Facies and Late Triassic Fossils in the Roisan Zone, Austroalpine Dent Blanche and Mt Mary-Cervino Nappe System, NW Alps
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Swiss J Geosci (2016) 109:69–81 DOI 10.1007/s00015-016-0207-6 Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps 1 1 2 3 Gloria Ciarapica • Leonsevero Passeri • Franco Bonetto • Giorgio Vittorio Dal Piaz Received: 11 August 2015 / Accepted: 7 January 2016 / Published online: 12 February 2016 Ó Swiss Geological Society 2016 Abstract The Roisan zone is a metamorphic cover unit Roisan zone could have been deposited in a more distal exposed along the ductile shear zone between the Dent area. Blanche s.s. and Mont Mary-Cervino Upper Austroalpine outliers, Aosta Valley, north-western Italian Alps. It is Keywords Austroalpine covers Á Sedimentology Á characterized by the occurrence of dolostones, pure mar- Stratigraphy Á Foraminifers Á Dasycladales bles, marbles with quartz, calcirudites and ophiolite-free calcschists. Locally, dolostones preserve alternances of thick massive beds and thinner levels of planar stromato- 1 Introduction lites and other sedimentary structures and textures typical of a carbonate platform. In Mt Grand Pays they contain The Roisan zone is the most extended sedimentary cover unit Dasycladales and foraminifers referable to the Norian. Pure associated to the Austroalpine Dent Blanche and Sesia– marbles and marbles with quartz grains are tentatively Lanzo nappe system, the upper part of the subduction-related referred to the end of Triassic–Early Jurassic, thin-bedded collisional wedge in the northwestern Alps. As shown in marbles and calcirudites to the Early and Middle Jurassic, Fig. 1, classic Argand’s (1908, 1909) Dent Blanche fold- calcschists from Middle Jurassic to Late Cretaceous. This nappe is currently replaced by a lot of Austroalpine outliers Roisan succession is quite similar to the one of Mt Dolin, derived from the distal part of the Adria passive continental in the Swiss part of the Dent Blanche nappe, where the margin, ribbon continents or extensional allochthons (Bal- same Triassic foraminifer association has been reported. le`vre et al. 1986; Dal Piaz 1999; Dal Piaz et al. 2001, 2003, There, the fossils were found only in reworked pebbles, 2015; De Giusti et al. 2004; Manzotti et al. 2012, 2014; contained in calcirudites of presumed Jurassic age. Some Beltrando et al. 2014). Among them, two main groups of differences exist between the two successions: calcirudites second order nappes–also known as tectonic units, klippen or are abundant in the Mt Dolin and sporadic in the Roisan subnappes–can be recognized, based on contrasting meta- zone, whereas calcschists are very thick in the Roisan zone. morphic features and structural location within the nappe As consequence the Mt Dolin succession can be considered stack: (1) Upper Austroalpine outliers, represented by the settled down in the proximity of the faults related to the blueschist and greenschist facies Dent Blanche s.s., Mt pre-oceanic rifting of the Piedmont basin, whereas the Mary-Cervino and Pillonet thrust system, including the Roisan zone; (2) Lower Austroalpine outliers, represented by the Mt Emilius, Glacier-Rafray, Etirol-Levaz and other Editorial handling: W. Winkler. eclogitic basement slices, all free of cover units. The former & Gloria Ciarapica group is thrusted over the entire ophiolithic Piedmont zone, [email protected] the latter is sandwiched inside it (Balle`vre et al. 1986; Bal- le`vre and Merle 1993; Dal Piaz 1999). 1 ` ` Universita di Perugia, Piazza Universita, 06100 Perugia, Italy Sedimentary covers, where present, are usually devoid 2 Regione Autonoma Valle d’Aosta, Aosta, Italy of preserved sedimentary structures and fossils. For- 3 Accademia delle Scienze, via Maria Vittoria 6, Torino, Italy aminifers were found only in some reworked pebbles in the 70 G. Ciarapica et al. Fig. 1 Tectonic map of the Dent Blanche and Mt Mary-Cervino nappe system and surrounding units (modified from Dal Piaz et al. 2015) Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary… 71 Swiss part of the Dent Blanche Nappe system (Mt Dolin, of the Alpine derivatives from Permian granitoids and Weidmann and Zaninetti 1974). Diplopores (Dasycladales) gabbros (Diehl et al. 1952; Dal Piaz et al. 1977, 2015; Dal were also found in an unspecified zone of Valtournenche Piaz 1992a, b, 1993, 1999; Pennacchioni and Guermani by Kienast as reported by Balle`vre et al. (1986). 1993; Gardien et al. 1994; Burri et al. 1998; Bussy et al. During the 85th National Congress of the Italian Geo- 1998; Bucher et al. 2003–2004; Baletti et al. 2012; Man- logical Society we presented an extended abstract on the zotti et al. 2012, 2014; Manzotti and Zucali 2013). These first discovery of Late Triassic microfossils in the Roisan basement units are poorly to pervasively reworked by the zone (Ciarapica et al. 2010). Now we intend: (a) to give a polyphase Alpine metamorphism which is documented by full description of all the lithofacies recognised in the relics of a relative high-P imprint, intermediate between Roisan zone, (b) to define the succession of these lithofa- epidote-blueschist and glaucophane-bearing greenschist cies, (c) to compare the Roisan succession with that of Mt facies conditions (Ayrton et al. 1982; Balle`vre et al. 1986; Dolin (Valais) where the same species of microfossils were Dal Piaz and Martin 1988; Pennacchioni and Guermani found (Weidmann and Zaninetti 1974). These researches 1993;Ho¨pfer 1997; Oberha¨nsli et al. 2004; Manzotti 2011; were carried out during field and laboratory refinements of Baletti et al. 2012; Manzotti et al. 2014) of Upper Creta- the Foglio Monte Cervino (070) of the Carta Geologica ceous age (75–73 Ma in the Pillonet klippe, Rb–Sr and d’Italia at 1:50.000 scale (CARG Project; Dal Piaz et al. 40Ar/39Ar dating: Cortiana et al. 1998), and by a green- 2015), printed during the assessment of this note. schist facies regional overprint at decreasing pressure. Eocene (48–43 Ma) Rb–Sr ages have been recently pro- vided by Angiboust et al. (2014) for the Dent Blanche 2 Roisan zone and related basement units frontal thrust over the ophiolitic Combin (Tsate´) zone. The polydeformed mylonitic basement rocks intimately asso- The Roisan zone is a metamorphic, strongly transposed and ciated to the Roisan metasediments and figured in the map fragmented sedimentary unit of supposed Mesozoic age. Monte Cervino are mainly derived from igneous protoliths This meta-sedimentary unit is discontinuously exposed of the Arolla Series and minor paraschists. along the kilometric ductile shear zone between the Upper Other remnants of the Mesozoic cover are preserved in Austroalpine Dent Blanche s.s. and Mont Mary-Cervino the north-western edge of the Dent Blanche nappe (Mt outliers (Figs. 1, 2-1; Dal Piaz 1999; Bucher et al. 2003– Dolin area), over the frontal part of the Arolla unit (Hagen 2004; Manzotti et al. 2014), and extends from the Roisan 1948; Ayrton et al. 1982; Burri et al. 1998; Angiboust et al. village, north of Aosta, to the Mont Blanc du Creton (Val- 2014; Manzotti et al. 2014). These remnants consist of tournenche), through the Col de l’Arpeyssaou, Col de St. Triassic dolostones, marbles and a thick sequence of sed- Barthe´lemy, Grand Pays, Col Salve´-Tsa de Chavalary, Cima imentary breccias of supposed Jurassic age. Late Triassic Bianca-Tsan Lake and Col de Saleron area (Stutz and Mas- foraminifera were found in some dolomitic pebbles (Wei- son 1938; Diehl et al. 1952; Elter 1960; Dal Piaz 1976, dmann and Zaninetti 1974). 1992a, b; Canepa et al. 1990; Manzotti 2011; Manzotti et al. No traces of Mesozoic metasediments are reported from 2014; Dal Piaz et al. 2015). The Roisan zone is generally the eclogitic Lower Austroalpine outliers, derived from a interpreted as the former sedimentary cover of the pre-Tri- Variscan or older metamorphic basement and Permian assic lower-to-upper crust Mont Mary basement units (Elter intrusives. Conversely, debated scattered remnants of cover 1960; Canepa et al. 1990), but its primary substratum cannot units occur within the Eclogitic Micaschist Complex of the be precisely established due to the pervasive tectonic contact Sesia–Lanzo zone (Fig. 1), including quartzites, dolo- between them. Isolated minor slices reappear in the Point des stones, marbles and metapelites (Scalaro unit, Venturini Cors eastern spur (Gosso and Martinotti in Dal Piaz 1976), 1995; Rubatto and Gebauer 1997). Comparison with these Ho¨rnli ridge of the Matterhorn, and left slope of Zmutt val- Permian and/or Mesozoic cover slices is beyond the aim of ley, within the NE extension of the Mont Mary-Cervino this note and sent to future developments, as well as the nappe (Bucher et al. 2003–2004; Dal Piaz et al. 2015). classic Lower Austroalpine successions in the Central- Similar successions of supposed Mesozoic age are also Eastern Alps and related palaeostructural restoration (e.g., recorded in the Pillonet klippe (Dal Piaz 1976). Roesli 1946; Masini et al. 2011). The Dent Blanche s.s. and Mont Mary-Cervino nappes share two main pre-Triassic basement units, separated by a mylonitic contact (Stutz and Masson 1938; Pennacchioni 3 Roisan lithofacies and Guermani 1993): (1) the Valpelline Series, that is a fragment of Variscan lower crust consisting of granulite to The Roisan zone is characterized by the occurrence of amphibolite facies felsic and mafic rocks; (2) the under- various types of dolostones, marbles and ophiolite-free lying Arolla Series, a larger tectonic unit mainly consisting calcschists, which are intimately associated with grey- 72 G. Ciarapica et al. Fig. 2 Dolostones of the Roisan zone. 1 The Roisan zone (R), with lagoon-tidal flat cycles. 3 Detail of a laminated level situated between dolostones, marbles and calcschists, is interposed between the Mont thick massive banks nearby the road to Tsa de Chavalary.