Discussion on the article “Remarks on the Permian-Triassic transition in Central and Eastern Lombardy (Southern Alps, Italy)” by G. Cassinis, M. Durand and A. Ronchi

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

The Monte Orfano Conglomerate (MOC), exposed in the foothills of the Southern Alps (northern Italy), is one of the few outcrops of sediments documenting the Cenozoic tectonic evolution of the Alpine retrowedge. Calcareous nannofossil biostratigraphy allowed us to constrain the upper part of the MOC, formerly attributed to the Early-Middle Miocene in the type-locality, to the earliest Miocene (Neogene part of the NN1 nannofossil zone). A likely latest Oligocene age is therefore suggested for the bulk of the underlying conglomerates, whose base is not exposed. Deposition of the MOC can be framed into the post-collisional tectonic uplift of the Alps, documented in the Lake Como area by the Como Conglomerate (CC) at the base of the Gonfolite Lombarda Group, and supports the correlation with Upper Oligocene clastic sediments cropping out further to the East, in the Lake Garda and in the Veneto-Friuli areas (“molassa”). The remarkable difference in petrographic composition between the western (CC) and eastern (MOC) clastics deposited in the Alpine retro-foreland basin highlights the synchronous tectonic activity of two structural domains involving different crustal levels. Whilst the bulk of the CC, that straddles the Oligocene/Miocene boundary, records the tectonic exhumation of the Alpine axial chain crystalline complexes, the coeval MOC consists of detritus deriving from the Alpine retrowedge superficial crustal section (Triassic to Paleogene sedimentary rocks), and constrains the onset of the post-collisional deformation phase of the Southern Alps as not younger than the latest Oligocene

Reassessing the biostratigraphy and the paleobathymetry of the Gonfolite Lombarda Group in the Como area (northern Italy)

Calcareous nannofossil and foraminiferal analyses have been carried out on outcrops from the type-area of the Gonfolite Lombarda Group (Como, northern Italy). In these marine fine- to coarse-grained clastics, rapidly accumulating at the southern front of the uprising Alpine range during the Oligo-Miocene, a scarce, but reliable, sequence of calcareous nannofossil events has been observed, allowing to refine the previous age assignments. Planktonic foraminifera were found to be extremely rare and provided limited biostratigraphic information. The Villa Olmo Conglomerate and the Chiasso Formation contain the Last Occurrence (LO) of Sphenolithus distentus and the First Occurrence (FO) of Triquetrorhabdulus carinatus, which are characteristic of the nannofossil zones NP24 and NP25 (Chattian), respectively. The lower part of the Como Conglomerate was deposited during the zone NP25, whilst the upper part of the Como Conglomerate straddles the Chattian/Aquitanian boundary in zone NN1. The deposition of the Prestino Mudstones also occurred during zone NN1. However, the upper part of this formation has been dated as Burdigalian during nannofossil zone NN2. The deposition of the upper part of the Val Grande Sandstone has been assigned to the NN3 zone owing to the presence of the taxon Sphenolithus belemnos, which is restricted to NN3. The upper part of the investigated section is characterized by the deposition of the Lucino Conglomerate and its fine-grained members (Lucinasco and Lurate Caccivio Mudstones). The Lucinasco Mudstones have been dated as late Burdigalian corresponding to zone NN4, whilst the overlying Lurate Caccivio Mudstones were deposited during the Langhian part of the zone NN5, based on the presence of S. heteromorphus and the absence of H. ampliaperta. On the whole, the base and the top of the outcropping Gonfolite Lombarda Group result from our study to be younger than hitherto proposed, allowing to resolve certain previous conflicts with the few radiometric dates available for clasts from the Gonfolite Lombarda Group. The depth of deposition was upper bathyal during the Chattian and the Aquitanian and shallowed to neritic during the deposition of the Langhian Lurate Caccivio Mudstones

Pennsylvanian-Early Triassic stratigraphy in the Alborz Mountains (Iran)

New fieldwork was carried out in the central and eastern Alborz, addressing the sedimentary succession from the Pennsylvanian to the Early Triassic. A regional synthesis is proposed, based on sedimentary analysis and a wide collection of new palaeontological data. The Moscovian Qezelqaleh Formation, deposited in a mixed coastal marine and alluvial setting, is present in a restricted area of the eastern Alborz, transgressing on the Lower Carboniferous Mobarak and Dozdehband formations. The late Gzhelian–early Sakmarian Dorud Group is instead distributed over most of the studied area, being absent only in a narrow belt to the SE. The Dorud Group is typically tripartite, with a terrigenous unit in the lower part (Toyeh Formation), a carbonate intermediate part (Emarat and Ghosnavi formations, the former particularly rich in fusulinids), and a terrigenous upper unit (Shah Zeid Formation), which however seems to be confined to the central Alborz. A major gap in sedimentation occurred before the deposition of the overlying Ruteh Limestone, a thick package of packstone–wackestone interpreted as a carbonate ramp of Middle Permian age (Wordian–Capitanian). The Ruteh Limestone is absent in the eastern part of the range, and everywhere ends with an emersion surface, that may be karstified or covered by a lateritic soil. The Late Permian transgression was directed southwards in the central Alborz, where marine facies (Nesen Formation) are more common. Time-equivalent alluvial fans with marsh intercalations and lateritic soils (Qeshlaq Formation) are present in the east. Towards the end of the Permian most of the Alborz emerged, the marine facies being restricted to a small area on the Caspian side of the central Alborz. There, the Permo-Triassic boundary interval is somewhat similar to the Abadeh–Shahreza belt in central Iran, and contains oolites, flat microbialites and domal stromatolites, forming the base of the Elikah Formation. The P–T boundary is established on the basis of conodonts, small foraminifera and stable isotope data. The development of the lower and middle part of the Elikah Formation, still Early Triassic in age, contains vermicular bioturbated mudstone/wackestone, and anachronostic-facies-like gastropod oolites and flat pebble conglomerates. Three major factors control the sedimentary evolution. The succession is in phase with global sea-level curve in the Moscovian and from the Middle Permian upwards. It is out of phase around the Carboniferous–Permian boundary, when the Dorud Group was deposited during a global lowstand of sealevel. When the global deglaciation started in the Sakmarian, sedimentation stopped in the Alborz and the area emerged. Therefore, there is a consistent geodynamic control. From the Middle Permian upwards, passive margin conditions control the sedimentary evolution of the basin, which had its depocentre(s) to the north. Climate also had a significant role, as the Alborz drifted quickly northwards with other central Iran blocks towards the Turan active margin. It passed from a southern latitude through the aridity belt in the Middle Permian, across the equatorial humid belt in the Late Permian and reached the northern arid tropical belt in the Triassic

Subsurface magnetostratigraphy of Pleistocene sediments from the Po Plain (Italy): Constraints on rates of sedimentation and rock uplift

We used facies analysis to reconstruct the Pleistocene sedimentary evolution of seven cores from the central-northern Po Plain, Italy. The cores record an overall regressive sequence consisting of shallow-water marine and fluvial-deltaic deposits overlain by fully continental sediments. We used magneto-stratigraphy to date marine and fluvial-deltaic sediments to the early Pleistocene and continental sediments to the middle-late Pleistocene. Sediment accumulation rates were similar to 30-40 cm/k.y. in the early Pleistocene, whereas relevant unconformities and/or an overall reduction in sediment accumulation rates characterized the middle-late Pleistocene. A simple Airy compensation model was applied to restore actual sediments elevations to elevations at times of deposition expressed in meters above current sea level. The correlation of isostatically corrected sedimentary facies to a sea-level curve obtained from classic oxygen-isotope studies shows that an event of rock uplift on the order of similar to 70-120 m occurred in the middle-late Pleistocene. Literature studies of vegetational cyclicity, used in conjunction with the sea-level curve, allowed us to link sedimentary facies to climate variability. We propose that the onset of fully and persistently continental sedimentation occurred in response to the waxing of the first major Pleistocene glaciation in the Alps, currently correlated to marine isotope stage 22 at ca. 0.87 Ma, and that the episode of uplift occurred (at least in part) as a consequence of erosion and crustal rebound of the Alpine chain triggered by Pleistocene glacial-interglacial cycles

First report of Claraia in the Servino Formation (Lower Triassic) of the western Orobic Alps, Italy.

A fossiliferous horizon containing Claraia is reported for the first time 8 m above the base of the Servino Fm. in the we¬stern Orobic Alps (Lecco, Lombardy). The specimens have a broad morphological variability which mostly concerns the sculpture, and thus several morphospecies can be recognized [C. cf. aurita (Hauer, 1850), C. cf. bittneri Ichikawa, 1958, C. tesidea (Leonardi, 1929), C. intermedia (Bittner, 1901), C. radialis (Leonardi, 1929), C. cf. clarai (Emmrich, 1844)]. These taxa have mostly been classified into C. in¬termedia by applying a species-population concept. The Claraia horizon of the Orobic Alps can be correlated with the transitional layers between the C. clarai and C. aurita subzo¬nes of the Werfen Fm. (Siusi Member) in the Dolomites. Thus, in this area, the first Triassic marine transgression with age-diagnostic fossils of the Servino Fm. may have an age ranging from the latest Griesbachian to the early Dienerian