Voltzia Recubariensis from the Uppermost Angolo Limestone of the Bagolino Succession (Southern Alps of Eastern Lombardy, Italy)
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Geo.Alp, Vol. 10 2013 61 - 70 Voltzia recubariensis from the uppermost Angolo Limestone of the Bagolino succession (Southern Alps of Eastern Lombardy, Italy) Peter Brack1 & Evelyn Kustatscher2 1ETH Zurich, Departement Erdwissenschaften, Sonneggstrasse 5, CH-8092 Zurich, Switzerland, e-mail: [email protected] 2 Naturmuseum Südtirol, Bindergasse 1, 39100 Bozen, Italy and Departement für Geo- und Umweltwissenschaften, Paläontologie und Geobiologie, Ludwig-Maximilians-Universität München, e-mail: [email protected]; Abstract Bagolino is the Global Stratotype Section and point for the base of the Ladinian Stage, well known for its ammonoids, bivalves and conodonts, while plant remains have so far not been described. Recently a conifer shoot of Voltzia recubariensis was found in the Anisian Angolo Limestone of the Rio Riccomassimo section east of Bagolino. This discovery opened a taxonomic discussion. A detailed literature research confirmed the validity of the name Voltzia recubariensis against V. agordica. The plant remain confirms also the presence of emerged land near Bagolino during the Anisian, probably to the south. It confirms also that V. recubariensis was one of the most common taxa of the Anisian of the Southern Alps and suggests that the flora of the western border area of the Tethys ocean must have been quite uniform. 1. Introduction The Anisian stratigraphic successions of eastern (Brack et al., 2005). Only few ammonoid-bearing Lombardy and Giudicarie rank among the clas- horizons are known also from older Anisian stra- sic localities since the early research on the Al- ta, i.e. from the uppermost part of the Angolo Li- pine Middle Triassic (Lepsius, 1878; Bittner, 1881; mestone (Monnet et al., 2008). Hitherto even less Mojsisovics, 1882). Additional stratigraphical and frequent in the Anisian successions of this area paleontological results were gained during sub- are findings of sufficiently well preserved plant re- sequent studies, especially in the last 50 years mains. So far only Bittner (1881, p. 247) mentions (e.g., Assereto, 1963, 1971; Assereto & Casati, “Voltzia recubariensis Massal. spec.” from nodular 1965; Gaetani 1969, 1993; Balini, 1998; Balini et al., limestones with brachiopods below the village of 1993; Brack & Rieber, 1986, 1993; Brack et al., 1999; Prezzo. Unfortunately no drawing or figure exists Mietto et al, 2003). As a result of this research, the of Bittner’s specimen, thus the attribution could Global Stratotype Section and point for the base not be verified. of the Ladinian Stage has been established in In this paper we describe a Voltzia recubariensis the stratigraphic succession at Bagolino (Brack shoot from the Anisian (uppermost Angolo Lime- et al., 2005). The main fossil taxa described from stone) of the Rio Riccomassimo section east of the upper Anisian to upper Ladinian units (Prez- Bagolino. This is the first unequivocal report of zo Limestone, Buchenstein Formation, Wengen this species also from the western Southern Alps, Formation) include ammonoids, thin-shelled but its distribution hitherto remains restricted to “pelagic” bivalves (e.g., Daonella) and conodonts the Anisian of the Southern Alps. 61 2. Geological setting The Middle Triassic sediments of eastern Lom- between and northwest of these areas are thrust- bardy and Giudicarie (Fig. 1) are part of the S- and ed and folded. Along the borders of the Adamello SE-directed fold and thrust belt of the Southern batholith they were cut and metamorphosed dur- Alps. Internal deformation varies strongly in these ing the emplacement of the Eocene-Oligocene rocks but largely undeformed stratigraphic suc- magmas. Another array of Triassic rocks is pre- cessions are exposed along the western flank served south of a prominent thrust-related struc- of lower Val Camonica and also around Dosso tural high between Val Camonica and Giudicarie. dei Morti in Giudicarie. The Triassic successions The uplifted portion north of the Val Trompia Line Figure 1: Geological overview map of the Brescian Prealps and adjacent areas with classical localities for the middle/upper Anisian (Middle Triassic) stratigraphy in eastern Lombardy-Giudicarie (modified after Brack et al., 2005). The black frame marks the Bagolino area as shown in Fig. 2 (big circle: Rio Riccomassimo section). 1) pre-Permian basement; 2) Permian to Lower Triassic including lowermost Anisian units; 3) mainly lower/middle Anisian units (Angolo Lst., Camorelli-Dosso dei Morti–M. Guglielmo Lst.); 4) upper Anisian–Ladinian pelagic successions (Prezzo Lst., Buchenstein and Wengen Fms.); 5) Ladinian/Carnian platform carbonates (Esino Lst., Breno Fm., Gorno Fm.) and age equivalent intra-platform deposits (Pratotondo Lst., Lozio Shales); 6) Ladinian/Carnian shallow intrusive rocks; 7) Norian–Rhaetian shallow water carbonates and basinal equivalents; 8) Tertiary Adamello plutonics; 9) major tectonic lines (faults and thrusts). 62 Geo.Alp,Vol.10 2013 Figure 2 : Geological sketch map of the vertical to slightly overturned Middle Triassic succession in the surroundings of Bago- lino (see Fig. 1 for location of map). The GSSP site marking the base of the Ladinian Stage is indicated. The Rio Riccomassimo section (circle) represents the easternmost outcrops of Middle Triassic rocks south of the Val Trompia Line. is called the “Massiccio dei Laghi”. Immediately classical Dosso Alto section, known since Lepsius south of this fault follow generally south-dipping (1878). Further east the upright to overturned Permian to Triassic sediments. succession is dissected by a system of steeply dip- The Triassic sediments of Bagolino are located ping conjugate faults. Displacements along these south of the Val Trompia Line and belong to a ca. faults range between several hundred metres 3 km thick and WSW-ENE trending prism of steep- and a few centimetres (visible, e.g., in the GSSP- ly dipping to slightly overturned strata. The strati- outcrop in the Caffaro riverbed at Romanterra). graphic succession ranges from Upper Permian Close to the hamlet of Riccomassimo the east- redbeds (Verrucano Lombardo) in the north to a ernmost preserved Middle Triassic sediments are thick portion of upper Triassic Dolomia Principale truncated by a fault. A possibly original continua- in the south (Fig. 2). tion of these strata is found northeast of Ponte di In the western part of this prism, the Middle Trias- Cimego, in a position displaced by more than ten sic units dip around 60° towards SE and host the kilometres along the Giudicarie Line s.s. (Fig. 1). 62 Geo.Alp,Vol.10 2013 Geo.Alp,Vol.10 2013 63 3. The Riccomassimo section east of Bagolino Along the Rio Riccomassimo creek, between its been observed recently in debris from the lower junction with the Caffaro River (at ca. 500 m asl) part of the formation. and the road connecting Riccomassimo with In the sections around Bagolino a prominent Bagolino (at 870 m asl), the easternmost Middle brachiopod lumachella occurs at the top of the Triassic sediments south of the Val Trompia Line Angolo Limestone (at Rio Riccomassimo this layer are exposed (Fig. 3). The section is overturned and can be identified in the topographically deepest dips ca. 60° towards northwest. Stratigraphically outcrops west of the creek). The fauna of the ca. it ranges from the Anisian Angolo Limestone to 2–4 metres thick “brachiopod bed” is dominated the Upper Triassic Dolomia Principale. Steeply dip- by articulated and broken valves of Coenothyris ping Permian sediments and volcanic rocks occur vulgaris. In the area including Val Trompia, Val north of a reverse fault limiting the Triassic succes- Sabbia and Giudicarie, this conspicuous interval sion at the level of the Bagolino – Riccomassimo can be traced over a few tens of kilometres. This road. The sediments at the transition of the Car- level seems to coincide with a rapid deepening nian Val Sabbia Sandstone / San Giovanni Bianco of the originally rather shallow depositional en- Fm. and the basal Dolomia Principale are also cut vironment (e.g., Monnet et al., 2008). by a supposedly northward dipping reverse fault. Above the “brachiopod bed” a succession of pe- The stratigraphic succession between these faults lagic strata comprises the Prezzo Limestone and has remained relatively intact. Because of the the Buchenstein Formation. The limestone and dense vegetation the exposures on the forested shale layers of the Prezzo Lst. are nodular in the slopes as well as along Rio Riccomassimo are best lower part and more evenly bedded in the upper visible during winter or in early spring. section. Siliceous nodular limestone layers with The Middle Triassic units of the Rio Riccomassimo only thin marly partings are characteristic for the section are comparable with the well exposed Buchenstein Formation. As in Romanterra the Bu- successions at Romanterra and Dosso Alto, a few chenstein Fm. hosts the same conspicuous green- kilometres further west (Fig. 2). ish volcanoclastic layers known as “Pietra verde”. The visible portion of the Angolo Limestone at In eastern Lombardy and Giudicarie the Anisian- Rio Riccomassimo reaches around 300 m and cor- Ladinian pelagic succession (Prezzo Lst., Buchen- responds to the upper two thirds of the complete stein Fm.) is known for its ammonoid faunas. The succession at Dosso Alto. There the formation is base of the Ladinian Stage is located in the lower ca. 500 metres thick and follows on top of lower- part of the Buchenstein Formation. (for further most Anisian evaporates (Carniola di Bovegno). information see Brack et al., 2005). West of Rio Riccomassimo the Angolo Limestone Siliciclastic layers of the Wengen Formation are can be examined along a trail following a water more than 40 m thick at Romanterra. In the Rio pipe at ca. 750 m level north of “Paradisi” (Fig. 3). Riccomassimo section, as well as at Dosso Alto It consists of irregularly thick bedded and partly only a few of these layers occur. In both sec- nodular, strongly bioturbated limestone layers. tions the Wengen Formation is largely replaced Marly intercalations are usually thin but there by slope deposits of a carbonate platform (Esino are also intervals with up to 5 cm thick marls.