Rivi'ta lt.rlienr di Paleonrologra e StrariSra[i.r \ottÌnle tul pagrne 4Z-68 Aprile 2001
THE LO\TER PERMIAN IN THE OROBIC ANTICLINE (SOUTHERN ALPS, LOMBARDY): A REVIE\T BASED ON NE\í STRATIGRAPHIC AND PETROGRAPHIC DATA
DARIO SCIUNNACH
Received. September 5, 200a; accepred January 31, 2A01
Key uords: Southern Alps, Permian, geological mapping, vol- points to distinct :nd/or non-coeval rolcanic rcririty rt the western canic successions, s,rndstone petrographl', chronostratigraphy. edge of the Collio Basin (Orobic Anticline) with respect to the cen- tral-eastern sector (Trabuchello-Cabianca to Camuna Antìclines). Riassunto.Un rilevamento geologico dj facies, eseguito in scala Sandstone petrography, indicatrng provenance from over- 1:10 000 nel Permiano Inferiore dell'Anticlinale Orobica nell'ambito whelming neovolcanic sources, is fairly unrform both vertically and del Progetto CARG, ha consentito di ricostruire l':rrchitettura del baci- laterall,v, and thus sugsests high sediment;rtion rares for the Collio no e di pervenire ad una suddir'ìsione della successione str:tign{icr Formation, in agreement wìth the short time span recently assigned to rneglio clocumentata rispetto al quadro disponibile della lerteratura. this unit bv radiometric data from the Camuna Anticline. Different All'eruzione parossistica di benmoreiti nel nrembro vulcanico della sandstone composition in the Orobic and Camuna Anticlincs is Forrnazionc di Collio seguì un'attività vulcanica sinsedimentaria dì tipo ànother line of evidence agaìnst physical continllitv of thc Colljo polimodale (comprendente lave mugearitiche e tufì riolitici), concomi- Basin. tante con la deposizione del soprastante membro arenaceo-vulca- noclastico. Dopo la progradazione delle facies dj delta-conoide prossi- male del Conglomerato del Pontcranica, una possibììe ingressionc Introduction. Lrarina e suggerira dr rari Foraminiferi nel membro arenaceo-pelitìco sommitale della Formaz.ione di Collio. The Italian program for geological mapping at the L'evoluzione del magmatismo ben si inquadra in uno scenrrio 1: 50 O0O scale (CARG Project: Catenacci 1995) can be paleogeodinamico dj trascorrenzr crostale; l'asscnza di rioliti nel mem- seen as an opportunity reconsider bro vulcanico induce a ritenerc che I'attività vulcanica:rl mrrqinc occì- to both the lithos- dentale del Bacino del Collio (Anticlinale Orobica) fosse distinta e/o tratigraphic framework and the tectonic evolution of the non coev:1 rispetto al settore centro-occidentale (dalì'Anticlinale di Lombardy Southern Alps. A prominent structural fea- Trabuchello-Cabianca all'Anticììnale Camune). ture of the Southalpine range is represented by the Oro- La petrografia delle arenarie, che indica una soverchìante bic Anticlines, an alignment o{ three anti- pror-enienza c1e aree sorgenti neovulcaniche, è assai uniforme in senso E-\fl trending sia verticale, sia laterale, c sugeerisce trlti tassi di sedimentezione per la clinoria named Orobic stricto sensu, Trabuchello-Cabi- Fornazione di Collio, in accordo con la breve fincstra-tempo recente- anca and Cedegolo, west to east (Fig. 1); in each anticli- mente assegnata a questa unità da datazioni radiometriche orrenute rn norium, a volcanic to sedimentary succession spanning Anticlinele Camuna. l,a clìfferenza di cornposizione tra le arenarie delle Anticlinali Orobica c Camuna è un altro elenrento: sfavore della con- the Upper Carboniferous to the lowermost Anisian tinuità fisica del Bacino del Collio. non-conformably overlies the Hercynian crystalline basement. The most important part of this succession in
Abstact. 1 : 1 Facies mapping at the 0 0OO scale of the Lon er Per- terms of voiume, economic interest and palaeotectonic rnian in thc Orobic Anticline, carried out in the framework of the CARG Project, allor.ed rcconstruction of the basin archìtecturc and significance is surely the Lower Permian (Collio Forma- refined subdir-ision of the stratigraphic succession with respect to the tion and heteropic conglomerates: Fig. 2), deposited in avajlable framer.ork. Paroxl.snal effusìon of be.nmoreites in the lower strongly subsiding, fault-bounded continental basins r-olcanic member of the Collio Formation x.as follorved bv r srn.c.li- and containing uranium minerals in the Novazza-Val mentaryr poll-rnodal volcanic activltlr (mu€learite flou.s to rhyolite tuffs) during deposition of th. olcrlyinia àrcnrccou\-\.olctrniclastic Ve dello district (Cadel 1986; Cadel er aI. 1987). Mapping member. After progradation of the Ponteranica Conglomerate pruxì of this series should rely on a detailed stratigraphic rnal fen-delta facies, a possìble marine transsression is suggested br framework, that however has turned out difficult to r:rre foraminifers, ncr.lv-found in the upper aren:rceous-pclitic mem- establish all over the outcrop area, mostly due to 1) ber of the Collio Form:tion. N4agm;rtìc evolution fits into a palaeogeodynamic scenarìo of stron€l lateral variations of facies and thickness of the continental r.renching; lack of rhvolites in the lor.er r.olcanic member clastic bodies, 2) scarcity of time-diagnostic fossils and
- Struttura Analisi e Informazjon j TerritoriaL, Regione Lornb.rrdi.r, Pirzza Duca d'Aosta I 20124 Milano (Italy). E-mail addrcss: Dario Sci- unn.reh,,, region.. lonrb.rrdir.ir 48 D. Scìunnach
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r1g. r Location of the Lower Permian outcrops in the Lombardy Southern Alps (simplifìed:rfter Montrasio 1990): solid, horìzontaÌly stripped and dotted areas correspond, respectivcl,v, to volcanic, :renaceous-pelitic and conglomeratic facies. OA : Orobic Anticline, T-CA : Trabuchello-Cabianca Anticline, CeA : Cedegolo Anticline, CaA : Camuna Antìcline. The box ìncludes the area of Fig. 3.
G. z ORG'CANÎCUNE TRABUCHELLOCABIAI\CAANNCLINE CEDEGOLOANNCUNE CAMUNAANTICLINE Í= J(,3< ,t LiJ z À< DOSSO DEI \=- O Z IJJ upper z. <: Sedimentary alO Upper Sedimeniary t.- F UJ Member zzF(9 Member (map unìts a Upper Sedimentary 2 4 to 8 of Cadel et al., > Member J 1 996) É. x ("Collio Beds") o< g É. Lower Volcanic Member O ^ = (ìniervals tn Lower Lower LR1 to UR6+, I Volcanic r -o : Member __l mîn I rnitc O t^ J -()Member r '13 of Cadel c) - - et al., 1996) (J () (J C'f\GLOIUERATO -.ìf BASALE É. U L t z METAMORPHIC I IICTAIÍADDUI''u*=ilft;"t I METAMORPHIC BASEMENT BASEMENT o( ,1 (J -s Fig. 2 Available stratigraphic framework for the Lorver Permian in the Orobic to Camuna Anticlines (mostly after Do4, 1.935; Casati 8c Gnac- co|tnr7967; Cassìnis et al. 1988; Cadel et al. 1996). Formations are in all capitals, members:in capital/smali letters. VBG : Val Biandi- no Granodìorite and associatcd post-Variscan plutons. Age constr.rint: after Fig. l0 and rclatcd discussìon in text. The Lou,er Permian in the Orobic Anticlìne 49 marker beds, 3) intense Alpine deformation. The type thick, commonly with imbricated pebbly lags at their base (estìmated pebble composition: mìlky quartz: 9a-99'L, metamorphic rocks 1- areas and best exposures of the Collio Formation occur - 107n, r.olcanic rocks - tr.). Deposition in elluvial fans and high-ener- is in the Camuna Anticline (Fig. 1); the present p.rper gy braided streams is indjc;rted; focused on stratigraphic and petrographic results 3) strongil' bioturbated siltstones to mediurn-grained sand- obtained on the Lower Permian of the Orobic Anticline stones, wìne red jn colour and rich in detrital mica flakes, ("Pietra during field and laboratory work carried out in the Simona-like" facies, although much oìder). This finer-grained facies, seemingl;' deposited by floods jn overbrnk sertings, supports thc mapping the forthcom- framework of the geological of inference that the describcd facies are rernnants of a vast. articulated ing sheet 076 "Lecco" of Lhe ne* I:50 OOO geological alluviel system, dissected by subsequent erosion; t1'pical channel ìag to map of Italy. mernder plarn [acicr :re lacking. Compositionally similar clastics, non-con- Geological framework. formably overlying the metamorphic basement west of Lake Como (Manno, Alpe Logone, Bocchetta di San The Orobic Anticline ("Val Stabina Anticline" of Bernardo, Bedero, Invorio) are ascribed to late West- Porro 1.932; "Mezzoldo Unit" of Schònborn 1992) phalian to possibly early Stephanian times due to plant extends for abor-rt 25 km west to east, from the Taceno remains (\funzo & Maglia 1942; Jongmans 1960) ; how- district in Valsassina (Sciunnach et al. 2000 and ref. ever, an early Cisuralian age (Asselian to Sakmarian) for therein) to the Monte Pesherolo carbonate ridge in the the unfossiliferous Congiomerato Basale cannot be Brembana Valley (Casati 8. Gnaccolini 1967), peaking in ruled out due to stratigraphic position. altitude with the Piz-zo derTre Signori (2554 m a.s.l.). At In the Orobic Anticline, the only outcrops of the core of the fold, the Hercynian crystalline basement Conglomerato Basale are found at Cima dei Siitri and is exposed in the Valsassina, Ornica, Caprile (or Val- Monte Arele (Casati Ec Gnaccolrm 1967), north of the n.roresca) and Mezzoldo "windows". The metamorphic Monte Pegherolo ridge and well outside the study area. basement consists of metapelites and metapsammites Thus, in the present paper this unit will be discussed (phyllites, micaschists and paragneisses with minor only marginally. That possibly huge volumes of Con- quartzites, amphibolites and marbles) in greenschist glomerato Basale were erased along the tectonic surface facies ("Gneiss di Morbegno" Auct.); relics of higher- displacing the base of the Collio Formation (see below) grade parageneses (kyanite, staurolite; Cadel et aL.1996) is destined to remain an undemonstrated assumption. xre 1ocally preserved. Leucocratic orthogneisses The Collio Formation (up to 1250 m-thick in the ("Gneiss Chiari di Corno Stelia") are commonly found Orobic Anticline) overlies either the Conglomerato at the non-conformable contact n'ith the sedimentary Basale or, more commonly, the crystalline basement. In succession (Boriani & Colombo 1979).In Valsassina, the former case the boundary is sharp and paracon- the metamorphic basement is intruded by Ìate Hercy- formable; in the latter, it typically corresponds to a blan- nian plutons, commonly known as Val Biandino Gran- ket of black catacl:rsites, up to a few metres-thick, yield- odiorite (Pasquaré 1967) and Vle Biagio Granite (De ing abundant tourmaline (Zhang et al. t99+). In the Sitter & De Sitter-Koomans 1949). Radiometric rges three Orobic Anticlines, the Collio Formation is usual- (Tlroni et aI. 1992 and ref. therein) constrain these intru- ly subdivided into a lower member, in which volcanic sions as largely Early Permian. A swarm of roughly par- products prevail by far over sedimentary rocks ("Vol- allel'stocks and dykes, striking about 60'N and thus canic Member" Auct.), and rn upper member, mainly nrrtching l Permirn tectonic trend widespread in the consisting of clastic sedimentarl' rocks with volcanic Orobic Alps (Cadel 1986). represents the typical out- intercalations ("Sedimentary Member" Auct., "Arena- .rnn nrtrern /Fia l\ \- -b' " / ' ceous-shaley Member" Auct.); both members have to be The oldest sedimentar.. unii, non-conformably regarded as informal lithostratig::aphic units. At the overlying the crystalline basement, is the Conglomerato western and eastern boundaries oi:the outcrop area, the Basale, up to160 m-thick in the Trabuchello-Cabianca "Sedimentary Member" passes laterally and upsection io Anticline (Cadel et al. 1996) but very poorly exposed in conglomerate wedges (Ponteranica and Dosso dei Galli the Orobic Anticline. Conglomerates, respectively) that arre instead formal lithostratigraphic units of formation rank. To the east, At le;rst three lithofacics r.ithin the Conglomerato Basale are easìly recognised in the field: the Dosso dei Galli Conglomerate (which includes the 1) "eporphyric" phyllarenitic conglonler;ìtes, with subangular Pietra Simona Member) is sealed by the Auccia Vol- metamorphic pebbles prevailing or.er quartzite pebbles and commonlr'' canics, in turn overlain with angular unconformity by supported by a coarse-grained, poorly-sorted micaceous sandy rnatrix. the Verrucano Lombardo redbeds (Fig. Z). Strong later- frns; Deposition occurred in talus cones to proximal alluviaÌ al variability of facies and local incornpleteness of the 2) "aporphyric" quartzose orthoconglomerates, paraconelom- crates ,rnd very coarse-grained sandstones with sparse granules ;rnd stratigraphic succession are probably due to syndeposi- pebbles ("typical" facies), in poorìy distinct la1'ers l0 to over 100 cm- tional uolift of fault-bounded blocks and erosion in con- 50 D. Sciunnach tinental setting. These factors cause the Collio Forma- braidplain, passing - east of the Giudicarie Line - to the tion, along with the above-mentioned associated units, Val Gardena Sandstone floodplain (Massari er aL 1994). to be perceived by mosr field geologists simply as the Both the non-conformable contacts on the crystalline stratal package bracketed between the crystalline base* basement observed west of Introbio (Garzanti 8r Sciun- ment -t- the Basal Conglomerate at the base and the Ver- nach 1.997; Sciunnach 2OO1) and the angular unconfor- rucano Lombardo at the top. Under this respect, the mity with the Collio Formation (Casati Ec Gnaccolini whole Lower Permian may be regarded as an unconfor- 1962) document a prolonged hiatus before deposition o{ mity-bounded allostratigraphic unit of group rank. the Verrucano Lombardo and a major shift in the subsi- Sedimentary structures and the few fossils (large- dence patterns, seemingly passing from tectonically- 1y plant remains and tetrapod footprints) documenr drìven and localised to thermo-tectonically driven and deposition in endorheic lacustrine basins in the Camuna widespread (Cassinis et al. 1988); the described archi- (Cassinis 1966) and Trabuchello-Cabianca Anticlines tecture indicates that regional subsidence outpaced the (Cassinis et al. 1986). Sandier facies and minor pelites vertical slip rate of normal faults (Doglioni et a1. 1998). are exposed in the Orobic Anticline, pointing to prevail- Oxidation and pedogenesis are reported from both base- ing distal fan-delta environmenrs. Strongly variable ment rocks (Sciunnach 2001) and Collio sediments facies and thickness can be seen as the result of syntec- (Cadel et al. 1996) underlying the Verrucano Lombardo tonic deposition on the hanging walls of fault-controlled redbeds. An intraformational hiatus, not easily perceived basins, accommodating the clastic sediments from foot- in the field, is suggested by the abrupt compositional wall-sourced fans; this architecture indicates that the shift in sandstone composition from Petrofacies P1 to vertical slip rate of the faults outpaced regional subsi- P2 (Sciunnach et al. 1996). dence (Doglioni et al. tllS). Tectonic deformation probably started already in the Permian and peaked dur- Methods of study. ing the Alpine Orogeny; it results in large-scale, open folds u,'idespread and faults, as well as in pervasive frac- Facies mapping. In the framework of the CARG ture to slaty cleavages developed in volcanic to coarse Project, an area of about 75 km2 was mapped at the clastic and pelitic rock types, respectively. Small-scale 1:10.000 scale in the southern Orobic Anticline (Fig. 3). polyharmonic folds are common sand-mud couplets, in Since the stratigraphic reference framework (Casati 8e and recumbent to isoclinal folds - transposing the sedi- Gnaccolini 1962) considered only two subdivisions in mentary bedding facies. - occur in all the sedimentary the Collio Formation (a volcanic lower member and a The Verrucano Lombardo, ranging in thickness sedimentary upper member), contrasting with much from 150 to nearly 450 m over most of its outcrop area more detailed subdivisions obtained in the adjacent Tra- (Assereto & Casati 1965), consists of pebble ortho- and buchello-Cabianca Anticline (Cadel et aI. 7996), a pre- paraconglomerates, coarse-grained sandstones and mica- liminary facies mapping has been carried our, aimed at ceous siltstones commonly arranged in decametric FU- defining a sufficient number of mappable stratigraphic cyclothems; rounded volcanic pebbles are roughly as intervals. The complex problems of stratigraphic nomen- abundant as subangular qrartzite pebbles, whereas mera- clature posed by the Lower Permian in the Southern Alps morphic pebbles are very rare. ln Valsassina, angular are beyond the scope of this paper; the rank of the Co1- cobbles of Gneiss Chiari are however observed; rare gra- Iio Formation, although needing reconsideration, will nophyre pebbles might have been derived from subvol- not be questioned, and all the lithostratigraphic names canic acidic rocks similar to the those presenrly exposed here proposed have to be regarded as informal. at Cuasso al Monte (Buletti 1985), but their local con- Six lithofacies, systemarically recognised in the centration might even suggest provenance from local Lower Permian of the Orobic Anticline, are described sources nearby. here below (volcanic lithofacies V1 to V3 and sedimen- Colour of all the grain size fractions is typically tary lithofacies S1 to 53). A similar, but wider, specrrum wine red; greenish reduction spots are observed onÌy of facies is proposed in Forcella et al. (ZOOt) for all the locally. The Verrucano Lombardo was deposited in a vast three Orobic Anticlines. Fìo l Selected facìes of the Lower Permian volcanics and cl:rstics in the Orobic Anticiine. Volcanic member of the Collio Formation: A) r.eld- ed lapillistone in u'hich only subtle variations in grain size and degree of welding hint at the original plane bedding (V1 facies, Cedri- no Pass); B) laminated welded tr"iff (V1 facies, Pizzo dr Giovanni); C) intraformational volcanic breccia (V1 facies, Pizzo di Giovanni); D) welded accretionary lapìlli (V1 facìes, Biandino Vallel- - photo by M.P Confalonieri); E) eraded pyroclastic to epìclastìc deposit (V2 facìes, Pizzo di Giovanni). Lower Permian clastics: F) volcaniclastic paraconglomeratc (S1 facies, Cusio - hanrmer head on the left for scale); G) Ponteranica Conglomerate (S3 facìes, Biandino Vallev - photo by M.P Confalonieri); H) medium-grained sandstone r.ith abundant flattened :rnd imbricated mudclasts (S2 facies, Przz.o di Giovanni). Tlce Lotoer Permian in the Orobic Anticline 51 ry. m; . '*"4 52 D. Sciunnach V1 - Intermediate to acidic welded tuffs and lapilLstones, with 300+350, livetime : 5O seconds, international standards variable amoLÌnt of porphyrocrvsts and degree of welding (Fìg. aA) up and metallic cobalt for calibration) microprobe analyses to n-rassive porphyries, ranging in colour from whitish to deep red to were carried out on a polished thin section of the basal brownish-green; comparatir.elv rare ignimbrites, displavins pumiccous lithics and shearedfìamme, do hou'ever occur. This facies was rneanr to cataclasite. inciude also intraforrnatìonal breccias (Fig. 4C) consistìng of 100%' Sandstone petrography. 300 points were counted angular blocks rvelded in an .rphanitic mrtrix ("block- and ash-flor. on each standard thin section, and point-counting data cleposìts"sezsa Cas & ${rright 1987), commonly too thin to be mapped were recalculated following the Gazzi-Dickinson at the 1:10.000 scaie. A rough bedding underlined by thin tuffaceous method (Dickinson D7A; 1985). Grain size was semi- interlavers (Fig. aB) is locally observed, but in gencral is pen'asively quantitatively eyaluated according al. overprinted by the Alpine cleavage. to Garzanti et V2 - Volcaniclastic breccias and arenites to reworked tuffs (2001). ("epiclastìcs"; Fisher 1961) rvith poorly-sorted, angular lapilli exclu- sivei,vvolcanic in origìn embedded ìn a "salt-and-pepper" sandv matrix (Fig. aE); abundant plagioclase is recognised in the arenites. The Pradini Valley section revisited. V3 - Massive, mostly basic lava flows, deep green to alnost black ìn colour. Black femics and saussurrìtìsed pale grccn plagiocLase Tectonic basal cont.rcts on the crystaliine base- are locally observed; amygdalar vacuoles are n,idespread. ment, severe Alpine deformation and poor exposure of S1 - Mcdium- to coarse-grained, grev to pink volc:nic arcnirc:, its sedimentary upper part, commonly hamper a prec;se displaying abundant fresh detrital feldspar and scattercd white mica flakes, locally concentratcd on the bedding pl;rnes in the finer-greined thickne ss estimate of the Collio Formation from base to fraction. Lavers are up to several tcns of cm-thick, mostl,v homoge- top. One of the very few places where a continuous sec- neous in grain siz-e but locally displaying a rough grading, either nor- tion is exposed is the Pradini Valley talweg (Fig. 5), on n-ral or inverse. Subordinatc intercalations of pelites, pvroclastics and the southern slope of rhe Pizzo dei Tre Signori and on conglomerates occur (Fìg. 4F). the northern limb of the Orobic Anticline (base of sec- 52 - Prevailing fine- to medium-grained, dark grs- r-oìcrnìc tion at 1490 m a.s.l., Gauss-Boaga co-ordinates 5 093 920 arcnites, in layers a few cm to less than 2O cm-thick, fining upr.:rrds to black pelites rich rn r.hite mica flakes (sand/nud ratio > 1). A variety - 1 541 220). This section, 976.5 m-thick, was first meas- of seclìmentarl- structures (channelised beds underlìneci by clay chips ured and described in Casati & Gnaccolinr (1967).In the at theìr base, normally graded beds, parallel and los-angle lamination, present paper, their interval 1 is split into intervals 1-8. climbing and load-casted ripples, lenticular bedding, slumprnes, con- Twenty-five samples were collected for petrographic and volute lamination, Joad structures evolving to balis and pìllorvs) is dis- geochemical analyses from the measured section. played (Fìg. 4H). 53 - PebbÌe to cobble conelomerates, alternating to coarse- The composite section of the Zatto-Monte Caval- grained pebbly sandstones, deposited ìn proximel allur.ial fans (Fig. lo Valleys north of Mezzoldo (Casati 8r Gnaccolini aG). ln the Pizz.o dei Tre Signori area this facies was formally intro- 1967) can be useful for comparison. In both sections, duced as a distinct unit, occupying a defìnite stratigraphic position however, the basal contact with the basement is tec- (Ponteranica Conglomerate; Casati Er Gnaccolini 1965), whereas in tonised and the thickness of interval t has to he req:rd- the Trabuchello-C:rbianca Anticline the term "Monte Aga Congiome- ed as a minimum value. rate" has been recentl,v introduced (Cadel et al. 1.996). The recognised levels are, top to bottom: The stacking pattern of facies observed in the field - Verrucano Lombardo. Orthoconglornerates and pariìcon- suggested their lumping into three mapping units: a vol- glomerates supported b1' r.e.o'.o..r" sand natrix, in amalglmated beds canic member, in which facies V1 is exclusive, an arena- with pebbiv lags at their base; angular boulders of Collio sedimentar,v ceous-volcaniclastic member, enclosing all facies except rocks, up to 3O cm in length, are observed at the very base. Sample VP S3, and an upper arenaceous-pelitic member, in which 20 (0.4 m abor.e the base). facies 52 is exclusive. These mapping units, exposed all - Collio Fomation. over the study area and easily identified even under poor 20) Interbedded fìne to medium-grained sandstones and exprrsure, correspond to the whole Collio Formetion as pelites, reddish in the lor.er part, greenish to darh grey upwards, with described in the previous literature; facies 53 corre- parallel to cross- and trough-beddìng, abundant mudclasts and load sponds to the Ponteranica Conglomerate. structures. Thin intercalations of black pelites are common; sand- stones are nore abundant at the base. Svmmetrical ripples are observed Geochemistry. Major element geochemical analy- 5 m below the top. Samples VP 17 (3 m above the base), VP 18 (30 m ses on volcanic rocks where carried discs out on powder below the top), VP 19 (0.4 m below the top) (196 rn); using an automatic Philips P\í 1400 X-Ray fluorescence - Ponteranica Conglomerate. spectrometer. The results were corrected according to a 19) Grey-greenìsh orthoconglomerates, paraconglomerates wide range of international and natural geostandards; at and pebbly sandstones, roughiy bedded to nrassive, with voìcanic and least five standards were used for each selected element. subordìnately metamorphic pebbles, up to 25 cm in size. Sampìes VP Loss on ignition was determined on powders kept at 15 (75 m above the base), VP 16 at the top (185 m); 1000 'C overnight. Both WDS (kV : 15.0, nA * 300, - Collio Formation. livetime : 40+80 seconds, natural amphibole and 18) Greenish sandstones, locally containing abund,rnt volcanic rhodonite as standards) and EDS (kV : 20.0, nA : pebbles, alternating to parallel-laminated pelites and greenish tuffs The Luu,'r Permian in the Orubic AnticÌine 53 2o of ,3: c) yo (d o o oL ;-: o c) q: (g .9 (ú o = ;ó0) c =o --^q o <-: c: o a 990- È x(d0= (l >ú): o "'c c -F, =ao (ú o .:'6 o a= A U' ='rt è(6oo E ;89 cìi Y P3 : 99oq) 6 a '- a) th g o Èoo (ú 6-0 "9 oo qL* ';= () o c) ;: óXo.> !LCC tt) CPdd 99!!1tr(l c-; E o (ú \ >. /aa (ú g (ú o;o) o(Ú0 o oo(Ú .:'(ú > > (o) + _ , I, : < ! ^ ; - -; -; 9.- =TùùOùÈOLAC oLI a a a a o a a-aa a ie 6 {6 e@e@6{06é6€6 @@@e 6: : 383:E A€e9O@6O@@e oc@e &' @eeel e6O@€@eQeCA ao@e €: '6 @eecY eóC@€O6@goA aa@e o: :^ 'i-FÉ*$k @coel eACAg66A9A6 @@ae o.- 14k3ff*k @a@as e@e6O€6o6e@ 6@6e @.o a, N cr) @ c! @ Elew3w crNvcto (rî. \> \O\< q) G o E 6 '-o a g) o- - The Pradini Vallev section (A - or.ervìel.; B : redrar.r.r after A: numbers in circles refer to the described interyals, dots indicate sam- pling sìtes; C : mcesured section). VL - Verrucano Lombardo. J,+ D. Sciunnacb with inversely-graded pumiceous lapilli. $later-escape structures are ACVP4 ACVP1 ACVP3 ACVP2 common in sand-mud couplets. Samples VP 8 (25 m below the top), VP 9 at the top (86 m); sio2 58,35 55,25 1Z) Grey sandstones with black pelite veneers, commonly dis- Tio2 nA4 1,10 0,85 2,03 playing parallel to cross-lamination and containing abundant mud- At2O3 16,20 20,1 9 '1 6,40 18,02 clasts, with thin tuf{aceous intercalations. Samples VP 4 (15 m above FeOiol 5,70 6,12 10,02 the base), VP 5 (85 m above the base), VP 6 (0.3 m below the top), VP 6,49 ./ at tne toP (/uJ mr; MnO o,26 0,28 0,05 0,14 16) Greenish sandstones, frequently purple and reddish in the Mgo )24 0,55 4,44 lower part, passing to parallel-laminated pebbly sandstones with CaO eoo 3,80 0,43 7,93 scoured base or less commonly to conglomerates yielding volcanic pebbles; up to 15 cm-thick intercalations of purple tuffs also occur. Na2O o, ll 3,89 5,07 Both normally and inversely graded beds are common. Sample VP 3 (5 Kzo 4,21 3,61 0,95 m below the top) (73.5 m); Pzos o24 o,23 0,1 B 0,53 15) Reddish sandstones, pebbly sandstones, tuffs and volcanic t. L. 10,93 6.41 2,00 8,53 breccias. Sample VP 21 at the top (1,1.5 m); 14) Deep green, massive lava flows, with plagioclase microliths Tot 99,54 99,30 99,48 and sparse amygdalar car.ities filled by carbonate crystals. Basal contact on the underlying sandstones is sharp and mildly erosional; up to 0.7 Tab. t - Geochemistry of the Collio volcanìcs in the Pradini Valley m above the base, the lavas are fractured, vuggy and somehow foliated section; samples are listed in stratigraphic order. I.L. : jgni- parallel to bedding. Sample ACVP 2 (4 m above the base) (20 m); tion loss. 13) Grey sandstones, locally pebbly and coarse-grained, in beds 1 to 20 crn-thìck irnode : 6 cm) alternating to biack pelite veneers, commonly with parallel lamination. Fractured microconglomerates, 10 cm-thick. ar the r-cry base. Samples VP 13 (0.6 m above the base), VP ites), yielding relatively high Ai and low Mg content. 12 (10 m below the top) (71 m); Proximal stratigraphic position of sample ACVP 4 to 12) Red to purpie porphyries. Sample ACVP 3 frorl-r the mid- the basai cataclasites results in a stronger alteration, tes- dle parr r5 mr: tified by high I.L. values, but not in a severe disruption 1 1) Grey to brownish sandstones and tulfs rich in plagioclase of the originai texture. Similar geochemistry of the two ("saìt-and-pepper"). Sarnple VP 1'l at the top (9.5 m); sampies does not allow to recognise differentiatìon 1O) Red to purple pyroclastic breccìa. Sampìes VP 10 (2.5 m suc- above the base), VP 11 (1.5 m below the top) (6 m); trends, from base to top, within the thick voicanic 9) Red to purple porphyries. Sample ACVP1 (4.3 m belon- the cession extending from intervals 3 to 9. top) (26 m); The lower volcanic member is capped by a pyro- 8) Cliff-forming whitish to whitish-grey volcanics (40.6 m); clastic breccia (interval 10) where deeply altered and 7) \fhite to pink porphyries, with sparse tuffaceous intercala- resorpted plagioclase phenocrysts are hardly distin- tions and unfolded quartz veins (15.5 m); guished from the feisitic to aphanitic groundmass of 6) Pinkish to reddish bedded porphyries n-ith cavities fìlled bv individual clasts. *,hit. l^l^-ir. r ì vR riil\nr\. \? 5) Fine-grained, pseudonodular porphyries alternating with IJpsection, porphyries in thinner layers (ACVP 3) s.elded tuffs in up to 2 cm-thick beds (5.3 m); alternating to volcanic arenites (interval 12), can be clas- 4) Welded tuffs with folded quartz veins (6 m); sified as acidic, high-K calc-alkaline volcanics (dacites) 3) Aphyric welded tuffs ranging in colour from light grcy to with 1ow Mg content. The silica excess due to qLrarrz pinkish to greenish, in thick, poorly distinct beds, intensely folded and veins might explain the shifting of this sample away cleaved. Sample ACVP .1 at the top (5.6 m); 2) Black catacirsite including thin slivers of tectonised vol- from rhe Na-alkaline suite. canrc.. Samplc Vl'2.rt thc top (2 m): Next, interval 14 consists of a 4O m-thick 1) Tectonised volcanics, injected b;, black cataclasite veins, mugearite lava flow (ACVP 2,25.9.1). These basic vol- encasing arnygdaloid basement blocks up to 60 cm in diamete r. Sample canics, belonging to the Na-alka1ine suite, yielded high VP 1 at the base (2.2 m); Al and Ti and low Mg content; in thin section, they dis- - meramorphic b:sement. Muscovite paragneisses with quartz play a homogeneous intersertal texture, where felted rods, injected by subparallel veins of black cataclasites in the topmost plagioclase laths (250 pm on the average) are enclosed in 2 metres. an altered matrix consisting of chlorite and ferroan dolomite (Fig. 6C). In outcrop, no evidence has been found of subaqueous deposition of the andesitic lavas Petrography and geochemistry of the Collio volcanics. (breccias to pillow-structures); contact on the underly- Four major element geochemical analyses were ing sediments is sharp, mildly erosional, and underlined carried out on samples from the Pradini Valley section by flow textures and cavities. Still upsection, thin tuffa- (Tab. 1). Samples from both the base (interval 3, sample ceous beds, in which q::l^rtz, plagioclase and commonly ACVP 4) and top (interval 9, sample ACVP 1) of the spherulitic felsitic fragments are enclosed in an aphyric lower volcanic member, over 110 m-thick, can be classi- matrix, are intercalated to the prevailing sandstones and fied as intermediate, Na-alkaline volcanics (benmore- pelites (sample VP 4; intervals 15-18). The Lo'u.rcr Permìan in the Orobic Anticline 55 '',r;.r;tr.::r$ .:lr:li::: ì"]:il:ii:,.. , .a:r.iì:r::' ii."' ,r!!s ',,,,.:l'.,.-'l .,& s*F- * ".-,%" " :-s' 'qF* î {,, I : ,i., ,tit-.,, *' ''i l:l' ' ..,"':.t;,1*rll.. : rri:'1.ì.,i::1. ' I :r::. : ::': :*litr,'... Fig. 6 Ilepresentative photomìcrographs of facies described in the Pradini Valley section. A - basal cat:rclasite, n'ith a clayey matrix (C1, largc- l,v consisting of illite) encasing tectosilicate lithons wjth felsitic texture (Lt) part\. replaced by ankerite patches (Ank) srown in over- sized secondarv pores (VP2; piane polariscd light,27x); B : weathered intermediate wclded tuff (benmoreite; facies V1) displaying feldspar phenocrl'5t5 locally tr:rnsformed into chessboard-albite (ChAb) (ACVP1; cross-polarised light,27x); C : sericitised lar-a llow (mugcarìtc; facies V3) with e plagioclase lathn-ork enc:rsing large calcite-chlorite pseudonrorphs (Ps) after fen'ric phenocrysts (amphi- boles?) (ACVP2; cross-polrrised light, 27x);D : ver,v coarse-Elrained, moderateiy sorted volcanic arenitc (Ponteranica Conglomerate; facies 53) rich in volcanic lithics displaving spherulitìc textures (Sph) and containìns emba,ved quartz (B-Qz) :rs rvell as lcached biotitc (Bt) phcnocr,vsts (VP 1 5; cross-polarised light, 2Zr); ll : black shale (fecies 52) includìng imbricatecl carl:on-rich chips (VP Z; cross- polerised light,25r); F, G - bioclasts in a mediurn-grained, moderately sorted volcanic arcnìte (VP 18; plane polarised lìght); F - sn.rall g,rstropod fr,rgment, 12'{x; G : furamjnjfer fr.rsmcnt 1cl. Hemigordiur sp.; dct. bv R. Rettori),260x. 56 D. Sciunnach -: U aE iE:E=?2t.4.,3; É ÉÉÉÉÉ F o Eà- -:\{9r;=azi"- É o aóó RRRRRRF : a = i i a/. / | a È u[È ÉaaÉaÉqÉÉÉrÉss Orc :Ì;.- - 'r t: = = EdEd{== PÉ=.;{PddrdQdoo EZIÚ6.:-;Èl-aí=' -:.=- >;>>> "t P í= 9 i: : r y. í il - Y =! ui.=:_.; oooo9o6oooonoo >-É = ó=,a=.:x I r, N- N- R33&3PB Ntsqqi,6\N-N-oNNo-N i-:i=-rit=Zr= (,@ -oqlJo; g{-sí"-i;==*Ét > 3 5 ic- -e 'a a t a . @@600@o -E oltr;{:---'1-.==. o- o- o- o- o- o- o_ 333.3333E.333333 dX -=--lF-.9i.ùI3-E oooo-oo ooro E o rr É;; : -o-" ^o ::.=^".6===^I= -- NOTN ooNo@Noo -Ò -"IjFa7.=9-t-ó-; = N n-N-N-@@-@N- 66È@O@@@@OO@O@ ,2=v+'='==_;=>+ È ooooooo dooooooctocioooc; L ! .= ; - ^ - ' 4==');ó;l3t:= o oN@oNot oNúoo@o606sooo - a) u. -- N- O- O- m- -- O- O- ó- -- r- N- ó- O- O- N- O- O- O- s- N- O- O- .9r a + EE * I o oooooóo oooooooooooooo -- 1eùla=1ÉL:=-=- =2 ".& ., tr =ú:===',,=-;< @soo@No o- N- @- o- o- o- N- ó- o- o- ts- @- N J ts ooo6@òis úsooo@o-Nt-oso -- a.::i.::'^- Éi! @@Nr@N@ N@@@F@róNF6r@N .9F 5 ! *:l r i'::t+==-> = N- 6- O- or60@o@-óNooots TiiiiÉ;ò'7oY oooo@oo -9Sc.i@Noo-ajrj E o, o- o- ÈNoso-ss oooNoNoNNoó NOOO_óO J-vrooi JoGoi' in rí-sF-o)(or(o-o.r$oc') oNo+ NNNO--óóOOr (o- cD- \ cr)- o- r- c!- r- o- o_ c{ @Ot-LOO$O-c.)OO) o-oN-oÈ-È-N- ró6oo-N NNO@ON@v@N$NO- \t c! '-N-N o-È-N-N-O-N-o óNorN-oossso-È-os Lo -ss nisct@:oósctod99só cDr.-r@co0o$o)$No)o -i-i-i;-i.^;_;^i; NC' ooo O O-O aooNoNo oNNOÈONONOOóóO CtCDONCDcOIt-TCDOO o;c.io o-NiosNo-d-d-o- Cr)- r^ r- cD- t-_ \ st- rO- O- (O- O- --òi rC\t* ! È È oN-o- ONNNONO ó- r- o, ó ó o- o- ó- o- ó- ó- ó- o- o- crc9-cDCr)O)O(OCDO)OO È Nosòiooo rN@r-N-OO@ìNOV cD- CD- N- ro- sf_ o- tr)- O- O- -^ -- \ xso- OOOOOTl)cOTOc{OO È o- o- o- o- o- o- ó- N- S- N- r(\$o @ *-S O-orrNv NO@óO@NN@SóS@6 oó-N-o-N-ts-ó- ÓÈOOÓNÓÓON a rsrooN- ooN--;Jooo tsNÓOts óóooNN o -rN-O oo--,o-ooooo o- o- o- o- o- N- o- È- o- N- N- ó- O- O- O- N- r- O- O- O- O- a @@-j NO@NNO@ NNNNSSONNO-NNO I g? cf) ON-N ddR ? ? 3"*dq E iS;'cr5 atr<ù>>Òz:<-o- -o ,(Ú a s:v The Lotaer Permian in the Orobic Anticline 57 F,DS and \lDS analyses were carried out on the have occurred during burial diagenesis, through the leaching of Ca and basal catacl.isite IVP 2t. indicating the clayey compo'i- Mg ions from the volcanic detritus. The adjoining Triassic dolostones and limestones, lf ing in tectonic contact wìth the Orobic Anticlìne tion (illite) for the dark, aphanitic matrix encasing vol- along the Valtorta Line (Fig. 3), are unl:ikely to have pervasively fed canic lithons (Fig. 6A), as well as the occurrence of large with carbonate the Permian succession, that had already experienced patches of nearly stoichiometric ankerite (Tab. 2), but strong compaction and porosìty loss at depth; moreover, an Alpine age failing to reveal tourmalinisation (compare with Zhang for the precìpitation of the authigenìc ferroan dolomite u.ould not explain is carbonate- et aL.1994). why the overlying Verrucano Lombardo almost free (Sciunnach et. air.1996). Composition of the Lower Permian sandstones is Petrography of the Lower Permian sandstones. consistent with provenance of detritus from overwheim- Al1 the studied sandstone samples from the Collio ing volcanic sources, along with a subordinate input Formation (n : 19) and the Ponteranica Conglomerate from the Hercynian crystalline basement (metamorphic (" : 6) can be classified as mostly medium-grained, and granitoid rocks) and minor supply from older cla- moderately to ver)- poorly-sorted volcanic arenrtes stic units (e.g., Conglomerato Basale). Total quartz, (average detrital modes: Q : 10*5, F :2A-r7,L: gradually increasing up$/ards in the Pradini Valley sec- 70-r8;Trb.3) end c;ìn thus be described comprehensive- tion (r : A.773, sign. Iev. < 1'h), records a stepwise ly. Moderately sorted sandstones are medium- to very increase passing from interval 17 to 18. However, this coarse-grained. petrographic signal is in general too gradual to allow dis- crimination of petrologic intervals within the Los.er is mainlv monocrystalline, although pol.u-crystalline Quartz Permian sandstones, that cluster into a single Petrofacies grains are also common (C/Q : 0.12+0.13)i ernbayed pseudo-hexa- ronal crystals of volcanic origin (Fig. 6D) represent nearlv 207. of P0 (Fig. Z). Since the pioneering work of Dickinson 8r total monocrvstalline quartz. Polycrystalline gr:ins dìsplay sutured Rich (1972), perrologic intervals identified by means of boundaries, elongated and isooricntecl crystallites and common phyl- peculiar petrofacies have been successfuily employed as Iosilicate inclusions, suggesting a mostly metamorphic origìn. correlation tools in sterile clastic successions (Zuffa et Feldspars comprise prevailing plagioclase and subordinate alka- lifeldsper (P/F : 0.81+0.11). Plagioclase commonly occurs in fresh, a.1. 1995 and ref. therein). ansular to subansular erains prcsening twinning 138.0+20.0% on Despite marked lateral variability of facies and total platioclase) and zoning (10.5+7.0""), thus hinting at provenance wide grain size range of the analysed samples, composi- from neo"olcanic rocks (Zuffa 1985) exposed not far from the basin. tion of the Lower Permian sandstones is remarkably uni- Among elkalifeldspars, chessboard albite ìs n-idespread, also pointing to a volcanic source; mìcrocline is instead negligible. Pervasively serici- form in both the Orobic and the Trabuchello-Cabianca tised feldspars are found only in deeply dìagenised samplcs, where epi- Anticlines, that is, over an area extending for over 35 km matrìx (Dickinson l97O) cross-cuts the boundaries of unstable grains. west to east (Sciunnach et aI. 1999a).In the Camuna Voicanic lithics arc mostly vitric (17.0+10.0% of rock volune), but Anticline, arkosic and more quartzose compositions are also fragments of felsite (8.0+5.0%) and microlitic lava (2.5+1.5%) are common (Fig. 6D). Hypabyssal lithics derived from subvolcanic instead reported (Cassinis et a.I. 1978), suggesting rocks are also r.idespreacl (2.0+1.0% of rock volume), r.here;rs grani- greater contribution from unroofed basement rocks. toid rock fragments are scanty. In general, parametamorphic lithics prevail over gneissìc rock fragments, that are instead more abund;rnt in the coarser-grained samples (.r A.7A6, sign. ler'. < 0.1%). Sandstone - Stratigraphy of the Lower Permian in the Orobic Anti- ro .elire {rrqme.ts or'crrr 1.1g1s25 the absence of carbonate lithic grains fails to support cannibalism of carbonate crusts as an inrportant cline. process (Cassinis ct al. 1986). Thc abundrnce of pseudonorphic grains and rnostly volc;rnic pseudomatrix (Dickinson 197A; 16.5+6.5"k ol The vertical organisation of facies recognised all rock voiume) hampers assessment of the original sand compo.ition. over the study area allowed construction of a refined that may be nrisinterpreted if pseudom;rtrix is overlooked. Accessory stratigraphic framework for the Lower Permian in the minerals cornprise prevailing rrica flakes (muscovite to paragonite and Orobic Anticline (Fig. S). Each of the lithostratigraphic "leached" biotite) and subordin.rre zircon, apatìtc. rutìle, tourmaline and sphene. Intraformationll oversized mudclasts (up to 3% of rock units listed below in ascending stratigraphic order, volurre) locally display slaty cleavages ,rs an effect of post-deposition- ,ì.- D^-,-..-; ,,.ca Conglomerate, needs to be al anchimetamorphism. Sparse phosphate nodules are observed only in regarded as an informal member of local significance, 18, where rare (foraminifers and small ga- sample VP bioclasts but also reflects a distinct stage of volcanic ectivity vs. stropods) also occur. Epìmatrix, ìncluding celadonite, is abundant (20.5+13.0% of rock volurne) and so intensely recrystellised that sedimentation. phyllosilicate flakes commonly cross-cut grain boundaries. Early dìa- genetic quartzose to feJdsprthic cenrent is minor (2.5ol. of rock vo- 1. Volcanic member of the Collio Formation. Lume on the average), consistent with poor sand sorting (Winn et al. 1984): rnaximum abundance is recorded in moderatell-sorted sand- 1a. In the lower part, observed in all sections, Poor preservation of primary pores under rapid burial and se- stones. welded tuffs and lapillistones (whitish in colour in the diment compaction is also indicated. Quartz veins and ferroan dolomite patches are widespread (B.A+n.5'/,' of rock volume); pre- lower part, reddish at the top probably due to oxidation cipitation of these authigenic carbonates in secondary pores should in continental setting) are associated to porphyric ign- 58 D. Sciunnach P2 (n=14) Verrucano Lombardo PROVENANCE CATEGORIES Fig.7 - QrFI di.rgrrm rDickin'on i985) for the Lower Permian P1 (n= 4) Verrucano Lombardo llXj continentalblock sandstones of the Orobic P0 (n=25) Collio Formation, Magratic arc Anticline (Petrofacies PO) Pnntaranina f-l vr/r compared to the Carbonifer- Conglomerate W.tH aecycled orogen ous-lowermost Permian? Conglomerato Basale CflP (n=S) Conglomerato Basale (Petrofacies CflP) and to the largely Upper Permian Verrucano Lombardo sand- stones (Petrofacies P1, P2). Modified aiter Sciunnach et al. (1999a). Poìygons are one standàrd deviation cach side o{ the mean; n : number of sampìes. F L imbrites, dark bedded tuffs and graded pyroclastic section, although a regional unconformity is suggested flows. Massive aphyric volcanics with a dark-green in map view. Volcanism occurred cpisodically and alter- glassy paste, in bodies of poorly-defined geometry, nated to prolonged stages of volcanic quiescence during locally occur. Accretionary lapilli (Moore & Peck 1962) which clastic sedimentation took place. This member were found in cobbles from the Collio volcanics in the can be further subdividcd, easily in continuous sectlons Biandino Valley (Fig. aD; Confalonieri 1993). These but hardly where outcrops are spJrse. in two parts: constituents are particularly common - although not 2a. Prevailing fine-grained, thin-bedded sand- exclusive - in pyroclastic flow deposits and are reliable stones with subordinate siltstones and rare conglomer- indicators of subaerial erllption (Cas & Wright 1987) . ates: all rock types are typically dark grey in colour. This 1b. In the upper part, observed only under lower part corresponds to intervals 11-13 of the Pradini favourable outcrop conditions and seemingly discontin- Valley section (thickness : 85.5 m), to interr-als 9-13 ol uous, intraformational volcanic breccias, consisting for the Zatto vrlley section (thickness : 76.5 m) and nearlv 100% of volcanic clasts, display highly vari.rble includes facies S1, 52. thickness. This mapping unit corresponds to intervals 1- 2b. Prerailing fine-grained, thin-bedded grel 1O of the Pradini Valley section (thickness : 113 m; 1a sandstones alternate with greenish tuffs, epiclastics, lava : 707 m, 1b : 6 m), to intervals 1-8 of the ZattoYalley flows, ignimbrites end black siltstones (Fig. 6F,); section (thickness : 128 rn;1a:81.5 m, 1f :96.5 m) inversely-graded pumiceous pvroclastic flows are pxrti- and invariably displays a V1 facies; thickness possibly cularly well preserved on the southern slope of thePizzo exceeds 25a m in the Ornica area. dei Tre Signori. This lithozone corresponds to intervals ' The basal volcanic succession (1a) was probably 14-18 of the Pradini Valley section (thickness : 397 m), deposited at very high accumulation rates during a stage to intervals 14-2a of theZatto Valley section (thickness of paroxysmal volcanic activity possibly lasting just a :295.2 m) and includes facies Vi, V2, V3, S1 and 52. few thousand years; the topmost voicaniclastic breccias A fossiliferous horizon was found on the Averara- (1b) are likely to have been accumulated during cata- Valmoresca road cut (689.6 m a.s.l.). Deformed compo- strophic intracalderic collapses. A very short timc sprn site moulds of smooth-shelled bivalves (Fig. l) were col- for the whole member would be consistent with both lected from a grey to brownish, fine-grained rnicaceous the homogeneous geochemistry of the welded tuffs sandstone overlain by a thin interval of welded tuff; a from base to top and the inconspicuous non-volcanic tectonic contact with the Verrucano Lombardo shortly clasts in the overlying breccias. follows upwards. This member was deposited as a whole in a rapid- ly subsiding, newly-formed sedimentary basin. Occur- 2. Arenaceous-volcaniclastic member of the Collio Formation. rence of both massive, structureless deposits of granular Prevailing coarse-grained arenites with minor sediment and plane bed, horizontally-laminated pelites, alternating to epiclastics, commonly welded deposits, documents either subaerial (sheetfloods) or tuffs, and basic lava flows. Basal contact on the underly- subaqueous (slurry rnass flows) fan-delta processes. ing member is paraconformable in the Pradini Valley Dacitic volcanics in the lower part seem to suggest The Lozaer Permian in tbe Orobic Anticline 59 -!> j 9! f,: !-o .4 : :. S N óÉe oo33393"=ó s. VERRUCANO $ LOMBARDO 3 d @.cPce@,.,:- .-aAgq, ,&"&e. s*eg# ARENACEOUS.PELITIC g @ @ s w s s @ a @ s @ a @ |aatl t.^^j"","",,ffi**l*ale @ a @ @ @ o mY:\fitrffiff|rlfl@ a @ :=- € s o basalbasar cataclasilcatacrasite r | , .lK-rì\-F-:r< -'.?^Al *Éli3ll\ t l'-*?gl F l{{{l a inf erred Permia \ normal,iltlii,o,l,t,i''"' fauli \HlètH F?31 ':i:; l$$rl iAAal tJNSI t ;iÉ':"1%%%1..", l;*fl db L H, 1.,""..1 l,*u.*"| r ", ,e,rapodroo,prn, 'N[.tNr,'ir:l{atl = -tr r0 Ò: ì l@asl leoallaaal È l-^^ll$$$l , \ roraminirers \ l;"$! li??l J*'.gr, lrrrl t t \ \., l%??lo e 1.,%*,1 l.:,:.:l sl- bivalves lo I _qlp-..--*i#F I l..f..l . sampres rrom spa t'l-.1..Él fe&el j-*:J r,{,i,\. |mugearile, "andesite" sampresrrommeasuredsect'|ons--"':::j:"::::"" t 1;-@--ll -- l lliiJl ' "\ F.îJ ** :*" : I --- ' l;i;;;i; I lsandstone,siltstone r,ffi l l* tl -îÀÈ g lt werded rurr, isnimbrire la"a"'l :il:[,1""';J;ytt METAMoRPHTc N!M-' coarse-gralneo r-vM paragnerss' mrcascnlst^^^hi^_ f;t;l ij(ix\b] l.;;- I sandstone, epiclastite BASEMENT Fig. 8 - Strrtigraohic framenork for the Collìo Formation in the Orobic Anticline, as obtained from correlation of measured sectjons (the Pra- dini Valley section near Valtorta and the Monte Cavallo-Zatto Valleys near Mezzoldo) and 8 stratimetric logs. Localisation of the major inferred Permian normal faults, as well as thickness of the volcanic and sedimentary basin fill, tend to indicate the Ornica area as a depocentre. Box displaying SiOl content refers to samples ACVPl-1. magma differentiation with respect to the underlying morphic rock fragments (10+15%) and milky quartz volcanic member, although basic lava flows, up to 2O m- (5+ 10%). Mostly poorly-sorted, pebbly to cobbly con- thick'and found 92 m above the base of the member in glomerates alternate to coarse-grained sandstones in the Pradini Valley section, shortly mark an inversion of thick, poorly-defined beds. A proximal fan-delta envi- the trend. Similar small "andesite" flows are reported ronment is indicated. Colour, ranging from purple to from the lower part of the "Sedimentary Member" of the pinkish to deep green due to variable oxidising condi- Collio Formation also in the Trabuchello-Cabianca tions, is invariably purple west of the Camisolo ridge, Anticline (Cadei et aL. 1987). The arenaceous-volcani- suggesting emergence of the fan-delta, that was instead clastic member is likely to have been deposited at va- in part or episodically submerged to the east. Three- riable accumulation rates (higher under episodic vol- dimensionally and exceptionally well-preser-ved plant canic input, lower when clastic rocks were deposited remains are found north of the study area, although at during stages of volcanic quiescence). present they do not provide biostratigraphic informa- tion (Kerp et al. 1.996). This mapping unit corresponds to interval 19 of the Pradini Valley section (thickness : 3. Ponteranica Conglomerate (Casati & Gnaccolini 1965; 185 m) and to facies 53. 1 967). Composition of coarse-grained sandstones inter- Sandstones and pelites of the "Sedimentary Mem- calated to the conglomeratic red beds of the Acquaduro ber" pass laterally to conglomerates of similar composi- Valley, south-east of Introbio (Fig. 3), is consistent with tion: volcanic clasts (75+80%) prevail in fact over meta- the Lower Permian petrofacies PO (Sciunnach et al. 60 D. Sciunnacb 1999a; Sciunnach 2001). These red beds, displirying lower textural maturity and mineralogical stability than the Verrucano Lombardo, are thus correlated with the Ponteranica Conglomerate, and are restored to the west- ern edge of a clastic wedge that pinches out towards a structural high (Sciunnach 2001). They ,'rre in turn sealed by a continuous blanket of Verrucano Lombardo red beds, not always obviously discriminated: criterir origi- nally proposed in Casati & Gnaccolini (1965) havc been integrated with further observations in Tab. 4. These previously Llnrecognised outcrops of Pon- ter;rnica Conglomerate correspond to the cobble con- glomerates displaying crystalline components and a matrix "classified as sr,rbangular coarse s:rndstone or fine conglomerate" described by Schónborn (1992) in the Introbio area. There, the Ponteranica Conglonlerate overlies intraformational breccias "consisting complete- ly of volcanics" (Schónborn 1992) ascribcd herein to the volcanic member of the Collio Formation (1b), as are the underlying bedded cinerites, up to about 2O m-thrck in the Acquaduro Valley (Fig. 8) and mentioned in Gae- tani ct al. (1987) as a sedimentary facies of the Collio Formation. As a who1e, the Ponteranica Conglomerate records a stage of fan progradation at the end of volcanic activity in the Orobic Anticline area. 4. Upper arenaceous-pelitic member of the Collio Forma- tion. Prevailing grey to greenish sandstones, largely fig. 9 - Deformcd composite moulcls of smooth-shelled bir.alves, medium to fine-grained and rich in sedimcntary struc- ,\'erar:r. lv{oulds A, C seen comparable to the scnus Anthraconauta (Paproth 1966), r.hereas ne;.Lrl1- equil.rtenl tures, are interpreted as deposited in perennial lake moulds B, D, E resemble the undeteririned fresh-r.ater basins (Cassinis et al. 1986; Cadel et al. 1996). Tetrapod bivirlves pictured bv Berruti (1962). Scele bar in cm. footprints were obserr.cd in the Scioc Valley (Ornica), fror.r.r wl.rere they had been reported also in Casati Ec Gnaccolini (1967:- plate 8, fies. 1,2), a few metres below Discussion. the unconformable contact with the Verrucano Lombar- do.'Rccentl;r, tetrapod footprints from the same stratr- Magmatic evolution. graphic level have been found in the Inferno Lake sur- Geochemic:rl analyses of the Collio r-olcanics in roundings, about 2 km north of the study area (Santi & the study :rrea indicate a benmoreitic composition for Krieger 2000). Rare for;rr.ninifers with calcareous tests the lower pyroclastic flow deposits, er-olving to a more l:If ,-^-,:,.. L 1..:.: -) (Miliolacea) and sn-rall gastropod fragments are reported rrLrJrLu \uJllrrl/ conruoririon ip rh,. lo*errnost here for the first time from the upper part of thc Pradi- :ìrenaceous-volcaniclastic member. These mrqmrtìc ni Val1e1'section (Fig. 6F, G). This mapping unrt corre- products are however abruptly followed upsection by sponds to interval 20 of tl-re Pradini Vallcl- section mugearite lava flows and rhyolite tuffs. In the Tra- (thickness : 196 m), to inten'a1 21 of the Zatto Valle,v buchello-Cabianca and Camuna Anticlines, prevailing (thickness : 246 n't) and to facies 52; it is heteropic, at rhyolites and subordinete "andesites" rrc reported (Pey- lcrst ìn prrt, s'ith the Ponteranica Conglomerate and ronel Pagliani 1965; Cadel 1986), although intermcdiate appcars to har.e been deposited at lower sedimentrtion alkaline volcanics also occur (Cortesogno et al. 1998). ratcs ît thc cnd oi the volc.rnic,rcrivity. Clrstic input In general, Earlv Permian magmatism in the South- seeminglv outp:1ced subsidencc, as suggested b1' pro- ern Alps displays a prevalent calc-alkaiine signrture, ,rressivet]'."'''''''.'.''''b.-..'' infillini' of the lake basin and reduction of the althoueh a subduction context can be ruled out bascd on water body documented by footprints and mud-cracks the regional tectonic scenario. Partial me lting is likely to towards the top. have occurrcd at the mantle/crust transition (Corteso- The Lozaer Permiart in the Orobic Anticline PONTERANICA VERRUCANO ern Alps (Cassinis et a1. 1988). A mantle plume source CONGLOI\,4EMTE LOI\,4BARDO finds no actualistic analogue because magmatism is scat- volcan c - 75./., quatlz 40+707o, tered over an exceedingly wide area; however. it might composition metamorphic = 15oÀ, volcanic 30+60%, quattz = 10Yó metamorphìc < 1% account at least in part for the rapid uplift of the Her- cynian range and widespread volcanism shortly pre-dat- E average 1 0+20 cm, average 4+1 0 gra n size cm, E maximum 60 cm maximum 20 cm ing normal faulting. The cauldron subsidence model is 9 clearly insufficient to account for the geometry of vol- C'z sortrng poor to very poor moderate to poor o canic bodies at basin scale, although it can be successful ro u nd ness angular to subrounded subangular to rounded to justify the geometric relationships between volcanics violet to pink re (g and epiclastics ar a more local scale (Cadel 1986; Cadel colouf to wine d reenish on reduction) et a|. 1996). detrital The magmatic evolution recorded by the Collio mooes 410+20 F1 0+20 170+80 Q35+68 F5+15 L20+60 Formation in the Orobic Anticline can be framed in a composite vs two-step regional trend of volcrnic rcrivity: 0.00 + 0.32 0.38 + 0.50 1. paroxysmal eruption of lithospheric mantle 2 o )lagioclase vs magmas enriched in Al and alkalis by crustal contamina- F tot. feldspar 0.55 + 0.90 0.25 + 0.30 o z tion, finding their way to the surface through "leaky" a sorting mooerale Ìo very poof moo e rate wrench faults; rou ndness angular to subanOUlaf subangular to subrounded 2. isothe rm upwelling, enhanced by stretching and unloading of the upper crust, causing the ascent of deep- wine red (greenish colour grey ro oeep green on reduction) er-seated, less differentiated magmas; emplacement of basic lava flows, coupled with ongoing eruption of re sid- PELITE occurrence negl gible cycircal ual acidic magmas, seemingly determined a late stage of 'ri lt-like" bimod.ri volc.rnism. Tab. 4 - Distinctive facics characters of the Ponteranica Conslomer- The hypothesis according to which the Valsassina ate vs. Verrucano Lornbardo. intrusions would represent the magma chambers of the Collio volcanics (Merla 1933; Cadel 1986), although rea- gno et al. 1998). Self-lìmitìng magmar;c sroping of the sonable, is not supported by outcrop evidence. A review continental crust, resulting in both mixing and mingling of the geochemical data available in the literature on the of different magmas, as well as in their high Al content, Val Biandino plutonic cornplex (De Capitani 1982; De occurred during the ascent of magmas stored at different Capitani Ec Liborio 1988) and the Collio volcanics lithospheric levels; their local underplating - consisrenr (Origoni Giobbi et al. 1981; Cadel 1986; Cortesogno et with the lorv Mg content in the studied sampies - caused al. 1998) fails to supporr derivation of the two magmar- a variable degree of differentiation even among plutons ic bodies from a common parent magma. Substantial and volcanic centres close either in space or time. lack of rhyolites in the volcanic member of the Orobic A system of "leaky" wrench faults, cutting through Anticline is a first line of evidence against physical con- the whole lithosphere and marking a transcurrent plare tinuity of the Collio basal volcanics; rhyolites are in fact boundary, is the most likely tectonic setting for magma- the dominant volcanic rock type in the Trabuchello- tism (Massari 1988; Rottura er r1. 1992). Both the distri- Cabianca (Cadel 1986) and Camuna Anticlines (Pey- bution of the volcanic districts and the geometry of sin- ronel Paglian;1965). gle plutons, in which subparaliel stocks and dyke swarms prevail over .:ntral necks (e.g. Val Biandino), are Sedimentary cycles. consistent with a fissure -guided rather than centr.rl mag- matism. Linear cauldron boundaries, striking approxi- The Upper Paleozoic succession of the Southern mately 60'N, have been reported from the Novazza-Val Alps can be subdivided into three sedimentary cycles, Vedello district (Cadel 1986 and ref. therein). corresponding to distinct lithostratigraphic units bound- Other tectono-magmatic models are less viable. ed - either at the base or top - by major unconformities Rising and partial melting of the asthenosphere related (Conglomerato Basale, Collio Formation and Verrucano to an aborted rifting does not match the close time rela- Lombardo) as weil as documenting different palaeotec- tionships between regional frulting rnd volcanism tonic and palaeogeographic scenarios (Sciunnach et al. (Favre Er Stampfli 1992; Breitkreutz & Kennedy 1999), 1999a). The subdivision into just two cycles (Conglom- as well as the wide range of aluminous magmatic prod- erato Basale + Co11io Formation vs. Verrucano Lombar- ucts deriving from the lithospheric mantle, although it do: Cassinis et al. 1988) seems insufficient to explain the might explain the "Texan long-horn" cross-sectional pat- strong differences between the Conglomerato Basale tern characterisine the Permian basin fills in the South- phyllarenitic subarkoses and quartz conglomerates, 62 D. Sciunnach deposited in a complex alluvial system including taìus ening suggested by Schònborn (1992). Under this cone to floodplain facies and fed by crystalline sources, respect, the volume (150 km3) and surface (600 km2) and the Collio Formation volcaniclastics, deposited in estimates for the lower Collio Formation volcanic confined lacustrine basins under active tectonic control plateau reported by Cadel (1986) are likely to represent and fed by the newly-emplaced volcanic succession. minimum figures. Thickness of the overlying intraformational brec- cias (1b), capping at various places the volcanic facies Basal cataclasite. (1a), strongly varies (6 to nearly 1OO m between the Pra- Major interpretation problems are posed by the dini Valley and Zatto Vailey sections) as an effect of belt, locally over four metres-thick, of black cataclasites deposition in small fault-controlled basins. These brec- marking the tectonic boundary between the crystalline cias can be interpreted as coeval with both the end of the basement and the Collio volcanics. Simiiar fault contacts early stage of paroxysmal volcanism, and the major are not observed anywhere at the basement/Conglomer- phase of widespread normal faulting of the Lower Per- ato Basale, Conglomerato Basale/Collio volcanics or mian volcanic plateau, which created the endorheic basement/Verrucano Lombardo contacts. To date no basins accommodating the subaqueous and much finer- conclusive evidence has been produced to obtain an esti- grained sediments of the arenaceous-volcaniclastic mate of the offset along this cataclastic surface, which member. At that time, the voicanic caps o{ the uplifted has long been disputed (Dozy 1935; Casati 1968). An footwalls were intensely eroded. Alpine displacement, although more or less implicitly The thickest basin fill, set in a rough palaeomor- admitted by Vennekers (1932), Casati Ec Gnaccolini phology controlled by block-faulting, corresponds to (1967) and Casati (1968), is at odds with the strati- the arenaceous-voicaniclastic member. Sharp superposi- graphic relationships observed between Introbio and tion on facies 1b of fine-grained sandstones and Primaluna (Valsassina), where the cataclastic belt is mudrocks (2a), yielding bivalve remains near Averara, unconformably sealed by the Verrucano Lombardo seemingly points to momentarily reduced clastic input (Figs. 3, 8) and does not continue into major faults and/or to the spreading of shallow basin floors. Later around the basement/Verrucano Lombardo boundary. (2b), a tectonic subsidence exceeding 100 m/My was Tourmalinisation of the basal cataclasites in the Tra- matched by deposition of a thick pile of coarser-grained, bucheÌlo-Cabianca Anticline has been explained in terms subaerial to subaqueous inass flow volcaniclastics. of penecontemporaneous metasomatism of the Collio Ongoing volcanism is suggested by a bimodal, "rift-like" sediments (Zhang et al. 7994). Tourmaiine was also association fostered by lithospheric thinning through found in fault rocks pre-dating the Verrucano Lombardo the active wrench faults, also causing extensive erosion in Valsassina (Sciunnach 2001). In the Novazza-Val of the hanging-wall source areas. Contrasting sandstone Vedello district, fault zones sealed by the Verrucano composition in the Orobic and Camuna Anticlines is a Lombardo were mineralised during the Late Permian second line of evidence against physical continuity of (Origoni Giobbi et al. 1981; Cadel 1986), and grains of the Collio Basin, although it may be accounted for also tourmaline-bearing cataclasite occur in the very first Ver- by deeper dissection of the eastern basement highs rucano Lombardo beds in the Pizzo dei Tre Signorr area. and/or lesser contribution from the volcanic plateaux All these lines of evidence constrain the age of cat- exposed there. aclastic deformation as Artinskian (age of the younger The Ponteranica Conglomerate) as well as the deformed rocks: see the chronostratigraphy paragraph coeval coarse-grained clastics from other localities of the below) to Middle?-Upper Permian (age of the older Orobic Alps, marks a stage of progradation of imma- undeformed rocks), and suggest that faulting was asso- ture, cobbly facies, deposited in fan-delta setting at least ciated with a major phase of metasomatic mineralization partly below lake 1eve1. Vhether this event was triggered of the host rocks. by climatic trends (increased rainfall) or renewed tec- tonic activity (relative uplift of source areas) can be only tentatively inferred. Preservation of fresh feldspar in the Basin history. interbedded sandstones is at odds with a humid climate, The lower volcanic member, regionally forming a whereas the invariably posìtive - although very low - morphologic plateau (Cadel 1986; Cadel et a\. 1996), correlation coefficients of polycrystalline quartz and extended well outside the boundaries of the tectonic alkalifeldspar, as well as of granitoid and orthogneissic troughs accommodating the Coilio sediments. It is near- rock fragments, vs. stratigraphy (r : 0.081+0.170, sign. ly aOO m-thick in the Monza 1 AGIP weli (Casati & lev. > 10%) hint at unroofing of basement rocks. The Poliani 1986), located today about 50 km south of the second line of evidence, however, is not unequivocal Collio Basin. This distance can be restored to a total 130 because all the mentioned grain types - with the possible km to the south according to the values of Alpine short- exception of alkalifeldspar - are originally coarse-grained The Louer Permian in tbe Orobic Anticline 63 Fig. i0 - Compilation of the available geochronological and mag- netostratigraphic data for the Permian of the Orobic to LOPINGIAN Camuna Anticlines. Tìme scale after Mennìng (1995); TATARIAN petrologic intervals (Dickin- son & Rich 1972) as in Fig. Z 263 are displayed ìnstead of lllawarra ,'.''.-_"*.'b'*r,','lirhosr r:r'or:'rhic unìrs. LVM ITIIII II : lower volcanic member of 265 Reve rsal 267 (Menning, 1 995) the Collio Formation; AV : Auccìa Volcanìcs. 270.5 273.5 277 _2nn 6 (Schaltegger & Brack. 1 999) _ cea (Schaltegger & | -"" Brack,1999) I L 297 (Cader, 1986; Hunziker in Cassinis et al.,1988) ASSELIAN and their abundance is positively correlated with grain basin. Actually, a marine transgression in the Collio size; correlation coefficients range from 0.089 (poly- Basin would explain both reduced clastic input and finer crystalline quartz; sign. lev. grain size of the clastic sediments, as well as the remark- (orthogneissic rock fragments; sign. lev. < 0.1%). Vo1- able input of Ca and SO4 ions, deposited as evaporites in canic activity seemingly ceased at this stage, as suggest- sabkha settings. The occurrence of benthic foraminifers ed by the lack of volcanic intercalations. is the most important line of evidence supporting the During deposition of the finer-grained uppcr are- marine transgression model, which, on the other hand, is naceous-pelitic member, subsidence was more wide- not indicated by any significant change in sandstone spread and reduced, as documented by fairly consrant modes and is poorly documented by sedimentary struc- stratigraphic thickness; this is consistent with ceasing tures (symmetrical ripple-marks may occur in lacustrine normal fault activity. Occurrence of tetrapod footprints, as well as in shallow-marine settings). The marrne trans- as well as of shallow-water sedimentary structures (wave gression model, if correct, would imply that 1) the Col- ripples, clay chips) in the upper part of the member sug- lio basin was not only a hypersaline, but also a coastal gests progressive infilling of the lake basin and reduction lake; it was eventually reached by an at least temporary of the *ater body even under reduced clasric input. seaway as a result of either a pronounced eustatic rise or Preservation of abundant fresh detrital feldspars is con- rapid subsidence/erosion of its damming ridge, and 2) sistent with both short transport and arid climate. Arid altitude of the late Collio lake level cannot have exceed- climates would also explain the occurrence of ferroan ed the amqlitude of a first-order sea-level rise, that is, | carbonate layers, crusts and nodules, interpreted as aDout luu' ^^ m. pseudomorphs after evaporites, in the upper "Sedimen- The hiatus at the top of the Collio Formation may tary Member" of the Trabuchello-Cabianca and Cedego- be as long as 1O My (see below). This angular unconfor- lo Anticlines (Cassinis et al. 1986) . There, conspicuous mity has been traditionally interpreted as documenting footprints are found approxin-rately at the same srrati- the effects of active tectonics in terms of strike-slip graphic level as the ferroan carbonate and Ca-sulphate faulting, intraplate wrenching, and inversion of the Late lenses (Forcella et al. 2001). Carboniferous-Early Permian basins (Cadel et al. 1,996). An alternative explanation might be the encroach- Critical revision of the apparent tilting directions of the ment of coastal environments onto the former lake Collio Formation with respect to the overlying Verru- D. Sciunnach cano Lombardo might suggest that folding. and not Lopingian in recent works (e.g., Cassinis 8c Neri 1999), simple tilting, occurred at this stage (Cadel et aI. 1996). seemingly contrasts with: 1. reported occurrence of the Illawarra reversal within the roughly coeval Val Gardena sandstones of the Perinian chronostratigraphy in the Orobic Alps: a review. eastern Southern Alps (Dachroth 1,976: Mauritsch & Becke 1983; Dachroth 1988). This major palaeomagnet- A largely Early Permian (Autunian) age for the ic event occurred during the early Taterian (Khramov Collio sediments is indicated by rnacrofloral remains 1963; Gialanella et al. 1997), the base of which has been (Cassinis 1966; Casati 8r Gnaccoliní,l967; Remy Ec extended to ages as oid as 267 My and - accordingly - Remy 1928). U/Pb zircon ages from the lower volcanics around the Vbrdian/Capitanian boundary in the new range from 280+287 (Cadel 1986) to 283'r1l My (Schal- Permian time scales (Menning 1995; Menning Ee Jin tegger & Brack 1999). Rb/Sr and K/Ar ages from the 1998; Glenister et aL.7999). basal volcanics are 287 My (J.K. Hunziker, unpublished 2. occurrence of a hiatus at the boundary with the data in Cassinis et al. tlss). Palynomorphs from the Lower Triassic Servino Formation, documented by Collio sediments are ascribed to the Artinskian-Roadian another, abrupt increase in mineralogical stability and (Cassinis & Doubinger 1991). U/Pb zircon ages of textural maturity from Petrofacies P2 to Petrofacies S1 280.5-f l My were obtained from the overll.ing Auccia (Sciunnach et al. 1996), as well as by progressive onlap volcanics (Schaltegger & Brack 1999). All of these age of tidal flat facies onto the Verrucano Lombardo conti- constraints fit in the time scale of Menning (1995), plac- nental floodpiain (Sciunnach et al. 1999b). Such a hiatus ing the Sakmarian/Artinskian boundary at 283 My (Fig. is likely to be confined almost entirely to the uppermost 10) . If so, the whole Collio Formation should have been as the Praso deposited in the latest Sakmarian (Tastubian) to mid- Permian, suggested by facies correlation of Tesero Artinskian, during a time span possibly as short as 5 My oolitic limestone of eastern Lombardy with the and by or less. Slightly older ages (Cadel 1986; Cassinis et al. Horizon of the Dolomites (Cassinis et al. 1993), 1988) might partly reflect a polyphase cooling history of the occurrence ol Claraia bivalves of latest Griesbachi- the lrte hercynian magmas, resulting in inherited ztcon an-early Dienerian age in the Ca' San Marco Member of cores. the Servino Formation (Posenato et al. 1996; Sciunnach Very high sedimentation rates for the Collio cla- et al. 1999b). stics are consistent n'ith the single petrofacies characte- A further line of evidence - although less conclu- rising r complete sandsrone section. commonly exceed- sive - is the occurrence of an intraformational hiatus ing 800 m in thickness, independent of facies lariations within the Verrucano Lombardo, documented by the and grain size. Each petrofacies reflects in fact a peculiar, sharp increase in mineralogical stability from Petrofacies temporary balance of interplaying factors such as source P1 to Petrofacies P2 (Sciunnach et al.1996). rocks, tectonic context, climate, sedimentary processes The tectonic event documented by the angular and drainage patterns (Johnsson 1993). Such an equilib- unconformity between the Collio Formation and the rium is unlikely to last for more than some My, because Verrucano Lombardo has long been considered coeval significant variations of just one of the factors men- with the Saalian "phase". Actually, not only regional cor- tioned above (e.g., unroofing of basement rocks; block- relation of tectonic phases is a questionable stillean con- faulting; increasing rainfall; relative sea-level rise; river cept (Sengór 1990), but in this case poses also timing cdptures) may cause conspicuous and systemàtic problems. The Saalian "phase" wrs in fact originally changes in sand mineralogy. Concurrent variations of ascribed to the hiatus between the lower and upper ts/o or more factors are unlikely to compensate, and will Rotiiegendes. Such a hiatus, :rccording to recent Perm- rather result in a new temporary equilibrium. ian tirne scales, would fall around the Asselian-Sakmari- In the Upper Permian to Carnian succession of an, probably before the Collio Formation started to be the Southern Alps, the estimated lifetime of each petro- cleposited. The hiatus between the Coilio Formation and facies is about 4-t1 My on the average. Two petrofacies the Verrucano Lombardo (Kungurian-Wordian) would are recognised in fact in the Upper Permian, three in the rather correlate with a major unconformity within the Lower Triassic, three in the Anisian (Sciunnach et al. upper Rotliegendes, recognised between the Mùritz and 1996) and three to five in the Carnian (Garzanti et al. Havel-Elbe subgroups and spanning a time interval of 8 2001). A similar duration for petrologic inter-val P0 to 20 My (Ufirnian to Kazanian after Menning 1995; would match the time constraints indicated in Schal- Kungurian to Kazanian after Pokorski \997; Kiersnows- tegger & Brack (1999). kr 1997). This event surely coincides in time with the A major unanswered question is the duration of end of the Early Permian volcanism (thin volcanic ìnter- the hiatus marking the Collio Formation/Verrucano calations in the lower Verrucano Lombardo are reported Lombardo boundary. The supposed latest Permian rge only by Cadel 1986) and with a major shift in the subsi- of the Verrucano Lombardo, tentatively confined to the dence patterns, from localised to regional. Moreover, it The Lo-o,er Permian in the Orobic Anticline 65 might correspond to a stage of widespread fault-bound position between the Orobic and Camuna Anticlines mineralization of the Lower Permian rocks. (Cassinis et al. 1978) is another line of evidence against physical continuity of the Collio Basin. 4. Facies architecture, geochemistry of the mrg- Conclusions. matic products and sedimentary evolution fit into a palaeogeodynamic scenario of continental wrenching 1. Field mapping at the 1:10 000 scale of the Lower (Arthaud Ee Matte 1977). By Sakmarian-Artinskian Permian in the Orobic Anticline allowed recognition of times the Flercynian orogen was being disrupted by systematically superposed facies, better defined subdir.i- transtensional faults bounding strongly subsiding conti- sion of the stratigraphic succession with respect to the nental basins. The major faults were "leaky" (Massari available framework, and reconstruction of the basin 1988) and drove to the surface calc-alkaline magmas architecture, including inferred location of the depocen- produced by partial melting of a lithospheric mantle tre and of the major master faults. Easily-recognised metasomat;sed by the Hercynian subduction (Corte- mapping units also correspond to distinct, subsequent sogno er al. 1998). Stretching and unloading of the con- stages of volcanic activity vs. tectonic subsidence and tinental crust in wrench settings also favoured a later, sedimentation patterns. limited upwelling of less differentiated magmrtic prod- 2. Geochemistry of the volcanic rocks, prevailing ucts. in the lower part of the Collio Formation, is highly vari- 5. An entirely continental setting for the upper able along section. Early paroxysmal effusion of inter- Collio Formation in the Orobic Anticline can be que- mediate welded tuffs (benmoreites) was followed by a rtioned in the light Ulr 'r- Lllq Ul)!Uvql)r: - " Ul 'r calcrreout synsedimentaryr, polymodal volcanic activity charac- ',b,'L foraminifers (Miliolacea) 3O m below the Collio/Verru- terised by both differentiated (dacites to rhyolites) and cano unconformity, in beds deposited under reducinq primitive products (mugearite flows). Such an evolution conditions and displaying symmetrical war-e ripples. seemingly contrasts with features of the Early Permian Foraminifers adapted normal salinity actualiy fit in volcanism in the Trabuchello-Cabianca and Camuna to the salt lake model (Cassinis et al. 1986), br.rt rhey do Anticlines, where "andesites" are found at the base of the require least seaway. marine trans- volcanic plateau, and rhyodacites to rhyolites prevail by an at temporary A is supported by other sedi- far over other volcanic rock types. Seemingly, the vol- gressìon et this stage poorly canic centres feeding the western edge of the Collio rnentary evidence and is at odds with all the palaeogeo- although Basin were thus distinct and/or non-coeval with those in graphic reconstructions proposed to date, the central-eastern sector; occurrence of rare clasts of coeval fusulinid limestones are found in the surrounding intermediate volcanics in the lowermost rhyelils5 sf th. areas of the Carnic Alps (Trogkofel Group), the north- Novazza-Val Vedello district (G. Cadel, pers. comm. ern Adriatic offshore (Amanda lbis well: Sartorio & 2001 \ m:v sltql'esr rh:t the volcanic member of the Oro- Venturini 1988), and Tirscany (Kalher & Kalher 1969; bic Anticline (intervals 1-9 of the Pradini Valley section) Engelbrecht et al. tlSs). Perhaps the present-day Salton was older. Sea (an intramontane salt lake, confined to r tectonic 3. Petrography of sandstones, fairly uniform both depression controlled by the San Andreas strike-slip vertically and laterally, invariably falls in the "r'olcanic fault svstem, and separated from the California Gulf by arc" field of Dickinson (1985). This does not imply, of the Coiorado River delta) might represent a good actu- course, that the Collio clastics were derived strictll. alistic analogue for the late Collio Basin. from a volcanic arc, but surely from an overwhelming neovolcanic source (ZuÍfa 1985), represented by the Acknoledgenents. Lower Permian volcanic plateaux. Homogeneous pe- Mucb of the field u'ork was carriecl out nith G.B. Silctto, A. trography of a clastic succession approaching lOOO m in Pjccin and students (U. Acerboni, D. trttilani, M. Milesi, M. Maggionì, variability sandstone thickness, independent of strong in T. Pozz,a, M. Ricottj, ìv1. Rota). The author is deeply indebted to M.P facies :rnd grain size, contrasts with the number of Confalonieri for hclp durins some field trips, to A. Grcgnanin for pro- petrofacies recognised in the overlying and thinner clas- viding and dìscussing the geochemical d:ta, and to F. Massari and M. tic units of Late Permian to Carnian age, and suggests Menning for providìng rare bibliograph,v. Useful discussions with C. Breitkreutz, G. Cassinis, F. Forcella, G. Gosso, R. Rettori and L.A. high sedirnentation rates for the Collio Formation, in Ronchi are also acknowledged. Technical assist:rnce was provided by agreement with the short time span recently assigned to D. Biondelli (VDS microanaiysis), R. Crespi (photomicrographs), C. this unit by radiometric data obtained in tl-re Camuna Malinverno (thìn sections), G. Manarolla (fossil preparation), S. Ren- Anticline (Schaltegger Er Brack 1999). Unfortunately, to esto (iossil photographs) and A. Rizzi (EDS microanali.sis). An earli cr drrlt of rhc rlrnuscript h,rs been rcmarhably improved by the con- date any biostratigraphic control on radiometric aqes structive criticism of G. Cadel, M. Gaetani and E. Garzanti. Field .rnd has been hampered by the poor fossil content of the unit laboratory n.ork supported by CARG grants. Proiect MURST-1999 and by the scarce biostratigraphic value of the discov- "L'eredità litosferica mesozoica tetidea nei processi subduttivi e colli- ered taxa. The remarkable difference in sandstone com- sionali del Mediterraneo centro-orientale". 66 D. Sciunnach RE,FERENCES Arthaud F. Ec Matte P. (1977) - Late Paleozoic strike-slip fault- Cassinis G. & Neri C. (1999) - Outline of the Permian stratig- ing in southern Europe and northern Africa: Result of a raphy in the Southern Alps. International Congress on right-lateral shear zone between the Appalachians and the Continental Permian of the Southern Alps and Sar- the Urals. Geol. Soc. Am. 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Sciunnach, Rivista Iulíana di Paleontolopia, vol. 107, n. I v. Mo,lÌnera v. delte Noci c.le Fîntana NW I T. Troggia T. ACSUAdUr0 l Alpe d'Olino Alpe cich *-\l sa'te N Alpe Bargiac i l--\-'- a """"rb r '+q"\ cDa ou "'G lInffiffiwje@'ì'1 SE sw corna crande v, Frasnida c0sta canale della Mattaia V. dCIll'lnfr !AVà ,l I zucco della 'í I Stremereggia Ì -:--.i i_ Èi't i !"-r'--r.. !l ,ilì1 Ji13 P$nciflale ffi (Norian] \'{ Ì1ì A' B' NE wNw rsE pianidell'Avaro Srno V. Pianella crasselli T1 I i LEGEND A A Geoloelcal crwiecd /X Pradlnl Valley secdot I (hntermry dePo*s \ \ *dbnentary bed&r: FreAlplt* and AIPt AnHal 6 Nortan (! Camldr d Boretgp sefllno Forna{on t Verrrrcrno Lomb. (r Fosl locdlde UpBer arenaceougp Ponteranlca C.onCrq l}lr$p|rlte to rande Arsreceos.volcanlt Yolca*lc mtrnber {t Pmt"Vartcan plstot o 2km Vaùcan metamory Study N Ceological map of the Orobir in the framework of the fon Sheet 076 "Lecco" of the 1 A New Geological MaP of ltal) - C.:..':r:al map o: ::: .-:. ::-- :'::::-:;:: ir the iramework of the forthcomini givcn io :::. ,: i3 C00 gco..: ::. : :': : -::'t Gauss-Boaga co-ordinrtcs are i as the map' wilh no vertical t S::..' --'i the geoìc-ri;:. j: i' - :: ::: :ìì r: iilt- sanlc {l (Sheers 84' C4)' .-.. - :.1., the 1 : i:. : : : : :: :' : : I :: : F'' :iorale 5 095 000 I 551000 LEGEN} A A Geolo*lcalcrw*.durr kadlnl Vr||ey xdon 4,,__**I*t ^,, \ / I I qnt€rnarr denods (undtftrenttated) tQt) \ srntlY dlPPlns tS{S"} kktenarybed&rg $eply dbplng{{é'90'} PeAIphr and A.lphe faulr An$alr h Norlrn {undl&rsdrt d} (T3} f""-"""1 Cgmhúr ó lovqno {C,{e*l{rn-Anldan} {î2} ls+*4 I 1 $tudy N Ceological map of the Orobic Anticline in the fiamework of the forthcoming Sheet 076 "Lecco" of the 1: 50 000 A New Geological Map of ltaly {CARG} Fig.,r - G..-.,r:ical map oi ih.- Orobic Anricline in the framework o{ the forthcoming Sheet 0/6 "l ecco" of the rr;r l: 5-î C00 gcolc.:ic:l map of ltal1.. Gauss-Boaga co*ordinarcs arc givcn for the NE and SrV corners. Sc:..- oi the geologrr:l ;:oss-secrions is the same as the map, with no vertical exaggcration. Topographic 'r-:.s jca r:on the 1 :53.3I I Carra Tecn Regionale (Sheets 84, C4).