RICERCA ITALIANA     

Research program

Stratigraphic-paleogeographic and geochemical-petrographic characterization of the events around the Triassic-Jurassic boundary: an integrated approach

University Co-ordinator

Università degli Studi di TRIESTE - SCIENZE GEOLOGICHE, AMBIENTALI E MARINE - TRIESTE(TS)

Research Unit Leader

Daniele MASETTI

Description

The researchers at the University of Trieste, D. Masetti (carbonate stratigraphy), M. Ponton (structural geology) and R. Romano (biostratigraphy) are currently involved in the CARG project, Sheet 049 Gemona. The presence, among extra-university personnel of F. Podda and S. Russo, actually involved as surveyors in the same Gemona Sheet, it will allow a close integration between the CARG and the proposed research project, especially with respect to the field work in the Carnian Prealps. This in-house expertise will be complemented by collaborations with personnel at other research institutions and private companies. Andrea Cozzi (Ph.D. in Geology at the Johns Hopkins University and currently research scientist at the ETH Zürich), will be responsible of the facies characterization and geochemical investigation of the drowning sequences. The ENI E&P involvement in this research project through Dr. R. Fantoni (regional geology) and Dr. P. Ronchi (carbonate diagenesis) will allow to integrate the borehole and seismic data from the Po and Venetian plains and the northern offshore Adriatic with the outcrop data.

The Sinemurian drowning of the carbonate platforms in the eastern Southern Alps
Preliminary results obtained by the Trieste research unit (PRIN 2001) showed the existence of a stratigraphic hiatus corresponding to most of the Pliensbachian in the Friuli Platform and in some areas of the Trento Platform, from the Valsugana to its eastern margin with the Belluno Basin (Romano et al., 2004). The forthcoming research plan is structured as follows:
- characterization of the areal distribution of the Pliensbachian hiatus in the eastern sector of the Trento Platform and in the Friuli Platform. For the latter, data acquisition will be integrated by the CARG project database;
-an E-W oriented transect from the early-drowned Friuli Platform to western Slovenia, where lower Jurassic platform carbonates were unaffected by drowning events, will be attempted;
- detailed facies analysis of the pre- and post-drowning succession, with focus on lateral variation of the characteristics of the unconformity with respect to underlying and overlying rocks;
- comparison of drowning styles between the eastern Trento Platform and the Friuli Platform;
- the opportunity of accessing the ENI subsurface database will enable to assess the presence of the Sinemurian drowning surface, and consequent depositional hiatus during the Middle and Late Liassic, in the part of the Trento platform found today under the Venetian plain. This revision of the Jurassic stratigraphy in the boreholes will allow to produce an evolutionary model of the Jurassic continental margin in the Southern Alps, improving the preliminary model by Fantoni et al. (2004);
- given the almost complete absence in the eastern Southern Alps of paleolatitude data in the Late Triassic-Early Jurassic interval, it is proposed to start a pilot study, in collaboration with the Research Unit of Milan, to explore the paleomagnetic signal in both shallow water and basinal sequences. This could potentially provide important clues on the paleoenvironmental changes that occurred in this time interval, also in lieu of the promising results recently published by Muttoni et al. (2005) on coeval sections in Lombardy;
- an integrated biostratigraphic study is required to precisely characterize the Sinemurian drowning event. Below the drowning unconformity, the Lower member of the Calcari Grigi Fm contains only dasycladalean algae and benthic foraminifera useful for a biostratigraphic calibration. As of now the biostratigraphic range of these taxa is reasonably well understood (Barattolo & Romano, 2004). In the last years a large number of micropaleontological studies has concentrated on refining the biozonation in shallow water carbonate successions. The last calibration of such biozones has been achieved in Lombardy (Botticino area), where on top of the drowning surface (Corna Fm) rich ammonite faunas, Sinemurian in age, starting from semicostatum zone, were recovered. To further improve this calibration, it is planned to closely resample the key sections in the Botticino area, in order to assess the changes in the micropaleontological associations through the pre-Sinemurian bioevents. Additionally, there is need for pre-Sinemurian marker horizons in order to subdivide the pre-Semicostatum interval. Appropriate markers will be looked for in the Chizzola (Adige Valley) and Mt. Cengio (Asiago Plateau) sections of the Lower Member of the Calcari Grigi Fm. These sections, located respectively in the Western and Eastern sector of the Trento Platform, are characterized by a cyclic alternation of subtidal mudstones and calcarenites and green marls. Preliminary studies showed the presence of a good palynomorph content. The integration of biozonations of different taxa (pollens in the marls, and benthic foraminifera and calcareous algae in the limestones) should result in a refining of the biostratigraphic zonations in the interval Hettangian-Lower Sinemurian. The close integration of these results with the newly acquired data for the CARG project – Sheet no. 49 (Gemona), will allow to extend these new biozonations from the Trento to the Friuli Platform. The Calcari Grigi Fm is characterized, in vast areas of the Friuli Platform, by the intercalation with basinal siliceous limestones (Soverzene Fm), attesting for platform drowning events later partially or totally sutured by the progradation succession of the Calcari Grigi platform. Preliminary studies within the CARG project have targeted these drowning sequences using calcareous nannoplankton and radiolarians.
An advanced state of this research will focus on the detailed study of key stratigraphic sections (e.g., Mt Verzegnis, Val Lavaruzza). The aim would be to integrate the biostratigraphic data from calcareous algae and benthic foraminifera in the shallow water part of the sections (Calcari Grigi Fm) with the data coming from the calcareous nannoplankton and radiolarians in the deep water part of them (Soverzene Fm). In this way to goals would be achieved: 1) to calibrate the shallow and deep water biozonations, and 2) to investigate the geologic record in the deep water sections of the drowning of the Calcari Grigi platform during the interval Hettangian - Pliensbachian.
In synthesis, this articulated biostratigraphic study of the Botticino sections (Brescia Prealps), Mt. Cengio and Chizzola (Venetian Prealps), Mt. Verzegnis and Val Lavaruzza (Friulan Prealps), should originate a new, high resolution biozonation scheme for dasycladalian algae and benthic foraminifera, biostratigraphically calibrated with palynomorphs biozonations for the pre-Sinemurian interval, and with the nannoplancton and radiolarian ones for the post-Sinemurian interval. This newly improved biostratigraphic scheme would be indispensable to date the Liassic shallow water platform deposits, allowing to define the presence of depositional hiatuses between the top of the platform and the overlying basinal facies. Particular care would be devoted to clarify the dependence of the taxonomic variability to the paleoenvironmental changes, recognizing local effects that could potentially alter the chronostratigraphic assignments. The research skills to carry on such an integrated biostratigrahic study are to be found within the different research units participating to this project, and in part within the CARG project, with which the Trieste unit is actively involved.
Geochemical variations across the Sinemurian and T/J boundary drowning events (Cozzi, ETH Zürich)
- The geochemical study of the drowning sequences will focus primarily on Carbon stable isotopes, that will be used to construct 13Ccarb and, especially for the basinal sections, 13Corg profiles through the drowning sequence, aiming to reconstruct any perturbation of the Carbon cycle in coincidence with the drowning event. The nature of the drowning surface will be studied with detailed petrography, cathodoluminescence microscopy and microsampling for single component stable isotope analysis (C and O). Strontium isotopes will be collected both for correlation with other worldwide well calibrated sections and for understanding possible environmental changes at time of the platform drowning (increased hydrological cycle and runoff);
- the chemostratigraphic study of the T/J boundary can be a valid substitute for assessing fossil-poor sequences. As shown by Galli et al. (2005), a negative shift of both 13Ccarb and 13Corg to values of -2‰ occurs in coincidence with the T/J boundary. This distinctive negative peak, if combined with the Late Triassic (Gawlick & Böhn, 2000; Muttoni et al., 2004) and Jurassic (Jenkins et al., 2002) chemostratigraphic data available today, becomes and extremely powerful marker, as it can be used to indirectly import in fossil-poor sections high resolution biozonations from well dated Tethys sections (e.g., ammonoids, conodonts, benthic foraminifera, palynomorphs, magnetostratigraphy). In this way it will be possible to compare the vertical facies evolution in both platform and basinal sections, evaluating the effects of the T/J boundary extinction event on the ‘carbonate factory' and establishing a relative chronology of the different drowning events. It should be possible, therefore, to discriminate between platform drownings caused by the T/J biotic crisis and others possibly related to sudden downfaulting, as suggested by Carulli et al. (1998, 2003), or else by environmental changes (changes in seawater salinity, circulation, nutrients, etc.). Last, it will be possible to resolve the decadal-long discussion about the true age of the base of the Calcari Grigi Fm and the top of the Dolomia Principale and Dachstein Limestone formations;
- where the negative shift in both 13Ccarb and 13Corg will be found, additional sampling for Boron stable isotopes (11B) will be performed. B isotopes are dependent on the pH of seawater, which is in turn controlled by the pCO2 in the atmosphere. Therefore, variations in atmospheric CO2, as suggested to have occurred in coincidence with the T/J boundary, Jurassic and Cretaceous carbonate platform drownings, could be directly assessed in the case of the eastern Southern Alps.