High-resolution Seismic Study Research Articles

High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal

The Azambuja fault is a NNE trending structure located 50 km north of Lisbon, the capital and most populous city of Portugal. The fault has been considered as a possible source for the historical, large earthquakes. Understanding this fault is a priority in seismic hazard evaluation of this region. The fault has a clear morphological signature. Miocene and Pliocene sediments are tilted eastward and cut by steeply dipping mesoscale fault segments, presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate a Quaternary slip on the fault of 0.05–0.06 mm/year. However, no direct evidence of the Azambuja fault affecting the Pleistocene or Holocene sediments was found so far. Here, we present the findings from high-resolution seismic reflection studies using both P- and S-waves over the Holocene deposits. The detection of small-throw faulting in ductile sediments is a challenging task. We show that multiple signatures, like perturbations in the reflection hyperbolae visible in shot and CMP gathers, interruptions of reflectors in stacked sections, lateral seismic velocity variations obtained by horizon velocity analysis, all at coincident locations, strongly suggest that the activity of the Azambuja fault has affected the Holocene sediments in the study area. The lateral velocity variations are corroborated by wavepath eikonal traveltime tomography and velocity analysis supported by seismic modeling. By means of 2D viscoelastic modeling, we explain the absence of fault-related diffractions and negligible back-scattered energy from the fault. Using data from nearby boreholes, we find that the 15 ka old alluvium cover has indeed been disturbed by the presence of shallow fault strands. Considering the estimated vertical throws and the empirical relationships between fault length, co-seismic rupture and magnitude, a slip rate of 0.07 mm/y, slightly larger than previously thought, is expected for this fault.

Reading the sediment archive of the Eastern Campeche Bank (southern Gulf of Mexico): from the aftermath of the Chicxulub impact to Loop Current variability

This is the first high-resolution seismic study showing how the Chicxulub impact shaped the eastern slope of the Campeche Bank in the south-eastern Gulf of Mexico. The induced shock wave fractured Cretaceous strata causing the collapse of the upper slope and shelf over a length of ca. 200 km. Failed material was either transported downslope or remained in parts on the accommodation space created by the collapsed. In the Cenozoic, the East Campeche Plastered Drift developed within the created accommodation space, controlled by the inflowing surface current from the Caribbean, which forms the Loop Current. The internal reflection configuration of the drift shows that the closure of the Suwannee Strait in the Late Oligocene and the closure of the CAS in the Mid to Late Miocene controlled the variability of the southern Loop Current in time. Since the Loop Current transports heat and moisture from the western Atlantic warm water pool into the North Atlantic and further to NW Europe by the Gulf Stream, the drift represents an archive for controlling factors that influenced climate of the northern hemisphere. This first high-resolution seismic reflection study from the eastern Campeche Bank expands the understanding of destructive processes that a meteorite impact induces into the earth system. Furthermore, these data document that the East Campeche Plastered Drift bears the potential to understand the link between the climate variability of the northern hemisphere and oceanic processes in the equatorial western Atlantic.

Delineation of Coal Bed Geometry using High Resolution Seismic Reflection Data for Uninterrupted Directional Drilling Exploration

Abstract The utilization of innovative technology by the coal mine industry requires proper guidance of sub-surface information so that it can provide uninterrupted exploration with less time and minimum cost. Sufficient knowledge about the coal beds that is variation of thickness, depth, and associated structures will facilitate the mine engineer to run machinery systematically. Conventional geological reports based on borehole data may not sufficient to adopt state-of-the-art drilling technology. To address these issues, high-resolution seismic survey (HRSS) was conducted to delineate the thickness of coal seams and mapping of structural features which are associated with coal seams. An effort was made to demonstrate the high-resolution seismic reflection study for picking of minor faults locations and thickness of coal seams in the central part of the Sohagpur, Gondwana basin that occurs at the junction of the Mahanadi and Son-Narmada rift systems in central India. The study reveals thick Barakar Formation and coal seams at a depth range of 300 to 550 meters. The thickness of coal seams and the location of minor faults were interpreted with the help of attribute analysis from the seismic sections. The obtained results from the seismic survey were useful for the selection of potential target zones which are favourable for drilling.

PROSPECTS OF OIL AND GAS POTENTIAL OF THE JUNCTION ZONE OF THE NORTH-EAST OF THE PRECASPIAN BASIN AND THE SOUTHERN SEGMENT OF THE URAL FOLD SYSTEM (AKTOBE URALIAN FOREDEEP)

The identification of promising areas of exploration to develop the resource base of the Aktobe region is an urgent issue due to the gradual depletion of proven hydrocarbon reserves. The subsalt complex of the Aktobe Uralian Foreland is a prospective object, where prospecting and exploration work was carried out unevenly in the 1950s and 1980s, and most of the seismic survey data of 2004-2008 are low-informative. The subsalt complex of the studied area has been poorly studied by high resolution seismic studies and deep exploration drilling and previously belonged to the territories with low hydrocarbon potential. On the basis of the conducted research, data on the structural and tectonic zoning of the junction zone of the Precaspian basin and the southern segment of the Urals Foredeep are summarized, the geological structure of the Aktobe Uralian Foreland area is substantiated, favorable geochemical criteria of oil and gas potential are revealed.

Depositional mechanism of the upper Pliocene-Pleistocene shelf-slope system of the western Malta Plateau (Sicily Channel)

A high resolution seismic stratigraphic study of the western edge of the Malta Plateau (central Mediterranean Sea) was conducted to reconstruct the depositional mechanisms of the shelf-slope system since the end of the Messinian salinity crisis (MSC; 5.3 Ma).The accurate interpretation of a large grid of variable-resolution seismic reflection profiles (multichannel and Sub Bottom) allows us to identify and map a Plio-Quaternary sedimentary sequence overlying the Messinian evaporites. The stratal pattern of this sequence is the result of a clinoform prograding system that constitutes the internal structure of a sedimentary shelf developed in a ramp about 2° slope with bathymetries that do not exceed 200 m. This type of shelf is a physiographic feature where the shelf edge separates littoral from bathyal realms, differing from the definition of continental shelf that implies the presence of a continent-ocean margin.The evolution of the western edge of Malta Plateau differs from adjacent sectors. Unlike to the nearby Gela Basin, where the Plio-Quaternary sedimentary sequences were affected by the thrusting of the Sicilian-Maghrebian chain until Middle Pleistocene, in the Malta Plateau the post-Messinian sedimentation was mainly controlled by subsidence and sea level fluctuations. In particular, the Malta Plateau represents a back bulge depozone where moderate fluvial sediment supply together with continuous subsidence favoured the progradation of the upper Pliocene-Pleistocene shelf-slope system at bathymetries not exceeding 200 m.

The Santo Antônio Bank: a high-resolution seismic study of a deflected ebb-tidal delta located at the entrance of a large tropical bay, eastern Brazil

The Santo Antonio Bank (SAB) is a large sand accumulation on the inner continental shelf of Salvador City, next to Todos os Santos Bay (TSB) entrance, eastern Brazilian coast. Since the sixteenth century, chroniclers report this feature as a navigational hazard. Previous studies have tried to determine the origin and volume of sands stored within this feature without success. In the present study, high-resolution shallow seismic surveys were used to investigate the architecture, origin, and evolution of the SAB. One hundred seventy kilometers of high resolution shallow seismic lines was acquired during December 2014 and February 2015 using a Meridata® sparker system operating on frequencies between 0.3 and 1.5 kHz, with a maximum 200 J of energy. Three major stratigraphic units were identified: the bedrock, a transgressive unit, and a highstand unit. The bedrock comprises sedimentary rocks of the Northern Camamu Sedimentary Basin and high-grade metamorphic rocks of the Precambrian basement. The transgressive unit is interpreted as the result of deposition in an estuarine environment infilling incised valleys. The highstand unit represents the SAB itself. A well-preserved proto-bank exists at the core of the SAB marking the early stages of bank development. This proto-bank drowned in place possibly during the 8.2-ka event. Present-day SAB and tidal prism volumes show that the bank reached a maturity phase of development. The volume of siliciclastic sands stored in the SAB is 5.98 × 108 m3. The SAB is potentially a major source of sands for beach nourishment for the urban beaches of the city of Salvador and metropolitan area, thus increasing the resilience of the city to future sea-level rises.

Effects of igneous bodies on modification of modern slope morphology: Insights from the continental slope offshore Dongsha Islands, South China Sea

A statistical analysis for the morphological parameters extracted from numerous seismic profiles, and a highresolution seismic study of the southeastern slope of the Dongsha Islands (South China Sea) with water depth between approximately 500 and 3 100 m, has revealed the variation of morphological features due to the intrusion of igneous bodies and associated sedimentary processes. Three types of the continental slope are distinguished: (1) a rough and steep slope with multiple igneous bodies (Type 1), (2) a relatively smooth and gentle slope with the single igneous body (Type 2), and (3) a smooth and gentle slope without igneous bodies (Type 3). These igneous bodies, formed in the post-seafloor spreading of the South China Sea, are often characterized by high positive seismic amplitudes, and chaotic reflections with complex shapes. The igneous bodies in Type 1 separated the slope into two or more upper sub-sags and a lower main-sag, in which the sub-sags and main-sag could be filled with sediments transported by alongslope bottom currents at the same time. Whereas, the igneous body in Type 2 just separated the slope into an upper sub-sag and a lower main-sag, in which the sediments could be transported into the lower main-sag only after the upper sub-sag has been filled up. Type 3 represents a normal slope with common clinoform progradation. The modern slope morphologies in the study area are the results of adjustments of the continental slope due to the intrusion of igneous bodies and associated sedimentary processes. The distinctions among three types of modern slope morphologies indicate different depositional conditions and adjustments of slope morphologies.

Deepwater Turbidite Lobe Deposits: A Review of the Research Frontiers

Deepwater/deep-marine turbidite lobes are the most distal part of a siliciclastic depositional system and hold the largest sediment accumulation on the seafloor. As many giant hydrocarbon provinces have been discovered within deepwater lobe deposits, they represent one of the most promising exploration targets for hydrocarbon industry. Deepwater exploration is characterized by high cost, high risk but insufficient data because of the deep/ultra–deepwater depth. A thorough understanding of the deepwater turbidite lobe architecture, hierarchy, stacking pattern and internal facies distribution is thus vital. Recently, detailed outcrop characterizations and high–resolution seismic studies have both revealed that the deepwater lobe deposits are characterized into four–fold hierarchical arrangements from “beds”, to “lobe elements”, to “lobes” and to “lobe complex”. Quantitative compilations have shown that hierarchical components of lobe deposits have similar length to width ratios but different width to thickness ratios depending on different turbidite systems. At all hierarchical scales, sand–prone hierarchical lobe units are always separated by mud–prone bounding units except when the bounding units are eroded by their overlying lobe units thus giving rise to vertical amalgamation and connectivity. Amalgamations often occur at more proximal regions suggesting high flow energy. A mixed flow behavior may occur towards more distal regions, resulting in deposition of “hybrid event beds”. These synthesized findings could (1) help understand the lobe reservoir distribution and compartmentalization therefore benefit the exploration and development of turbidite lobes within the deep marine basins (e.g. South China Sea) and (2) provide rules and quantitative constraints on reservoir modeling. In addition, the findings associated with deepwater turbidite lobes might be a good starting point to understand the sedimentology, architecture and hierarchy of turbidites in deep lacustrine environment.

High Resolution Seismic Reflection Studies in Godavari Coal Fields: Mapping of Coal Seams and Associated Structural Features

Abstract High Resolution seismic Reflection technique is one of the most suitable geophysical methods for mapping the shallow coal seams and associated structural features, which will provide essential input for adopting proper methodology for exploitation and mine planning of coal reserves. In the present study, depth of the coal horizons were determined and the geometry of the faults in Ramagundam area, Gondwana basin is illustrated which is located at the junction of the Bastar and Dharwar cratons. The High Resolution seismic surveys conducted along eight parallel profiles with coverage of surface area one km2 in the study region. The study reveals (1) seismic sections up to 1000m depth with thick, gently dipping Barakar Formation, (2) thick coal horizons at a depth range of 200 to 500m, and (3) NW-SE striking and gently dipping normal faults are observed in coal horizons. The obtained results will provide better knowledge of the coal reserves in the study area.

First Discovery of Channel-Levee Complexes In A Modern Deep-Water Carbonate Slope Environment

Abstract New high-quality high-resolution seismic data along the western slope of the Great Bahama Bank reveals a present-day channel–levee complex developed in a pure carbonate setting. This complex grew over two buried complexes separated by erosion surfaces, suggesting both the continuity of downslope gravity-driven processes along this carbonate slope, and channel migration through avulsion, processes similar to what happens along siliciclastic slopes. Complex morphology and geometry are similar to analogs described in siliciclastic systems, but the size of the presented carbonate complex is smaller by a factor of ten. Integrating high-resolution seismic and core studies shows that this complex was built by the stacking of gravity-flow deposits, including turbidites. It presently is inactive and buried by deposits from hemipelagic fallout or low-energy density processes channeled by the gully network; Recent sediments are reworked by along-slope bottom currents dominated by internal tides. The discovery of these channel–levee complexes has implications both on the conceptual models describing the behavior of carbonate slope systems and on hydrocarbon exploration by enhancing the reservoir-bearing potential of carbonate slopes.

Bottom topography, recent sedimentation and water volume of the Cerro Prieto Dam, NE Mexico

The Cerro Prieto dam, a relatively small water reservoir in NE Mexico, is one of the main resources of potable water for Monterrey, a city with a population of about fourmillion inhabitants. A high resolution seismic study using non-linear parametric sub-bottom echo-sounder SES-2000 was carried out in this water reservoir. High resolution acoustic data interpretation shows that the thickness of recent sediments due to siltation of the reservoir reaches 3.5-4.0m. It shows a high recent sedimentation rate (1-2 up to 14cm/ year). Based on the echo-sounder data, the first bathymetric map and a digital model of recent sediment thickness were designed. A significant (5-12%) difference between the volume capacity value used by National Commission of Water (CNA) and acoustic survey results was revealed. Differences between the CNA and acoustic data indicate storage losses from 12-17 up to 30million cubic meters. The results obtained through study such as this one, could be useful to improve a water resources management.

High-resolution seismic imaging of the Sohagpur Gondwana basin, central India: Evidence for syn-sedimentary subsidence and faulting

Gondwana sedimentary basins in the Indian Shield preserve a rich record of tectonic, sedimentary and volcanic processes that affected Gondwanaland. The Gondwana rocks were deposited in the linear rift basins that were formed during Permian–Cretaceous time, similar to their neighbours in Australia, Africa and Antarctica. In this study, we illustrate how Gondwana tectonics affected the Sohagpur Gondwana basin that occurs at the junction of the Mahanadi and Son–Narmada rift systems in the central India, through a high-resolution seismic reflection study along six profiles, covering the central part of the Sohagpur basin. The study reveals (1) ∼1000 m thick, gently dipping Barakar Formation, (2) thick coal seams at a depth of 350–550 m, and (3) NNW–SSE to NW–SE striking steeply dipping normal faults defining rift geometry. These results indicate that the Sohagpur basin contains a thick Lower Gondwana sedimentary succession with a high potential of coal resources and were affected by extensional tectonics. The rift structure in the study area is a syn- to post-sedimentary deformational structure that was formed arguably in response to tectonics that pervasively affected Gondwanaland.

Mega ultra low velocity zone and mantle flow

Mantle flow in Earth's interior has been inferred from a variety of geo-disciplines. Two continental-scale, nearly antipodal, large low shear velocity provinces (LLSVPs) at the base of the mantle, thought to be dense and chemically distinct likely play a significant role in mantle dynamics and plume generation, and hence are targeted in a high-resolution seismic study. We analyze broadband SPdKS waveforms using a 2.5D axi-symmetric finite difference wave propagation algorithm PSVaxi. Here we find patches of greatly reduced seismic wave speeds at the core–mantle boundary, i.e., ultra-low velocity zones (ULVZs), within the Pacific LLSVP, including the largest ULVZ detected to date, roughly 250×800km in lateral dimension and 10–15km thick, in an apparent hole in the LLSVP. The presence of this ULVZ in the LLSVP hole is well explained by dynamically merging, chemically-distinct piles containing ULVZs at their margins. The consequence of these merging piles may be to initiate anomalously large, infrequent plumes, as well as to provide a means to transfer isotopes to the surface.

High resolution seismic study of the Holocene infill of the Elkhorn Slough, central California

The seismic analysis of the sedimentary infill of the Elkhorn Slough, central California, reveals a succession of three main seismic units: U1, U2, U3, with their correspondent discontinuities d2, d3. These units are deposited over a paleorelief representing the channel location at least 8k years ago. The location of that paleochannel has not changed with time, but the geometry of the infilling sedimentary packages has done so through the years. Discontinuities d2 and d3 show a relic island or relative high in the center of the Slough that separated the sedimentation into two main small basins at least 3k years ago. There is evidence of erosion in the last two sedimentary units showing that the present erosive pattern began decades ago at minimum. We have correlated radiocarbon data of selected cores with the high resolution chirp profiles and reconstructed the infill for the Elkhorn Slough.In the most recent unit, the occurrence of numerous lateral accretion surfaces on both ends of the main channel is discussed within their environmental setting, tidal currents and the net ebb flux of the area. We have interpreted the presence of gas in the sediments of the slough, with a gas front located at the tops of units 2 and 3, which are discontinuities that reflect an effective seal for the gas. Our data shows no obvious evidence for seepage, although the shallow presence of some of the fronts points out the fragility of the environment in the present erosive conditions.

Dynamics of post-slab breakoff in convergent plate margins: a "jellyfish" Model

The Earth9s subduction zones are characterized by a complex architecture of the subducting slabs, including necking, tearing, detachment from the surface plate, breakoff or ultimate slab loss, in contrast to the idealized models as popularly envisaged. Here we address the fate of the subducted slab after its breakoff by employing a simple two dimensional numerical modeling. Our results agree well with the tectonic scenario and geometric evolution of slab breakoff and reveal that during the sinking of broken slab fragments, the shape of the low viscosity slab (1-1000 times more viscous than the surrounding mantle) evolves into an inverted plume, similar to the “<i>jellyfish</i>” model proposed in some recent studies on deep subduction slabs. Our results confirm that the penetration criteria of the jellyfish into the lower mantle are primarily governed by the viscosity contrast between the lower to upper mantle. An evaluation of the various parameters suggest that the broken slab fragments may achieve a diverse type of morphology, correlating well with a similar diversity observed from high resolution seismic tomographic studies.

Tracking deep mantle reservoirs with ultra-low velocity zones

Some regions of the Earth's lowermost mantle exhibit anomalous seismic properties within a thin zone, less than tens of kilometers in thickness, that directly overlies the core-mantle boundary (CMB). These regions have been dubbed Ultra-Low Velocity Zones (ULVZs) due to their greater than 10% drop in seismic velocities. High resolution seismic array studies have found small, localized ULVZs (e.g., 10 km thick and 50-100 km wide) with a large increase in ULVZ density (~ 10%) relative to the background mantle. Many studies note that ULVZ material may be chemically distinct, though P-to-S-wave velocity reductions are sometimes consistent with partial melting. The apparent absence of ULVZs in many regions of the CMB is consistent with having a distinct chemical signature, regardless of melt content. However, it is unknown how a small volume of very dense ULVZ material can be locally elevated, particularly in the presence of large-scale compositional reservoirs predicted by seismology, geochemistry, and geodynamics. We perform ultra-high resolution, kilometer-scale, thermochemical convection calculations for an entire mantle system containing three distinct compositional components in order to investigate how a ULVZ interacts with large-scale lower mantle compositional reservoirs. We demonstrate that convection can dynamically support small scale accumulations of dense ULVZ material, consistent with the size and density inferred from seismology. Furthermore, we show that ULVZs preferentially reside at the boundaries of large compositional reservoirs, which periodically break apart and merge together in response to changes in downwelling patterns. As they do, ULVZ material migrates and recollects in a systematic fashion. ULVZ material can become entrained in mantle plumes forming from reservoir boundaries, contributing to isotopic anomalies found in hotspot volcanism. Thus ULVZ detection helps to constrain large-scale mantle convection patterns, the locations of compositional reservoir boundaries, and the evolution of geochemical reservoirs.

Initiation, development, and benefits of Lithoprobe — shaping the direction of Earth science research in Canada and beyondThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.Lithoprobe Contribution 1480.

Lithoprobe is Canada’s national, collaborative, multidisciplinary, Earth science research project established to develop a comprehensive understanding of the evolution of northern North America. It is regarded internationally as one of the most successful national geoscience projects ever undertaken. Part of Lithoprobe’s success derives from its history and organizational structure. A one-year Phase I program (1984–1985) was highly successful. Phases II to V carried the project forward until 2005, when funding terminated. Lithoprobe was organized as a decentralized research network of multidisciplinary scientific studies and collaborating scientists. About 1500 scientific publications were generated. Lithoprobe’s legacy also includes economic and social benefits. New and improved understanding of Earth history provides petroleum and mining companies with an enhanced knowledge base. Lithoprobe demonstrated the applicability of high-resolution seismic reflection studies to mineral exploration. Development of a portable seismic refraction recorder and a long-period magnetotelluric system led to technology transfer to Canadian companies. Very high-resolution seismic reflection studies, applied first by Lithoprobe, are proving effective for exploration for uranium deposits in Saskatchewan. In the cratonic areas of Canada, Lithoprobe seismic and magnetotelluric studies have provided significant new information relevant to exploration for diamonds. On the west coast of Canada, Lithoprobe studies provided data and a framework for better understanding the mega-thrust earthquake hazard in the region. Lithoprobe established a highly effective public outreach strategy that involved the print and electronic media as well as material for educational purposes. Unequivocally, Lithoprobe has enhanced the already strong reputation of the Earth sciences in Canada.

Evidence for current-controlled morphology along the western slope of Hatton Bank (Rockall Plateau, NE Atlantic Ocean)

A multibeam bathymetric and high- (airgun and sparker) to very high-resolution (Topas) seismic study of the western slope of Hatton Bank (NE Atlantic), located between 600 m and 2,000 m water depth, has revealed a highly variable range of current-controlled morphological features. Two major seabed areas can be distinguished: (1) a non-depositional area corresponding to the top of the bank and (2) a depositional area in which the Hatton Drift has developed. Both areas are characterised by distinct morphologies associated either with rock outcrops and rocky ridges or with smooth surfaces, slides and bedforms controlled mainly by bottom currents interacting with the topography of the bank. The water depth separating the morphological areas probably coincides with the boundary of the Labrador Sea Water and the Lower Deep Water. Morphological features identified in the study area include contourite channels (moats, furrows and scours), fields of sediment waves, edges of contourite deposits, ponded deposits, scarps, gullies, ridges, depressions, slides and slide scars. These morphological features do not necessarily reflect present-day conditions but may have been associated with past current events, consistent with earlier interpretations.

Downslope-migrating sandwaves and platform-margin clinoforms in a current-dominated, distally steepened temperate-carbonate ramp (Guadix Basin, Southern Spain)

During the Late Tortonian, platform-margin-prograding clinoforms developed at the south-western margin of the Guadix Basin. Large-scale wedge-shaped deposits here comprise 26 rhythms of mixed carbonate–siliciclastic bedset packages and marl beds. These sediments were deposited on a shallow-water, temperate-carbonate distally steepened ramp. A downslope-migrating sandwave field developed in this ramp, with sandwaves moving progressively down the ramp to the ramp-slope, where they destabilized, folded and occasionally collapsed. Downslope sandwave migration was induced by currents flowing basinwards. During the Late Tortonian, the Guadix Basin was open north to the Atlantic Ocean via the Dehesas de Guadix Strait and connected east to the Mediterranean Sea through the Almanzora Corridor. According to the proposed current circulation model for the Guadix Basin for this time, surface marine currents from the Atlantic entered the basin from the northern seaway. These currents moved counter-clockwise and shifted the sediment on the ramp, forming sandwaves that migrated downslope. The development of platform-margin prograding clinoforms by the basinward sediment-transport mechanisms inferred here is known relatively poorly in the ancient sedimentary record. Moreover, these wedge-shaped geometries are similar to those found in some shelves in the Western Mediterranean Sea and could represent an outcrop analogue to (sub)-recent, platform-margin clinoforms revealed by high-resolution seismic studies.

Megaturbidite deposits in the Holocene basin fill of Lake Como (Southern Alps, Italy)

For the first time, limnogeological investigations have been carried out in Lake Como, the deepest lake of the Alps, combining a bathymetric survey (multibeam Simrad 3000) with a high-resolution seismic reflection study (single-channel 3.5 kHz sub-bottom profiler) and a coring campaign (gravity corer). This data set enables detailed characterization of the sedimentary subsurface in the western branch of the lake, the Como branch, which has a typical fjord morphology. This paper focuses on the deepest part of the Como branch (Argegno basin), in which up to 3.5-m-thick turbidite deposits are identified. The basin fill of the Como branch is characterized by well-layered draping and onlapping pelagic sediments that are locally affected by creeping and that are intercalated with mass-wasting deposits, in particular with two large debris-flow deposits evolving into megaturbidites in the deepest part of the basin. The multibeam data together with the acoustic-facies distributions and the volumes of these two major sedimentary deposits MT1 (∼ 3 × 10 6 m 3) and MT2 (∼ 10.5 × 10 6 m 3) indicate that they resulted from large slides at the northern tip of the Como branch along the steep slopes of a sub-lacustrine plateau. The estimated ages of these events, around the mid-12th (MT1) and early 6th (MT2) centuries, are extrapolated from mean sedimentation rates based on radiocarbon ( 14C) and radionuclide ( 137Cs) analyses from short cores in the Argegno basin. Possible trigger mechanisms leading to these catastrophic events in the Como branch include a combination of steep-slope overloading, with significant lake-level fluctuations related to Holocene climate change and/or earthquake shaking. The tentative age assignment places the age of both major mass movements near two other events: MT1 near the occurrence of a major earthquake in the Po Plain in 1222 AD (Intensity IX MCS, macroseismically derived magnitude 6.2) and MT2 near a megaturbidite triggered in ∼ 700 AD in a proglacial lake in southeastern Switzerland [Blass, A., Anselmetti, F., Grosjean, M., Sturm, M., 2005. The last 1300 years of environmental history in the sediments of Lake Sils (Engadine, Switzerland). Eclogae Geologicae Helvetiae 98, 319–332]. Since dangerous, tsunami-like waves (seiches) can be generated by large sub-aqueous landslides leading to such megaturbidites in this fjord-like basin, future studies are required: 1) to constrain the age of these catastrophic events; 2) to document the stabilities of the steep slopes in the Como branch, 3) to assess the expected frequency of such catastrophic events and 4) to model the propagation of large waves in the Como branch and their potential damage along the highly populated lake shore.