Sedimentology (2018) 65, 1097–1131 doi: 10.1111/sed.12416 The start-up of the Dolomia Principale/Hauptdolomit carbonate platform (Upper Triassic) in the eastern Southern Alps MARCELLO CAGGIATI* , PIERO GIANOLLA*, ANNA BREDA†, BOGOMIR CELARC‡ and NEREO PRETO† *Physics and Earth Sciences Department, University of Ferrara, Via Saragat, 1, Ferrara 44100, Italy, (E-mail: [email protected]) †Geosciences Department, University of Padua, Via Gradenigo, 6, Padua 35131, Italy ‡Geological Survey of Slovenia, Dimiceva ulica 14, Ljubljana SI-1000, Slovenia Associate Editor – Gregor Eberli ABSTRACT Wide carbonate platform environments developed on the western passive margin of the Tethys during the Late Triassic, after a major climate change (Carnian Pluvial Episode) that produced a crisis of high-relief microbial car- bonate platforms. The peritidal succession of this epicontinental platform (Dolomia Principale/Hauptdolomit, Dachstein Limestone) is widespread in the Mediterranean region. However, the start-up stage is not fully under- stood. The original platform to basin depositional geometries of the system have been studied in the north-eastern Southern Alps, close to the Italian/ Slovenian boundary where they are exceptionally preserved. Sedimentologi- cal features have been investigated in detail by measuring several strati- graphic sections cropping out along an ideal depositional profile. The analysis of the facies architecture allowed reconstruction of the palaeoenvi- ronments of the Dolomia Principale platform during its start-up and early growth stages in the late Carnian. The carbonate platform was characterized by an outer platform area, connected northward to steep slopes facing a rela- tively deep basin. Southward, the outer platform was connected to inner sheltered environments by a narrow, often emerged shelf crest. Behind this zone, carbonate sedimentation occurred in shallow lagoons and tidal flats, passing inward to a siliciclastic mudflat. The Dolomia Principale platform was initially aggrading and able to keep pace with a concomitant sea-level rise, and then prograding during the late Carnian. This stratigraphic interval was correlated with the Tuvalian succession of the Dolomites, allowing depiction of the depositional system on a wide scale of hundreds of kilome- tres. This large-scale depositional system presents features in common with some Palaeozoic and Mesozoic carbonate build-ups (for example, the Per- mian Capitan Reef complex, Anisian Latemar platform), both in terms of architecture and prevailing carbonate producers. A microbial-dominated car- bonate factory is found in the outer platform and upper slope. The recovery of high-relief microbial carbonate platforms marks the end of the Carnian Pluvial Episode in the Tuvalian of Tethys. Keywords Carbonate factories, carbonate platforms, facies model, micro- bialites, Triassic, Western Tethys. © 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists 1097 1098 M. Caggiati et al. INTRODUCTION platform are well-known only from Norian and Rhaetian times, and mostly for local settings fac- During the Triassic, the Tethys saw the progres- ing poorly oxygenated intraplatform basins and/ sive development of carbonate platforms of or open pelagic areas (Iannace & Zamparelli, increasingly larger scale and relief, from isolated 2002; Cozzi & Hardie, 2003; Jadoul et al., 2004). occurrences of carbonate build-ups during the Conversely, the early-stage marginal systems are Induan and Olenekian (Lehrmann et al., 1998, known only from Julian Alps, and remain poorly 2005), to kilometre-scale isolated and epiconti- studied, with a few significant exceptions (Gia- nental high-relief platforms in the Anisian, Ladi- nolla et al., 2003; Celarc & Kolar-Jurkovsek, nian and lower Carnian (Stefani et al., 2010). The 2008). This means that the facies, depositional majority of these carbonate platforms are mostly geometries and carbonate producers of the larger made of microbialites rather than metazoan skele- and most long-lived Tethyan carbonate platform tons (Blendinger, 1994; Russo et al., 1997; Kies- system of the Triassic are nearly unknown in its sling, 2009; Martindale et al., 2015). The spread early stage. and increase in scale of Triassic carbonate plat- The aim of this work was to contribute forms is particularly well-documented in the towards filling this gap, by proposing a compre- Dolomites of Northern Italy (Gaetani et al., 1981; hensive facies model of the late Carnian portion Bosellini, 1984; Stefani et al., 2010). In this of the Dolomia Principale/Hauptdolomit carbon- region, where the continuity and resolution of ate platform system in a palaeogeographic set- the sedimentary record is exceptional, it can be ting facing a branch of the Tethys ocean (the shown that the growth of Triassic carbonate plat- Tarvisio Basin: Gianolla et al., 2003; Gale et al., forms was a process in steps, with frequent inter- 2015). Furthermore, this study aimed to provide ruptions (De Zanche et al., 1993; Gianolla et al., new data on the type of this late Carnian carbon- 1998). One of these halts in carbonate production ate factory at its seaward margin. The start-up is related to a major climate change and carbon stage of the Dolomia Principale/Hauptdolomit isotope excursion, the Carnian Pluvial Event carbonate platform has been investigated in the (CPE; Simms & Ruffell, 1989; Dal Corso et al., eastern Southern Alps (Fig. 1) by analysing sev- 2015). During the CPE, microbial carbonates were eral sections with multi-disciplinary methods replaced by skeletal grains and ooids, and carbon- including facies analysis, biostratigraphy and ate platforms could not achieve a substantial sequence stratigraphy. depositional relief (Stefani et al., 2010; Gattolin et al., 2015). At the end of this event, a single, continental-scale carbonate platform developed GEOLOGICAL SETTING nearly everywhere in Western Tethys during the Late Carnian and Norian/Rhaetian. The platform system has been analysed in the The Dolomia Principale/Hauptdolomit is one western Julian Alps and eastern Carnia (north- of the most known lithostratigraphic units of the east Italy and north-west Slovenia; Fig. 1) that Alpine Upper Triassic (Guembel, 1857; Lepsius, are part of the eastern Southern Alps fold and 1876; Bosellini, 1967; Fruth & Scherreiks, 1984; thrust belt (Castellarin et al., 2006). The investi- Bosellini & Hardie, 1985; Carulli et al., 2000; gated area represents one of the best places to Berra et al., 2010, and references therein), link the almost barren Upper Carnian inner plat- mainly representing a monotonous thick succes- form system with its eteropic, biostratigraphi- sion of inner platform facies related to this wide cally well-constrained, basinal counterpart epicontinental platform bordering the Triassic (Fig. 2), and the margin system is indeed Tethys. exposed for more than 5 km in a ESE–WNW It is recorded from the Southern Alps to the direction. Despite the fact that the whole region Austroalpine nappes (for example, Northern Cal- is affected by polyphase tectonics resulting in a careous Alps, Drau Range; Tollmann, 1977, complex structural framework, the study area 1980; Mandl, 2000), as well as in the Trans- falls into a single tectonic unit in which dis- danubian Range (Fodolomit;} Kovacs et al., 2011; placements are relatively limited. Major faults Haas, 2012; Haas et al., 2015) and External bounding the unit are represented by the (cur- Dinarides–Albanides (Glavni/Main Dolomite; rently) dextral strike-slip Fella–Sava Line to the Vlahovic et al., 2005; Dozet & Buser, 2009). north, and by the Val Resia–Val Coritenza fault Despite its vast extent, the margin to slope system to the south and to the east as the hang- facies of the Dolomia Principale/Hauptdolomit ingwall (Ponton, 2010; Gale et al., 2015). © 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists, Sedimentology, 65, 1097–1131 The start-up of the D.P. carbonate platform 1099 Fig. 1. Location of investigated area and simplified structural framework of the eastern Southern Alps. (A) The area is part of a single thrust-unit and tectonic displacements inside it are quite limited. Analysed sections are displayed in the enlargement below. (B) After Assereto et al. (1968), Bianchin et al. (1980), Venturini (2002), Pon- ton (2010), Zanferrari et al. (2013) and Gale et al. (2015). MATERIALS AND METHODS maps from literature were missing or incorrect (for example, Fig. 3). The start-up stage of the Dolomia Principale/ A preliminary macrofacies analysis was carried Hauptdolomit carbonate platform was investi- out on the field for each geological section, using gated in detail by measuring ten composite a centimetre to decimetre-scale resolution. Micro- stratigraphic sections placed along the ideal facies analysis has been performed in the most platform-dip profile (Fig. 1; Table S1) and by interesting intervals, with 155 thin sections anal- analysing the depositional record with a multi- ysed by transmitted light microscopy. In strongly disciplinary approach. In order to support the dolomitized samples, thin section images have physical correlation of stratigraphic sections; been post-processed by filtering and/or modifying which are scattered over a ca 150 km2 area, the band colour curves in order to improve the visi- Mesozoic cover was mapped where geological bility of sedimentological textures. © 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists, Sedimentology,