Sedirnentary andpalaeotecronic evolution ofsorne Permian continental basins iii the central Southern Alps, Italy.
O. Cxssr~ís * & C. NERI ** * Dipartimento di Scienze della Yerra dell7iniversitú,
Strada !Vuova 65,1 - 27100 Pavia, Italia
** Dipartimenta di Scienze Geologiche ePaleontalagiche dellVniversitñ,
Corso Ercole P dEste 32,1 - 44100 Ferrara, Italia
ABSTRACT
The authors have dealt with the sedimentary and palaeotectonic evolution of some selected Permian continental basins located in the central South-Alpine domain, immediately to tbe west and east of the Giudicarie Line. In this area the Permian System can be divided into two well differentiated tectono-sedimentary cycles separated by a marked unconformity. The first one (generally spanning late Early Permian to Ufimian times) consists of lacustrine and alluvial fan deposits (Collio Fm, Tregiovo Fm, etc.) as well as of volcanics, both infilling intermontane grabens or half-grabens separated by meta- morphic and igneous highs. The boundary faults generally have 55W-NNE and E- W trends and often coincide with long-lived tectonie lineaments reactivated as late as the Alpine orogeny(such as theTrompiaLine, ValsuganaL., GiudicarieL., etcétem). The second cycle (Late Permian, from the Tatarian (7) to the P/’T boundary or almost) is represented by fluvial clastics (Verrucano Lombardo and Val Gardena Sandstone) and, only to the east of te Adige Valley, by sulphate evaporites to the shallow marine carbonate succession of the Bellerophon Fm. These deposits form a widespread blanket which covers both the basins of the first cycle and the surrounding highs. The continuity of tectonic control is documented by strong changes in thickness (from a few tens of metres on the Adige-Giudicarie highs to several hundreds of metres in te depocentral areas, such as Cadore-Comelico). A reorganisation of the Permian tectonics occurs at te boundary between te
Cuadernos de Geología Ibérica, n> 16, 145-176. Editorial Complutense, Madrid. 1992 146 G. Cassinis & C. Neri two cycles, which is probably correlatable to the “Middle” Permian tectonics of Hercynian Europe (i.e. post-Saalian, Altmark, Palatine movements). The new configuration is attested to te different geometry of the rock bodies pertaining to both cycles as well as to the frequent switching of depocentres (stmctural highs of the first cycle sometimes acted as strongly subsiding areas during the second one). Key-words: Permian, Collio Basin, Tione Basin, Tregiovo Basin, sedimentary evolution, palaeotectonics, lower cycle, upper cycle, Southern Alps, Italy.
RESUMEN Este trabajo describe la evolución sedimentaria y paleotectónica de algunas cuencas sedimentaríaspérmicas del área suralpina central, localizadas al oeste y al este de la Línea de Giudicarie (Italia). La sucesión pérmica de los Alpes meridionales, comprende dos ciclos tecto- sedimentarios mayores, separados por una clara discordancia. En el sectorconsiderado, el ciclo inferior(correspondiente al intervalo Pérmico inferior-Ufemiense) está constituido por depósitos fluvio-lacustres (Fm Collio, Fm Tregiovo, etc.) asociadosavulcanismo, generalmentede tipoácido. Estosmateriales constituyen elrellenode «grabens» o «semigrabens», separados poraltos estructurales. Las fallas limitantes tienen, en general, una dirección SSO-NNE y E-O, y a menudo coinciden con antiguas zonas de fractura, reactivadas al final de la orogenia alpina (Linea del Val Trompia, L. de Valsugana, L. de Giudicarie, etcétera). El segundo ciclo (Pérmico superior, desde el Tartaniense (7) al limite PAl? o cercano a él) está representado por sedimentos fluviales (Areniscas de Verrucano Lombardo y Val Gardena). Tan sólo al este del valle Adige, existen evaporitas y carbonatos marinos de la Fm Bellerophon. Los depósitos del segundo ciclo forman un cuerpocontinuo lateralmente, que recubretantolas cuencas del primerciclocomo los umbrales que existian durante el mismo. La persistencia de una tectónica distensiva está registrada por la existencia de importantes variaciones de espesor (desde algunas decenas de metros en los umbrales de Adige-Giudicarie, hasta cientos de metros en las áreas de depocentro, como la de Cadore-Comelico). El límite con el segundo ciclo está caracterizado por una reorganización de la tectónica del Pérmico, probablemente correlacionable con algún movimiento del Pérmico medio de la Europa hercinica (movimientos post-saálicos, Altmark o palatinos). La nueva confirmación queda reflejada tanto por ladiferente geometría de los cuerpos rocosos de ambos ciclos, como por la frecuente variación de los depoceníros (los altos estructurales del primer ciclo, a veces actúan como áreas de gran subsidencia durante el segundo). Palabras clave: Pérmico, Cuenca del Collio, Cuenca del Tione, Cuenca de Sedi,nentary aud palaeotectonic evolution... 147
Tregiovo, evolución sedimentaria, paleotectónica, ciclo inferior, ciclo superior, Alpes meridionales, Italia.
RIASSUNTO
II presente lavoro descrive levoluzione sedimentaria e paleotettonica di alcuni bacini continentali permiani del Sudalpino centrale, localizzati immediatamente ad ovest e ad est della Linea delle Giudicarie. La successione permiana delle Alpi Meridionail comprende due cicli tettono- sedimentan maggiori, separati da una chiara discordanza. Nel settore considerato, il ciclo inferiore (databile in buona parte allintervallo Artinskiano-Ufimiano) consiste di depositi fluvio-lacustri (Fm di Colijo, Fm di Tregiovo, ccc.) associati a vulcaniti generalmente acide e quasi; questi materiali costituiscono fi riempimento di graben, in qualche caso asimmetrici, separati da alti strutturali. Le faglie bordiere hanno in genere direzioni SSW-NNE eE-W, e spesso coincidono con lineamenti ancestrali pié volte riattivati fino allorogenesi alpina (Linea della Val Trompia, L. della Valsugana, L, delle Giudicarie, ece.). II secondociclo (Permiano Superiores.!., dal Tatariaino(?) sino al limite superiore o quasi del Periodo) é rappresentato da sedimenti fluvialie, ad est della Val dAdige, dalle evaporiti solfatiche e dai carbonati marmni della Fm a Bellerophon. 1 depositi del secondo ciclo formano un corpolateralmentecontinuo che ricopre sia i baciniche gli aIddel ciclo precedente. La persistenza di una tettonica distensiva é documentata da vistose variazioni di spessore (da poche decine di metri in alcuni settori dellalto dellAdige-Brenta a pareechie centinaia di metri nel depocentro cadorino). II limite tra i due cicli é caratterizzato da una riorganizzazione della tettonica permiana, probabilmentecorrelabileaglieventiMedio-PermianidellEuropaErcinica (movimenti post-Saaliani, di Altmark, o Palatini), e documentata sia dalIa diversa geometria dei corpi appartenenti ai duecicli, sia dalIa disattivazione di alcune linee e dallinversione o nascila di altre. Parolechiave:Permiano,Bacino diCollio,BacinodiTione,Bacinodi Tregiovo, evoluzione sedimentaria, paleotettonica, ciclo inferiore, ciclo superiore, Alpi Meridionali. Italia.
INTRODIJCTJON The Permian System of the Southern Alps can be divided into two main very distinct teetono-sedimentaiy cycles, separated by a marked unconformity (ITALtAN IOCP 203 GROIJP, ed., 1986). (Fig. 1). ‘¡he lower cycle (1) is represented by voicanies and subordinate aiiuviaj-lacustrine deposits,locally assoeiated within intermontanebasins,inmost of the Southern Alps 148 O. Cassinis & C. Neri
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- Sedi~nentary andpalaeatectcnic eva/u/lan.. 149 as well as by paralic-marine terrigenous to carbonate sequences inthe Carnia area. me upper cycle (2) consisis of widespread clastic fluvial sediments and,onlyeast of te Adige Valley, of te evaporitie - carbonate successionoftheBellerophonFoimation. Recent research, in collaboration with J. Doubinger from Strasbourg, have lcd to more accurate dating of both cycles in the central South-Alpine domain (see Cassinis & Neri, 1990). According to palynological data, the Collio and Tregiovo Formations, included in Cycle 1, may be attributed to the late Early Permian and probably, aboye alí with regard to Tregiovo Fm, also to the earliest Late Permían. Following the Cis-Uralian standardgeological seale, their depositional range seems
cii
Fig. 2—Location of [he PermianCollio, Tione and Tregiovo basins on both sides of the Giudicarie Line, central Southern Mps, in relation to other basins and to síructuralhighs. Fig. 2—Situación de las cuencas pérmicas de Collio, Tione y Tregiovo, a ambos lados de la línea Giudicarie, y su relación con las otras cuencas y los umbrales estructurales. 150 G. Cassinis & C. Ne,i
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CA- a”- nw a <00 2 0 Co Co 0taL>.C0 0,-. cia o Ce tAJ Ce e --4 .44 -jo- 1 Ú 1 ¡ Al u N V 1 V’d U 3 d LI E LI Sedinzentary and palaeotectonic evolution... 151 to span Artinskian to Ufimian times (Figs. 2 and 3). If we refer to te European continental stratigraphy, the aboye units could be correlated to te medium and higher levels of te Saxonian or te Upper Rotliegend. Biostratigraphic papers, contaiing alist anddescriptionof the palynologicalassemblages, are in preparation by J. Daubinger. From alí the geological data, also including regions extraneous to te one considered, te beginning of Cycle 2 must be located in te Late Permian; te age of its base (which may be diachronous) is still undefined. The dating of the unconformitybetween the two Permian cycles is therefore not yet known with precision; bowever, it should be within the «Middle» Permian, where it is probably younger than Ufimian and older tban Tatarian. Re validity of ¡bis dating unfortunateiy meets witb a serious obstacle: tbe searce stability of the existing geological time- scales, aboye ah withregard tothe Early¡ Late Permian transition’, dueto the paucity of data as well as to tbeir problematie and often imprecise correlation. LOWFP CYCLE ‘¡he products,bot sedimentary andvolcanic, of Cycle 1 (Hg. 1) havebeen laid down witin fault-bounded intermontane basins essentiahly separated by basement highs. ‘¡hese basins are characterized by quite a similar tectono-sedimentary evolution, which will be described here with reference to some selected examples, viz. te better known Collio, Tione and Tregiovo basins, locatedin the areabetween eastern Lombardy and sout-westem Trentino-Alto Adige (Fig. 2). Fig. 3—Chronostratigraphic sehemes as applied Lo the Permian continental basins examined. Ttricknesses areapproximaíe. me stratigraphicclassification. notdrawnto any seale,is based on data taken from several documentsof International Working C,roups. whichare includedin Newsletters 0 12-16, 1987-90), as wc]1 as ofolber scientists; it largeíyreflects theCis- ofSCPS (Perrnophiies, N Ural/Russplatform standard seale (only the interval from Tatarian up to thetopPermian additionally shows, on the right, the USSR Tethyan subdivision, according to Rotíjar, 1989 and in Nakazawa, 1990). Both two-fold and three-fold subdivision of the Permian are reponed. Fig. 3—Esquemacronoestratigráfico,utilizado enlascuencascontinentales pérmicasestudiadas. La clasificación estratigráfica (no a escala), se basa en datos de diversos documentos de Grupos Internacionales de Trabajo, incluidos en los boletines de informacióndel SCPS (Permophiles, n0 12- (6, 1987-90), asícomo de diversos investigadores. Esta clasificación refleja en granparte (a escala tipo de la plataforma(Ás-Ural/Rus. (tan solo en el intervaloTatariense- parte superior del Pérmico. se expresa además, ala derecha, la subdivisión rusapara el Tethys, segónKotljar, 1989y Nakazawa, 1990). Se indican aquí tanto la subdivisión Pérmica doble como la triple. 152 G. Cassinis & (1 Neri Colijo Basin (Fig. 4) Tbis is die westemmost of the basins mentioned aboye. To te south and east it surrounds te Adamello Tertiary magmadc intrusion from te lower Catnonica Valley to the Giudicarie Line, with an approximately E-Worientation changing to a NNE-SSWdirection along the western sideof the GiudicarieLine. This structural disposition, although accentuated by te Alpine tectonies, probably refiects the original geometry of te basin. The basin was originated as a result of collapse associated with a strong volcanicity, following te l-Iercynian orogenesis, that gaye risc to a rhyolitic ignimbrite plastron anó tuff cover resling unconformably on te metamorphic basement. Locally, however, some thin btmds and wedges of Basal Conglomerate occur in between the aboye volcanics and metamorphic complex. Oenerally these deposits, rich in lithic basament fragments, are found on the Permian mountain ridge thai from the upper Trompia Valley, through Mt. Muffetto, continues into te Hg. 4,—TentativeW-E cross-sectionthrongbdiclaíe-Hercynian Boario andcollioBasins (Perínian lower cyc!e) between dic Canionica and Chiese valleys. Ihis discontinuous non-palinspastic reconstruction shows both an intense coeval dislocation of the whole area In mínor stuctural highs aud lows, giving riscto notable varialions in thickncss andlithology, and¡he occurrence nfmagotalie resurgence sites ja ihe casrera sector (Caifaro - Gindicarie). (After Cassiais, ¶985, modificó>. Hg. 4—Corte idealizado, condirección oeste-este, através de lascuencas tardi-hercínicas de Boario y Collio (ciclo inferior pérmico) entre los valles dc Camonica y Chiese. Esta reconstrucción discont¡nua, no-palimpástica, presenta por una parte una dislocación coetánea de todo el área en umbrales y surcos estructurales menores, dando lugar a variaciones importantes de espesor y lUología, así corno a la existencia de una reacuvación magmúbea en el sector oriental (CalTaro- Giudicarie). (Modificado de Cassinis, 1985). Sedimentar and palaeotectonic evolution... 153 Camonica Valley. Their age is still uncertain, but probably belongs to the Early Permian. The incipient deformations of the Collio Basin were accompanied by some reddish clastie lenses, mainly of current-reworked volcanic detritus, within and on topofthetuffs(Fig. 5). According toOri, Dalia and Cassinis (1988), alluvial tanto lacustrine sediments form the bulkof the overlying Collio Formation which consists of fanglomerates, sandstones and shales (Fig. 5). Shale deposits prevailinthe central parts ofte basin, whereas sandstones and conglomerates are more frequent in tbe marginal and other tectonically active arcas. Fanglomerates include conglomerates, without clear organisation, pebbly sandstones and pebbly mudstones. me lack oforganisation and virtual absence of erosional bases indicate mass-flow as the principal sedimentary process. ‘[be fanglomerates pass into flat-bedded reddish-green sandstones. Parallel lamination, small seale (ripple-) cross-lamination, normal grading and rain-drop marks are trie most common sedimentary structures. Thesestructures, caupled with dic associated fanglomera¡es, can be related to a sand fiat environment in ¡he dista) part of alluvial fans. By sand fiat, we suggest that this area was dominated by unchannelised sheet fiood deposits. Shaly facies may include very thin fine-sandstone layers, which show parallel and ripple-cross lamination. Mud-cracks and rain-drop prints are common struc- tures. Shalesareblack and contain an abundantquantity oforganic matter, Icaves and other plant remains. This facies represents low energy mudflat to distal lacustrine environments. Moreover, the aboye sedimentary structures suggest that the basin was never veiy deep and was very often subaerial. Interstratified with the aboye terrigenous deposits, there are one or more volcanie bodies generally interpreted as rbyolitic lavas (Peyronel Pagliani, 1965; Cassinis, Origoni Giobbi & Peyronel Pagliani, 1975) (Fig. 5). meir source area is located to the east of tbe Caffaro Valley, where subvolcanic bodies and a thicker volcaniesequence cropoul (Cassinis, 1988).Thesebodies become suddenly ¡hinner westwards, so that only some of them can be followed through te basin and used as “key bcds”. ‘[bese igneous products testify to good continuity of marginal volcanism during te deposition of te Collio Formation. Between the Trompia andCaffarovalleys the sedimentary «Collio» is generally organised in two sequences starting upwards from each of the two mentioned volcanie units. Each sequence shows a general upwards decrease in grain size and bed thickness passing from distalfan to shallow lacustrine facies (Ori et al., 1988). These sequences may be trie product of two major tectonie movements. ‘[he Dosso dei Galli Conglomerate Formation follows aboye. It overlies and grades laterally into the upper part of the «Collioss. ‘Phis new unit, the thickness of 154 O. Cassin,s & C Ner, COLLIO Fm., M. DASDANA 200 400 00SS0 DEI GALIA LACIJSTRINE G gI. MEMBER A tAO USTRINE MtJDFLAT SANDF L A1 ‘5. 00 SA NDE LA T DISTAL FAN VaLCANIO ci KEY BED a’--—-- o DISTAL FAN m o O 29 09 CONGLOMERATE and RRECCIA SANDSTONE —iii MUOSTONE and SHALE Liiiiiii LAVA-FWW PYROCLASTIO DEPOSIT curron¡ ripples Sltitnp siructures 4 Mud cracks Sioturbation Planí remalos Sedbnentary ¿md pa/aco/ectanit- es’olution..- 155 which ranges from O to 450 m, is made up of afluvial fan deposits which can be divided mío twa members. According to Oh et al. (1988), the lower member (A) consists of coarsening upwards sequences locally interfingering with lacustrine deposits(«Collio»)or stackedone aboye theother. ‘[heseC.U. sequences show three main facies. ‘[he underlying deposits are made up of sandstone beds wit normal grading, parallel and erass-lamination. Aboye fal]ow fine conglomerate ¡o coarse sandstone lenses, still containing te same sedimentary structures. The uppermost deposits are composed of coarse-grained red-grey-green unsorted conglomerates wit ilí-defined stratification. (Fig. 6). ‘[be aboye general sequence represents te progradation of an alluvial fan. ‘¡he upper conglomerates correspond to proximal debris flow deposits tbat pass down- stream into channeled tongues due to traction and turbulent currents. In te more distal part, sheel flow sandstones interfinger witb lacustrine sbales of tbe Cojijo Formation (Ori et al., 1988). ‘[he upper member (B) is composed ofthick unsorted conglomerates interbed-. ded with coarse sandstones, both reddish in colour (Hg. ‘7). ‘[bis unit represents the most proximal part of the progradational alluvial fan system. Another unit known as «Membro della Pietra Simona» (Assereto & Casati, 1965; Cassinis, 1966a, 1966b), which in te Trompia-Caffaro area is normally developed below member B, consists of intensively bioturbated sandy-shaly red beds and can be interpreted as reas lateral to te main alluvial fan lobes orto inactive reas on trie fans (Fig. 7). According to Oh andDalla(seeOrietal., 1988), thetopoftheDossodeiGalli Conglomerate is marked by an erosional surface, only visible where it is superim- posed by te Auccia Volcanies. ‘¡he erosion was probably due to the normal (autocyclie) evolution of the alluvial Lan systems. Clast composition in the Dosso dei Galli Conglomerate reflects te saurce area, which palaeocun-ents showto have 1am to the south-soutwest. Upwards, metamor- phic pebbles generally replace volcanic detritus. In te Caffaro Valley and towards te Lower Giudicarie sorne conglomerates are practically monomict, being almost entirely composed ofvolcaniclitologies. Thesebodies, which are generally tliinner Fig. 5—Litholog, with sedimentological interpretation, of theCollio Formation on the eastern side of Mt. Dasdana. A¡ ¡he bottorn of ¡he section crop out slralificd tuffs and a widespread basal conglomerateand sandstone deposit starting the sedimentary history ot the local basin. For more details see explanation in dic text. (After Ori ci a/., 1988). Ñg. 5—Columna litológica e interpretación sedimentológica dc la Formación ColIjo en el área oriental del Monte Dasdana. En la base afloran tobas estratificadas así como un depósito basal de conglomeradosy areniscas, querepresentan el comienzo de la historia sedimentaría de estacuenca local. Para más detalles, ver explicación en el texto. (Según Ori etal., 1988). 156 O. Cassinis & C. Neri u ~ — di -,o. .0 ‘o 5-e di ‘o O di ~ 50 O ~ a “-5 .4 Li.. Mass f10w e -— 00•0~ ~O ‘o 0 — depos¡ts di O 5-O di di ‘o .4 .5 - 0 >4— di di ci di - Z -2 •0. E O di 40 — ci - 5 .4 0 ~ -5 u di E di a> E -— - n .9 ~9co E o ~ a o Channelised a> U -jr E a, <1 30 deposiís c a, Ú fl o o- a, Cfl ~tE~ a O-s.S >4 0 1 -D 6 tfl&~ w .0 • a o ci, .5 >.. 0 ca 2 ~ O di o di o 5- -~ 20 -~ O ~t ‘-dici di Odi Sheet f10w -~ di — -b ~ 5-a deposits E ~ di O O ~— ~ diZ 5½ .c a ej o 0 ‘5-> . lo Cdi 0 .2 O ~ Li -~ ¿E Lacustrine 7 deposits E ‘o ci ‘o O - ‘o LL~ 5-0 0 5-e — o u..-— ~di~> di o mOJ Sedimcntary andpalaeotectonic evalution... 157 than the preceding ones in te ‘[rompia Valley, can be interpreted as the product of volcanie rocks located in te eastemmarginal area of the Collio Trough,and eroded and brought in from the east. AUGCOA 40v.ck QQ NG 21SF qQ~ 2000 1950 mi CONO LOME~ATE ~ SANOSTOIIJC ‘o SHALC ‘ — 44 - -vb,->-, -‘1- PORPHSRmC fis» e JI $0’ m biót>,bal,on Fig. 7—Geologicalcross-section of tlie Dosso dei Galli Conglomerate in the valley of the Malghe Dasdana Cama. (Aher Cassinis, 1969, modified). 1 valle de Malghe DasdanaComa. (ModificadoFig. 7—CortedetransversalCassinis, 1969).delconglomerado Dosso dei Galli ene ‘[he Auccia Volcanics, ranging from O to 140 m, represent tbe conclusion of effusive activity in the basin (Fig. 8). They mainly consist of violet rhyolitic ignimbrites (Peyronel Pagliani & Clerici Risari, 1973), like the volcanie body underlying te Collio Fonnation. Locally, as inthe Mt. Crestoso area, te lack ofte Auccia Volcanies is apparently related ,at least in parÉ, to erosion by te overlying fluvial deposits of te Verrucano Lombardo. In conclusion, theCollioBasin inW-Ecross section is asymmetric, withatypical half-graben shape represented by a steep margin on te eastemside and amore gentle slope on the western margin (Ori & DalIa, 1984). ‘[he eastern border (towards the Giudicarie Line) is a site of volcanie activity, whereas te westem margin, not volcanic, is a saurce area of detritus, as is proved by te thickness of alluvial fan deposits. Furthermore, a southem source is inferred from palaeocurrents measured from ripple-marks in the Collio Formation. ‘[he westem side of te basin dips towards the depocentre area matehed with 158 O. Cassinis & C. Ner, Fig. 8.—Oeological cross-section of theAuccia Volcanies in ¡he area betwccnDosso dciGaIli and GiogodellaBala, belowandaboye theManiva-Croce Dorniniroad. (AfterCassinis, 1968, modified). Fig. 8—Corte transversal de los materiales volcánicos Auccia en el área entre Dosso dei GaIli y Giogo della Bala, por debajoy por encima de la carretera de Majóva a Croce Dornini. (Modificado dc Cassinis, 1968). some synsedimentary faults (Cassinis, 1964, 1966 b, 1985). Slumping events were provoked by slope instability and faulting. In the Collio Formation, alluvial fan depositsare concentrated aboyethe most extensive volcaniebodies that reached the westem slope of the basin. Ibis fact suggestsapronounced upliftofte western area (and probably elsewliere) indueed by tectonism and related magmatism. ‘[he erosion-deposition relationships are practically the same in the Dosso dei CaRi Conglomerate (Ori u aL, 1988), since in te Mt. Crestoso area te unu only consists of member B that erosionally rests on the «Collioss. Downdip the boundary becomes an intertingering between the lower member A and the underlying formation. As shown in Fig. 3, the beginning of progradation corresponded to a major break in the history of the basia. Tione itasin (Fig. 9) Owing to the non gradual, often aLso sudrien lateral facies and thickness variations ofthe materials containedtherein, te presenceofalate- I-lercynian trough clearly stands out inan area to te 5W of the Brenta Group. friere, north of Honda, this depression corresponds to a subsiding sector delimited by stmctural highs, formed during the Early Permian and named «Tione Basin» (Cassinis eta!., 1982). ‘¡he boundary faults were also active throughout Triassic times and were reactivated again during the Alpine orogenesis, taking on aspects different from original ones (see also Peloso & Vercesi, 1982). Sedimentarv andpalaeatectonic evolution... 159 Re TioneBasin contains acidie volcanies with occasionalterrigenous interca- lations similarto the «Collio» sediments. Re thickness of the totalsuccession is up to 600-700 m. ‘[he basin shows a rectangular shape oriented 55W-NNE, and therefore aGiudicarie trend not toe dissimilar to te one observed in te NEpanof the «Collio» cropping out to te south. 5(4’ tI Gagg>o mONjE 845<5 tI Loo NC y - y VOLcANICS sj ~ALLUVIAL- LACUSTRINE DEPOSITS. cRYSTALLINE BASEMENT conglomorate (a). sand- sihatone ano sha Hg. 9—Idealised SW-NE Perniian cross-sectson throughtheTione Basin (lower cycle), in westem Trentino. (AfterCassinis etal., 1982, madified). Fig. 9—Corte idealizado, en la zonaoccidentaldeTrentino, delPérmicode lacuencadeTione(ciclo inferior), con dirección SO-NE. (Modificado de Cassinis etal., 1982). Aceording to Cassinis et al. (1982), a steep margin with an approximate E-W direction represents te soutem boundary of te trough in the Tione area. ‘[he eastem boundary runs along or immediately west of te Sabion Line, where te Permian System is only represented by te Ven-ucano Lombardo. Rus, ¡he present area in frontof te Mt. Sabionoverthrust was locatedon te eastem side oftebasin, giving rise to a structural high along awide escarpment. The northem and eastem limits are difficult to define owing to a general lack of outcrops. ‘[owards ¡he nort it is possible ¡o suggest tat te late-Hercynian intrusivemass ofMt. Sabion was pan of a higb and ¡bat ¡bis represented ¡he northem boundary of ¡be basin. To ¡he wes¡ ¡he trough may have continued beyond ¡he SouthGiudicarie Line auct. because, in so far as this line can be followed clearly (e.g. in te surroundings of Tione), the contact between te Permian outcrops and ¡be metamorphic basement is tectonie. However, on general grounds, te area nowoccupied byte Adamello pluton seems to have essentially corresponded ¡o a structural high (Cassinis, 1985). In conclusion, ¡he aboye data allow te suppositionthat te Tionedepressxon was mostly characterised by magmatie activity coupled wit tectonicmovements, lateral orclase to ¡he Atbesian megacaldera present¡o ¡he east.During quiet teetono- volcanie times conglomerates and finer deposits were laid down. Unfortunately, 160 O. Caxsinis & C. Neri owing ¡o ¡he rich vegetationand numerous faul¡s in ¡he area examined, it has not yet been possible to recognise Ihe exact geometric relationships. Generafly, these sedimentary bodieswould seem ¡o occupy differentpar¡s of¡he Permian succession. Tregiovo Basin (Fig. 10) (with notes on ¡be surrounding Athesian volcanics) TheTregiovobasin is afault-bounded small troughlocatedat¡he western border of ¡he ¡he so-called «Athesian volcanic p¡atform», which may be regarded as a megacaldera infilled by a very ¡bick effusive sequence known as «Atesine» volcanies or«BozenerQuartzporphyr>s. ‘[his majorvolcanic «basin» is limited ¡o ¡he west by ¡be Giudicarie Line, to the south by the \~alsugana Line, ¡o Ihe north approximately by the Punes Line; ¡he eastern margin is determined by aN-Stectonic alignmen¡running a little westoíthe Badia and Cisnion valleys; its northern segmen¡ corresponds ¡o ¡he «Wiirz Joch Ridge» of Wopfner(1984). ‘[he volcanie sequence of this megacaldera may reacha thickness of about 2000 m aud shows a quite complex vertical evolution from basal andesitie lavas ¡o upper rhyolitic ignimbrites, through daci¡ic- rhyodaci¡ic lavas aná ignimbrites (for recent reviews see DAmico, 1979,1986). According lo DAmico(1986) and Bargossi and DAmico (1989) the magmatic associationis clearly calk-alkaline. In DAmico etal. (1980), and DAmico and Del Moro (1988) radiometric ages relative¡o agreat part of this sequence range from abou¡ 272 Ma (both lower and upper rhyodacitic ignimbrites) ¡o 267 Ma (upper rhyolitic ignimbrites). Synvolcanic ¡ectonicactivity hasbeen documented (e.g. Brandi etat, 1970, 1976; Bargossi,D’Amico& Scipioni, 1983),resultinginiateral changes in thicknessand in Ihe petrographic associa¡ion of ¡he effusive success¡on. Intravolcanie sedimentary clastic bodies(conglomerate andsandstone) mayoccurwi¡hin Ihe megacaldera; ¡hey represení Ihe infillings of small fault-controlled basius. The ‘fregiovoFormation is onlyone of such clastie intercalations, al¡hough it is probably tite most importaní one, dueto i¡s stratigraphicposition near ¡he endof¡he volcanic manifesta¡ions. Rich fossil assemblages (plan¡ remains, palynomorphs, tetrapod footprints) permil its correlation with other Alpine and extra-Alpine Permian continental basins. ‘[he ‘[regiovo Fm is inserted in ¡he uppermos¡ panof the MonteLuco volcanie succession,croppingout alongaNNE-SSEstrikcbetween ¡heGiudicarieand Foinna Lines. ‘[his succession (Bargossi & DAmico, 1989) consisís oía “Lower Group’ composed of conglomerates with pebbles derived from ¡he metamorphic basement and basal effusive producís, iníercalated with rhyodacitic to rhyolitic lava flows and covered by thick (800 m) rhyodacitic lavas. The “Upper Group’ is mainly repre- Sedimentary and palaeotectonic evolution... 161 sented by rhyodacitic to rhyolitie ignimbrites. ‘¡he peculiarity of ¡be LowerGroup, lacking in andesitie produc¡s and rich in rhyodacitic lavas (whichare quite searce in Éhe true Athesian ama) Ieads Bargossi and DAmico (1989) to propose a tectonie separation of¡he Mt. Lucodis¡ric¡from ¡he A¡hesianone, which only ceased during ¡he deposition of ¡he Upper Group. w E TREGIOVO FORMATION a - alluvial fan conglamerates VGS: VAL GARDENA SANOSTONE a~ and sandstones b - lacustrine shales -- ignimixite; y y ph: lower rhyolitic ignimixite; ////// paJaensol pSI: rhyadacitic ignimbrite l’ig. 1 0—Idealised W-EPermiancross-sect¡on throughwestern partofEheTregiovoBasin, Bolzano/ Trento provinces. (After Cassinis & Neri, 1990). Fig. lO——Corte idealizado, en las provincias de Bolzano/Trento. del Pérmico de la zonaoccidental de la cuenca de Tregiovo. con dirección O-E. (Segón Cassinis & Neri, 1990). 162 0. Cassinis & C. Neri ‘[he ‘[regiovo Fm was deposited along ¡be sou¡b-eastem borderof¡he mentioned volcanic dis¡rict, between ‘[regiovo and Lauregno, within a ¡ec¡onically depressed N-S trending belt. Itis limited a¡¡he base and ¡he top by ¡wo rhyoli¡ic ignimbrite uni¡s (Gianno¡¡i, 1963; Uleigral, 1969; Bargossi et al., 1983; Bargossi & DAmico, 1 989).The occurrence of¡he upperignimbriteunit is denied by Mostler(1966), Klau andMostler (1983) and AstI andBrezina (1986), according to whom ¡he «‘[regiovo» is directly overlain by ¡he Val Gardena Sandstone. l-lowever, recent ticíd work by te presen¡ au¡horsalso supports ¡he existenceofavolcanic unil,a few ¡cris of metres lhick, intercalated between ¡he ‘[regiovo beds and ¡he overlying Val Gardena Sandstone (localities: N4iauneri, Pertmeri). ‘¡he upper pan of ¡he ignimbri¡ic unit displays an intense pedogenic alteration before ¡he deposition of the Val Gardena clasties. Also¡he Rio Pescara section, from whicb AstI andBrezina (1986) describe a Tregiovo-Gardena stratigraphic eontac¡, confirms ¡his conclusion. In fact, ¡he contactexaminedby these autbors is tectonised; la¡erally¡he ValGardena Sands¡one is clearly underlain by a volcanic body (kindly interpreted by L. Cortesogno from Genova Universityas arhyolitic ignimbrite, pers. com.), of unknown thickness as only the top (abou¡ 2 m) emerges frorn ¡be water. - The ‘[regiovo Forniation muy be divided into ¡bree infonnal members (Fig. 10). 1) Re lower one (40-50 m) is made up of unsorted sund- lo clast- supported conglomerates wit angular to poorly rounded volcanie pebbles (from a few cm to sorne dm in size). Only towards the top ¡bis unit evolves lo coarse sandstone with scarce trough cross- bedding and parallel lamination, withruditic pockets and lenses. ‘[he prevailing depositional mechanism seems ¡o have been inass-fiow. It foílows ¡ha¡ te sedimen¡ary setting may be interpreted as a proximal alluvial fan at ¡he foo¡ of fault- controlled scarps. ‘[he upper pan of the metnber records mid- ¡o distal alluvial fan facies, dominated by channelised ¡o unchannelised stream flows. 2) Re seeond member (80-100>? m) represents the classical facies of ¡he ‘[regiovo beds, consis¡ing of thin-bedded, platy dark grey ¡o black muds¡one wi¡h mm-cm¡hick “varve-like” in¡ercalationsmudeop of siltstoneoc very fine sandstone. A carbonate fraction is very common throughout ¡bese beds and may be Iocally dominant. Silicification aud silicified concretions oceur al different levels. ‘Ube sedimentarys¡ructures are characterizedby normal grading, parallellamination and, more rarely, ripples and mud-cracks. Ihe fossil contents include plant remains, os¡racods, conchostracans; several horizons with «lacertoid» tetrapod footprints have been recently discovered and sampled by MA. Conti, N. Mario¡¡i, U. Nicosia (University of Rome) aud P. Mietto (Universi¡y of Padova) in ¡he lower part of ¡he unil. No clearly apparent internal organization into facies sequences seems ¡o exist within ¡his mono¡onous succession. ‘[he sedimen¡ary environmení can be referred Sedimentary and palaeotectonic evolution.. 163 ¡o a shajlow, low-energy lacustrmne se¡ting charaeterised by frequent episodes of subaerial exposure. 3) ‘¡he upper member is documented by a few outcrops separated by wide unexposed areas. ‘[bus, due ¡o ¡bis condition and ¡he intense Alpine ¡ectonies affecting ¡he sector iii question, it is difficult ¡o reconstrucí lis vertical sequence and thickness. ‘[he lower part is represented by .shaly-silty mudstonewi¡h sandy intercalations ranging in ¡bickness froin a few cm lo about 20-40 cm. ‘[bese lilbofacies are organised into thickening- and coarsening-upward (C.U.) sequences, 4-5 m thick,in which ¡he mud fraction is dominant. Sorne outcrops attributable to ¡he mid-upper part of the member are sÉilí characterised by a similar facies organisalion, witb a distincí increase in ¡be sand/ mud ratio; the sandstone layers are platy. Following a C.U. ¡rend, these facies are capped by a massive coarse sands¡onebody, of ilí-definedtickness (over 10 m 7), strongly disturbed by tectonies. Member 3 probably records Ibe repeated progradation pbases of distal afluvial fan «sheet sands»(such as ¡bosedescribed by Ori etal., 1988,in the typical «Collio» area) towards ¡he basin centre. Upwards, more proximal facies return. According to ¡he strarigraphic column of Klau aud Mostler (1983), the ¡op parÉ of the niember is represented by mass-flow conglomeratic deposi¡s pertaining ¡o proximal alluvial fans (no¡ observed in ¡he field by tIte wri¡ers). Member 3 has not been observed alí over ¡he Tregiovo Basin. For example, ¡he upper parÉ of ¡he Tregiovo Formation nearLautrgno (Pertmeri) consists of shaly «varve-like» faciesof member 2; these contain sornehorizons, 0.5-1 m thick,made up of fiatpebble breccias with clasts derived from ¡he same lacustrine facies; ¡here are alsoslumped ordebris-flowlevelswi¡b plas¡icallydeformed slabs morethan 1 m large, also dedved fmm ¡he «shaly» ‘[nigiovo beds. MosÉ probably ¡bese represcnt the response to a reactivation of the boundary faul¡s, whieh are alsoresponsible for ¡he progradation of ¡be marginal alluvial fan lobes. On ¡he basis of the stratigraphic features described aboye, te structural evoluton of thc Tmgiovo Basin can be summansed as follows: a> initial block-faulting leading ¡o ¡be collapse of ¡he basin and ¡o the deposition of the scarp-foot conglomerates of member 1; b) atectonically quiel time in which ¡be monotonous muddy lacustrine facies (member 2) was deposited; c) a reactivation of synsedimentary ¡ectonics, producing ¡he basinward progradation of ¡he marginal alluvial fan coarse clasties and episodesof gravitational resedimentation wi¡hin the basin; d) fmally, ¡heextrusionofthe last volcanicproduc¡s(upperrhyoliticignimbrite) capping ¡he sedimentary suecession. 164 O. Cas.sinis & C. Neri UPPER CYCLE lii the central-eastern South-Alpine areate seeondcycle (Fig. 1) consistsof¡he following uni¡s (from ¡be base upwards): a) basal conglomerates, sometimes distinguished as independent litas- tra¡igraphic units (Val DaoneConglomerate,SestoConglomerate,Tarvisio Ereccia); ¡hey res¡ unconformably on a substíate formed witbin ¡he investigated area by ¡he products of ¡he firsí Permian eycie as well as by ¡he Hercynian metamorphic basement; b) continental clastic Verrucano Lombardo-Val Gardena Sands¡one, repre- senting the samelithosome known witbdifferentnames in Lombardyand in te area eas¡ of ¡he Adige Vafley; e) EellerophonFormation, deposited only in te area casÉ of¡be Adige Valley and consisting of avery complexarray of lithofacies rangingfrom sulphateevaporite aud rnarly-sihy dolomite deposited in -a coastal sabkha environrnent lo shallow marine fossiliferous limestone; it interfingers with and laterally replaces parÉof the Val Qardena Sandstone. Unlike ¡hedeposits of¡he lowercycle, those of ¡he secoad onelorm acontinuous, widespread body in a large sector of the Sou¡hern Mps, extending from the eastern border of¡he Monte Generoso- Como High (Cassinis etaL, 1988) to the Carnie Alps and, furthcr¡o ¡he east, to Yugoslavia.‘[bey are separated from ¡he generally Lower Permian units by a pronounced unconformity, relatedtoanoveralí reorganisationof the synsedimentary ¡ectonieswhich will be discussed in detail in the nextsection. AÉ the top tbey are overlain by te marine Lower Triassic sediments pertaining to te Servino (Lombardy) and Werfen (eastem Sou¡hern Alps) forma¡ions. As regards ¡he «basal conglomerates» the age, stra¡igraphic setting and geologi- cal meaningof ¡hese bodies, in¡erpre¡ed as alluvial fan lobes deposited atte foot of faulí-controlted scarps, havelongbeendebated. Sorneau¡horsplaced ¡bern a¡theend of ¡he first cycle.‘[he Sesto Conglomerate andTarvisio Breccia, considered by Kah- ler (1986). Fltigel (1986), FlÍlgel aud Kraus (1988) as Cisjanskian in age, have been interpretedby tese authors as almost isochronous bodiesresultingfromthetectonic upliftanddisintegration of¡he LowerPermian carbonate platforms a¡ the endof ¡heir depositional his¡ory. However,accordingtotheltali-an [GCP2O3Group(1986),Cassinisera/.( 1988), Ven¡urini (1990), ¡hey represent ¡he lowermost deposits of ¡he second Permian cycle; ¡heir age probably changes from place to place, due to synsedimentary tec- tonics ami the palaeostruetural setting. In tis interpretation ¡bey grade upwards into te braided stream deposits of ¡he Val Ordena Sandstone wi¡hout any appreciable time gap. Sedimentary and palaeotectonic evolution... 165 The sedimentary evolution of te units pertaining to te second cycle has been described in detail by several workers (e.g. Bosellini & Dal Cm, 1968; Bosellini & Hardie, 1973; Italian IGCP 203 Group, 1986; Ori etaL, 1988; Ori, 1988; Massari et aL, 1988; BroglioLoriga et aL, 1988; Noé, 1988). Altogether, ¡he Verrucano Lombardo - Val Gardena Sandstone and tbe Belle- rophonFormationconstituteamajortining-upwardstransgressivesequence, slowly evolving from coarse, piedmontclasties at the base ¡o shallow marine fossiliferous limestone at ¡be top. ‘[he base of this sequence is represented by mud- ¡o clast- supported conglom- erates, frequentlyunchannelisedandinterbeddedwkb variableamountsofmudstone, usually rich in pedogenic structures such as deeply-reaching desiccation cracks and calcrete horizons; ¡hese facies are interpreted in Éerms of an alluvial fmi This unit is overlain by red ¡o grey sandstonesandgravels forming cross-cu¡¡ing lenticular channel filís in which ¡he dominantdepositional structuresare represented by trough cross-bedding (braided stream deposits); it evolves upwards into a meandering river setting, characterised by the vertical s¡acking ofanumber ofpoin¡- bar sequences with a lateral accretian pattern (epsilon cross-bedding). The fluvial sequence is capped by a mud- dominated unit witb sparse ribbon channels and shee¡ sands; tese deposits are interpretedas ¡he record of terminal fan systems encroach~ng onto ¡he coastal plain. According ¡o Ori (1988) ¡he Permian nvers of ¡he Southern Alps were exotie in nature; flowing across a semiarid-arid plain ¡bey were affected by a progressive loss of water due to evaporation and infiltration, until ¡bey became unable lo cnt channels. Before reacbing ¡be sea, ¡bey vanished iii coastal tenninal fans. East of ¡he Adige Valley, te red clasties are gradually covered by te marginal marine sediments of¡he lower part ofte Bellerophon Fm, representativeofvarious coastal se¡tings and pa¡¡ems of sedimentation. AÉ the base of ¡his formation, ¡he pro- minent depositional¡heme is ¡he «sabkha cycle» described by Bosellini and Hardie (1973), eharacterised by a shallowing-upwards trend evolving from subtidal dolo- mileto cbicken-wire gypsnm of¡be supratidal portion of ¡be sabbka. Following an overalí transgressive ¡rend, the marginal marine deposits are replaced upsection by shallow wa¡er limestone, wi¡h rich faunal and floral remains (forams, calcareous algae, bivalves, gastropods, nautiloids; see Broglio Loriga et al., 1988 forareview). Such an ideal transgressive sequence may be divided into a numberof minor ¡rans- gressive-regressive cycles, which may be interpre¡ed as eusta¡ically controlled 3rd order depositional sequences, althougb tlieir vertical evolution and facies organisa- tion is also strongly influenced by synsedimentary ¡ectonies. As a result, clastie, evaporite and carbonate repea¡edly interfinger. ‘[bree depositional sequences have been recognized within Cycle2 in the westemDolomites (Farabegoli & Viel, 1982; 166 0. Cassinis & C. Nerí Massari et al., 1988; Neri & Massari, 1990). Four sequences are recorded in ¡be Carnie mps where a basal sequence occours, apparently inexistent wes¡ of the Comelico region. AI¡bough synsedimentary teetonies were sÉilí active, Cycle 2 seems ¡o testify, from ¡he base upwards, ¡o aprogressive flattening of¡hepalaeotopography, inherited as well as due ¡o new fault movements (reactivation and new formation may be involved). At ¡he end of the Permian ¡he depositional area extended alí over the Sou¡hern Alpseas¡of¡he Mt. Generoso High.Itwas characterised by avery low relief with wide facies belts which graded into each other lateralLy. On Uds fiat latid ¡he «Werfen transgression», more or less coinciding with ¡he Permian¡Triassic bound- ary, encroached. ‘[bis generated a fast shift of ¡be coastline towards westem Lombardy. ‘[he geometry, ¡hickness and faciespattem of the deposits of Cycle2 give some indications about ¡be “Middle” Permian reorganisation of structural Érends. ‘[he most subsident areas were represented by: a) eastern Lombacdy, where the Verrucano Lombardo reaches a maximum thickness ofabou¡ 500 m, even on previous EarlyPermian highs, such as the Val Ca- monica Ridge; b) the eastem sector of¡be Sou¡bern Alps: panof ¡he Westem Dolomites, te. Mt. Putia, wit a ¡hickness of more ¡han 500 m; ¡he Cadore-Comelico area, where a ¡bickness of ca. 800 mis reached locally; pan ofCarnia, i. e. ¡he Paularo area. It is noted tat in ¡he Cadore-Comelico depocentre ¡he succession directly overlies ¡he me¡amorphic basement, which was a relative high during Cycle 1. ‘[bese two main depocentres were separated by a prominent N-S trending ancient high located between the Adige Valley and ¡he Giudicarie Line (Adige- Brenta Ridge), which is subdivided into ininor lows and highs, with thicknesses ranging from 50 m ¡o about 150-200 m. O¡herimportant highs were repíesented by the area south of ¡he ValsuganaLine (thickness of the Upper Permian deposits nol in excess of 100 ni), which also acted as a highduring Cycle 1, and by sorne sectors ofNECarnia (Pramollo, etc.;see Venturini, ¡986, 1990). Moreover, ¡bepalaeocurrent pattem of ¡be Val Gardena Sandstone, with frequent 5- directed transpon, seems ¡o sugges¡ ¡he existence of a northern lnsubric high. THE UNCONFORMITY BETWEEN LOWER AND IJPPER CYCLES As stated in the previous pages, ¡he boundary between ¡be two Permian cycles is marked ‘ay an unconformity (Figs. 1 and 3), in ah prohability due to te Mid-. Permian ¡ec¡onic event. It results in ¡he superposition of ¡he widespread fluvial Sedimentary and palaeotectonic evolution.. 167 Verrucano Lombardo-Val Gardena Sands¡one litosome onto the more localised lacustrine-alluvial fan sediments and subaerialvolcanics infilling te «taphrogenic» basins of Cycle 1. Cycle 2 is characterised by a general reorganisation of ¡be tectonie Unes responsible for¡he palaeo¡opography and ¡he subsidence pattem. Several impor¡ant faults whiehcontrolled te palaeogeography of Cycle 1 were deactivated(e. g. Funes Line, probably Val Trompia Line, etc.); o¡bers were reactivated wit a different s¡ructural behaviour,de¡ermining Ihe shiftof depocentres. Someexamples are given at ¡he end of ¡he preceding section. ‘[he time gap associated with tis unconformity is still not fully known, due ¡o ¡he fact ¡bat the standard classification of tis geological interval has noÉ yet been clearly defined and to ¡he low resolution and general paucity of te fossil record (mainly restricted¡o palynomorphs) in ¡he lowerpartof¡be ValGardenaSandstone. However, ¡he base of Cycle 2 seems diachronous, being older in Carnia tan in the Dolomites; moreover, within a single area, ¡he age also depends on ¡he struc¡ural setting, the onset of sedimentation being usually later on palaeohighs. Palynolog¿ca¡ dating of [hetop o?Cycle 1, dueto J. Doubinger(in Cassinis & Neri, 1990), indicates that ¡he age of¡he unconformity hastobe later ¡han ¡he earliest «Middle» Permian (according ¡o ¡he ¡bree-fold subdivision of ¡he Period), and probably older ¡han ‘[atarian(al¡hough no certain data are available abou¡ te time of the basal sequence of Cycle 2 from Camia, believed to be ¡be oldest deposits of ¡he upper cycle) (Fig. 2). ‘[hus ¡he unconformity in question is more recent¡han ¡he main Saalian «phase». Probably it is ¡o be correlated ¡o pos¡-Saalian events, such as ¡he first Al¡mark movementsoíHoffmann, Kamps andSchneider(1989), probably corresponding ¡o ¡he «Palatine phase» sensa Kozur (1980). In ¡he Sou¡hern Alps this eventtook place a¡ ¡he endof te Permian volcanism, e-of¡he lower cycle,although some authors (Cadel, 1986; Dickins, 1988) note¡he occurrence of minor igneous products in tbe lower pan of ¡he upper cycle. CONCLUDING REMARKS In this work we have discussed ¡he history of some continental basins in te Soutern Alps. Two tec¡ono-sedimentary cycles have been documented. 1) CycIeI 1.1. Thedepositional history andthe struc¡ural evolution of¡beproducts ofthis. 168 (3. Cassinis & C. Neri cycle may be described with reference¡o ¡he Collio Basin, ¡he besÉ known and most remarkable example among ¡bose considered. Re structural organisation of ¡he ‘[regiovo Basin is substantially similar, albeit on a smaller seale.Other basins supply less clear evidence. ‘[he main evolu¡ionary stages ofthe Collio Basin can be summarised as follows (Ori el aL, 1988) (Hg. 11): a) Extensive volcanie activi¡y produced a basal ignimbritic body draping the crys¡alline basement. b) Block-faulting, whichalso controlled ¡he volcanicity, led to the formation of an half-graben; main faults roughly correspond ¡o ¡he Giudicarie Line (east) and ¡o te Val ‘[rompia Line (south). Alluvial fans, on ¡he basin margin, and lacustrine ~ B ~‘00 c Ljj] VERRUCANO LOMBARDO D LÚIiI AUCOLA VOLCANIOS ~FTfl DOSSO DEI GALLI COCMLCLIERA TE ELiiI] 001110 FORMATION F71 BASAl. VOCCANIO DEPOSIT Fig. II —Majorevolutionary stagesof Permian volcanic-sedimcntary deposition intheCollio Basin. Explanarion in ¡he text for more det.ails. (After Ori «tal., 1988). Fig. II -—Principales etapas evolutivas dc la sedimentación volcanoclástica pérmica en la cuenca Collio. Para más detalle ver explicación en el texto. (Segón Ori el al., 1988). Sedhnentary and pa/aeatectonic evalution... 169 deposits (Collio Fm) were bat laid down atthis stage. Volcanie activity (lavaflows, dikes, tuff.s) was sÉil] iii force on te steep eastem niargin of te basin. e) As a result of majar teetonic effeets (uplift of te basin margin), marginal aluvial fan systemsprograded mio the basin, progressively filhing it up (Dosso dei Galli Conglomerate). d) ‘[he sedimentaryhistory of the basin is closed byte outpouring of te last volcanie products, represented byte rhyolitic ignimbrites andother minor effusive manifestations knownas «Auccia Volcanies». e) After s¡age d>, te whole suceession underwent a phase of tectonie defor- mation and subsequení erosion, resuhing in an unconformiíy (locally angular) between ¡he Auccia Volcaniesaudte overlying Verrucana Lombardo.Frequently, thick palaeosals are developed atthetop afilie dcposi¡s ofthe lowercycle during¡his phase. 1.2. As discussed in ¡he previouspages, ¡he main faults controlling the basin margins and ¡he volcanie districts in ¡he differentparts ofte overalí areafallmio twa majar groups. ‘[he firsí, froni NNE- 55W ¡o N-S trending, is subs¡antiaily parailel to te Giudicarie Line. ‘[he second is directed abaut E-W, and the main faulis approximately coincide wit or are parallel ¡o ihe presení Val ‘[rompía Line, Valsugana Line, ‘[anale and Pusteria segments of ¡he Periadriatie Lineament, etc. ‘[hus te quated lines seem to represen¡ ancestral faulí lineamenis which have beenrepeatedly reactivated until te Alpine orogenesis. ‘[he Hercynian structural grain, ¡herefore, has played an importantrole on the future Mesozoicand Tertiary reorganisatian. 2) Cyde 2 ‘[he Mid-Upper Permian clasties forming ¡he basal parÉ of tus cycle is very widely distributed geographically; ¡hey also extend outside te Sou¡hem Alps (Siavenia, etc.), overlying ¡he LawerPemiian marine sediments (‘[ragkofe] Group) which crop out in Carnia and in the Karawanken Mts. mis sugges¡s a regional relative uplift due tate previously discussed «Mid-Permian» ¡ectonies, accompa- nied by an overall regression. ‘¡he following depositional historyof te sedimentary succession pertaining to Cycle 2 is characterised by a general transgressive trend wbich lcd ¡he coastline to reach ¡he Adige Valley at te end of ¡be Permian. ‘[his may be interpreted as ¡he responsetoageneral peneplanationconcomi¡anttoacantinuingregional subsidence, inducedby ¡ensional kctomcs. The whole succession may be furter divided into some ¡bird arder cycles, resulting in te repeated interfingering of continental red beds (Val Gardena Sandstone) andmarginal lo fufly marine units (Belleropbon Fm). 170 0. Cassinis & C. Neri The evolution of these cycles is controlled by eus¡a¡ic fluctuations (Neri & Massari, 1990) as well as being influenced by persistent tectonie activity. In fact, ¡he InÉter determined ¡he position ofpalaeohigbs(with reduced,mainlycontinental successions) and strongly subsiding areas. 3) Geodynamic setting ‘[he geodynamic setting of ¡he basins examined is still controversial, as it belongs la te more general aral still debated interpretation of the «Late» andlor «Pos¡»-Hercynian events of ¡he whole European area. Re data supplied from ¡he Permian sedimentary and volcanic successions of¡he central Southern Alps under- ¡me ¡he fbllowing facts. 3.1. ‘[he stmctural evolution of te basins is controfled by tensional tectonies, assupportedby manyauthors (cg.Cassinis etal., 1980).’[his has been interpreted as ¡he response to simplecrustalextension (Wopt’ner, 1984), orasdue totranstensional movementsin ageneral strike-slip context. ‘[he lat¡erin¡erpre¡ation, advanced forthe Southern Alps by Venturini (1983), Cadel (1986). Massari (1988) and others, is gaining force since it mayexplaiti ¡he coexistenee of distensive and compressional effects (which may control ¡he repeated reactiva¡ions of ¡he paleohighs), ¡he abrupí shiftingofdepocentres, ¡he frequently observed inversionofthe highsinto subsiding areas and viceversa. ‘[he strike-slip interpretation fits in witb te Permo-Carbon- ifernus recons¡ructions of Arihaud and Maue (1911). Ziegler (1982, 1988) aud others, which affirm the existence of a widespread zone of dextral megashear be¡ween ¡he Atlas and ¡he ‘[omquist Line. 3.2. According ¡o most authors (cg. Bargossi & DAmico, 1989) ¡he central South-Alpine Permian volcanics are mainly ealk-alkaline in nature. ‘[he common alkalinecharactershownby teessen¡ially rhyolitic volcanic andsubvolcanic bodies cropping ou¡ in ¡he Collio Basin (between ¡he Daone and ‘[rompia Valleys) as well as in ¡he Tione Basin (lower Rendena Valley)seems referable toan intense deuterie hydrothermal-pneumatholiticalteration (Peyronel Pagliani, 1965; Peyronel Pagliani & Clerici Risari, 1973; Cassinis etaL, 1975; Origoni Giobbi, Peyronel Pagliani & Zanchini Camerini, 1979), even if, according ¡o ¡he la¡¡er authors, ¡he present chemical composition should be very clase to ¡bat of ¡he primary magmatie melÉ. A numberof modeis have beenproposed for the causes of meltingaud volcanic chronologicaldistribution in ¡he light of different geodynamic interpreta¡ions used ¡o explain ¡he I-lercynian orogeny and ¡he following related events. MasÉ of tese models involve type A subduc¡ion (Vai etaL, 1984; etc.) or typeB (SÉille & Bulet¡i, 1987; Mercalli & Oberhánsli, 1988; Di Battistini etaL, 1990), duringLate Palaeozoic times. Sedimentary and palaeotectanic evolution... 171 Cabellaeta/. (1988)pointed outthe prablemarising from ¡heverylong timespan which separated I-lercynian tectogenesisfrom te beginning ofte volcanieactivity. Indeed, ane should remember tat ¡he area investigated shows the age of the Hercynian metamorphic events is tobe around350-315 Ma, according ¡o Del Moro, Sassi and Zirpoli (1980, 1984) aud others, while plutonism and volcanism are respectively assigned to about 295-274 Ma and 272-267 Ma, on ¡he basis of data given by Borsi, Ferrara and ‘[ongiorgi (1966), Borsi, Del Moro and Ferrara (1972), Borsi, D’ amico andDelMoro (1974), lyAmico etal. (1980), DAmicoandDel Moro (1988, and personal comniunication of ¡he la¡¡er au¡har). Cabella et aL (1988) sugges¡ed an alternative view based on a post-orogenie genesis of ¡he magma, probably linked to mantie diapirism. ‘Uhe emplacement of ¡he igneous bodies may be connected with ¡he transition between the compressional ¡ate distensive phase which eharacterised late- ¡o post- Hercynian times. ACNOWLEDGEMENTS ‘[he authors are grateful ¡o the editorial board of ¡he Review for ¡he transíation into Spanish of ¡he abstract and the captions of the figures. 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