Mid-Carnian'' Transgression in the \Testern Southern Alps (Lombardi Italy): Stratigraphic and Paleogeographic Implications

$Mid-Carnian'' Transgression in the \Testern Southern Alps (Lombardi Italy): Stratigraphic and Paleogeographic Implications$

Paleontologia e Stratigrafia

EVIDENCE OF A ''MID-CARNIAN'' TRANSGRESSION IN THE \TESTERN SOUTHERN ALPS (LOMBARDI ITALY): STRATIGRAPHIC AND PALEOGEOGRAPHIC IMPLICATIONS

FABRIZIO BERRA'I & FLAVIO JADOUL**

Recehed. April 2a, 20A1; accepted JanuatT 8, 2002

Key zoord: Carnian, stratigraphli paleogeographr.; sequence presence of two stratigraphic markers: stratigraphl Lombardy, Southern Alps. a) Dark grey shales and siltstones ("Black Pelites"), considered previousll. as the northern closure of the Gorno-Lower S. Giovanni Riassunto. Lo studio descrive gli eventi stratìgrafico-sedimen- Bianco Fms., but re-interpreted as the srestern pinch-out of the Lozio tologici che cararrerrzzano 1a trasgressione "medìo-Carnìca" sviluppara Shale depositional s,vstem. The Earl,v Carnian Lozio Shale s.as deposit- tra la piattaforma carbonatica (Fm. di Breno) e la soprastante succes- ed first in the Valle di Scalve-Lozio trough and iater covered the car- sione carbonatico-pelitica-evaporitica (Fm. di S. Giovanni Bianco) in bonate platform (Breno Fm.). Prealpi Bergamasche. Due marker stratigrafici regionali documentano b) Fossiliferous, open subtidal limestones, marls and burrowed questa trasgressrone. marlv limestones ("Bioclastic Horizon") of the northern Bergamasc a) Le "Pelìti Nere", in precedenza considerate come Fm. di Alps. Gorno o F. di S. Giovanni Bìanco basale, sono reinterpretate come la The spreading of shales and siltstones represents the first chiusura verso occidente dell'Argillite di Lozio (bacino della Val di transgressive stage of the last Carnian sequence in Lombard)r, after the Scalve) che ha interrotto lo sviluppo e ricoperto la piattaforma car- "mid- Carnian"(Julian substage) regional carbonate platform crisis bonatica (F. di Breno). Quest'unità, che rappresenta la fase iniziale di (top of the Valcamonìca Breno Fm.). The "Bioclastic Horizon" records una trasgressione regionale (età presunta la sommità del Carnico infe- the zf represented by normal, open marine facies, identified and cor- riore), documenta la crisi regionale delle piattaforme carbonatiche ed related throughout the Bergamasc Alps. evìdenzia l'ìnizìo dell'ultima sequenza deposizionale del Carnico lom- Di{ferent petrographic and chemical characters between the bardo. Lozio Shale - "Black Pelites" and the Gorno-San Giovanni Bianco Fms. b) L'Orizzonte Biocl:stico, costituisce una peculiare asso- suggest different source areas: the former units are characterized by ciazione litofacies d'ambiente subtidale aperro (calcari e marne spesso clasts derived from a metamorphic-intrusive area (placed northward molto {ossiliferi) presente nella successìone transìzionale alla F. di S. and westward), whereas the latter units are characterized by prevailìng Giovanni Bianco della Lombardia centro-orientale. Questo marker volcaniclastic material. regionale rappresenta l'estensione maggiore raggiunta dagli ambienti A climatic change (from arid to relatively humid conditions) subtidali nella successione carnica della Bergamasca (zf). ma,v be ìnvo\ed to explain the crisis of the "mid-Carnian" carbonate Le caratteristiche chìmico-petrografiche delle "Peliti Nere"- platforms in the western Southern Alps and the regional spreading of Argillite di Lozio sono differenti rispetto a quelle delle coeve, più fine-grained terrigenous material. meridionali, Fm. di Gorno, Arenaria di Val Sabbia, S. Giovanni Bianco e suggeriscono l'esistenza di due diverse aree d'alimenrazione per i silicoclastici del Carnico Lombardo. Le prime sembrano derivare in Introduction prevalenza dall'erosione di rocce metamorfiche ed intrusive (poste a nord e ad ovest del settore studiato), mentre i silicoclastici delle The thick Carnian succession of the Lombardy seconde derir.ano in gran parte dallo smantellamento d'edifici vulcanì- ci meridionali. Basin develops between the Ladinian and the Norian car- L'analisi stratigrafica e delie facies hanno permesso dì ipotiz- bonate platforms systems. The Carnian formations crop zare un temporaneo cambiamento climatico (da condizioni aride a rel- out along an east-west belt, where they are preser-ved in ativamente umide) in prossimità della fine del Carnìco inferiore; esso different tectonic units (Forcella &Jadoul 2000), consist- è uno delle cause della crisi delle piattaforme carbonatiche e della dif- ing mainly of Anisian to Upper Carnian rocks. fusione regionale di sedimenti terrigeni fini (prodelta deltizi) in precedenza confinati alle aree costiere meridionali e nel becino setten- The Carnian of Lombardy is characterized by ter- trionale della Val di Scalve (Argillite di Lozio). rigenous to carbonate depositional systems (Assereto Er Casati 1965; Gnaccolini 8r Jadoul 1988), ranging from Abstract. A "mid-Carnian" transgressive succession, developed fluvial dominated deltas with volcaniclastic input (Val between the Breno carbonate platform and the semiarid coastal carbonates-sabkhas facies of the S. Giovanni Bianco Fm., is recorded in Sabbia Sandstone, lower S. Giovanni Bianco Fm.; Gnac- the northern Bergamasc Alps. This episode is characterized b1 rhe colini 1983, Garzantí &Jadoul 1985) and coastal lagoons

Regione Lombardia, Struttura Analisi e Informazìoni Territorialì, Via F. Filzì 22,20124 Milano. E- mail: [email protected] ': Dipartimento di Scienze della Terra, via Mangiagalli 31,2A83, Milano, Italy. E- mail: flavio..jadoul@unimì.it DA F. Berra & F. JatÌoul

Simplified tectonic sketch of the eastern Bergamasc Alps, s.ith the geographic and structural position of the sections studied (from published data and new geological mapping).

(Gorno Fm., Gnaccolini 1986, 1987),to subtidal-periti- graphic sections measured along an easr-west transect dal carbonate platforms (Breno Fm., Gnaccolini & (Fig. 3), together with observations from other outcrops Jadoul 1988) and semiarid coastal systems (San Giovan- framed in i:10.000 geological maps (Regione Lombardia ni Bianco Fm., Garzanti et al. 1995). Four regional depo- CARG project). Two main paleogeographic settings, sitional sequences were recognrzed by Gaetani et al. expressed sedimentologically in a "lower" and an "upper (1998) within this succession. Several types of facies lithofacies association", can be recognized in western associations, characterized by different scale cyclicity. and central Lombardy at a regional scale. The early were described by Gnaccolini Ec Jadoul (1990) and Carnian "lower lithofacies association" is represented, Garzanti et al. (1995). from south to north, by fluvio-deltaic, lagoonal and per- 'Ihis study focuses on the phl.sical stratigraphy itidal carbonate platform successions. The "upper associ- and paleogeographic setting of the middle-upper por- ation" (mainly late Carnian) is represented by coastal tion of the Carnian succession in the northern Bergam- siliciclastics and sabkha carbonates (with gypsum lenses asc Alps (Val di Scalve Ladinian-Carnian through, Valle toward the top), capped by carbonate intraformationai Seriana), dealing in de tail with the stratigraphic relation- breccias (Castro Fm., Jadoul et aI. 1992). Toward the ships between the southwestern shallow-water sedi- northeast (Val di Scalve-Lozio) this well-known stratr- ments (S. Giovanni Bianco Fm.) and the more basinal graphic succession shows a transition to a different set- deposits prevailing toward the northeast (upper Lozio ting, dominated by fine-grained subtidal siliciclastics Shale). The latter unit, previously considered to be (Lozio Shale, Assereto Ee Casati 1968) in the lower-mid- Ladinian (Rossetti 1967) or late Ladinian-early Carnian dle portion where the subdivision in the two lithofacies (Forcella Er Jadoul 2000), has now been dated as exclu- association is not evident, due to a less pronounced sively Carnian by Balini et al. (2000). facies diversification. The fine-grained siliciclastics of Facies analysis n'as carried out mainly within a sin- the Lozio Shale, with limestone intercalations toward gle tectonic unit (Vigna Vaga and equivalent tectonic the top, are covered by peritidal carbonates and sabkha units of the Seriana and Scalve valleys, Forcella 1988), facies referred to an "undifferentiated Carnian st4ccession" which preserves part of the Carnian basin, allowing (Forcella & Jadoul 2000). reconstruction of a WSV to ENE section, only slightly Particular attention was paid to the middle part of affected by AIpine rectonics rFrg. lr. the Carnian succession (boundary between depositionai sequences C3-C4 o{ Garzanti et aI. 1995 Gaetani et al. 1998) at the transition from the "lower" to the "upper Stratigraphy lithofacies association" toward the southwest (Presolana-Valle Seriana), and to the stratigraphic evolu- The str:tiqr:nhic*r"''' framework of the studv area tion of the upper Lozio Shale toward the northeast (Fig. Z) was reconstructed on the basis of several strati- (Valle di Scaive). The elaboration of the stratigraphic 11 id Carnian transgression

data (Fig. 3) records important facies changes, as Nrell as detail in the ULA: the occurrence of stratigraphic events, recognizable in 2a) "Black Pelites". Black shales and siltstones (up most of the studl'area, which represent local and region- to 25 m thick) prevail with dm - layers of fine-grained a1 marker horizons. sandstones with ripple marks, hummocky and parallel 1) "Lower lithofacies association" (LLA). It is char- laminations. The fine-grained sandstones (comparable acterized by shallow subtidal to peritidal carbonate plat- with the upper Lozio Shale) are rich in white mica and form cycles (Breno Fm., Annunciara Mb., Assereto & rare garnet (Fig. a); grains of I'olcanic origin are scarce. Casati 1965 of sequence C3 of Gaetani et al.1998 in the A ferruginous silty laver (up to 1 m thick), s'ith iron- western part of the study area (Ave, Valzurio, Rifugio rich carbonate matrix, vielding large microbial oncoids Albani sections), whereas toward the northeast it is rep- (up to 1.5 cm) is intercalated (marker bed in the Ave, resented by pelagic limestones (Prarorondo and Valzurio and Rifugio Albani secrions, Fig. 3; 5b, 5c). In Sommaprada Lmst., Balini et al. 2000) followed by a the Rifugio Albani secrion, Pb, Zn, CaF2 and BaS04 thick, monotonous succession of black shales and silt- mineralizations occur at the passage between the rop of stones (lower-middle Lozio Shale; Assereto E Casati the Breno Fm. and the "Black Pelites", where the pres- 1968). Toward the southeasr (Val Camonica), in a lower ence of an unconformity is documented locally (silcrete tectonic unit (Parautoctono Bergamasco; Gaetani & paleosoil of Rodeghiero 1977). Further west and south-

Jadoul 1979), the srratigraphic evolution is different: westward, the "Black Pelites" were nor deposited, .as shallowing-upward carbonate cycles, locally with shale recorded by a gradual transition from the Breno and/or intercalation at the base (Gnaccolini & Jadoul tlS8; Gorno Fms. to the fan delta siliciclastics of the lower S. 199A) or alternations of dark grev marls and limestones Giovanni Bianco Fm. (Garzanti et al. 1995), capped by (Gorno Fm.) represent the LLA. lagoonal-tidal carbonates and coastal plarn - sabkha 2) Upper Lithofacies association" (ULA). Toward deposits (upper S. Giovanni Bianco Fm.). Localll' (lower the northwest (Valle Seriana-Presolana) the Breno Fm. Camonica Valley) "Black Pelites" might correspond to is covered by black fine siliciclasrics representing the the siltstones and dolomitic marls presenr at the base of 1 t base of the ULA ("Black Pelites", Fig. 3). The boundary the 5. Grovanni^' Bianco Fm. iGarzanti er al. 1995; Fig.3. between the Breno Fm. and the "Black Pelites" is sharp. Val Supine secrion). In the uppermost Breno Fm. thin tuffitic intercalations Toward the east, the "B1ack Pelites" connect with are present. In the Camonica valley the ULA begins the Val di Scalve trough (characterized by a basinal evo- with green siltstones, dolomitic marls and dolostones at lution since the Ladinian), where the Lozio Shale was the top (lower S. Giovanni Bianco Fm. after Garzanti et deposited (Fig.3, Epolo section). The lithological fea- aI. 1995). The following horizons can be recognized in tures of "Black Pelites" and Lozio Shale are very similar,

Relolive seo level Southern Cenlrol corbonole North-weslern R"bù;;b*ll qènr C nnai rènnaa ,o epiconlinentol oreo plotforms ond sequences +- bosln - o= (-)

C '-- o c \-z o O >'- O rc OO o Sfudy oreo !o a) Goetoni et ol, o ( r ee8) o o Block Peliles Corbonole po{eosoils/residuol breccios Evoporites Perilidol doloslones Sobkho dolostones Dork grey bosinol limestones J al Fluviol cooslol ploin sondstones/siltstones Dork grey bosinol volconiclostic sondstones - Bosinol dork morly sholes/si stones Corbonote plofforrn morgin iI Fluviol-deltoicvolconÌclosficsondsiones Subtido limestones Logoonor dork gre! morls/ reslores Grev perilidol limestones

- Stratigraphic section along;rn ideal southrvest -northeast transect across the eastern Berganasc Alps. Numbers refer to the projection \il/engen of the stratigraphic sections of Fig.3. 'ù,'e: Fm., PV: Perledo Varenna Fm., Pr: Pratotondo Lmst., So: Sommaprada Lmsr., Es: Esino Fm., Br: Breno Fm. (LC: Lingua dì Campolungo Member), CMB: Calcare Metall:ifero Berg;amasco, VS: Val Sabbia Sandstone, Go: Gorno Fm., Lo: Lozio Shale (BP: "Black Pelites"), SGB: S. Gior-anni Bianco Fni. L22 F. Berra & F. Jadoul

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@ lR, -D sa'à ?Rcs \ qcq"f ( r Ri rS tq Re -( A iI Eò 'o' ) tLozio\iE Lozio Shole Breno Fm, S, Giovonni Bionco Fm, ;snore[

Fig. 3 - Correlations among the stratigraphic sections measured in the study area. Note that the "Black Pelites" can be correLated toward the west, whereas eastward they pass to the Lozio Shale. The "Bioclastic Horizon" is recognized all over the study area. M id- C arnian transgre s sion 123

180 160 140 o tr 120 I 100 o BO 60 40 20 0 0 '10 20 30 40 50 SiOz/MgO 180 160 140 o 120 o 100 s BO 60 40 20 0

180 160 140 120 o 100 o- o BO s 60 tr "Black Pelites" 40 20 0 100 SiOz/TiOz

Fig. 4 - a) Microfacìes (normal and polarized light) of a fine-grained sandstone of the Lozio Shale: note the presence of garnets (arrows) denoting the erosion of a metamorphic basement. Sample X10. b) Diagrams of different ratios from chemical analysis of fine-grained ' J2OZ, siliciclastics from the Lozio Shale, Gorno and San Giovanni Bìanco Fms. Note the similar distribution of the Gorno and San Giovan- ni Bianco samples and the di{ferent pattern of the Lozio Shale

despite the fact that the thickness of the latter unit co and Gorno Fms. (which provided similar results) and reaches about 200-300 m in the type area (Asserero Ed the "Black Pelites", which show a different parrern. Casati 1965) and that the beginning of the deposition of These observations indicate that the source area of the the Lozio Shales began before the deposition of the "Black Pelites" and Lozio Shale is probably different "Black Pelites". compared to that of the coeval southern successions. Chemical analyses of fine-grained terrigenous sed- 2b) Sabkha dolostones (SD). In the Presolana- iments from the "Black Pelites", the Gorno Fm. (distal Valle Seriana area (Fig. 3, 5a), the "Black Pelites" are cov- part of the southern volcaniclastics, passing laterally to ered by 18 to 35 m of bedded, grey, vuggy microspar the Val Sabbia Sandstones) and from the southernmost dolostones ("SD" lithozone of the basal S. Giovanni B. San Giovanni Bianco sandstones were compared, in Fm., Fig.3,5a), which can be traced to the Epolo sec- order to detect analogies or differences between the tion. Further eastward, in the Valle Marcia Section, these composition of the fine-grained clastics (Fig. a). The lithofacies are probably substituted by subtidal marls compared ratios indicate a different distribution, whose and limestones (Fig. 3). meaning is still unclear, between the San Giovanni Bian- 3b) "Bioclastic Horizon". Above the "SD" lithozone, 121 F. Berra & F. Jadoul M i d - C a t n i a n tran sgress i on 125

Fig. 6 - Microfacies of the "Broclastic Horizon" in the Ave Section. a) Calcified star-shaped secondarv anhl'd;it. in a bioclastic r.ackestone. Sam- p1e AE13, X 10; b) Bioclastic packstone with coated and rounded grains (brachìopods, pelecvpods) and superficial ooids, sample AE26, Xl0; c) Bioclastic peckstone u-ith red algae. crìnoids, brachiopods, sampìe AE2l, X20; d) oolitic grainstone with quartz grains at the nucleus of ooids, sample AE28, X 10. lenticular bioclastic/lithoclastic calcarenites/rudites, Eastward, the "Bioclastic Horizon" becomes dark grey bedded calcarenites and burrowed limestones thicker as the limestones intercalate with marls and with marlf intercalations (20-60 m thick, Fig. 3, 5a) crop shales in the upper Lozio Shale of Val di Scalve (Fig.3). out. Limestones are represented by oolitic-bioclastic Lithofacies consist of cyclic intercalations of marly lime- packstones to floatstones-rudstones with well-rounded stones and thick-bedded dark grey bioturbated lime- reworked intraclasts and lithoclasts (Fig. 5f, g. h and Frg. stones and ooidal and/or bioclastic fine calcarenites 6b, c) and, less frequentl,v, grainstones (Fig. 6d) . Skeletal (with crito#s, pe@rtg. 7a). The upper bound- grains are represented mainly' by open subtidal marine ary with the overlying typical bedded grey microbialitic fauna: crinoids, echinoderms, pelecypods, brachiopods, dolostones of the S. Giovanni Bianco Fm. is transitional. red algae, sponges, bryozoans (Fig. 6c), rare corals and The fossiliferous horizon (SGB2, Lithosome VIII both benthic (Trocbammind sp., Aulotortus sp., A. fried- of Garzanti et a1. 1995) occurring in Val Supine (Fig. 3) /1, (Kristan-Tolmann)) and sessile foraminifers (Zoly- is considered to be the southernmost evidence of this pammìnd gregaria \íendt, Fig. 5e). Generally, skeletal unit. grains and oolites are reworked and affected by micro- The "Bioclastic Horizon" is not well dated (sever- boring, microbial coating and micritization (Fig. 5 f, h) . al samples were processed for conodonts, by Alda Nico- This kind of syndepositional-ear1y diagenetic alteration ra, Milano, without results). A palynofacies study is indicative of relatively low sedimentation rates and of (Maria Teresa Galli, Milano) was carried out on the an intense reworking by currents. Most of the skeletal "Black Pelites" and the "Bioclastic Florizon": seven sam- grains derive probably from patch reefs (not preserved ples were processed, but only one yielded identifiable in the study area) and oolitic bars; grains $rere mainly palynomorphs (AE23, Fig. 3 Ave section) supporring a resedimented as tempestites. The facies association and Carnian age. the locally cyclic stacking pattern indicate a storm-dom- On the base of cephalopod faunas present near the inated, shallow carbonate-siliciclastic ramp depositional top of the underlying Breno Fm. (Rifugio Albani sec- system. tion. Fig. 3), and ascribed to the Julian (Allasinaz 1968, This lithofacies association is r-ell-expressed Aonoides Zone reviewed by Balini et al. 2000;, we ten- mainly toward the west of the study area (Val1e Seriana- tatively refer the "Biack Pelites" to the latest early Carn- Presolana); the physical correlation, supported by ian (Hardenbol et al. 1998; possibly in the Austriacum detailed geological mapping, has identified this horizon Zone, Krystyn 1928) and the overlying "Bioclastic Hori- as a re gional marker. zon" close to the early-late Carnian boundar,v.

Fìo 5 Valzurìo stratigraphic section (the location of the samples is in Fig.3): a) Vier. o{ the measured section, with the n.rain stratigraphic units; b) the oncoidal horizon in the "Black Pelites"l c) detail of the oncoìdal mìcrobialite, sample VZ6, X12; d) fossiliferous intercalation in the "Bioclastic Horizon"r e; oncoidai floatsrone x-ithTol,pammina gregaria 1ffendt, sampleVZ9n,X1.2: f; biochstic packstone\rudstone n-ith coated, ren-orked bioclasts (sponges, brachiopods, echinoids) and micritized ooids, from the "Bioclastic Hori- zon", sample VZ9i; X20: g) bioclastic packstone with crinoids, pelecypods, br1-ozoans and oolìtes, sample VZ9f, X12; h) bioclastic packstone n'ith micritized bioclasts, (Auloconus permodiscoìdes (Oberhauser), red algae. crinords, small gastropods) and ooids. Sam- ple J212, X12- 126 F. Berra ù F. Jadoul

Fig. 7 - Macrofacies and microfacies of the Lozio Shale of Val di Scah.e-Lozio area. a) Bioclastic floatstone with echìnoids and small authigenrc . K-feldspar (white) on a bioclast, Valle Marcia sectioni X 10; b) burroning in a silty layer of the Lozio Shale, Valle Marcia section; c-d) Slump overfolds in the Lozio Shales at Lozio (Concarena structural unit).

Discussion upper part) denote a coarsening- and shallowing-upward trend that indicate a progressive filiing of the basin. Lithostratigraphy and paleogeography According to the petrographic and chemical characteris- The analysis of the succession highlights the pres- tics, sediments of the Lozio Shale and the coeval Val ence of four stratigraphic units, discussed below, that Sabbia and Gorno Fms. derived, at least partially, from represent important elements in the reconstruction of different source areas (Fig. 4). Considering the geo- the Carnian sedimentary evolurion of the Bergamasc graphic distribution of these units, it is possible to rec- Alps. ognrze a carbonate barrier (Breno Fm.) which acted as a Lozio Shale. This unit, up to 300 m thick, is con- threshold for the southern volcaniclastic input, prevent- sidered to be the expression of a fine-grained siliciclastic ing its expansion toward the north, where the character- monotonous sedimentation in a restricted subsident istics of the terrigenous marerial are different (Fig. 8). trough (relict of the Ladinian Valle di Scalve-Lozio Breno Formation. This carbonate platform succes- basin), bordered south$.ard and eastward by platform sion, partly coeval with both the Lozio Shale and the carbonates (Breno Fm.). The fine-grained sandstones Gorno Fm, separates these two terrigenous depositional and siltstones rich in white mica (that intercalate in the systems. In the latter unit, an intercalation of Breno- M id,- C arn ian tran s gr e s s i on 127 derived carbonates is significant just south of the graphic position, we presume an age close to the early - Presolana area (Val Camonica), whereas, in the lower late Carnian transition. The microfacies associations Lozio Sha1e, limestones are absent, except for the upper- (Fig. 5, 6), with prevalent normal marine fauna, are most part. To explain this different exportation of car- indicative of a wide, open marine, subtidal bay bordered bonate mud from the platform, a controi exerted by the by carbonate platforms and by a delta plain and coastal windward and leeward margins of the platform and an sabkha southward. This succession is interpreted as the asymmetric distribution of the tidal channels may be expression of a major transgressive event that produced invoked. The dominating winds probably blew from the a second paleogeographic change, after the "mid Carn- northward basinal areas, resulting in the development of ian" carbonate platform crisis. This transgressive trend tidal plains and channels toward the southern quadrants occurs also toward the south (Val Supine, Fig. 3), where (leeward) and sweeping the carbonate muds toward the bioclastic facies are not associated with "Black Pelites", Gorno coastal lagoons. The absence of resedimented, but intercalate within a semiarid coastal lagoon-sabkha coarse-grained carbonates both in the Lozio Shale and succession (S. Giovanni Bianco Fm., Garzanti et al. the Gorno Fm., is indicative of the existence of very 1995). The transgressive episode is probabiy of regional low-angle slopes (ramp type) between the platform and importance: in the Austroalpine Domain, the bioclastic the adjacent subtidal areas. The stratigraphic evoiution limestones of the Cluozza Sandstone record an episode and distribution of this carbonate platform denote a of open marine conditions, related to a short-lived trans- prevalent aggradational trend with a progradation both gressive stage, which u/as responsible for the improve- southv/ard (Lingua di Campoiungo Member, Assereto ment of the living conditions (Frank 1986, Hochuli Er & Casati 1965; Fig. 2) and toward north-northwest Frank 2000). (Camino-Concarena massif, Arano informal unit, pers. The following progressive restoration of doiomi- comm., Forcella 2001) at the top. tized inner platform to sabkha facies (upper S. Giovanni "Black Pelites". This informal unit has been referred Bianco Fm.) characterizes the top of the succession in to the lower siliciclastic member of the San Giovanni the Ave, Valzurio and Rifugio Albani sections (Fig. 5a) Bianco (Garzanti et aI. 1.995) or the Gorno Fm. (Desio and reflects a regional distribution of coastal plain facies. et a1.1"970, "Blacle Shales" facies of Rodeghiero 19ZZ). According to lithofacies associations and petrography, The Carnian siliciclastics characteristics and provenance we interpret the "Black Pelites" as a tongue of the Lozio Shale that spread (westward and probably southward) The Carnian succession of the Lombardy Basin is over the early Carnian carbonate platform (Breno Fm., characrerízed by a mixed terrigenous-carbonate sedi- Fig. 2,3), which bordered the Val di Scalve basin, possi- mentation, documented by the interfingering of coeval bly during the latest early Carnian (? Austriac'tmZone peritidal, lagoonal, sabkha carbonates, and siliciclastic after Krystyn 1978). This unit increases in thickness (up fluvio-deltaic facies. In the southern part of the Lom- to 40 m) toward the northeast (Val di Scalve) and corre- bardy Basin it is possible to identify two volcaniclastic lates with the uppermost Lozio Shale or with pelites at inputs from the south ("Southern Mobile Belt", Brusca the top of shaliow water carbonates (local member of er al. 1982, Garzanti & Jadoul 1985) during the early the Breno Fm., Valle Marcia section, Fig. 3). The spread- Carnian (Val Sabbia, Gorno Fms.) and, in part, the late ing of the pelites over former, carbonate highs, locally Carnian (lower S. Giovanni Bianco Fm.). Most of the emerged (Rifugio Albani), was probably due to a relative San Giovanni Bianco siliciclastics record a prevalent vol- sea level rise, later followed by the deposition of fine- caniclastic input with minor material derived from the grained sediments. basement. An increase of grains derived from metamor- Toward the south, the "Black Pelites" event may cor- phic-intrusive rocks is documented toward the top of respond to the development of the second fluvio-deltaic the formation, denoting a progressive deep erosion of a depositional system of the Carnian (siliciclastics of the volcanic arc (Garzanti 1.985a, b; 1986; 1988). Lower S. Giovanni Bianco Fm., Garzanti et al. 1995). The terrigenous material of the "Black Pelites" and "Bioclastic Horizon". This informal unit has been siliciclastics of the upper Lozio Shale, is consistently referred by previous authors to different stratigraphic rich in white mica and minerals such as garnet (Fig. a) units: Gorno Fm. and/or lower S. Giovanni Bianco Fm. which denote a provenance from the erosion of a meta- (Desio er aI. 1.970; Rodeghiero 1977), base of the upper morphic basement. S. Giovanni Bianco Fm. (Garzanti et ai. 1995). This par- The existence of sandstones derived from a north- ticular facies, with a peculiar paleontological content, ern basement high located in the Central Austroalpine represents the most continuous stratigraphic marker in (upper part of the Carnian succession, Clsozza "black the study area. IJnfortunately, no significant fossiis were shales" and sandstones and Mergel Member of the Fanez found. The presence of microfacies very rich in Aulo- Fm.; Frank 1986, Hochuli & Frank 2000) may suggest a tortids and the dasycladacean alga Clypeina besici Pantic similar source area for the fine siliciclastics of the Lozio indicates a generic Carnian age. According to the strati- Shale - "Black Pelites", even if the Insubric Line prevents \28 F. Berra & F. Jadctul

'.'.'.Emoraadlnndc Fig.8 -Schematic paleogeographic -îîî- reconstructron! not pahnspas- 4# Corbonote plotform tic (map and section) at the beginning oI the deposition of "Black Pelites" in the west- lO0km ern Southern Alps and in the Central Austroalpine (Cluoz- za Sandstone). The black hatches correspond to out- cropping areas, the grey -ones to interpretation. Subsurface 11 .r, data in the Po Plain are placed accordìng to Brusca et al. 19 82.

-j"vERoNA \ Wesfern Soulhern A/ps t

Son Giovonni Bionco Breno LOZIO Fnnaz Fmaraor{ Formofion Formotion Shole Formotion ond

S Wesfern Soufhern Alps Insubric Line

a precise paleogeographic reconstruction (Fig. 8). The (Garzanti et al. 1995: Passoni 1.997; Anna Paganoni, presence of another basement high is documented west- pers. comm), coinciding with the arrival of shales and ward according to the petrographv of the Carnian Lier- siltstones on the former carbonate highs. We suggest na Fm. (eastern shores of Como Lake; Garzanti Ec Pagni that this event may be controlled, at least partially, by a Frette 1990). that is characterized by scarcity of volcani- climate change to more humid conditions, that favored clastic. Therefore, we suggest for the Lozio Shale-"Black the development of a rich flora on the coasts of the Pelites" the existence of different sources, from north- western alpine Tethys and the delivery of siliciclastics. A ern and/or western basement highs of the Lombardy return to arid conditions is recorded by sabkha dolo- Alps (Fig.8). stones and evaporites of the San Giovanni Bianco Fm. In different portions of the Alpine chain a "mid - Carnian" humid episode was invoked by Garzanti et al. (1995) in Paleocli mate considerations the Lombardy Basin, Gianolla et al. (1998), Keim et al. The crisis of carbonate production close to the (2001) and Preto & Gianolla (2001) in the Dolomites transition from Early to Late Carnian in the Vestern and by Hochuli & Frank (2000) in the Centrai Aus- Tethys and the following input of fine-grained terrige- troalpine. This event can be referred to the episode rec- nous sediments (study area; Hochuli & Frank 2000 in the ognized by Simms & Ruffel (1990). Flowever this inter- Central Austroalpine; De Zanche et al. 2000 in the Julian pretarion was later rejected by Visscher et al. (1994) on Aips, and Keim et al. 2OOl in the Dolomites) allow some the basis of palynological evidence. These different considerations on the mechanisms that controlled the results can be interpreted suggesting that the humid observed strati graphic-paleogeo graphic evolution. episode could be local and documented on the coasts of In the succession studied, an increasing abun- the Vestern Tethys and in part of the Pangean continent, dance of plant fragments is recorded at the passage from probably due to the atmospheric circulation and the the Gorno Fm. to the San Giovanni Bianco Fm. control exerted by rhe orography of the continent. M i d - Carn i an tran sgress i on 129

Sequence stratigraphy the Carnian cycles recognizedby the different Authors A different number of Carnian third-order cycles (De Zanche et al. 1.993; Garzanti et al. 1.995, Gaetani et al. 1998, aL.1.998, can be recognized in southern-central Lombardy as Gianolla et Hochuli & Frank 2000) are opposed to the northeastern sectors (Fig. 2). Toward local and not exportable to other portion of the Alps. the south, four sequences (Garzanti et aI. 1,995:' Gaetani et al. 1998) were recognized. Toward the northeast, the Conclusion early Carnian cycles are not represented in the basinal Pratotondo Lmst. and Lozio Shale succession: in this The Carnian sediments of the \flestern Southern area ir is possible to identify only two sequences. The Alps of Lombardy identify two palaeogeographic only Carnian sequence that can be clearly traced from domains with different stratigraphic evolution. Toward the Lozio Basin to the southern Lombardy is represent- the south and west, the Carnian succession is repre- ed by sequence C4 of Gaetani et al. (tllS). The lower sented by a fluvial-deltaic, lagoonal to peritidal carbon- boundary, sharp and well defined in the Presolana-Val1e ate platform setting (respectively Val Sabbia, Gorno, Seriana sections and locally marked by an unconformity, Breno Fms.) covered by carbonate, sabkha and evapor- is placed between the peritidal limestones-dolostones itic facies (S. Giovanni Bianco Fm.), whereas toward with tepees, caliches, oncoids and microbialitic lamina- the northeast the coeval succession consists of basinal tions at the top (Breno Fm. of Presolana, Valle Camon- to shallow water dark grey shales and siltstones (Lozio ica) and the "Black Pelites". The top of the sequence is Shale), with minor intercalations of shallow-water lime- not preserved in the study area due to the presence of stones, covered by mainiy sabkha carbonates and pelites alpine thrust surfaces developed at the top of the S. Gio- (S. Giovanni Bianco Fm.). In the transitional area vanni Bianco Fm.; in other areas the maximum thickness between the two palaeogeographic domains (northeast ls uD to l)u-luu m. of the Lombardy Basin), two stratigraphic markers The lower portion of the TST (possibly inciuding ("Black Pelites" and the "Bioclastic Horizon"), deposit- part of the LWST) is characterized locally (Presolana- ed close to the F,ar|y-Late Carnian boundary, mark a Seriana areas) by a shallowing-upward parasequence nes/ stratigraphic-paleogeographic setting after the cri- (high frequency sequence after Mitchum & Vagoner sis of the early Carnian carbonate platform. The "Black 1991), 25-50 merers thick, with a subtidal lower pelitic Pelites", according to lithofacies association, chemical portion ("Black Pelites") and an upper dolomitic and petrographic evidence) are not interpreted as the (lagoon-sabkha facies of SD lithozone, Fig. 3). The northern closure of the Gorno Fm. lagoon but thought overiying main TST is well recognizable in a1l the sec- to represent the western pinch-out of the Lozio Shale tions studied and is represented by the open subtidal deposited, since the beginning of the Carnian, toward normal marine facies of the "BioclasticHorizon" (msfl, the east and only later covering the Breno carbonate also recognizable in few proximal successions of the S. platform westward. The sharp lower boundary of this Giovanni Bianco Fm. (Val Supine, Lithosome VIII of S. horizon is interpreted as indicative of a relative sea level Giovanni Fm., Garzanti et al. 1995). The highstand sys- rise, necessary to create the accommodation space and tems tract (HST) is represented by the regressive trend responsible for the development of poorly oxygenated (shallow water dolostones and locally evaporites) typical sea bottoms. A climatic event, responsible for environ- of the upper S. Giovanni Bianco Fm. At a regional scale, mental changes and foilowed by a regional sìliciciastic the mfs represented by the "Bioclastic FIorizon" can also progradation may be invoked to explain the crisis of be recognized both in the Central Austraolpine and in "mid-Carnian" subtidal to peritidal carbonate platforms the Julian Alps: in these areas, the different sequences in the Southern Alps (Breno, Durresrein, Conzen containing this episode are characterized by a different Fms.). progressive number in the sequence order, denoting cor- Considering the difference in composirion of the relation problems. According to recent literature, the southern clastics during the Early Carnian, ar leasr r'wo only depositional sequence that can be recognized all different main source areas can be invoked. Whereas over the Alps, is the one containing the msf correspon- the source area for the Val Sabbia, Gorno and S. Gio- ding to the "Bioclastic Horizon". The difficulty in iden- vanni Bianco units is well defined (Southern Mobile tifying the Triassic sequences over large distances, may Belt, Brusca et aI. 1982), two different basement highs be related in part to the fact that the data are not based could represent the source of the Lozio Shale and on the coastal onlap curve, but on the transgressive and "Black Pelites" sediments: a northern (Austroalpine regressive trends recognized in the stratigraphic sec- Domain) and/ or a western high (Como area, Fig. 8). tions. Considering the different sedimentary supply, the The main rransgressive trend is then confirmed presence of carbonate factories (that can produce differ- by the regional distribution of a normal marine, subti- ent amount of sediments according to climate, position dal succession ("Bioclastic Horizon"), which towards with respect to the coast, amount of nutrients) and the the east becomes fine-grained and thicker within the regional subsidence, it is therefore possible that some of Lozio Shale through. A similar transgressional stage is 130 F. Berra G F. Jadoul documented in the Central Austroalpine (Frank 1988). and the Paleotethys located eastward.

The passage to marine conditions implies the con- Acknozaledgments. The Authors wish to thank Prof. A. Gre- nection to a wider marine basin: considering the Carn- gnanin (Milano) for his help in performing chemical analysis of the ian paleogeography of the Southern Alps (Brusca et al. siliciclastics, Dott. M. T. Galli (Milano), Prof. S. Cirilli (Perugia) for 7982; Garzanti et al. 1995, this paper, Fig. 8) the con- the discussions on palynofacies and A. Nicora (Milano) foi the conodonts studies. The manuscript benefited from careful reviews by nection of the study area with the open sea was likely Prof. E. Garzanti, Prof. H. Visscher., Dott. P Gianolla and Prof. M. where the reahzed through the region of the Dolomites, Gaetani. Financial support by Centro CNR per la Geodinamica Carnian carbonate platforms did not close the connec- Alpina e Quaternaria di Milano, 60% Università di Milano (Resp. tion between the coeval environments of Lombardy Prof. F. Jadoul) and CARG project of Regione Lombardia.

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