Turbidite Units Research Articles

Sub-bottom sonar profiles from Loch Ness, Scotland: implications for deglaciation of the British-Irish Ice Sheet

Recent studies of the sedimentary record of the Japan Trench indicate unique earthquake-induced depositional characteristics. To correlate such deposits to historical earthquakes, we studied the stratigraphy of two sedimentary cores obtained from basins of the Japan Trench at 7400 m and 7600 m water depth using paleomagnetic secular variation (PSV). Because of their high sedimentation rates, results show that excellent geomagnetic records are preserved in the sediments in spite of the great water depth. Paleomagnetic secular variation, especially declination during the past 3000 years revealing a distinctive feature of large oscillation, offers an opportunity to make precise stratigraphic correlations. Identification of boundaries between thick fine turbidite intervals and hemipelagite intervals was made based on rock magnetic properties. Successful applications of extraction of paleomagnetic data solely from the hemipelagic interval demonstrate that stratigraphic positions of turbidite in paleomagnetic secular variation record are mutually well correlated in PSV variations. Even over long distances between sites, the turbidites induced by earthquakes can be correlated exactly. The turbidite ages were determined by referring to the PSV master curve. Especially, the ages of turbidite units regarded as induced by the Jogan earthquake can be inferred from two cores that closely approximate its documented age.

Sedimentation of submarine fan deposits in the Pindos foreland basin, from late Eocene to early Oligocene, west Peloponnesus peninsula, SW Greece

Turbidite facies distribution and palaeocurrent analysis of submarine fan evolution in the Pindos foreland basin of west Peloponnesus peninsula (SW Greece) indicate that this part of the foreland was developed during Late Eocene to Early Oligocene in three linear sub‐basins (Tritea, Hrisovitsi and Finikounda). The basin fill conditions, with a multiple feeder system, which is characterized by axial transport of sediments and asymmetric stratigraphic thickness of the studied sediments, indicate that the Pindos Foreland Basin in this area was an underfilled foreland basin. Sediments are dominated by conglomerates, sandstones and mudstones. The flow types that controlled the depositional processes of the submarine fans were grain flows, debris flows and low‐ and high‐density turbidity currents. The sedimentary model that we propose for the depositional mechanisms and geometrical distribution of the turbidite units in the Tritea sub‐basin is a mixed sand‐mud submarine fan with a sequential interaction of progradation and retrogradation for the submarine fan development and shows a WNW main palaeocurrent direction. The Hrisovitsi sub‐basin turbidite system characterized by small‐scale channels was sediment starved, and the erosion during deposition was greater than the two other studied areas, indicating a more restricted basin topography with a NW main palaeocurrent direction. The Finikounda sub‐basin exhibits sand‐rich submarine fans, is characterized by the presence of distinct, small‐scale, thickening‐upward cycles and by the covering of a distal fan by a proximal fan. It was constructed under the simultaneous interaction of progradation and aggradation, where the main palaeocurrent direction was from NNW to SSE. Copyright © 2012 John Wiley & Sons, Ltd.

Paleogeographic reconstruction of a late Paleozoic arc collision zone, southern Mongolia

Permian sedimentary sequences exposed in southern Mongolia record final closure of the paleo–Asian Ocean and suturing of the terranes of northern China with the southern margin of a vast accretionary complex (the Altaids) in Mongolia. Detrital zircon U-Pb geochronology results presented here are the first of their kind in southern Mongolia. Geologic mapping, stratigraphic interpretations, and provenance data including U-Pb zircon geochronology suggest that sedimentary strata at two localities in southern Mongolia, Bulgan Uul and Nomgon, were once part of the same closing ocean basin. The Upper Permian sedimentary deposits at Bulgan Uul record an upward-shallowing marine succession that is unconformably overlain by Lower Triassic fluvial and alluvial strata. The Bulgan Uul marine succession is composed of distal turbidite fan deposits in the lowest portion of the section, with interbedded sandstone and limestone interpreted as shallow-marine deposits at the top of the section. Exposures of Permian-aged distal turbidite units at Nomgon are similar in stratigraphic architecture, sandstone provenance, and detrital zircon age distributions to those documented at Bulgan Uul. Paleocurrent measurements, sandstone provenance data, and U-Pb ages from detrital zircons collected from both study locations document southeastern transport directions for sediment derived from extinct Carboniferous and Ordovician–Silurian arcs of the southern Altaids. Results are consistent with depositional models for remnant ocean basins and indicate diachronous west-to-east closure of the paleo–Asian Ocean (a northern segment of Paleotethys) in the Late Permian. Finally, basin reconstructions place the coeval turbidite deposits at Nomgon to the southeast of Bulgan Uul during the Late Permian. These correlative turbidite successions at Nomgon are currently northeast of Bulgan Uul, offset by ∼250 km of left-lateral strike-slip faulting across the East Gobi fault zone.

Coupled fluid flow and deformation modeling of the frontal thrust region of the Kumano Basin transect, Japan: Implications for fluid pressures and decollement downstepping

Results of coupled fluid flow and deformation modeling illustrate the influence of frontal thrust slip on excess pore pressures and decollement downstepping in the Kumano Basin transect, offshore Japan. Although permeable trench sediments and a buried sandstone layer provide potential fluid escape pathways, the numerical modeling suggests that excess pore pressure could build beneath the frontal thrust within hemipelagic clay of the Shikoku Basin facies. Simulated excess pore pressures range between 45% and 77% of the overburden weight 3 km landward of the deformation front, and are very sensitive to the permeability of the Shikoku Basin clay and the coefficient of friction of the fault zone. Excess pore pressures increase as the fault coefficient of friction is increased, suggesting that volume change due to shear within the footwall sediments is an important contributor to excess pore pressures. Raising the coefficient of friction also increases shear within the sediments beneath the fault zone, suggesting downward migration of the decollement zone. This shearing focuses in the weaker Shikoku Basin clays rather than an underlying turbidite unit, suggesting that lithologic differences play an important role in controlling deformation location.

An integrated lithogeochemical approach to detecting and interpreting cryptic alteration around the Elura Zn-Pb-Ag deposit, New South Wales, Australia

ABSTRACT Hydrothermal alteration around the sediment-hosted Elura Zn-Pb-Ag sulphide deposit has produced detectable and systematic chemical changes that are also reflected in subtle mineralogical features (cryptic alteration). Iron carbonate development accompanied by potassic alteration, the destruction of albite and the absence of chlorite are the dominant mineral alteration effects in the surrounding turbidites. Key elements enriched in the primary dispersion zone are Zn, Pb, Ag, As, Rb, Tl, and particularly Sb. Sodium is strongly depleted in the alteration system. Cryptic alteration and primary dispersion can be detected up to 350 m from the orebody below the weathering front. Pearce Element Ratio (PER) and General Element Ratio (GER) techniques more clearly identify and quantify this alteration and could assist in vectoring towards high intensity alteration adjacent to ore during exploration drilling. These data assessment techniques avoid the problems of closure when comparing element contents for altered and unaltered samples and allow modelling of the mineralogical controls on chemical variation. There are important differences in the alteration effects observed in the fine and coarse fractions of the enclosing turbidite units. For vectoring purposes, the best lithotype to sample is the shale component.

Sediment mounds and other sedimentary features related to hydrate occurrences in a columnar seismic blanking zone of the Ulleung Basin, East Sea, Korea

A mound related to a cold vent in a columnar seismic blanking zone (CSBZ) was formed around site UBGH1-10 in the central Ulleung Basin (2077 m water depth), East Sea, Korea. The mound is 300–400 m wide and 2–3 m high according to multi-beam bathymetry, 2–7 kHz sub-bottom profiler data, and multi-channel reflection seismic data. Seafloor topography and characteristics were investigated using a remotely operated vehicle (ROV) around site UBGH1-10, which is located near the northern part of the mound. The origin of the mound was investigated through lithology, mineralogy, hydrate occurrence, and sedimentary features using dive cores, piston cores, and a deep-drilling core. The CSBZ extends to ∼265 ms two-way traveltime (TWT) below the seafloor within a mass-transport deposit (MTD) unit. Gas hydrate was entirely contained 6–141 m below the seafloor (mbsf) within hemipelagic deposits intercalated with a fine-grained turbidite (HTD) unit, characteristically associated with high resistivity values at site UBGH1-10. The hydrate is commonly characterized by veins, nodules, and massive types, and is found within muddy sediments as a fracture-filling type. Methane has been produced by microbial reduction of CO 2, as indicated by C 1/C 2+, δ 13 C CH 4 , and δ D CH 4 analyses. The bowl-shaped hydrate cap revealed at 20–45 ms TWT below the seafloor has very high resistivity and high salinity, suggesting rapid and recent gas hydrate formation. The origin of the sediment mound is interpreted as a topographic high formed by the expansion associated with the formation of the gas hydrate cap above the CSBZ. The lower sedimentation rate of the mound sediments may be due to local enhancement of bottom currents by topographic effects. In addition, no evidence of gas bubbles, chemosynthetic communities, or bacterial mats was observed in the mound, suggesting an inactive cold vent.

Chapter 42 The deep-marine glaciogenic Gaskiers Formation, Newfoundland, Canada

Abstract In eastern Canada, the Neoproterozoic Gaskiers Formation (Fm.) consists of a thick diamictite–turbidite succession (250–300 m thick) that occurs within the deep-marine predominantly volcaniclastic turbidite units of the Conception Group (4–5 km thick). These rocks are well exposed on the coast of the Avalon Peninsula in eastern Newfoundland. The thick succession is considered by some to represent the final Neoproterozoic glacial event, known as the Gaskiers Glaciation c. 582–585 Ma. The Gaskiers Fm. appears to have accumulated in a volcanically-influenced arc-related basin within the confines of the Proterozoic peri-Gondwanan Avalon terrane, before rifting away from the Gondwanan margin in the early Palaeozoic, but it is not clear whether it was adjacent to the West African Craton or the Amazon Craton. It has also been argued that Avalon had already rifted from Gondwana by the late Proterozoic. Glacially striated clasts, dropstones, and chatter-marked garnets identify a glaciated source area, and clasts and matrix are of volcanic origin; pyroclastic flows and volcanic bombs are also present. As a result, there is a growing consensus that the Gaskiers Fm. records local glaciation of a high relief volcanic topography, owing its origin to the episodic downslope reworking of volcanic and glacial debris into a deep, rapidly subsiding basin. The volcanic nature of these deposits implies that local volcanic activity was coeval with deposition. A minimum age of the Gaskiers Fm. is constrained by a diverse assemblage of Ediacaran-type fossils in the upper Conception Group dated at c. 565±3 Ma and in the lower St. John's Group, which overlies the Conception Group. A maximum age for the Gaskiers is provided by a U–Pb date of 606 +3.7/–2.9) Ma in the Harbour Main volcanic rocks underlying the Gaskiers Fm. A thin limestone bed has also been identified at two localities in Conception Bay directly overlying the diamictite yielding strongly negative δ 13 C values.

Validation of Hurst statistics: a predictive tool to discriminate turbiditic sub-environments in a confined basin

ABSTRACT Turbidite sequences within confined basins constitute important hydrocarbon reservoirs world-wide and, for this reason, the discrimination of sedimentary sub-environments based on an objective statistical method is of interest for pure and applied science. We investigated the potential use of the Hurst test as a statistical tool to discriminate sub-environments within geologically complex turbiditic units that fill a confined basin with well-exposed facies transitions and onlaps, at the scale of several stacked reservoirs (Cengio and Bric la Croce–Castelnuovo Turbidite Systems in the Tertiary Piedmont Basin, Oligocene, northern Italy). In vertical stratigraphic sections, the Hurst test determines the degree of clustering of low and high values of sedimentological variables, such as bed thickness, grain size and sand/mud ratio, which are dependent on sub-environments of deposition. We applied the Hurst test to depocentral and marginal sub-areas across the basin (parallel and perpendicular to the main palaeocurrent direction), documenting a different clustering of thick and thin beds, and of high and low values of the sand/mud ratio, in the depocentre–distal sector with respect to the onlap areas. A new field (onlap sub-environment) could thus be added to the classification diagram of turbidite settings based on the Hurst index. The Hurst phenomenon (clustering of high and low values of the selected variables) was also able to distinguish between proximal and distal (depocentral) lobe settings, and to recognize the fingerprint of the different depositional lobes (fully confined aggrading, prograding, backstepping). The map of turbidite sub-environments obtained by interpolation of the Hurst index is quite comparable to the field-observed facies map, providing impressive robust validation of the Hurst statistics. This method seems to represent a very promising predictive tool for subsurface studies of turbiditic oil fields based on core and log analyses.

Timing of initiation of extension in the Tianshan, based on structural, geochemical and geochronological analyses of bimodal volcanism and olistostrome in the Bogda Shan (NW China)

This paper describes an olistostrome formation and accompanied bimodal volcanic rocks occurring in the Baiyanggou area, south of Bogda Shan. The main lithotectonic units consist of olistostrome, volcanic rocks and turbidite. The olistostrome is tectonically underlain by Upper Carboniferous limestone and sandstone along a NEE-trending detachment fault. Paleo-growth fault is locally observed. The olistostrome unit includes plenty of blocks of limestone, sandstone, rhyolite and volcaniclastic rocks, and a matrix of graywacke. Limestone blocks are dated as Pennsylvanian-Bashkirian in age by the coral and brachiopod fossils that are extensively recognized in the Upper Carboniferous strata. The volcanic unit consists of pillowed and massive basalt and rhyolite, the latter occur as an 8- to 10-meter-thick layer above the olistostrome unit. The turbidite unit is mainly composed of chert, siliceous mudstone and sandstone, within which the Bouma sequence can be locally recognized. Meter-wide gabbro and diabase dykes intrude these three units. Geochemically, rhyolites are characterized by high ACNK value of >1.1, depletion of Ba, Nb and Sm, and enrichment in Rb, Th and Zr. Basaltic rocks are rich in K2O, they show a LREE-enriched pattern and depletion in Ba, Nb and Zr, and enrichment in Ti, Ce and Hf, similar to continental rift-type tholeiite series. A gabbro porphyrite intruding the olistostrome was dated at 288 ± 3 Ma by a sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb method, and a rhyolite at 297 ± 2 Ma by a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) zircon U–Pb method. The Baiyanggou olistostrome and accompanying bimodal volcanic series are linked to an extensional setting that developed in the south of the Bogda Shan. Several lines of evidence, e.g. occurrence of large-scale strike-slip shear zones, large number of mantle-derived magmatic rocks and available geochronological data, demonstrate a significant geodynamic change from convergence to extension in the Chinese Tianshan belt, even in the whole Central Asian Orogenic Belt. The extension in the Chinese Tianshan belt is initiated at ca. 300 Ma, i.e. around Carboniferous–Permian boundary times, and the peak period of intra-plate magmatism occurred in the interval of 300–250 Ma.

Quantification of the degree of confinement of a turbidite-filled basin: A statistical approach based on bed thickness distribution

Bed thickness distribution within turbidite systems is related to basin geometry and to magnitude and duration of depositional events. Cumulative distribution of turbidite bed thicknesses is often interpreted in terms of a power law. Alternatively, these distributions have been described by a lognormal mixture model. Changes in the power-law exponent can be related to the rheological properties of the gravity-driven flows and to the geometry of the basin. This study attempts to quantify the degree of confinement of well-exposed turbidite units of the Oligocene Cengio and Bric la Croce–Castelnuovo Turbidite Systems (Tertiary Piedmont Basin, Italy). Focus is on the behaviour of bed thickness distribution. The succession is suitable for this study because it represents the infill of a progressively widening basin, during quiescence of intrabasinal tectonics. In this basin the cumulative frequency distribution of turbidite bed thickness follows a segmented power-law relationship. The deviation from the trend observed for the thickest beds is associated with a deficiency of beds in a certain thickness range. This behaviour suggests existence of a thickness threshold that segregates the deposits of the fully confined flows that could deposit thick aggrading beds, with minor downcurrent flow transitions, from the rest of the bed thickness population. This threshold decreases from the lowermost (and most confined) to the uppermost units of the Cengio Turbidite System (CTS). No thresholds were identified for the uppermost Bric la Croce–Castelnuovo Turbidite Systems (BCTS) that spread over a wider basin than the CTS, suggesting unconfined deposition. The BCTS was presumably larger than the area that could be covered by the turbidity currents. When integrated with data on sedimentology and basin morphology (well-log and seismic data), bed thickness statistics can provide general guidance about interpretation of depositional settings, degree of confinement, erosion/amalgamation and bypass of turbidites, improving and assisting the development of reservoir models.

The Lower Triassic Sulphur Mountain Formation in the Wapiti Lake area: lithostratigraphy, conodont biostratigraphy, and a new biozonation for the lower Olenekian (Smithian)Earth Science Sector (ESS) Contribution 20080714.

The Sulphur Mountain Formation in the Wapiti area is subdivided into the Phroso, Meosin Mountain (new), and Vega members, which collectively span much of the Lower Triassic. The Smithian conodont faunas are particularly well developed and form the basis of three partly new conodont zones and two new subzones: the lachrymiformis Zone, meeki Zone, and mosheri Zone, with the last subdivided into the phryna and milleri Subzones. An informal lowermost Smithian nepalensis interval is introduced, although it has yet to be found in the area. The Phroso Member is latest Griesbachian, Dienerian, and, in its uppermost part, Smithian, specifically the lachrymiformis and meeki zones: these are shown to be equivalent to the Euflemingites romunduri ammonoid Zone. The Meosin Mountain Member is assigned to the meeki Zone in its lower part and the phryna Subzone of the mosheri Zone in its upper part: at least, the upper part of the mosheri Zone is equivalent to the Anawasatchites tardus ammonoid Zone. The Mackenzie Dolomite Lentil is at least partly older than the Meosin Mountain Member, and age equivalence of subsurface turbidite units in the Montney Formation should not be assumed. The lowermost Vega Member is also assigned to the phryna Subzone, and higher parts of that member span both the milleri Subzone of the Smithian and the entire Spathian. The following new conodont taxa are described: Neogondolella ? joanae , Novispathodus latiformis , Scythogondolella lachrymiformis , Sc. phryna , and Sc. rhomboidea ; several other new species are kept in open nomenclature.

Flow transformations in slumps: a case study from the Waitemata Basin, New Zealand

AbstractThe process by which slumps transform into other flow types is an understudied phenomenon; generally, sedimentologists have assumed that this is how many debris flows and turbidity currents form, yet there is a paucity of information relating to the specific processes involved. This paper aims to redress this imbalance and investigates the processes of slump flow transformation using a well‐exposed example, where the precursor slump and flows to which it was transforming have been preserved in the outcrop. A detailed field investigation of the Lower Miocene, Little Manly Slump, located within the Waitemata Basin, New Zealand, reveals a complex bi‐partite deposit, comprised of a lower slump‐debrite unit and an upper turbidite unit, separated from one another by an impersistent contact. Reconstruction of slump evolution, from preserved strain indicators, shows that slump motion was unsteady and non‐uniform, and that the slump arrested rapidly. Flow transformation is directly linked to slump evolution and progressed through the multiple processes of body transformation, fluidization transformation and surface transformation. The resultant flow comprised a very dense lower unit with slump and debris flow phases overlain by a turbidity current. This study shows that flow transformation did not result in en masse transformation to a debris flow by a single process, but rather it was characterized by partial transformation of the slump to generate a three‐phase flow. Density is thought to be the key parameter in controlling flow transformation efficiency, which appears to have varied rapidly as a function of slump unsteadiness and non‐uniformity.

Variations in sediment thickness and type along the northern Philippine Sea Plate at the Nankai Trough

Abstract We documented regional and local variations in basement relief, sediment thickness, and sediment type in the Shikoku Basin, northern Philippine Sea Plate, which is subducting at the Nankai Trough. Seismic reflection data, tied with ocean drilling program drill cores, reveal that variations in the incoming sediment sequences are correlated with basement topography. We mapped the three‐dimensional seismic facies distribution and measured representative seismic sequences and units. Trench‐parallel seismic profiles show three regional provinces in the Shikoku Basin that are distinguished by the magnitude of basement relief and sediment thickness: Western (<200–400 m basement relief, >600 m sediment thickness), Central (>1500 m relief, ∼2000 m sediments), and Eastern (<600 m relief, ∼1200 m sediments) provinces. The total thickness of sediment in basement lows is as much as six times greater than that over basement highs. Turbidite sedimentation in the Shikoku Basin reflects basement control on deposition, leading to the local presence or absence of turbidite units deposited during the middle Oligocene to the middle Miocene. During the first phase of sedimentation, most basement lows were filled with turbidites, resulting in smooth seafloor morphology that does not reflect basement relief. A second phase of turbidite deposition in the Eastern Province was accompanied by significant amounts of hemipelagic sediments interbedded with turbidite layers compared to the other provinces because of its close proximity to the Izu–Bonin Island Arc. Both regional and local variations in basement topography and sediment thickness/type have caused lateral heterogeneities on the underthrusting plate that will, in turn, influence lateral fluid flow along the Nankai accretionary prism.

Dynamics of a Transgressive Prodeltaic System: Implications for Geography and Climate Within a Pennsylvanian Intracratonic Basin, Colorado, U.S.A.

Detailed sedimentological analysis of a prodeltaic turbidite unit within the Pennsylvanian Minturn Formation provides insight into the nature of fan-delta deposition, and allows reconstruction of paleogeographic and paleoclimatic conditions within a tectonically active cratonic basin. The formation, a thick succession of fan-delta deposits, exhibits a complex stratigraphic architecture of alluvial, fluvial, and deltaic fithofacies that were deposited in the Central Colorado Basin (CCB), U.S.A., a fault-bounded depression within the Ancestral Rockies. An ~ 20 to 35-m-thick, regionally extensive, unconformity-bounded prodelta unit was deposited during a large eustatic sea-level rise that temporarily interrupted coarsegrained fluvial-deltaic deposition. The unit consists of dark-green shale and sandstone event beds with tool marks produced by abundant plant debris. The sandstone event beds were previously interpreted as turbidites because they contain current ripples, parallel lamination, and sole marks consistent with deposition from turbidity currents in the absence of storm-generated surface gravity waves. We have found, however, that proximal prodeltaic deposits contain evidence for combined oscillatory and unidirectional flow (i.e., asymmetric hummocky cross stratification (HCS), quasi-planar lamination, and combined-flow ripple stratification), indicating deposition under large waves in relatively shallow water. Paleohydraulic calculations indicate that formation of the HCS beds requires a fetch of at least 75 km to generate the necessary wave periods. Given the position of the CCB in the lee of the ancestral Front Range relative to the prevailing trade winds, and the narrow seaway to the south, storms either tracked across the Cordilleran seaway from the west or formed in situ within the CCB. The former hypothesis is consistent with paleoclimate models that purport the development of summer monsoonal circulation patterns over western equatorial Pangea during the late Paleozoic, and regional observations of southeastward eolian bedform migration directions.

Deep water gravity core from the Marsili Basin (Tyrrhenian Sea) records Pleistocenic–Holocenic explosive events and instability of the Aeolian Archipelago, (Italy)

A 4.8 m long gravity core was recovered on a relative topographic high in the northern part of the Marsili Basin (southern Tyrrhenian Sea) at a water depth of 3200 m. The core was taken in order to decipher the sedimentary record of the past volcanic events of the nearby Aeolian arc. A succession of thin (2 cm to 5 cm thick) fine-grained turbidites, mainly of volcaniclastic origin, topped by hemipelagic mud layers and a number of primary tephra layers were recovered by the core. The most prominent turbidite occurs in the lower part of the core at 385 cm. It consists of a 20 cm-thick, thinning-upward, pebble to sand-sized bed. Grain-size analysis and component compositions in the 0.063–0.250 mm size fractions were determined on thirty samples taken from primary tephra beds and the silty–sandy basal part of the volcaniclastic turbidite units. SEM scans and glass fraction chemical analyses were successively carried out on a selection of 17 samples. To aid source correlation and comparison, sub-aerial tephras of the Lower Pollara (Salina, 24 ± 3.6 ka), Gabellotto-Fiumebianco (Lipari, 8.5 or 11.5 ka), Monte Pilato (Lipari, 749 or 580 AD) and Secche di Lazzaro (Stromboli, ~ 5 ka) eruptions were also analyzed with the same procedure. Primary tephra respectively belonging to the eruptions of Lower Pollara, Gabellotto-Fiumebianco and Vesuvius (AP eruptions 3.5 ka–79 AD) were identified in the core at the expected relative stratigraphic depths. Two turbidite beds composed of monogenic glass shards were also identified and interpreted as the remobilisation of primary tephras of Secche di Lazzaro (Stromboli, 5 ka) and Pilato (Lipari, 580 or 749 AD). Tephrochronology results indicate that the cored sequence formed in the last 30 ka suggesting an average sedimentation rate of 0.15–0.17 mm/y. The thick pebbly sandy turbidite unit in the lower part of the core has component and glass composition compatible with the Lower Pollara volcanic sequence of Salina Island. In view of the grain-size and thickness of the turbidite unit, we suggest that it represents the deposit of a large failure event. The tephra corresponding to the Lower Pollara event lies below the turbidite unit, separated by 16 cm of hemipelagic mud, indicating that the collapse took place sometime after the eruption.

Sedimentation of the Phyllopod Bed within the Cambrian Burgess Shale Formation of British Columbia

We provide the most detailed sedimentological log to date through the Phyllopod Bed of the mid-Cambrian Burgess Shale Formation of British Columbia, based on millimetre-scale logging of a suite of thin sections. The sedimentary facies is dominated by alternations of homogeneous mudstone and a coarser-grained, laminated, variably sandy and shelly mudstone that is locally micronodular. Most boundaries between these two lithologies are gradational, and discrete fining-upwards turbidite units were rarely recognized. Such a pattern is interpreted to indicate rapid sedimentation of up to decimetre-thick units at this location from pulsatory, quasi-continuous density currents consistent with earlier proposals of exceptional preservation through rapid burial; the density currents responsible were probably largely akin to mud-rich slurries, helping explain the transport and entombment of the fossils. The homogeneous mudstone units are characterized by numerous distinctive lenses of pyrite framboids or subeuhedral crystals, previously interpreted as small ripples. Their 3D shape, however, suggests an origin as subspherical early diagenetic aggregates; their present morphology is consistent with the high levels of compaction inferred from the preservation of fossils.

Compound seismic modelling of the Ainsa II turbidite system, Spain: Application to deep-water channel systems offshore Angola

Seismic models of outcrops are essential for qualifying petroleum targets, because they bridge a critical gap in both resolution and scale between architectural geometries observed in outcrops and in seismic data. The paper describes how a seismic model of the deep-marine Ainsa II turbidite system of the St. Vicente Formation in the Southern Pyrenees, Spain is developed and used as an analogue for exploration prospects in the Quifangondo Formation, Lower Congo Basin offshore Angola. First, geometry and lithology information from the Ainsa II outcrop were utilised to construct a detailed earth model. The Ainsa II turbidite system consists of at least five clastic turbidite units bounded by erosional surfaces, where only the youngest unit is steep-sided. The input geo-model (earth model) includes stratigraphic information, such as truncations, onlaps, thickening/thinning of beds, condensed sections and detailed lateral changes in lithology. Second, subsurface petrophysical properties (velocity and density logs) necessary to build the forward seismic models were extracted from an offshore well in Angola. This allowed us to utilise the outcrop as a seismic analogue for a relatively narrow, laterally aggrading turbidite system offshore Angola. The seismic models showed that it is difficult to image and identify amalgamated beds inside a larger unit, particularly if beds have predominantly the same facies associations and weakly dipping basal boundaries (as in the lower part of the Ainsa II system). Second, seismic modelling helped to highlight areas where it is difficult to distinguish between seismic onlap and true stratigraphic onlap. Information which decides when evaluating stratigraphic traps. We also found that lateral degradation in amplitude may represent a lateral change in lithology (e.g. from sandy channel deposits to levee-overbank heterolithics). Integration of outcrop mapping, well data, 3D seismic interpretation and forward seismic modelling, has enabled us to study deep-water depositional elements at a sub-seismic scale. This has resulted in enhanced prediction of the lithology distribution and architecture of the turbidite system offshore Angola, and gives new insights into the 3D geometry of the Ainsa II system.

Holocene sedimentary processes in the Andøya Canyon system, north Norway

The Andøya Canyon system is the most pronounced of the ten canyon systems identified on the Norwegian continental margin. None of these canyon systems is linked to any present active drainage area and are interpreted to have been basically non active since the last glacial maximum and the deglaciation period. Our knowledge on these canyon systems is however still limited as no high-resolution stratigraphical or acoustic data sets are available from these areas. The main target for analysing the long term history of the Andøya Canyon has been to study the distal accumulation part of this system in the Lofoten Basin. The Lofoten Basin area is characterized by repeated stacks of extensive turbidite units where the details of the stratigraphical succession can be studied in undisturbed settings. About 3000 km 2 of high-resolution multibeam data (EM300) along with three gravity cores have been collected from the Lofoten Basin area. The analyses carried out on the cores revealed that the predominant process results in single turbidite units interbedded with thin units of hemipelagic sedimentation. The radiocarbon dating revealed that turbidites have been transported through this channel system at least twice during the Holocene. The two uppermost dated turbidite sequences give a maximum age of 7.9 ka 14C BP for the lower sequence and an age of ca. 4.0 ka 14C BP for the upper sequence. The upper sequence is of a similar age as the Trænadjupet Slide failure whereas the lower sequence indicates an age close to the Storegga Slide failure event, taking into consideration the erosive lower boundary of that sequence. The location of the investigated cores, which are far away from both the Trænadjupet Slide and the Storegga Slide, indicate that the instability in these canyon systems during the Holocene period has most probably been influenced by these large-scale failure events. What triggered the release of these turbidites is still not understood, but earthquakes or tsunami waves associated with the mega scale Trænadjupet Slide and the Storegga Slide events are considered to be the most likely mechanism at least during the Holocene Interglacial period.

The anatomy and provenance of thick volcaniclastic flows in the Cretan Basin, South Aegean Sea

Santorini volcano has been the largest source of volcaniclastic sediment in the Eastern Mediterranean during the late Quaternary. A dozen cores from the Cretan Basin, south of Santorini, have sampled two megabeds that consist of gravity emplaced volcaniclastic sequences. The uppermost megabed U consists of a succession of five (U 5–U 1) base cut out turbiditic units. Lower megabed A is a single turbiditic event. Only the uppermost U 2 and U 1 turbidites are separated from the underlying beds by hemipelagic marls. The texture and composition of the U and A megabeds closely match the texture and composition of the fine, vitric ash of the “Minoan” deposits on Santorini islands, dating from about 3500 yr BP. These megabeds are therefore attributed to rapid accumulation of separate gravity flows fed by the “Minoan” eruption, except for the upper U 2 and U 1 turbidites deposited from subsequent gravity flows transporting eroded volcaniclastic sediments. With the exception of the margin south of Santorini, dozens of cores retrieved around the margins of the Cretan Basin have a continuous late Quaternary succession that shows no evidence for massive sediment remobilization into the deeper basin, including the passage of the “Minoan” tsunami. Extensive high-resolution 3.5 kHz records revealed the acoustic character, architecture and distribution of the U and A megabeds and four underlying late Quaternary volcanogenic megabeds in the Cretan Basin. The acoustic facies of megabeds are typical of megaturbidites and consist of an upper, transparent, lower velocity layer that corresponds to the fine-grained upper turbiditic silt and clay section and a lower, strongly reflective higher velocity section that corresponds to the lowest, coarser-grained base of the turbidite that is developed over a sharp erosional surface. Penetration of the high-resolution records reveals the existence of at least six megabeds. Correlation with core lithology and the physical properties of the various lithofacies, including down-core velocity profiles, has allowed us to determine the thickness and volumes of the upper four megabeds which are: U ≤ 9 m thick, volume 3.7 km 3; A ≤ 25 m thick, volume 12.2 km 3; B ≤ 22 m thick, volume 10.3 km 3; C ≤ 15 m thick, volume 8 km 3. These thick megabeds are the uppermost products of repeated explosive eruption of Santorini in the late Quaternary. Calculated sedimentation rates from and after the “Minoan” eruption are 9.4 m/1000 yr that rise to over 15.7 m/1000 yr if megabed B was also deposited during this eruption.

Facies and facies association of the siliciclastic Brak River and carbonate Gemsbok formations in the Lower Ugab River valley, Namibia, W. Africa

The Neoproterozoic Zerrissene Turbidite Complex of central-western Namibia comprises five turbiditic units. From the base to the top they are the Zebrapüts Formation (greywacke and pelite), Brandberg West Formation (marble and pelite), Brak River Formation (greywacke and pelite with dropstones), Gemsbok River Formation (marble and pelite) and Amis River Formation (greywacke and pelites with rare carbonates and quartz-wacke). In the Lower Ugab River valley, five siliciclastic facies were recognised in the Brak River Formation. These are massive and laminated sandstones, classical turbidites (thick- and thin-bedded), mudrock, rare conglomerate and breccia. For the carbonate Gemsbok River Formation four facies were identified including massive non-graded and graded calcarenite, fine grained evenly bedded blue marble and calcareous mudrock. Most of these facies are also present in the other siliciclastic units of the Zerrissene Turbidite Complex as observed in other areas. The vertical facies association of the siliciclastic Brak River Formation is interpreted as representing sheet sand lobe to lobe-fringe palaeoenvironment with the abandonment of siliciclastic deposition at the top of the succession. The vertical facies association of the carbonate Gemsbok Formation is interpreted as the slope apron succession overlain by periplatform facies, suggesting a carbonate slope sedimentation of a prograding depositional shelf margin. If the siliciclastic–carbonate paired succession would represent a lowstand relative sea-level and highstand relative sea-level, respectively, the entire turbidite succession of the Zerrissene Turbidite Complex can be interpreted as three depositional sequences including two paired siliciclastic–carbonate units (Zebrapüts-Brandberg West formations; Brak River–Gemsbok formations) and an incomplete succession without carbonate at the top (Amis River Formation).