An investigation of upper mantle heterogeneity beneath the Archaean and Proterozoic crust of western Canada from Lithoprobe controlled-source seismic experiments

Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas.Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background).The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.     

Subglacial tunnel valleys in the Alpine foreland: an example from Bern, Switzerland

The morphology of the Alpine and adjacent landscapes is directly related to glacial erosion and associated sediment transport. Here we report the effects of glacio-hydrologic erosion on bedrock topography in the Swiss Plateau. Specifically, we identify the presence of subsurface valleys beneath the city of Bern and discuss their genesis. Stratigraphic investigations of more than 4,000 borehole data within a 430 km²-large area reveal the presence of a network of >200 m-deep and 1,000 m-wide valleys. They are flat floored with steep sided walls and are filled by Quaternary glacial deposits. The central valley beneath Bern is straight and oriented towards the NNW, with valley flanks more than 20° steep. The valley bottom has an irregular undulating profile along the thalweg, with differences between sills and hollows higher than 50–100 m over a reach of 4 km length. Approximately 500 m high bedrock highlands flank the valley network. The highlands are dissected by up to 80 m-deep and 500 m-broad hanging valleys that currently drain away from the axis of the main valley. We interpret the valleys beneath the city of Bern to be a tunnel valley network which originated from subglacial erosion by melt water. The highland valleys served as proglacial meltwater paths and are hanging with respect to the trunk system, indicating that these incipient highland systems as well as the main gorge beneath Bern formed by glacial melt water under pressure.     

Nature,origin and evolution of a Late Pleistocene incised valley‐fill,Sunda Shelf,Southeast Asia

Understanding the stratigraphic fill and reconstructing the palaeo‐hydrology of incised valleys can help to constrain those factors that controlled their origin, evolution and regional significance. This condition is addressed through the analysis of a large (up to 18 km wide by 80 m deep) and exceptionally well‐imaged Late Pleistocene incised valley from the Sunda Shelf (South China Sea) based on shallow three‐dimensional seismic data from a large (11 500 km²), ‘merge’ survey, supplemented with site survey data (boreholes and seismic). This approach has enabled the characterization of the planform geometry, cross‐sectional area and internal stratigraphic architecture, which together allow reconstruction of the palaeo‐hydrology. The valley‐fill displays five notable stratigraphic features: (i) it is considerably larger than other seismically resolvable channel forms and can be traced for at least 180 km along its length; (ii) it is located in the axial part of the Malay Basin; (iii) the youngest part of the valley‐fill is dominated by a large (600 m wide and 23 m deep), high‐sinuosity channel, with well‐developed lateral accretion surfaces; (iv) the immediately adjacent interfluves contain much smaller, dendritic channel systems, which resemble tributaries that drained into the larger incised valley system; and (v) a ca 16 m thick, shell‐bearing, Holocene clay caps the valley‐fill. The dimension, basin location and palaeo‐hydrology of this incised valley leads to the conclusion that it represents the trunk river, which flowed along the length of the Malay Basin; it connected the Gulf of Thailand in the north with the South China Sea in the south‐east. The length of the river system (>1200 km long) enables examination of the upstream to downstream controls on the evolution of the incised valley, including sea‐level, climate and tectonics. The valley size, orientation and palaeo‐hydrology suggest close interaction between the regional tectonic framework, low‐angle shelf physiography and a humid‐tropical climatic setting.     

Development of an equatorial carbonate platform across the Triassic-Jurassic boundary and links to global palaeoenvironmental changes (Musandam Peninsula,UAE/Oman)

The Triassic-Jurassic boundary is marked by one of the ‘big five’ mass extinctions of the Phanerozoic. This boundary event was accompanied by several carbon cycle perturbations, potentially induced by the opening of the Central Atlantic and associated volcanism, and accompanied by an ocean acidification event. Continuous carbonate successions covering this interval of environmental change are however rare. Here data from a shallow-marine equatorial mixed carbonate-siliciclastic succession is presented, that was studied on a regional scale. Four sections that are 48 km apart were examined on the Musandam Peninsula (United Arab Emirates and Sultanate of Oman). The system was analysed for its sedimentology, vertical and lateral facies changes, and stable carbon and oxygen isotopes. Strontium isotope analysis was used to determine the position of the Triassic-Jurassic boundary horizon. The studied ramp experienced an episode of demise during the Late Triassic, followed by a restricted microbialite dominated ramp, containing large amounts of siliciclastic facies. During the Latest Triassic the diverse carbonate factory revived and flourished across the Triassic-Jurassic boundary. No clear evidence for a biocalcification crisis or an ocean acidification event across the Triassic-Jurassic boundary is visible. Lateral facies heterogeneities can be observed across the studied interval, attributed to hydrodynamic activity, including tropical storms, crossing the extensive shelf area. Although evidence for synsedimentary tectonic activity is present, the vertical stacking pattern is largely controlled by changes in relative sea level. The refined chronostratigraphy accompanied by the detailed environment of deposition analysis allows for a refinement of the regional palaeogeography. The neritic equatorial carbonate ramp has archived a negative carbon isotope excursion preceding the Triassic-Jurassic boundary that has also been reported from other study sites. The lack of evidence for a biocalcification crisis across the equatorial Triassic-Jurassic boundary indicates that the Tethys did not experience a distinct global acidification event.     

A Middle Paleozoic shear zone in the Sierra de Valle Fértil,Argentina: Records of a continent-arc collision in the Famatinian margin of Gondwana

Geological, petrological and structural observations were obtained along a 30-km-long traverse across a segment of the Valle Fértil shear zone, central-western Argentina. On a regional scale, the shear zone appears as numerous discontinues belts over 25 km in width and is approximately 140 km in length, extended on the western section of the Sierras Valle Fértil – La Huerta mountain range. The steeply dipping shear zone with a vertical mylonitic lineation is composed of amphibolite facies ribbon mylonites and amphibolite to greenschist facies ultramylonites derived from Early Ordovician plutonic and metasedimentary parent rocks. Locally, syn-kinematic retrogression of mylonites formed greenschist facies phyllonites. During the later stages of deformation, unstrained parent rocks, mylonites, ultramylonites and phyllonites were affected by pervasive cataclasis under low greenschist facies conditions associated with localized faulting. One new ⁴⁰Ar/³⁹Ar age on biotite and published ⁴⁰Ar/³⁹Ar ages on amphibole in the shear zone yield an average cooling rate of 6.2 °C/Ma for a time period that crosses the Silurian–Devonian boundary. Since in metasedimentary rocks the youngest zircon's rims dated at 465 Ma marks the beginning of cooling, nearly continuous uplift of rocks within the shear zone occurred over a minimum time span of 55 Ma. During the period of active deformation, dip-slip movement can explain uplift of several kilometers of the Early Ordovician arc crust. The Valle Fértil shear zone, which was formed near above the inferred suture zone between the Famatinian arc and Cuyania microcontinent, is a major structural boundary nucleated within the Early Ordovician crust. The simplest geodynamic model to explain the evolution of the Valle Fértil shear zone involves the collision of the composite Cuyania/Precodillera microcontinent against the Famatinian arc.     

Late Pleistocene baldcypress (Taxodium distichum) forest deposit on the continental shelf of the northern Gulf of Mexico

Approximately 13 km south of Gulf Shores, Alabama (United States), divers found in situ baldcypress (Taxodium distichum) stumps 18 m below the ocean surface. These trees could have only lived when sea level fell during the Pleistocene subaerially exposing the tectonically stable continental shelf. Here we investigate the geophysical properties along with microfossil and stratigraphical analyses of sediment cores to understand the factors that lead to this wood’s preservation. The stumps are exposed in an elongated depression (~100 m long, ~1 m deep) nested in a trough of the northwest–southeast trending Holocene sand ridges and troughs with 2–5 m vertical relief and ~0.5 km wavelength. Radiocarbon ages of the wood were infinite thus optically stimulated luminescence (OSL) dating was used to constrain the site’s age. Below the Holocene sands (~0.1–4 m thick), separated by a regional erosional unconformity, are Late Pleistocene mud-peat (72±8 ka OSL), mud-sand (63±5, 73±6 ka OSL), and palaeosol (56±5 ka OSL) facies that grade laterally from west to east, respectively. Foraminiferal analysis reveals the location of the terrestrial-marine transitional layer above the Pleistocene facies in an interbedded sand and mud facies (3940±30 (1σ) ¹⁴C a BP), which is part of a lower shoreface or marine-dominated estuarine environment. The occurrence of palaeosol and swamp facies of broadly similar ages and elevation suggests the glacial landscape possessed topographic relief that allowed wood, mud and peats to be preserved for ~50 ka of subaerial exposure before transitioning to the modern marine environment. We hypothesize that rapid sea-level rise occurring ~60 or ~40 ka ago provided opportunities for local flood-plain aggradation to bury the swamp thus preserving the stumps and that other sites may exist in the northern Gulf of Mexico shelf.     

Sediment-supply-dominated stratal architectures in a regressively stacked succession of shoreline sand bodies,Campanian Desert Member to Lower Castlegate Sandstone interval,Book Cliffs,Utah–Colorado,USA

Strongly progradational regressive stacks of shallow marine sandstones are ubiquitous in modern and ancient coastal depositional systems. Many ancient examples form prolific hydrocarbon and freshwater reservoirs in the subsurface. One of the best areas in the world to study progradational shallow marine successions is the Campanian Book Cliffs of Utah and Colorado, where the Desert Member to Lower Castlegate Sandstone interval served as a foundational data set for early sequence stratigraphic models. A strongly progradational stack of 17 parasequences comprises the Desert–Castlegate interval. Parasequences are 6·5 to 20·7 m thick. Normally regressive coarsening-upward successions are abundant, as are flat-topped, rooted foreshore sandstones. Conformable facies contacts mark the transition between the laterally adjoining nearshore terrestrial and shallow marine deposits which are genetically, temporally and spatially linked. The width of the shoreface to inner shelf facies belts varies from 4·8 to 19·9 km per parasequence, with a mean of 12·6 km. Solitary tongue shoreline trajectories are all very low to low angle ascending regressive, varying from +0·0004° to +0·171°. Stacked shoreline system trajectories are also dominantly low angle ascending regressive, with only two descending regressive trajectories, one of which intersects the depositional slope. The predominance of ascending regressive shoreline trajectories and normal regression, rarity of high frequency sequence boundaries, regressive surfaces of marine erosion and descending regressive shoreline trajectories, and absence of third-order sequence boundaries, incised valley fill deposits and no prolonged and regionally extensive sediment bypass, all point towards increasing sediment supply as the dominant driver of the Desert–Castlegate stratal architectures, while reduced accommodation (i.e. decreasing tectonic subsidence) played a secondary role.     

Late Quaternary multiple incised valley systems: An unusually well‐preserved stratigraphic record of two interglacial valley‐fill successions from the Arno Plain (northern Tuscany,Italy)

Un‐fragmented stratigraphic records of late Quaternary multiple incised valley systems are rarely preserved in the subsurface of alluvial‐delta plains due to older valley reoccupation. The identification of a well‐preserved incised valley fill succession beneath the southern interfluve of the Last Glacial Maximum Arno palaeovalley (northern Italy) represents an exceptional opportunity to examine in detail evolutionary trends of a Mediterranean system over multiple glacial–interglacial cycles. Through sedimentological and quantitative meiofauna (benthic foraminifera and ostracods) analyses of two reference cores (80 m and 100 m long) and stratigraphic correlations, a mid‐Pleistocene palaeovalley, 5 km wide and 50 m deep, was reconstructed. Whereas valley filling is chronologically constrained to the penultimate interglacial (Marine Isotope Stage 7) by four electron spin resonance ages on bivalve shells (Cerastoderma glaucum), its incision is tentatively correlated with the Marine Isotope Stage 8 sea‐level fall. Above basal fluvial‐channel gravels, the incised valley fill is formed by a mud‐prone succession, up to 44 m thick, formed by a lower floodplain unit and an upper unit with brackish meiofauna that reflects the development of a wave‐dominated estuary. Subtle meiofauna changes towards less confined conditions record two marine flooding episodes, chronologically linked to the internal Marine Isotope Stage 7 climate‐eustatic variability. After the maximum transgressive phase, recorded by coastal sands, the interfluves were flooded around 200 ka (latest Marine Isotope Stage 7). The subsequent shift in river incision patterns, possibly driven by neotectonic activity, prevented valley reoccupation guiding the northward formation of the Last Glacial Maximum palaeovalley. The applied multivariate approach allowed the sedimentological characterization of the Marine Isotope Stage 7 and Marine Isotope Stage 1 palaeovalley fills, including shape, size and facies architecture, which revealed a consistent river‐coastal system response over two non‐consecutive glacial–interglacial cycles (Marine Isotope Stages 8 to 7 and Marine Isotope Stages 2 to 1). The recurring stacking pattern of facies documents a predominant control exerted on stratigraphy by Milankovitch and sub‐Milankovitch glacio‐eustatic oscillations across the late Quaternary period.     

The palaeo‐valley infilling glaciogenic Sarah Formation,an example from Rahal Dhab palaeo‐valley,Saudi Arabia

The Sarah Formation is a glaciogenic sedimentary unit deposited along the Gondwana margin during the latest Ordovician ice age and represents a major hydrocarbon reservoir in northern Saudi Arabia. Large‐scale glacial palaeo‐valleys cut into the Qasim Formation and were infilled by the Sarah Formation. Post‐glacial transgression in the earliest Silurian resulted in the deposition of the Qusaiba Shale Member and associated organic‐rich basal source rocks, which cap the Sarah Formation infilled palaeo‐valleys. This unique setting makes the Sarah Formation an important emerging exploration target in Saudi Arabia. This study focuses on the facies and depositional architecture in seismic‐scale outcrops of the Sarah Formation in north‐western Saudi Arabia. The Rahal Dhab palaeo‐valley provides a 100 km long dip‐oriented cross‐section which has been covered by 24 vertical sections, sedimentary architectural analyses at metre to kilometre scale and by three cored shallow boreholes. In the Rahal Dhab palaeo‐valley, the Sarah Formation was deposited in a proglacial setting that ranged from marginal marine to offshore prodelta and is made up of three units: (i) the Sarah Sandstone; (ii) the Sarah Shale; and (iii) the Uqlah Member. This study shows the relationships between these three units and architectural controls on reservoir quality in this system. This paper contributes to the regional understanding of the Sarah Formation, and the new depositional model of the Rahal Dhab palaeo‐valley provides an outcrop‐reservoir analogue for hydrocarbon exploration in adjacent areas.     

Landslides triggered by the 1949 Khait earthquake, Tajikistan, and associated loss of life

Earthquake-triggered landslides are a major geological hazard in Central Asia. In July 1949, the M7.4 Khait earthquake triggered many hundreds of landslides in a mountainous region near the southern limit of the Tien Shan Mountains, central Tajikistan. These landslides involved widespread rock-slope failure as well as large numbers of flowslides in loess that mantles the steep slopes of the region. In the Yasman valley hundreds of loess landslides coalesced to form a massive loess flow (est. vol. 245 Mm³) that travelled up to 20 km on a slope of only 2°. In an adjacent valley, the Khait landslide involved transformation of an earthquake-triggered rockslide into a very rapid flow by the entrainment of saturated loess into its movement. It travelled 7.41 km over a vertical distance of 1421 m with an estimated average velocity of ~30 m/s. We estimate its volume as 75 Mm³, an order of magnitude less that previously published estimates. The Khait landslide was simulated using DAN. The number of casualties due to earthquake-triggered landslides in the epicentral region was considerable. Approximately 4000 people were killed in the Yasman valley loess flow as 20 villages (kishlaks) were overwhelmed. In the Khait landslide alone we estimate ca. 800 people lost their lives when the villages of Khait and Khisorak were overrun by rapidly moving debris. Our data indicates that a total of approximately 7200 people were killed by earthquake-triggered landslides in the epicentral region of the Khait earthquake and that, in terms of loss of life, the 1949 Yasman valley loess flow was one of the most destructive landslides in recent history.     

Interaction of an antecedent fluvial system with early normal fault growth: Implications for syn‐rift stratigraphy,western Corinth rift (Greece)

Continental ‘overfilled’ conditions during rift initiation are conventionally explained as due to low creation of accommodation compared with sediment supply. Alternatively, sediment supply can be relatively high from the onset of rifting due to an antecedent drainage system. The alluvial Lower Group of the western Plio–Pleistocene Corinth rift is used to investigate the interaction of fluvial sedimentation with early rifting. This rift was obliquely superimposed on the Hellenide mountain belt from which it inherited a significant palaeorelief. Detailed sedimentary logging and mapping of the well‐exposed syn‐rift succession document the facies distributions, palaeocurrents and stratigraphic architecture. Magnetostratigraphy and biostratigraphy are used to date and correlate the alluvial succession across and between fault blocks. From 3·2 to 1·8 Ma, a transverse low sinuosity braided river system flowed north/north‐east to east across east–west‐striking active fault blocks (4 to 7 km in width). Deposits evolved downstream from coarse alluvial conglomerates to fine‐grained lacustrine deposits over 15 to 30 km. The length scale of facies belts is much greater than, and thus not directly controlled by, the width of the fault blocks. At its termination, the distributive river system built small, stacked deltas into a shallow lake margin. The presence of a major antecedent drainage system is supported by: (i) a single major sediment entry point; (ii) persistence of a main channel belt axis; (iii) downstream fining at the scale of the rift basin. The zones of maximum subsidence on individual faults are aligned with the persistent fluvial axis, suggesting that sediment supply influenced normal fault growth. Instead of low accommodation rate during the early rift phase, this study proposes that facies progradation can be controlled by continuous and high sediment supply from antecedent rivers.     

Elevator tectonics and orogenic collapse of a Tibetan-style plateau in the European Variscides: the role of the Bohemian shear zone

Variscan collision of peri-Gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the Bohemian Massif. This crustal thickness resulted in a highly elevated Bohemian plateau with a topographic height >3–4 km. The Bohemian plateau was covered with unmetamorphic Paleozoic strata, all of which are today well preserved in the Tepla–Barrandian unit because of crustal-scale vertical slip along the Bohemian shear zone (BSZ). The BSZ forms a subvertical, ca. 500-km long and up to 2-km wide belt of dip–slip mylonites which show several 90° deflections in map view. Tepla-Barrandian-down movements were active under retrograde metamorphic conditions, starting with granulite and ceasing with greenschist facies conditions. As slip along the BSZ was largely vertical and led to a minimum throw of 10 km, this type of crustal-scale deformation is referred to as elevator tectonics. The elevator-style movements caused the juxtaposition of the supracrustal Tepla–Barrandian lid (the “elevator”) against high-grade rocks of the extruding orogenic root. The BSZ has further governed the foci of mantle-derived plutonism. New U–Pb zircon and monazite TIMS dating of six plutons suggest that emplacement of mantle-derived melts along the BSZ lasted for at least 20 m.y., starting with the emplacement of the Klatovy granodiorite at 347 +4/−3 Ma and ceasing with the emplacement of the Drahotin pluton at 328 ± 1 Ma. When taking into account the new ages of synkinematic plutons, the simultaneous vertical slip along the individual segments of the BSZ (North, West, and Central Bohemian shear zone) is bracketed to the period 343–337 Ma. Elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating-up of the orogenic root. The overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow, vertical extrusion, fast uplift, and exhumation of the orogenic root.     

Stability analysis of the 2007 Chehalis lake landslide based on long-range terrestrial photogrammetry and airborne LiDAR data

On December 4th 2007, a 3-Mm³ landslide occurred along the northwestern shore of Chehalis Lake. The initiation zone is located at the intersection of the main valley slope and the northern sidewall of a prominent gully. The slope failure caused a displacement wave that ran up to 38 m on the opposite shore of the lake. The landslide is temporally associated with a rain-on-snow meteorological event which is thought to have triggered it. This paper describes the Chehalis Lake landslide and presents a comparison of discontinuity orientation datasets obtained using three techniques: field measurements, terrestrial photogrammetric 3D models and an airborne LiDAR digital elevation model to describe the orientation and characteristics of the five discontinuity sets present. The discontinuity orientation data are used to perform kinematic, surface wedge limit equilibrium and three-dimensional distinct element analyses. The kinematic and surface wedge analyses suggest that the location of the slope failure (intersection of the valley slope and a gully wall) has facilitated the development of the unstable rock mass which initiated as a planar sliding failure. Results from the three-dimensional distinct element analyses suggest that the presence, orientation and high persistence of a discontinuity set dipping obliquely to the slope were critical to the development of the landslide and led to a failure mechanism dominated by planar sliding. The three-dimensional distinct element modelling also suggests that the presence of a steeply dipping discontinuity set striking perpendicular to the slope and associated with a fault exerted a significant control on the volume and extent of the failed rock mass but not on the overall stability of the slope.     

Structural and petrophysical characterization of mixed conduit/seal fault zones in carbonates: Example from the Castellas fault (SE France)

The Castellas fault in SE France affects carbonate rocks with a plurimeter scale offset along 1.5 km of outcrop. In order to decipher the structural control on fault petrophysical and hydraulic character, we performed high resolution field structural mapping, laboratory porosity-Vp measurements, and thin sections analysis of deformations and diagenesis. Field mapping shows that the fault zone architecture displays strong lateral variations at the hectometer scale characterized by core thicknesses of 0–5 m, one or several slip planes, and varying fracture patterns within the damage zone. The fault zone heterogeneity may be related to the magnitude of the throw, the position along the fault and the affected rock facies. Laboratory measurements revealed a strong porosity reduction correlated to a Vp increase, related to the cementation of pore volumes within a decameter area around the fault plane. This fault-sealing occurred mainly through a chemical diagenesis related to fluid circulation within fault-zone heterogeneities. A 3D conceptual model of a mixed conduit/seal fault zone is proposed, characterized by sealed impermeable fault tips and more or less permeable units within which flow can occur either perpendicular or parallel to the fault strike.     

The Todagin Creek landslide of October 3, 2006, Northwest British Columbia, Canada

The Todagin Creek landslide is located at 57.61° N 129.98° W in Northwest British Columbia. A seismic station 90 km north of the landslide recorded the event at 1643 hours coordinated universal time (UTC; 0943 hours Pacific daylight time (PDT)) on October 3, 2006. The signal verifies the discovery and relative time bounds provided by a hunting party in the valley. The landslide initiated as a translational rock slide on sedimentary rock dipping down slope at 34° and striking parallel to the valley. The landslide transformed into a debris avalanche and had a total volume estimated at 4 Mm³. An elevation drop of 771 m along a planar length of 1,885 m resulted in a travel angle (fahrb?schung) of 21.3°. The narrowest part of the landslide through the transport zone is 345 m. The widest part of the divergent toe of the landslide reaches a width of 1,010 m. Landslide debris impounded a lake of approximately 32 ha and destroyed an additional 67 ha of forest. The impoundment took 7 to 10 days to fill, with muddied waters observed downstream on October 13. No clear linkage exists with precipitation and temperature records preceding the landslide, but strong diurnal temperature cycles occurred in the days prior to the event. The Todagin Creek area appears to have an affinity for large landslides with the deposits of three other landslides >5 Mm³ observed in the valley.     

Quaternary depositional patterns and sea-level fluctuations, northeastern North Carolina

A detailed record of late Quaternary sea-level oscillations is preserved within the upper 45 m of deposits along an eight km transect across Croatan Sound, a drowned tributary of the Roanoke/Albemarle drainage system, northeastern North Carolina. Drill-hole and seismic data reveal nine relatively complete sequences filling an antecedent valley comprised of discontinuous middle and early Pleistocene deposits. On interfluves, lithologically similar marine deposits of different sequences occur stacked in vertical succession and separated by ravinement surfaces. Within the paleo-drainage, marine deposits are separated by fluvial and/or estuarine sediments deposited during periods of lowered sea level. Foraminiferal and molluscan fossil assemblages indicate that marine facies were deposited in a shallow-marine embayment with open connection to shelf waters. Each sequence modifies or truncates portions of the preceding sequence or sequences. Sequence boundaries are the product of a combination of fluvial, estuarine, and marine erosional processes. Stratigraphic and age analyses constrain the ages of sequences to late Marine Isotope Stage (MIS) 6 and younger (∼ 140 ka to present), indicating multiple sea-level oscillations during this interval. Elevations of highstand deposits associated with late MIS 5 and MIS 3 imply that sea level was either similar to present during those times, or that the region may have been influenced by glacio-isostatic uplift and subsidence.     

Three‐dimensional to two‐dimensional cross‐strata transition in the lower Pleistocene Catanzaro tidal strait transgressive succession (southern Italy)

Sandstone tidal cross‐strata are the predominant sedimentary feature of strait‐fill stratigraphic successions. However, although widely described in numerous studies, tidal strait‐fill two‐dimensional and three‐dimensional cross‐strata have rarely been reported to occur in discrete intervals which are laterally adjacent or vertically stacked, and the meaning of this stratigraphic architecture has not yet been fully investigated. Understanding of the processes responsible for changes in the internal features of modern and ancient tidal bedforms is essential in order to predict lateral and vertical heterogeneities in analogous reservoir strata. This facies‐based study aims to interpret the three‐dimensional to two‐dimensional cross‐strata transition observed in the lower Pleistocene mixed siliciclastic/bioclastic sandstone filling the Catanzaro Strait, in southern Italy, during a continuous phase of tectonically driven marine transgression. Tidal cross‐strata disappear in the uppermost interval of the studied succession, where mudstone strata prevail. This stratigraphic trend is interpreted as the evidence of an important change in the tidal strait hydrodynamics due to a phase of relative sea‐level rise. At the beginning of the transgression, three‐dimensional tidal dunes migrated throughout the ca 3 to 4 km wide and ca 30 km long, WNW–ESE‐oriented Catanzaro Strait, due to strong tidal currents amplified through the seaway and flowing in semi‐diurnal phase opposition. As the intermediate phase of transgression enlarged the seaway width, the tidal current strength decreased as tidal water exchange occurred over a larger cross‐sectional area. The progressive reduction of the bed shear stress modified three‐dimensional tidal dunes into an extensive two‐dimensional bedform field. At the end of the transgression, the further widening of the Catanzaro Strait into a ca 10 to 12 km wide marine passageway changed the tidally dominated strait into a non‐tidal open shelf. The results of this research suggest the presence of a ‘critical cross‐sectional area’ in the narrowest strait‐centre zone which controls the activation and deactivation of tidal current amplification along a marine seaway.     

Sedimentary characteristics of palaeolake deposits along the Indus River valley,Ladakh, Trans‐Himalaya: Implications for the depositional environment

This study is an attempt to contribute to the data set of granulometric studies of sediments by measuring the sedimentary structure and texture, along with statistical parameters, of cold and arid lake systems. The palaeolake sequence along the River Indus on the western fringe of the Tibetan Plateau in Ladakh sector was selected in order to shed light on depositional environmental changes within the lake from post‐last glacial maximum to 5 ka. The River Indus was blocked by Lamayuru dam burst during the deglaciation, after the Last Glacial Maximum (LGM) and the subsequent increase in water level led to the formation of the Saspol–Khalsi palaeolake. This lake was ca 55 km in length, extending from Nimo to Khalsi, had a surface area of 370 km² and was in existence until 5 ka. Two sections (Saspol and Khalsi) separated by an aerial distance of 35 km show a similar trend in sediment character due to their deposition in the same lake system. Grain‐size studies show a polymodal nature of sediments for both of the sections. However, sediments of the lower/downstream section (Khalsi) show a poorer degree of sorting, and coarser grain size and high energy depositional condition as compared with the sediments of Saspol section (positioned upstream) due to the location of the sections within the lake system. It was noted that, in high‐altitude arid regions, the sedimentological characteristics of large‐sized valley lakes may vary greatly, horizontally as well as vertically, owing to local stream input, inflow intensity from the catchment, outflow velocity of water channels, lithology and valley widths at the different sites.     

Facies model of a fine‐grained,tide‐dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert,Egypt

Existing facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.