The Lombardian Gonfolite Group in central Brianza (Como and Milano Provinces, Italy): Calcareous nannofossil biostratigraphy and sedimentary record of neo-alpine tectonics

Abstract

The sedimentary succession cropping out in central Brianza, in an area embracing the territories of Besana Brianza, Briosco (Milan Province), Arosio and Inverigo (Como Province), is properly ascribed to the Lombardian Gonfolite Group. It consists of prevailing turbidite sandstones and marls, with conglomerate beds occurring in the central part of the section.A precise biostratigraphic dating of the succession was obtained through the analysis of calcareous nannofossils. The occurrence of reliable index species allowed us to recognise the NN3 to NN6 nannoplankton zones (Burdigalian to Serravallian). Since in the type-areas between Varese and Como the youngest dated units of the Gonfolite Group are Burdigalian in age, the studied succession documents a more recent part in the history of the south-alpine foredeep, that might have been recorded in outcrop elsewhere in Lombardy only by the ill-dated Gurone Sandstone and Bizzozzero Mudstone (Varese area). The deformation of the studied succession seemingly requires a single event of tectonic buckling, that is reliably constrained as late Serravallian to Tortonian (late "Lombardic Phase").Provenance signals point to roughly contemporaneous exhumation of the Pennidic nappes and active stacking of thrust sheets in the uprising Southern Alps. This phase of tectonic activity took place around the Langhian/Serravallian boundary (early "Lombardic Phase"). RIASSUNTOLa successione sedimentaria affiorante in un piccolo settore della Brianza centrale, che abbraccia il territorio dei Comuni di Besana Brianza, Briosco (Provincia di Milano), Arosio and Inverigo (Provincia di Como), mostra una chiara appartenenza al Gruppo della Gonfolite Lombarda. Essa è rappresentata da prevalenti arenarie e marne in facies torbiditica, con strati conglomeratici limitati alla parte centrale della colonna stratigrafica.Una datazione biostratigrafica precisa della successione è stata possibile grazie all'analisi dei nannofossili calcarei. La presenza di specie-indice affidabili ci ha consentito di riconoscere le zone a nannoplancton dalla NN3 alla NN6 (Burdigaliano-Serravalliano). Poiché nelle località-tipo tra Varese e Como le unità più recenti riconosciute all'interno della Gonfolite sono burdigaliane, la successione studiata documenta uno stadio più recente nell'evoluzione dell'avanfossa sudalpina, che potrebbe essere stata registrata in altri affioramenti in Lombardia solo dalle Arenarie di Gurone e dalle Peliti di Bizzozzero (Varese), unità la cui datazione rimane tuttavia poco precisa. La deformazione della successione in esame sembra compatibile con un singolo evento di piegamento, che si può ricondurre in modo convincente al Serravalliano superiore-Tortoniano (tarda "Fase Lombarda").I segnali di provenienza indicano una sostanziale contemporaneità tra l'esumazione delle falde Pennidiche e l'impilamento attivo dei sovrascorrimenti nelle Alpi Meridionali in via di sollevamento. Questa fase di attività tettonica ebbe luogo attorno al limite Langhiano/Serravalliano ("Fase Lombarda" iniziale).

The Continental Crust as a Source of Sand (Southern Alps Cross Section, Northern Italy)

“…The south-vergent South Alpine belt began to form well before the intrusion of Oligocene plutons (Brack 1981;Doglioni and Bosellini 1987) and was completed in the Neogene, with accretion of orogenic turbidites as young as the Serravallian at the mountain front (Castellarin et al 1992;Sciunnach and Tremolada 2004).…”

Precollisional development and Cenozoic evolution of the Southalpine retrobelt (European Alps)

“…Since the early Burdigalian, deformation has also affected the Cenozoic clastic units. Folding and N-vergent back thrusting occurred south of Lake Como (Sciunnach and Tremolada, 2004) and in the Varese-Ticino areas (Bernoulli et al, 1989;Malusà et al, 2011a). Folds and thrusts affecting the Gonfolite clastic wedge are in turn unconformably covered by Messinian ("Ghiaie di Sergnano"; Fantoni et al, 2004) and Pliocene (Felber et al, 1994) fluvial conglomerates and marine sediments that mark the end of late Neogene shortening.…”

The Monte Orfano Conglomerate revisited: stratigraphic constraints on Cenozoic tectonic uplift of the Southern Alps (Lombardy, northern Italy)

“…Although the MOC strongly deviates from the average petrography and sedimentology of other GLG formations (compare with Gunzenhauser 1985;Gelati et al 1988;Carrapa and Di Giulio 2001;Sciunnach and Tremolada 2004), its paleotectonic significance and age are the same. Also the MOC represents, in fact, a coarse clastic formation deposited after Alpine collision in the Oligo-Miocene foreland basin at the front of the Southern Alps, later displaced by Middle to Late Miocene thrusting.…”

“…Timing of tectonic evolution and deformation in the Southern Alps can be constrained by studying the clastic sediments of the South-Alpine foredeep, that recorded the uplift of the Alps at its southern margin for nearly 20 My, from the Oligocene to the latest Miocene (Pieri and Groppi 1981;Dalla et al 1992;Schumacher et al 1997;Sciunnach and Tremolada 2004).…”