Veneer Of Sediments Research Articles

Sediment transport pathways in a dredged ria system, southwest England

The Fowey Ria system, southwest England, comprises the River Fowey catchment, the Fowey estuary, the cliffs and bays adjacent to the ria mouth, and part of the inner continental shelf of the English Channel. Previously, large quantities of sediment were introduced into the upper ria by ore mining activity. Today, in common with other rias, the Fowey receives a low riverine sediment input. Material from maintenance dredging in the lower ria is dumped in a spoil ground outside the ria mouth. The sediments of the system are investigated using an integrated approach to determine sediment distribution and sediment transport pathways. Surface sediments are analysed for grain size and mineralogy. Grain size trend analysis is used to examine sediment dispersal patterns away from the locus of deposition in the spoil ground. Archived data are used to investigate the seabed morphology and to determine long-term (100 year) bathymetric changes. Within the ria, mixing of sediment from several sources occurs. In the upper reaches, riverine and locally-eroded sediment is transported seawards towards the main area of commercial activity. Sand and finer-grained material moves into the ria from offshore. The bed of the inner continental shelf comprises interfluves covered by a thin veneer of sediment, with a natural composition of locally-derived lithic fragments and biogenic material. The area is mainly low/non-depositional in character, except within the partially-infilled palaeovalley and its tributaries. Sediments dumped in the spoil ground disperse in a complex pattern: coarse-grained material is moved by the action of waves and tidal currents towards the southwest and northeast; fine-grained material is transported either to the east or the west, depending upon the prevailing wave and tidal current regime. Because of its geomorphology, the lower ria acts as an efficient sediment trap, retaining (a) riverine material and sediment eroded from the upper reaches; and (b) sediment entering the ria from offshore. Despite being subjected to major anthropogenic disturbance from past mining and present-day dredging activities, the Fowey Ria conforms to the general sediment model for southwest England rias [Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries – Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69–111]. A conceptual model of sediment transport pathways for the Fowey Ria system is presented as the basis for further investigations.

A Zona Tectônica Teixeira Terra Nova - ZTTTN - fundamentos da geologia regional e isotópica

A sigmoidal-shape linear tectonic occurs from Teixeira (Paraiba, to the north-northeast)- to Terra Nova (Pernambuco, to the south-southwest) as a line diagonal to theTransversal Zone, which is the central tectonic portion of the Borborema Province, thus dividing this province into two different litho-structural and tectonic domains. To the north and northwest occur the Early Neoproterozoic Riacho Gravata (RG) and the Neoproterozoic Pianco-Alto Brigida (PAB) terranes, both of them characterized by supracrustal rocks with low-grade of regional metamorphism. To the south-southwest occur the Alto Pajeu terrane (TAP, Early Neoproterozoic, orthognaisses and mesozonal supracrustal rocks) and the domain of its basement, the Alto Moxoto terrane (TAM, Paleoproterozoic in age), with high grade metamorphic rocks. This proposedtectonic zone is mostly composed by orthogneisses (from TAP and its Paleoproterozoic basement) pierced by an almost continuous series of intrusive granitic and syenitic rocks (plutons and stocks) of the Ediacaran period (ages between 630 and 570 Ma). During the evolutionary history of this linear portion of the plateau of the Borborema province there are some (direct and indirect) records of polycyclic geologic events of the Paleoproterozoic, Early Neoproterozoic, Ediacaran, Cambrian and Cenozoic era (a sedimentary veneer, evolution of a supergenic crust and regional uplift) with interesting implications for the regional geology, geography and social-economic development. In this paper the emphasis will be given to the isotopic data (Rb-Sr, Sm-Nd, mostly) along this special and the domains that it separates, as well its role on the regional tectonic framework. For this purpose, we take into consideration an expressive amount of available geological and isotopic data, obtained along the last decade (most of them not published) from different sources. Additionally, some first interpretative hypotheses will be postulated, where this tectonic (ZTTTN) would be formed as by-product of three parallel Neoproterozoic subduction zones.

Geodynamic and metabolic cycles in the Hadean

Abstract. High-degree melting of hot dry Hadean mantle at ocean ridges and plumes resulted in a crust about 30km thick, overlain in places by extensive and thick mafic volcanic plateaus. Continental crust, by contrast, was relatively thin and mostly submarine. At constructive and destructive plate boundaries, and above the many mantle plumes, acidic hydrothermal springs at ~400°C contributed Fe and other transition elements as well as P and H2 to the deep ocean made acidulous by dissolved CO2 and minor HCl derived from volcanoes. Away from ocean ridges, submarine hydrothermal fluids were cool (≤100°C), alkaline (pH ~10), highly reduced and also H2-rich. Reaction of solvents in this fluid with those in ocean water was catalyzed in a hydrothermal mound, a natural self-restoring flow reactor and fractionation column developed above the alkaline spring. The mound consisted of brucite, Mg-rich clays, ephemeral carbonates, Fe-Ni sulfide and green rust. Acetate and glycine were the main products, some of which were eluted to the ocean. The rest, along with other organic byproducts were retained and concentrated within Fe-Ni sulfide compartments. These compartments, comprising the natural hydrothermal reactor, consisted partly of greigite (Fe5NiS8). It was from reactions between organic modules confined within these inorganic compartments that the first prokaryotic organism evolved. These acetogenic precursors to the bacteria diversified and migrated down the mound and into the ocean floor to inaugurate the "deep biosphere". Once there they were protected from cataclysmic heating events caused by large meteoritic impacts. Geodynamic forces led to the eventual obduction of the deep biosphere into the photic zone where, initially protected by a thin veneer of sediment, the use of solar energy was mastered and photosynthesis emerged. The further evolution to oxygenic photosynthesis was effected as catalytic [Mn,Ca]-bearing molecules that otherwise would have been interred in minerals such as ranciéite and hollandite in shallow marine manganiferous sediments, were sequestered and invaginated within the cyanobacterial precursor where, energized by light, they could oxidize water. Thus, a chemical sedimentary environment was required both for the emergence of chemosynthesis and of oxygenic photosynthesis, the two innovations that did most to change the nature of our planet.

Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand

Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root.

A geophysical study of the northern Svalbard continental margin

SUMMARY In the summer of 1999, the first systematic seismic profiles were acquired across the northern Svalbard continental margin east of 15 ◦ E. Approximately 1470 km of multi-channel seismic reflection data as well as sonobuoy wide-angle data were collected up to 82 ◦ N. With few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement. The uppermost sedimentary deposits of the inner shelf yield P-wave velocities of 2 km s −1 and higher, indicating erosion and compaction due to a former ice load. The inner shelf east of Hinlopen Strait has only a thin veneer of over-consolidated sediments above the acoustic basement. Beneath the outer shelf, up to 3.5 km of sedimentary deposits cover the down-faulted acoustic basement. The continental slope is heavily eroded due to bottom current activity and slumping. At about 30 ◦ E the morphology of the continental slope has a smooth appearance. Shelf progradation only in the vicinity of glacial troughs crossing the shelf (associated with submarine fans) indicates main sediment transport by ice streams during former glacial periods. The maximum sedimentary thickness in the Sophia Basin is more than 9 km, and the Nansen Basin has a sediment thickness of 4.5 km close to the margin. Gravity modelling along the seismic profiles was performed to constrain the position of the continent‐ocean transition. Existing sedimentary thickness and structural maps were extended over the area investigated. The new data provide no evidence for the presence of former extensive subaerial volcanic sequences (seaward-dipping reflectors), which would have been emplaced during the break-up along the margin. Thus, we consider this part of the margin as non-volcanic.

Effects of shoreline and bedrock irregularities on the morphodynamics of the Alexandria coast littoral cell, Egypt

Beach-nearshore profiles combined with beach and surficial sediment samples were analyzed in conjunction with wave, current, littoral drift and sea-level data to determine the effect of bedrock on morphodynamic processes within the littoral zone of Alexandria on the Mediterranean coast of Egypt. This 14.5-km-long littoral cell is bounded by pronounced embayments and pocket beaches separated by headlands which prevent bypassing of beach sands, in effect making this cell a large, semi-closed basin. The compartmented nature of this cell acts together with the rough irregularity of the rocky seafloor to trap a thin veneer of sediment (<3 m thick), showing proportional mixing between two sedimentary provinces. A modern fine-grained sediment facies consisting of mixed carbonate/siliciclastic sand flanks most of the nearshore zone down to a depth of 8–10 m. Beyond this depth, considered to be the depth of closure, a relict late Pleistocene to mid-Holocene coarse-grained facies composed of biogenic carbonate sand is found. Along a short section of the coastline (km 3–6), the coarser sediment also occupies the nearshore zone. Over most of the study area the two sediment types are mixed in various proportions, largest mixing coinciding with poorest sorting. Profile analyses revealed seasonal changes in sediment volume along the coast which closely follow the cyclicity of seasonal changes in wave climate. The present shoreline orientation, headlands and rough, irregular rocky seabed are reflected in the erosion/accretion pattern, sediment characteristics, and the reversibility of longshore currents and littoral drift. Although there is a marked deficiency in the sediment balance, the sand budget for this cell, including artificial material (2.339*106 m3) has increased slightly by 0.041*106 m3 year−1 as a result of engineering works carried out to widen the coastal road (Corniche). In addition to the physical properties of the bedrock (degree of induration), the accelerating sea-level rise during the Holocene and human influences, the modern morphology of the coast, the erosional seabed features in the nearshore zone, and the texture of seabed sediments are all controlled by the original geometry of the coast which consisted of an elevated subaerial ridge.

Large earthquakes kill coral reefs at the north‐west Gulf of Aqaba

AbstractDown‐faulting at the north‐west margins of the Gulf of Aqaba is inferred to have triggered a catastrophic sedimentary event at 2.3 ka that killed the Elat fringing coral reef. Whereas segments of the Holocene reef were perfectly fossilized and preserved beneath a veneer of siliciclastic sediments, other segments were abraded, settled by nomads, and later re‐submerged under 4 m of water. Repeated damage triggered by down‐throwing earthquakes degenerate the fringing reefs of the north‐west end of the gulf. Conversely, on the north‐eastern and southern parts of the gulf, where earthquakes uplift the margins, modern reefs are thriving, attached to uplifted fossil reef terraces. Therefore, coastal subsidence moderates the development of fringing coral reefs during the late Holocene sea‐level stand still.

The Fringing Reef Coasts of Eastern Africa—Present Processes in Their Long-term Context

Keywords: Kenya, Tanzania, fringing reef, reef platform, sediment, sea-level change, climatevariability, shoreline change, Late Pleistocene, Holocene Sea-level changes through the Quaternary era have provided recurrent opportunities for the biosphere to significantly shape the coastal geomorphology of eastern Africa. Key agents in this shaping have been the calcium carbonate-fixing biota that have constructed the ocean-facing fringing reefs and produced the extensive backreef sediments that form the limestone platforms, cliffs and terraces that characterise these coasts. Today\'s reefs comprise tough, algal-clad intertidal bars composed largely of coral rubble derived from their ocean front. They provide protection from wave attack to the inshore platforms with their sediment veneers and their beach and beach plain sands that are susceptible to erosion. If the eastern African coasts are subjected to the rise of sea-level that is predicted at the global scale during the coming century, the protective role of the reef bars will be diminished if their upward growth fails to keep pace. Favourable ocean temperatures and restraint in the destructive human pressures impacting the reef ecosystems will facilitate such growth. Western Indian Ocean Journal of Marine Science Vol.2(1) 2003: 1-14

Massive recycling of nitrogen and other fluid-mobile elements (K, Rb, Cs, H) in a cold slab environment: evidence from HP to UHP oceanic metasediments of the Schistes Lustrés nappe (western Alps, Europe)

Nitrogen and hydrogen isotopic compositions together with N, K, Rb, Cs and H2O contents were measured on several high-pressure (HP) to ultrahigh-pressure (UHP) metasediments from the Schistes Lustrés nappe (western Alps) and on unmetamorphosed sedimentary protoliths from the Apennines (Italy). These samples represent a sequence of pelagic sediments subducted to different depths down to 90 km along a ‘cold’ geothermal gradient (∼8°C/km). Nitrogen isotopic composition (δ15N between +2.6 and +4.8‰) does not show any specific evolution with increasing metamorphic conditions and can be considered constant during subduction. Large variations of the N content (between 169 and 1721 ppm N) together with K, Rb and Cs content are observed but the constancy of K/N (14), K/Rb (385) and K/Cs (10 190) molar ratios in protoliths and metamorphic rocks indicates that none of these fluid-mobile elements was lost through devolatilization processes. This suggests that fluid circulation was limited to sample scale and that the rocks behaved as closed systems during subduction. This interpretation is also supported by the small range of δD values (from −54.1 to −78.0‰). The present results indicate that N, K, Rb and Cs were retained in the subducted sedimentary veneer at least down to the depth locus of island arc magmatism. Based on the correlation between N and K contents, the flux of sedimentary N recycled in subduction zones is estimated at 7.6×1011 g/yr. Mass balance calculations strongly support the fact that nitrogen is efficiently recycled to the deeper mantle.

Analysis of surface compositions in the Oxia Palus region on Mars from Mars Global Surveyor Thermal Emission Spectrometer Observations

We examine the distribution of Thermal Emission Spectrometer (TES) derived surface compositions in the Oxia Palus region on Mars through high‐spatial‐resolution mapping and integration with Mars Orbiter Camera and Mars Orbiter Laser Altimeter data sets. We also fit the basalt and andesite‐weathered basalt interpretations of TES surface type 1 and type 2 materials into multiple working hypotheses to describe the origin of surface compositions. A mixing/transition from surface type 1 to type 2 is observed in low‐albedo regions of southern Acidalia Planitia and may represent either (1) an influx of basaltic sediment from the southern highlands, deposited on and mixed with andesitic volcanics; (2) an influx of water‐transported basaltic sediment from the southern highlands that was altered and later deposited as a thin sedimentary veneer; or (3) different degrees of weathering of indigenous basalt, marking the geographic extent of submarine alteration of basaltic crust. TES spectra of low‐albedo intracrater materials reveal that surface type 1 compositions form a central core on crater floors, while type 2 compositions form a surrounding arc on the dark downwind sides of crater walls. Intracrater floor materials are interpreted as eolian sediment, and wall materials are interpreted as either eolian sediment or eroded material from in‐place crater wall lithologies. Surface type 1 and type 2 compositions are also observed in adjacent low‐albedo wind streaks; however, a mixing trend is not as evident. The Mars Pathfinder landing site and Ares and Tiu Valles source regions are sufficiently blanketed by fine‐grained dust to prohibit the analysis of surface rock compositions.

Facies architecture of the mixed carbonate/siliciclastic inner continental shelf of west-central Florida: implications for Holocene barrier development

Sediment vibracores and surface samples were collected from the mixed carbonate/siliciclastic inner shelf of west–central Florida in an effort to determine the three-dimensional facies architecture and Holocene geologic development of the coastal barrier-island and adjacent shallow marine environments. The unconsolidated sediment veneer is thin (generally <3 m), with a patchy distribution. Nine facies are identified representing Miocene platform deposits (limestone gravel and blue–green clay facies), Pleistocene restricted marine deposits (lime mud facies), and Holocene back-barrier (organic muddy sand, olive-gray mud, and muddy sand facies) and open marine (well-sorted quartz sand, shelly sand, and black sand facies) deposits. Holocene back-barrier facies are separated from overlying open marine facies by a ravinement surface formed during the late Holocene rise in sea level. Facies associations are naturally divided into four discrete types. The pattern of distribution and ages of facies suggest that barrier islands developed approximately 8200 yr BP and in excess of 20 km seaward of the present coastline in the north, and more recently and nearer to their present position in the south. No barrier-island development prior to approximately 8200 yr BP is indicated. Initiation of barrier-island development is most likely due to a slowing in the Holocene sea-level rise ca. 8000 yr BP, coupled with the intersection of the coast with quartz sand deposits formed during Pleistocene sea-level highstands. This study is an example of a mixed carbonate/siliciclastic shallow marine depositional system that is tightly constrained in both time and sea-level position. It provides a useful analog for the study of other, similar depositional systems in both the modern and ancient rock record.

Stratigraphic framework of sediment-starved sand ridges on a mixed siliciclastic/carbonate inner shelf; west-central Florida

Seismic reflection profiles and vibracores have revealed that an inner shelf, sand-ridge field has developed over the past few thousand years situated on an elevated, broad bedrock terrace. This terrace extends seaward of a major headland associated with the modern barrier-island coastline of west-central Florida. The overall geologic setting is a low-energy, sediment-starved, mixed siliciclastic/carbonate inner continental shelf supporting a thin sedimentary veneer. This veneer is arranged in a series of subparallel, shore-oblique, and to a minor extent, shore-parallel sand ridges. Seven major facies are present beneath the ridges, including a basal Neogene limestone gravel facies and a blue-green clay facies indicative of dominantly authigenic sedimentation. A major sequence boundary separates these older units from Holocene age, organic-rich mud facies (marsh), which grades upward into a muddy sand facies (lagoon or shallow open shelf/seagrass meadows). Cores reveal that the muddy shelf facies is either in sharp contact or grades upward into a shelly sand facies (ravinement or sudden termination of seagrass meadows). The shelly sand facies grades upward to a mixed siliciclastic/carbonate facies, which forms the sand ridges themselves. This mixed siliciclastic/carbonate facies differs from the sediment on the beach and shoreface, suggesting insignificant sediment exchange between the offshore ridges and the modern coastline. Additionally, the lack of early Holocene, pre-ridge facies in the troughs between the ridges suggests that the ridges themselves do not migrate laterally extensively. Radiocarbon dating has indicated that these sand ridges can form relatively quickly (∼1.3 ka) on relatively low-energy inner shelves once open-marine conditions are available, and that frequent, high-energy, storm-dominated conditions are not necessarily required. We suggest that the two inner shelf depositional models presented (open-shelf vs. migrating barrier-island) may have co-existed spatially and/or temporally to explain the distribution of facies and vertical facies contacts.

The Neogene and Quaternary Clay‐with‐flints north and south of the English Channel: comparisons of distribution, age, genetic processes and geodynamics

AbstractWeathered residual accumulations termed Clay‐with‐flints (Argiles à silex) overlying Cretaceous deposits are widespread in southern England and northwestern France. Geological and pedological studies and some field surveys indicate their distribution, nature, age and origin. In France, the microfaunas preserved in the hollow flints of the Clay‐with‐flints demonstrate the preservation of the parent Chalk stratigraphy in the sections studied, i.e. the Clay‐with‐flints corresponds to in situ weathering profiles. However, in England and in France, an important part of the clay component is derived from a thin veneer of basal Tertiary sediment overlying sub‐Tertiary marine erosion surfaces of various Palaeogene transgressions. In England as in France, the youngest generation of Clay‐with‐flints can be dated from Late Pliocene to Quaternary. It formed after removal of different Cenozoic deposits on many plateaus and during the downcutting of Pleistocene rivers. The most visible effect of subsequent weathering profile development is irregular dissolution of the underlying Chalk to produce deep karstic pipes into which the Clay‐with‐flints has slumped; other pedological processes include rubification, clay illuviation and cryoturbation. Clay‐with‐flints also provides evidence of Quaternary palaeoenvironments, Cenozoic continental palaeosurfaces, and their vertical deformation. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Reply to comments by Corbett and Cable on our paper, “Seepage meters and Bernoulli's revenge”

We thank Corbett and Cable (2003) for questioning our results and for pointing out the great number of studies that have employed various forms of seepage meters. It was the ever-increasing use of seepage meters and the spurious results we obtained that led us to warn the seepage meter community of possible defects in the method. Apparently our choice of an eye-catching title served its purpose. It was never our intent to perform the kind of engineering style quantitative tests that we hope this exchange will stimulate. We purposely used larger collection bags, because preliminary experiments with smaller bags showed they filled to capacity in less than 24 hr. For reasons of efficiency and the large spatial nature of our meter distribution, it was not practical to sample on hourly or multi-hour schedules. Our purpose therefore was to obtain 24-hr measurements that would average about four tidal intervals. Because our monitoring wells vividly showed tidal pumping and water transport are limited to the porous and permeable Pleistocene limestone, we chose to permanently seal our meters directly to the rock. Lee-type meters, on the other hand, must be placed in the sedimentary veneer that caps about 25% of the limestone in eastern Florida Bay (Prager and Halley 1999). Unlike most other areas where meters have been used, ground water entering the Lee-type meters in the Florida Keys ultimately comes from and is driven by tidal pumping and leakage from the underlying limestone. We are nevertheless surprised and pleased that the Lee-type meters of Corbett and Cable produced seepage rates paralleling those we obtained directly from the rock. Clearly, their meters were placed either in permeable shelly or marine grass-covered shell and lime mud sediments where permeabilities are greater than on bare lime mud substrates. Such areas are riddled with

Incremental Growth of Plains-Type (Compactional) Folds in a Cratonic Environment as Revealed by the Sedimentary Sequence

The enigma of the origin and development of plains-type folds, as they were christened in the early 20th Century, essentially has been solved. The folds, a considerable distance from the tectonic disturbance, were formed by draping of sediments over differentially displaced Precambrian basement fault blocks. These Precambrian basement fault blocks controlled the location, size, and shape of the folds. Forces were transmitted through the rigid basement causing readjustment along the indigenous fracture/fault pattern formed much earlier. In the U.S. Midcontinent, the crystalline basement is overlain by a thin veneer of sediments, and once the structures were formed, they continued to develop as evidenced by features in the overlying sediments. As the stress was transmitted through the basement and then relaxed, the fault blocks moved differentially in concert to these outside forces. Sediment compaction and nondeposition over structural topographic highs reacted accordingly to form the features as seen today. To determine the structural history, structural closure on different horizons on the anticline is plotted in their appropriate stratigraphic position at depth. This gives a ‘compaction line’ for each tectonically coherent segment. Similar segments show a relatively straightline with offsets at major unconformities indicating breaks in the continuum. It is at these breaks that the section can be stretched until the compaction line matches as a continuum with the resulting gap giving the approximate amount of missing section for that part of the rock column. Conversely, the amount of closure on a structure at depth for each line segment can be estimated by extrapolating downward in that segment. This technique to determine depth of burial and thus the amount of missing stratigraphic section from well data at numerous locations has been compared with estimates made by other methods and the results are similar. Where no other data are available or for quick estimates, then, it is proposed that this approach will give reasonable results and that the values can be used as a constraint in basin modeling.

An evaluation of the success of dredging as remediation at a DDT‐contaminated site in San Francisco Bay, California, USA

Lauritzen Canal, a portion of San Francisco Bay near Richmond, California, USA, was heavily contaminated with dichlorodiphenyltrichloroethane (DDT) and dieldrin as a result of releases from a pesticide-formulating firm. In 1996 and 1997, 82,000 m3 of contaminated sediment was removed from the canal by dredging. This study evaluated the success of the dredging based largely on body burdens of DDT and its metabolites (sigmaDDT) in resident biota, with some data on sediment- and water-contaminant levels and sediment toxicity testing. Sediment disturbance during dredging introduced a pulse of sigmaDDT into the Lauritzen Canal ecosystem, and body burdens of fish and invertebrates increased 2- to 76-fold, depending on the species. Approximately 1 1/2 years after remediation, 11 of 14 indicators showed contamination comparable with or worse than the contamination that existed prior to dredging. Monitoring of mussels up to four years postdredging suggests some modest improvement, although the sigmaDDT body burden of canal mussels remained far above the norm for San Francisco Bay. The elevated sigmaDDT body burdens in biota that persisted for years after remediation reflect recent exposure and are not merely a result of slow metabolic elimination of the sigmaDDT pulse associated with dredging. Sediment sigmaDDT concentrations were low immediately after dredging, but within months, the canal bottom became covered with a veneer of fine sediment as contaminated as that that had been removed. The source of this material has not been conclusively established, but we suspect it came from slumping and erosion from the flanks of the canal beneath docks and around pilings where dredging was not done. In retrospect, either capping in place or more thorough dredging may have been more successful in reducing pesticide exposure of the biota, although there were difficulties associated with both alternatives.

A note on needle‐ice mound formation in the High Drakensberg, Southern Africa

AbstractSmall needle‐ice mounds are described from the High Drakensberg. They develop infrequently during periods of several days of uninterrupted cold conditions. Ice growth incorporates sediment and stones. An accumulated segregated ice length of &gt;41 cm was measured, with clasts heaved ∼14 cm. During subsequent thaw phases, the ice ablates from the top downwards, forming sediment veneers and cappings. These mounds contribute towards turf exfoliation, thereby exposing sediment to deflation. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Tectonic significance of the cold seepage zones in the eastern Nankai accretionary wedge – an outcome of the 15 years’ KAIKO projects

The tectonic significance of the cold fluid venting in the eastern Nankai accretionary wedge has been elucidated based upon results of precise mapping and submersible observation executed since 1984 under the French–Japanese cooperative projects (KAIKO I, II, KAIKO-Nankai and KAIKO-Tokai) in conjunction with studies of the Japan Trench, Sagami Bay and other regions. The very sharp boundary of benthic communities associated with temperature anomalies indicates the occurrence of focused upflow of nutrient-rich fluids through cracks in the carbonate crust existing on a near-surface horizon of permeable sediment. The linear alignment of venting zones is a surface expression of the shallow detachment faults. Methane-rich fluids expelled from the conduit along a fault migrate in the overlying permeable sediment and react with circulating sea water containing sulfate to produce hydrogen sulfide and carbonate precipitate. En echelon or zigzag patterns of the cracks are developed in the carbonate crust under the influence of tectonic shear stress. It has been concluded that bivalve communities grow on a thin veneer of sediment above the cracks of carbonate through which fluid containing hydrogen sulfide diluted with sea water flows out. On the other hand, bacterial mats seem to flourish either on thin sediment overlying the fault conduit or over a thick cover of sediment without carbonate crust. In the former case, bivalves cannot grow because venting fluids are too dense with methane and hydrogen sulfide to maintain the oxygen-rich condition of the bottom water. In the western Nankai Trough fine-grained less permeable sediment cover seems to prevent circulation of fluids, whereas methane appears to be widespread in the deeper sedimentary layer there. Major distinctions in modes of seepage among sites are presumed to be caused by: (1) the tectonic regime causing channels of upward fluid venting; (2) rates and composition of venting fluids affected by mixing rates of sea water; and (3) the permeability and thickness of the near-bottom sediment.

Lower Pleistocene to present structural deformation and sequence stratigraphy of the continental shelf, offshore Eel River Basin, northern California

Seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore Eel River Basin of northern California, using a closely spaced, high-resolution multichannel seismic grid. Geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences, producing shifting sedimentation patterns in the offshore basin during the last ∼1 Myr. Estimated timing of unconformity formation correlates generally with a deep-ocean global δ 18O curve, both before and after a ∼500-kyr transition from 41-kyr to 100-kyr dominated cycles. We suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide. However, regional tectonic influences on sequence development are also observed. Folding associated with Gorda–North America Plate convergence ended progressively from south to north between ∼1.0 Ma and ∼500 ka, in response to northward migration of the Mendocino Triple Junction (MTJ). Since ∼500 ka, continued encroachment of the MTJ produces rotation of preexisting structures, uplift of the Table Bluff Anticline (TBA), related periods of channel incision south of the TBA, and generally reduced sediment preservation across the shelf. MTJ-related uplift may also have induced formation of Eel Canyon. Over the past ∼1.0 Myr, a dominant northern sediment source becomes progressively less important; a southern source, probably the paleo-Eel River, has become dominant since ∼750 ka. Seismic unconformities fall into two categories: (1) irregular surfaces of limited mappable extent, interpreted as incision/exposure surfaces forming during relative sea-level lowstands, and (2) smooth, laterally extensive regional unconformities, interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea-level transgressions. A sequence stratigraphic model for sediment deposition and preservation on the Eel River shelf predicts that preserved sediment on the Eel margin is dominated by fluvially derived highstand silts and muds, deposited by longshore-directed currents and waves. Lowstand sediments are preserved only in fluvial channels infilled during late lowstand/early transgression. Preserved transgressive sediments are likely limited to a thin veneer of well-sorted coarse sediment or shell debris, deposited above the transgressive ravinement. Future deep coring will be required to confirm these predictions.

Key Largo Limestone revisited: Pleistocene shelf-edge facies, Florida Keys, USA

New dates and analysis of 12 deep and 57 shallow cores allow a more detailed interpretation of the Pleistocene shelf edge of the Florida Platform as found in various facies of the Key Largo Limestone beneath the Florida Keys. In this study a three-phase evolution of the Quaternary units (Q1–Q5) of the Key Largo is presented with new subdivision of the Q5. (1) In the first phase, the Q1 and Q2 (perhaps deposited during oxygen-isotope stage 11) deep-water quartz-rich environment evolved into a shallow carbonate phase, (2) Subsequently, a Q3 (presumably corresponding to oxygen-isotope stage 9) flourishing reef and productive high-platform sediment phase developed. (3) Finally, a Q4 and Q5 (corresponding to oxygen-isotope stages 7 and 5) stabilization phase occurred with reefs and leeward productive lagoons, followed by lower sea levels presenting a sequence of younger (isotope substages 5c, 5a) shelf-margin wedges, sediment veneers and outlier reefs. The Key largo Limestone provides an accessible model of a carbonate shelf edge with fluctuating water depth, bordering a deep seaward basin for a period of at least 300 ka. During this time, at least four onlaps/offlaps, often separated by periods of karst development with associated diagenetic alterations, took place. The story presented by this limestone not only allows a better understanding of the history of south Florida but also aids in the interpretation of similar persistent shelf-edge sites bordering deep basins in other areas.