Trento Platform Research Articles

An Early Jurassic oligohaline ostracod assemblage within the marine carbonate platform sequence of the Venetian Prealps, NE Italy

Micropalaeontological investigation of one of a number of black shale horizons within the carbonate platform sediments of the Calcari Grigi Formation (Trento Platform, NE Italy) has yielded the first recorded oligohaline, early Jurassic, ostracod assemblage. The shale is dated as late Sinemurian on the basis of large benthic foraminiferal biostratigraphy in the sediments above and below. The shale is devoid of foraminifera and ammonites, which supports a non-marine context. Three previously unknown ostracod species are identified, however, the assemblage is dominated (>95%) by a single taxon which is erected as a new genus and species (Phraterfabanella tridentinensis Whatley and Boomer gen. et sp. nov.) which further supports the interpretation of a “stressed” environment. The two remaining ostracod taxa are assigned to Klieana and Limnocythere both considered to represent non-marine or very low salinity conditions. This new genus includes probable members from the Rhaetian of Hungary and France as well as from the Liassic of France. The new genus is shown to be one of the earliest representatives of the Cytherideidae, a family which survives to the present and includes the modern pandemic, euryhaline species Cyprideis torosa.The discovery of such an assemblage within an otherwise marine sequence indicates temporary, physical isolation from marine influence. Sedimentological, faunal and geochemical evidence suggests that the organic rich shale may indicate high organic carbon supply via continental runoff in this tropical palaeogeographical setting. Resulting dysaerobia within the sediment would have created a particularly stressed environment.

Burrows and body fossil of decapod crustaceans in the Calcari Grigi, lower Jurassic, Trento platform (Italy)

Crustacean burrows are largely known and well described in the literature but a concurrence of abundant burrows and fossil remains of decapods represents an exceptional circumstance in the geological record. Well-preserved burrows of decapod crustaceans in the Pliensbachian of the Trento carbonate platform, northern Italy, reveal several ichnoassemblages of Thalassinoides and Ophiomorpha from 2 to 16 cm in diameter forming complex burrow systems in the lagoonal facies of the Rotzo Member. These burrows have been found both in the lower surface of beds and within shallowing-upwards cycles of fossiliferous limestones (Lithiotis facies), which reveal peculiar taphonomic characteristics. The abundant concentration of decapod crustacean burrows suggests intense burrowing activity of such organisms during the Early Jurassic as indicated also by an exuvia of Phlyctisoma (Erymidae), although we do not know if it was an active tracemaker. Many shallowing-upward parasequences occur and burrow systems were concentrated mainly in their lower part suggesting intense biological activity in deeper and quiet subtidal conditions; conversely, biologic activity was interactive with physical agents in the middle part of cycles and progressively replaced by wave or tidal-induced physical agents in the upper part where intertidal/supratidal conditions were formed.

FIRST RECORD OF EVERTICYCLAMMINA REDMOND 1964 (E. PRAEVIRGULIANA N. SP.; FORAMINIFERA) FROM THE EARLY JURASSIC OF THE VENETIAN PREALPS (CALCARI GRIGI, TRENTO PLATFORM, NORTHERN ITALY)

The rich fossil content of the Lower Jurassic carbonate successions of the Trento Platform (northeastern Italy) have been studied since the beginning of the nineteenth century. This detailed study on several stratigraphic sections cropping out in this area, all spanning the Liassic Rotzo Member (Calcari Grigi formation, Venetian Prealps), provides us new biostratigraphic data about the genus Everticyclammina Redmond 1964. A new species of Everticyclammina, E. praevirguliana n. sp., is formally described from the Calcari Grigi, where it occurs in levels attributed to the late Sinemurian to Domerian age. Everticyclammina praevirguliana n. sp. thus becomes the first representative of the genus in the Early Jurassic; this record consequently enables the stratigraphic distribution of the genus to be lowered to the Liassic. Hence the plexus of Everticyclammina evolving communities now bridges the Liassic (late Sinemurian) to Early Cretaceous (Aptian) interval.

Segmentation and linkage of the Lessini Mountains normal faults, Southern Alps, Italy

The Lessini Mountains on the southern margin of the Italian Southern Alps exhibit two Tertiary graben systems at an angle of about 40°. The NNW-trending system is orthogonal to the ENE-trending Tertiary extension direction. The NNE-trending system parallels the neighbouring Jurassic western margin of the Trento Platform and experienced sinistral transtension during the Paleogene. The interaction between the two normal fault trends defines a ca. 8 km long and 6 km wide rhomb-shaped structure containing igneous intrusions and dolomitized rock bodies, suggesting that the structure controlled syn-extensional fluid migration. At a smaller scale, fault segments of the NNE-trending set show two basic types of transfer zones, i.e. conjugate and synthetic. Due to the sinistral and normal oblique-slip on these faults, left steps of en échelon segments had a kinematics similar to that of pull-apart structures. The location of intrusion-centres within such transfer zones led to the collapse of the roof of the magma chambers and consequent deep basins development. In contrast, right-stepping synthetic en échelon normal-oblique fault segments developed relay ramps.

Cymbriaella, a new foraminiferal genus (textulariina) from the early jurassic of the venetian prealps (Northeastern Italy)

The new genus Cymbriaella (type-species Cymbriaella lorigae n. gen. n. sp.), herein introduced, has been recorded from the Lower Jurassic (Liassic) Calcari Grigi formation (Trento Platform, Venetian Prealps, Northeastern Italy). The new genus has been placed in the family Hauraniidae SEPTFONTAINE (Textulariina).

Tertiary extension in the southern Trento Platform, Southern Alps, Italy

The southern Trento Platform (Southern Alps, NE Italy) was the locus of abundant mafic volcanism associated with Paleogene extensional tectonics. During the Pal eocene and throughout the Eocene, a NNW–SSE trending graben (the Alpone‐Agno Graben (AAG)) developed in the eastern Lessini Mountains. The graben was filled with basaltic volcaniclastics, calcarenites, and lava flows. The AAG is a half‐graben system probably bounded by rotational listric normal faults. Most faults in the graben dip towards the WSW. Paleostress analysis within the AAG shows that during the Eocene the direction of horizontal extension was ENE–WSW. The volcanic activity continued into the Oligocene; however, the relation between Oligocene tectonics and volcanism remains unclear. Gravimetric and magnetic anomalies in conjunction with the crustal thickness variations suggest a locus of lithospheric thinning slightly off‐axis from the AAG. This can be explained by an overall asymmetric extensional geometry dominated by a WSW dipping detachment fault. The Paleogene geodynamic scenario of the Trento Platform suggests that the AAG developed as a foreland extensional response to the active collisional convergence between the European and Adriatic plates. Neogene compressional tectonics reactivated some of the Paleogene normal faults as strike‐sup faults.

Large-scale strike-slip displacement of the Drauzug and the Transdanubian Mountains in early Alpine history: Evidence from permo-mesozoic facies belts

Both the Drauzug (Italy, Austria) and the Transdanubian Mountains (Hungary) show great differences in facies compared to the geological units that presently surround them, i.e. proximal facies contrast with distal facies in the Permo-Triassic. Lower Liassic strata in the Drauzug and the Transdanubian Mountains indicate an extensional regime causing typical structural features such as tilted blocks, fault scarps, and drowned carbonate platforms. The Permo-Mesozoic facies zones provide markers for the paleogeographic fitting of the Drauzug and the Transdanubian Mountains with areas today lying some 300–400 km to the west: the Drauzug corresponds to the Lombardian basin and the westernmost part of the Northern Calcareous Alps while the Transdanubian Mountains correspond to the westernmost part of the Lombardian basin, the Trento platform, and the Belluno trough. The Drauzug and the Transdanubian Mountains (together with the South Alpine realm) were displaced to the east along strike-slip faults during the Middle Jurassic to Early Cretaceous opening of the Central Atlantic and Ligurian-Piemont oceans and the simultaneous subduction of the Vardar ocean at the eastern margin of Apulia. Finally in Late Oligocene and Miocene times the Southern Alps were displaced back to the west along the dextral Periadriatic fault system.

The Dilemma of Gaps in Carbonate Stratigraphic Sequences: A Case History from the Jurassic of the Venetian Alps, Italy

The Lower Ammonitico Rosso (L.A.R.) of the western Venetian Alps is a thin, red nodular limestone that overlies a thick Late Triassic to Middle Jurassic (Aalenian) shallow-water platform sequence. It is thought to represent a Middle Jurassic (Bajocian) drowning event whereby the Trento Platform became a submerged plateau. The L.A.R. is problematic in that it (1) is rich in ammonites and other pelagic fauna; (2) contains stromatolites, oncolites, and shallow-water sedimentary structures; and (3) directly overlies platform sediments that contain complex brecciated fabrics filled by cement and Bajocian-age sediment. These seemingly contradictory features have generated much debate as to the sequence stratigraphy of the Trento Platform. New evidence suggests that the L.A.R. was deposited in a shallow-water environment following a period of subaerial exposure. In an east-west transect from the platform edge to platform interior, a clear transition is exhibited from high- and low-energy open-marine facies to restricted lagoonal facies. High-energy open-marine facies include well-sorted skeletal- and peloidal-rich sands possessing low-angle to planar cross stratification and thin, fairly sorted ammonite- and belemnite-rich gravels. Sands include both shallow-water and pelagic fauna; gravels commonly contain ripup clasts from underlying sediments. Low-energy open-marine facies consist of buff colored wackestones and packstones that containmore » ammonites, belemnites, pelagic bivalves, and peloids. With respect to paleogeography, the abrupt transition from open-marine facies at the platform edge to lagoonal facies in the platform interior indicates that thrombolites, stromatolites, and oncolites accumulated in a shallow restricted environment.« less

Ein Äquivalent des Misone-Kalks (Pliensbachium) am Ostrand der jurassischen Trento-Plattform

Ein Äquivalent des Misone-Kalks (Pliensbachium) am Ostrand der jurassischen Trento-Plattform

A Jurassic “Tongue of the Ocean” infilled with oolitic sands: The Belluno Trough, Venetian Alps, Italy

The Belluno Trough is a narrow and elongate basin on the northwestern corner of the Apulian Plate continental margin. It acquired topographic identity in Early Liassic time, during the breakup of a widespread Upper Triassic carbonate shelf. At the beginning (Liassic), the Belluno Trough was a starved depression accumulating euxinic mudstone and bounded by two shallow-water carbonate banks (Trento and Friuli Platforms). This typical Bahamian physiography lasted until Late Liassic time, when the shallow-water Trento Platform collapsed tectonically and suddenly drowned, tilting northward and breaking along an old tectonic line; it became therefore a submerged pelagic plateau. In the meantime, the western edge of the Friuli Platform, now the windward margin of an oceanic platform, turned into a prolific, super-producing, oolite “factory”. Quite soon the oolite sands of the Friuli margin started to flow down the slope, reaching the basin through numerous entry points and gradually building a complex of coalescent deep-sea fans, which, prograding westward, infilled the Belluno Trough. The fans terminated westward against the faulted edge of the Trento Plateau, on which above a karst-like surface, locally encrusted with manganese, red nodular ammonitic limestone was slowly accumulating. Northward, where the topographic relief decreased, the oolite fans could spill over the escarpment onto the edge of the plateau. By the end of Middle Jurassic time, the Belluno Trough was no longer a submarine depression but a gentle slope connecting the Friuli Platform to the deep, pelagic Trento Plateau. In Callovian time, probably because of a short drop in sea level, large parts of the Friuli Platform were exposed to subaerial diagenesis. During the subsequent Oxfordian transgression, the resulting rocky bottom became the substratum for an extensive growth of reefal organisms (hydrozoans, corals, rudists), which fringed the Friuli Platform for the whole Malm and Cretaceous times.