Turbidite Deposits Research Articles

Calcareous nannofossil biostratigraphy of the External Dinarides flysch (Vrčić‐Staravasa Pag Island, Croatia): A key to an Eocene tectono‐stratigraphic and paleoenvironmental interpretation

It is largely accepted that in the Dinaric shallow foredeep basin, the onset of Cenozoic synorogenic sedimentation is diachronous along strike and shows orogen‐parallel southward younging, creating a large uncertainty in the age of the Dalmatian flysch. Calcareous nannofossil biostratigraphy was studied to investigate a turbiditic succession exposed in Pag Island (Croatia) with the main purpose of constraining its age, which is still a matter of debate. The age assignment for the turbiditic deposition bears important implications for better understanding of the geodynamic and paleoenvironmental evolution of the External Dinarides. We logged a well‐exposed sedimentary section in the south‐western limb of the Pag anticline and performed calcareous nannofossil quantitative analyses revealing a high species diversity and abundant assemblage. The age diagnostic species indicate CNE14‐CNE15 biozones, suggesting that flysch deposition occurred in the Lutetian–Bartonian. This age can be further restricted to the CNE14 (42.37–40.51 Ma) by means of the lowest occurrence of Reticulofenestra reticulata and the highest occurrence of Sphenolithus furcatolithoides. Vertical facies variation and paleoecological indications suggest an increase in paleobathymetric depth during deposition. Our data support a post‐Lutetian age of folding and thrusting for the Pag Island region and suggest a relatively short time span during the middle‐late Eocene period for the main contractional stage in this sector of the External Dinarides. This result has important implications for the evolution of the Dinaride foreland basin system in Cenozoic times.

Cenomanian to Coniacian sea-level changes in the Lower Benue Trough (Nkalagu Area, Nigeria) and the Eastern Dahomey Basin: palaeontological and sedimentological evidence for eustasy and tectonism

Abstract The Benue Trough formed in close relation to the opening of the South Atlantic and experienced sea-level fluctuations of different magnitudes during the Cenomanian to Coniacian interval. We identify depositional environments from outcrop sections and a drilling as control record. Lines of evidence for the interpretation include facies analyses, foraminiferal assemblage composition (P/B-ratio) and the presence of planktonic deep-water indicators. While the analysis of the well data from the Dahomey Basin indicates a continuous deep-water (bathyal) environment, the succession in the Nkalagu area of the Lower Benue Trough evolved in a different and more complex way. Beginning with latest Cenomanian shoreface to shelf deposits, a long period of subsidence lasted until the middle Turonian when pelagic shales and calcareous turbidites were deposited at upper to middle bathyal depths. These conditions continued during late Turonian and Coniacian times. The general deepening trend of the Lower Benue Trough was mainly controlled by tectonic subsidence and was superimposed by eustatic sea-level changes, resulting in periodically changing palaeowater depths. We were able to identify eight sea-level rises and falls that can be attributed to 405 kyr eccentricity cycles. The amplitudes of the sea-level changes were most likely in the range of several tens to a few hundred metres. The deposition of carbonate turbidites at Nkalagu was probably triggered by eustatic sea-level lowstands.

A giant sandy sediment drift in early Silurian (Telychian) and its multiple sedimentological prosess

Abstract This study describes a regional-scale sedimentary feature in the Upper Yangtze region of China that is identified as a Telychian age (438 Ma) sediment sandy drift that is 500–800 km long, 100–150 km wide, and 10–20 m thick. Graptolite zones is applied to recognize Telychian sandstone beds and reconstruct an isochronal stratigraphic framework for the study area. It is possible to observe numerous traction structures in Telychian sandstone beds, including ripples, cross-bedding, cross-laminations, wavy-laminations and horizontal-laminations and Bouma sequence. The palaeocurrent direction is towards SSW (200°–240°), which is measured by sedimentary structures (cross-bedding, cross-laminations and ripples) in outcrops. Besides, the alignment of elongate particles in thin section, and the distribution characteristics of heavy minerals also indicate a SSW palaeocurrent direction. This direction is parallel to palaeo-coastline, which is a characteristic feature of bottom current. A multiple sedimentary processs is identified based on analysing facies of each sections. Contourite-dominated sequences are observed in channel-related seaway while turbidite sequences are observed in shelf near palaeo-coastline. Based on provenance analysis, the Telychian drift (channel reworked sandy drift) is the production of a mixed depositional system (first turbidite deposition and then bottom current deposition). The turbidite sand on the shelf provides material source for the sandy drift, and the deposit is transported from the shelf to the channel (parallel to ancient coastline). The provenance direction is identified by analyzing the distribution of heavy minerals. The unstable heavy minerals content is higher in the turbidites samples (near provenance region) than the channel reworked sandstone samples (far away from provenance region). This Telychian deposit is a rare example of an ancient channel reworked sandy drift.

Morphology of retrogressive failures in the Eastern Rhone interfluve during the last glacial maximum (Gulf of Lions, Western Mediterranean)

The Gulf of Lions (NW Mediterranean Sea) is a SW-NE oriented passive continental margin formed since the Oligocene. It presents small to large scale mass movement features suggesting a long history of seafloor instability. Of particular interest are the two surficial large mass-transport deposits along the Rhone turbiditic levee, known as the Rhone Eastern and Western Mass-Transport Deposits (REMTD and RWMTD). With the help of the recently acquired multi-beam bathymetric, sub-bottom profiler, high-resolution seismic and sedimentological data, we investigate the morphology, timing, kinematics, and possible triggering mechanisms of the source area of the REMTD, which we refer to as the Eastern Rhone Interfluve Slide (ERIS). ERIS has an estimated run-out distance of approximately 200km. It covers an area of about 700 km2 and the volume of the mobilized material is approximately 110km3. Our data reveal four individual glide planes within the ERIS complex which were most likely generated by retrogressive failures. The basal surfaces of the ERIS coincide with high-amplitude seismic reflectors similar to those previously interpreted as the expression of condensed sections on the upper slope. The turbiditic sequences sandwiched between the condensed sections likely control the localisation of potential weak layers favouring the failures. AMS radiocarbon dating yields an age of approximately 21 ka cal BP for the failures, which falls within the peak of the Last Glacial Maximum. The toe area of the ERIS is incised by several active listric faults rooted in the Messinian strata, which control the location of the slide scarps. The combination of several factors such as slope steepening, halokinesis, and excess pore pressure generation due to rapid turbiditic sedimentation during the Last Glacial Maximum are considered as the possible candidates for the triggering of the failures in the investigated slope.

Lithological control on multiple surface ruptures during the 2016–2017 Amatrice-Norcia seismic sequence

On August 24th 2016, a Mw 6.0 earthquake started the Amatrice - Norcia (Central Italy) seismic sequence, generated by the extensional tectonics along the Apennines, that had its apex with the Mw 6.5 October 30th mainshock. As a unique documented case reported in Italy, complex surface faulting occurred during both earthquakes along the Mt. Vettore fault. Multiple surface faulting was accompanied at depth by the development of a km-scale normal fault-propagation fold. This fold was characterized by breakthrough and by surface rupture within thick carbonatic layers only in the central and north-western area (Mt. Vettore). On the contrary, the fault remained blind where flexural slip was active in sandy-silty turbiditic deposits in the south-eastern area (Mt. Gorzano). We explain the different faulting behaviour with the occurrence of more rigid and competent lithologies in areas characterized by breakthrough and with the occurrence of weak lithologies in areas characterized by blind faulting. Overall, the entire seismic sequence appears as a gradual gravitational adjustment of the hangingwall block, slipping along a NW-trending and 80 km long fault system. In particular, the following crustal blocks, partially overlapping and with different length (30, 40 and 22 km, respectively), progressively collapsed during the sequence: the Amatrice sector during the August 24th 2016, Mw 6.0 event, the Norcia-Visso sector during the October 26th 2016, Mw 5.9 and the 2016 October 30th Mw 6.5 event, and the Campotosto Lake sector during the four January 18th 2017, M > 5 events. The progressive involvement of these three rock volumes, during the seismic sequence is here explained by the occurrence of a low angle detachment that limited the maximum potential depth of the mainshocks and consequently the dimensions of involved rock volumes, therefore limiting the magnitudes of the mainshocks.

An effective method for shear-wave velocity prediction in sandstones

Shear-wave velocity is essential in various seismic exploration applications, including seismic modeling, amplitude variation with offset analysis, multicomponent seismic data interpretation and other exploration applications. This study presents a simple but effective method for S-wave velocity prediction from P-wave velocity based on granular media model. In the proposed method, the soft-sand, intermediate stiff-sand and stiff-sand models are unified with the expression of the soft-sand model with an effective coordination number. The shear modulus of dry rocks can be related to bulk modulus through the coordination number in the unified model. Elastic-wave velocities of water-saturated rocks at low frequencies can be predicted from the moduli of dry rock using Gassmann’s equation. Thus, the coordination number can be inverted from P-wave velocity of saturated rocks by combining the unified granular media model and Gassmann’s equation. Eventually, S-wave velocity of saturated rock is computed with the inverted effective coordination number. The numerical results indicate that the predicted S-wave velocities agree well with the measured velocities for the laboratory data and well logging data. The proposed method is applicable to sandstones with lithification of large ranges because the unified granular media model accounts for the media between Hashin–Shtrikman lower bound and upper bound. Moreover, the new method is quite suitable for the prediction of S-wave velocity in sandstones deposited in deep-water environment, particularly for turbidite sediment.

Changes in deep-sea benthic foraminiferal fauna caused by turbidites deposited after the 2011 Tohoku-oki earthquake

The impact of the 2011 Tohoku-oki earthquake and subsequent turbidites on the deep-sea environment was investigated by conducting sedimentological, radionuclide, and benthic foraminiferal analyses of core sediments collected from water depths of 3230 and ~3570 m, 5 and 17 months after the earthquake. Sedimentological profiles and patterns of excess 210Pb, 134Cs, and 137Cs indicate that the analyzed sediment core records deposits before the earthquake, turbidite deposits just after the earthquake, and deposits after the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident, which caused the release of a large amount of radioactive material four days after the earthquake. Core 4W-2012 F-RN was collected at 3585 m water depth 17 months after the earthquake. Within this core, an event deposit layer (6.5 cm thick) consisting of turbidites contains highly diverse and possibly reworked foraminiferal assemblages. Overlying these turbidites are sediments with high 134Cs concentrations that were deposited after the FNPP1 accident. Overall foraminiferal and species abundances show a marked decrease in the sediments above the deposited turbidites. The abundance of opportunistic and competitive species shows a gradual upward increase in the uppermost sediment layers, and these species are dominant in the top 1 cm of the core. Our findings demonstrate that although foraminiferal assemblages in the sediment overlying the turbidites related to the 2011 earthquake might include allochthonous assemblages, turbidites deposited after the earthquake caused a marked decrease in benthic foraminiferal diversity and allowed opportunistic species to colonize in a low-diversity assemblage within 17 months. These foraminiferal faunal changes, together with analysis of sedimentological structure, provide valuable information for identifying these types of sedimentation events in core sediments from the deep-sea floor.

A NEW TEICHICHNOID TRACE FOSSIL SYRINGOMORPHA CYPRENSIS FROM THE MIOCENE OF CYPRUS

ABSTRACTA new teichichnoid trace fossil, Syringomorpha cyprensis from the Miocene of Cyprus, is proposed as a vertical burrow composed of an arcuate-like tube with horizontal parts to subhorizontal distally and vertical to subvertical parts proximally and triangular spreiten in the inner corner of the tube. Previously, this ichnogenus was represented only by the lower Paleozoic, shallow marine S. nilssoni, which disappeared after the Cambrian. Syringomorpha cyprensis marks the reappearance of similar behavior, in a deep-sea environment with pelagic, contouritic, and turbiditic sedimentation, influenced by frequent turbiditic and bottom current flows. Both ichnospecies of Syringomorpha could be produced by the same taxonomic group of probable worm like organisms (polychaetes?). Energy conditions were a stronger influence on the distribution of S. cyprensis tracemaker rather depth.

Risks inherent in the Cenozoic stratigraphic plays in basins of the Uruguayan continental margin

Abstract The genesis and evolution of the Uruguayan Continental Margin are linked to the geological processes that resulted in the breakup of the Western Gondwana and the opening of the Atlantic Ocean. Intense hydrocarbon exploration over the last decade has identified several plays in the Mesozoic–Cenozoic sedimentary basin infill. The main exploration milestone was the drilling of the Raya x-1 well (in the Pelotas Basin), which targets an Oligocene turbidite deposit. The well encountered no hydrocarbon accumulations but has contributed to the knowledge of a frontier area and has raised questions regarding the geological risks associated with the Cenozoic prospects in the Uruguayan Continental Margin. In this study, the exploratory potential of a set of stratigraphic plays identified in the Cenozoic sedimentary succession of the Uruguayan Continental Margin basins was analysed, through the interpretation of 2D seismic sections. A significant risk appears to be related to migration and charge of the Cenozoic reservoirs from Cretaceous source rocks because of the lack of major faults and the development of thick transgressive sedimentary packages, especially in the Pelotas Basin. The best areas for vertical charge of the Cenozoic reservoirs resides on the southwest of the Uruguayan Continental Margin (distal part of the Punta del Este Basin) and in a portion of the central region (southernmost part of the Uruguayan Pelotas Basin).

Influence of volcanic and hydrothermal activity on organic matter enrichment in the Upper Triassic Yanchang Formation, southern Ordos Basin, Central China

Abstract Based on the field investigation of oil shales in the southern Ordos Basin, detailed observations, and systematic sample analyses of core samples from a well that penetrated an oil shale, the influence of volcanic and hydrothermal activity on the Chang 8–Chang 7 members of the Yanchang Formation on organic matter (OM) enrichment was geochemically and petrologically analyzed. According to maceral analyses and rock pyrolysis, the kerogen types of the samples in the study area are widely distributed. Those of the oil shale with the highest OM abundance are mostly type II-I, and are immature or in a stage of low-maturity. Six layers of tuff and tuffaceous mudstone were identified by core observation and element characteristics; they are characterized by higher total rare earth element (∑REE) and δCe values and Th contents, and lower (La/Yb)N and δEu values. Mineral assemblages of anhydrite–pyrite–marcasite and relevant diagrams of the elemental geochemistry confirmed the existence of hydrothermal deposits. The high Yn/Hon and U/Th ratios and total sulfur (TS) contents indicate that the Chang 7 Member experienced four episodes of intense hydrothermal activity. Early in the Chang 73 Member, volcanic ash and hydrothermal fluids brought in a large amount of nutrients, which not only increased primary productivity, but also increased the salinity of the water column, enhanced anoxia in the sedimentary environment of the lake bottom, promoted the preservation of OM, and finally formed the oil shale layer with the richest OM. However, the strong volcanic eruption in the late Chang 73 Member led to changes in the regional climate, causing it to dry and become colder and resulting in a decrease of primary productivity. Earthquakes triggered by volcanism resulted in turbidite deposits, which destroyed the preservation and diluted the enrichment of OM. This study reveals in detail the influence of volcanic and hydrothermal activity on OM enrichment during the evolution of the basin, and thereby deepens our theoretical understanding of OM enrichment.

Evidence for provenance change in deep sea sediments of the Bengal Fan: A 7 million year record from IODP U1444A

Previously published geochemical and mineralogical data have revealed conflicting views on the provenance of the deep-sea sediments in the Bengal Fan. Here we report results of a study for a 330-m sequence consisting of both turbitide and semi-pelagic sediments recovered at the Site U1444A during the IODP 353 Expedition. A novel technique using the optically and thermally stimulated luminescence (OSL and TL) signatures from quartz and K-feldspar has been employed to characterise the mineral compositions of the sediments. We show that the quartz luminescence sensitivity clearly differentiates the four sedimentary units. There is a distinct increase of quartz luminescence sensitivity in Unit 2 during the time interval of ~3.5–0.5 Ma. Analyses of the luminescence signal components in quartz and the ratio of the K-feldspar and quartz also point to a change in Unit 2 relative to the other three units. We interpret such changes in the mineralogical compositions through the sequence as reflecting a change in the provenance over the last 7 million years in the western Bengal Fan. We attribute the quartz with enhanced luminescence sensitivity in Unit 2 to the contribution from the sources associated with Indian rivers. We infer that the delivery of the sediments from Indian peninsula to IODP Site U1444A is facilitated by a weakening in the turbidite deposition which is dominated by weathering products from the Himalayas. We speculate that the relatively high elevation of the 85°E Ridge may act against the flow through and the associated deposition of turbidity current originated from the Himalayas.

Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

The primary stratigraphic fabric of a chaotic rock unit in the Zermatt Saas ophiolite of the Western Alps was reworked by a polyphase Alpine tectonic deformation. Multiscalar structural criteria demonstrate that this unit was deformed by two ductile subduction-related phases followed by brittle-ductile then brittle deformation. Deformation partitioning operated at various scales, leaving relatively unstrained rock domains preserving internal texture, organization, and composition. During subduction, ductile deformation involved stretching, boudinage, and simultaneous folding of the primary stratigraphic succession. This deformation is particularly well-documented in alternating layers showing contrasting deformation style, such as carbonate-rich rocks and turbiditic serpentinite metasandstones. During collision and exhumation, deformation enhanced the boudinaged horizons and blocks, giving rise to spherical to lozenge-shaped blocks embedded in a carbonate-rich matrix. Structural criteria allow the recognition of two main domains within the chaotic rock unit, one attributable to original broken formations reflecting turbiditic sedimentation, the other ascribable to an original sedimentary mélange. The envisaged geodynamic setting for the formation of the protoliths is the Jurassic Ligurian-Piedmont ocean basin floored by mostly serpentinized peridotites, intensely tectonized by extensional faults that triggered mass transport processes and turbiditic sedimentation.

Integrating heavy-mineral, geochemical and biomarker analyses of Plio-Pleistocene sandy and silty turbidites: a novel approach for provenance studies (Indus Fan, IODP Expedition 355)

AbstractA multidisciplinary mineralogical, geochemical and biomarker study of Indus Fan sediments cored during International Ocean Discovery Program (IODP) Expedition 355 to the Laxmi Basin was carried out to define the different compositional signatures of sand, silt and clay. Upper Pliocene – lower Pleistocene turbidites from sites U1456 and U1457 were selected as the best candidates for this study. The integrated dataset presented here was obtained by coupling traditional and innovative bulk-sediment and single-mineral techniques on the same samples. Turbiditic deposits mostly consist of medium to fine silt, including rich and diverse heavy-mineral assemblages. Such a fine grain size forced us to push the limits of high-resolution quantitative heavy-mineral analysis down to as low as 5 μm. Heavy-mineral analysis allowed us to establish a Himalayan origin of the detritus in the studied turbidites. Heavy-mineral concentrations are higher in channel-fill than in overbank deposits. Mineralogical and geochemical data concur in revealing that fast-settling ultradense minerals such as zircon are preferentially concentrated in channel-fill deposits, whereas the top of overbank deposits are notably enriched with slow-settling platy phyllosilicates. Biomarker analysis represents a most suitable complementary technique that is able to investigate the provenance signature of the finer sediment fraction, largely consisting of clay. This technique allowed us to identify a largely terrigenous origin of organic matter at Site U1456 and an open marine origin at Site U1457. The latter site lies closer to the Laxmi Ridge, where thermal maturity increases with depth to reach the early oil window (127°C at c. 320 m below the seafloor).

Textural features and transportation mode of Nile Delta coastal lagoon surficial sediments (Lake Burullus, Ramsar Site)

The present study aims to give a comprehensive account of the sedimentological characteristics and sedimentary processes action on Nile Delta coastal lagoon. The study carried out on 15 surficial sediments samples collected from Lake Burullus. Through a detailed sedimentary texture study and using applications (CM diagram, bivariate diagrams and multivariant linear discriminant functions) sediment source, transportation and accumulation of Lake Burullus are suggested. Results revealed that the increased coarse fractions northward near inlets were due to wave actions which indicate the high-energy transporting environments. Moreover, Areas of drains effluents, fine fractions are trapped and deposited directly after the entering of the lagoon, then it well-ordered by hydraulic sorting. Mean grain size is mostly controlled by sediments sources. Sediment sorting is mainly poor indicating turbulent conditions. Skewness results indicate that lagoon is classified as environments undergoing depositional trends spatially southward. The CM divulges that the deposition takes place by homogenous suspension and rolling. With strong freshwater processes signatures, turbidite deposition, and siltation are more than truncation in the main transportation mode.

Turbidite sedimentology, biostratigraphy and paleoecology: A case study from the Oligocene Zuberec Fm. (Liptov Basin, Central Western Carpathians)

Abstract Outcrops of a thick turbiditic succession are exposed on the northern bank of the Liptovská Mara reservoir near Liptovská Ondrašová and Ráztoky. The section consists of rhythmic, predominantly thin- to medium-bedded turbidites of the Rupelian age. Their biostratigraphy is based on the calcareous nannofossils. Facies associations of these deposits represent different components of depositional lobe deposits in the turbidity fan system, including mainly the lobe fringe and lobe distal fringe/inter-lobe facies associations and locally the medium bedded deposits of the lobe off-axis facies association. This interpretation is supported by statistical analysis. The deep-sea turbiditic deposits contain trace fossil associations, which include deep-tier fodinichnia and domichnia up to shallow-tier graphoglyptids. Paleocurrent measurements indicate that the majority of sedimentary material was transported from SW and W.

Estrellichnus jacaensis from the Eocene Jaca Basin of NE Spain: new locality and new ethological interpretation

New occurrences of Estrellichnus jacaensis in the deep-marine turbidite deposits of the Eocene Hecho Group (Fiscal, Huesca, NE Spain) are described. Most of them include specimens preserved as ‘urban fossils’ located in several villages of the south-central Pyrenees. Well-preserved morphological features of the new specimens studied and their interactions with accompanying ichnotaxa allow reinterpreting the constructional process and functional significance of Estrellichnus and its rejection as a graphoglyptid. The comparison of this ichnogenus with lebensspuren recorded in oceans and seas around the world has allowed finding a plausible current analogue and proposing a likely tracemaker.

Interaction of a deep-sea current with a blind submarine canyon (Mar del Plata Canyon, Argentina)

The Mar del Plata (MdP) Canyon at the Argentine continental margin is incorporated into a major contourite depositional system, built by the incursion of southern-sourced water masses affecting the seafloor at different waters depths. The new sedimentological, morphological and hydro acoustic data provide novel insights into contour and turbidity current interactions in mid-slope (blind) canyons, which do not have a connection to the shelf or an onshore river system. Such canyons are capable to record climate-related ocean stratification changes, current variability, and slope stability. Three sediment cores were obtained along the MdP Canyon thalweg covering the last 20,000 years and compiled with two cores from the adjacent Ewing Terrace. Turbidity-current activity within the MdP Canyon was limited to the time interval from Last Glacial Maximum (LGM) to the late deglacial. During the LGM and early deglacial, turbidites reached both the proximal sector and the distal northern flank of the canyon. During the late deglacial only the proximal sector was characterized by turbidite deposition. Similarities in mineralogy and grain-size data indicate that the material transported by the turbidity currents originated from the mid-slope Ewing Terrace. Glacial turbidity-current activity was most probably favored by increased sediment supply along the Ewing Terrace from a shallowed and/or enhanced glacial Antarctic Intermediate Water (AAIW) nepheloid layer. These sediments were trapped by the MdP Canyon, in particular at the head area. During the late deglacial, a displacement or limited AAIW nepheloid layer resulted in less sediment transfer along the Ewing Terrace and immediate accumulation in the MdP Canyon head restricting turbidite deposition to the proximal sector of the canyon. In general, contourite-turbidite interactions provide valuable information on variations in thermohaline circulation such as AAIW distribution and current strength.

Tectonostratigraphic framework and depositional history of the deepwater Ulleung Basin, East Sea/Sea of Japan

AbstractThe Ulleung Basin, East Sea/Japan Sea, is a Neogene back‐arc basin and occupies a tectonically crucial zone under the influence of relative motions between Eurasian, Pacific and Philippine Sea plates. However, the link between tectonics and sedimentation remains poorly understood in the back‐arc Ulleung Basin, as it does in many other back‐arc basins as well, because of a paucity of seismic data and controversy over the tectonic history of the basin. This paper presents an integrated tectonostratigraphic and sedimentary evolution in the deepwater Ulleung Basin using 2D multichannel seismic reflection data. The sedimentary succession within the deepwater Ulleung Basin is divided into four second‐order seismic megasequences (MS1 to MS4). Detailed seismic stratigraphy interpretation of the four megasequences suggests the depositional history of the deepwater Ulleung Basin occurred in four stages, controlled by tectonic movement, volcanism, and sea‐level fluctuations. In Stage 1 (late Oligocene through early Miocene), syn‐rift sediment supplied to the basin was restricted to the southern base‐of‐slope, whereas the northern distal part of the basin was dominated by volcanic sills and lava flows derived from initial rifting‐related volcanism. In Stage 2 (late early Miocene through middle Miocene), volcanic extrusion occurred through post‐rift, chain volcanism in the earliest time, followed by hemipelagic and turbidite sedimentation in a quiescent open marine setting. In Stage 3 (late middle Miocene through late Miocene), compressional activity was predominant throughout the Ulleung Basin, resulting in regional uplift and sub‐aerial erosion/denudation of the southern shelf of the basin, which provided enormous volumes of sediment into the basin through mass transport processes. In Stage 4 (early Pliocene through present), although the degree of tectonic stress decreased significantly, mass movement was still generated by sea‐level fluctuations as well as compressional tectonic movement, resulting in stacked mass transport deposits along the southern basin margin. We propose a new depositional history model for the deepwater Ulleung Basin and provide a window into understanding how tectonic, volcanic and eustatic interactions control sedimentation in back‐arc basins.

Chemosymbiotic Bivalves from the Lower Miocene Kurosedani Formation in Toyama Prefecture, Central Honshu, Japan

Many molluscan fossils have been found from the turbidite deposits of the lower Miocene Kurosedani Formation at Kakuma in Toyama City. They include six chemosymbiotic bivalves, namely Solemyidae gen. et sp. indet., Nucinella sp., Luinoma cf. acutilineatum (Conrad), Conchocele yatsuoensis sp. nov., Pliocardia kawadai (Aoki) and Adulomya chitanii Kanehara. This is the oldest record of chemosymbiotic species in the Japan Sea region. Nucinella has been unknown from the Cenozoic deposits in the Japan Sea borderland and from the Recent Japan Sea. Moreover, the vesicomyids P. kawadai and A. chitanii are shared with those from the lower Miocene Honya Formation in Fukushima Prefecture and from the middle Miocene Nupinai Formation in eastern Hokkaido. This supports our hypothesis that the invasion of chemosymbiotic species from the Pacific side to the Japan Sea took place soon after the formation of a deep-sea basin in the Japan Sea.

Geology and Oil and Gas Bearing Potential of the East Canadian Continental Margin

Within the East Canadian continental margin, three oil and gas bearing regions have been identified from north to south: the Labrador Sea shelf, the margin of the Grand Banks of Newfoundland, and the Nova Scotia continental margin. In each of these regions, a number of sedimentary basins are distinguished and completely submerged. The following large sedimentary basins are distinguished on the Labrador Sea shelf: the Saglek, Hopdale, and Havke; on the margin of the Grand Banks of Newfoundland, the following basins are known: the Jeanne d’Arc, Flemish Pass, and Orphan. The Scotian Shelf distinguishes the Nova Scotia and Sable basins. It is noteworthy that at high latitudes, such as the Labrador Sea region, the age of productive sediments begins with more ancient rocks (since the Paleozoic) than in the basins to the low latitude (since the Mesozoic), due to which the stratigraphic range of oil and gas bearing in the high latitude basins is much wider. The oil and gas bearing potential in the entire region is mainly related to the continental slopes and turbidite deposits occurring within them. Late Jurassic and Early Cretaceous reservoirs will be the target sediments during deep drilling. Widespread Late Cretaceous and Early Tertiary formations with oil and gas potential are quite realistic, since they do not lie at deep horizons. The oil and gas bearing potential on the margins of basins is associated with Late Cretaceous and Tertiary sediments, alluvial fan deposits, and salt diapirs.