Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova A.A. 2015

Transcription

1 MORPHODYNAMIC EVOLUTION OF MEANDERING CHANNELS IN TIDAL LANDSCAPES: SEDIMENTOLOGY AND STRATAL ARCHITECTURE Ph.D. candidate: LARA BRIVIO, III course Tutor: Prof. MASSIMILIANO GHINASSI, Co-Tutor: Prof. ANDREA D ALPAOS Cycle: XXVIII Abstract Meanders constitute one of the fundamental components of tidal systems, as related tidal point bars are ubiquitous features in lagoonal sedimentary successions. Unlike their fluvial counterparts they have scarcely been studied and their morphodynamics, internal architecture and sedimentary facies have scarcely been explored. To fill this gap three study sites were selected in the Venice Lagoon (Italy) and analysed on the basis of a multidisciplinary approach, which comprises the comparison of historical photos, the interpretation of high-resolution sub-bottom profiles, core logging analysis and 3D modelling. The main results stemmed out for these sites highlight that: i) the evolution of tidal bends occurs following abrupt changes in hydrodynamic conditions linked with the evolution of the whole network; ii) lateral tributaries can strongly influence the evolution of bends modifying local mechanisms of sediment and flow distribution; iii) the migration of tidal point bars occurs under aggradational conditions both in intertidal and subtidal setting. 1. Introduction Lagoons represent the result of a tight interaction between marine and terrestrial environments. Numerous processes control the evolution of these complex areas, which consist of three fundamental components: salt marshes, tidal flats and tidal channel networks. Tidal channels provide a pathway for the tide to propagate and are, thus, a primary control on the sedimentation and ecology of these environments (Hughes, 2012). Although meanders are ubiquitous features of the tidal landscape, very few papers have analysed their morphodynamic evolution (Marani et al., 2002; Solari et al., 2002; Fagherazzi et al., 2004) or internal architecture (Barwis, 1978), which is relatively unexplored and commonly investigated using facies models developed for fluvial meander bends. Towards the goal of providing new insight into the sedimentological and stratal architecture of tidal meanders, three study sites from the Venice Lagoon (Adriatic Sea, Italy) were selected and investigated through: i) comparison between historical aerial photos; ii) analyses of sedimentary cores; iii) highresolution geophysical investigations. 2. The study sites and methods The Venice Lagoon formed over the last 7500 years and, with an area of 550 km 2, it constitutes the largest Mediterranean brackish water body. It is connected to the Adriatic Sea through three inlets: Lido, Malamocco and Chioggia. The lagoon is subjected to a semidiurnal tidal regime, with an average tidal range of about 1 m and peak tidal amplitudes of about 0.75 m. Two of the selected study cases are located in the northern Venice Lagoon (S. Felice area); whereas the third site is located in the southern Venice Lagoon, close to the Malamocco inlet. Site 1. It is represented by the Gaggian channel, which consists of a m wide active channel forming a bend with a wavelength of about 600 m. It is an intertidal channel and drains a salt-marsh, located between Burano and Punta Sabbioni. The investigated segment of the channel is roughly 900 m long and is located along a bend showing a radius of about 200 m. The channel receives numerous tributaries, which drain both the bar top and the outer bank zone. On the outer bank, two main tributaries (T w and T e ) enter the channel near the bend apex, connecting the Gaggian channel with a tidal flat located northward. Medium to coarse-grained sand is common in the deepest part of the Gaggian channel, whereas fine-grained silty sand occurs in shallower areas and along the thalweg of the T w and T e tributaries. High-resolution seismic data were acquired along 27 transects oriented both parallel (2) and transverse (25) to the main channel axis. The main bounding surfaces, separating different sedimentary units, were defined and correlated using Move TM software, in order to defining the paleomorphology of the meander bend at different depositional stages. 1

2 Site 2. It is located between Burano and Punta Sabbioni and is represented by an inactive meander loop, which drained a small portion of the San Felice salt marsh. The loop is made by a 6 m wide, mudfilled channel showing a radius of curvature of about 13 m. A total number of 150 cores were recovered using a manual corer equipment, which prevents sediment compaction. On the point bar, 13 cores were collected along a regularly spaced grid (5 m per side), whereas 27 were collected in selected sites in order to refine acquired data. Core logging was carried out following the basic principles of facies analyses in order to typify different deposits. The high-resolution sedimentary logs were placed in a virtual space and correlated using the software Move TM (Midland Valley), that allows to obtain the threedimensional representation of all cores in a georeferenced space. Every contact between different types of deposits was associated to a plain point of a specific horizon, and the surfaces bounding different deposits were consequently defined in space. Site 3. It consists of a sub-tidal channel ranging in width between 70 and 100 m and showing a wavelength of about 1 km. In order to provide a fully 3D architecture of the meander bend deposits, subbottom profiler investigations were carried out boating over the channel and related overbank areas. Seismic data were acquired along 28 transects oriented both parallel (3) and perpendicular (25) to the channel of the meander. The main bounding surfaces separating different sedimentary units were defined by interpreting onlap, downlap, and toplap geometries. Using a 3D modelling software (Move TM ), the interpreted seismic sections were correlated, with the goal of defining the paleo-morphology of the meander bend. Using a hand auger coring sampler 8 cores were collected in some selected sites, in order to calibrate the sub-bottom sections. Core logging was carried out following the basic principles of facies analyses to define the main processes associated with bar accretion. 3. Results and Discussion Site1. The analysis of geophysical data reveals the presence of three sedimentary units (hereafter called Unit 1, 2 and 3), bounding by two laterally extensive key surfaces. Unit 1 overlies the channel basal surface, which locally occurs between 7 and 10 m below mean sea level and overlies alluvial Late Pleistocene deposits. This unit reaches 6 m in thickness along the seaward side of the inner bank, while thins in the landward side of the bend. It consists of inclined deposits dipping toward the outer bank, which register an ebb- tide transport direction in the longitudinal section. It documents the older stage of the meander bend evolution, when sedimentation occurred along the inner bank due to the occurrence of a bend-apex helicoidal flow. Unit 2 occurs in the landward side of the bend. It erosionally overlies Unit 1 and locally the Caranto deposits, reaching the maximum thickness of about 4 m. Inclined beds dip toward the outer bank at about and agree with dominance of ebb currents. Some minor internal truncations are present. The erosive nature of the basal surface suggests an increase in water discharge, which heralded the seaward propagation of a sediment wave (Unit 2), which mainly accreted above Unit 1 along the landward side of the inner bank. Unit 3, up to 3.5 m thick, erosionally overlies Unit 2 along the outer bank of the bend. Beds dip at about 10 toward the inner bank and define two lobes at the outlet of the Tw and Te tributaries. The proximal part of such lobes includes channelized deposits, with a concentric fill geometry. Accumulation of these lobes caused flux concentration against the inner bank of the main channel, triggering its degradation. Stacking of unconformity-bounded units shows that evolution of the bend occurs under abrupt variations in sediment and water discharge. Differently from fluvial bends, where confluence bars are commonly removed during flood events, lobes developed at the outlet of lateral tributaries can cause aggradation along the outer bank and modify flow distribution along the bend. Site 2. The abandoned channel succession of the S. Felice area consists of five main type of deposits: i) salt marsh deposits, consisting of brown mud, with millimetric laminae of fine to very fine sand; ii) point bar deposits, made of grey-bluish fine sand grading upward into sandy mud; iii) channel lag deposits, characterized by massive grey-bluish medium sand, with mud clasts, wood fragments and shells; iv) channel plug deposits, made up of massive, dark grey-bluish, organic-rich mud; v)sub-tidal platform deposits, consisting of grey-bluish mud with shell-rich sandy layers. Spatial interpolation of the 2

3 boundaries between these different types of deposits allowed to define a 3D architectural model of the point bar deposits. In a section parallel to the point bar axis, this model reveals the geometry of the trajectory of channel thalweg and bar brink zone during channel bend expansion. Specifically, migration of the channel thalweg defines a flat, sub-horizontal trajectory, whereas the bar brink zone describes a clearly rising trajectory. These features show that bar migration occurs under aggradational conditions, which causes a progressive increase in the channel cross section that allows the section itself to accommodate an increasing tidal prism. Site 3. Sub-bottom profiler investigations provided insights on stratal architecture and sediment distribution across the study bend. The internal architecture, defined by the interpolation of the main bounding surfaces, shows that the bar brink zone describes a rising trajectory during bar migration, pointing to lateral accretion under aggradational conditions. Facies analyses on the recovered sedimentary cores highlights that: i) bar top deposits are made of wave-reworked, massive grey sandy silt, mixed with very fine to fine sand with abundant shells; ii) point bar deposits are made of heterolytic beds, which consist of alternating mud and fine to medium sand, with abundant plant debris and shells; iii) outer bank deposits are made of massive dark grey mud, with organic laminae and abundant shells, locally in life position. Sedimentary features of these deposits indicate that the bar continually migrated in subtidal setting, where the bar top was winnowed by waves during the main storm events. 4. Conclusive remarks The main remarks stemmed out from the present study can be summarized as follows: 1) The evolution of tidal bends occurs following abrupt changes in hydrodynamic conditions, which do not necessarily reflect local conditions, but can be related with morphodynamics changes of surrounding areas. 2) Differently from fluvial realm, where confluence bars are commonly removed during flood events, a significant amount of sediments can accumulate at the outlet of lateral tributaries, in tidal meander bends. These accumulations can influence the main distribution of flow and sediment along the bends. 3) Tidal point bars tend to migrate under aggradational conditions both in intertidal and sub-tidal settings as attested by the rising trajectory defined by the bar brink zone during bar expansion. References BARWIS, J. H Sedimentology of some South Carolina tidal-creek point bars, and a comparison with their fluvial counterparts In: Miall A. D. (ed.). Canadian Society of Petroleum Geologists. FAGHERAZZI, S., GABET, E. J. and FURBISH, D. J The effect of bidirectional flow on tidal channel planforms. Earth Surf. Process. Landforms, 29, HUGHES, Z. J Tidal Channels on Tidal Flats and Marshes In: DAVIS Jr. R. A. DALRYMPLE R. W. (ed). Principles of Tidal Sedimentology. Springer, Netherlands, 621 pp. MARANI, M., LANZONI, S., ZANDOLIN, D., SEMINARA, G. and RINALDO, A Tidal meanders. Water Resour. Res., 38, SOLARI, L., SEMINARA, G., LANZONI, S., MARANI, M. and RINALDO, A Sand bars in tidal channels, part two, Tidal meanders. J. Fluid Mech., 451, SUMMARY OF ACTIVITY IN THIS YEAR Courses: 2013: L. SALMASO, L. CORAIN, S. BONNINI, R. ARBORETTI: Statistica applicata alla sperimentazione scientifica (2013). Department of Geosciences, University of Padua 3

4 E. CALANDRUCCIO: Seminari di Inglese parlato, Dipartimento di Geoscienze, Università degli Studi di Padova. L. GULICK: Seminari di Inglese scientifico, Dipartimento di Geoscienze, Università degli Studi di Padova. V. MASELLI: Seismic and sequence stratigraphy, Dipartimento di Geoscienze, Università degli Studi di Padova. R. CARNIEL: Analisi spettrale e dinamica di serie temporali, Dipartimento di Geoscienze, Università degli Studi di Padova. R. J. ANGEL: Scientific Communication, Dipartimento di Geoscienze, Università degli Studi di Padova. 2014: A. KROON, T. AAGAARD, T. J. ANDERSEN, J. BARTHOLDY, V. B. ERNSTSEN, M. FRUERGAARD, M. AUSTIN, T. SCOTT, H. DE SWART: Coastal and Estuarine Morphodynamics, Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen. J. BARTHOLDY, M. S. SEIDENKRANTZ, L. O. BOLDREEL, V. B. ERNSTSEN, J. B. JENSEN: Marine Geoscience, Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen. 2015: N. SURIAN, F. FERRARESE: Corso GIS, Dipartimento di Geoscienze, Università degli Studi di Padova. Communications: BRIVIO, L., GHINASSI, M., D ALPAOS, A. and MARANI, M Stratal architecture of tidal meander bend deposits in the NE sector of the Venice Lagoon (Italy). In: XI GeoSed Congress September, Rome, Italy. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., FONTANA, A., RONER, M. and HOWES, N D internal architecture and morphodynamic evolution of tidal point bars: inferences from the Venice Lagoon (Italy). In: 31 st IAS Meeting of Sedimentology June, Krakow, Poland. RONER, M., D ALPAOS, A., GHINASSI, M., BRIVIO, L., FEDI, M., BELLUCCI, L., VIGLIOTTI, L., FRANCESCHINIS, E., AND REALDON, N Signatures of the past in tidal environments: insights from the southern Venice lagoon (Italy). In: 31 st IAS Meeting of Sedimentology June, Krakow, Poland. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., FONTANA, A. and RONER, M Tidal point bars: morphodynamic evolution and 3D modelling of internal architecture. In: XII GeoSed Congress September, Cagliari, Italy. RONER, M., D ALPAOS, A., GHINASSI, M., BRIVIO, L., FEDI, M., BELLUCCI, L., VIGLIOTTI, L., GILLI, A Evolution of salt marshes during the last millennium in the Venice lagoon (Italy). In: XII GeoSed Congress September, Cagliari, Italy. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., RONER, M., FONTANA, A. and HOWES, N Sedimentary features of an expansional tidal pointbar in the Northern Venice Lagoon (Italy). In: International Congress GeoSUB 2015 Underwater Geology October, Trieste, Italy. GHINASSI, M., D'ALPAOS, A., BRIVIO, L., FINOTELLO, A., RONER, M., CARNIELLO, L., GASPAROTTO, A. and HOWES, N Stratal architecture and flow distribution in a translating tidal point bar, Northern Venice Lagoon (Italy). In: International Congress GeoSUB 2015 Underwater Geology October, Trieste, Italy. RONER, M., D ALPAOS, A., GHINASSI, M., BELLUCCI, L., BRIVIO, L., FEDI, M., VIGLIOTTI, L Analysis of the morphodynamic evolution of a salt-marsh system in the last 1000 years: insights from the southern Venice lagoon. In: International Congress GeoSUB 2015 Underwater Geology October, Trieste, Italy. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., RONER, M., FONTANA, A. and HOWES, N Morphodynamics and sedimentary features of an expansional tidal point bar (Northern Venice Lagoon, Italy). In: 9 th International Conference on Tidal Sedimentology - Tidalites November, Puerto Madryn, Argentina. Posters: GHINASSI, M., D ALPAOS, A., BRIVIO, L. and MARANI, M Stratal architecture of tidal point bars and comparison with their fluvial counterparts. In: 10th International Conference on Fluvial Sedimentology (ICFS) July, Leeds, UK. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., CARNIELLO, L. and MARANI, M Morphodynamic evolution and stratal architecture of tidal meander bends through geophysical investigations and numerical modelling: inferences from the Venice Lagoon (Italy). In: European Geosciences Union, General Assembly 2014 (EGU). 27 April-02 May, Vienna, Austria. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., CARNIELLO, L. and MARANI, M Role of lateral tributaries in the morphodynamic evolution of tidal meander bends: inferences from the Venice Lagoon (Italy). In: 19 th International Sedimentological Congress 2014 (ISC) August, Geneva, Switzerland. BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., RONER, M., CARNIELLO, L., GASPAROTTO, A. and HOWES, N Planform evolution, flow velocity distribution and sedimentary features of a translating tidal point bar (Venice Lagoon, Italy). In: 9 th International Conference on Tidal Sedimentology - Tidalites November, Puerto Madryn, Argentina. 4

5 Publications: FIDOLINI, F., GHINASSI, M., ALDINUCCI, M., BILLI, P., BOAGA, J., DEIANA, R. and BRIVIO, L Fault-sourced alluvial fans and their interaction with axial fluvial drainage: An example from the Plio-Pleistocene Upper Valdarno Basin (Tuscany, Italy). Sedimentary Geology, 289, BRIVIO, L., GHINASSI, M., D'ALPAOS, A., FINOTELLO, A., CARNIELLO, L., MARANI, M., CANTELLI, A. and HOWES, N. Observational stratal architecture challenges current understanding of tidal meander morphodynamics, submitted to Pnas BRIVIO, L., GHINASSI, M., D ALPAOS, A., FINOTELLO, A., FONTANA, A. and RONER, M. 3D internal architecture and morphodynamic evolution of tidal point bar in saltmarsh setting: inferences from the Venice Lagoon. In preparation BRIVIO, L., GHINASSI, M., D ALPAOS, A. and FINOTELLO, A. Planform evolution and stratal patterns of a subtidal channel bend. In preparation RONER, M., D ALPAOS, A., GHINASSI, M., BELLUCCI, L.G., BRIVIO, L., FEDI, M.E., VIGLIOTTI, L. Morphodynamic evolution of a tidal system during the last millennium in the southern Venice lagoon. In Preparation. RONER, M., D ALPAOS, A., GHINASSI, M., BELLUCCI, L.G., BRIVIO, L., FEDI, M.E., FINOTELLO, A. Dynamics of salt-marsh landscapes under high sediment delivery rates: the case of the southern Venice lagoon. In Preparation. Visiting period: 26 th June 31 st October 2014: Visiting period at Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen (DK). Teaching activities: Teaching assistant: 25 hours, Geologia del Sedimentario, Laurea Triennale in Scienze Geologiche (2013/2014). Field assistant: 15 th - 17 th May 2014 Tuscany, Sedimentology (Prof. M. Ghinassi), Laurea Magistrale in Geologia e Geologia Tecnica (2013/2014). Field assistant: 20 th - 22 nd May 2015 Tuscany, Sedimentology (Prof. M. Ghinassi), Laurea Magistrale in Geologia e Geologia Tecnica (2014/2015). Teaching assistant: 25 hours, Geologia del Sedimentario, Laurea Triennale in Scienze Geologiche (2015/2016). In progress. Other: 13 th - 16 th October 2014: Fieldwork in Skallingen Peninsula, Denmark. 5