Origin of Taiwan Canyon and its effects on deepwater sediment

The continental slope of the Taiwan Shoal, which has cultivated numerous submarine canyons, is located in a passive continental margin environment. However, the trend of the Taiwan Canyon, with its 45° intersection angle, is obviously different from that of the erosion valley downward along the continental slope. A distinct break is present in the lower segment of the Taiwan Canyon, which then extends from west to east parallel to the continental slope until finally joining the Manila Trench. By utilizing multiple-beam water depth data, high-resolution seismic data, and sediment cores, this study describes the topographic characteristics of the Taiwan Canyon and provides a preliminary discussion on the origin of the Taiwan Canyon and its effect on deepwater sediment. The terrain, landform, and sediment of the Taiwan Canyon exhibit segmentation characteristics. The upper segment is characterized primarily by erosion, downward cutting with a V shape, and wide development of sliding, slumping, and other gravity flow types. The middle segment is characterized mostly by U-shaped erosion-sedimentation transition and development of an inner levee. The lower segment is characterized primarily by sedimentation and development of a sediment wave. The bottom current has a significant reworking effect on the interior sediments of the canyon and forms reworked sands. The formation and evolution of the Taiwan Canyon is closely related to sediment supply, gravity sliding(slumping), faulting activities, and submarine impaling. Given the sufficient terrigenous clastic supply, the sediments along the continental shelf edge continuously proceed seaward; gliding and slumping in the front edge provide driving forces for the formation of the canyon. Faulting activities result in stratum crushing, and the gravity flow takes priority in eroding the relatively fragile stratum. Thus, the direction of the extension of the canyon crosses the surrounding erosion valley obliquely. Seamounts are formed through submarine impaling. Owing to seamount blocking, the lower segment of the canyon is turned toward the east–west direction. Large amounts of sediments overflow at the turning, forming sediment waves.     

Sediment delivery,resuspension, and transport in two contrasting canyon environments in the southwest Gulf of Lions

Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35 km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5 mg/l. In Cap de Creus canyon head (located ∼14 km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20 mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists.     

Dense shelf water cascades and sedimentary furrow formation in the Cap de Creus Canyon,northwestern Mediterranean Sea

To investigate the processes by which sediment is transported through a submarine canyon incised in a continental margin affected by recurrent dense shelf water cascading events, several instrumented moorings were deployed in the Cap de Creus Canyon from September 2004 to September 2005. This was done as part of the EuroSTRATAFORM Program that investigated sediment transport and accumulation processes in the Gulf of Lions. Results obtained in this observational study confirm that major cascading events can effectively contribute to the rapid export of sediment from the shelf and upper slope to deeper environments, and suggest that the associated strong currents carrying coarse particles are able to erode the canyon floor and generate sedimentary furrows. During winter 2004–2005, persistent northerly winds and the absence of river floods contributed to decrease the buoyancy of coastal waters and to dramatically enhance the intensity of dense shelf water cascades in the Gulf of Lions. Under such conditions, cascading continuously affected the entire Cap de Creus upper canyon section for more than a month and sustained cold temperatures and down-canyon steady currents >60 cm/s (up to 100 cm/s), showing periodic fluctuations that lasted between 3 and 6 days. Increases in suspended sediment concentrations were associated with dense shelf water cascading outbursts, but the magnitude of the concentration peaks decreased with time, suggesting a progressive exhaustion of the resuspendable sediments from the shelf and canyon floor. Grain size analyses of the particles caught by a near-bottom sediment trap show that dense shelf water cascades are able to transport coarse sediments (up to 65% sand) in suspension (and presumably as bed load), which have the potential to abrade the seafloor and generate erosive bed forms. The orientation of a large field of “wide” (i.e., widths about 1/2 spacing indicative of erosive formation) sedimentary furrows recently observed in the Cap de Creus Canyon clearly coincides with the preferential direction of highest velocities measured by the moored current meters, indicating a causative relationship between contemporary dense shelf water cascades and furrow formation.     

High‐frequency sediment transport responses on a vegetated foredune

Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd.     

26Al and 10Be dating of late pleistocene and holocene fill terraces: a record of fluvial deposition and incision,Colorado front range

Cosmogenic ²⁶Al, ¹⁰Be, and ¹⁴C dating of fluvial fill terraces in steep canyons of the Colorado Front Range provides a temporal framework for analysing episodic aggradation and incision. Results from Boulder Canyon show that terrace heights above the modern channel (grade) can be divided into: (1) Bull Lake (≳100 ka; 20–15 m above grade); (2) Pinedale (32–10 ka; 15–4 m above grade); and (3) Holocene age (<4 m above grade). No pre‐Bull Lake deposits are preserved along Boulder Canyon, and only three small remnants >15 m above grade record Bull Lake deposition. Well‐preserved terraces of Pinedale age suggest that the range of terrace height above grade reflects short‐term fluctuations in the river profile during periods of rapidly changing stream load and power. Net river incision apparently occurred during transitions to interglacial periods. Soil development and stratigraphic position, along with limited cosmogenic and ¹⁴C dating, suggest that ∼130 ka terraces in Boulder Canyon correlate with the Louviers Alluvium, and that 32 to 10 ka fills in the canyon correlate with the Broadway Alluvium on the adjacent High Plains. Late Pleistocene incision rates (∼0·15 m ka⁻¹) along Boulder Canyon exceed pre‐late Pleistocene incision rates, and are higher than middle to late Pleistocene incision rates (∼0·04 m ka⁻¹) on the High Plains. This study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments. Copyright © 2002 John Wiley & Sons, Ltd.     

Case study of Pampa,Texas, multicell storms

Analysis of the Pampa, Texas, multicell storms showed prestorm environmental conditions of a strong horizontal convergence of moisture, mixing ratio 12 g/kg at the 850 mb level in the Texas Panhandle areas and strong winds with shear veering with height. Rapid-scan satellite imagery showed that the clouds penetrated above the tropopause and cloud-top temperatures were at least 4 to 9° C colder than the temperature of the tropopause. This formation and collapsing of high-mass-density overshooting cloud tops above the tropopause is characteristic of tornadoes in the middle portion of the United States.     

Canyon topography effects on ground motion at Feitsui damsite

The free-field accelerograms along Feitsui Canyon are analyzed and modeled by a numerical scheme to study the effect of canyon topography. Since six strong-motion accelerometers (SC1–SC6) were deployed along the Feitsui Canyon in 1991; there are 14 earthquakes (4.9≤M_L≤6.6) recorded by these stations until June 1996, but only five triggered all six stations. The maximum PGA value is 68.6 cm s⁻² recorded at station SC1. According to the present data, the effect of the dam on the ground motions at canyon stations can be negligible. The amplitude of ground motion on the slopes of the canyon is bigger than that at its trough. The integral equation method is applied to a two dimensional model of Feitsui Canyon to study the effects of the canyon topography. We choose the ground motion of SC3 or SC4 station at the trough of the canyon as the input motion for the model, which is then used to predict the ground motion at the other five stations. Apart from the earthquake close to the damsite, the simple model can reproduce the observed accelerations at all frequencies below 4 Hz. Overall, the numerical method can well predict the ground motion along the canyon, although the high-frequency simulation is underestimated.     

High energy coastal sedimentary deposits; an evaluation of depositional processes in Southwest England

Sediments left by coastal flooding have been observed worldwide and have been variously ascribed to the action of storm surges and tsunami waves. To date, no study has attempted to unequivocally establish on stratigraphical, sedimentological, and palaeoecological grounds the means by which these two different processes might be distinguished in coastal sedimentary sequences. This paper examines the evidence for historical storm surges and tsunamis and shows that both high magnitude events have been documented over the past 250 years in southwest England. Sand layers of varying thickness are present within Holocene lagoonal and peat sequences of several shallow lakes of the Scilly Isles. Detailed analysis of Big Pool, St Agnes, indicates that the basal peats date from around 1000 BP. Within the basal peats are numerous thin sand layers. Above the basal peat is an extensive sand layer 15 to 40 cm thick. The base of this latter layer probably dates from the early to mid 18th century. Particle size, grain surface morphology, diatom, and mineral magnetic studies are used to try and determine the most likely mode of deposition. The results of all analyses are inconclusive, but the weight of evidence suggests that the increasing frequency of thin sand layers in the upper part of the basal peat may be related to the increasing frequency and intensity of Atlantic storms during the Little Ice Age superimposed upon a rising sea level. The thick sand layer may have been deposited by the tsunami wave generated by the Lisbon earthquake of November 1,1755. Due to the difficulties in distinguishing depositional processes in coastal environments known to have been affected by storm surges and tsunami waves, it is suggested that generally accepted sedimentological techniques are inadequate for discriminating depositional processes in coastal environments.     

Sediment dispersal from a typical Mediterranean flood: The Têt River,Gulf of Lions

This paper describes an integrated study of a typical Mediterranean flood event in the Gulf of Lions. A flood with a 5-year return interval occurred in the Têt River basin and adjacent inner-shelf in the Gulf of Lions, northwest Mediterranean, during April 2004. Data were collected during this flood as part of event-response investigations of the EU-funded Eurostrataform (European Margin Strata Formation) project. Southeasterly storm winds led to a flood which directly modified the inner-shelf hydrodynamics. Sediment delivery to the coastal zone during this flood represented more than half of the mean annual discharge of the Têt River to the Gulf of Lions. This river transported a large amount of sand in suspension, representing 25% of the total suspended load, and as bedload representing 8% of the total load, during this event. Sand introduced in the nearshore was transported northwards during the peak storm and nourished a small delta. Fine sediments were separated from coarse sediments at the river mouth, and were advected southwards and seawards by the counter-clockwise general circulation. Fine-grained sediments were transported via a hypopycnal plume along the coast towards the southern tip of the Gulf of Lions and the Cap Creus canyon. The along-shore currents, which intensified from north to south of the Gulf of Lions, particularly between the Cap Creus promontory and the Cap Creus canyon, favoured the transfer of fine-grained sediments from the continental shelf of the Gulf of Lions towards the continental slope. Our results show that floods with a few-year return interval in small coastal rivers can play a significant role in the transport of sediments on microtidal continental margins and their export from the shelf through canyons.     

Precambrian volcanics associated with the taum Sauk Caldera,St. Francois Mountains,Missouri, U.S.A.

Petrological studies of 12 volcanic rock units in the northeast segment of the Taum Sauk Caldera, the major structural feature in the western part of the St. Francois Mountains, indicate that they were probably derived from the same magma chamber. These calc-alkalic rocks become progressively silica and alkali rich and calcium poor from the base to the top of the stratigraphic column. In the part of the northeast segment of the caldera studied in detail, the extrusives are over 5 thick and have a volume of over 500 km³. Rock units consisting of ash-flow tuffs, bedded airfall tuffs and lava flows were apparently deposited within a single episode of volcanic activity, since no signs of extensive erosion were observed among them. Although the rocks are completely devitrified, the preservation of pyroclastic and flow features is excellent. These volcanics are exposed representatives of a 1.3–1.4 b.y. old belt of volcanics and associated plutons which extends from southern Ohio to the Texas Panhandle any may represent a belt of continental accretion.     

PAH concentrations in Coquina (Donax spp.) on a sandy beach shoreline impacted by a marine oil spill

The BP MC252 well failure in the Gulf of Mexico, April 2010 caused concern for crude oil and polycyclic aromatic hydrocarbon (PAHs) exposure along the sandy beaches of the Florida Panhandle. We began collections of Coquina clams (Donax spp.) from the surf zone of Florida Panhandle beaches to monitor PAH contamination to compliment analysis of surf zone sand samples. These clams had higher levels of PAHs relative to ambient sand, and this allowed us to continue to monitor PAH levels after sand concentrations fell below limits of detection. PAH levels in the Coquina tissues were highly variable, perhaps indicative of the heterogeneous distribution of oil and tar on the beaches and exposure to tar particles. Overall, PAH levels decreased continuously in both sand and Coquina tissues, reaching limits of detection within one and two years respectively after oil landed on Florida Panhandle beaches. Our work suggests these surf zone molluscs may be used to monitor pollutant exposure along high energy sandy beach shorelines.     

Assessing the importance of tropical cyclones on continental margin sedimentation in the Mississippi delta region

Recent research on the Mississippi margin indicates notable seasonal variation in seabed dynamics. During years with minimal tropical-system activity, sediments initially deposited from late spring to early fall are remobilized by wind-driven currents and wave energy during extra-tropical weather systems in the winter. This research reveals the profound significance of tropical cyclones on Louisiana Shelf sedimentation. The amount of material delivered to and advected across the shelf by recent tropical cyclones is considerably larger than that related to winter storm systems. In Fall 2004, the river-dominated shelf of Louisiana was impacted by three tropical systems in less than a month, including Hurricane Ivan. Ivan, with maximum sustained winds in excess of 74 m s⁻¹ (144 knots) and a minimum measured central pressure of 910 mbar, was the eighth most intense Atlantic hurricane on record at the time. In order to assess the impact these tropical systems had on the continental margin west of the Mississippi delta, seabed samples were collected from box cores in October 2004 and analyzed for particle-reactive radionuclides ²³⁴Th, ⁷Be, and ²¹⁰Pb. Radiochemical data and observations from X-radiographs indicate event-driven sediment deposits ranged from 4 to 30 cm on the shelf and 2–6 cm in the Mississippi Canyon. These deposits exhibit distinct radiochemical signatures and differ visually and texturally from the underlying sediment. The well-developed physical stratification and graded nature of the deposits observed in core X-radiographs suggests that the sediment could have been deposited from sediment-gravity flows. Inventories of ⁷Be and ⁷Be/²³⁴Th_xs ratios reveal this series of cyclones transported considerably more material to the outer shelf and slope than periods of minimal tropical-system activity. When compared to seasonal depositional rates created by winter storms, tropical-cyclone-related event deposits on the middle and outer shelf are up to an order of magnitude greater in thickness. The number and thickness of these event deposits decrease with distance from the delta and suggest that only the most severe tropical systems are likely capable of redistributing significant quantities of sediment to more distal portions of the shelf and slope. These severe-event-driven deposits may account for as much as 75% of the sediment burial budget on decadal time scales within Mississippi Canyon. Higher than average tropical cyclone activity, predicted by the National Hurricane Center over the next decade, may be the major mechanism controlling sediment transport and deposition on the Mississippi River continental shelf and in Mississippi Canyon.     

Decadal record of sediment export to the deep sea via Eel Canyon

An active upper-canyon system, Eel Canyon, was studied to determine its role as a conduit and/or sink for terrigenous material over decadal timescales and to assess the sedimentary record preserved by transport processes. These data are used to (1) link seasonal fluctuations in sediment transport and deposition to preserved stratigraphic signatures, and (2) assess sediment storage and removal in the upper Eel Canyon (100–850 m water depth) over decadal timescales. Previous research has shown that upper thalwegs commonly experience gravity-driven flows during winter (November–March), due to increased sediment supply from Eel River flooding and intense storms that produce energetic wave/current conditions. Thick winter deposits composed of recently discharged fluvial sediment are formed in upper thalwegs, with distinct short- and long-lived radioisotopic and textural signatures (detectable ⁷Be and ²³⁴Th_xs, lowered ²¹⁰Pb activity, elevated clay content, and physical structures). Box and kasten cores were collected in the upper canyon (thalwegs and walls) to measure these signatures in recent and preserved winter deposits, and to calculate 100-yr accumulation rates. Non-bioturbated deposits (that have signatures indicative of rapid accretion by gravity-driven flows during the winter) are common in the upper canyon thalwegs. Short-lived radioisotopes (⁷Be and ²³⁴Th) show that sediment delivery to the upper thalweg varies temporally, sometimes beginning at the onset of river flooding, and at other times beginning during fall/early winter dry-storm events. In contrast, bioturbated deposits (which do not have signatures indicative of rapid deposition) are found on canyon walls.Non-bioturbated winter deposits are easily identified in the decadal record of thalwegs by decreases in ²¹⁰Pb activity and increases in clay content. Stacking of multiple years of winter deposits (∼10 cm preserved per winter) results in non-steady-state ²¹⁰Pb profiles and high decadal accumulation rates. However, down-core changes in ²¹⁰Pb profiles show that slope failures are actively redistributing these winter deposits. Partial or total removal of multiple winter deposits appears to happen periodically (every ∼13 yr), which will inhibit preservation of the longer decadal record. 100-yr accumulation rates were calculated in the thalwegs from the resulting ²¹⁰Pb profiles (i.e., the result of winter accretion and decadal removal by failures). Accumulation rates are much higher in thalwegs (1–6 cm/yr) than walls (0.1–0.8 cm/yr), which is likely the result of differing sediment delivery processes (via gravity-driven flows and nepheloid layers, respectively). At least 2.6±1.4% of the Eel River sediment budget is accumulating in the upper canyon over 100-yr timescales. However, this value greatly underestimates the total amount entering the canyon system because minimum accumulation rates were used in many areas (due to limited core length) and slope failures are moving sediment out of the budget area.     

Overbank deposition along the concave side of the Red River meanders,Manitoba, and its geomorphic significance

Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada.     

The effect of canyon topography on strong ground motion at Feitsui damsite: Quantitative results

The free-field accelerograms along Feitsui Canyon are analysed and modelled by a numerical scheme to study the effect of canyon topography. Six strong-motion accelerometers (SC1–SC6) were deployed along the Feitsui Canyon in 1991. The observations suggest that the amplitude of ground motion at the trough of the canyon is smaller than that on its slopes. The integral equation method is applied to a two-dimensional model of Feitsui Canyon to study the effects of the canyon topography. We choose the ground motion of SC3 station at the trough of the canyon as the input motion to the model, which is then used to predict the ground motion at the other five stations. The simulated and observed seismograms agree well in displacement and velocity, although the simulated acceleration at high frequency is underestimated. The simple model can reproduce the observed accelerations at all frequencies below 4 Hz.     

The performance of grass filter strips in controlling high‐concentration suspended sediment from overland flow under rainfall/non‐rainfall conditions

There is little information on the performance of vegetative filter strips (VFS) in filtering high‐concentration sediment from subcritical overland flow. Flume experiments on simulated grass strips were conducted using combinations of three slope gradients (3°, 9° and 15°), five 1‐m‐wide slope positions (from upslope to downslope), two flow rates (60 and 20 L min^‐1 m^‐1) and sediment concentrations of 100–300 kg m^‐3 under simulated rainfall and non‐rainfall conditions. The results showed that sediment deposition efficiency increased with VFS width as a power function. Rainfall significantly reduced sediment deposited within VFS. Higher sediment concentration corresponded to a larger sediment deposition load but reduced deposition efficiency. Flow rate had a negative effect on deposition efficiency but no effect on deposition load. Sediments were more easily deposited at the upper slope position than downslope, and the upper slope position had a higher percentage of coarse sediments. The deposited sediment had significantly greater median diameters (D₅₀) than the inflow sediment. A greater proportion of coarse sediments larger than 25 µm in diameter were deposited, and particles smaller than 1 µm and of 10–25 µm had a better deposition performance than particles of 1–10 µm. Rainfall reduced the deposited sediment D₅₀ at a slope gradient of 3° and had no significant influence on it at 9° or 15°. A higher sediment concentration led to a smaller D₅₀ of the deposited sediment. Rainfall had no significant effect on overland flow velocity. Both the deposited sediment load and D₅₀ decreased with increasing flow velocity, and flow velocity was the most sensitive factor impacting sediment deposition. The results from this study should be useful to control sediment flowing into rivers in areas with serious soil erosion. Copyright © 2013 John Wiley & Sons, Ltd.     

Origin of a bouldery diamicton,Kunlun Pass,Qinghai-Xizang Plateau,People's Republic of China: gelifluction deposit or rock glacier?

On the west side of the military road to Tibet in the Kunlun Shan, a major body of diamicton is moving slowly downslope from the ridge crest at 4800 m in a northerly and easterly direction. The material is derived from Middle Pleistocene till deposits and the underlying Pliocene alluvial gravels. More than 10 per cent of the material is composed of boulders longer than 2 m, 45 per cent has long axes between 0·5 and 2 m, while the matrix is a poorly sorted sandy loam. The mean annual air temperature is −7°C to −5°C and the mean annual precipitation is under 300 mm a⁻¹. The diamicton lacks a vegetation cover, in contrast to meadow tundra on the surrounding slopes. The diamicton mantles the north slope of the ridge, but splits into at least 16 separate tongues which are moving down fluvially graded valleys. The average slope of the landform is about 19°, while the mean slope of the fronts of the tongues is 21°. With one exception, the slope of the fronts does not exceed 25°, unlike true rock glaciers. The diamicton is up to 40 m thick in valley 4. The active layer was 12 to 30 cm deep in July at 4780 m, increasing to 1·5 to 2 m at about 4650 m. Ice contents in the permafrost may reach 57 per cent but 30 per cent is more usual The larger boulders act as braking blocks on the upper slopes of the landform and are frozen into the permafrost. The lower parts of the landform move at under 3 cm a⁻¹, whereas the fine-grained material in the active layer moves past the braking blocks on the upper slopes at up to 30 cm a ⁻¹. There is no direct evidence for flowage of the icy diamicton forming the deposit. It is therefore best referred to as a gelifluction slope deposit, and is the longest and most spectacular of such deposits described so far in the world. © 1998 John Wiley & Sons, Ltd.     

Three thousand years of flank and central vent eruptions of the San Salvador volcanic complex (El Salvador) and their effects on El Cambio archeological site: a review based on tephrostratigraphy

The volcanic events of the last 3,000?years at San Salvador volcanic complex are reviewed using detailed stratigraphic records exposed in new excavations between 2005 and 2007, at El Cambio archeological site (Zapotitán Valley, El Salvador), and in other outcrops on the northern and northwestern sectors of the complex. The sequences that overlie Tierra Blanca Joven (cal. 429?±?107?ad), from the Ilopango caldera, comprise the Loma Caldera (cal. 590?±?90?ad) and El Playón (1658?C1671) deposits and the San Andrés Tuff (cal. 1031?±?29?ad), related to El Boquerón Volcano. The surge deposits within the El Playón, San Andrés Tuff and overlying Talpetate II sequences indicate the significance of phreatomagmatic phases in both central vent and flank eruptions during the last 1,600?years. Newly identified volcanic deposits underlying Tierra Blanca Joven at El Cambio extend the stratigraphic record of the area to 3,000?years?bp. Paleosols interstratified with those deposits contain cultural artifacts which could be associated with the Middle Preclassic period (900?C400?bc). If correct, human occupation of the site during the Preclassic period was more intense than previously known and volcanic eruptions must have affected prehistoric settlements. The archeological findings provide information on how prehistoric populations dealt with volcanic hazards, thousands of years ago in the eastern Zapotitán Valley, where several housing projects are currently being developed. The new stratigraphic and volcanological data can be used as a basis for local and regional hazard assessment related to future secondary vent activity in the San Salvador Volcanic Complex.     

Magnitude of Hurricane Ike storm surge sedimentation: implications for coastal marsh aggradation

There is a paucity of information on the regional distribution and magnitude of hurricane storm surge sedimentation. This study assesses the spatial extent and magnitude of Hurricane Ike's (2008) storm surge sedimentation and discusses implications for the role of hurricanes in marsh aggradation. The characteristics of the storm surge deposit, including thickness, inland penetration, volume and mass, were determined for 15 transects across marshes bordering the Gulf of Mexico in south‐eastern Texas and south‐western Louisiana. The deposit is up to 0·85 m thick, extends up to 3·6 km inland, and has an estimated volume of about 13·7 million m³ and an estimated mass of about 16·2 million metric tons. This level of sedimentation is one to two orders of magnitude larger than other potential sources of marsh sedimentation, including annual riverine inputs and inputs from alongshore sediment transport. The study findings add support to a growing body of evidence that hurricanes may be the predominant sediment source for long‐term aggradation of many coastal marshes bordering the Gulf of Mexico. Copyright © 2012 John Wiley & Sons, Ltd.     

Characteristics and sedimentary processes of lamina-controlled sand-particle imbricate structure in deposits on Lingshan Island,Qingdao, China

Imbricate structure is a common sedimentary structure and is well developed in sandy sediments.Here,we report a laminacontrolled fine-sand-particle imbricate structure in a set of very fine grained sedimentary rocks(fine sandstone,siltstone,and mudstone)at the Dockyard and Qianceng Cliff areas of Lingshan Island,Qingdao,Shandong,China.Sets of up to 300 laminae are found in stratigraphic profiles in these areas.The laminae are generally less than 1 mm thick,with most being 0.3–0.4 mm thick and the thinnest being0.1 mm.The dip angle of the imbricate structure varies widely,between 0°and 90°,with an average dip angle of about 40°–50°,which is higher than that of imbricate riverbed gravels(about 34°).The dip angle is a function of the shape and sorting of the particles,as well as the hydrodynamic conditions under which these fine-grained sediments were deposited.Several profiles show well-developed multiscale,soft-sediment deformation structures.Flute casts,load casts,and groove casts are also common.Fragments of carbon remains occur widely and commonly constitute stringers several millimeters thick and up to 10 cm long,together with fine clasts.Vitrain lenticles are also common.Based on the imbricate structure of the Lingshan Island deposits,it is shown that in addition to paleocurrent analysis,the imbricate structure can be used to infer information about fluid properties,transport characteristics,and sedimentary processes of the depositional environment.A near-bottom underflow,either of authigenic origin or derived from cold river water in winter in a delta interdistributary bay or delta-front environment,is inferred to have provided the hydrodynamic setting in which this imbricate structure formed.The imbricate structure,together with other sedimentary structures and features,shows that the sedimentary rocks on Lingshan Island were deposited in an inland,shallow-water environment,such as a delta,and not in a deep-water or submarine continental-slope environment.