Transparent Facies Research Articles

Oligocene magmatic accretion and transtensional tectonics in the Central Basin Fault rift of the West Philippine back-arc basin: New insights from high-resolution multichannel seismic data

High-resolution multichannel seismic transects that span from oceanic crust domains, and the Central Basin Fault rift to the Kyushu-Palau Ridge are utilized to investigate the dynamic mechanism of the Oligocene tectonic evolution in the West Philippine Basin based on an analysis of sismostratigraphic sequences and the geometries of acoustic substrates. In the oceanic domains, the physiography is characterized by ridges and troughs, and the pelagic stratigraphic succession with transparent facies covers the irregular top of the basement. Relatively homogeneous intracrustal reflections probably correspond to an area of abundant magma supply during the major spreading period. However, a widespread discrete, layered and high-amplitude intracrustal reflector is present in the Central Basin Fault rift that can be interpreted as an area of weaker and unstable magma supply after the major spreading period. The first group of reflectors shows typical low-angle fault shapes; the second and third groups of layered intracrustal reflectors are characterized by spatially limited high-amplitude seismic reflections. Inclined and mounded features of the intracrustal reflectors are interpreted as the result of tectonic processes, suggesting the presence of later magmatic accretion and transtensional tectonics within the acoustic substrate. Additionally, in the Kyushu-Palau Ridge, the anticline that appears within the stratigraphic infills of the basins serves as evidence of transtensional faulting. Overall, we propose that the Oligocene tectonic evolution of the West Philippine Basin occurred during a later transcurrent period due to Oligocene magmatic accretion followed by transtensional tectonics. The changes in the magmatic supply rate and tectonic processes of the Central Basin Fault rift may have originated from the compressive process within the Kyushu-Palau Ridge.

Traces of a prehistoric and potentially tsunamigenic mass movement in the sediments of Lake Thun (Switzerland)

Mass movements constitute major natural hazards in the Alpine realm. When triggered on slopes adjacent to lakes, these mass movements can generate tsunami-like waves that may cause additional damage along the shore. For hazard assessment, knowledge about the occurrence, the trigger and the geomechanical and hydrogeological mechanisms of these mass movements is necessary. For reconstructing mass movements that occurred in or adjacent to lakes, the lakes’s sedimentary record can be used as an archive. Here, we present a prehistorical mass-movement event, of which the traces were found in an alpine lake, Lake Thun, in central Switzerland. The mass movement is identified by large blocks on the bathymetric map, a chaotic to transparent facies on the reflection seismic profiles, and by a mixture of deformed lake sediments and sandy organic-rich layers in the sediment-core record. The event is dated at 2642–2407 cal year BP. With an estimated volume of ~ 20 × 106 m3 it might have generated a wave with an initial amplitude of > 30 m. In addition to this prehistorical event, two younger deposits were identified in the sedimentary record. One could be dated at 1523–1361 cal year BP and thus can be potentially related to an event in 598/599 AD documented in historical reports. The youngest deposit is dated at 304–151 cal year BP (1646–1799 AD) and is interpreted to be related to the artificial Kander river deviation into Lake Thun (1714 AD).

Morphosedimentary, Structural and Benthic Characterization of Carbonate Mound Fields on the Upper Continental Slope of the Northern Alboran Sea (Western Mediterranean)

Carbonate mounds clustering in three fields were characterized on the upper continental slope of the northern Alboran Sea by means of a detailed analysis of the morphosedimentary and structural features using high-resolution bathymetry and parametric profiles. The contemporary and past benthic and demersal species were studied using ROV underwater imagery and some samples. A total of 325 mounds, with heights between 1 and 18 m, and 204 buried mounds were detected between 155 to 401 m water depth. Transparent facies characterize the mounds, which root on at least six erosive surfaces, indicating different growth stages. At present, these mounds are covered with soft sediments and typical bathyal sedimentary habitat-forming species, such as sea-pens, cerianthids and sabellid polychaetes. Nevertheless, remains of colonial scleractinians, rhodoliths and bivalves were detected and their role as potential mound-forming species is discussed. We hypothesized that the formation of these mounds could be related to favorable climatic conditions for cold-water corals, possibly during the late Pleistocene. The occurrence on top of some mounds of abundant rhodoliths suggests that some mounds were in the photic zone during minimum sea level and boreal guest fauna (e.g., Modiolus modiolus), which declined in the western Mediterranean after the Termination 1a of the Last Glacial (Late Pleistocene).

New insights in glaciers characterization by differential diagnosis integrating GPR and remote sensing techniques: A case study for the Eastern Gran Zebrù glacier (Central Alps)

In GPR profiles, ice is usually imaged as a mostly electromagnetic transparent facies. However, diffraction events, as well as internal layering, can be also observed. In some cases, the bedrock below glaciers is masked by dense diffractions usually interpreted as the effect of liquid water pockets inside the so-called warm ice. However, the actual physical meaning of such GPR facies is not always obvious, because it can be related also to mixed debris and ice deposits.We adopted a strategy well known in medical sciences and referred as “differential diagnosis” in order to infer which is the actual meaning of a high scattering facies imaged within the Eastern Gran Zebrù glacier (Central Italian Alps) and, more generally, of all the internal glacier features. In fact, in many cases, there is no direct information to limit the subjectivity of geophysical interpretation; therefore, we provide all the discriminative hypotheses based on both independent and integrated criteria including GPR attribute analysis, imaging effects, reflection analysis, GPR frequency evaluations combined with geomorphological and remote sensing data obtained by two photogrammetric UAV and thermal infrared surveys.On the basis of the differential diagnosis, we concluded that the high scattering zone embedded within the studied glacier is most likely related to a mixture of ice and debris probably formed during a past shrinking phase. Beside this case study, this approach could be helpful in other GPR glaciological surveys, in which the target is related not only to the bedrock detection, but also to a detailed analysis of the internal facies of a glacier.

Latest Miocene restriction of the Mediterranean Outflow Water: a perspective from the Gulf of C\xe1diz

The Mediterranean-Atlantic water mass exchange provides the ideal setting for deciphering the role of gateway evolution in ocean circulation. However, the dynamics of Mediterranean Outflow Water (MOW) during the closure of the Late Miocene Mediterranean-Atlantic gateways are poorly understood. Here, we define the sedimentary evolution of Neogene basins from the Gulf of Cádiz to the West Iberian margin to investigate MOW circulation during the latest Miocene. Seismic interpretation highlights a middle to upper Messinian seismic unit of transparent facies, whose base predates the onset of the Messinian salinity crisis (MSC). Its facies and distribution imply a predominantly hemipelagic environment along the Atlantic margins, suggesting an absence or intermittence of MOW preceding evaporite precipitation in the Mediterranean, simultaneous to progressive gateway restriction. The removal of MOW from the Mediterranean-Atlantic water mass exchange reorganized the Atlantic water masses and is correlated to a severe weakening of the Atlantic Meridional Overturning Circulation (AMOC) and a period of further cooling in the North Atlantic during the latest Miocene.

Linking the high-resolution acoustic and sedimentary facies of a transgressed Late Quaternary alluvial plain (Gulf of Trieste, northern Adriatic)

We use geophysical and sedimentological data to study the high-resolution acoustic stratigraphy and sedimentology of a Late Quaternary alluvial plain (located in the Gulf of Trieste, northern Adriatic) which was transgressed during the post-LGM sea-level rise. With sub-bottom sonar profiling we determined six acoustic facies, of which the top five acoustic facies (A-E) were sampled with a gravity corer. We performed core descriptions, radiocarbon dating and granulometric analysis with a laser particle analyser. The acoustic facies and their corresponding sediments are associated with three general sedimentary environments: Last Glacial to Younger Dryas alluvial (E-B), Early Holocene transgressive (Ab) and Holocene shallow marine (Aa). Acoustic facies E with low-amplitude chaotic reflection geometries is represented by cross-stratified sandy mud deposited by braided or wandering rivers. Acoustic facies D with high-amplitude and high- frequency sub-horizontal reflection geometries is represented by graded deposits (sandy mud and sandy clay grading into clay) of braided or wandering rivers. Acoustic facies C and B with individual low to middle amplitude reflections in an otherwise transparent facies are represented by overbank fine-grained sediments deposited by a meandering river systems. The top surface of AF B presents the Younger Dryas paleosurface in the geophysical record. Acoustic facies Ab with onlapping and concordant middle to low amplitude reflection geometries is represented by bioclastic transgressive sandy mud containing brackish mollusc assemblages. Acoustic facies Aa is acoustically transparent and contains bioclastic sandy mud with shallow marine mollusc assemblages. Our work provides a reference for future studies of the Late Pleistocene-Holocene transition on well-preserved low-gradient mid-to-low latitude continental shelves.

Transgressive deposits and morphological patterns in the equatorial Atlantic shallow shelf (Northeast Brazil)

The aim of this work is to identify and characterize the Late Quaternary morphological patterns of the west coast of the Ceará shelf, Northeast Brazil, based on Landsat 8, sedimentology, and seismic data. The shallow shelf has three regions: the Acaraú high (AH), unconsolidated floor (UF), and Itapagé bank (IB). The AH occurs in the inner shelf (0–5 m water depth) and shows various bedforms and in part a rocky bottom. The UF (>5 m) has many subaqueous dunes with bioclastics, structures for catching lobsters, and an escarpment. The IB is an extensive feature (delimited by the 20-m isobath) with large dunes and a shipwreck. Three seismic boundaries were identified as follows: transgressive surface (TS), subaerial exposure (SE), and miscellaneous surface (MS = TS + SE). Four seismic units were defined as follows: U1 — transparent facies with low amplitude, underlying the SE; U2 — chaotic to parallel facies with moderate to high amplitude, overlying the SE and underlying the TS; U3 — chaotic facies above the SE or TS and below the TS or sea floor, with the largest spatial extension; U4 — periodically occurring chaotic facies above the TS and below the seafloor. U1 is probably related to the falling stage systems tract or previous highstand systems tract. U2 and U3 belong to the transgressive systems tracts and U4 is associated with bedforms of the highstand systems tract. Ancient topography favored the development of transgressive deposits and modern bedforms. The AH, escarpment, and IB are morphostructures influenced by Acaraú sub-basin faults. The inner shelf is characterized by shallow geomorphology strongly influenced by structural inheritance (Precambrian and post-breakup of Pangea), bedrock control, and the Holocene transgressive systems tract (the largest thickness occurs with subaqueous dunes).

Hormuz salt distribution and influence on structural style in NE Saudi Arabia

This study uses previously unpublished reflection seismic data and wells to map part of the western margin of the Hormuz salt basin for the first time, and to link Hormuz facies distribution to the evolution of major structures in NE Saudi Arabia. Most of these major structures host giant or supergiant oil fields in Mesozoic reservoirs. This study is based on seismic interpretation of structural style because the Hormuz occurs at up to 10 km or more depth present day and is not penetrated by any wells. In the eastern part of the study area, seismically transparent zones with structural elements diagnostic of salt tectonics pass laterally into layered seismic facies with local clinoform geometries. The transparent facies are interpreted as mobile salt, the layered facies as immobile evaporite basin-margin strata. The layered facies display onlap and fault-bound relationships with older basement domains, and in map view the boundary between layered and transparent seismic facies at the Hormuz level forms embayments and promontories on the west margin of the salt basin. Areas of mobile salt underlie domal and periclinal structures, such as Karan, Hasbah, Dammam and Khursaniyah. These structures display steeply dipping reflections at depth that can be interpreted as salt pillow flanks, with base salt locally interpretable, and have plan-view aspect ratios of 2 or less. Beyond the limits of layered seismic facies, between the embayments and westwards towards the Arabian Shield, seismic and well data indicate that the major structures are not salt cored, including Berri, Manifa, Safaniya, Jauf, Juraybi'at and Haba.

Incised channel morphology and depositional fill of the paleo-Seomjin River in the continental shelf of the South Sea, Korea

This study is the first attempt at describing and interpreting incised-channels on the continental shelf of the South Sea, Korea. The distribution, pattern, shape, and infilling of submerged channels incised across the continental shelf of the South Sea have been investigated based on approximately 1940 line-km of chirp and sparker data. The incised-channel system of the paleo-Seomjin River on the continental shelf is approximately 109 km long, 800–5000 m wide, and is more than 890 km2 in area. Meandering and straight types of channel are dominant on the inner shelf, with a change to straight and braided planforms on the mid to outer shelf. Sparker seismic data suggest the paleo-channels formed under fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). Channel fills are divided into five types, based on erosional surfaces and internal seismic reflectors: (I) transparent to semi-transparent incised channel fill; (II) parallel to sub-parallel incised channel fill; (III) complex incised channel fill; (IV) divergent incised channel fill; and (V) chaotic incised channel fill. The acoustically transparent facies of the channel fill type (Type I) suggest the presence of basin muddy deposits in a low energy environment, which were dominant in the inner shelf. These muddy sediments were likely during a period of higher sea level during the Holocene transgression. Types II, III, and IV are characterized by onlap, downlap, and wave or tidal ravinement surfaces, respectively, indicating that they were deposited during a transgressive stage, which type V represent basal incision surface. The evolution of paleo Seomjin River has been strongly controlled by sea level change and sediment transport processes since the LGM.

3D seismic analysis of buried tunnel valleys in the Central North Sea: tunnel valley fill sedimentary architecture

Abstract Fourteen overlapping 3D seismic reflection data sets, originally acquired by the oil and gas industry, were used to image and investigate complex networks of buried tunnel valleys in the UK sector of the Central North Sea. More than 180 individual tunnel valleys were observed and mapped to horizontal resolutions of 12–50 m and vertical resolutions of 8–10 m. The results presented here identify three recurring seismic facies that locally define stratigraphic organization from base to top of the buried tunnel valley fills: (i) high-amplitude disrupted units, (ii) a unit containing gently dipping disrupted reflections or transparent facies and (iii) well-layered continuous reflections. A comparison to similar works in the region is used to infer tunnel valley infill during changing depositional conditions from high-energy subglacial meltwater to quiet glaciolacustrine or glaciomarine environments.

High-resolution geophysical observations of the Yermak Plateau and northern Svalbard margin: implications for ice-sheet grounding and deep-keeled icebergs

High-resolution geophysical evidence on the seafloor morphology and acoustic stratigraphy of the Yermak Plateau and northern Svalbard margin between 79°20′ and 81°30′N and 5° and 22°E is presented. Geophysical datasets are derived from swath bathymetry and sub-bottom acoustic profiling and are combined with existing cores to derive chronological control. Seafloor landforms, in the form of ice-produced lineations, iceberg ploughmarks of various dimensions (including features over 80 m deep and down to about 1000 m), and a moat indicating strong currents are found. The shallow stratigraphy of the Yermak Plateau shows three acoustic units: the first with well-developed stratification produced by hemipelagic sedimentation, often draped over a strong and undulating internal reflector; a second with an undulating upper surface and little acoustic penetration, indicative of the action of ice; a third unit of an acoustically transparent facies, resulting from debris flows. Core chronology suggests a MIS 6 age for the undulating seafloor above about 580 m. There are several possible explanations, including: (a) the flow of a major grounded ice sheet across the plateau crest from Svalbard (least likely given the consolidation state of the underlying sediments); (b) the more transient encroachment of relatively thin ice from Svalbard; or (c) the drift across the plateau of an ice-shelf remnant or megaberg from the Arctic Basin. The latter is our favoured explanation given the evidence currently at our disposal.

Comment on “Lost tsunami” by Maria Teresa Pareschi et al.

] A numerical simulation recently published byPareschi et al. [2006a] (hereinafter referred to asPAR2006) suggests the possible occurrence of a majortsunami that should have hit the eastern Mediterranean inthe early Holocene. The catastrophic collapse of the easternflank of the Etna Volcano that formed the Valle del Bove[Calvari and Groppelli, 1996; Calvari et al., 1998] isreported as the trigger of this tsunami that generated apelagic turbidite nicknamed ‘‘Homogenite’’ [Kastens andCita, 1981] or ‘‘Augias turbidite’’ [Cita et al., 1984; Hieke,1984] throughout the eastern Mediterranean. Since itsdiscovery, this deposit characterized by an acousticallytransparent facies on seismic profiles has been related tothe 3500 years old collapse of the Santorini Caldera[Kastens and Cita, 1981]. Several papers published in thelast 25 years followed this interpretation [see Cita andRimoldi, 2005, and references therein]. So there is a generalagreement about the tsunamigenic origin for the widespreadhomogenite deposits, while a big discrepancy exists be-tween the timing of this event and the formation of the Valledel Bove which cannot be younger than 7600 calibrated yr.B.P. [Calvari and Groppelli, 1996].[

Shallowly buried, enigmatic seismic stratigraphy on the New Jersey outer shelf: Evidence for latest Pleistocene catastrophic erosion?

Chirp seismic profiles reveal a prominent seismic facies boundary 0–20 m below the seafloor within an 8.6 × 10.2 km area of the New Jersey outer shelf. This irregular boundary separates seismically transparent facies from underlying stratified facies. The irregularities form two populations of incisions trending northeast and east-northeast. Stratified blocks within the transparent facies, occasionally within presumed incisions, together with lithologic evidence, indicate that some of the transparent facies was formed by disruption of the stratified facies. Preservation of steep (50°–90°) incision flanks implies that deposition of the transparent facies closely followed disruption. Catastrophic erosion and redeposition following the multiple breaching of glacial lake dams to the north ca. 19–12 ka constitute the likeliest mechanisms both for producing facies-boundary incisions and emplacing the transparent facies, implying that local forcing can generate erosional unconformities on periglacial shelves in the absence of base-level change. This facies boundary adds complexity to the already complex, shallowly buried stratigraphic record of the last glacio-eustatic cycle on the New Jersey continental shelf, long considered typical of periglacial shelves worldwide.

Spatially irregular sedimentation in a small, morphologically complex lake: implications for paleoenvironmental studies

A sub-bottom acoustic survey of Devil Lake on the Canadian Shield in southern Ontario reveals three acoustic facies: (I) a moderately acoustically transparent, laminated sequence interpreted as a glacilacustrine deposit in glacial Lake Iroquois or a subsequent phase in water depths up to 200 m greater than at present, (II) a transitional more transparent, less layered facies interpreted as being deposited in a more distal glacial lake from erosion of sediment in the watershed exposed by the failure of the ice dam and lowering of the glacial lake before stabilization by the development of forests, and (III) an acoustically transparent facies with similar transmissivity to the water column, interpreted as Holocene gyttja. Each is spatially variable in extent and thickness in response to those processes. There is only a very weak relation between sediment thickness and the water depth in which it was deposited. Wave processes prevent deposition in water depths less than about 6 m and evidence of erosion to the greatest depths of the lake (>40 m) is pervasive. The data demonstrate the value of acoustic survey in assessing lacustrine processes and the history of lakes, and the significance of such documentation in planning a coring program and in interpreting the results.

Post-glacial depositional environments in a mid-high latitude glacially-overdeepened sea loch, inner Loch Etive, western Scotland

A high-resolution seismic and gravity coring survey has been conducted on inner, and part of outer Loch Etive, a 30 km long, 150 m deep sea loch on the west coast of Scotland. The seismic reflection profiles reveal a 30–60 m thick sediment pile that has accumulated on the floor of the loch since the last (Younger Dryas stadial) glacial re-advance 11–10 ka BP. Four seismic facies have been identified grouped into two seismic sequences, A and B, which are separated by a distinct reflector termed ‘E1’. A highly reflective facies is interpreted as direct river input, an acoustically well-laminated, draped facies interpreted as fine-grained river sediments and a well-laminated to transparent facies interpreted as glaciomarine and glaciolacustrine. Throughout the loch, methane gas-rich sediments occur producing a chaotic seismic facies. The transition from glacially-influenced to river-dominated deposition is displayed by the seismic reflector E1. Core analysis identified eight lithofacies. The oldest sediments are grey muddy sands and olive sandy silts, overlain by younger laminated greeny-grey muddy sand and greeny-grey and greeny-black homogeneous watery sandy mud. Abundant shell material is present throughout, notably the bivalve Arctica islandica, from which a 14C age of 9490±90 yr BP was obtained from the laminated greeny-grey muddy sands. The sediments are interpreted as being of glaciomarine and glaciolacustrine (Younger Dryas to early-mid Holocene) and more modern riverine-derived origin (mid-latest Holocene).

Resistivity and georadar mapping of lacustrine and glaciofluvial sediments in the late-glacial to postglacial Store Amose basin, Denmark

Resistivity profiling, resistivity sounding, georadar profiling and analyses of borings are used in a sedimentological and geomorphological study of shallow subsurface deposits in the proximal (southeastern) part of the Store Amose lacustrine basin in eastern Denmark. Areas of low, intermediate and high resistivities are located by resistivity profiling. Low-resistivity areas (type I) correspond to a thick unit of water-saturated lake sediments (fine detritus gyttja and siltlclay) which were deposited in a large lake system from the late Weichselian to the late Holocene. In intermediate-resistivity areas (type II), a relatively thick top layer of fresh-water peat and drifted gyttja or dry gyttja occurs. Fine-grained lake sediments, drifted gyttja and peat are equivaIent to transparent facies and low penetration (max. c. 2 m) in georadar profiles. Resistivity soundings indicate a shallower high-resistive substratum of meltwater sand and gravel in type II areas than in type I areas. The lake and fen sediments fil1 a highly irregular lake bottom relief, interpreted to have been formed in glaciofluvial channel systems and as dead-ice holes during the final Weichselian deglaciation. Sandy and gravelly domes and ridges, interpreted as proximal glaciofluvial deposits, are exposed at the ground surface in high-resistivity areas (type III). Convex surfaces of glaciofluvial bars and sides of meltwater channels are distinct georadar reflectors. Shallow, glaciofluvial bars were islands in the Holocene lake system and surrounding intermediate-resistivity areas represent shore zones of these islands (possible settlement areas).

Holocene depositional history of the Fluviá—Muga prodelta, northwestern Mediterranean Sea

Seismic analysis of the Fluviá-Muga prodelta, located on the Gulf of Rosas continental shelf, northwestern Mediterranean Sea, is a prograding oblique-sigmoidal depositional wedge. The internal seismic pattern of this wedge records its sedimentary history developed during the Holocene sea-level highstand. Three major types of seismic facies have been identified within the prodeltaic wedge: (1) continuous stratified facies, that characterizes the proximal prodelta environment; (2) discontinuous stratified facies, that describes the middle prodelta environment; and (3) a transparent facies representing the distal prodelta environment. The growth pattern of the prodelta is chiefly controlled by the seasonal character and low water discharge of the Muga and Fluviá rivers, the multiple changes in their mouth locations, the local shelf-current regime developed in the Gulf of Rosas embayment and the predominance of gravity-induced sedimentary processes.

Holocene Prodeltaic Depositional History in the Gulf of Rosas, Northwestern Mediterranean Sea: ABSTRACT

Prodeltaic sediment in the Gulf of Rosas, northwestern Mediterranean Sea is supplied mainly by the seasonally controlled Fluvia and Muga rivers and, until the seventeenth century, by a distributary of the southern located Ter River. These rivers, which had several distributaries in the past have developed a 120-km{sup 2} delta plain in a wave-dominated coast, prograding offshore by beach ridge development. High-resolution seismic profiles (3.5 kHz) and gravity cores were examined to evaluate the transgressive and highstand progradational history of prodelta deposits in the Gulf of Rosas. They cover an extension of about 365 km{sup 2} between the 20-m and 120-m isobaths. They extend 23 km offshore and, toward the south lie with the prodeltaic body of Ter River. Over a major flat erosional unconformity, related to postglacial Versilian transgression, the following pattern of seismic facies is seen progressing offshore: (1) stratified facies (5-15 m thick) with closely spaced southeastward-prograding reflectors, (2) chaotic facies (4-21 m thick) in which lenses with reflectors of low continuity and high acoustic amplitude predominate, and (3) thin transparent facies (2-7 m thick). An area of gas-charged sediment (90 km{sup 2}) is located between the 20 m and 80 m isobaths, mainly affecting the stratifiedmore » facies, from 2 to 6 m below the sediment/water interface. Sediment stratigraphy is well correlated, with the pattern of seismic facies showing a complex pattern of coarsening- and fining-upward sequences. Since the last Holocene transgressive stage and the following establishment of the present sea level highstand, a regressive depositional wedge-shaped sequence has been prograding on the Gulf of Rosas continental shelf.« less

Mechanisms of sediment transport and deposition: Sediment sequences and accumulation during the Holocene on the Thermaikos plateau, the continental slope, and basin (Sporadhes basin), northwestern Aegean Sea, Greece

Detailed 3.5-KHz seismic reflection profiling carried out on the Thermaikos plateau, the slope and the basin (Sporadhes basin), northwestern Aegean, has shown that during the Holocene, the area received fine-grained sediments discharged mainly by the rivers Axios, Aliakmon and Pinios. A well-developed oblique sigmoid progradation pattern has been observed on the seismic profiles taken across the deltaic Holocene sequences of these rivers. Three seismic facies have been identified within the oblique-sigmoid pattern: (1) Dark homogeneous facies that represent fine sand deposits along the delta front (topsets), (2) stratified facies that represent alternating muddier or sandier horizons (foresets), and (3) transparent facies in which the muddy sediments predominate (bottomsets). The base of the Holocene (reflector C) has been found beneath the oblique-sigmoid pattern. The thin homogeneous and transparent seismic facies throughout the slope and the basin has been interpreted as the Holocene muddy sediment sequence in the area. Bottom samples from the outer Thermaikos plateau have shown that it is floored by relict terrigenous sand at depths of 90–110 m. The sediment and clay mineral distribution and the presence of relict sand confirm that during the Holocene period, the fine-grained sediment dispersion and deposition was effected by the water-mass circulation. The majority (80–90%) of the sediments dispersed from the rivers during the Holocene has been captured by the extensive shelf (Thermaikos plateau), having deposited from the near-bottom nepheloid layer. Minor amounts (10–20%) of very fine-grained material have escaped and deposited from suspension (hemipelagic settling) in the slope and basin of the studied area.