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Between Scylla and Charybdis (part 1): the sedimentary dynamics of the modern Messina Strait (central Mediterranean) as analogue to interpret the past
No full textThe present paper reviews a number of geophysical and geological datasets acquired in recent and less recent times on the modern Messina Strait, aiming at reconstructing the sedimentary dynamics of this complex system. In a second paper, the results achieved with the present study are used as proxy for interpreting the analogue lower Pleistocene succession cropping out along the margins of the modern strait. The tide-dominated Messina Strait separates the Italian peninsula and Sicily in the central Mediterranean Sea. This 3-km-wide marine passageway is governed by bi-directional tidal currents flowing along the strait axis, because of semi-diurnal tidal phase opposition, so that high tide in the Ionian Sea corresponds with low tide in the Tyrrhenian Sea and vice versa. Tidal currents cyclically accelerate passing through the strait central restriction and decelerate towards the strait exits, following reverse directions during each tidal phase. This hydrodynamics generates specific by-pass areas, sediment routes and accumulation patterns in the modern strait, based upon the varying bed-shear stress exerted onto the strait bottom by the converging/diverging M2 tidal waves. Recent multibeam-based investigations on the modern Messina Strait bottom reveal subaqueous morpho-bathymetric features, which allow a number of areas with specific morphologic characters and sedimentary processes to be identified: (i) the ‘strait-centre zone’ is the narrowest and shallowest by-pass sector, where the tidal currents reach their maximum strength eroding and scouring the substrate. (ii) Towards NE and S, two larger ‘dune-bedded zones’ occur. Here, clastic sediments transported as bed load and in suspension are accumulated and continuously reworked forming characteristic bedform fields, whose features reflect complex current patterns due to a strong tidal asymmetry. (iii) The ‘strait-end zones’ represent distal depositional areas, where decelerating currents rework fine-grained smaller moribund dunes. (iv) The ‘strait-margin zones’ are diffusely characterised by mass-wasting processes along the steepest sublittoral sectors, where sediment instability are caused by earthquakes or sea storms. Fan-deltas prograde from the gentler-sloping coastal segments, represent the main sediment entry points with episodic, high-magnitude discharge and are influenced by the tidal circulation only along their delta-front parts. However, a dogmatic zone partition, based on a ‘mutually-evasive’ mechanism of sediment distribution and consequent decrease in the transport capacity towards the strait ends is not everywhere applicable to the Messina Strait sedimentary dynamics. The examination of measured tidal ellipses reveals that although flood and ebb tidal currents are approximately equal in speed, they are collinear only across the narrowest zone, being separated as the strait enlarges. This flow separation has possibly generated two main populations of flood and ebb bedforms, which migrate at high angle and with reverse direction in the two dune-bedded zones of the strait. Moreover, because of a strong water-mass stratification due to a marked difference in salinity, the denser flood constituent dominates over the lighter ebb current. As a consequence, the largest dunes observed in both the strait opposite depositional areas have northward-oriented lee faces, whereas superimposed smaller dunes have southward-oriented fronts, reflecting flood and ebb tidal constituents, respectively. Erosional features in the central sill, uncharacteristic bedforms having orientations not explained by modern currents and coarser grains in some places implying thresholds of motion larger than the modern currents all suggest that different, most likely, larger currents, probably occurred at some point in the past. It is suggested here that these features reflect an early stage of strait connection, during the initial sea-level rise after the Last Glacial Maximum. The data reviewed in the present work summarise the complex sedimentary dynamics of the modern Messina Strait and allow a comparative analysis of the ancient strait exposed along the modern margins, also suggesting insights for other examples of tidal straits developed under similar oceanographic conditions
Geoturismo in Basilicata: il Parco Letterario “Carlo Levi” ad Aliano quale veicolo di divulgazione di elementi di geologia del sedimentario.
No full textThe small Gilbert-type delta of Grassano (an example of regressive coastal deposits of the Bradanic Trough in the Irsina 50.000 sheet)
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Sedimentary features of the Lower Pleistocene mixed siliciclastic- bioclastic tidal deposits of the Catanzaro Strait (Calabrian Arc, south Italy)
No full textThe Plio-Pleistocene infill of the Catanzaro Basin (central Calabria) includes a Lower Pleistocene (Calabrian), ~80 m thick interval made up of mixed (bioclastic-siliciclastic) sand-sized sediments. The Catanzaro palaeo-strait (Fig. 1) is interpreted as an E-W-elongated, narrow linear basin linked the Ionian and Tyrrhenian seas, producing a marine seaway or strait which was tidally-dominated during the Lower Pleistocene. Two main fault systems bound the basin: an ENE-WSW to WNW-ESE-trending system to the North and an E-W-trending system to the South, both showing an en-échelon-type pattern (TANSI et alii, 2007). The Plio-Pleistocene infill of the Catanzaro Basin unconformably overlies the Upper Miocene conglomerates and evaporites and, locally, the Paleozoic crystalline basement. From the bottom to the top, the Catanzaro Basin infill consists of a Pliocene unit (about 100 m thick) made up of limestone and marls, and erosively overlain by the mixed succession focus of this work. This succession is top-truncated by Quaternary terraced deposits and/or by surfaces of modern exposure (CHIARELLA, 2011).
Mixed deposits of the Catanzaro Strait consist of a series of vertically-stacked bidirectional cross strata (Fig. 2) which exhibit a sedimentary facies association typical of tidal dominated environments, including herringbones, bundles and re-activation
surfaces (e.g., LONGHITANO et alii, 2012a, b)
Tidal sedimentation preserved in volcaniclastic deposits filling a peripheral seaway embayment (early Miocene, Sardinian Graben)
No full textThe Sardinian Graben System was a part of a NE-SW-oriented extensional basin, rotated counter-clockwise into a N-S-elongate basin, as consequence of the eastward migration of the Apennine orogenic front, in the western Mediterranean during the Neogene. Starting from the early Miocene, the Sardinian Graben was inundated by marine waters, turning progressively into a seaway, characterized by a tidal circulation as consequence of the connection between the Atlantic Ocean to the west and the Paratethys Ocean to the east. In this work, we investigate an area located marginally to the mid-seaway, whose well-exposed volcaniclastic deposits record the local expression of a tidal amplification occurring in a coastal peripheral embayment of the wider Sardinian Seaway. The studied succession is ca. 140 m thick and includes three main units: (i) the 20-m-thick lowermost unit consists of fluvio-lacustrine sandstones and conglomerates belonging to lower delta-plain and delta-platform environments; (ii) the second unit is 60–70 m thick and includes heterolithic sandstones and mudstones, exhibiting a variety of tidal sedimentary structures, and lies on the previous deposits through a tidal ravinement surface; these two units are mostly volcaniclastic in composition, reflecting the dominance of a magmatic source over other extrabasinal components; (iii) the uppermost unit is ca. 50 m thick, erosionally overlies the previous deposits and is made up of shoreface sandstones and open-shelf mudstones, whose composition indicates even less volcaniclastic elements and the prevalence of other clastic alongshore-derived components. Based on the results of the facies analysis, the study succession is interpreted as the infill of an incised valley along the southern flank of a structural high. The valley was excavated during a phase of relative sea-level lowstand (Aquitanian?) preceding a subsequent stage of major transgression (Burdigalian). Initially, a fluvial system impinged the valley from the west favoring the progradation of a deltaic system in a shallow-marine embayment. During an early stage of transgression, the isolation of a part of this coastal area generated by the building of a barrier island, produced the onset of a tidal-flat sedimentation over the previous deposits. A late transgression occurred through the inundation of this coastal area by marine waters and the consequent back-stepping of beach-barrier and open-shelf strata. The sedimentological features of this stratigraphic succession indicate as this valley was filled in a tectonic setting with a high rate of accommodation, where the tidal influence progressively increased during sediment accumulation, possibly due to the marginal position respect to a wider tide-dominated marine conduit. The present paper thus: (i) documents for the first time a tidal signature in the lower Miocene strata of Sardinia; (ii) indicates new possible relationships with other, coeval seaway successions of the western and northern Mediterranean area; (iii) suggests constrains for palaeogeographic reconstructions; (iv) and throws the basis for future researches on the Sardinian Seaway
Three-dimensional to two-dimensional cross-strata transition in the lower Pleistocene Catanzaro tidal strait transgressive succession (southern Italy)
No full textSandstone tidal cross-strata are the predominant sedimentary feature of strait-fill stratigraphic successions. However, although widely described in numerous studies, tidal strait-fill two-dimensional and three-dimensional cross-strata have rarely been reported to occur in discrete intervals which are laterally adjacent or vertically stacked, and the meaning of this stratigraphic architecture has not yet been fully investigated. Understanding of the processes responsible for changes in the internal features of modern and ancient tidal bedforms is essential in order to predict lateral and vertical heterogeneities in analogous reservoir strata. This facies-based study aims to interpret the three-dimensional to two-dimensional cross-strata transition observed in the lower Pleistocene mixed siliciclastic/bioclastic sandstone filling the Catanzaro Strait, in southern Italy, during a continuous phase of tectonically driven marine transgression. Tidal cross-strata disappear in the uppermost interval of the studied succession, where mudstone strata prevail. This stratigraphic trend is interpreted as the evidence of an important change in the tidal strait hydrodynamics due to a phase of relative sea-level rise. At the beginning of the transgression, three-dimensional tidal dunes migrated throughout the ca 3 to 4 km wide and ca 30 km long, WNW-ESE-oriented Catanzaro Strait, due to strong tidal currents amplified through the seaway and flowing in semi-diurnal phase opposition. As the intermediate phase of transgression enlarged the seaway width, the tidal current strength decreased as tidal water exchange occurred over a larger cross-sectional area. The progressive reduction of the bed shear stress modified three-dimensional tidal dunes into an extensive two-dimensional bedform field. At the end of the transgression, the further widening of the Catanzaro Strait into a ca 10 to 12 km wide marine passageway changed the tidally dominated strait into a non-tidal open shelf. The results of this research suggest the presence of a 'critical cross-sectional area' in the narrowest strait-centre zone which controls the activation and deactivation of tidal current amplification along a marine seaway
Why does a clinoform stop to progade? An example form the Tricarico mixed succession, Southern Apennines (Italy)
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A mixed bioclastic/siliciclastic flood-tidal delta in a micro-tidal setting: depositional architectures and hierarchical internal organization (Pliocene, Southern apennines, Italy)
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