Orobic Basin Research Articles

Evidence of Early Permian extension during the post-Variscan evolution of the central Southern Alps (N Italy)

In the central Southern Alps (N Italy) some well-preserved Permian extensional structures, that exceptionally escaped the Alpine deformation, have been recently identified. Their analysis offers important insights for the interpretation of the post-Variscan tectonics affecting the Southern Alps during the Permian. We describe here a previously unknown fault system related to the development of the Early Permian Orobic Basin, where large exposures of the Variscan basement preserve their original tectonic contacts with the overlying Lower Permian cover. The fault system consists of Low-Angle Normal Faults (LANFs) accompanied by High-Angle Normal faults (HANFs), these last entirely developed within the volcanic, volcaniclastic and terrigenous deposits. The studied structures occurring in the upper part of the Gerola Valley, share several features with Early Permian normal faults already recognized in other areas of the central Southern Alps. The low-angle normal fault planes are characterized by a continuous layer of coarse-grained fault breccias, locally impregnated by tourmalinites. The fault rocks have been invariably observed along the fault surface at the basement-cover contact all across the study area. This newly identified fault system with its characteristic combination of low- and high-angle normal faults suggests a tectonic regime characterized by pure extension in the central Southern Alps, rather than by a transtensional regime during the Early Permian. The provided data give new insights into the Early Permian geodynamic scenario, which is discussed in light of the transition from the Pangea B to the Pangea A configuration.

Metasomatism by Boron-Rich Fluids along Permian Low-Angle Normal Faults (Central Southern Alps, N Italy)

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper Carboniferous-Permian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

Oncoids and groundwater calcrete in a continental siliciclastic succession in a fault-controlled basin (Early Permian, Northern Italy)

Lower Permian continental deposits of the fault-controlled Orobic Basin (Central Southern Alps; Northern Italy) include alluvial fan facies interfingering with muddy basin-floor deposits, consisting of three facies associations: heterolithic fine-grained siliciclastic facies, laminated sandstone facies, and oncoidal limestone facies. Besides oncoidal and microbial limestones, carbonates occur as nodules in sandy tabular beds within the laminated sandstone facies association. Microfacies analyses distinguish several types of oncoidal carbonate (consisting of an alternation of microbial carbonate and fibrous calcite) and carbonate nodules. Each type of carbonate has been characterized in terms of δ18O and δ13C. The two types of carbonate in the oncoids record a stable δ18O and a slightly varying δ13C, whereas the isotope composition of the calcite in nodules is completely different. Carbonate nodules in sandy beds of the laminated sandstone facies association have a diagenetic origin as indicated by cross-cutting relationships between nodules and lamination; the nodules are interpreted as groundwater calcrete, formed in the subsurface at the top of the unconfined water table. The exclusive sedimentation of oncoidal carbonate facies within siliciclastic deposits indicates that when oncoids were formed in ephemeral shallow ponds, siliciclastic input was minimal. The sedimentological and geochemical characteristics of the studied succession and the stable isotopic composition of the oncoids (the absence of covariance between δ18O and δ13C excludes deposition in evaporating basins) indicate persistent stable conditions for sufficient time to permit growth of centimeter-sized oncoids. Oncoids are interpreted to have formed in spring-fed ponds and outflow channels, with flowing, clean water, at the toe of major alluvial fans. Episodes of rapid delivery of sand and silt-sized sediments by flash floods, with an oscillating water table, caused the observed facies alternation. The precipitation of calcareous cements close to the water table surface produced nodules in sandy layers. Carbonate precipitation within laminated sandstone reduced porosity and permeability, causing a strong compartmentalization in the well-bedded continental succession.

Low-angle normal faults record Early Permian extensional tectonics in the Orobic Basin (Southern Alps, N Italy)

Well-preserved SSE-dipping low-angle normal faults (LANF) active during the Early Permian (Cisuralian) were recognized along the northern margin of the Orobic Basin (central Southern Alps, N Italy). These faults, which escaped most of the Alpine deformations, exhumed the Variscan basement during the deposition of the upperpart of the Lower Permian succession (Pizzo del Diavolo Formation).Fault planes show evidence of frictional processes typical of the upper crust associated with hydrothermal circulation, responsible for the deposition of cm to m thick tourmalinite and Uranium mineralization.The recognized LANFs interacted with high-angle normal faults producing half grabens that stored the Lower Permian deposits, where synsedimentary fault activity in their hangingwall is testified by abrupt vertical and lateral facies changes, thickness variations and by soft-sediment deformations. Mesoscopic structures, exposed in the hangingwall of a major LANF (the Aga-Vedello Fault system) along a synthetic high-angle normal fault, include conjugate normal faults, horst-and-graben, domino-style planar and listric faults, which clearly record synsedimentary deformations testified by liquefaction and dewatering structures, typical of pre-consolidation hydroplastic conditions. This exceptional record indicates deformations at shallow crustal level which occurred during the Early Permian along high-angle normal faults soling into the LANFs, forming the northern boundary of the Orobic Basin.The outcrop continuity, the perfectly preserved relationships among high- and low-angle normal faults together with the synsedimentary record of fault activity and the occurrence of mesoscopic faults developed during the deposition of the sediments, make this case-study an excellent reference for the analysis of extensional tectonics in synsedimentary conditions.In addition, the occurrence of large LANF systems, typical of a stress regime characterized by a vertical s1, suggests that the Lower Permian Orobic Basin was dominated by pure extension at least in the study area, alternatively to existing interpretations, which favor a transtensional origin of the basin. Strike-slip tectonics can be responsible for a later partial tectonic inversion of the basin, as testified by the angular unconformity with the overlying Upper Permian succession (Verrucano Lombardo), marking a Middle Permian stratigraphic gap.

The Variscan evolution in the basement cobbles of the Permian Ponteranica Formation by microstructural and petrologic analysis

This work reconstructs the metamorphic evolution recorded in the cobbles of the Ponteranica Formation to be compared with the evolutions of the basements of the central Southern Alps and of the clasts of Aga and Vedello and Dosso dei Galli conglomerates. The Ponteranica Formation is part of the lower volcano-sedimentary cycle of the Permian sequences of the western Orobic basin. These sequences cap the Variscan basement in which different tectonometamorphic units were recognized and contoured. We sampled all the crystalline rock types preserved as clasts such as different types of metapelites, meta-intrusives, granitoids, and tourmalinites and reconstructed the relative chronology of the superposed equilibrium assemblages. Only for metapelitic rocks the PT paths were reconstructed on the ground of mineral chemical analyses and geothermobarometry. The cobbles of the Pontereanica Formation record two types of metamorphic evolution. In Type 1 cobbles, the M1 metamorphic assemblage is characterised by white mica, biotite, garnet, staurolite, ± kyanite and is interpreted as the result of the record of the Variscan collision. This assemblage is followed by M2 assemblage characterised by white mica, chlorite, ± biotite that represents the Variscan exhumation. In type 2 cobbles, relict andalusite and possible cordierite characterise the pre-M2 metamorphic re-equilibration that developed under a high thermal regime. The PT paths recorded in cobbles are consistent with the tectono-metamorphic evolutions recorded in the basement units suggesting that during the deposition of conglomerates, the type of tectono-metamorphic units exposed to erosion were equivalent to that presently exposed in the Orobic Alps.The evolution of type 1 cobbles is similar to the evolution recorded in the clasts of Aga and Vedello conglomerates, whereas the clasts of Dosso dei Galli Conglomerate recorded only the effects of the Variscan tectonic burial. The occurrence of tourmalinite cobbles may imply an age up to Upper Permian / Lower Triassic for the Ponteranica Formation. In this view, the age of the Permian conglomerates of central Southalpine decreases westward with the increase of the thermal maturity of the metamorphic imprints recorded in clasts. These observations are consistent with a westward migration of the extensional tectonics responsible for the basin opening.

TETRAPOD FOOTPRINT PALEOECOLOGY: BEHAVIOR, TAPHONOMY AND ICHNOFAUNA DISENTANGLED. A CASE STUDY FROM THE LOWER PERMIAN OF THE SOUTHERN ALPS (ITALY)

Vertebrate tracks are linked to the depositional environment where they were formed. Several studies hypothesized a paleoenvironmental control on vertebrate track ichnocoenoses, although this issue was never analyzed thoroughly. A new study of the sedimentology and tetrapod ichnology of two key stratigraphic sections in the Pizzo del Diavolo Formation of the lower Permian continental Orobic Basin of Southern Alps (Italy) tested the link between tetrapod ichnocoenoses and depositional environment. Behavior, taphonomy, and ichnocoenoses of three different lithozones (P-PDV, U-PDVa, U-PDVb) were analyzed and compared, two new census methods for tetrapod tracks were tested (“track/slab” and “weighting size”) and the first was applied to our specimen sample. The possible biostratigraphic meaning of track occurrences/relative proportions were discussed and excluded. Results indicate a predominance of the ichnogenus Erpetopus in a distal floodplain environment (P-PDV), a diverse ichnocoenosis in a proximal floodplain environment (U-PDVa), and a predominance of the ichnogenus Dromopus in a scarcely diverse ichnocoenosis in a floodplain/marginal lacustrine environment (U-PDVb). This encourages further detailed studies on tetrapod track paleoecology, in order to refine and give utility to the concept of tetrapod ichnofacies.

Siderite deposits in northern Italy: Early Permian to Early Triassic hydrothermalism in the Southern Alps

We present a minero-petrographic, geochemical and geochronological study of siderite orebodies from different localities of the Southern Alps (northern Italy). Siderite occurs as veins cutting the Variscan basement and the overlying Lower Permian volcano-sedimentary cover (Collio Fm.), and as both veins and conformable stratabound orebodies in the Upper Permian (Verrucano Lombardo and Bellerophon Fms.) and Lower Triassic (Servino and Werfen Fms.) sedimentary sequences of the Lombardian and the Venetian Alps. All types of deposits show similar major- and rare-earth (REE)-element patterns, suggesting a common iron-mineralizing event. The compositions of coexisting siderite, Fe-rich dolomite and calcite suggest formation from hydrothermal fluids at relatively high temperature conditions (≥250°C). Geochemical modelling, supported by REE analyses and by literature and new δ13C and δ18O isotopic data, suggests that fluids responsible for the formation of siderite in the Variscan basement and in the overlying Lower Permian cover were derived from dominant fresh water, which leached Fe and C from volcanic rocks (mainly rhyolites/rhyodacites) and organic carbon-bearing continental sediments. On the basis of U-Th-Pb microchemical dating of uraninite associated with siderite in the Val Vedello and Novazza deposits (Lombardian Alps), the onset of hydrothermalism is constrained to 275±13Ma (Early–Mid Permian), i.e., it was virtually contemporaneous to the plutonism and the volcanic-sedimentary cycle reported in the same area (Orobic Basin). The youngest iron-mineralizing event is represented by siderite veins and conformable orebodies hosted in Lower Triassic shallow-marine carbonatic successions. In this case, the siderite-forming fluids contained a seawater component, interacted with the underlying Permian successions and eventually replaced the marine carbonates at temperatures of ≥250°C. The absence of siderite in younger rocks suggests an Early Triassic upper limit for the iron pulse in the Southern Alps, which would thus predate the Middle Triassic magmatism. Based on the overlap between hydrothermalism, extensional tectonics and, in part, magmatism, the genesis of siderite in the Southern Alps may be related to plutonic activity and/or magmatic underplating occurring since the Permian in a geodynamic scenario preluding the opening of the Neo-Tethys.

An unusual Amphisauropus trackway and its implication for understanding seymouriamorph locomotion

Vertebrate ichnology has proved to be a valuable tool for reconstructing tetrapod locomotion. Grounding on 1, the discovery of a new, exceptionally preserved Amphisauropus trackway (Permian Orobic Basin, Northern Italy), showing continuous tail and pedal scratch impressions; and 2, a comparative analysis of track and bony records, we provide a first synapomorphy-based attribution of Amphisauropus tracks to their most plausible producers, seymouriamorph reptiliomorphs, and propose a detailed reconstruction of the locomotion biomechanics in this group. The markedly sinuous and continuous tail impression with a relatively constant width, the curve arrangement of pedal scratches and the inferred position of head, trunk and tail during the locomotion suggest large lateral movements, while vertical movements were nearly absent, consistently with a primitive sprawling gait. We also document the functional prevalence of the hindlimbs and provide a detailed description of femur biomechanics during step cycle. Trackway parameters suggest that the speed was quite slow: a comparison with a large database of Amphisauropus trackways from Italy and Germany proves that this peculiar preservation was gait- and substrate-related, rather than trackmaker-related. In all, this study sheds light on the locomotion of Seymouriamorpha and aimed to contribute to a full integration of track and skeletal data in exploring locomotory biomechanics of extinct tetrapods.

New occurrences of tetrapod ichnotaxa from the Permian Orobic Basin (Northern Italy) and critical discussion of the age of the ichnoassociation

AbstractThe late Cisuralian (Artinskian–Kungurian) is a key time interval for Permian tetrapod evolution. In all the main low‐latitude Pangean sites, the fossil footprint record clearly shows a diversification and dispersal of non‐synapsid amniote tracks compared to the early Cisuralian (Asselian–Sakmarian). However, data on latest Cisuralian (i.e. late Kungurian) sites are quite fragmentary. A new ichnotaxonomic study was carried out in the central part of the Orobic Basin, where the highly‐fossiliferous Pizzo del Diavolo formation crops out and can potentially cover this gap in knowledge. The following tetrapod ichnogenera were identified: Amphisauropus, Batrachichnus, cf. Dimetropus, Dromopus, Erpetopus, Hyloidichnus, Limnopus, cf. Merifontichnus and Varanopus; they can be attributed to seymouriamorph and temnospondyl amphibians; pelycosaur synapsids; and diapsid, parareptile and captorhinid reptiles. Dimetropus and Merifontichnus are identified for the first time in the Permian of Italy. The ichnoassociation, in agreement with the radiometric dating of the underlying Cabianca formation, suggests a late Kungurian (latest Cisuralian) age for the Pizzo del Diavolo formation. It includes the youngest and most diverse non‐synapsid amniote ichnofauna of the Cisuralian with five different ichnogenera and possibly six ichnospecies, and thus constitutes a key area for the study of eureptile and parareptile dispersal at low latitudes of Pangaea in the time span that immediately pre‐dates the first therapsid synapsid radiation.

Stratigraphic Architecture of a Transtensional Continental Basin In Low-Latitude Semiarid Conditions: The Permian Succession of the Central Orobic Basin (Southern Alps, Italy)

Abstract Continental successions in fault-controlled extensional–transtensional basins store a wide range of clastic deposits from various depositional environments, representing a challenging target for hydrocarbon exploration. Recent advances in field-based facies analysis and genetic interpretation of these sediments show that these successions are characterized by great variability in terms of volume, geometry, facies, and stacking patterns. This variability derives from a number of factors that govern the sedimentary processes, such as tectonic setting, basin size and morphology, type and amount of sediments, and climate. The study focuses on a well-exposed Early Permian continental succession (Southern Alps, northern Italy), which was deposited in an extensional–transtensional tectonic regime under a semiarid to arid climate. The study area is characterized by a thick (more than 1000 m) succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo) Formation resting on up to 800 m of prevailing pyroclastic flows (Cabianca Volcanite). These units are unconformably covered by the Upper Permian alluvial sediments of the Verrucano Lombardo. Sedimentological features reflect a proximal fan system where mass-flow phenomena dominate, a distal fan-terminal setting with prevailing sandy sheet-flow processes and a silty floodplain environment, where sheet-flood events dominate. Laterally continuous siltstones and dark shales with carbonate nodules and layers of microbial carbonates in the most depressed part of the basin indicate alternating conditions of shallow lake and desiccated floodplain. Syndepositional tectonic activity is documented in the succession by rapid vertical and lateral facies changes. In this study, we present a data-constrained reconstruction of the depositional architecture of the basin, a description of depositional processes and depositional environments, and a model of facies distribution. Outcrop data, integrated with laboratory analyses, provide a detailed understanding not only of the small-scale elements of the system (sub-seismic scale) but also of the entire basin, focusing on how those elements are interrelated. The studied succession can be considered an interesting outcrop analogue for subsurface extensional–transtensional basins, potentially useful to predict reservoir properties for similar subsurface settings.

The Gerola Valley site (Orobic Basin, Northern Italy): A key for understanding late Early Permian tetrapod ichnofaunas

A taxonomic study has been carried out on the historical Gerola Valley locality (Pizzo del Diavolo formation, late Cisuralian), which represents one of the best tetrapod ichnosites of the whole Southern Alps (North Italy). With respect to previous studies, the ichnoassociation is now enlarged and lists the following taxa: Amphisauropus, Dromopus, Erpetopus, Hyloidichnus, Limnopus, Varanopus. The optimal preservation of the material, which in some cases shows even traces of the skin, allowed the recognition of a new morphotype of Varanopus. The invertebrate ichnoassociation is newly described and is representative of impoverished Scoyenia and Mermia ichnofacies. A detailed facies analysis of the uppermost arenitic–pelitic lithofacies of the Pizzo del Diavolo formation, which revealed the most abundant fossil content (Locality 1) and of two new localities (pelitic facies, Localities 2, 3) shows a transition from shallow lacustrine to floodplain and alluvial fan environments under a generally dry climate. Besides the taxonomic revision of vertebrate tracks, the final aims of our study are: i) the depositional, palaeoenvironmental and climatic reconstruction of this sector of the Orobic Basin through an integration of palaeontological data and facies analysis; and ii) the regional correlation of this key stratigraphic succession with other coeval ichnofaunas in Pangaea.

U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

AbstractU–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons (c. 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

Transition between terrestrial-submerged walking and swimming revealed by Early Permian amphibian trackways and a new proposal for the nomenclature of compound trace fossils

Exceptionally preserved Early Permian tetrapod trackways from the Orobic Basin (Central–Western Southern Alps) offer a unique opportunity to investigate in detail locomotion in fossil vertebrates that lived on continental European landmasses. Herein are reported the results of a study on several tetrapod trackways that display a large variety of behavioral, gait and substrate related extramorphologies. They clearly document the transition from terrestrial–underwater walking to swimming and are assigned to the compound ichnotaxon Batrachichnus C Lunichnium. The use of the “C” symbol is here introduced for the first time as nomenclatural indication of a Compound trace. Producers were probably small-sized temnospondyl or lepospondyl (microsaurs) amphibians. Comparisons with living urodelan anatomy and mechanics provide evidence for conservatism of locomotor mechanics in evolutionary history among amphibians. The derived model for locomotor kinematics in Early Permian amphibians provides a reference for interpreting transitional land-to-water trackways. The shift from walking to swimming behavior in early tetrapods, as in extant urodelan amphibians, is described as a complex balance between different dynamics.

Hyloidichnus bifurcatus Gilmore, 1927 and Limnopus heterodactylus (King, 1845) from the Early Permian of Southern Alps (N Italy): A New Equilibrium in the Ichnofauna

Studies on Early Permian tetrapod ichnofauna emphasized the scarcity of forms from Italian sites. A revision work on the entire collections revealed the presence of Hyloidichnus bifurcatus Gilmore, 1927 and Limnopus heterodactylus (King, 1845). The ichnoassociation now lists seven ichnogenera: Amphisauropus, Batrachichnus, Dromopus, Erpetopus, Hyloidichnus, Limnopus, Varanopus. These new data enlarge the ichnoceonosis, adding tracks of medium-size captorhinomorphs (Hyloidichnus) and temnospondyls (Limnopus) to the Italian ichnofauna, previously characterized by scarcity of predators and amphibians. Radiometric ages give a strong age constraint to the ichnoassociation (Early Kungurian), allowing useful correlations to contemporary successions all over the world. The main difference is the absence of Ichniotherium and Dimetropus, and this could have a stratigraphic or paleoenvironmental significance. The fauna is similar in two main basins, Collio and Orobic. It differs solely in the proportions between ichnotaxa, with a predominance of areoscelid traces (Dromopus) in the Collio Basin and of captorhinomorph traces (Erpetopus, Varanopus, Hyloidichnus) in the Orobic Basin. This datum could reflect slightly different environments, seasonal in the Collio Basin (alluvial plain) and more arid in the Orobic Basin (playa-like). The lack of some forms in smaller basins of the Athesian Volcanic Complex is probably due to a bias.

Syndepositional tectonics recorded by soft-sediment deformation and liquefaction structures (continental Lower Permian sediments, Southern Alps, Northern Italy): Stratigraphic significance

The Lower Permian succession of the Central Southern Alps (Lombardy, Northern Italy) was deposited in fault-controlled continental basins, probably related to transtensional tectonics. We focussed our study on the stratigraphic record of the Lower Permian Orobic Basin, which consists of a 1000 m thick succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo Formation, PDV) resting on the underlying prevailing pyroclastic flows of the Cabianca Volcanite. The PDV consists of a lower part (composed of conglomerates passing laterally to sandstones and distally to silt and shales), a middle part (pelitic, with carbonates) and an upper part (alternating sandstone, silt and volcanic flows). Syndepositional tectonics during the deposition of the PDV is recorded by facies distribution, thickness changes and by the presence of deformation and liquefaction structures interpreted as seismites. Deformation is recorded by both ductile structures (ball-and-pillow, plastic intrusion, disturbed lamination, convolute stratification and slumps) and brittle structures (sand dykes and autoclastic breccias). Both the sedimentological features and the geodynamic setting of the depositional basin confidently support the interpretation of the described deformation features as related to seismic shocks. The most significant seismically-induced deformation is represented by a slumped horizon (about 4 m thick on average) which can be followed laterally for more than 5 km. The slumped bed consists of playa-lake deposits (alternating pelites and microbial carbonates, associated with mud cracks and vertebrate tracks). The lateral continuity and the evidence of deposition on a very low-angle surface along with the deformation/liquefaction of the sediments suggest that the slump was triggered by a high-magnitude earthquake. The stratigraphic distribution of the seismites allows us to identify time intervals of intense seismic activity, which correspond to rapid and basin-wide changes in the stratigraphical architecture of the depositional basin and/or to the reprise of the volcanic activity. The nature of the structures and their distribution suggest that the magnitude of the earthquakes responsible for the observed structures was likely higher than 5 (in order to produce sediment liquefaction) and probably reached intensity as high as 7 or more. The basin architecture suggests that the foci of these earthquakes were located close to the fault-controlled borders of the basin or within the basin itself.

Vestiges of lost tectonic units in conglomerate pebbles? A test in Permian sequences of the Southalpine Orobic Alps

AbstractMicrostructural analysis andP–Testimates of metamorphic pebbles in Permian conglomerates of the Central Southern Alps, representing the erosion product of the collapsing Variscan chain, are the discriminating tools for determining the metamorphic sequences representing potential sources of the conglomerates. In the selected case, basement units are precisely outlined on the basis of qualityP–T–d–tpaths that allow reconstruction of their metamorphic evolutions (tectonometamorphic units); this facilitates individuation of the basement sources with much better confidence. The lower Permian volcaniclastic sequence of the Eastern Orobic Basin, which overlies the Variscan Val Vedello basement, comprises the Aga and Vedello conglomerates, which are the oldest deposits containing a considerable amount of up to metre-sized metamorphic pebbles. Microstructural and mineral chemical data on metamorphic pebbles of the Aga and Vedello conglomerates were used to infer quantitative pre-PermianP–Tevolutions, which are compared with those of the tectonometamorphic units constituting the surrounding Southalpine metamorphic basement. Two types ofP–Tevolution are recorded in the metamorphic pebbles of Aga and Vedello conglomerates: Type 1 is characterized by an amphibolite-facies imprint, followed by greenschist retrogression; Type 2 is characterized by three successive greenschist-facies re-equilibrations. The Type 1P–Tevolution of metamorphic pebbles matches with that of the adjacent tectonometamorphic unit of the Val Vedello basement. Type 2 is similar to those recorded in units B and C of the North East Orobic basement, and it differs from that of the adjacent Val Vedello basement. This suggests that the Aga and Vedello conglomerates were fed by two different basement sources: one consisting of the present day Val Vedello basement, and the other compatible with units B and C of the North East Orobic basement. According to theP/Tratios of theTmax–PTmaximprints, both basement sources recorded the Variscan collision but at a different crustal level. The age (c.278 Ma) of the Aga and Vedello conglomerates constrains the minimum exhumation age for their basement sources.

Early Permian vertebrate ichnoassociation in the Scioc Valley, Orobic Basin (Lombardy, Northern Italy)

Early Permian vertebrate ichnoassociation in the Scioc Valley, Orobic Basin (Lombardy, Northern Italy)

Permian tetrapod footprintsfrom W Orobic Basin (Northern Italy). Biochronological and evolutionary remarks

Abstract Numerous tetrapod footprints have been found in the Permian deposits of the Western Orobic Prealps(Upper Gerola Valley, Sondrio and Lecco Provinces, Lombardy, Italy). The footprints come from the uppermost levels of the Collio Fm. and are ascribed to Amphisauropus latus Haubold 1970, A. imminutus Haubold 1970, Dromopus lacertoides ( Geinitz 1861) and Varanopus curvidactylus Moodie 1929. The association is closely comparable to the association known from the lower portion of the Collio Fm. cropping out within the Collio Basin, in the Brescia region and to the similar and coeval Early Permian associations of Central Europe and North America.