig. 1A for location); in the easternmost Val Gardena Sandstone (where the rock-fragments are invariably <35), they dominate in the western Dolomites (Butterloch gorge, near Cavalese in Val di Fiemme) and become exclusive, towards the east, from Cadore to Friuli (Sciunnach et al, 1996).

During the Verrucano Lombardo sedimentation, the climate was characterized by semi-arid to arid conditions (Massari et al, 1988; Ori, 1988), presumably in subtropical paleolatitudes.

Given the lack of fossils, the age of the Verrucano Lombardo is still subject to uncertainty and discussion. However, as the unit generally rests over Lower Permian volcanic and sedimentary deposits and below Lower Triassic fossiliferous marine sediments, attribution to the Upper Permian is clearly plausible (Fig. 5). Owing to the age outlined for the previous lower cycle (generally related to a Sakmarian-Artinskian interval), and due to correlation with the more indicative lateral sections of the Trentino-Alto Adige, the Verrucano Lombardo probably developed diachronously during Late Tatarian times, reaching up (or almost) to the Permian-Triassic boundary. However, from Lake Como to the Giudicarie region, the presence of a P-T boundary is not yet supported from paleontological data.

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According to Massari et al. (1994) and to Massari and Neri (1997), this second Upper Permian cycle continues up to the Lower Triassic (Induan-Olenekian) and slightly younger units. As already indicated, the contact with the previous cycle 1 is marked by a stratigraphic gap of unknown duration, probably extending into Middle Permian (Guadalupian) times. The gap, which is well documented by extensive erosion surfaces and palaeosol profiles, could be tentatively estimated as spanning approximately 14 to 25 Ma in various places. This time-interval is consistent with the marked evolutionary changes recorded in the tetrapod footprints of the Southern Alpine Lower and Upper Permian sedimentary deposits.

2.2. Lower Triassic

On the top, the Verrucano Lombardo continental red beds are followed by shallow-marine sediments of the Servino Formation, Early Triassic in age. The so-called "Servino" is the Lombardian equivalent of the Werfen Formation (Fig. 5), representing the Lower Triassic deposits in the eastern Southern Alps (as well as, with different regional names but with substantially similar fossil assemblages, facies pattern and sedimentary evolution, a large part of Alpine Europe) (Broglio Loriga et al, 1990).

The Servino Formation generally consists of mixed or alternating terrigenous and carbonate deposits laid down in depositional settings ranging from the offshore portion of a wave-dominated shelf to supratidal mud-flats. Thickness is variable, ranging from approximately 120 to 200 m or more; it decreases rapidly along the western shore of Lake Como, where the unit is reduced to discontinuous lenses west of Acquaseria (Sciunnach et al., 1996), and changes abruptly in the Trento region across the Giudicarie Line, probably due to previous Permian paleotopography, increasing towards the east in the Dolomitic Alps.

Orobic Anticlines

The base of the Servino Formation is characterized from Lake Como to the Manina mine section by discontinuous oligomict conglomerates, breccias and sandstones (Prato Solaro Member; Sciunnach et al, 1996, 1999b), which lack extensively towards the southeast, in the Brescian Prealps and nearby, where the basal Servino is represented by quartzarenites and other lithotypes (Cassinis, 1968b; De Donatis and Falletti, 1999), which will be described below (Figs. 1 and 6).

The middle part of the formation consists of mature quartzose sandstones. It yielded in only one place of the western Orobic Alps, about 7 m above its base, many specimens of the bivalve Claraia, which led Posenato et al. (1996) to relate the first Triassic marine transgression, in this area, to an age presumably ranging from the latest Griesbachian to the early Dienerian. The chronostratigraphic meaning of these fossils will be discussed farther herein. In the upper part of the Servino Formation, which is represented by bioclastic yellowish carbonates and green to reddish carbonatic pelites, occurrence of the ammonoids Dinarites and Tirolites, and the foraminifers Meandrospira pusilla, Glomospira and Glomospirella indicates a Spathian age (Sciunnach et al, 1996: p.33).

QFL detrital modes for the Servino Formation display three distinct petrofacies (Sciunnach et al, 1996; Garzanti et al., 2003). The petrological interval S1 corresponds to the Prato Solaro Member of the Servino Formation and is essentially made up of sublitharenites (Fig. 4.C). It is markedly enriched in detrital quartz at the expense of lithic grains. Metamorphic pebbles (identified as Gneiss Chiari in the Prato Solaro and Comasira sections, both in Fig. 1A. 1) occur at the very base of the Member. Further details may be taken from the above-cited works. In essence, this geological step represents the conclusive geodynamic evolution of the previous Verrucano Lombardo depositional stage. However, the Prato Solaro Member does not crop out everywhere with its typical features and, as already indicated, it is also locally (e.g. at Ca' San Marco) lacking. In contrast, petrofacies S2, with a further increase in detrital quartz, and petrofacies S3 exibit subarkosic and arkosic trends respectively (Fig. 4.C).

Between Lake Como and Brembana Valley (Fig. 1A) the conglomeratic basal beds of the Prato Solaro member yielded to the authors of this paper many ventifacts: wind worn phenoclasts (Fig. 7). They are mainly quartz pebbles, having already acquired a certain roundness, during fluvial transportation, before their fashioning (ridges and facets, pits) by aeolian sand blasting. At Alpe d'Oro they were not largely dispersed, indicating that their last reworking was rather short. Such an interpretation is evidenced everywhere by the good preservation of the secondary ridges and of the polarity expressed by smooth upper and rough lower faces. They testify for a clearly arid climate in the depositional area just before the first Triassic transgression (Durand, 1972; Smith and Edwards, 1991; Durand, 2006).

Brescian Prealps

The Servino Formation of eastern Lombardy, between the Camonica and Caffaro Valleys, crops out prevalently in the Brescian Prealps, south of the Alpine Adamello intrusion. Generally it is well exposed along the Fontanelle Valley, near Collio in upper Val Trompia (Figs. 1A for location, 6 and 8). The basal part, resting on the Verrucano Lombardo continental red beds, consists of waverippled thin clastics, 3-4 m-thick, which are overlain by metre-thick layers of yellowish oolitic dolostones alternating with siltstones (marked as "PD Hor." in the the small columns 8-10 of Fig. 6; Fig. 9.I). This oolitic unit (named improperly, for its composition, as "Calcare di Praso") can be recognized in many parts of the investigated area and can be followed and also locally mapped to the eastern side of the Rendena Valley, in western Trentino. The lower and upper contacts of this member were interpreted as erosional (Cassinis et al., 1990: Fig. 8, and 1993: surface 2 in the logs III and IV of Fig. 9). A grey to bluish well-bedded unit, with marls, siltstones, thin sandstones and dolomitic calcarenites, containing Claraia aurita (Hauer) and foraminifers of the Rectecornuspira-Cyclogira group, occurs above. These strata may be interpreted, according to Neri (in Cassinis et al., 1990), as a lithostratigraphic equivalent of the Siusi Member in the Werfen Formation of the Dolomites, and thus ascribed to a latest Griesbachian-early Dienerian age. In this context, we wish also to point out that Yin (1985) related the acme zone of Claraia aurita (Hauer) in the Western Tethys (Alps) essentially to a Dienerian interval.

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[FIGURE 6 OMITTED]

Upwards follows the so-called "Gastropod Oolite" member (indicated as "GO Mb." in the columns 7-10 of Fig. 6; Fig. 9.E), which represents the most typical facies of the Servino Formation in eastern Lombardy. It is made up of lenticular bedded, cm- to dm-thick, locally dolomitized oolitic coquinoid limestones, passing laterally to red calcarenites and mudrocks. The unit is practically unknown west of the Camonica Valley, along the eastern outcrops of the Orobic Anticlines (CA: Cedegolo anticline). Pelitic (marls, marly siltstones) sets alternating with fine-grained sandstones forming thin wave-ripple beds or lenses, of red colour, crop out above. Some dm-thick oolitic calcarenites (storm layers) occur in the lower part. This unit, which shows an overall fining-upwards trend suggesting a lagoonal to muddy tidal-flat setting, can be correlated with the Campil Member of the Dolomites (Neri in Italian IGCP 203 Group 1986, pp. 162163; Cassinis et al, 1990; Twitchett, 1999; Fig. 6.10 of this paper: CMb.). The overlying "Myophoria beds" Auct. (Fig. 9.F and H) consist of grey and bioturbated marly and silty limestones with intercalations, dm-m thick, of bioclastic and oolitic calcarenites. Fossils are represented by Natiria costata, Neoschizodus ovatus, and so on; Dinarites spp. and other Spathian fossils (Turbo rectecostatus, Bakevellidae, etc.) also occur in other parts of the Brescian province, such as in the Passo di Croce Domini and Passo Valdi sections (Figs. 1A.9-10 for location and 6.9-10; Fig. 9.G), where the foraminifer Meandrospira pusilla was displayed at the transitional beds with the overlying member. The already named Val Fontanelle section ends with pelitic rocks (marls and siltstones) of grey, green and red colour, and subordinate intercalations of thin sandstone and carbonate rocks. Upwards, this uppermost member is delimited by the yellowish Carniola di Bovegno (Fig. 2), ?Upper Scythian to Lower Anisian in age.

In the Brescian Alps and surroundings, between the Trompia and Camonica Valleys (Fig. 1A), the Servino Formation is locally a well-known host rock for iron ore, exploited by mining works for many centuries until the last two decades. Mineralization occurs at different levels, as typical strata-bound deposits and in vein-type discordant bodies, generally characterized by manganesiferous siderite and subordinate barytes and sulphides (Rodeghiero and Zuffardi, 1985, 1988; Cassinis et al., 1997a). In particular, the mineralization events appear to be connected with well-defined tectonically active zones, marked by fractures and paleofaults cutting the crystalline basement, the Permian and Lower Triassic rocks, the latter sutured by the ore-bodies present in the Servino Fm. In this context the Val Trompia lineament, which was a southern boundary-fault of the Lower Permian Collio Basin (Fig. 3), can be considered as a significant example. Therefore, the distribution of the Servino iron-bodies seems to have been strictly conditioned by the Permian paleotopography (Cassinis, 1985; De Donatis et al., 1991; Cassinis et al., 1997a; De Donatis and Falletti, 1999).

3. Permian-Triassic Boundary

Several studies have been carried out on the P-T boundary in the eastern Southern Alps (especially in the Dolomites) both from chronostratigraphical and sedimentological viewpoints. They essentially arose from the Field Conference on the "Permian and Permian-Triassic boundary in the South-Alpine segment of the Western Tethys", held in Brescia in 1986, which led to a historical review by Broglio Loriga and Cassinis (1992) and to a large number of Italian and other investigations, based on the macro- and microfloras, tetrapod footprints and marine fossil fauna from the cited time interval. In contrast, very few data are available on the P-T boundary of Lombardy. Exceptions are due to the research regionally carried out by Cassinis (1968b), Assereto et al. (1973), Cassinis et al. (1993), Sciunnach et al. (1996, 1999a) and De Donatis and Falletti (1999).

Therefore, in this context, an outline of the P-T transitional sections selected by the writers in the Lombardy study areas is required. This aims to elucidate some coeval geological events, as well as to confirm again the presence of a gap and, possibly, to evaluate its chronological range.

Orobic Alps

Along the Orobic Anticlines, between the Verrucano Lombardo red beds and the Servino Formation, Sciunnach et al. (1996) pointed out the presence of pinkish quartzose conglomerates to sublitharenites and white subarkoses from northern Valsassina to northwestern Brembana Valley (Valtorta area) and, further to the east, in western Scalve Valley (Manina mine section) (Figs. 1A.14, for location, and 6.1-4; Figs. 7.a-j and 10.A-F). This unit was mentioned by previous authors (Merla, 1933; Gaetani et al., 1987; Dallagiovanna et al., 1987; Schonborn and Laubscher, 1987; Schonborn, 1992; Schonborn and Schumaker, 1994) under different names (such as the Alpe d'Oro quartzites; Figs. 1A.2, for location, and 6.2; Fig. 7.b-e and i.j), but was formally defined by Sciunnach et al. (1996), owing to problems of nomenclature and exposure, as the "Prato Solaro Member" from a locality of the Muggiasca Valley where the member is complete and well-exposed along the S.P. 62 roadcut, below the Prato Solaro hamlet (below Parlasco) (Figs. 1A.1 and 6.1). The type-section may be estimated as about 30- 40 m thick. The basal contact with the Verrucano Lombardo is paraconformable and includes locally metamorphic ("milky" polycrystalline quartz, gneisses) and sedimentary pebbles of presumed Late Carboniferous age. Later on, lithological data were also provided from the nearby Mulini Valley section (Sciunnach et al., 1999b).

The wind worn pebbles (ventifacts) found in the lowermost beds of the Prato Solaro Mb., on the type-section and elsewhere in Orobic Alps, evoke those recorded in the Early Triassic from many other places of Germany, England, France, Spain, Sardinia and Bulgaria. These latter can be related to an episode of maximum aridity developed probably in late Dienerian-early Smithian times (Durand, 2006; Bourquin et al., this volume).

Nevertheless, according to Sciunnach et al. (1996), most of the texturally submature to mature sediments of the Prato Solaro Member were deposited in a fan-delta setting, where marine transgression would be proved by wave rippled mature subarkoses and the presence (Merla, 1933) of the bivalves Neoschizodus laevigatus (Goldfuss) and Unionites canalensis (Catullo) which, according again to Sciunnach et al. (1996: p.32), generally indicate an early to mid-Griesbachian age. Actually these two cosmopolitan taxa range on the entire Early Triassic, and the first one even until the Ladinian in the Germanic Basin.

[FIGURE 7 OMITTED]

As regards the Claraia beds found less than 5 km SE from the Prato Solaro Member type-section, in the lower part of the overlying Ca' San Marco Member, but only about 8 m above the basement, there is some discrepancy between their latest Griesbachian--early Dienerian age proposed by Posenato et al. (1996) and the age proposed above for the ventifact beds. This problem deserves further studies, but it should however be noticed that, as pointed out by Posenato et al., these Claraia beds of the Valsassina "do not yield any of the index-species of the Dolomites"--all specimens were referred to C. intermedia (Bittner)--and thus had to be dated on the basis of correlations with Iranian sequences. Claraia can display a very high genetic plasticity (Broglio Loriga et al., 1983) and is also clearly facies dependent, proliferating in stressing, mainly dysaerobic, settings (Wignall and Hallam, 1992), which reduces its chronostratigraphic value. So, it must be emphasized that Yin (1985) distinguishes only two Claraia zones useful at the world scale within the Early Triassic, among which the second (Claraia aurita --C. stachei--Eumorphotis multiformis Zone), that include C. intermedia, ranges from the late Griesbachian up to the Smithian.

[FIGURE 8 OMITTED]

Towards the east, however, in contrast with the aforementioned geometry, the top of the Verrucano Lombardo at Ca' San Marco is unconformably onlapped at a low angle by the middle Servino (Garzanti, 1990). A slightly angular contact between the Upper Permian Verrucano Lombardo and the overlying Prato Solaro Member of the Servino Formation has been also supposed locally by the authors of this paper in the Piani dell'Avaro area (above Cusio, in the upper Brembana Valley) (Fig. 1A.3, for location, Figs. 6.3, 7.a, 10A and 10B).

Further to the east, the Manina mine section (Figs. 1A.4 and 6.4)--confined to the Cedegolo Anticline, in western Scalve Valley--is characterized again by the Prato Solaro Member (Sciunnach et al, 1999a).

Brescian Prealps

East of the Orobic Alps, around the southern border of the Adamello magmatic intrusion of Palaeogene age (42-30 Ma, according to Del Moro et al, 1986), Upper Permian to Lower Triassic rocks occur extensively from the Camonica to the Giudicarie Valleys (Fig. 1A for location). However, in recent years, only the latter outcrops have been the topic of comprehensive stratigraphical and sedimentological research (De Donatis and Falletti, 1999), which was locally integrated with previous data (Cassinis, 1968b; Cassinis et al, 1990, 1993). In general, the former authors confirmed for the Servino of the investigated area the presence of six lithostratigraphic units, similar to those already described above, in Val Fontanelle. Only the basal part of the formation shows somewhat different features, which can be summarized in the area between Anfurro and Monte Guglielmo (i.e. in the southwestern corner of the Camonica Valley) as represented, according to De Donatis and Falletti (1999), by quartzarenites, bioclastic and oolitic arenites, sandstones and siltstones with wave-ripples, forming cm- to dm-thick beds locally alternating with burrowed pelites (Figs. 1A.5, 1A.7, for location, and 6.5, 6.7; Fig. 9.AC). The Servino Formation is clearly visible also in the Rondeneto Valley, near M. Muffetto (Figs. 1A.6 and 6.6; Fig. 10.G and H), where the basal part, which rests paraconformably on the Verrucano Lombardo continental red beds locally through a surface bearing dolomitic nodules of probably pedogenetic origin, may be interpreted as deposited in fluvial and coastal environments, bordered by mud-dominated areas. This unit marks the inception of the Early Triassic transgression and excludes indirectly the continuity of the Prato Solaro Member to the south of the Orobic Anticlines. The age of this geological event is not clearly documented by paleontological evidence. However, the correlation by De Donatis and Falletti (1999) of these first local deposits with those observed at the base of the Val Fontanelle section could support the same age for both the respective units, which, from the already above-cited presence in the latter of latest Griesbachian-early Dienerian pelecypods, should be probably ascribed to a slightly earlier Triassic interval.

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As regards the interpretation of the P-T boundary in the Brescian Alps, however, another problem arises from the chronostratigrafic classification of the Praso oolitic horizon at the base of the Servino Formation, which is very reduced but widespread in the investigated area (Cassinis et al., 1993) and, as already recorded in this paper, also in the nearby Trento region, where it can be followed up to the Upper Rendena Valley, east of Massimeno (Malghe Movlina). As recognized in the well-exposed Pass Valdi section by Cassinis (1968b), the very base of the Servino consists of a thin horizon made up of grey sandstone covered by a m-thick oolitic unit which can be correlated to the Tesero Member of the Werfen Formation of the Dolomites (Cassinis et al., 1993) (Figs. 1A.8-11 for location; 6.8-10; 8.I-IV; Fig. 9.I). In more detail, according to Neri, red to grey medium- to coarse-grained sandstones, which represent the top of the Verrucano Lombardo, are overlain by a unit ("b"), 1-2 m thick, consisting of pelites alternating with fine sandstones; wave- and current ripples are the dominant structures (Fig. 8 in this paper), and depending on the amount of pelite, the stratification pattern may be represented by wavy or flaser bedding.

In the Praso section (Val Daone, Trento: Figs. 1A.11, location, and 8.IV), the base of this unit is marked by a "lag" of breccia including pelite clasts. Bioturbation, mainly represented by vertical burrows, may occur, while no significant fossil have been found until now.

The depositional setting of unit "b" may be referred to a marginal marine setting, probably to coastal lagoons bordered by muddy tidal flats; moreover, unit "b" may be regarded as the record of the first stages of the early Scythian transgression, responsible for the superposition of "paralic" deposits on the continental, fluviatile Verrucano Lombardo.

According again to Neri (in Cassinis et al., 1993; Fig. 8 of this paper), unit "b" is overlain by sandstones with carbonate cement and sandy dolostones ("c1"), forming a body ranging in thickness from a few decimetres to about 1-1.5 m, with amalgamated hummocky cross-lamination, suggesting a shallow shelf (shoreface) depositional setting. The body "c1" grades upwards into a dolomitized oolitic unit ("c2"), exhibiting both hummocky structures and bidirectional cross-bedding (indicative of tidal control) (Fig. 8). The "c2" oolite may be correlated to the Tesero Member on a simple lithostratigraphic basis. Surface 2 in Fig. 8, separating the "paralic" unit "b" from the fully marine (shoreface) base of unit "c", is obviously characterized by a jump in facies and may be regarded as a shoreface ravinement surface generated during the westward shifting of the transgression. It is at least a moderately heterochronous surface, younger to the west and older to the east.

Following upwards are fine-grained clastic sediments, mixed or alternating with carbonate rocks, dolomitized, and yielding pelecypods and subordinate gastropods.

Due to the lack of paleontological data, the age of the above units is yet unknown. However, on the basis of their stratigraphic position and wider correlation, attribution to an Induan-early Olenekian time interval could be generally justified.

Concluding Remarks. From the aforementioned review, the Permian-Triassic boundary in central and eastern Lombardy is extensively marked by a hiatus, which is of as-yet-uncertain duration (Fig. 2). In fact, the first Lower Triassic transgressive marine deposits of the Servino Formation rest undoubtedly paraconformably, or locally (Ca' San Marco, Piani dell'Avaro? and in other places) also unconformably, on the underlying Upper Permian continental fluvial red beds of Verrucano Lombardo, but the major problem is to date the temporal distribution of this gap.

[FIGURE 10 OMITTED]

Our overview, based essentially on the lithostratigraphic record of central and eastern Lombardy (Fig. 6) in a general scenario inclusive of the western sector (Sciunnach et al., 1996), displays a large number of discontinuities and sharp changes possibly connected with an inherited paleotopography and more or less coeval tectonic activity, generating lateral and vertical striking contrasts for a well-defined Permian-Triassic boundary. In fact, as already stated, the Upper Permian Verrucano Lombardo, the Lower Permian volcanic and sedimentary units, and the Variscan crystalline basement along with its Permo? - Carboniferous siliciclastic cover, were affected in some places (such as to the west of the Orobic Alps) and in different ways by erosion, which prevents us from knowing the real duration of the debated PTB gap before the overlying unconformable Triassic deposits.

In contrast, the main marine transgression, even if based on some fossils (Claraia) occurring towards the east of the Adige Valley (Trento-Bolzano), in the Siusi Member of the Werfen Formation, probably developed between Induan and early Olenekian times. However, in the Brescia and Trento regions, the "Calcare di Praso" (along with the underlying thin clastic unit) seals a hiatus probably shorter than that regarded in the western Orobic Anticline. In conclusion, the PTB gap of central and eastern Lombardy should be interpreted as discontinuous in time and space, generally spanning the latest Permian (top of Lopingian) and not yet a defined part of the Early Triassic.

4. Permian and Early Triassic geodynamic evolution: an overview

As recorded above, significant events mark the geodynamic evolution of the Lombardy Southern Alps from late- to post-Variscan to early Mesozoic times. During the Early Permian, at the end of the Variscan orogenesis, a transtensional geodynamic regime developed along the southern Paleoeuropean border, due to a dextral transform margin between Laurasia and Gondwana (Arthaud and Matte, 1977). The coeval depositional record, represented by calc-alkaline, low- to high-K, intermediateto-acidic volcanics and alluvial-to-lacustrine deposits, infilling fault-bounded, subsiding, strike-slip or pull-apart basins, was accompanied by widespread granitoids and subordinate gabbros from asthenospheric upwelling and crustal contributions. In more detail, the Collio Basin of the Brescian Alps (Figs. 1B, for location, 2 and 3) can be appropriately interpreted as a pull-apart basin because it has a typical sigmoid shape, which is clearly evident both in the eastern sector, where the basin changes direction, and in the western sector, especially if the latter is considered together with the Boario Basin (BB in Fig. 1B). Thus the Collio Basin seems to originate from the stress field, which interacts between two main Permian right strike-slip faults, i.e. the Giudicarie line to the east and another hypothetical parallel fault, the Val Camonica line, to the west (Fig. 3).

During Late Permian to Anisian times, an extensional regime, linked to plate reorganization and likewise to the opening of the Meliata back-arc basin to the east (Ziegler and Stampfli, 2001), gave rise to a second sedimentary cycle, following a stratigraphic gap marked by extensive erosion surfaces and palaeosol profiles, but of as-yet-unknown duration, tentatively estimated at about 15-25 Ma, probably extending into Middle Permian (Guadalupian) times. This "Mid-Permian Episode" (Deroin and Bonin, 2003) was characterized by specific tectonic, magmatic, thermal and basinal features. An angular unconformity can be observed in places. In the Southern Alps, this general structural reorganization was accompanied by a change in the depocentral areas; we may consider this reorganization as a consequence of a larger wavelength extensional regime. However, such a geological event was also assumed by some authors, also in other countries, as due to compressional movements or tilting (Cassinis, 1964; Prost and Becq-Giraudon, 1989; Cadel et al, 1996; Cassinis and Perotti, 1997; Sciunnach, 2001b; and so on).

The aforementioned upper cycle consisted of the Middle p.p.?--Upper Permian continental, fluvial red beds of Verrucano Lombardo, lacking in volcanic activity, and the Lower Triassic shallow-marine, varicoloured deposits of the Servino Formation. In this context, the former unit gave rise to a widespread blanket over the Lower Permian cyclic deposits and the surrounding metamorphic and igneous structural highs; moreover, if compared with the products of the lower cycle, it generally appears more widely distributed although less thick. The siliciclastic and carbonate sedimentation of the latter unit, linked to a rapid transgression from east, followed a progressive peneplanation of the former irregular topography.

According to Ori (1988) and other authors, during the Verrucano Lombardo deposition the climate was dominated by semi-arid to arid conditions, which persisted throughout the Early Triassic up to the Carniola di Bovegno Formation, which is generally ascribed to latest Spathian-early Anisian times. In this context, the maximum of aridity is recorded by wind-worn quartz pebbles (ventifacts) present in the basal beds of the Servino Formation of the Orobic Alps. On the scale of the West European Plate, such climate indicators, referred to mid-Scythian times, appear as powerful tools for correlations within non-marine formations, devoid of any biostratigraphical marker, that straddle the Permian-Triassic boundary (Durand, 2006). Therefore, in contrast with previous authors, our current research leads us to suggest that the P-T boundary of central and eastern Lombardy is marked by a gap generally running from the top of the Permian (Lopingianp.p.) to part of the Scythian (up to early Olenekian locally).

In conclusion, this work displays in the Lombardy stratigraphic record of the Permian to Lower Triassic a significant link between the eastern Trentino-Alto Adige and the western Lugano-Varese regions. In fact, the restored scenario of the South-Alpine segment examined shows a fairly good similarity, as it is generally made up, in the lower part, of continental volcanic and alluvial to lacustrine deposits unconformably capped by the Verrucano Lombardo-Val Gardena Sandstone fluvial red beds, and is marked on the top, again through an unconformity, by the marine sediments of the Lower Triassic Servino and Werfen formations. This rather similar evolution is probably, at least in large part, consistent with the structural setting of the investigated area, widely confined to active tectonic lineaments matching with the present Lake Como and the Giudicarie belt, respectively. This Permian-Triassic regional structure, which separated an uplifted sector (M.Grona-Lugano) to the west from a more subsiding basin (Val d'Adige-Dolomites) to the east, could also explain some geological features of Permian history, such as the presence of different facial deposits and of new gaps (e.g. between P1 and P2 petrofacies of Verrucano Lombardo as indicated by Sciunnach et al., 1996, in the Orobic Alps), as well as the presumed slightly younger age of the Lower Triassic Servino Formation, due to the westwards progressive and relatively rapid transgression over a more stable region. Therefore, in this context, we can remark on the particular geodynamic significance of central and eastern Lombardy during the late- and post-Variscan evolution of the Southern Alps.

Acknowledgements

This paper is devoted to Carmina Virgili, who carried out extensive and critical studies on the Western Mediterranean Upper Carboniferous, Permian and Triassic deposits, which have been presented in a great number of national and international meetings not only in Spain but also in other parts of the world. We also wish to remember her talent as an organizer and the generous and intense energy she infused in her work. Certainly, her life will give a remarkable example for future generations. We wish to thank S. Bourquin and N. Sheldon for their critical reviews.

Received: 05/03/06 / Accepted: 19/09/06

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G. Cassinis (1), M. Durand (2), A. Ronchi (1)

(1) Dipartimento di Scienze della Terra, Universita di Pavia, Via Ferrata 1, 27100 PAVIA, Italy cassinis@unipv. it /ausonio.ronchi@manhattan.unipv. it

(2) 47 rue de Lavaux, 54520 LAXOU, France, mada.durand@wanadoo.fr
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Author:Cassinis, G.; Durand, M.; Ronchi, A.
Publication:Journal of Iberian Geology
Article Type:Report
Date:Jan 1, 2007
Words:8664
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