Trento Platform Research Articles

Success and demise of exceptionally preserved terebratulide brachiopod accumulations in a Jurassic (early Pliensbachian) tropical lagoonal setting (Southern Alps, Italy): brachiopod response to environmental changes

During the Early Jurassic, the shallow marine carbonate platforms of the western-Tethys margins were characterized by highly diverse benthos including larger foraminifera, sponges, bivalves, gastropods, brachiopods, echinoderms, and dasycladalean calcareous algae. In this paper, we document examples of such assemblages within the lower Pliensbachian part of the Rotzo Formation (upper Orbitopsella Zone) of the Southern Alps, Italy. This carbonate succession was deposited in a complex mosaic of marine and brackish habitats within a tropical lagoon of the Trento Platform area. Large terebratulide brachiopod shells form autochthonous accumulations comprising exceptionally well-preserved monospecific assemblages of Lychnothyris rotzoana. These brachiopod-bearing successions were analysed in terms of biotic components, microfacies analysis, shell biofabric (three-dimensional arrangement of skeletal elements), and taphonomic signatures to understand brachiopod response to changing conditions within a highly variable lagoonal palaeoecosystem. Findings show that terebratulide shell accumulations are dominated by adult specimens and juveniles are rare. The brachiopods thrived during low energy conditions that resulted in the accumulation of highly temporally-condensed shell beds. Stabilized by microbialite encrustations, the shells were not re-oriented during the subsequent rapid burial. The abrupt demise of these communities was possibly related to rapid environmental change, and causal factors are discussed. The medium-term response of brachiopods to the relatively instable ecosystem of the tropical lagoon shows that they were not able to adapt to continuous perturbations, and that continuing stress severely compromised the resilience of benthic taxa.

AN INSIGHT INTO THE SYSTEMATICS OF PLICATOSTYLIDAE (BIVALVIA), WITH A DESCRIPTION OF <em>PACHYGERVILLIA ANGUILLAENSIS</em> N. GEN. N. SP. FROM THE <em>LITHIOTIS</em> FACIES (LOWER JURASSIC) OF ITALY

The Lithiotis facies represents an Early Jurassic global bioevent characterized by a remarkable spread of gregarious bivalves, which produced large sedimentary bodies in tropical shallow-water marine environments. The most peculiar and common genera Lithiotis, Cochlearites and Lithioperna, with aberrant and extremely elongated or strongly flattened shells, have been studied since the second half of the nineteenth century. Despite numerous systematic studies, their phylogenetic relationship with the other bivalve families is still uncertain. The Lithiotis facies yields other bivalve genera, among which a large multivincular mytiloid, provisionally determined as Isognomon (Mytiloperna) sp. ind. or Mytiloperna sp., is recorded in the literature. This taxon is here studied from a systematic point of view to clarify its taxonomic position and solve the open nomenclature adopted in the past. Here, we propose a new genus Pachygervillia and a new species Pachygervillia anguillaensis. The stratotype is located in the lower part of the Rotzo Formation (Calcari Grigi Group, Lower Jurassic), while the type locality is in the Lessini Mountains (Verona Province, Trento Platform, Southern Alps). This new species is characterized by a thick aragonitic inner shell layer with a fibrous, irregular, spherulitic, prismatic microstructure combined with a nacreous middle layer, both also occurring in species of the genera Lithiotis and Cochlearites of the family Plicatostylidae. This microstructural layering is here proposed as the main taxonomic character of the family, which is here emended and divided into the following two subfamilies: Plicatostylinae, yielding Lithiotis and Cochlearites with stick-like shells, and Pachygervilliinae nov. subfam., yielding Gervilleioperna, Lithioperna, Pachygervillia n. gen., and Pachyperna, previously placed within the subfamily Isognomoninae.

Microstructures and sclerochronology of exquisitely preserved Lower Jurassic lithiotid bivalves: Paleobiological and paleoclimatic significance

Lithiotids are enigmatic, large-sized bivalves that formed an important biotic component of tropical shallow marine environments during the Early Jurassic. Lithiotis problematica and Cochlearites loppianus are the most peculiar lithiotids, characterized by stick-like shells of predominantly aragonite which is generally calcitized or replaced by sparry calcite. Uniquely preserved specimens of these two species from the upper Sinemurian-Pliensbachian (Lower Jurassic) Rotzo Formation (Trento Platform, northern Italy), containing large parts of pristine shell (based on SEM, cathodoluminescence, and μXRF analysis), were selected for a sclerochronological and sclerochemical study, allowing to describe in detail the lithiotid microstructures, to decipher seasonal patterns and to investigate their functional and paleoenvironmental significance.We show that the outer shell layer of lithiotids, rarely preserved, consists of a calcitic simple prismatic microstructure with an asymmetrical thickness between the two valves, whereas the inner layer is aragonitic and is mainly composed of a fibrous irregular spherulitic prismatic fabric, which allowed a very fast shell growth. The latter microstructure is currently unknown in other mollusc shells. We recognized diurnal, fortnightly and annual growth increments, documenting a maximum annual growth of about 25 mm/yr in ventral direction. Stable isotopes show a clear annual periodicity suggesting seasonal changes in the paleoenvironment, which also affected the shell microstructures. During the warm season, first-order prisms are very elongated and show a massive structure without growth breaks, whereas during the cold season prisms are short and with growth cessations. Our results highlight the unique adaptation of lithiotid bivalves that allowed them to dominate the tropical shelf seas during the Early Jurassic.

LA SUCCESSIONE GIURASSICA (PLIENSBACHIANO - KIMMERIDGIANO) DELLE ALPI FELTRINE (BELLUNO)

The Feltrine Alps, a small mountain relief located few km north of Feltre (Belluno) consist of an over 600 m thick carbonate succession that spans in age from Late Triassic to Early Cretaceous. The Feltrine Alps belong to the paleogeographic unit called "Trento Platform", a structural high which formed in Early Liassic time, laterally limited by two subsiding basins: the Lombardian Basin to the west and the Belluno Trough to the east.
 The present work concerns only the Pliensbachian to Kimmeridgian interval. Nine stratigraphic sections were measured in detail and rich ammonite faunas of Toarcian, Aalenian and Bajocian ages were collected and studied.

Distinguishing core and flank facies based on shell fabrics in Lower Jurassic lithiotid shell beds

Lower Jurassic larger bivalves, mostly represented by the monospecific lithiotid genera Lithiotis, Cochlearites and Lithioperna, formed large shell accumulations in the shallow-water carbonate Tethyan and Panthalassa margins. A quantitative analysis of lithiotid accumulations from the Trento Platform (northern Italy) was carried out in order to distinguish the distribution of autochthonous, parautochthonous and allochthonous lithiotids in the accumulation core and flanks. Various representative accumulations are characterized with respect to systematic content, shell cover, shell density, disarticulation, shell fragmentation and orientation of individual shells. Lithioperna shows a high shell cover within the accumulation core while that of Cochlearites is variable and Lithiotis is rare. All three lithiotids occur in high densities in the accumulation flanks. As expected, autochthonous individuals of all three genera are frequent in the accumulation cores where they are preserved in life position. In the flanks, most bivalves are parautochthonous except for Lithioperna which can grow in life position. All bivalves in allochthonous shell beds are highly fragmented and disarticulated. The studied traits are potentially useful as proxies for distinguishing core or flank facies when the accumulation does not crop out as a whole.

SEDIMENTOLOGIA E STRATIGRAFIA DELLE SUCCESSIONI RETICHE AL CONFINE FRA LOMBARDIA E TRENTINO

This paper deals with Rhaetic facies at the Lombardy-Trentino border, in the Giudicarie Region. This area is characterized by important facies changes crossing strike slip faults. Two coeval sections, showing remarkable facies differences, are examined in detail, in order to describe the transition between the Rhaetic Lombardy Basin and the peritidal Trento Platform. The western section shows clear lombardian features and includes two marly formations: the Argillite di Riva di Solto and the Calcare di Zu. They are characterized by asymmetric marl-limestone cycles, developed on a metric scale, and show a shallowing upward evolution, from poorly oxygenated fairly deep environments to tidal flats. The upper portion of this section is referred as a new unit, the Tremalzo limestone, that is characterized commonly by a clear peritidal bathymetric cyclicity. The wholly carbonate eastern section is considered entirely Tremalzo limestone. It shows a similar evolution, but it was deposited in shallower environments on the western margin of the Trento Platform. Both sections are mud- dominated, but storm waves and tidal currents at times produced lenticular grainy beds. A detailed sedimentological analysis shows that these facies were deposited in a ramp system, the dynamics of which were mainly controlled by eustatic fluctuations and climatic cycles.

New constraints on the last aragonite–calcite sea transition from early Jurassic ooids

Calcite and aragonite seas are commonly distinguished based on the prevailing primary mineralogy of ooids and carbonate cements over time. Secular oscillations of these seas are usually attributed to changes in ocean chemistry and paleoclimate. While the veracity of such oscillations has been verified by independent data and modeling approaches, the timing of the transition from one ocean state to the other remains poorly resolved. Here, the timing of the last aragonite–calcite sea transition is estimated by assessing the preservation of Early Jurassic ooids from the Trento Platform in northern Italy. Point counting of ooid-bearing limestones from four distinct stratigraphic levels provides a contrasting pattern: Hettangian and Sinemurian ooids are all poorly preserved and were probably predominantly originally aragonitic, whereas Pliensbachian and Toarcian ooids are excellently preserved, suggesting a primary calcitic mineralogy. Although calcitic ooids may have already been common in the Late Triassic, it is proposed that the last aragonite–calcite sea transition occurred in the Early Jurassic between the Sinemurian and Pliensbachian, at least in this subtropical region. Therefore, the selective extinction of aragonite-secreting organisms at the end-Triassic mass extinction cannot be attributed to secular changes in ocean chemistry.

Apparent polar wander path for Adria extended by new Jurassic paleomagnetic results from its stable core: Tectonic implications

As a continuation of a systematic paleomagnetic research in the northern part of stable Adria, which provided a well-defined apparent polar wander (APW) path for the Cretaceous-Eocene, we present new paleomagnetic results for the Jurassic. These new data were obtained from 15 geographically distributed localities from the Trento platform (eastern Southern Alps) using standard paleomagnetic approach. The Lower Jurassic shallow water carbonates are not considered for tectonic interpretation, due to inconsistent inclinations. The Middle and Upper Jurassic Rosso Ammonitico provided excellent kinematic constraints.With the new Jurassic results, an APW is now defined for Adria for the 167–40Ma interval. It is well constrained for timing of important changes, like the speed and the sense of rotations. This APW suggests that the CW rotation and southward shift changed to the opposite at 155.1±5.3Ma, signifying a dramatic change in the life of the Neo-Tethys, from opening to closing. The latter is manifested in fast CCW rotation and northward movement of Adria up to 102.9±2.4Ma and moderate displacements in the same manner in post-103Ma times. Combined data sets from stable Istria and the foreland of the Southern Alps point to an approximately 15° CW rotation with respect to Africa at the end of the Cretaceous, and an about 25° in the CCW sense, after the Eocene.

An extensional syn-sedimentary structure in the Early Jurassic Trento Platform (Southern Alps, Italy) as analogue of potential hydrocarbon reservoirs developing in rifting-affected carbonate platforms

This work focuses on the 3D modeling and structural analysis of the Monte Testo syn-sedimentary structure, developed in the Early Jurassic Calcari Grigi Group of the Trento carbonate platform (Southern Alps, Italy). Significant changes in the facies architecture of the platform sedimentary units, occurred across a global perturbation of the Carbon cycle at the Sinemurian-Pliensbachian boundary, are associated with evidences of syn-sedimentary tectonics. In particular, an early cemented oolitic sedimentary body with a high initial porosity (Loppio Oolitic Limestone) was broken-up and tilted by a pulse of rifting and overlain by tight marls and marly limestones (lower Rotzo Formation) that display sharp changes in thickness across the syn-sedimentary faults. This complex setting creates conditions potentially favorable to hydrocarbon accumulation. In this work, the Monte Testo structure is presented as a conceptual analogue of a hydrocarbon reservoir that may develop thanks to the overlap of the effects of extensional tectonics and climate change-induced modifications in the carbonate platform facies. A 3D geo-model was realized to obtain information about the genesis and tectonic evolution of the structure. Hence, a potential porosity distribution in the 3D model was evaluated showing that such extensional structure, which has a vertical extent of 500 m and covers an area of 15 km2, could have been associated to a total pore volume of 2.24 × 107 m3 at the time of its formation. Results suggest that in rifting contexts the combined effect of syn-sedimentary faulting and facies variations related to perturbations in the global carbon cycle could generate potential reservoirs in carbonate platforms.

The Loppio Oolitic Limestone (Early Jurassic, Southern Alps): A prograding oolitic body with high original porosity originated by a carbonate platform crisis and recovery

The Loppio Oolitic Limestone is a lithostratigraphic unit of the Early Jurassic Trento Platform in the Southern Alps, Northern Italy, which deposited over an area of ca. 3500 km2. It appears as a roughly tabular or wedge-shaped sedimentary body with thickness gradually increasing from 0 to ca. 100 m toward the western platform margin. We investigated the sedimentology, petrography and bulk carbonate carbon isotope geochemistry of the Loppio Oolitic Limestone in order to shed light on its depositional setting and origin. The Loppio Oolitic Limestone is made almost exclusively of oolitic grainstone, and can be subdivided in two parts. In the lower part, ooids are poorly sorted and sedimentary structures are scarce or absent. In the upper part, sorting becomes good and sedimentary structures are common. The vertical succession of sedimentary structures and the upward increase in sorting suggest a shallowing upward trend within the oolite. A reddened surface, meteoric cements and dinosaur footprints occur at the top of the unit, testifying for a subaerial exposure which is also confirmed by carbon and oxygen stable isotopic data. In terms of sequence stratigraphy, the Loppio Oolitic Limestone represents a Highstand Systems Tract, bounded at the top by a subaerial exposure surface. Bulk carbonate stable carbon isotope curves across the Loppio Oolitic Limestone from 7 stratigraphic sections could be correlated over distances of tens of km on the whole Trento Platform. This correlation suggests that the deposition of ooids was nearly synchronous across the platform. A negative excursion of carbon isotopes with magnitude of ca. 1‰ VPDB was identified within a lime mudstone unit (“Nodular lithozone” of the Monte Zugna Formation) immediately below the Loppio Oolitic Limestone, which can be correlated to a global perturbation of the carbon cycle in the mid-Sinemurian. The flooding of a wide area of formerly peritidal carbonate platform below the wave base was interpreted as due to an ecological crisis that caused a drop of carbonate production. We suggest that the subsequent recovery of carbonate production is marked by the shallowing upward succession of the Loppio Oolitic Limestone, which quickly occupied the accommodation space formed in consequence of the crisis, thus preventing the platform drowning. The Loppio Oolitic Limestone deposited as an initially highly porous oolitic sand that was then topped by a clayey interval (base of the Rotzo Formation), giving origin to a structural and stratigraphic configuration that could be favourable for the accumulation of hydrocarbons in the subsurface. The recurrence of similar facies superpositions, formed in consequence of perturbations of the carbon cycle with documented climatic effects, is discussed with regard to the Tethysian record of Mesozoic carbonate platforms.

Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys

High-resolution carbonate carbon-isotope stratigraphy of Lower Jurassic marine shallow-water limestones are compared with the distribution of the Lithiotis Fauna on the Trento Platform (Southern Alps, Italy). The Lithiotis Fauna is the first example of globally distributed mound-building bivalves in the geological record and experienced global diffusion in the Early Jurassic. A set of carbonate carbon-isotope excursions of 2–3‰, illustrating three distinct negative shifts followed by positive rebounds, are recorded in the isotope stratigraphy and can be correlated with the global negative δ13C shift of the Sinemurian–Pliensbachian boundary Event (S–P Event) and to the subsequent phase of C-isotope perturbation that characterized the lower Pliensbachian. In the studied stratigraphic sections, the S–P Event likely triggered eutrophic conditions illustrated by the presence of organic-rich facies and by fossil associations characteristic of poorly oxygenated waters. After the eutrophic phase, the amelioration of environmental conditions was marked by a positive ~3‰ rebound of the δ13Ccarb values, and by the occurrence of marine stenotypic faunas. On the Trento Platform, the stabilization of the δ13Ccarb values coincided with the appearance of the Lithiotis Fauna that subsequently became widely distributed in the entire range of platform environments and thrived in the late Pliensbachian when metric-scale bivalve mounds were generated. During the same time, the maximum proliferation of the Lithiotis Fauna is recorded both in the Tethyan and Panthalassa regions. Hence, the reported relationships between the δ13Ccarb data and the distribution and ecological characteristics of the genera contained in the Lithiotis Fauna suggest that the S–P Event and its aftermath, possibly coupled with the undergoing coeval continent reorganization that led to the opening of the Hispanic Corridor, could have set the stage for the rapid diffusion of these unusual bivalves across many parts of the globe.

Tectonostratigraphic evolution of the Jurassic extensional basins of the eastern southern Alps and Adriatic foreland based on an integrated study of surface and subsurface data

Successions that characterize the eastern southern Alps have been compared with coeval units drilled in the Alpine foreland (Po and Veneto plains, northern Adriatic Sea). The eastern southern Alps are composed of a carbonate platform-plateau, drowned in the Early Jurassic (Trento platform and plateau); a basin formed in the Early Jurassic (Belluno Basin) and a carbonate platform that lasted from the Jurassic to the Cretaceous (Friuli platform). Integration of stratigraphic and geophysical data illustrates the extensional architecture of the Alpine foreland subsurface. At the beginning of the Jurassic, peritidal successions were widespread everywhere except for the Belluno Basin. A reorganization of the Early Jurassic paleogeography affected the southern Alps around the Sinemurian–Pliensbachian boundary: the Pliensbachian successions were deposited in the central-western areas of the Trento platform whereas, elsewhere in the same platform and in the northern Friuli platform, Pliensbachian units are missing, replaced by an unconformity surface covered by crinoidal sands that have also been found in the subsurface. The Belluno Basin is recognizable in seismic profiles under the Veneto Plain. Southward (northern Adriatic Sea and Po plain), the basin between the Trento and the Friuli platforms, here called the “northern Adriatic Basin,” possesses a stratigraphy different from that of the Belluno Basin. The northern Adriatic Basin drowned later, and seismic profiles indicate that it was wider and bounded by groups of small synsedimentary faults instead of the major faults displacing the Belluno Basin. The northern Adriatic Basin can be interpreted as the northeastern extension of the Umbro-Marchean Basin of central Italy.

Environmental control and dynamics of Lower Jurassic bivalve build-ups in the Trento Platform (Southern Alps, Italy)

Bivalve molluscs show a short and peculiar Early Jurassic evolutionary phase characterized by the Lithiotis sensu lato fauna, mainly represented by the three monospecific genera Lithiotis, Cochlearites and Lithioperna. These bivalves with stick-like or flattened large shells are mostly restricted to the Pliensbachian. They have a wide geographical distribution, occurring both in Tethyan and Panthalassian carbonate platforms, where considerable biogenic accumulations formed. They were common on the Trento Platform (Southern Alps), where they occur in the Rotzo Formation (Calcari Grigi Group). This formation was deposited in a tropical lagoon which was closed seawards by oolitic shoals and bars and bordered landwards by marshes. The Lithiotis s.l. fauna of the Trento Platform seems to be restricted to the upper Pliensbachian. It appeared after a eutrophic phase with recurrent low oxygen and brackish conditions and flourished in meso- to oligotrophic waters.The stratigraphic and geographic distributions of each genus are compared to the environmental development of the Rotzo Formation in order to highlight limiting environmental factors and ecological requirements. Lithioperna occurs throughout the whole formation; it is the most eurytopic genus, well adapted to dysaerobic habitats and with higher ecomorphic plasticity than the other two genera. It frequently occurs in the life position, giving rise to densely packed fan-like or book-like aggregates. Lithiotis required aerobic and euhaline marine conditions and is common in the middle part of the formation where mesotrophic conditions prevailed. This genus is rarely preserved in the theoretical vertical life position probably due to lower shell stability and a less effective attachment modality. Cochlearites is the most common genus of the Rotzo Formation. This genus inhabited euhaline to slightly hypersaline marine environments occurring in the upper part of the formation, prevailing in oligotrophic conditions.Lithiotis s.l. fauna gave rise to biogenic accumulations with an elevation of a few decimeters to several meters above the sea-bottom and with a lateral extension ranging from ten to several hundred meters. Therefore, these accumulations represent biological build-ups, which document four developmental phases: colonization, aggradation, progradation and demise. The colonization phase is represented by pioneering individuals which were cemented or byssally attached to a bioclastic, coarse-grained storm layer, which constituted the hard substrate for larval attachment. The aggradation phase produced the core of the build-up and its elevation above the sea-bottom. The core is mostly represented by individuals which acted as bafflers, commonly with an apparently chaotic fabric. The Lithiotis s.s. and Cochlearites cores are generally mud-supported and contain loosely packed to dispersed specimens, occasionally preserved in life position. The Lithiotis s.s. and Cochlearites build-ups can be defined as bivalve mounds. The cores of the Lithioperna build-ups are skeletal-supported with shells mostly preserved in life position, which characterize the bivalve reefs. The prograding phase produced lateral additions to the build-up flanks of oblique and imbricated articulated individuals. The flanks grew centrifugally from the core and represent the most conspicuous part of the build-ups, ranging from 10 to 20 times the size of the core and occasionally extending over several hundred meters.

Dinosaur tracks in a marginal marine environment: the Coste dell’Anglone ichnosite (Early Jurassic, Trento Platform, NE Italy) di Fabio Massimo PETTI, Massimo BERNARDI, Paolo FERRETTI, Riccardo TOMASONI & Marco AVANZINI

Bipedal tridactyl dinosaur footprints dominate the Early Jurassic Coste dell’Anglone tracksite, located on the eastern slope of the Mt. Biaina-Mt. Brento chain (Trentino Alto-Adige, NE Italy). The site yielded 544 tridactyl footprints, arranged in 20 long trackways, and belongs to the upper part of the Calcari Grigi Group (lower portion of the Rotzo formation). All the tracks can be attributed to smalland medium-sized theropods. Trace fossils were discovered in a peri tidal carbonate succession deposited in a marginal area of the Trento carbonate Platform, which has so far been considered fully marine (subtidal) in origin. The Coste dell’Anglone outcrop represents one of the most extensive Early Jurassic dinosaur tracksites in the Southern Alps and is probably the last dinosaur occurrence on the Trento Platform. The ichnological and sedimentological analyses of the track-bearing sequence indicate a depositional setting corresponding to a tidal flat embayment, positioned on the westernmost sector of the Trento Platform, close to the Lombardy pelagic basin. Stratigraphical data indicate that dinosaur populations lived in this area until the Sinemurian. A comparison with the Early Jurassic theropod tracks global record is here provided, along with an investigation into the possible trackmaker. The tracks share unambiguous similarities with the tridactyl tracks discovered at the Lavini di Marco tracksite, in the basal portion of the Calcari Grigi Group (Monte Zugna formation, Hettangian) and with theropod tracks from the Lower Jurassic of Poland, France and the United States. This new discovery therefore supports calls for a re-examination of the palaeogeographic reconstruction of the Southern Alps during the Early Jurassic

Pliensbachian gastropods from Venetian Southern Alps (Italy) and their palaeobiogeographical significance

Abstract: The Pliensbachian gastropods described by De Toni in 1912, coming from an isolated boulder at the foot of Mt Vedana (eastern margin of Trento Platform, Venetian Southern Alps, Italy) are revised. The fauna consists of 13 species representing nine families and eight superfamilies. Despite the low number of species, the assemblage represents the most diverse Early Jurassic gastropod fauna known for the Venetian Southern Alps. The boulder yielding the material was thought to derive from the upper part of the Early Jurassic Calcari Grigi Group, a carbonate platform unit extensively cropping out in the Mt Vedana area. The sedimentological analysis indicates a prevalently bioclastic wackstone-floatstone, reflecting a lime-muddy deposit undergone to an early consolidation. This and the high content of ammonoids, which is unusual for the Calcari Grigi Group, are typical aspects of a condensed pelagic sediment, presumably a fissure filling at the top of the carbonate platform succession. Palaeobiogeographical comparisons show that the fauna is composed of species occurring exclusively in pelagic limestones of the western Tethys. By contrast, it shows no relationships with the coeval faunas of the adjacent Trento platform and of the other western Tethyan carbonate platforms. These lines of evidence and the facies analysis would testify to the Pliensbachian drowning of the eastern margin of the Trento platform. In the wider context of the palaeobiogeographical history of Early Jurassic western Tethyan gastropods, the species from Vedana belong to a faunal stock which is typical for pelagic, mainly postdrowning sediments. Thus, appearance and diffusion in space and time of this stock were probably regulated by the direction, rate and pattern of the Neotethyan rifting. A new subgenus, Proarcirsa (Schafbergia) subgen. nov., and three new species, namely Ataphrus (Ataphrus) cordevolensis sp. nov., Guidonia pseudorotula sp. nov. and Proarcirsa (Schafbergia) zirettoensis sp. nov. are erected.

TESTATE AMOEBAE FROM THE EARLY JURASSIC OF THE WESTERN TETHYS, NORTH‐EAST ITALY

Abstract: Fossil testate amoebae and their non‐marine finds are rare so their ecological importance through Earth history is poorly understood. The Lower Jurassic shallow water black‐shales of Trento Platform (north‐east Italy) are rich in micro‐organisms and contain a thecamoebian and ostracod assemblage representing the first known record of Early Jurassic oligohaline forms from the European mainland. The thecamoebians are represented by the genera Difflugia, Pontigulasia and Centropyxis. The present discovery of Lower Jurassic thecamoebians in fine carbonate organic‐rich deposits indicates, for the first time, that these sediments can preserve testate amoebae very well. The occurrence of difflugid testate amoebians confirm a transitional marine‐terrestrial habitat, outside large bodies of water, and suggests occasional eutrophication in ephemeral restricted aquatic environment in the Sinemurian–Pliensbachian Trento Platform.

The response of two Tethyan carbonate platforms to the early Toarcian (Jurassic) oceanic anoxic event: environmental change and differential subsidence

AbstractChemostratigraphic analyses (87Sr/86Sr, δ13Ccarb) of limestones from two Jurassic platform‐carbonate sequences in Italy (Trento and Campania–Lucania Platforms) illustrate previously established trends found in pelagic sediments and skeletal carbonates from biostratigraphically well‐calibrated sections elsewhere in Europe. Chemostratigraphic correlations between the platform‐carbonate successions and appropriate intervals from well‐dated reference sections allow the application of high‐resolution stratigraphy to these shallow‐water peritidal carbonates and, furthermore, elucidate the facies response to the Early Toarcian Oceanic Anoxic Event (OAE). Lower Jurassic (Toarcian) levels of the western Trento Platform (Southern Alps, Northern Italy) contain spiculitic cherts that appear where rising carbon‐isotope values characterize the onset of the OAE: a palaeoceanographic phenomenon interpreted as driven by increased nutrient levels in near‐surface waters. There is a facies change to more clay‐rich facies at the level of the abrupt negative carbon‐isotope excursion, also characteristic of the OAE, higher in the section. The Campania–Lucania Platform (Southern Apennines, Southern Italy) records a change to more clay‐rich facies where carbon‐isotope values begin to rise at the beginning of the OAE but the negative excursion, higher in the section, occurs within oolitic facies. Although, in both examples, the Early Toarcian OAE can be recognized by a change to more clay‐rich lithologies, this facies development is diachronous and in neither case did the platform drown. Although the Trento Platform, in the south‐west sector studied here, was adversely affected by the OAE, it did not drown definitively until Late Aalenian time; the Campania–Lucania Platform persisted throughout the Jurassic and Cretaceous. Differential subsidence rates, which can be calculated using comparative chemostratigraphy, are identified as a crucial factor in the divergent behaviour of these two carbonate platforms: relatively fast in the case of the Trento Platform; relatively slow in the case of the Campania–Lucania Platform. It is proposed that where water depths remained as shallow as a few metres during the OAE (Campania–Lucania Platform), dissolved oxygen levels remained high, nutrient levels relatively low and conditions for carbonate secretion and precipitation remained relatively favourable, whereas more poorly ventilated and/or more nutrient‐rich waters (Trento Platform) adversely influenced platform growth where depths were in the tens of metres range. The stage was thus set for drowning on the more rapidly subsiding western margin of the Trento Plateau and a pulse of oolite deposition post‐dating the OAE was insufficient to revitalize the carbonate factory.

Evolution of a poly-deformed relay zone between fault segments in the eastern Southern Alps, Italy

Abstract In the eastern Southern Alps (NE Italy), Liassic north–south extensional structures are prominent. The southern Trento Platform also experienced extension during the Palaeogene, when reactivation of some pre-existing faults occurred, coupled with nucleation of new faults. During Neogene shortening, these structures were reactivated once again, but with strike-slip kinematics. In this framework, the Gamonda–Tormeno restraining stepover represents the final result of an overlap zone which evolved through time. In the first stage (Lias to Early Cretaceous) a prominent splay developed at the tip of the Gamonda Fault by lateral propagation and breaching of independent segments. At the same time, there was kinematic interaction between the antithetic Gamonda and Tormeno faults, followed by diachronous motion on crossing faults and the development of a narrow graben. During the second stage of extensional tectonics (Palaeocene to Early Oligocene), the reactivation and propagation of the overlapping faults along with the generation of new faults led to deepening of the graben. In the third stage (Miocene to present), the final structure of a strike-slip restraining stepover was accomplished. Due to the mechanical stratigraphy and complex inherited architecture of the relay zone where stratigraphic sequences with different rheological properties are juxtaposed, the style of shortening is different in the western and eastern sides of the stepover. The Gamonda–Tormeno structure represents a unique example of how a relay zone may change through time.

Trophic regimes of benthic foraminiferal assemblages in Lower Jurassic shallow water carbonates from northeastern Italy (Calcari Grigi, Trento Platform, Venetian Prealps)

Benthic foraminiferal distribution patterns in the Lower Jurassic Rotzo Member (Trento Platform, northeastern Italy) are analysed in terms of trophic gradients to provide a better understanding of the benthic communities during the time of the initial diversification of Lower Jurassic large foraminifera in the Southern Alps. The Rotzo Member deposited on the Trento Platform records a complex mosaic of distinct shallow water ecosystems. The changes in the foraminiferal assemblages of the Rotzo Member are considered to be primarily a reflection of gradients in trophic resources and oxygen levels. The comparative analysis of benthic foraminiferal assemblages (characteristic taxa, number of species, test size, reproduction strategy), the associated sediments (lithofacies) and the macrofossil and microfossil content allow three trophic regimes to be identified during the time of deposition of the Rotzo Member: (a) eutrophic, (b) mesotrophic, and (c) oligotrophic. (a) The eutrophic regime, marked by episodically dysaerobic bottom conditions and high organic matter supply, occurs stratigraphically below and within the Orbitopsella spp. assemblage (Sinemurian pro parte-Carixian). Gradual change in the benthic foraminiferal assemblage may have been caused by a combination of reduced nutrient supply and increased bottom-water circulation which led to a mesotrophic regime (b). The latter, which has been recognised in the basal and middle part of the Rotzo Member alternating with the eutrophic assemblage, is characterised by a foraminiferal assemblage dominated by Orbitopsella sp. A major foraminiferal faunal turnover across the Carixian/Domerian boundary, corresponding to the disappearance of Orbitopsella spp., indicates a significant environmental change. An oligotrophic regime (c) became established towards the upper part of the Rotzo Member. New taxa appeared and diversity of the foraminiferal faunas was higher than in the previous ecosystems. This part of the member, characterised by the maximum lateral and vertical development of large Lithiotis bivalve mounds, reflects low and more stable nutrient supply, which provided optimal conditions for the more specialised benthic communities. Evolution of these benthic foraminiferal faunas from the Late Pliensbachian onwards was obliterated by events that led to the drowning of the Trento Platform, which is marked by a regional unconformity.

Enigmatic dinosaur trackways from the lower jurassic (Pliensbachian) of the Sarca valley, Northeast Italy

Four large limestone blocks with dinosaur footprints have been discovered in a large landslide situated in the Sarca Valley (northeast Italy). The trampled sediments have been ascribed to the upper part (Pliensbachian) of the Lower Jurassic Calcari Grigi Formation, which in the Sarca Valley consists of oolitic‐bioclastic limestones. These carbonates were originally deposited on the western margin of the Trento Platform, a paleogeographic structural high that was located in the northern tropical belt at the westernmost end of Tethys. Several footprints form trackways of quadrupedal dinosaurs. The data suggest the presence, within the ichnoassociation, of possible prosauropods and basal Thyrephora. This inference is consistent with the chronological and geographical distribution of both prosauropods and Thyreophora.