Large-scale Slumping Research Articles

The 2019 Brumadinho tailings dam collapse: Possible cause and impacts of the worst human and environmental disaster in Brazil

On 25th January 2019, the tailings dam of the Brumadinho iron mine operated by Vale S/A failed catastrophically. The death toll stood at 259 and 11 people remained missing as of January 2020. This tragedy occurred three years after Mariana’s tailings dam rupture – the most significant tailing dam disaster in Brazilian history. Thus far, a systematic investigation on the cause and effect of the failure has yet to be conducted. Here, we use satellite-driven soil moisture index, multispectral high-resolution imagery and Interferometric Synthetic Aperture Radar (InSAR) products to assess pre-disaster scenarios and the direct causes of the tailings dam collapse. A decreasing trend in the moisture content at the surface and the full evanescence of pond water through time (2011–2019) suggest that the water was gradually penetrating the fill downwards and caused the seepage erosion, saturating the tailings dam. Large-scale slumping of the dam (extensional failure) upon the rupture indicates that the materials of the fill were already saturated. InSAR measurements reveal a dramatic, up to 30 cm subsidence in the dam (at the rear part) within the past 12 months before the dam collapse, signifying that the sediments had been removed from the fill. Although the information on the resistance level of the tailings dam to infiltrations is not available, these pieces of evidence collectively indicate that the seepage erosion (piping) is the primary cause for the chronic weakening of the structure and, hence, the internal “liquefaction” condition. Upon the collapse, the fully saturated mud tailings flowed down the gentle slope area (3.13 × 106 m2), where 73 % were originally covered by tree, grass or agricultural tracts. The toxic mud eventually reached the Paraopeba River after travelling 10 km, abruptly increasing the suspended particulate matter (SPM) concentration and the toxic chemical elements in the river, immediately affecting the local livelihoods that depend on its water. The Paraopeba River is a major tributary of the San Francisco River, the second-longest river in Brazil reaching the Atlantic Ocean. We anticipate that the environmental repercussions of this toxic seepage will be felt throughout the entire basin, especially riverine communities located downstream.

Early Cretaceous slumps and turbidites with peculiar soft-sediment deformation structures on Lingshan Island (Qingdao, China) indicating a tensional tectonic regime

Some sections on Lingshan Island, in the Yellow Sea, close to the city of Qingdao, show the Early Cretaceous Lingshandao Formation. The exposed succession consists mainly of turbidites, some of which show uncommon soft-sediment deformation structures, and contains well-developed, large-scale slumps that consist of sets of turbidites. The sediments were deposited as part of lobes of turbidites and hyperpycnites (deposits of flood-generated sediment-laden bottom currents derived from an estuary) in a deep-sea environment that formed part of an inter-arc basin with a tensional tectonic regime.Some of the slumps have dimensions of the order of metres to tens of metres; they consist of the turbidites that build most of the succession. The soft-sediment deformation structures in the turbidites represent a wide variety of types, most of which are common in subaqueous environments with a high accumulation rate. The most interesting are, however, structures that indicate active tectonic tension. These structures are boudins, which are present with two subtypes. The most common are pinch-and-swell structures, sometimes leading to isolated parts of the affected sandy layer, enveloped by shales of the under- and overlying shale layers. The second type consists of domino boudins, which have, as far as we are aware of, not earlier been described from sediments that were still unlithified during their formation.The presence of the boudins and associated structures, which indicate synsedimentary stretching related to an extensional tectonic setting, form an additional argument in the long-lasting debate about the question of whether a compressional or an extensional tectonic regime dominated in the area during the Early Cretaceous.

Três Forquilhas Valley in Southern Brazil - evidence for the uplift of the volcanic plateau

Os padrões de drenagem e a encosta foram estudados no vale do rio Três Forquilhas que é situado próximo da localidade de Terra de Areia, entre o mar e a borda nordeste do planalto vulcânico, no Rio Grande do Sul. Esta área foi estudada para explicar por que a escarpa do planalto vulcânico é mais próxima ao mar nesta sua por- ção nordeste e estabelecer a história erosional desta porção da escarpa. Aqui dois rios relativamente curtos, os rios Três Forquilhas e Maquiné, seguem a Sinclinal de Torres em direção ao Atlântico. Esses rios, provavelmente, iniciaram a erodir significativamente a sinclinal e a borda do planalto a partir do Mioceno Médio a Superior, em resposta à tectônica Andina de soerguimento tectônico de colisão ao longo da maior parte da América do Sul. A erosão subsequente, provavelmente, ocorreu em pulsos e não em uma fase uniforme em ambos os divisores de águas, muito como as chuvas torrenciais atualmente, que a cada dois ou três anos “impulsionaram” a erosão local da escarpa. O estudo da sequência estratigráfica no Terciário quando da sedimentação da Bacia de Pelotas adjacente à área de estudo, nos ajudou a estabelecer esta história erosiva local (sete discordâncias entre as sucessões de depósitos, variações de espessura entre estas discordâncias e a grande escala de espessuras). O estudo de uma seção transversal do continente em direção ao litoral identificou dois paleovales em rochas do Mesozoico abaixo do nível de base dos atuais rios Três Forquilhas e Maquiné. Há um controle estrutural forte da drenagem em todas as escalas. Essas considerações sobre os processos que se desenvolvem sobre a vertente contribuem, significativamente, para a compreensão sobre a evolução da drenagem e a origem da escarpa do planalto vulcânico.

The Mianwali and Tredian Formations: An Example of the Triassic Progradational Deltaic System in the Low-Latitude Western Salt Range, Pakistan

The Nammal Gorge, Salt Range, Pakistan, preserves a record of the interacting Triassic open-marine carbonate sedimentation, cyclic siliciclastics progradation and fluctuating relative sea level. The sediments at the bottom in the Mianwali Formation, show bivalves and ammonoids rich wacke- to packestone lithofacies (i.e. ML 1 and ML 2), which indicates an open-marine depositional setting. The overlying shale of the Mianwali Formation (i.e. ML 3), has ammonoids and gastropods, the lower part of which indicates a relatively deep-marine depositional setting. Upwards, the ML 3 grades into the sandstones and shales (i.e. ML 4), which show ripple marks, mudcracks, bioturbation, graded, and cross bedding and syndepositional deformation. These features indicate deposition in delta lobe/shoreline sub-environments, with a high siliciclastic input. The lower part of the overlying Tredian Formation contains interbedded shale and sandstone (i.e. TL 1), having large scale slumps, which indicate deposition in cyclic, proximally located varied sub-environments of the delta topsets/channels and in the flood plain environments. The overlying thick, coarse, conglomeratic sandstone (i.e. TL 2) has planar, trough cross bedding and lenticular channelized bedding, which indicates a fluvial to continental environment. Further up, in the Tredian Formation, the presence of dolomite (i.e. TL 3) represents marine transgression. The facies association in the Mianwali and Tredian formation indicates many coarsening upward cycles representing the progradation of individual delta lobes. This study therefore proposes that, subsequent to the Permian-Triassic paraconformity, an open-marine depositional setting established in this area that gradually built up into a progradational deltaic system.

FACIES ARCHITECTURE OF KOOKFONTEIN SHELF EDGE DELTA, TANQUA-KAROO BASIN (SOUTH AFRICA): IMPLICATIONS FOR FACIES ANALYSIS AND MODELLING

In recent years, the construction of accurate geological models and subsequent upscaling for reservoir simulation are commonly built from an integration of subsurface and surface datasets. Surface data can supply additional information on facies variability and small scale heterogeneity needed for a robust reservoir model, yet impossible to attain from subsurface data. This study therefore presents an outcrop characterisation study of the Permian Kookfontein deltaic succession using a combination of detailed sedimentary log, gamma-ray log and photopanel analysis. Based on texture and sedimentary structures, twelve depofacies are recognised which are broadly grouped into four lithofacies associations i.e. sandstone facies, heterolithic facies, mudstone facies and soft-sediment deformation facies. Following a hierarchical procedure, these depofacies and lithofacies form the basic building blocks for the flooding surface-bounded facies succession (i.e. cycle). Lateral juxtaposition of observed vertical facies variations across each cycle in an inferably basinwards direction exhibits upward change in features, i.e. decrease in gravity effects, increase in waves and decrease in slope gradient of subsequent cycles. This systematic upward transition in features, grading vertically from distal to proximal, represents a deposition of mid-slope to shelf-margin succession exhibiting an overall upward thickening and coarsening with progradational stacking pattern typical of a normal regressive prograding delta. However, in detail, cycles show some anomalies from a purely thickening and coarsening upward succession. Deposition of each cycle is believed to result from: 1. primary deposition by episodic and probably sporadic mouthbar events governed by stream flow dynamics; and 2. secondary remobilisation of sediments under gravity. The architecture and geometry of the ensuing depo-system is interpreted to have been a river-dominated, gravitationally reworked and wave-influenced shelf edge Gilbert-type delta. Widespread distribution of soft-sediment deformation structures, their growth-style and morphology within the studied succession are empirically related to progradation of Gilbert-type mouthbars over the shelf break as well as the slope gradients of the Kookfontein deltaic clinoformal geometry. Apparently, low-slope gradients may have favoured mostly loading and dewatering structures rather than large-scale slumps. Although shale content (shaliness) estimation from gamma-ray logs offers a quick and straightforward way for assessing reservoir potentiality of encountered facies in the field, this is rather crude and therefore its reliability requires detailed petrographic characterisation for confirmation. The described internal heterogeneity in this work is below the resolution (i.e. mm-scale) of most conventional well-logs, and therefore could supplement well-log data especially where there is no borehole image and core data. The combination of ‘descriptive’ facies model and schematic geological model for our specific delta make the results of this study applicable to any other similar ancient depo-system and particularly subsurface reservoir analogue.

3D seismic geomorphology and sedimentology of the Chalk Group, southern Danish North Sea

Abstract: Classically, the North Sea Chalk is interpreted as having been deposited under quiet, homogeneous pelagic conditions with local redeposition in slumps and slides. Recent observations of highly discontinuous reflection patterns on 2D and 3D seismic reflection data from the NW European Chalk Group have led to a revision of some general ideas of chalk deposition, with the suggestion that long-lived, contour-parallel bottom currents exerted a primary influence on the development of intra-chalk channels, drifts and mounds. This study proposes an alternative explanation for the formation of selected intra-chalk seismic and stratal discontinuities, interpreting these as being caused by gravity-driven processes that developed in response to intense syndepositional tectonics. Submarine mass-transport systems identified in the study area include large-scale slumps, slides, debris flows and turbidites. The last occur in sinuous channel systems flanked by large master levees, with the channel fill exhibiting well-developed secondary banks and overbanks on the outer bends of the channel thalweg. This first documentation of channelized density-flow deposits in the North Sea Chalk has important consequences for the interpretation and prediction of redeposited chalk units, emphasizing at the same time the strength of detailed 3D seismic discontinuity detection for subsurface sedimentary-systems analysis.

Geochemical modeling of sedimentary rocks in the central Hokkaido, Japan: Episodic deformation and subsequent confined basin-formation along the eastern Eurasian margin since the Cretaceous

Late Cretaceous to Paleogene tectonic episodes around the eastern Eurasian margin are described utilizing one-dimensional (1D) basin modeling technique on the basis of organic maturation; vitrinite reflectance and T max parameter values of the Rock-Eval pyrolysis. Fine-grained marine sediment of the Yezo Supergroup in central Hokkaido is one of the most extensive Cretaceous constituents along the plate margin. To evaluate organic maturation within the Yezo Supergroup, a surface section (Hakkin-zawa) and a deep borehole (MITI Yubari) were selected, from which quite different maturation trends were obtained. Notably low and constant maturation was suggested throughout the thick Hakkin-zawa section, and it was confirmed on the basis of various biomarker analyses. It requires anomalously rapid burial, probably related to thrust-stacking or large-scale slumping, followed by prompt tilting/exhumation event. In sharp contrast, Late Cretaceous strata in the MITI Yubari do not indicate coeval tectonic disturbances in a short period. A wider view of the ancient convergent margin suggests that deformation of the forearc propagated westward during the Cretaceous. High maturation levels in the uppermost thrust sheet around the MITI Yubari are optimized in 1D geochemical modeling on the assumption that a thick missing Paleogene unit has been eroded out as a result of thrust-forming contraction scheme emerged during the Neogene. A similar Cenozoic burial pattern is adopted for the once tilted and exhumed Hakkin-zawa section in order to match the present maturation levels. Although such an active subsidence of a ‘forearc’ is generally interpreted as an effect of the subduction erosion, heavy mineral composition of the shallow marine to fluvial Paleogene implies uplift and exhumation of ultramafic rocks to the east (trench-side). Thus the confined basin-formation in central Hokkaido is to be understood in a different tectonic framework that may reflect transcurrent motions on the plate margin.

Three-Dimensional Splay Fault Geometry and Implications for Tsunami Generation

Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.

The record of historic earthquakes in lake sediments of Central Switzerland

Deformation structures in lake sediments in Central Switzerland can be attributed to strong historic earthquakes. The type and spatial distribution of the deformation structures reflect the historically documented macroseismic intensities thus providing a useful calibration tool for paleoseismic investigations in prehistoric lake sediments. The Swiss historical earthquake catalogue shows four moderate to strong earthquakes with moment magnitudes of M w=5.7 to M w=6.9 and epicentral intensities of I 0=VII to I 0=IX that affected the area of Central Switzerland during the last 1000 years. These are the 1964 Alpnach, 1774 Altdorf, 1601 Unterwalden, and 1356 Basel earthquakes. In order to understand the effect of these earthquakes on lacustrine sediments, four lakes in Central Switzerland (Sarner See, Lungerer See, Baldegger See, and Seelisberg Seeli) were investigated using high-resolution seismic data and sediment cores. The sediments consist of organic- and carbonate-rich clayey to sandy silts that display fine bedding on the centimeter to millimeter scale. The sediments are dated by historic climate and environmental records, 137Cs activity, and radiocarbon ages. Deformation structures occur within distinct zones and include large-scale slumps and rockfalls, as well as small-scale features like disturbed and contorted lamination and liquefaction structures. These deformations are attributed to three of the abovementioned earthquakes. The spatial distribution of deformation structures in the different lakes clearly reflects the historical macroseismic dataset: Lake sediments are only affected if they are situated within an area that underwent groundshaking not smaller than intensity VI to VII. We estimate earthquake size by relating the epicentral distance of the farthest liquefaction structure to earthquake magnitude. This relationship is in agreement with earthquake size estimations based on the historical dataset.

ACTIVE SLUMPING OFFSHORE AMASRA (SOUTHWEST BLACK SEA) AND ITS RELATION WITH REGIONAL TECTONICS

The Bartin earthquake of 3rd September, 1968 provided the first evidence for active thrust faulting at the southern margin of the Black Sea. 500 km of 2D seismic reflection profiles acquired in this area show the presence of large scale slumps and landslips due to oversteepenin g of the shelf sediments, as a result of thrust faulting. The onshore drainage pattern is affected by interaction of the faulting with the geology, and this in turn affects the offshore shelf. Kilometre-scale bathymetric features on the sea-floor indicate a previous mass movement of a relatively coherent block of sediment, which has subsequently been partially buried. The source location for the block is visible on the geological map of the area. Incisional and erosional features in Pliocene sediments far from today's coastline and drainage systems suggest that water level was lower at the time they were formed than at present day.

Armored mudstone boulders in submarine debris-flow deposits, the Hunghae Formation, Pohang Basin: An evidence for the large-scale slumping of adjacent area of a submarine channel or scar wall

In the Hunghae Formation of the Pohang Basin, some heavily armored, calcareous mudstone balls (boulders) occur in a chaotic conglomerate bed which was deposited from debris flow on a steep slope. Sparsely armored or unarmored mudstone balls are also found in the same bed. Also some isolated armored mudstone boulders occur in a homogeneous mudstone bed of the formation which is interpreted as hemipelagite. The armored mudstone balls were most likely formed by retrogressive failures of submarine channel or scar wall and subsequent rolling over sand and pebble pavement along the submarine channel floor prior to incorporation into the debris flow. Their occurrence also suggests that a large-scale slumping should be happened on the upper slope apron or lower prodelta including the submarine channel and be evolved into debris flows, resulting in the corporation of the boulders into them. Some armored mud boulders rolled along and passed away the channelized gravel pavement, and deposited on hemipelagic mud in lower slope, not having been reworked.

Large-scale slumping deduced from structural and sedimentary features in the Lower Palaeozoic Anglo-Brabant fold belt, Belgium

Distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult, especially when the scale of the slump folds exceeds outcrop scale. In the southeastern part of the single-phase deformed, Lower Palaeozoic Anglo-Brabant fold belt a comparison of cleavage/fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle Caradoc fine-grained turbidites in the core of a large synform was overturned prior to tectonic deformation. This overturning is attributed to large-scale slumping, which was most likely a result of middle Caradoc seismic activity. The exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias, making up less than 5% of the exposed thickness. If the beds were not overturned, large-scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall ‘stable’ sedimentary pile. This may explain why so few ancient large-scale slides and slumps have been reported: the small amount of internal deformation makes them very difficult to recognize, especially when dealing with poorly exposed areas. As such, large ancient slides and slumps may be more common than suggested by the geological literature.

ABSTRACT: A Comparison of Two Lower Miocene Carbonate Margins: The Zhujiang Carbonate Platform (Subsurface, South China Sea) and The Pirinç Platform (Outcrop, Southern Turkey)

Early Miocene carbonate platform complexes have been the subject of extensive exploration efforts in Southeast Asia (for example, the Gunung Putih carbonate complex, East Java Sea, and the Zhujiang and Liuhua platforms, Pearl River Mouth Basin). However, few outcrop equivalents of these early Miocene platforms have been published to date. In this study, we present an outcrop that is an excellent analog for this petroleum province. The Pirin outcrop is an early Miocene carbonate platform-to-basin transition exposed on the northern flank of the Mut Basin in southern Turkey. The carbonates were deposited during a time of relative tectonic quiescence in the early Miocene (Burdigalian) over a relict paleotopography inherited from Oligocene tectonic activity. The interval described here is interpreted to be influenced by three large, relative sea level cycles with 100–150-m amplitude. These generated a steep-edged platform with prograding slope and margin geometries during the highstands, whereas during the lowstands, thin carbonate platforms developed with onlapping geometries against the older underlying slump deposits. Stratigraphic relationships indicate that collapse of the platform margin occurred during late highstand to early lowstand times and was coincident with major sea level falls. During transgression, an isolated platform complex developed. Carbonate production on these platforms shut down around the maximum flooding, leading to terminal drowning. This may have been the result of eutrophication caused by the arrival of nutrients from an encroaching deltaic system, combined with a period of rapid relative sea level rise. The subsurface Zhujiang Platform (Pearl River Mouth Basin, China) is an early Miocene carbonate platform that developed along the flank of a basement high in the basin. A steep, prograding, carbonate-shelf margin developed with large-scale slumping and collapse features in places. Pinnacles and small isolated platforms grow across a zone seaward of the margin. A strikingly similar stratigraphic organization (steep shelf margin, slumps, and isolated platforms) is observed in the Pirin outcrop. These similarities are attributed to a common tectonostratigraphic setting (the rapid marine flooding of a complex relict topography during a time of relative tectonic inactivity) and a Burdigalian eustatic sea level curve with cycles of sufficiently large amplitude to have a significant impact on the platform morphology. At the exploration scale, the outcrop allows us to test three play concepts. From outcrop observations, the slump packets (play concept 1) do not seem to be sufficiently interconnected to make a viable prospect. The development of an enhanced porosity wedge (play concept 2) at the shelf margin will be testable with future outcrop studies. In addition, with future outcrop work, a predictive model for the development of porosity in the isolated platforms can be developed (play concept 3) to more successfully predict porosity prior to drilling. At the reservoir scale, observations of subseismic bedding patterns from the outcrop will help us construct more accurate reservoir models. Additionally, outcrop observations of the dimensions of facies belts and their lateral and vertical variability will improve the way in which we populate a reservoir model with facies and reproduce variability on a scale finer than the interwell spacing.

Sediments and sedimentation at the NE Faeroe continental margin; contourites and large-scale sliding

High-resolution seismic profiles of the NE Faeroe continental margin show a wedge-shaped sedimentary sequence of up to 2 km thickness on top of lower Tertiary basaltic basement. The sedimentary section can be divided into four major seismic sequences separated by unconformities. The uppermost upper slope deposit is interpreted as a contourite which formed later than the Mid-Miocene by the southerly flow of bottom water along the margin of the SW Norwegian Sea, which is subsequently deflected to the east by the NE Faeroe continental margin and Fugloy Ridge. A scoured channel about 85–90 m deep at about 1000 m water depth associated with the contourite partially follows and modifies a preexisting slump scar. Large-scale slumping and sliding of the middle and lower continental slope below 1500 m water depth have affected sediments of presumably Miocene, Pliocene and Quaternary age. A TOBI deep-tow side-scan sonar mosaic composed of ten lines recorded in 1995 semi-parallel to the slope, shows that mass flow deposits are partially covered by recent contourite sediments on the middle slope. The slump complex at the middle/lower continental slope is younger and has a steep, 250–300 m, irregular, deep main scarp and very large, sometimes rotated blocks near the main scarp. At the base of the slide, numerous 10–15 km long, narrow tracks with individual blocks at the end are found. Several types of debris flows have been mapped, some with a longitudinal flow fabric.

Landslides and relict ice margin landforms in Adventdalen, central Spitsbergen, Svalbard

Two characteristic landforms, landslide blocks and drainage channels, were investigated in Adventdalen, central Spitsbergen. The landslides in the middle reaches of Adventdalen comprise large-scale bedrock slumps which form a hummocky surface on the south slope of Arctowskifjellet. The fourteen recognized landslide blocks are divided into upper and lower sections, according to altitude. The drainage channels consist of tributary rivers to Adventelva which flow in two distinct directions, either parallel with or oblique to the direction of the main river. Glacial deposits were found to cover the ridges between these tributary channels. The upper and lower landslide divisions may indicate former positions of the ice surface, and the channels appear to have originated during the existence of lateral moraine ridges with high ice content. These geomorphological findings have allowed reconstruction of former ice marginal positions, and they strongly suggest the existence of stagnant ice or minor re-advance phases during the course of deglaciation in Adventdalen.

Calderas on Mars: characteristics, structure, and associated flank deformation

Abstract Calderas and flank structures of martian volcanoes yield insight into general questions of volcano structural evolution and the underlying magma chambers in an environment where erosion is minimal. We have documented, through detailed geological mapping, the structures, associated volcanological features, and the stratigraphical relationships between the flank structures and caldera events during the building of each martian edifice. Two fundamentally different types of calderas are identified on Mars (the Olympus type and the Arsia type) that may represent end member variations in the size and depth of magma chambers. Many of the flank structures adjacent to caldera rims are consistent with the predicted effects of magma chamber inflation as well as deflation that exert significant influences in the structural development of many volcanoes. Large-scale terracing and steepening of the upper flanks of the larger martian volcanoes may originate from magma chamber inflation and radial thrusting. Thus the endogenous component of volcano growth resulting from accumulated magma chamber growth may be significant. Many of the deepest calderas are associated with evidence for voluminous eruptions elsewhere on the flanks and along through-going fissures and appear to result largely from evacuation and deflation of magma chambers without extensive precursor inflation. Draining of the magma chamber in these cases may be aided by the lateral propagation of magma in the form of shallow dykes up to several hundred kilometres in length and the associated formation of linear fissures. Nested caldera sequences, related flank pits, large-scale slumping, terracing, and sector structure are frequently arranged in linear patterns and are part of through-going eruptive lines or fissures several hundred kilometres in length that characterize several martian shield volcanoes. Fissures this long are interpreted to be dykes propagated outward from shallow magma chambers that have followed a minimum regional stress orientation. Comparison of the observed shape and orientation of caldera structures with orientation and style of flank deformation, and with the predictions from theory, indicate that regional stresses have probably been an important influence on the caldera and flank structures of martian volcanoes. The minimum regional stress orientation may be controlled largely by regional slopes associated with the Tharsis region and Elysium regions, and, in the case of Tyrrhena Patera, pre-existing radial fractures associated with the Hellas basin.

Mid-Dinantian Waulsortian buildups in the Dublin Basin, Ireland

The sedimentary history and biostratigraphic setting of Waulsortian carbonate buildups of the Feltrim Limestone Formation (late Courceyan to early Chadian) within the Dublin Basin are described. There is no unique precursor or successor facies to this formation, and the massive Waulsortian banks are composed predominantly of peloidal, skeletal wackestones and lime mudstones with packstones near the tops of banks. These banks form tabular bodies of moderate relief and are interbedded with thin shales and argillaceous crinoidal limestones of inter-bank facies. In the southwest of the basin inter-bank facies are rare and the bank facies have abundant stromatactis cavities, and uniquely at Roselawn a fauna of rugose corals. All buildups in the Dublin Basin have Phase C and/or D component assemblages of Lees and Miller (1985) and are interpreted as accumulating in moderately shallow-water depths, near or within the photic zone. Isopachs for the Feltrim Limestone Formation show a NE-SW-trending axial depocentre where the Waulsortian facies is in excess of 400 m thickness. Deposition appears to have taken place on this “double-sided” ramp, in a manner similar to the model of Lees (1982) for Belgium and southern Britain. Soft-sediment deformation such as large-scale slumping, shale-injections and water-escape structures, not previously recorded from these rocks is widespread. The upper surface of the Feltrim Limestone Formation is fissured and displays a prominent erosion surface. Termination of Waulsortian facies deposition and influx of terrigenous sediment was caused by rapid uplift, attributed to Chadian tectonism. However, eustatic sea-level fall cannot be ruled out as a partial cause of the demise of the Waulsortian.

Submarine canyon development in the Izu-Bonin forearc: A SeaMARC II and seismic survey of Aoga Shima Canyon

SeaMARC II sidescan (imagery and bathymetry) and seismic data reveal the morphology, sedimentary processes, and structural controls on submarine canyon development in the central Izu-Bonin forearc, south of Japan. Canyons extend up to 150 km across the forearc from the trench-slope break to the active volcanic arc. The canyons are most deeply incised (1200–1700 m) into the gentle gradients (1–2°) upslope on the outer arc high (OAH) and lose bathymetric expression on the steep (6–18°) inner trench-slope. The drainage patterns indicate that canyons are formed by both headward erosion and downcutting. Headward erosion proceeds on two scales. Initially, pervasive small-scale mass wasting creates curvilinear channels and pinnate drainage patterns. Large-scale slumping, evidenced by abundant crescent-shaped scarps along the walls and tributaries of Aoga Shima Canyon, occurs only after a channel is present, and provides a mechanism for canyon branching. The largest slump has removed >16 km3 of sediment from an ∼85 km2 area of seafloor bounded by scarps more than 200 m high and may be in the initial stages of forming a new canyon branch. The northern branch of Aoga Shima Canyon has eroded upslope to the flanks of the arc volcanoes allowing direct tapping of this volcaniclastic sediment source. Headward erosion of the southern branch is not as advanced but the canyon may capture sediments supplied by unconfined (non-channelized) mass flows.

A Permian kuroko-type hydrothermal system, Afterthought-Ingot area, Shasta County, California; lateral and vertical sections, and geochemical evolution

The Afterthought and smaller kuroko deposits in the East Shasta mining district formed at the culmination of a late Permian mafic to felsic volcanic cycle. They were succeeded by Triassic elastic sediments and reworked tuff in a presumed rifting environment with local evidence of renewed mafic volcanism and large-scale slumping. The Afterthought host horizon is repeated in outcrop by folding. Fold closures are tight east of Ingot where preore volcanic rocks are more altered; they are open basins and domes west of Ingot where the preore volcanic rocks are less altered. Igneous pyroxene and titanomagnetite and secondary pumpellyite occur only in preore andesite and basalt more than 2.5 km from known kuroko deposits. In outcrops closer to the deposits, disseminated pyrite (type E) is locally abundant in preore dacite. The kuroko deposits comprise three ore types: A: banded, fine-grained sulfides and gangue; B: recrystallized, coarser grained sulfides and gangue with a consistent paragenetic sequence; and C: coarse-grained pyrite and barite. B and C appear to replace type A. A further type of mineralization, type D, consists of fine-grained pyrite occurring locally within silicified dacite on the Afterthought host horizon. The paragenetic sequence of type B implies a sequential introduction of metals in a fluid evolving toward higher H 2 S activity. Cu was introduced early as chalcopyrite and bornite; later fluids altered preexisting Cu minerals rather than introducing significant new Cu. Type B fluids probably cooled from >300 degrees C (chalcopyrite stage) to about 270 degrees C (sphalerite + barite stage). The last type B fluids dissolved much base metal sulfide and deposited calcite, quartz, and sericite. Types A, B, C, and E sulfides have a delta 34 S value range of 5 to 12 per mil, implying a common origin. Barites from type A and B ore have delta 34 S values near 30 per mil. Partially reduced contemporaneous marine sulfate (10-12ppm) is a possible sulfur source, but a concealed rock-sulfur source of high delta 34 S value is more probable. Type D pyrite, with delta 34 S values between -28 and 0 per mil, contains sulfur of a different, probably biogenic, origin. Hydrothermal carbonates formed from fluids with delta 13 C values from near 0 to -12 per mil, consistent with oxidation of organic C in the hydrothermal system. Hydrothermal and postore carbonates have delta 18 O values of 17 to 19 per mil and probably equilibrated with a single fluid after the deposition of the Hosselkus Limestone.

Paleogeography, sedimentation and tectonics of the upper cretaceous Anambra basin, southeastern Nigeria

A reinterpretation of the depositional environments of the Anambra Basin succession in the light of modern sedimentological models has necessitated a revision of the late Cretaceous paleogeography of southeastern Nigeria. The phases of transgressions and regressions in the Campanian in the Campanian to Paleocene within the Anambra Basin are reviewed, based on these new interpretation. Thus, it has been shown, for example, that the Mamu Formation and the overlying Ajali Sandstone are both part of a continued controlled Maastrichtian transgression, which started in the late Campanian, superimposed on a regional geoidal eustatic sea level rise, which lasted until the Paleocene. This contrasts with the interpretation favouring a regressive phase of a short-lived Maastrichtian transregression, as has been previously thought by some workers. Tectonic movements within the basin have been shown to have been active during the late Cretaceous sedimentary cycles, as demonstrated by variations in the thickness patterns across the basin. Lateral facies variations both within and across formation boundaries, from north to south, as well as the occurrences of a large-scale slumps and other types of syn-sedimentary deformation structures attributed to tectonic activity further attest to the active nature of the basin during the period of sedimentation. The pattern of subsidence is also thought to have been influenced by earth tremors and periods of “quiescene”, particularly in the northern Ankpa sub-basin.