Turbidite Beds Research Articles

Recognition of cyclic steps in sandy and gravelly turbidite sequences, and consequences for the Bouma facies model

AbstractPreservation of cyclic steps contrasts markedly with that of subcritical‐flow bedforms, because cyclic steps migrate upslope eroding their lee face and preserving their stoss side. Such bedforms have not been described from turbidite outcrops and cores as yet. A conceptual block diagram for recognition of cyclic steps in outcrop has been constructed and is tested by outcrop studies of deep water submarine fan deposits of the Tabernas Basin in south‐eastern Spain. Experimental data indicate that depositional processes on the stoss side of a cyclic step are controlled by a hydraulic jump, which decelerates the flow and by subsequent waxing of the flow up to supercritical conditions once more. The hydraulic jump produces a large scour with soft‐sediment deformation (flames) preserved in coarse‐tail normal‐graded structureless deposits (Bouma Ta), while near‐horizontal, massive to stratified top‐cut‐out turbidite beds are found further down the stoss side of the bedform. The architecture of cyclic steps can best be described as large, up to hundreds of metres, lens‐shaped bodies that are truncated by erosive surfaces representing the set boundaries and that consist of nearly horizontal lying stacks of top‐cut‐out turbidite beds. The facies that characterize these bedforms have traditionally been described as turbidite units in idealized vertical sequences of high‐density turbidity currents, but have not yet been interpreted to represent bedforms produced by supercritical flow. Their large size, which is in the order of 20 m for gravelly and up to hundreds of metres for sandy steps, is likely to have hindered their recognition in outcrop so far.

The spatial and temporal distribution of grain‐size breaks in turbidites

ABSTRACTGrain‐size breaks are surfaces where abrupt changes in grain size occur vertically within deposits. Grain‐size breaks are common features in turbidites around the world, including ancient and modern systems. Despite their widespread occurrence, grain‐size breaks have been regarded as exceptional, and not included within idealized models of turbidity current deposition. This study uses ca 100 shallow sediment cores, from the Moroccan Turbidite System, to map out five turbidite beds for distances in excess of 2000 km. The vertical and spatial distributions of grain‐size breaks within these beds are examined. Five different types of grain‐size break are found: Type I – in proximal areas between coarse sand and finer grained structureless sand; Type II – in proximal areas between inversely graded sand overlain by finer sand; Type III – in proximal areas between sand overlain by ripple cross‐laminated finer sand; Type IV – throughout the system between clean sand and mud; and Type V – in distal areas between mud‐rich (debrite) sand and mud. This article interprets Types I and V as being generated by sharp vertical concentration boundaries, controlled by sediment and clay concentrations within the flows, whilst Types II and III are interpreted as products of spatial/temporal fluctuations in flow capacity. Type IV are interpreted as the product of fluid mud layers, which hinder the settling of non‐cohesive grains and bypasses them down slope. Decelerating suspensions with sufficient clay will always form cohesive layers near to bed, promoting the generation of Type IV grain‐size breaks. This may explain why Type IV grain‐size breaks are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere. Therefore, Type IV grain‐size breaks should be understood as the norm, not the exception, and regarded as a typical feature within turbidite beds.

Deep-Water Ediacaran Fossils from Northwestern Canada: Taphonomy, Ecology, and Evolution

Impressions of soft-bodied Ediacaran megafossils are common in deep-water slope deposits of the June beds at Sekwi Brook in the Mackenzie Mountains of NW Canada. Two taphonomic assemblages can be recognized. Soles of turbidite beds contain numerous impressions of simple (Aspidella) and tentaculate (Hiemalora, Eoporpita) discs. A specimen of the frond Primocandelabrum is attached to an Aspidella-like holdfast, but most holdfast discs lack any impressions of the leafy fronds to which they were attached, reflecting Fermeuse-style preservation of the basal level of the community. Epifaunal fronds (Beothukis, Charnia, Charniodiscus) and benthic recliners (Fractofusus) were most commonly preserved intrastratally on horizontal parting surfaces within turbidite and contourite beds, reflecting a deep-water example of Nama-style preservation of higher levels in the community. A well-preserved specimen of Namalia significantly extends the known age and environmental range of erniettomorphs into deep-water aphotic settings. Infaunal bilaterian burrows are absent from the June beds despite favorable beds for their preservation. The June beds assemblage is broadly similar in age and environment to deep-water Avalonian assemblages in Newfoundland and England, and like them contains mainly rangeomorph and arboreomorph fossils and apparently lacks dickinsoniomorphs and other clades typical of younger and shallower Ediacaran assemblages. Fossil data presently available imply that the classically deep- and shallow-water taxa of the Ediacara biota had different evolutionary origins and histories, with sessile rangeomorphs and arboreomorphs appearing in deep-water settings approximately 580 million years ago and spreading into shallow-water settings by 555 Ma but dickinsoniomorphs and other iconic clades restricted to shallow-water settings from their first known appearance at 555 Ma until their disappearance prior to the end of the Ediacaran.

Palaeoenvironment of Eocene prodelta in Spitsbergen recorded by the trace fossilPhycosiphon incertum

Ichnological, sedimentological and geochemical analyses were conducted on the Eocene Frysjaodden Formation in order to interpret palaeoenvironment prodelta sediments in the Central Basin of Spitsbergen. Phycosiphon incertum is the exclusive ichnotaxon showing differences in size, distribution, abundance and density, and relation to laminated/bioturbated intervals. Large P . incertum mainly occur dispersed, isolated and randomly distributed throughout the weakly laminated/non-laminated intervals. Small P . incertum occur occasionally in patches of several burrows within laminated intervals or as densely packed burrows in thin horizons in laminated intervals or constituting fully bioturbated intervals that are several centimetres thick. Ichnological changes are mainly controlled by oxygenation, although the availability of benthic food cannot be discarded. Changes in oxygenation and rate of sedimentation can be correlated with the registered variations in the Bouma sequence of the distal turbiditic beds within prodeltal shelf sediments. Keywords: Trace fossils; Phycosiphon ; Eocene; prodelta; turbiditic sediments; Spitsbergen (Published: 20 October 2014) Citation: Polar Research 2014, 33 , 23786, http://dx.doi.org/10.3402/polar.v33.23786

Lower Triassic mixed carbonate and siliciclastic setting with Smithian–Spathian anoxic to dysoxic facies, An Chau basin, northeastern Vietnam

In the An Chau basin in northeastern Vietnam, the Olenekian (Lower Triassic) stage comprises mixed carbonate and siliciclastic shallow marine to marginal basin deposits that extend into the southern Nanpanjiang basin in South China. The upper Lang Son Formation, which is dominated by siliciclastic facies, is composed of storm- and wave-influenced shallow marine and slope deposits. The overlying and interfingering Bac Thuy Formation consists of tidal flat, wave-influenced carbonate platform, slope, and marginal basin plain deposits. The tidal flat deposits are composed mainly of lenticular, wavy-bedded, or thin flat-bedded carbonates containing desiccation cracks and ripples that are indicative of bidirectional paleocurrents. The platform carbonates are characterized by wave ripples, ooids, and abundant marine mollusks and microfossils; the molluscan fossil assemblage primarily contains Smithian ammonoids such as Owenites koeneni. The slope deposits are characterized by limestone breccia and slump beds. The marginal basin plain deposits consist of classical turbidite beds, minor limestone breccia, and hemipelagic mudstone and marl containing the Spathian ammonoids Tirolites and Columbites. The turbidite sets may represent frontal splay environments in the marginal basin plain. These Olenekian carbonate and siliciclastic facies are typical of a transgressive succession.The Smithian–Spathian boundary and the anoxic to dysoxic facies of the latest Smithian are characterized by organic-rich dark gray limestone and mudstone deposits intercalated in the succession of slope to marginal basin plain facies, which are composed mainly of gravity flow and hemipelagic deposits. The organic-rich deposits show no evidence of bioturbation and are characterized by low-diversity fauna. Clastics in the marginal basin plain deposits were apparently supplied from the west; hence, the paleocurrent direction of the siliciclastic gravity flows was approximately eastward.

Quantitative detection of stacking patterns of sheet-like turbidite beds using the Straub method:

Quantitative detection of stacking patterns of sheet-like turbidite beds using the Straub method:

Statistical analysis of turbidite bed thickness patterns from the Alpine fold and thrust belt of western and southeastern Greece

A statistical evaluation of turbidite bed thickness was performed for 11 turbidite successions which constitute part of foreland basins formed along the Alpine fold and thrust belt of western and southeastern Greece. The purpose was to compare the results from this study with previously published investigations and discuss if and how statistical approaches could contribute to the environmental analysis of turbidite deposits in tectonically complex areas. Bed thickness datasets exhibit a clear non-normal signal in their distribution behavior. Statistical analysis was focused on: i) fitting of widely used non-normal theoretical distribution models using robust non-parametric goodness-of-fit statistical tests, and ii) detecting the possible presence of asymmetric bed thickness cycles and/or non-random bed thickness clustering. Results indicate that the studied bed thickness datasets are well characterized by a bimodal lognormal distribution model probably reflecting deposition from both high- and low-density turbidity currents. Several datasets exhibit distributions characterized by power law tails. Exponential distribution types were not observed. All datasets exhibit non-random thinning-upward bed thickness trends probably due to tectonically induced gravity flows. Proposed methods for identifying depositional sub-environments using turbidite bed thickness statistics were found to have limited applicability. Observations of the present study may prove useful for the environmental analysis of turbidite deposits in fold and thrust belt areas characterized by limited outcrop exposures.

Paleoredox and pyritization of soft-bodied fossils in the Ordovician Frankfort Shale of New York

Multiple beds in the Frankfort Shale (Upper Ordovician, New York State), including the original “Beecher9s Trilobite Bed,” yield fossils with pyritized soft-tissues. A bed-by-bed geochemical and sedimentological analysis was carried out to test previous models of soft-tissue pyritization by investigating environmental, depositional and diagenetic conditions in beds with and without soft-tissue preservation. Highly-reactive iron (FeHR), total iron (FeT), δ34S, organic carbon and redox-sensitive trace elements were measured. In particular, the partitioning of highly-reactive iron between iron-carbonates (Fe-carb), iron-oxides (Fe-ox), magnetite (Fe-mag), and pyrite (FeP) was examined. Overall, the multi-proxy sedimentary geochemical data suggest that the succession containing pyritized trilobite beds was deposited under a dysoxic water-column, in agreement with the paleontological data. The data do not exclude brief episodes of water-column anoxia characterized by a ferruginous rather than an euxinic state. However, the highest FeHR/FeT values and redox-sensitive trace element enrichments occur in siltstone portions of turbidite beds and in concretions, suggesting that subsequent diagenesis had a significant effect on the distribution of redox-sensitive elements in this succession. Moderately high FeHR/FeT and FeP/FeHR, low organic carbon, enriched δ34S, and the frequent presence of iron-rich carbonate concretions in beds with soft tissue preservation confirm that pyritization was favored where pore-waters were iron-dominated in sediments relatively poor in organic carbon.

The stratigraphic record of the Alì-Montagnareale Unit (Peloritani Mountains, NE Sicily, Italy)

Some Internal Units of the western Alpine peri-Mediterranean orogenic system (Betic Cordillera, Calabria-Peloritani Arc) point out to a complex Alpine tectono-metamorphic evolution followed by an exhumation phase. Before the onset of the Alpine compressive tectonics, the rocks of these units belonged to the margins of a microcontinent, the Mesomediterranean Microplate, originated, together with the main plates, following the break-up of Pangaea. To these units, characterized by similar tectono-sedimentary evolution, belongs the Ali-Montagnareale Unit of the Peloritani Mountains (NE Sicily), where the Internal Units of the Sicilian sector of the Maghrebian Chain crop out. The Ali-Montagnareale Unit appears within two small tectonic semi-windows, under the phyllites of the Mandanici-Piraino Unit, and it proves to be formed by three groups of anchimetamorphic formations.The Lower Group is made up of two Upper Permian?-Upper Triassic siliciclastic formations, which grade into each other and which were deposited in alluvial environments. This group is followed by the Intermediate Group, consisting of Middle?-Upper? Triassic formations, indicating deposition in an evaporitic coastal plain evolving to a marine carbonate platform. The Upper Group consists of a Pliensbachian formation, made up of pelagic calcilutites with added carbonate turbiditic beds, followed by a prevailingly marly formation with strata of silicified limestones and episodic calcareous re-sediments. In this latter formation, the finding of Jurassic fossils and Globuligerina oxfordiana, which occurs up to the Valanginian, indicate that the stratigraphic succession spans from the late Early Jurassic to the Late Jurassic and probably reaches the Early Cretaceous.The stratigraphic units are generally separated by tectonic contacts which act as preferential detachment surfaces during compressive and exhumation tectonic phases. The tectonic elision of more or less large intervals of the original stratigraphic succession is diffuse in both tectonic windows.The succession of the Ali-Montagnareale Unit testifies to extensional tectonics, which in Middle-Late Triassic determined the evolution of sedimentation from continental to evaporitic and neritic environments, and, since the Early Jurassic, to calcilutite and marly pelagic sediments with added calcareous re-sediments. This sedimentary evolution records the break-up of Pangaea and the successive opening of oceanic basins. A similar tectono-sedimentary history may be recognized in many other Internal Units of the Alpine peri-Mediterranean Chains.

Distal turbidite fan/lobe succession of The Late Paleozoic Taean Formation, Western Korea

The Late Paleozoic Taean Formation is similar in depositional environment and age to sedimentary successions around the collision belt between the North and South China blocks. The studies on those successions would be helpful for figuring out the pre- and post-collision depositional setting and tectonic framework around the Korean Peninsula and mainland China. As the first step for such an approach to the Taean Formation in Anmyeondo, this study carried out analyses of sedimentary facies and architectural elements of it. The Taean Formation of turbidite origin consists of 6 sedimentary facies, which can be grouped into 4 facies associations (FA) representing axis lobes, off-axis lobes, fringe lobes and distal fringe lobes of a deep-water turbidite system. Based on assemblage and stacking pattern of 4 facies associations, three architectural elements are defined: forward stacking of depositional lobe, backward stacking of depositional lobe and fan/lobe fringe. The first two architectural elements are characterized by meso-scale thickening- and thinning-upward succession of facies associations 1 and 2. The last one is defined by disorganized assemblage of facies associations 3 and 4. Stratigraphic correlation on the level of architectural elements gives rise to division of the Taean Formation in Anmyeondo into 4 fining-upward stratigraphic units (FSUs) separated by thick mudstone beds of fan/lobe fringe architectural element. The stratigraphic units can be grouped into 2 turbidite members based on the composition of sandstone turbidite beds. The lower turbidite member A comprises FSU 1 of calcareous turbidite beds, whereas the upper turbidite member B comprises FSU 2–4 of non-calcareous turbidite beds. These sedimentological and stratigraphic features indicate that the Taean Formation in Anmyeondo represents distal fan/lobe depositional environments in a sand/mud-mixed turbidite system.

Magnetic-Fabric Analysis As A Tool To Constrain Mechanisms of Deep-Water Mudstone Deposition In the Marnoso Arenacea Formation (Miocene, Italy)

Abstract Mudstone-dominated marine successions are common in the geological record, yet a full understanding of their depositional processes is often hampered by a lack of generally accepted diagnostic criteria to distinguish between hemipelagic settling and deposition from a flowing medium. The Marnoso Arenacea Formation, a turbidite unit of Miocene age cropping out in the northern Apennines of Italy, offers the possibility to address some of these uncertainties. A relatively small (∼ 10%) but distinctive portion of the Marnoso Arenacea Formation is composed of white marlstone beds (WM beds) that have frequently been interpreted as due to hemipelagic settling of fine-grained particles (hemipelagites). The analysis of the anisotropy of magnetic susceptibility (AMS) revealed the presence in the WM beds of maximum susceptibility axes clustered within the depositional plane along the average paleoflow direction inferred from flute casts at the bases of the nearest turbidite beds, whereas the minimum susceptibility axes are oriented perpendicular to the bedding plane. This fabric is interpreted as largely sedimentary in origin (albeit a contribution from tectonic shortening cannot be excluded) and due to the alignment within the bedding plane of paramagnetic grains (e.g., muscovite) and possibly also ferromagnetic grains (magnetite) under low-velocity currents. The trend of the maximum susceptibility axes, and hence of the paleoflow direction, is approximately oriented NNW–SSE after correction for Apennines thrust-sheet rotation since the Miocene. These results suggest that the WM beds cannot be entirely due to hemipelagic settling, as often stated in the literature. A discussion of alternative depositional mechanisms leads us to conclude that the WM beds may have been deposited under the influence of contour currents and should therefore be referred to as muddy contourites.

Fault rate controls on carbonate gravity-flow deposits of the Liassic of Central High Atlas (Morocco)

The aim of this work is to describe and to explore linkages between fault activity and gravity-related sedimentary deposits during the early rifting of a carbonate depositional system. The study area, located at the southern edge of Central High Atlas (Morocco), hosts 900 m thick Liassic gravity-related sedimentary deposits, preserved at the hanging wall of one of the possibly inherited, reactivated Hercynian faults (Tizi n'Firest fault). This study documents gravity-flow deposits constrained by a high-resolution biostratigraphic framework (ca. 100 ka). These gravity-flow deposits evolve over time. Unstratified conglomerate beds are limited to the Sinemurian and are preserved near the fault. Incomplete, ungraded Bouma sequence beds are preserved preferentially at the transition between the Sinemurian and the Carixian while complete, graded Bouma sequences are recorded during the Carixian and Domerian. Fine-grained turbidite beds are observed dominantly during the Domerian. Incomplete, graded Bouma sequences are observed all along the section. Our work suggests that fault activity over time, associated to rifting, has led to this diversity of gravity-flow deposits. Fault activity plays a key role in the initiation of gravity flows, but also influences the type of gravity-flow deposits, by controlling the amount of sediment available, the sediment grain-size, the slope morphology and the basin deepening.

Turbidite paleoseismology in the Calabrian Arc Subduction Complex (Ionian Sea)

The Calabrian Arc subduction system is part of the Africa–Eurasia plate boundary, is one of the most seismically active regions in the Mediterranean Sea, and has been struck repeatedly by destructive historical earthquakes. In this study, we investigate the effects of historical earthquakes on abyssal marine sedimentation through the analysis of the turbidite record. We collected gravity cores in tectonically controlled basins where the eastern Mediterranean pelagic sequence is interbedded with resedimented units. Textural, micropaleontological, geochemical, and mineralogical signatures reveal three turbidite events in the last millennium. We dated the turbidite sequences from two different cores using different radiometric methods, whereas the average time interval between successive turbidite beds was estimated from pelagic sediment thickness and sedimentation rates; chronologies were refined through age modeling that provided age ranges (2σ) of each turbidite bed. The results suggest that turbidite emplacement was triggered by three historical earthquakes recorded in the area (i.e., the 1908, 1693, and 1169 events); their magnitude, epicentral location, and associated tsunamis support causative faults located in the Ionian Sea. The source for all the turbidites, as inferred from their mineralogy, is the metamorphic basement outcropping in southern Calabria and/or northeastern Sicily. Turbidite composition and cable breaks for the 1908 event have been used to infer likely traveling paths and seismogenic faults in the subduction system. Our findings suggest that Ionian Sea turbidites represent more than 80% of sedimentation and may be seabed archives of paleo‐earthquakes capable of reconstructing seismicity back in time, during several earthquake cycles.

The full range of turbidite bed thickness patterns in submarine lobes: controls and implications

A widely misused criterion to interpret lobe deposits in submarine fan systems at outcrop, and in core and well logs, is a thickening and/or coarsening upward profile. Lobe deposits from the Laingsburg depocentre, SW Karoo Basin, demonstrate that a full range of bed thickness patterns exists within lobes. When lobes are defined by their laterally extensive bounding surfaces that are marked by abrupt facies changes, five types of bed stacking patterns are identified: thickening upward, thinning upward, thickening then thinning upward, thinning then thickening upward, and constant. The abrupt bounding surfaces are interpreted to record avulsion of feeder distributive channels. The stratigraphic bed thickness pattern preserved in a lobe reflects the internal organization of smaller-scale lobe elements, rather than lobe-wide initiation and progradation as implied by a thickening-upward only pattern. The full range of bed thickness patterns in lobes can be used to understand the stacking of lobe elements, the evolution of sediment deposition through time and space, and the relative movement of depocentres.

Sedimentology of seismo-turbidites off the Cascadia and northern California active tectonic continental margins, northwest Pacific Ocean

Abstract Holocene turbidites from turbidite channel systems along the active tectonic continental margins of the Cascadia subduction zone (offshore Vancouver Island to Mendocino Triple Junction) and the northern San Andreas Transform Fault (the Triple Junction to San Francisco Bay), have been analyzed for sedimentologic features related to their seismic origin. Centimeter thick silt/sand beds (turbidite base) capped by mud layers (turbidite tail) and interbedded with hemipelagic silty clay intervals with high biogenic content have been characterized by visual core descriptions, grain-size analysis, X-ray radiographs and physical properties. Along the northern California margin in upstream single tributary canyons and channels, most turbidites are uni-pulsed (classic fining up) whereas downstream below multiple tributary canyon and channel confluences, most deposits are stacked turbidites. Because each set of stacked turbidites has no hemipelagic sediment between each turbidite unit and each unit has a distinct mineralogy from a different tributary canyon, we interpret that a stacked turbidite is deposited by several coeval turbidity currents fed by multiple tributary canyons and channels with synchronous triggering from a single San Andreas Fault earthquake. The Cascadia margin is characterized by individual multi-pulsed turbidites that contain multiple coarse-grained sub-units without hemipelagic sediment between pulses. Because the number and character of multiple coarse-grained pulses for each correlative multi-pulsed turbidite is almost always constant both upstream and downstream in different channel systems for 600 km along the margin, we interpret that the earthquake shaking or aftershock signature is usually preserved, for the much stronger Cascadia (≥ 9 Mw) compared to weaker California (≥ 8 Mw) earthquakes, which result in upstream uni-pulsed turbidites and downstream stacked turbidites. Consequently, both the strongest (≥ 9 Mw) great earthquakes and downstream confluences cause multi-pulsed and stacked turbidites that are typical for seismo-turbidites generated by a single great earthquake. Along both margins, earthquake triggering and multi-pulsed or stacked turbidites become an alternative explanation for amalgamated turbidite beds in active tectonic margins and show that multiple grain-size pulses and reverse grading are not unique criteria for hyperpycnites, thalweg levee turbidites, or mini-basin margin turbidites. The analyses of the turbidites along the Cascadia and northern California margins reveal common sedimentologic characteristics of turbidites triggered by great earthquakes that can be used to distinguish seismo-turbidites in other active tectonic margins around the world.

Stages of sedimentary prism development on a convergent margin — Eocene Tyee Forearc Basin, Coast Range, Oregon, USA

Architecture of ancient forearc basin successions can be difficult to reconstruct because of the widespread syn-depositional and post-depositional deformations experienced by many forearc basin-fills. For this reason various techniques have been used for reconstructing forearc basin-infill geometry, including geochemical correlation. The Tyee Basin succession exposed in Coast Range of Western Oregon, USA, is an Eocene forearc-fill that includes genetically related non-marine, shallow marine and deepwater clastic deposits and is gently deformed. Reconstruction of the depositional geometry of the Tyee Basin succession from detailed outcrop and subsurface data reveals two distinct stages of development for this active basin-margin. These stages are characterized by two different basin-margin clinoform architectures and also by a pronounced change in the character of the associated deepwater deposits. During the initial stage, the basin-margin clinoforms are smaller (<250m clinoform height) and strongly progradational, with clinoform topset dominated by fluvial deposits. At this stage thick sand-rich unconfined turbidite beds accumulated on the slope segment of the clinoforms and extended out onto the basin-floor. Large scale slope conduits such as slope channels or canyons, are notably absent in this stage. The second stage is characterized by larger clinoform height (>500m), a greater degree of topset aggradation with repeated fluvio-deltaic cycles on the shelf, and well-organized, large turbidite channels on the slope. The turbidite channels supplied medium-grained sands to the extensive, stacked basin-floor fans. The first stage described above marks the early development of a shelf-slope prism on the Tyee continental margin, and has been interpreted by some earlier workers as an unique category of basin-margin architecture, termed as a ‘submarine ramp’. However, this was only the initial stage of development of the Tyee margin and it was followed by a period of basin-filling when repeated fluvial and shallow marine shelf-transit cycles fed well-organized turbidite channels on the slope as well as Tyee Basin floor fans. The large volume of sediment deposited during the initial stage, resulted from of the unique geometry of the Tyee Basin, as influenced by the presence of pre-existing topography on the accreted oceanic basement underlying the Tyee succession.

Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology

Abstract. We summarize the importance of great earthquakes (Mw &amp;amp;gtrsim; 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which results in a margin stratigraphy of minor MTDs compared to the turbidite-system deposits. In contrast, the MTDs and turbidites are equally intermixed on basin floors along passive margins with a mud-rich continental slope, such as the northern Gulf of Mexico. Great earthquakes also result in characteristic seismo-turbidite lithology. Along the Cascadia margin, the number and character of multiple coarse pulses for correlative individual turbidites generally remain constant both upstream and downstream in different channel systems for 600 km along the margin. This suggests that the earthquake shaking or aftershock signature is normally preserved, for the stronger (Mw ≥ 9) Cascadia earthquakes. In contrast, the generally weaker (Mw = or &lt;8) California earthquakes result in upstream simple fining-up turbidites in single tributary canyons and channels; however, downstream mainly stacked turbidites result from synchronously triggered multiple turbidity currents that deposit in channels below confluences of the tributaries. Consequently, both downstream channel confluences and the strongest (Mw ≥ 9) great earthquakes contribute to multi-pulsed and stacked turbidites that are typical for seismo-turbidites generated by a single great earthquake. Earthquake triggering and multi-pulsed or stacked turbidites also become an alternative explanation for amalgamated turbidite beds in active tectonic margins, in addition to other classic explanations. The sedimentologic characteristics of turbidites triggered by great earthquakes along the Cascadia and northern California margins provide criteria to help distinguish seismo-turbidites in other active tectonic margins.

Arumberia-type structures in the Upper Vendian of the Urals

Problematic arumberia-type structures are widespread in Upper Proterozoic sedimentary rocks deposited in depositional environments subject to periodic desiccation and fluctuating salinity. Here we report two new occurrences of arumberia-type structures in the Krutikha Member of the Upper Vendian Chernyi Kamen Formation on the western slope of the Central Urals (Us’va River) and the Zigan Formation of the Asha Group in the South Urals (new road Sterlitamak-Beloretsk). The results of the study of these structures significantly expand paleontological characteristics of the Upper Vendian non-marine rocks and allow us to re-examine the paleontological importance of the hastily forgotten group of arumberia-type organisms. We distinguished six varieties of arumberia-type structures that for the sake of convenience were given formal Latin names in binominal nomenclature although they are not biological species. Two species—Arumberia banski and A. vindhyanensis—were erected by previous researchers, whereas the A. beckeri, A. ollii, A. usvaensis, and A. multykensis varieties are recognized for the first time. Despite similarity to erosional structures on the bottom of turbidite beds, erosion does not explain the genesis of the entire spectrum of arumberia-type structures including those preserved on ripple marks, with evidence of soft-body deformation, or with ribbon and filamentous fabric replaced by clay minerals. The diversity of arumberia could be only a manifestation of living systems. The species of Arumberia are distinguished by their different biostratinomy, the distance between the main structural units, the preservation of the main structural units, and the presence of additional structures between the main structural units. Most likely, the diversity of arumberia reflects not only taphonomic, but also ontogenetic and phenotypic variation. The level of morphological complexity and biostratinomic features of arumberia apparently exceed the organization of microbial mats and represent a non-actualistic type of microbial communities. The occurrence in extremely shallow-water depositional setting suggests that arumberia had evolved an adaption to periodic desiccation and that arumberia-type organisms, along with microbial mats, had been an important factor of sediment biostabilization in the Late Vendian non-marine environments.

The Global Boundary Stratotype Section and Point (GSSP) of the Carnian Stage (Late Triassic) at Prati di Stuores/Stuores Wiesen Section (Southern Alps, NE Italy)

The Global boundary Stratotype Section and Point (GSSP) for the base of the Carnian Stage (Late Triassic) is defined on the north flank of the Cordevole Valley, on the southern slopes of the crest separating the Badia/Abtei and the Cordevole valleys (46° 31' 37N, 11° 55' 49E) at an elevation of 1980 m, approximately 4.7 km south of San Cassiano/St. Kassian in the Dolomites (Southern Alps). The Prati di Stuores/Stuores Wiesen section (Province of Belluno, Veneto Region, Italy) is located a few hundred meters east of Pralongia, a locality northeast of Arabba (Belluno). The section consists of hemipelagites and thin turbidite beds (both siliciclastic and carbonate intraclastic), deposited below storm wave-base with a high but variable sediment accumulation rate (San Cassiano Fm.). The GSSP boundary is located at the base of bed SW4, 45 m above the base of the San Cassiano Fm. Bed SW4 yielded ammonoids including Daxatina canadensis (Whiteaves), which is the primary global marker for identifying the base of the Carnian Stage. The secondary marker for the Carnian Stage, the conodont Paragondolella polygnathiformis (Budurov and Stefanov), appears 70 cm above the first occurrence of Daxatina canadensis and at the base of a long normal-polarity magnetic zone (S2n) in close proximity to the boundary. Furthermore, the GSSP is located one ammonite subzone just above a level dated at 237.77 ± 0.14 Ma.

Searching for the seafloor signature of the 21 May 2003 Boumerdès earthquake offshore central Algeria

Abstract. Shaking by moderate to large earthquakes in the Mediterranean Sea has proved in the past to potentially trigger catastrophic sediment collapse and flow. On 21 May 2003, a magnitude 6.8 earthquake located near Boumerdès (central Algerian coast) triggered large turbidity currents responsible for 29 submarine cable breaks at the foot of the continental slope over ~150 km from west to east. Seafloor bathymetry and backscatter imagery show the potential imprints of the 2003 event and of previous events. Large slope scarps resulting from active deformation may locally enhance sediment instabilities, although faults are not directly visible at the seafloor. Erosion is evident at the foot of the margin and along the paths of the numerous canyons and valleys. Cable breaks are located at the outlets of submarine valleys and in areas of turbiditic levee overspilling and demonstrate the multi-source and multi-path character of the 2003 turbiditic event. Rough estimates of turbidity flow velocity are not straightforward because of the multiple breaks along the same cable, but seem compatible with those measured in other submarine cable break studies elsewhere. While the signature of the turbidity currents is mostly erosional on the continental slope, turbidite beds alternating with hemipelagites accumulate in the distal reaches of sediment dispersal systems. In perspective, more chronological work on distal turbidite successions offshore Algeria offers promising perspectives for paleoseismology reconstructions based on turbidite dating, if synchronous turbidites along independent sedimentary dispersal systems are found to support triggering by major earthquakes. Preliminary results on sediment core PSM-KS23 off Boumerdès typically show a 800-yr interval between turbidites during the Holocene, in accordance with the estimated mean seismic cycle on land, even if at this stage it is not yet possible to prove the earthquake origin of all the turbidites.