Ausonio Ronchi

Detrital zircon provenance from the Neuquén Basin (south-central Andes): Cretaceous geodynamic evolution and sedimentary response in a retroarc-foreland basin

The surface response, in terms of drainage pattern changes, to the Cretaceous geodynamic reorganization of the Andean subduction zone between 36°S and 41°S is reconstructed through the geochronology-based provenance study of alluvial detrital zircons. The age spectra obtained by 500 spot U-Pb ages record an eastward provenance of detritus coming from the foreland during the Early Cretaceous backarc extensional stage, followed by westward-sourced clastics coming from the Cordillera during the Cenomanian. This drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime, forcing the Neuquen Basin to evolve from a retroarc to a foreland stage. After this inversion, the clastic systems progressively returned to be mainly fed by the foreland, due to the uplift of the peripheral bulge as a consequence of the Late Cretaceous thrust front migration. This tectonic evolution of the Neuquen Basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the Benioff subduction zone.

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal *

Abstract Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings highlight the occurrence of an older melting event in the GHS during prograde metamorphism in the kyanite stability field before the more diffuse Miocene melting event. The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew during the same time span and they grew in different tectonic settings.

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.

New palaeontological discoveries in some Early Permian sequences of Sardinia. Biostratigraphic and palaeogeographic implications

New palaeontological material was collected during recent extensive fieldwork in the Early Permian Perdasdefogu and Escalaplano basins (SE Sardinia). For the first time, an accurate biostratigraphic position can be assumed for the sediments of these two basins, on the basis of their fossiliferous content (floras; ostracods). Regional and inter-regional correlations are proposed and palaeogeographical implications are analysed.

New hybondontoid shark from the Permocarboniferous (Gzhelian-Asselian) of Guardia Pisano (Sardinia, Italy)

Numerous isolated teeth, fin spine fragments and dermal denticles of a hybodont shark from a lacustrine limestone horizon at the top of lithofacies B of the Late Carboniferous to Early Permian succession of the Guardia Pisano Basin (Sulcis area, southwestern Sardinia, Italy) are assigned to a new species of the genus Lissodus Brough, 1935. Lissodus sardiniensis sp. nov. is erected on the basis of about 500 teeth, which show a unique feature of only one pair of lateral cusps that are bent in the direction of the prominent central cusp. Weak heterodonty allows distinction of symphyseal, mesial to anterolateral, and lateral teeth. Lissodus sardiniensis sp. nov. was a freshwater-adapted durophagous shark of bottom dwelling habit, an interpretation supported by general construction of the dentition and the morphology of the dermal denticles. The association with Acanthodes, diplodoselachid sharks and branchiosaurs allows the reconstruction of a five-level trophic chain for the Guardia Pisano Basin. The discovery of Lissodus in Sardinia is presently the southernmost known occurrence of that genus in the Late Palaeozoic of Europe. This new find adds significantly to knowledge of migration routes of aquatic organisms, especially freshwater sharks, between the single European basins in the Late Pennsylvanian, and changes in palaeobiogeography during the Early Permian.

A huge caseid pelycosaur from north−western Sardinia and its bearing on European Permian stratigraphy and palaeobiogeography

Skeletal remains, some loose on the surface and others still embedded, have been recovered from the uppermost part of an outcrop of the Permian Cala del Vino Formation located near Torre del Porticciolo (Alghero, Nurra, NW Sardinia). Taphonomic analysis suggests that all the elements pertain to a single individual; ongoing studies indicate the fossil represents a large caseid close (or referable) to Cotylorhynchus; otherwise restricted to a narrow geographic and stratigraphic zone of the central USA. The new finding, the first of a caseid in Italy and one of few in Europe, enlarges the known distribution of the family and provides a significant and key chronostratigraphic constraint for the continental succession of this area and, in turn, helps establish a stratigraphic framework for the Permian units cropping out in Italy and southern France.

Ichnosystematics of the Lower Permian Invertebrate Traces from the Collio and Mt. Luco Basins (North Italy)

The Lower Permian ichnofauna in the Collio Formation (Artinskian) in the Val Trompia (Brescian Prealps, North Italy) has been studied for a long time, but the studies have focused mainly on vertebrate prints. In this study, the invertebrate ichnofauna of the Collio Formation, and in the epiclastites of the Monte Luco Formation (Artinskian) cropping out in the Monte Luco area (Trentino Alto Adige region), is systematically analyzed for the first time. This ichnocoenosis consists of: Permichnium isp., Paleohelcura tridactyla, Diplichnites gouldi (Types A and B), Diplopodichnus biformis, Circulichnis montanus, Helminthopsis hieroglyphica, Gordia marina, Acripes cf. multiformis, Cruziana cf. problematica, Cochlichnus anguineus, Palaeophycus tubularis, Planolites isp. and ?Scoyenia isp. The ichnoassociation from the Collio Formation belongs to the Scoyenia ichnofacies, while that from the Mt. Luco Formation belongs to the Mermia ichnofacies. The latter, because of the lack of complete data, can be linked only to one of several submersion phases of the intravolcanic basin and is not referred to the complete continental Permian sequence in this zone. The composition of the ichnoassociation here analysed is similar to those of other European (especially France and Germany) and extra-European areas (especially North America and Argentina).

Early Permian silicified floras from the Perdasdefogu Basin (SE Sardinia): comparison and bio-chronostratigraphic correlation with the floras of the Autun Basin (Massif central, France)

ABSTRACT In the Perdasdefogu Basin (Ogliastra, Sardinia) plants occurring in angular cherts are preserved as siliceous permineralizations. The plant assemblage consists of the genera Sphenophyllum, Arthropitys, Astromyelon, Palaeostachya, Pecopteris, Scolecopteris, Stewartiopteris, Psaronius, Ankyropteris, Anachoropteris; furthermore, probable medullosan wood and ovules, cordaitean stems, leaves and ovules, and Dadoxylon wood have been recorded. Arborescent marratialean ferns are the dominant elements with Pecopteris and Scolecopteris leaves being the most common; the calamiteans are the second in abundance. Generally the chert blocks contain a large number of plant fragments — roots and leaves are being almost equally represented — suggestive of a coal ball-like plant accumulation. More rarely a chert block may consist of a single plant part, for example a piece of wood. The preservation and accumulation of the plants are closely comparable to the silicified plant assemblages described from the Lower Permian of Autun and the upper Pennsylvanian of Grand-Croix (Massif central, France) and from the Early Permian Döhlen Basin (Germany). It is certainly significant that the assemblage from the Perdasdefogu Basin is dominated by tree ferns and calamites like the coeval silicified vegetation from Autun. However, like in Autun, the permineralized flora contrasts with the underlying compression floras dominated by conifers and peltasperms. The Perdasdefogu macroflora record suggest a middle-upper Autunian age which corresponds to the Surmoulin and Millery formations of the Autun Basin. This is in agreement with the Asselian-Sakmarian transition as pointed out by the amphibian species found in the same formation, and perfectly correlatable with the same association found in Gottlob-lake (Thuringian Forest Basin). Because of its rich fossil content, the Perdasdefogu Basin represents a reference succession for the Autunian of the entire westernmost palaeo-Tethyan domain and its macrofloral record.

Palaeoecology of non marine algae and stromatolites: Permian of France and adjacent countries

Abstract Some Permian, continental basins from Algeria, Morocco, France, Italy, Germany and Poland reveal algal remains and stromatolites, mainly during Lower Permian, but also during Middle and Upper Permian. The algae belong to 8 morphogenera and 12 morphospecies. Two species are new. The taxonomic attributions are difficult, even if some species resemble living species. The algae make unorganized masses, or laminated builtups (stromatolites, oncolites, oolites). Algal masses and stromatolites are contained in fluviatile sediments (active or abandoned channels) and lacustrine deposits (playas, ephemeral lakes shorelines, lakes several meters or decameters deep). Considering the sedimentological context, the water salinity could range from karstic springs (hard water) to evaporic ephemeral lakes (playas).

Pennsylvanian floras from Italy: an overview of the main sites and historical collections

The paper provides an overview of the main Pennsylvanian sites in Italy yielding associations rich in plants and/or palynomorphs. So far in Italy, the principal outcrops are located in the Southern Alps, Tuscany and Sardinia. In the Western Southern Alps and bordering Switzerland, Westphalian outcrops are small and scattered. Nevertheless they yielded an abundant fossil flora, stored at the Museo Civico di Storia Naturale of Milan-, (Venzo and Maglia Collection). In the Carnic Alps-, (Eastern Southern Alps)-, continental deposits of Moscovian to Gzhelian age also occur near the border with Austria. They have produced a high number of preserved plant fossils, presently stored in the Museo Friulano di Storia Naturale of Udine. In Tuscany, the two main sections yielding Westphalian to Autunian floras are those of the Iano and Pisani Mountains. A rich collection of plant fossils from those sites is hosted at the Museo di Storia Naturale of Florence University and at the Museum of Natural History of Pisa University. In Sardinia, plant fossil sites are located in the south west and central east parts of the island. The San Giorgio Basin (Iglesiente subregion) and the Tuppa Niedda section (Arburese subregion) are late Westphalian – early Stephanian in age. In the Barbagia at Seui-Seulo and the Gerrei subregions, other continental basins yielded transitional “Stephanian- Autunian” fossil plant associations. The slabs are stored as part of the Lovisato Collection at the Lovisato Museum of the Chemical and Geoscience Department of Cagliari University. Smaller historical outcrops of Carboniferous age are also known from other Italian regions, such as Liguria.