Monegato, Giovanni

The Messinian succession of the Southern Alps is widely exposed in the Venetian-Friulian Basin, whilst more westwards in the Po Basin only isolated outcrops are preserved in the Lake Garda and Lake Como areas. The Alpine Messinian deposits are characterized by the lack of evaporites and Lago-Mare type sequences and, because of the lacking of marine deposits, their chronology is still loosely constrained. Pliocene marine deposits are located in very confined outcrops at the outlet of the main valleys, from Friuli to Piedmont. New investigations concerning many of these successions have yielded new chronological constraints for the stratigraphy of the upper Neogene in the Southern Alps area, as well as new provenance data, supporting the reconstruction of the drainage network evolution in response to the Messinian Salinity Crisis (MSC). In Friuli, deposits of early Messinian age exposed along the Tagliamento River show a petrographic evolution across the Messinian-Pliocene unconformity. The occurrence of clasts from the Paleozoic successions of the Carnic Alps (up to 15%), completely lacking in the Messinian deposits, pinpoints to a widening of the drainage basin during the MSC because of stream piracy and tectonic activity. Deposition continues with alluvial fan facies within the Tagliamento Valley and in the upper piedmont plain until the Late Pliocene. In the central Venetian sector, new provenance analyses on the Messinian conglomerate suggest a drastic drainage network reorganization, as a consequence of the MSC fluvial entrenchment. The Pliocene deposits at Cornuda, next to the present-day valley mouth of Piave River, indicate that the outlet of the river was still confined at that time several km to the east, and the southwestwards migration to the present location took place only later, during the Pleistocene. Westwards, in the catchment of the Brenta River an increase of crystalline rock fragments in the gross composition (from 30 to 60 %) is observed and interpreted as the enhancement of the crystalline basement exhumation, triggered by the system response to the sea level drop. In the Lake Garda area, new investigations on the Sirmione continental conglomerate, doubtfully ascribed to the Messinian, shows the existence of three superimposed units, different in petrographic composition and sedimentology. The lower body consists of poorly-sorted, matrix-supported, coarse-grained gravels, with blocks exceeding the 0.5 m size; sand and gravel petrographic analyses point to a provenance from the Adige catchment, with porphyries locally reaching the 80% of the whole petrographic composition in the pebble fraction. The middle unit displays clast-supported, crudely-bedded gravels with a better sorting and enrichment in carbonate pebbles (up to 70%). The upper body is totally characterized by well-sorted limestone and dolostone pebbles, pointing to a drainage confined in the Southern Alps. The geologic survey allowed to detect several clayey silt layers interbedded in the conglomerate lower and middle units, that have been sampled for pollen analyses. On the western bank of Lake Garda, the Mt. San Bartolomeo succession consists of Messinian?-Early Pliocene continental conglomerate with Melanopsis, overlain by Pliocene shallow-water marine clays and an upper conglomerate unit. The lower conglomerate is clast-supported and poorly-sorted and suggests alluvial fan/intra-valley deposition settings, locally confined. Calcareous nannofossil analyses allowed to refer the marine clays to the late Zanclean (biozones NN14 and NN15). Shallow-water marine settings have been inferred also for the upper conglomerate on the basis of the occurrence of marine algae (Tasmanaceae and Prasinophyceae) and stratigraphic data suggest an heteropic relationship with the marine clay. Petrographic analyses point to a local provenance for the whole Mt. San Bartolomeo succession, with dominant carbonates and subordinate porphyries and volcanoclastic rocks in the pebbles. The overall gathered data allow to cast light on the geomorphologic response of the fluvial systems to the MSC in the Venetian-Friulian Basin and on the Late Miocene paleodrainages in the Lake Garda area. In the Venetian-Friulian Basin the stratigraphic record documents the strong rejuvenation of the Tagliamento and Brenta rivers’ headwaters, leading up to a stream piracy and to the basement exhumation increase, respectively. Taking into account the post-Messinian tectonic shortening and the former Messinian coastline position, we can estimate roughly in the order of 0.1 m/yr the upstream migration of the erosional signal triggered by the MSC dramatic sea level drop (~1500 m). More westwards, in the Lake Garda area high percentage of porphyries in the Sirmione conglomerate suggests that the Adige River flowed in Late Miocene times along Lake Garda valley with likely the Chiese River as tributary of the main trunk. The huge sand-rich bodies observed in the Po Plain subsurface and related to the Garda entry-point could be therefore referred to the Adige River catchment.

The Central Apennines represents a wonderful record of a still active complex stratigraphic and structural history, as testified by recent earthquakes (L'Aquila 2009, Mw 6.1; Amatrice-Norcia 2016, Mw 6.5). Normal faults activity contributed to the formation of tectonic basins that hosted continental deposits, which recorded the long term traces at surface of large earthquakes. Moreover local seismic amplification has been observed, as result of peculiar geological and structural conditions. The field trip will illustrate examples of earthquake-induced effects due to the recent seismic sequence, in relation to the local stratigraphic and structural setting. The main steps of the geological evolution of Central Apennines, from Jurassic up to the Present, will be investigated which will help define contrasting responses to earthquakes between the cover terrains and the seismic bedrock. Evidences for regional uplift and active faulting, which interplayed with Quaternary climatic changes to feed the mountain chain into its present shape will be illustrated. The effects of recent earthquakes on towns and villages will be described considering the contributon of sediments characterization and the results of the recent advanced seismic microzonation studies.

Moulin and Benedetti (2018), https://doi.org/10.1029/2018tc00495810.1029/2018tc004958 present a new interpretation of the Neogene-Quaternary tectonic evolution of the Eastern Southern Alps (ESA) in Friuli. After the reinterpretation of literature field data by means of remote sensing analysis (Digital Elevation Model interpretation), they calculated deformation rates of the tectonic structures through age interpretation of geomorphological surfaces of the Veneto-Friuli piedmont plain. The authors linked the result of surface analysis to the thrust and fold architecture of the ESA basing on the Castellarin et al. (2006), https://doi.org/10.1016/j.tecto.2005.10.013 interpretation of TRANSALP project and the Friuli geological map at the scale 1:150,000 (Carulli, 2006). Discussing their new architecture of the ESA, the Authors finally yielded rates of Europe-Adria plates convergence and suggest fragmentation of Adria over the last 1–2 Ma. The present comment is aimed at discussing several critical points concerning: the use of the geomorphological and chronological data; the misinterpretation of the Digital Terrain Model; the reconstruction of the balanced geological cross section. Moreover, the application of a structural model defined in a certain area to another without considering peculiar structural complexities available in the literature results is geologically and methodologically questionable.

The end product of a three years long research project on the distribution of Messinian age sediments in Italy, carried out by the Operational Unit 5.2.10 of the Progetto Finalizzato Geodinamica CNR, is represented by a data bank and a series of graphic outputs published in 1983 in a 467 pages long volume. Publication n. 514 described 610 subsurface sections from commercial wells and 245 measured sections from land. Outputs included graphic logs and a number of maps, originally produced at the scale of 1:1.500.000, showing the location of the sections or wells, the presence of the various units identified, and their thickness. Maps showing numerical data were presented as mean values per surface unit, each unit being 10’ x 10’ wide (~320 km2). Most of the data presented, with special reference to the commercial wells, were unpublished. But now, 25 years later, they may well be discussed openly, revealing their terrific geodynamic implications. The short duration of the Messinian Salinity Crisis (MSC), now astrochronologically calibrated with unprecedented precision (initiation of the crisis at 5.96 Ma, intra-Messinian unconformity at 5.62, initiation of the Lago-mare biofacies at 5.41 Ma, termination of the crisis at 5.33 Ma) in agreement with the 0.5 my estimated in 1983, is such that these computer generated maps can still provide a number of paleogeographic information and contain a strong geodynamic message. Important variations in thickness are recorded in the various lithologic units referred to the Messinian. Total thickness ranges from negative, where erosional gaps exist (i.e. at the foot of the Alps) to over 1000 m. High thicknesses are recorded in two different situations: at the depocenter of backarc and wedge-top evaporitic basins and in the Apennine foredeep, where sedimentation was essentially not evaporitic and/or with clastic gypsum. The top of the Messinian formations (the Miocene/Pliocene boundary) documented in wells and land outcrops has a vertical range in excess of 7000 m. The minimum elevation recorded is -5365 m a.s.l. in the area of the Po delta. The maximum elevation is 1806 m a.s.l. in central Apennines. In Sicily the elevation of the top of the Messinian (base of the Pliocene) ranges from -1156 m to 721 m a.s.l. (mean value per surface unit). A number of new information is now available as a result of exploration and/or production wells and surface exposures obtained in the last 25 years. We present an updated view for the northern Italy limited to the Po Pain and the southern border of the Alps. ENI geologists greatly contributed to the subsurface geology thanks to the interpretation of over 30000 km of seismic profiles and 1800 wells. New contributions include: − individuation of superimposed buried paleosols, indicative of different paleoclimate conditions in the Messinian succession cored in the Malossa field − persistent occurrence of dinoflagellates of paratethyian affinity (Galeacysta etrusca zone) in the post-evaporitic Messinian − evaluation of the Messinian sea level drop reconstructed by the geometry of the depositional architecture in the Venetian basin − new stratigraphic data from land exposures in the Venetian-Friulian Basin and in the Lake Garda area From a geodynamic point of view, the Po basin is the foredeep of the Apennine chain (accretionary complex) but it has been also the foreland basin of the south-verging Alps. In a broader geodynamic perspective the Mediterranean is a small ocean basin, that as a result of plate motions lost its connections with the Indian Ocean in Middle Miocene times, becoming an W-E elongated gulf tributary of the Atlantic Ocean. Being surrounded by orogenic belts active in Neogene times and crossed by the Maghrebian-Apenninic chain where it reaches its greater N-S width (~1200 km), the Mediterranean basin behaves as an amplifier of the climatic signal with occasional catastrophic episodes, as the MSC. The rate of deposition during the short-lived stage of maximum dessication was three order of magnitude greater than both prior and after the crisis. This is by far the greatest sea level drop registered in the entire history of our planet (1500 m in a few thousands years) causing the deposition of one million km3 of salts, the annihilation of the entire marine fauna living in the Mediterranean basin prior to its dessication, the deep entrenchment of the major rivers that had to adapt their course to the substantial change undergone by base level of erosion, the creation of erosional surfaces on the passive-type basin margins (as the south-verging Alps in Late Miocene times). The Adriatic sea is now the shallowest basin of the Mediterranean, but in Messinian times it was the deep, rapidly subsiding depocenter of the Apenninic foredeep. Its NW prolongation extended as far as the foot the Western Alps arc, in the Piedmont basin, some 600 km far from the present day coastline. There, the marine fossiliferous sediments of early Messinian and early Pliocene age indicate bathyal depths, but Messinian evaporites are recorded (in outcrops and/or in wells) only in the Apennine side. The Alpine margin is conversely characterized by erosional surfaces (sequence boundaries) and lacustrine sediments. The hydrologic budget that is now, and supposedly was in Messinian times, strongly negative in the Mediterranean basin, could well be positive in this northernmost portion, where the Alpine chain reached elevations even greater than the present ones, and the connection with the NNW-SSE trending sector of the Apennine foredeep was prevented by the “dorsale ferrarese” structure. The coherent picture deriving by the old, large data bank and the new acquisitions suggest that a lake was in existence in the depocenter of the Apenninic foredeep, with some connections with the Paratethyan basins to the east. The water table of the lake was some hundreds (300-500 m) meters below the level of the global ocean. Sedimentary composition indicates that the source area was from the north, not from the west (as it is nowadays). The same is true for the northern Adriatic, where provenance of the clastics is from the north (Eastern Alps), not from the west (Western and Central Alps). Recent studies proved that the influence of the Po drainage system likely started only in the early-middle Pleistocene. Lithogenesis, orogenesis, and morphogenesis are different processes that in general occur in successive phases. The Messinian events are so drastic and short-lived that their reconstruction requires a number of precise observational data but also a broad 3D perspective. Isopach map reconstructions are planned to reach the goal of a fully acceptable interpretation.

Le indagini svolte nel settore gardesano meridionale evidenziano la persistenza della tettonica attiva nel Quaternario superiore legata alla compless interazione tra strutture ad andamento scledense e ad andamento giudicariense.

We carried out new geological, morphotectonic, geophysical and paleoseismological investigations on the Meduno Thrust that belongs to the Pliocene-Quaternary front of the eastern Southern Alps in Friuli (NE Italy). The study area is located in the Carnic Prealps, where a series of alluvial terraces, linked to both climatic and tectonic pulses characterises the lower reach of the Meduna Valley. In correspondence of the oblique ramp of the Meduno Thrust, the Late Pleistocene Rivalunga terrace shows a set of scarps perpendicular to the Meduno valley, often modified by human activity. In order to reconstruct the tectonic setting of the area and identify the location for digging paleoseismological trenches, integrated geophysical investigations including electrical resistivity tomography, seismic refraction and reflection, ground penetrating radar and surface wave analyses (HVSR, ReMi and MASW), were carried out across the scarps of the Rivalunga terrace. Geophysical surveys pinpointed that in correspondence of the oblique ramp, stress is accommodated by a transpressive thrust system involving all the seismo-stratigraphic horizons apart from the ploughed soil. Trenching illustrated the Meduno Thrust movements during Late Pleistocene-Holocene. Trenches exhibited both shear planes and extrados fracturing, showing deformed alluvial and colluvial units. 14C datings of the colluvial units show that the most recent fault movements occurred after 1360 CE and 1670 CE. The age of the deformed stratigraphic units compared with the earthquakes listed in current catalogues, suggests that the 1776 earthquake (Mw 5.8, Io = 8–9 MCS) could represent the last seismic event linked to the Meduno thrust activity. This study provided new quantitative constraints improving seismic hazard assessment for Carnic prealpine area.