Variability of late-quaternary Transgressive sedimentation in the Northern Adriatic Sea

Size: px

Start display at page:

Download "Variability of late-quaternary Transgressive sedimentation in the Northern Adriatic Sea"

Piers Hodge
5 years ago
Views:

1 Variability of late-quaternary Transgressive sedimentation in the Northern Adriatic Sea Ph.D. candidate: GIORGIA MOSCON, II course Tutor: Dr. ANNAMARIA CORREGGIARI, Dr. ALESSANDRO FONTANA, Prof. CRISTINA STEFANI Cycle: XXVIII Abstract During the last sea-level rise in the Northern Adriatic Sea different generation of transgressive sediments were deposited and drowned in-place. Several transgressive deposits have been analysed in order to characterise their sand composition and their evolution before and during the last eustatic cycle. New researches on transgressive deposits located north of the Po delta were carried out in order to understand their provenance; a quantitative compositional point-counting of large amount of samples were analysed according to the standard Gazzi-Dickinson technique. Whereas one deposit, offshore Ravenna, completely covered by VHR seismic profiles and multibeam bathymetry was investigated to define the stratigraphic evolution during its formation. In each profile key surfaces were digitized and Digital Elevation Models was shaped. Petrographic data, compared with literature data, confirm a different provenance of the deposits according with their position. Seismic data highlight a variation of the fluvial trend during the formation of the deposit. Introduction The Adriatic Sea is an epicontinental semi-enclosed basin characterized by a shelf with a very low axial gradient in the northern and central part and by a steeper gradient in the southern part. During the last sealevel cycle in the Adriatic basin were deposited: low-stand deposits formed during the Last Glacial Maximum (LGM, years BP; Storm et al., 2008), transgressive deposits formed during the last relative sea-level rise and high-stand deposits confined in a narrow belt parallel to the modern shoreline. Across the low-gradient northern shelf, the stepwise, high-amplitude last relative sea-level rise favoured the deposition and in-place drowning of different generations of transgressive barrier-lagoon systems. Where present, the paralic transgressive deposits rest on a transgressive surface and are topped by a wave ravinement surface (Trincardi et al., 1994; Correggiari et al., 1996; Cattaneo and Steel, 2003). Previously these transgressive deposits have been studied in detail because their sand portion is an optimal potential of sand for beaches nourishment. This research project focuses on the characterization of main transgressive deposits in the northern Adriatic Sea in order to highlight their evolution and provenance during the last transgressive cycle. Transgressive Deposits The transgressive deposits, drowned in-place and partly preserved, are present in the northern Adriatic shelf as elongated reliefs parallel to the present coastline, characterized by a low thickness. Their preservation is due to a combination of different factors such as the rate of sea-level rise, the low gradient of the shelf and the coastal dynamics. These bodies are ancient coastal wedges, the more preserved deposits are characterized by a paralic portion and a marine portion. They consist in well sorted sand, delimitated at the base by the transgressive surface and capped by the ravinement surface. Methods The transgressive deposits under examination are located in a narrow area between -35 to -10 m w.d., the transgressive deposits north of the Po delta have been characterized through compositional analysis, whereas one deposit south of the Po delta has been studied with VHR seismic profiles and cores. COMPOSITIONAL ANALYSES Petrographic analysis: 130 cores has been described and drawn to select the better sand samples. A quantitative compositional-point counting of large amount of sand samples has been carried out, under polarizing microscope, according to the standard Gazzi-Dickinson technique (Gazzi 1966; Dikinson 1970; Zuffa 1980). For each samples 300 points were determined and the percentage of the main 1

2 components (Quartz, Feldspar, Lithic fragments + Extra-basin Carbonates) was calculated and plotted in triangular diagrams. Our samples have been compared with literature data (Gazzi et al., 1973; Marchesini et al., 2000; Stefani 2002; Monegato et al., 2010; Piovan et al., 2010) to better correlate them with present day rivers composition flowing in the northeast alluvial plain. Xrf core analysis To integrate the sand composition 10 selected cores have been analysed with XRF methods to define their chemical composition and its variation within the vertical profile of the core. Each core was scanned at 0,5 cm resolution with the AVAATECH scanner at the NIOZ institute (Nederland). The elements: Al, Si, P, S, Cl, K, Ca, Ti, Cr, Fe, Mn, Co, Rh, were measured at an X-ray voltage of 10 kv, while the elements: Zn, Ga, Br, Rb, Sr, Y, Zr, Au, Pb, Bi and the elements: Sr, Zr, Nb, Mo, Ag, Sn, Te, I, Ba were measured with an X-ray voltage of 30 kv and of 50 kv, respectively. The count time of each measurement was 30s. From the XRF analysis were obtained profiles per each element. SEISMIC PROFILE INTERPRETATION and DIGITAL ELEVATION MODEL (DEM) In each seismic profile four key surfaces (transgressive surface at the base, A surface, B surface and C surface) have been identified, traced and digitized with SeisPrho (Gasparini, L. and Stanghellini, G., 2009), an interactive computer program for processing and interpretation of high-resolution seismic reflection profiles. The seismic profiles show two different seismic facies: 1) semi-transparent unit with an irregular base indicative of sandy sediments and 2) interbedded transparent seismic facies with reflectors due to deposits with fine grain-size and strong reflectors interpreted as peat layers. Key surfaces, border four different units that highlight four depositional events during the formation of the transgressive deposit. After the seismic analysis a DEM has been obtained for each key surfaces, showing the morphology and the pattern dispersion. DATING In the area south of the Po delta some peat layers, corresponding to the key surfaces, have been sampled and dated with 14C technique. With these data it was possible to define the age of the study body, around cal. years BP, and the rate of sedimentation per each unit. OCEANOGRAPHIC CRUISE (ASII2014) From 30 September to 13 October, during the oceanographic cruise ASCII2014, new data have been acquired in order to increase the transgressive deposits database. For this research project have been sampled several vibro-corer in the transgressive deposits north and south of the Po Delta. Moreover, VHR seismic profile have been acquired to enlarge the area of one deposit off-shore Venice. Results The North Adriatic transgressive deposits located offshore, from Trieste to the Po mouth, have been studied with sand analysis. The comparison between our data with literature data allowed us to distinguish three different petrofacies. Petrofacies A, located offshore Chioggia, highlights a quartzolithic signature similar to the Po river composition emphasized by high amount of heavy minerals; Petrofacies B, located offshore Trieste, shows an eastern Alpine composition underlined by the high percentage of terrigenous carbonates; Petrofacies AB, located offshore Venezia, highlights a mixed composition between the Petrofacies A and B with carbonate Alpine supply and quartzolithic component. The XRF core analysis have integrated the petrographic analysis; the previous data confirm high Ca content in cores from Petrofacies B and high Si content in cores from Petrofacies A. Moreover some sandy cores highlighted an evident variation in the Ba/Al and Ba/Ti ratio, these variations could identify sedimentary contributions from different environments. Whereas the transgressive deposit south of the Po delta has been characterized by seismic analysis. The deposit is delimitated at the base by the transgressive surface (ts). The digitalization of the key surfaces highlight 4 different units. The morphology of each key surface, could give information on fluvial pattern. 2

3 During the deposition of Unit 1 and 2, the fluvial progradation seems to have ENE direction, while with the deposition of the Unit 3 a variation to ESE fluvial trend has been detected. The deposit s evolution is due to filling of the most depressed areas first in the northern part and later in the south. To better characterize this deposit the top of the Unit 1, Unit 2 and Unit 3 have been dated, respectively years cal BP, years cal BP and years cal BP. These data allow us to estimate the rate of sedimentation of the Unit2 at about 600 years and of the Unit3 at less than 300 years. Discussion The integrated study of transgressive deposits gave new information about sand provenances and, for a specific deposit, fluvial setting evolution. Petrographic data confirm a different provenance of the sand sediment in north Adriatic according with their position, in addition highlight a potential sediment dispersion linked to opposite marine currents in respect to the present or a northward shifting of the Po river paleo-mouth during the last sea-level rise. The sands characterized by Petrofacies A are located northern of the Po delta, their position suggest a northward shift of the Po system, or a branch of the river, during their deposition. Moreover in the deposit south of the Po delta it was possible highlight the evolution of the coastal plain environment during the last transgressive cycle identifying different fluvial phase between the transgressive and the ravinement surface. The variation of the fluvial pattern from eastnortheast to east-southeast could indicate the begin of fluvial supply from north. A hypothesis on the fluvial trend variation may be associated with tectonic causes. Future work The new cores from ASCII2014 will be described and analysed in order to increase the petrographic data and dating samples. One transgressive deposit belonging to the Petrofacies A, off-shore Venice, will be analysed in detail with cores and new VHR seismic profiles. The data obtained from the XRF core analysis will be interpreted in order to define some correlation between elements, sediment, deposition environment of the deposit and 14C chronology. Results elaboration focus more attention on the applicative use of the analysed sand. References CATTANEO, A. & STEEL, R.J Transgressive deposits: a review of their variability. Earth-Science Reviews, 62, CORREGGIARI, A., ROVERI, M., TRINCARDI, F., Late Pleistocene and Holocene evolution of the North Adriatic Sea. Il Quaternario 9, DICKINSON, W.R Interpreting detrital modes of greywacke and arkose. J. Sediment. Petrol. 40, GASPERINI L., STANGHELLINI G., SEISPRHO: An interactive computer program for processing and interpretation of high-resolution seismic reflection profiles. Computers & Geosciences, vol. 35 (7) pp GAZZI, P., ZUFFA, G.G., GANDOLFI, G., PAGANELLI, L Provenienza e dispersione litoranea delle sabbie delle spiagge adriatiche fra le foci dell Isonzo e del Foglia: inquadramento regionale. Mem. Soc. Geol. Ital. 12, GAZZI, P Le arenarie del flysch sopracretaceo dell Appennino modenese: correlazioni con il Flysch di Monghidoro. Mineral. Petrogr. Acta 12, MARCHESINI, L., AMOROSI, A., CIBIN, U., ZUFFA, G.G., SPADAFORA, E., PRETI D Sand composition and sedimentary evolution of a Late Quaternary depositional sequence, northwestern Adriatic Coast, Italy. J. Sediment. Res., MONEGATO, G., STEFANI, C., ZATTIN, M From present rivers to old terrigenous sediments: the evolution of the drainage system in the eastern Southern Alps. Terra Nova,

4 PIOVAN, S., MOZZI, P., STEFANI C., Bronze Age Palaeohydrography of the Southern Venetian Plain. Geoarcheology, 25 (1) STEFANI, C., Variation in terrigenous supplies in the Upper Pliocene to recent deposits of the Venice area. Sedimentary Geol., v STORM, J.E.A., WELTIE, G.J., TERRA, G.J., CATTANEO, A., TRINCARDI, F Coastal dynamics under conditions of rapid sea-level rise: Late Pleistocene to Early Holocene evolution of barrier lagoon system on the Northern Adriatic shelf (Italy). Quaternary Science Reviews, vol 27, n , pp TRINCARDI, F., CORREGGIARI, A., ROVERI, M Late Quaternary transgressive erosion and deposition in a modern epicontinental shelf: the Adriatic semi-enclose basin.geo-marine letters, 14, ZUFFA, G.G Hybrid arenites: their composition and classification. Jour. Sed. Petrology, v

5 SUMMARY OF ACTIVITY IN THIS YEAR Courses: Inglese Accademico per Dottorandi (30 ore), CLA Padova. G. DE BOER, From Data to Information JERICO Deltares Summer School 2014, Delft (14-20 June). H. EYNATTEN, I. DUNKL, G. MEINHOLD: Sedimentary Provenance Analysis,Department of Sedimentology and Environmental Geology, Geoscience Center, University of Gottingen (23-25 June). Posters: Moscon, G., Correggiari, A., Remia, A., Stefani, C., Fontana, A., Calabrese, L., Luciani, P., Perini, L., Piazza, R., Pinato, T. Transgressive Deposits: a sand reservoir in Adriatic shelf (Depositi transgressive: una riserva di sabbia nella piattaforma adriatica). Coast Expo 2014 (16-18 Settembre, Ferrara, Italia). Moscon, G., Stefani, C., Correggiari, A., Remia, A., Fontana, A. Preliminary characterization of transgressive deposits in the Northern Adriatic Sea (Italy). IAS 2014 (18-23 Agosto, Geneve, Svizzera). Moscon, G., Stefani, C., Correggiari, A., Fontana, A., Remia, A. Petrographic signature of the more recent transgressive sands in the Northern Adriatic Sea. 2 nd Workshop WGSG 2014 (25-27 Giugno, Gottingen, Germania). Moscon, G., Correggiari, A., Remia, A., Stefani, C., Fontana, A. VHR reconstruction of a portion of late Holocene transgressive adriatic system tract. EGU 2014 (27/04-02/05, Vienna, Austria). Teaching activities: Teaching assistant: 25 hours, Geologia, Laurea di primo livello in Scienze Naturali (2013/2014). Other: XRF analysis to NIOZ institute (Nederland) from 7 to 12 July. 9 cores sediment composition analysis. ASCI14 Oceanographic Cruise with Urania vessels (30 September-13 October) in the Northern Adriatic Sea to collect more data (vibro-corer, gravity-corer, high resolution seismic profile, multibeam data). 5

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova A.A

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova A.A VARIABILITY OF LATE-QUATERNARY TRANSGRESSIVE SEDIMENTATION IN THE NORTHERN ADRIATIC SEA Ph.D. candidate: GIORGIA MOSCON, III course Tutor: Prof. CRISTINA STEFANI, Dr. ANNAMARIA CORREGGIARI, Dr. ALESSANDRO