MULTIORDER SEQUENCESTRATIGRAPHY IN THE TRIASSIC SYSTEM OF THE WESTERN SOUTHERN ALPS MAURIZIO CAETANI, MARIO GNACCOLINI, FLAVIO JADOUL, AND EDUARDO GARZANTI Dipa imenîo di Scienze della Teffa, l\a Mangiaqalli 34, 20133 Milano, Italy. ABsrRAcr: A hi€rarchy of depositional sequences, from long-term megasequences to transgressive.tegressive sequences to Milankovitchian cyclothems, is recognized in the w€ll-known stratigraphic record of the Southern Alps. The sedimentary succession deposited belweeo th€ Hercynian and Alpine orogenies can be subdivided into thee largely tectonically-controlled megasequences, with boundaíes occuniog in the Upper Triassic and loweÍnost Cretàceous successions. Pattemsof sediment accumulation in the Scythian-Camian palt ofthe Triassic succession were primarily controlled by successive episodes of transpression and transtension with associated volcanism, indirectly linked to the opening of ' Neotethysin the east. In the Norian-Rhaelian stages, however, subsidence markedly increased, leading to final dmwning and deposition ofpelagic sedimentsfollowing late Hettangian time. Transtensional movements are interpreted as related to the initial development of a nanow rift system connecting lhe Neotethys with the future site where the central Atlantic will open in Middle Jurassictime. Within the Triassic succession, five transgressive/regressive supersequenc€s punctuated by major transgressive episodes are interpretedas tectono-eustatic in origin, since they are on one hand characterized by different subsidence and slructural pattems, but on the other also broadly correlate with worldwide long-term changes in sea level. Besides eustasy and t€ctonic activity caused by Neotethyan rifting, sequ€nce architecture was also influenced by several factors, including climate, sedimentsupply and changesin the productiviîy of "carbonate factories"- In total, l6 sequences are recognized,with an average duration of 3 my. The first supersequence (S) is confined to Lower Triassic strata. \ùr'hile five sequences occur in the eastem Southem Alps, only three lo none of the sequences could be identified from eastem to westem Lombardy. The Middle Triassic supersequence (A-L) and the Camian supersequence (C) may be subdivided into five and four sequences, respectively.The fourth supersequence consists of one sequence only (Nl), whereas the fifth one, extending into the Upper Hettangian deposition (N2-H), can be subdivided into at least three sequences. The Triassic succession of the Southem Alps (Fig. 1) pro- vides an excellent caseto test the application ofsequencestrati- graphic concepts, since stratigraphic researchhas been active for more than a century. The Southem Alps are also the type- area for two Túassic stages: the Carnian and the Ladinian, and southern alpine Triassic rocks provided the first evidence for basic concepts in geology, such as the intertonguing of facies (Mojsisovics, 1879). Palaeontological tools are reflned enough to establish a fairly accurate and well-controlled biostratiFaphic scheme (Table l). Good resolution is obtained for Lower Triassic and Middle An- isian to Ladinian strata, which commonly contain ammonoids and conodonts. At other times, characterizedby widespread shallow-water sediments,less precise age information is pro- vided by foraminifers, brachiopods, algae and bivalves. Geographically, the SouthernAlps (Fig. l) can be subdivided into an eastem part, including the Dolomites as well as the Recoaro and Camia regions, and a western part, located be- tween Lake Garda and Lake Maggiore. A complete sequence stratigraphic analysis should consider the whole area, since Lower Triassic sedimentsare fully represented only in the E, whereas the Upper Triassic succession is better developed in the west. In the presentpaper, we will deal only with the west- ern SouthernAlps (Fig. 2). FACTORS CONTROLLING SEDIMENTARY EVOLUTION Besides global eustatic changes, sedimentaryevolution was controlled by the interaction of severalregional factors,includ- ing tectonismand subsidence, volcanism, nature and amountof sedimentsupply, and climate. Tectonism The area occupied today by the Southem Alps was sub- merged during the Triassic by a mostly shallow seaway linled to Neotethys (Fig.3). Since Permian times, when the peri- Gondwanan microplate fringe detached from Gondwana, the Southem Alps "hinge zone" has been dominated by tectonic movements caused by W-ward propagation of Neotethyan spreading(Marcoux et al., 1993a).At the westerncomer ofthe Neotethyan oceanic triangle, rotations and strike-slip displace- ments of microplates were active at several stages(Doglioni, 1987). Transtension and associated volcanism, which possibly resultedfrom local oblique convergence (Marinelli et al., 1980; Garzanti, 1985b), were active from Middle Triassic to at least mid-Carnian. After a major ieodynamic change occurring to- wards the close of the Carnian,the SouthernAlps were affected by renewed extensionuntil the Middle Jurassic time, when the region becamethe passivemargin of the Adria microplate,fac- ing the Piedmont-Liguria oceanicsea-way linking Neotethysto the Central Atlantic. The impact of tectonic extension can be evaluated by cal- culating subsidence rates through time. Besidesdecompaction problems, uncefainties are due to considerablediscrepancies betweenTriassictime scales, especiallywith regardto the Early and Middle Triassic. The estimatedduration of the Early Tri- assicranges, for instance, from a minimum of4.l m.y. (Harland et al., 1989), to a maximumof 10 m.y. (Haq et al., 1988), with intermediate figures of 5 and 7 m.y. given by Odin and Odin (1990) and Menning (1990) respectively. Recentefforts to cal- culate age figuresfrom counting ofsuperposed Milankovitchian cyclothems, on the other hand, are largely basedon weak as- sumptionsand circular reasoning. Moreover, datareported from îhe Latemar buildups in the Dolomites (Goldhammer et al., 1990), if extrapolated to the entire Ladinian, would imply a duration of over 20 m.y., which seems excessive (Brack and Rieber, l99l; 1993). The most reasonable estimat€s of accumulationrates,which approach gubsidence rates for this largely shallow-waîer suc- cession,are those calculatedaccording to the Haq et al. (1988) scale, which is adoptedherein also because it best approaches the accurate age oî 251.2 a 3.4 Ma recently obtained for the Permian/Triassic boundary (Claoué-Long et al., 1991). [Iow- ever, it should be noted that the estimatedagesof the Triassic/ Jurassic boundary (210 Ma and 208 Ma according to Haq et al. Mesozoic aDd Cenozoic SeqùenceStratigraphy of Europ€an Barins, SEPM Special Publication No. 60 Copyright O 1998. SEPM (Society for Sedim€nlary Geology), ISBN l-5657G&3-3