The Mediterranean Sea hosts two subduction systems along the convergent Africa-Eurasia plate boundary that have produced strong ground shaking and generated tsunamis. Based on historical descriptions ...and sedimentary records, one of these events, in 365 CE, impacted a broad geographical area, including tsunami evidence for distances of 700-800 km from the source event, qualifying it as a 'megatsunami'. Understanding how megatsunamis are produced, and where they are more likely, requires a better understanding of the different secondary processes linked to these events such as massive slope failures, multiple turbidity current generation, and basin seiching. Our sedimentary records from an extensive collection of cores located in distal and disconnected basins, identify turbidites which are analyzed using granulometry, elemental (XRF), micropaleontological, and geochemical data in order to reconstruct their coastal or marine source. The results show that the 365 CE basin floor sediments are a mixture of inner shelf and slope materials. The tsunami wave produced multiple far-field slope failures that resulted in stacked basal turbidites. It also caused transport of continent-derived organic carbon and deposition over basal turbidites and into isolated basins of the deep ocean. The composition of sediment in isolated basins suggests their deposition by large-scale sheet like flows similar to what has been caused by the Tohoku earthquake associated tsunamis. This is significant for rectifying and resolving where risk is greatest and how cross-basin tsunamis are generated. Based on these results, estimates of the underlying deposits from the same locations were interpreted as possible older megatsunamis.
Integrated sedimentological and micropalaeontological (foraminifers, ostracods, pollen) analyses of eight continuously cored boreholes, up to 140 m deep, reveal the depositional history and the ...palaeoclimatic evolution of southeastern Po Plain (northern Italy) over the last 150 ky. Age assignments are supported by the chronostratigraphic sequence framework calibrated by radiocarbon and pollen data, allowing detailed correlation with the oxygen-isotope record.
Facies analysis (including identification of 12 distinct microfossils associations) and detailed stratigraphic correlations across several tens of km document a cyclic sedimentation pattern (including continental, coastal and shallow-marine deposits), which defines two transgressive–regressive sequences, deposited over the last two interglacial–glacial cycles. Two prominent stratigraphic markers, corresponding to wedge-shaped coastal sand bodies, are recorded between 0–30 and 100–125 m core depths. These sedimentary bodies were deposited during the two major transgressive pulsations and subsequent sea-level highstands of the last 150 ky, assigned to the Holocene and the Tyrrhenian (oxygen-isotope Substage 5e), respectively.
The stratigraphic architecture of post-Substage 5e deposits shows consistent patterns of coastal evolution with changing sea-level position. Lowering of sea level between 125 and 70 ky (onset of Substages 5d and 5b, and Stage 4) resulted in extensive and repeated basinward shifts of facies, which can be observed across closely spaced unconformity surfaces associated to alluvial plain sedimentation (falling-stage systems tract). The general phase of sea-level fall was punctuated by short transgressive phases (Substages 5c, 5a and Stage 3 interstadials), which led to widespread deposition of organic-rich (lagoonal and swamp) deposits. Upper Stage 3 and Stage 2 deposits (lowstand systems tract) are replaced across a significant part of the study area by a hiatal surface. Holocene interglacial deposits are characterized by a retrogradational stacking pattern of coastal plain and littoral facies (transgressive systems tract), reflecting invariably the landward migration of a barrier–lagoon–estuary system. Subsequent highstand deposition (highstand systems tract) was characterized by extensive progradation of wave-influenced deltas and strandplains.
The two major transgressive surfaces, of Tyrrhenian and Holocene age, are considerably easier to identify than the other key surfaces for sequence stratigraphic interpretation. The lowermost transgressive deposits display a pollen signature diagnostic of warm climate (interglacial) periods, showing wide forest development and relatively high pollen concentrations. By contrast, the deposits overlying the unconformable surfaces related to successive phases of sea-level fall are fingerprinted by pollen spectra dominated by
Pinus and non-arboreal pollen types, with very low pollen concentrations, reflecting the onset of a stable cold climate vegetation characteristic of stadial to fully glacial conditions.
The good match between facies architecture, pollen distribution and global sea-level evolution strongly suggests that Late Quaternary sedimentation in the Po Basin developed under a predominantly glacio-eustatic control. Stratigraphic architecture of the Po Basin thus can provide a useful analog for interpretation and correlation in the stratigraphic record of very rapid glacio-eustatic (fourth-order) cycles, with frequencies of about 100 ky.
This study investigates Ionian Sea seismo-turbidite (ST) deposits that we interpret to be triggered by major historic earthquakes and tsunamis in the Calabrian Arc. ST beds can be correlated with the ...AD 1908 Mw 7.24 Messina, AD 1693 Mw 7.41 Catania, and AD 1169 Mw 6.6 Eastern Sicily earthquakes while two previously unknown turbidites might have been generated by the AD 1818 Mw 6.23 Catania and AD 1542 Mw 6.77 Siracusa earthquakes.
Textural, micropaleontological, geochemical and mineralogical signatures of STs reveal cyclic patterns of STa, STb, STc and STd sedimentary units for each earthquake with an associated tsunami. The STa unit contains multiple ST stacks with different mineralogy, geochemistry foraminiferal assemblages and sedimentary structures that are deposited from synchronous multiple slope failures and turbidity currents. The STb homogenite graded mud unit overlying the STa unit is deposited by the waning flows of the multiple turbidity currents that are trapped in the Ionian Sea confined basin. The STc laminated and marine-sourced unit results from seiching of the confined water mass that appears to be generated by earthquake ruptures combined with tsunami waves. The STd unit is a tsunamite cap deposited by the slow settling suspension cloud created by tsunami wave backwash erosion of the shoreline and continental shelf. This tsunami process interpretation is based on the textural gradation of the upper unit and a more continental source of the tsunamite cap which includes C/N>10 and the presence of inner shelf foraminifera with a lack of abyssal species. This interpretation is in agreement with the lack of a tsunamite cap for the turbidite likely linked to the AD 1542 historic earthquake that is not associated with a tsunami. The new sedimentologic criteria identifies the final seiche and tsunamite cap deposits of STs and provides a model that can now be tested in other locations to better understand the different depositional processes of seismo-turbidites in confined basins.
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•Influence of seismic shaking and tsunami wave loading on seismo-turbidite deposition•First discovery of turbidite units recording basin wide seiching in the Ionian Sea•Sediment characterization of tsunamite caps in the Ionian Sea•Model of seismo-turbidite sequence for confined basins•Multiple slope failures during 1908CE Messina and 1693CE Catania earthquakes
Integrated sedimentological and micropaleontological study of 16 cores and 137 piezocone penetration tests, approximately 40 m deep, in the Comacchio area enables the documentation of the ...depositional history of southeastern Po Plain in the last 30 ka, in response to fluctuating sea level. Sedimentation within an alluvial plain was the dominant feature across the entire study area during the pronounced sea level fall that culminated in the Last Glacial Maximum. Thin lowstand fluvial sediments form the lower part of a shallow incised valley above the Last Glacial Maximum unconformity, whereas a characteristic paleosol separates the last glacial alluvial plain deposits from the overlying postglacial deposits in the interfluves. Transgressive and highstand deposits show a well‐developed stacking pattern of retrogradational (coastal plain and estuarine) and progradational (deltaic) facies. Detailed reconstruction of transgressive paleogeography shows evolutionary features that can be useful for refined interpretation of coeval and ancient analogs. At relatively early stages of transgression (10.5–9 ka B.P.), sedimentation in a coastal plain was restricted to the incised valley, whereas nondeposition and pedogenesis took place on the interfluves. With rising sea level (9–6 ka B.P.), a wave‐dominated, barred estuary developed in the former topographic low. At peak transgression, after filling up of the estuarine systems with coastal, back‐barrier sediments, wide areas outside the valleys were flooded, aggradation extended onto the interfluve unconformity, and a shallow marine depositional environment developed across most of the study area. The depositional history during the subsequent highstand phase was dominated by progradation of the early Po Delta and reflects the complex interplay between high‐frequency sea level fluctuations, climate, subsidence, and autocyclic processes.
Detailed benthic foraminiferal analyses performed on the Holocene subsurface deposits of modern Po Delta evidenced a complex palaeogeographic evolution. Hierarchical R- and Q-mode cluster analyses ...allowed to distinguish four assemblages indicative of different marine environments and sub-environments. Temporal and spatial distribution patterns of benthic foraminifers reflect changes in Po River discharge during delta evolution. The capability of foraminiferal assemblages to track nutrients and sediment supply oscillations permitted to recognize four main evolutionary phases (A–D), which took place after the maximum marine transgression (ca. 5500 cal yr BP).
Prodelta and delta front deposits are accumulated in the western part of the study area between the first stages of progradation and the late Middle Ages (ca. 10th century AD); these are paralleled, in a more distal position, by open-marine sedimentation, replaced upward by prodelta sediments (phases A and B). Owing to the combined effect of a major avulsion (Rotta di Ficarolo) and the Adriatic circulation patterns, an organic-rich zone developed in the eastern part of the present Po Delta at the transition to the Modern Age (phase C), around 600 cal yr BP. For the fist time a fossil mud-belt, although its marginal fringe, along with its peculiar microfossil content (
Nonionella turgida assemblage), is recorded beneath a modern delta plain. During the recent past an outstanding rate of sedimentation affected the Po Delta, as clearly evidenced by the deposition of 14 meters fine-grained prodelta sediments between 600 and 380 cal yr BP.
Prodelta deposits were finally replaced by delta front sands after the Porto Viro cut (ca. 350 cal yr BP), which marked the onset of Modern Delta (phase D).
Despite abundant geologic literature about the Po coastal plain, stratigraphic architecture beneath modern Po River Delta is virtually unknown. In this paper, combined sedimentological, ...micropalaeontological and geochemical investigations of three continuous cores (cores 1 to 3), about 40 m thick, enable for the first time the detailed reconstruction of subsurface stratigraphy in the Po River Delta area, along with its late Quaternary palaeoenvironmental evolution.
Lowstand deposits, formed during the Last Glacial Maximum, are expressed as a fining-upward succession of fluvial-channel and related overbank deposits, capped by a stiff, overconsolidated horizon. Above this unconformable surface, the lower post-glacial succession (transgressive systems tract or TST) is less than 7 m thick and includes a deepening-upward succession of back-barrier, shoreface and inner-shelf deposits. On the other hand, the overlying highstand systems tract is considerably thicker (up to 27 m) and comprises the bulk of recent delta progradation (
i.e., vertically stacked prodelta, delta front and delta-plain deposits).
Stratigraphic correlations within early transgressive, back-barrier deposits show vertically stacked, thin shallowing-upward successions, reflecting an internal subdivision of TST into small-scale parasequences.
The homogeneous shallow-marine clay that accumulated at the turnaround from transgressive to highstand conditions can be palaeontologically differentiated into two packages (
Miliolidae-dominated and
Ammonia/Cribroelphidium-dominated), reflecting open-marine and river-influenced conditions, respectively. Timing of maximum marine ingression (maximum flooding surface-MFS) is recorded within the open-marine clays at about 5500 calibrated yr BP by minor changes in microfauna.
Geochemical characterization of Core 1 testifies that distinct changes in sediment provenance accompanied the palaeoenvironmental evolution of the study area under changing relative sea-level conditions. At lowstand times, the study area was an alluvial plain under the influence of Adige River, as suggested by comparatively high Ba/Al values within pre-Holocene fluvial deposits. Increasing upward values of Cr/Al
2O
3 and Ni/Mg within TST suggest an increasing sediment provenance from the Po River during early stages of transgression, via the longshore drift.
The onset of modern-age Po Delta in the study area is precisely identified in Core 1 at 25.10 m depth, and has a distinctive, double signature. In terms of microfaunal assemblages, Po River sediments are clearly marked by the development of a characteristic foraminiferal association dominated by the opportunistic species
Nonionella turgida, a taxon typically abundant in the marginal zone of present northern Adriatic Sea mud-belt. Po River deposits are also fingerprinted by a peculiar geochemical composition, corresponding to maximum Cr and Ni concentrations.
A multidisciplinary study of a 105-m-long core was carried out on the Tyrrhenian coast of Tuscany, Western Italy. Detailed description of sedimentary facies, foraminifer and ostracod assemblages, ...pollen, and
14C ages is presented in this paper.
Identification in core of two transgressive surfaces (TSs) as the most prominent stratigraphic markers allows subdivision of the Late Quaternary stratigraphic succession into two transgressive–regressive (T–R) sequences, attributed to the last 150 kyr BP. Sequence boundaries have no unequivocal physical expression in the core.
Detailed pollen analysis documents a direct relationship between vertical facies evolution and climate fluctuations. Coastal to shallow-marine sediments in the lower part of T–R sequences were deposited during the last two interglacial periods (OIS 1 and 5e), under rising sea-level conditions and during the following sea-level highstands. By contrast, alluvial sedimentation (upper part of T–R sequences) took place during periods of sea-level fall and subsequent sea-level lowstands, and was invariably linked to the onset of glacial periods (OIS 4–2 and 6, respectively).
This paper presents the first detailed facies documentation of a Late Quaternary incised-valley fill sequence from Italy. About 51 m of Holocene sediments are recorded beneath the present Arno River valley. Early transgression is documented in the lower part of the incised-valley fill by wave-dominated estuarine facies overlying lowstand fluvial deposits. Late transgression records the rapid landward migration of a beach-barrier system, followed by the establishment of an open-marine environment. Highstand sedimentation is represented by a shallowing-upward succession, which reflects progradation of the modern delta/strandplain.
The Quaternary deposits of the Lamone River Valley in the Northern Apennines are mainly composed of marine clays and sands. Micropaleontological investigations allow the reconstruction of the ...paleoenvironmental evolution of that succession by recognition of 13 foraminifer and ostracod biofacies diagnostic of upper slope to deltaic environments.87Sr/86Sr measurements were performed in well‐preserved fossils throughout most of the recognized biofacies. These are compared with the strontium (Sr) isotopic ratios of Pleistocene seawater. Samples collected within biofacies characteristic of slope to shallow marine environments, without significant riverine influence, provide Sr isotope ratios consistent with Pleistocene seawater. In contrast, fossils from shallow marine deposits of a river‐influenced zone may exhibit Sr isotope ratios lower than Pleistocene seawater. These anomalous ratios may be due to dilution by low‐87Sr/86Sr fluvial waters from the Lamone River that largely drain Tertiary sedimentary rocks.87Sr/86Sr ratios measured in brackish and freshwater ostracods from backshore sediments are substantially lower than that in Pleistocene seawater and support this interpretation. Sr isotope stratigraphy is an important tool for dating marginal marine sequences; however, caution should be used in applying this technique in shallow marine river‐influenced deposits.
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