Muddy Debris Flows Research Articles

Runout Scaling and Deposit Morphology of Rapid Mudflows

Prediction of runout distance and deposit morphology is of great importance in hazard mitigation of geophysical flows, including viscoplastic mudflows. The major rheological parameters of mudflows, namely, yield stress and viscosity, are crucial factors in controlling the runout and deposition processes. However, the roles of the two parameters, especially in mudflows with high inertia, remain poorly understood and are not accounted for in runout scaling relations with source volume. Here we investigate the effects of flow rheology on runout scaling and deposit morphology using small‐scale laboratory experiments and three‐dimensional numerical simulations. We find that yield stress and viscosity both influence flow velocity gained during downslope propagation of mudflows, which is strongly correlated with the runout distance; the role of yield stress is more significant than viscosity. High yield stress and low viscosity lead to an elongated deposit, where longitudinal propagation is more significant than lateral spreading. In contrast, high viscosity promotes the dominance of lateral spreading of the deposit, while low yield stress and moderate viscosity produce an initial elongate deposit, followed by a secondary surge that spreads laterally near the head of the deposit. Following appropriate scaling relations for viscosity and yield stress, a general scaling function is proposed to incorporate flow properties in the well‐known correlation of runout distance and source volume. Our findings regarding the inertia effects and the roles of yield stress and viscosity enhance our understanding of mudflows, muddy debris flows, and other viscoplastic geophysical flows.

Pore-throat characteristics of tight sandstone reservoirs composed of gravity flow sediments: Yingcheng Formation, Longfengshan sag, China

The Lower Cretaceous Yingcheng Formation in the Longfengshan sag (Songliao Basin, China) represents a typical tight sandstone reservoir that hosts large accumulations of hydrocarbons. The lacustrine sandstones mainly reflect gravity flow deposits making up various lithofacies displaying varying degrees of heterogeneity. This study reports on the links among gravity flow type, diagenetic overprinting and pore-throat characteristics that control reservoir quality. Core observations, especially grain size variations and related sedimentary structures are used to differentiate types of gravity flows. Microscopic observations, cathodoluminescence, scanning electron microscopy, pressure-controlled mercury injection tests and rate-controlled mercury injection tests are used to determine the mineralogy, sedimentary composition and pore-throat characteristics of the tight sandstones.In the study area, the gravity flows are divided into muddy debris flows, sandy debris flows, high-density turbidity flows, low-density turbidity flows and mud density flows. The petrophysical properties of these different gravity flow sediments reflect a range of permeability and porosity characteristics. Because of their mud-dominated textures and related weak compressive strength, mud density flows and muddy debris flows become ineffective as reservoirs during burial. In contrast, for approximately similar burial depths, turbidites and sandy debris flows show better reservoir quality. However, reservoir units with high porosities still possess considerable heterogeneity; this is especially true of some sandy debris flows that possess large amounts of secondary pores but which are not charged with hydrocarbons. In particular, in some poorly sorted sandy debris flows that are characterized by high contents of fine-grained matrix, some of the pore throats are blocked by matrix material. Moreover, the dissolution of feldspars and volcanic debris led to the formation of authigenic clays and quartz, which preferentially developed in pore throats with small radii. This process significantly reduced permeability.Overall, the permeabilities of the sandy debris flows are generally lower than those of the turbidites. Due to their relatively large-radius pore throats, turbidites can be categorized as potential effective hydrocarbon reservoirs in the study area. This research provides significant insights into the characteristics of pore throats within tight reservoirs consisting of gravity flow sediments in lacustrine basinal settings.

Delta-fed turbidites in a lacustrine rift basin: the Eocene Dongying depression, Bohai Bay Basin, East China

ABSTRACTSediment avalanche from delta ramp is one of the significant development mechanisms for a turbidite system in a lacustrine basin. To advance our understanding of deep-water sedimentary processes in a lacustrine delta ramp, delta-fed turbidites in the Eocene Dongying depression of the Bohai Bay Basin were studied using core data, 3-D seismic data and well log data. Sandy debris flows, muddy debris flows, mud flows, turbidity currents, slides, sandy slumps and muddy slumps were interpreted based on the identification of lithofacies. Data indicates that deep-water sedimentary processes in the study area were dominated by debris flows and slumps, which accounted for ∼68% and 25% (in thickness) of total gravity flow deposits, respectively; turbidity-current deposits only accounted for ∼5%. Mapping of turbidites showed that most were deposited after short-distance transportation (<20 km), restricted by the scale of deep-water areas and local topography. Channels, depositional lobes, debris flow tongues, muddy turbiditic sheets, slides and slumps were identified in a delta-fed ramp system. Slides and slumps were dominant at the base of slopes or at the hanging walls of growth faults with strong tectonic activity. Channels and depositional lobes developed in gentle, low-lying areas, where sediments were transported longer distances. Sand-rich sediment supply, short-distance transportation and local topography were crucial factors that controlled sedimentation of this ramp system. Channels generally lacked levees and only produced scattered sandstones because of possible hydroplaning of debris flow and unstable waterways. In addition to lobes, debris flow tongues could also be developed in front of channels. These findings have significant implications for hydrocarbon exploration of deep-water sandstone fed by deltas in a lacustrine basin.

Sedimentary differences between different segments of the continental slope-parallel Central Canyon in the Qiongdongnan Basin on the northern margin of the South China Sea

Submarine canyons have been the subject of intense studies in recent years because of their academic significance to studies of climate signals, eustasy and tectonic movement of the neighbouring area and their economic significance for hydrocarbon exploration in deep-water areas. The Central Canyon in the Qiongdongnan Basin runs parallel to the continental slope, and has a length, width and depth of 520 km, 2–15 km and 270–800 m. Using seismic and well log data, we analyses the sedimentary differences and controlling factors of the sediments in the different segments of the Central Canyon. The sediments within the Central Canyon can be subdivided into five phases of the secondary canyon filling. The most typical features of each phase of the secondary canyon are high- and low-density turbidity currents developed in the phase 1–2 of the secondary canyon, the muddy debris flows, turbidity currents and mass transport deposits in the phase 3, the high-density turbidity currents in the phase 4, the hemi-pelagic mudstone and mass transport deposits in the phase 5. There are four depositional models corresponding to the head area, upstream segment, middle reaches and downstream segment of the Central Canyon were established. It is characterized by muddy matrix-supported conglomerates and thickly massive sandstones with the feature of upward fining and thinning deposited in the head area, the thickly high net to gross sandstone infilled in the upstream segment, the thinly interbedded sandstones and mudstones and muddy mass transport deposits developed in the middle reaches, and the extensive hemi-pelagic mudstone infilled in the downstream segment. The transformation of the debris flows into turbidity currents and high-density turbidity currents into low-density turbidity in the head and upstream segment, supersaturated filling in the upstream segment, extensive development of mass transport deposits in the middle reaches, under-compensation filling and widely development of canyon wall collapse in the downstream segment are the main reasons contributing to the sedimentary differences between different segments of the Central Canyon.

Tracing multiple resedimentation on an isolated karstified plateau: The bauxite-bearing Miocene red clay of the Southern Bakony Mountains, Hungary

The Vöröstó (=Red Lake) Formation, located in the Southern Bakony Mountains and the Balaton Highlands (Hungary), is a red, clayey continental assemblage containing hard, up to fist-sized bauxite pebbles, occurring in a large apparent stratigraphic gap between underlying karstified Triassic carbonates and mid’ Miocene to Quaternary cover. The origin and the exact stratigraphic position of the assemblage have been controversial for a long time. In this study, petrographic observations on the bauxite pebbles revealed common features with Cretaceous bauxite deposits known in the region, whereas heavy mineral composition of the red clay matrix is similar to those known from the Eocene bauxite horizon of the region. Single grain zircon U–Pb ages obtained from the bauxite pebbles and their red clayey matrix show similar late Archean to Jurassic age components. Additionally, Cenozoic U–Pb ages are well represented in the mostly euhedral zircon crystals separated from the red clays, whereas this volcanogenic contribution completely missing from the bauxite pebbles. SEM morphology and related EDX chemical analysis of clay minerals indicate polygenetic, detrital origin for the red clays. The main source material of the bauxite bearing Vöröstó Formation is most probably local Cretaceous and Eocene bauxite deposits exposed during the middle-late Miocene. These tropical weathering products were partly decomposed and degraded, but dilution by siliciclastic contribution is negligible. We suggest a transport mechanism dominated by local redeposition of mostly pelitic and allitic material through seasonal muddy debris flows within a karstic landscape.

Pyritic event beds and sulfidized Fe (oxyhydr)oxide aggregates in metalliferous black mudstones of the Paleoproterozoic Talvivaara formation, Finland

The Paleoproterozoic, 2.0–1.9 Ga Talvivaara formation of Finland was deposited during the Shunga Event, a worldwide episode of enhanced accumulation of organic-rich sediments in the aftermath of the Lomagundi–Jatuli carbon isotope excursion. Sulfidic carbonaceous mudstones in the Talvivaara formation contain one of the largest known shale-hosted nickel deposits. In order to gain new insight into this Shungian sedimentary environment, sedimentological, petrographical and in situ S and Fe isotopic microanalyses were carried out on samples representing depositional and early-diagenetic conditions. The event-bedded lithology with tidal signatures in the organic-rich mudstones strongly indicates deposition from predominantly river-delivered mud on a highly-productive coastal area, below storm-wave base. The riverine supply of phosphorus, sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water. The stratigraphic upper part of the Talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds. The pyrite beds were deposited by seaward excursions of the concentrated, acidic Fe-rich river plume subsequent to droughts or dry seasons, which led to intense pyrite precipitation upon mixing with euxinic waters. δS34 and δFe56 values of the bedded pyrite (median δS34=−10.3‰ and δFe56=−0.79‰) are consistent with the reaction of dissolved Fe(II) with H2S from bacterial sulfate reduction. Organic-rich clayey Fe-monosulfide-bearing granules were transported from the muddy estuary, and enclosed in Fe (oxyhydr)oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river-delivered iron. The isotopic composition of these presently pyrrhotitic inclusions (median δS34=−3.3‰ and δFe56=−1.6‰) indicates microbial iron reduction. The Fe (oxyhydr)oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor. Their Fe (oxyhydr)oxide matrix was replaced by pyrite (median δS34=+5.8‰ and δFe56=+0.81‰) at shallow sediment depths with 34S and 56Fe-enriched porewater. Wavy-crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation. These results indicate episodic deposition at seasonal to multiannual time scales. δS34 and δFe56 values in the studied Fe-sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early-diagenetic origin, finding no support for the previously proposed local hydrothermal activity in the Talvivaara mudstones.

Sedimentary characteristics and facies model of gravity flow deposits of Late Triassic Yanchang Formation in southwestern Ordos Basin, NW China

Abstract Based on cores and logging data, and gravity flow theory, the sedimentary characteristics and facies model of gravity flow deposits in the Longdong area in southwestern Ordos Basin were analyzed. Five types of gravity flow deposits are recognized: slides, slumps, sandy debris flows, muddy debris flows and turbidity currents. Affected by multiple provenances, paleogeomorphology and triggering mechanisms jointly, the gravity flows resulted in sublacustrine fans and slump olistoliths in the basin center. Containing channels, the sublacustrine fans can be divided into three subfacies, inner fan, middle fan and outer fan and subdivided into six microfacies, main channel, main channel lateral margin, distributary channel, distributary channel lateral margin, inter-channel and sheeted turbidite sand. It is inferred that they are caused by seasonal floods. The slump olistoliths, with no channels, consist four parts: slide rock, slump rock, debris flow lobe and sheeted turbidite sand, and are inferred to be the product of collapse of break belts triggered by events like earthquakes. Combining facies models with types of gravity flow deposits can reveal the sedimentary characteristics and genetic models of gravity flow deposits in deep water more directly and provide theoretical basis for deep water oil and gas exploration.

Status and Trends in Research on Deep‐Water Gravity Flow Deposits

AbstractDeep‐water gravity flows are one of the most important sediment transport mechanisms on Earth. After 60 years of study, significant achievements have been made in terms of classification schemes, genetic mechanisms, and depositional models of deep‐water gravity flows. The research history of deep‐water gravity flows can be divided into five stages: incipience of turbidity current theory; formation of turbidity current theory; development of deep‐water gravity flow theory; improvement and perfection of deep‐water gravity flow theory; and comprehensive development of deep‐water gravity flow theory. Currently, three primary classification schemes based on the sediment support mechanism, the rheology and transportation process, and the integration of sediment support mechanisms, rheology, sedimentary characteristics, and flow state are commonly used. Different types of deep‐water gravity flow events form different types of gravity flow deposits. Sediment slump retransportation mainly forms muddy debris flows, sandy debris flows, and surge‐like turbidity currents. Resuspension of deposits by storms leads to quasi‐steady hyperpycnal turbidity currents (hyperpycnal flows). Sustainable sediment supplies mainly generate muddy debris flows, sandy debris flows, and hyperpycnal flows. Deep‐water fans, which are commonly controlled by debris flows and hyperpycnal flows, are triggered by sustainable sediment supply; in contrast, deep‐water slope sedimentary deposits consist mainly of debris flows that are triggered by the retransportation of sediment slumps and deep‐water fine‐grained sedimentary deposits are derived primarily from fine‐grained hyperpycnal flows that are triggered by the resuspension of storm deposits. Harmonization of classification schemes, transformation between different types of gravity flow deposit, and monitoring and reproduction of the sedimentary processes of deep‐water gravity flows as well as a source‐to‐sink approach to document the evolution and deposition of deep‐water gravity flows are the most important research aspects for future studies of deep‐water gravity flows study in the future.

Multiple failure styles related to shallow gas and fluid venting, upper slope Canadian Beaufort Sea, northern Canada

The continental slope of the Canadian Beaufort Sea presents an exceptional opportunity to study the relationship between the fluid venting and the formation of mass-transport deposits. The continental shelf was emergent and partially ice-free during the last glaciation and is underlain by widespread permafrost. Water-column backscatter has shown the locations of more than 40 active gas vents along seaward margin of the subsea permafrost at the shelf break and upper slope. New multibeam bathymetry and subbottom profiler data show shallow potentially late Holocene failures and mass-transport deposits on the upper slope. Upslope from a prominent headscarp, undulating seabed with apparent growth faults overlies an acoustically incoherent to stratified horizon at 50 m sub-bottom interpreted as a decollement surface over which progressive creep has occurred. Similar creep is present in places on the upper slope and in places seems to have evolved into small translational slides, involving more compacted sediment buried > 25 m, or into muddy debris flows where sediments buried < 25 m have failed. Much of the slope failed during a regional retrogressive event, the Ikit slump, likely initiated on steep channel walls on the lower slope. Characteristic ridge and trough morphology resulting from retrogressive spreading or rotational slumping are preserved on gradients < 2° on the upper slope, but appear to have been completely evacuated on gradients of 3° on the mid slope, where muddy debris-flow deposits are found. Correlations between radiocarbon dated cores and sub-bottom profiles show that the retrogressive failure occurred in the last 1000 years. This study implies that Holocene shelf break and upper slope stability in the Beaufort Sea are strongly linked to the dynamics of the permafrost and the presence of weak, gas-rich sediments. It demonstrates that creep deformation evolves into either muddy debris flows or translational slides, dependent on sediment strength. • Water column backscatter shows active gas vents near the subsea permafrost limit. • Creep with growth faults occurs over a gas-charged decollement zone. • Creep evolves to translational slides where thick, debris flows were thin. • A major < 1 ka retrogressive slump is the largest failure in the area.

Gravity flow sedimentation and tight oil exploration in lower first member of Shahejie Formation in slope area of Qikou Sag, Bohai Bay Basin

Abstract Based on observation of cores and thin slices and analysis of drilling, log and test data, the gravity flow deposits characteristics and formation conditions and characteristics of tight oil in lower first member of Shahejie Formation (Es1x) were systematically analyzed in Qikou Sag, Huanghua depression, Bohai Bay Basin. There mainly developed sandy debris flow, muddy debris flow and classical turbidity flow in the study area, and sandstone with boulder-clay is mostly sandy debris flow origin, muddy debris flow deposits are mudstone or silty mudstone with sandy lumps, the normal graded bedding is a reliable sign of classical turbidite. Affected by multi-source supply, slope break, different trigger mechanism and fluctuation of lake level, multi-gravity flows formed multi-level fans in mid-low slope to main sag, which can be divided into five micro-facies including submerged main channel, submerged distributary channel, lateral channel margin, mud between channels and turbidite sheet sand. Multi-level and multi-stage gravity flow sediments provide material foundations for tight oil and gas in the deep mud and sand-rich lake, where wide spread tight reservoirs and high quality source rocks contact closely vertically, forming multiple sets of sandwich-like source-reservoir-seal assemblages. Exploration shows that the submerged distributary channel and turbidite sheet sand of sandy debris flow are two main exploration facies of gravity flow in Es1x and important area of scale reserve increase in slope zones of Qikou Sag.

SPH modeling of fast muddy debris flow: numerical and experimental comparison of certain commonly utilized approaches

SPH (Smoothed Particle Hydrodynamics) is a particle, purely mesh-free Lagrangian method, proposed by different authors, well suited to the computing of highly transitory free surface flows of complex fluids in complex geometries. Different approaches have been proposed in order to better simulate the mutual interaction between particles and their interactions with boundaries. Therefore, the main target of this article is to discuss and explore the numerical performance of certain commonly utilized SPH approaches, based essentially on mass and momentum balances, in the simulation of a 2D fast mudflow in fast motion, composed of fluid and solid material, assumed to be just one equivalent phase (fluid-solid). The “Herschel-Bulkley”, non Newtonian constitutive equations, describing a viscoplastic material suitable to reproduce the rheological behaviour of mudflows, has been selected. Hence, a laboratory experimental test, already proposed in literature and, after properly scaling, representative of a real fast flow phenomenon, was considered for comparison with numerical outcomes carried out by a research code that has already been tested and discussed in previous papers. A simple but effective statistical approach was developed and applied in order to identify and utilize a numerical index suitable for the quantitative measurement of the degree of matching between numerical results and measurement data affected by experimental errors. More than thirty numerical experiments were performed, of which the most significant eleven simulations are discussed. Satisfactory results were achieved. As outcomes, it was verified that, in particular for the selected experimental test, Rusanov flux addition within the continuity equation with the proper choice of both the viscosity term of momentum and the SPH boundary conditions, is suited to enhancing the performance of this type of numerical simulation of a fast flow.

Geomorphology and kinematics of debris flows with high entrainment rates: A case study in the South French Alps

Debris flows consist of a fully saturated mixture of water, sediment and debris that can travel a distance of several kilometers. The entrainment processes occurring during the run-out phase are little discussed in the literature, especially from the discussion of observed events. In the early evening of 5 August 2003, a large debris flow occurred at the Faucon torrent in the Barcelonnette Basin (South French Alps). This debris flow caused substantial damage to several buildings located on the torrential fan. Some morphological aspects of this debris-flow event are presented through a careful field survey and the characterization of the transported sediment. First, the observations show that the triggering area corresponds to a strong incision in scree slopes; the grain-size distribution of the debris-flow deposits in this area is typical of granular debris flows. Second, the total volume of the debris-flow was estimated at 60,000 m 3. Channel scouring is responsible of the large difference between the mobilized volume and the deposited volume; an entrainment rate of 15 m 3.min −1 is estimated. Laboratory tests show that the 2003 debris-flow event started as a granular flow and then transformed into a muddy debris flow by incorporating clay-rich marly sediments along the flow track.

Depositional Characteristics and Distribution of Lake-Floor Fan of Paleogene Lower Member 3 of Shahejie Formation in Northwestern Part of Huanghekou Sag, Bohai Bay Basin

The lake-floor fan, as an important deposition of Paleogene in Huanghekou Sag, is characterized with both good physical character of reservoir and oil-bearing property, thus it has higher values for petroleum exploration. The lower Member 3 of Shahejie Formation, rich in sandstone, can be divided into lowstand system tract, extensive system tract and highstand system tracts based on seismic reflection signature, and core and logging data clearly. The sand bodies in the study area belonged to two kinds of lake-floor fan deposition formed under semi-deep-lake to deep-lake environment: deposition of the deep-water turbidite fan corresponded to lowstand system tract; deposition of the slump turbidite fan corresponded to extensive system tract and highstand system tract, which was formed by slumping fan delta front sand. Viewed from spatial distribution, the deep-water turbidite fan was mainly developed in the second-order fault downthrown of the basin, while the slump turbidite fan was distributed in the abrupt slope which located in the south of study area. The complex of lake-floor fan consists of various sediments’gravity flow deposits, including:(1)turbidity current deposit with characteristics of Bouma sequences; (2)mud-bearing sandy debris flow deposits with characteristics of dominated sand and mixed by mud; (3)sand-bearing muddy debris flow deposits with characteristics of dominated mud and mixed by sand; (4)gritty debris flow deposits with characteristics of massive gravel accumulation;(5)sandy slump deposits with characteristics of deformed sedimentary structure. During lower water level period(LSTs3L), most of the study area were erosion or sediment pass-by areas, and terrigenous clastic matters were directly transported into deep-water area under the second-order faulted belt, and then formed the deep-water turbidite fan. During higher water level period (ESTs3L, HSTs3L), the slump tuibidite fan, which resulted from various sediments’gravity flows, mainly developed in front of fan dalta in south. The sandstones of the lake-floor fan are the main reservoirs of oil in the northwestern part of the Huanghekou sag.

Dynamic taxonomies applied to a web-based relational database for geo-hydrological risk mitigation

In its 40 years of activity, the Research Institute for Geo-hydrological Protection of the Italian National Research Council has amassed a vast and varied collection of historical documentation on landslides, muddy-debris flows, and floods in northern Italy from 1600 to the present. Since 2008, the archive resources have been maintained through a relational database management system. The database is used for routine study and research purposes as well as for providing support during geo-hydrological emergencies, when data need to be quickly and accurately retrieved. Retrieval speed and accuracy are the main objectives of an implementation based on a dynamic taxonomies model. Dynamic taxonomies are a general knowledge management model for configuring complex, heterogeneous information bases that support exploratory searching. At each stage of the process, the user can explore or browse the database in a guided yet unconstrained way by selecting the alternatives suggested for further refining the search. Dynamic taxonomies have been successfully applied to such diverse and apparently unrelated domains as e-commerce and medical diagnosis. Here, we describe the application of dynamic taxonomies to our database and compare it to traditional relational database query methods. The dynamic taxonomy interface, essentially a point-and-click interface, is considerably faster and less error-prone than traditional form-based query interfaces that require the user to remember and type in the “right” search keywords. Finally, dynamic taxonomy users have confirmed that one of the principal benefits of this approach is the confidence of having considered all the relevant information. Dynamic taxonomies and relational databases work in synergy to provide fast and precise searching: one of the most important factors in timely response to emergencies.

SURGE OCCURRENCE CONDITION AND FLOW MODELS OF MUDDY DEBRIS FLOW

土石流に代表されるような土砂礫を高濃度に含む流れの周期的なサージ生成の機構について述べている．特に，桜島の野尻川や長谷川で観測されるような高濃度泥流では，短時間に複数のサージ流下が観測されている．このようなサージの生成機構において，流れの不安定性理論に基づく転波列生成条件について，ビンガム流体モデル，ダイラタント流体モデル，混合・粒子衝突流体モデルの生成条件を示した．実験結果および桜島の観測結果は，混合・粒子衝突流体モデルによく対応することを明らかにした．

Lake-mire deposition, earthquakes and wildfires along a basin margin fault; Rønne Graben, Middle Jurassic, Denmark

Regional uplift of structural highs took place in the greater North Sea region in early Middle Jurassic times, resulting in a change from marine shelf deposition in the Early Jurassic to widespread emergence, erosion and localized deposition in grabens. The Middle Jurassic Bagå Formation exposed on the island of Bornholm in the Baltic Sea was deposited in the Rønne Graben, which forms a dog-leg pull-apart basin linking the northern and southern branches of the Tornquist Zone. The studied succession is situated on the hanging wall immediately adjacent to the main eastern graben margin fault and the footwall consists of deeply weathered granite. It is the only locality along the northern branch of the Tornquist Zone where active tectonic influence on Middle Jurassic deposition can be demonstrated in outcrop. Small alluvial fans composed of poorly sorted washout sands and muddy debris flows with weathered granite boulders fringed the footwall scarp. Syn-depositional soft-sediment deformation and chaotic bedding structures interpreted as seismites caused by earthquake shocks are characteristic at several levels. On the hanging wall further away from the active fault the depositional environment was dominated by shallow freshwater lakes with deposition of clay. Lake deposition outpaced subsidence and open, oxygen-deficient freshwater mires were formed during periods of no or only incremental fault-movement. The mires were only occasionally isolated from clastic input due to doming and dense vegetation, resulting in accumulation of pure peat beds. The content of spores and pollen shows that the mires were covered by a vegetation of arborescent and herbaceous Filicopsida (ferns), Lycopsida (club mosses), Cycadales/Bennettitales, Ginkgoales, Caytoniales, Taxodiaceae or Cupressaceae, and Araucariaceae. Wildfires were common in the mires and also on the footwall probably followed by episodes of increased run-off and erosion of the upland. The presence of common washout sands, debrites, seismites and abundant lacustrine flooding surfaces indicating sudden drowning of the mires was caused by abrupt movements on the fault with associated earthquakes. The succession thus provides an example of temporal changes in deposition in response to changes in tectonic conditions. The study illustrates how regional uplift of structural highs caused a restriction of deposition to narrow grabens like the Rønne Graben. Facies patterns changed dramatically from laterally extensive uniform packages controlled by sea-level changes in the Early Jurassic to strongly localized deposition with marked facies changes controlled by faulting and earthquakes in the Middle Jurassic. The succession thus serves as an excellent example of tectonically controlled deposition under humid conditions in a continental graben.

Evidence of different contractional styles along foredeep margins provided by Gilbert deltas: examples from Bowser Basin, British Columbia, Canada

Abstract The northern margin of the early Middle Jurassic Bowser Basin in British Columbia contains large, solitary shelf-type Gilbert deltas and smaller, multistorey Gilbert deltas stacked against inferred active reverse faults. Analysis of the deltas provides evidence of tectonic structures that can no longer be directly observed. Bowser Basin developed as a foredeep in response to overthrusting of Stikinia by Cache Creek Terrane (oceanic crust) during accretion of Stikinia to the western margin of North America. The terrane boundary is currently represented by the King Salmon and associated thrust faults. The shelf-type Gilbert deltas consist of accretional conglomerate packages up to 100 m thick, that are overlain by and laterally associated with cyclothemic shelf facies. Gilbert deltas responded to low accommodation to sediment supply ratios by prograding to the shelf break of the basin, where they fed large volumes of gravel to slope gullies and the deep basin beyond. At one location, fault-block rotation resulted in oversteepening of foresets and internal discordances. Deposition was dominated by non-cohesive debris flows (50% to 60% of foresets) and intervening muddy debris flows. The coarse-grained sediments are texturally and compositionally mature, consisting predominantly of spherical radiolarian chert clasts derived from the overthrust Cache Creek accretionary prism. A spectacular succession at least 700 m thick near Cartmel Lake records the aggradational stacking of 30 small Gilbert deltas. Like the shelf-type Gilbert deltas, sediment gravity flows predominated. Creation of accommodation space for accumulation of the Gilbert delta stack was repeated many times; the locus of accommodation remained in much the same place over time. This implies a high degree of tectonic control over the geometry of the stacked Gilbert deltas. Accommodation to sediment supply ratios for this kind of Gilbert delta were high. Based on their thickness, geometry, and general association with shelf lithofacies, Bowser Basin shelf-type Gilbert deltas probably required relatively large drainage catchments, such as those associated with thrust ramps, fault transfer zones or blind thrusts. In one example, delta progradation direction was at a low angle (15 –27°) to the main structural trends and in this particular case we favour delta accumulation at a thrust fault transfer zone. In contrast, the stacked Gilbert deltas are inferred to be footwall deltas that accumulated close to an active, reverse fault where the stacking geometry indicates little or no basinward shift in the locus of deposition. Drainage basins were relatively small and were frequently rejuvenated. The Bowser Basin Gilbert deltas accumulated at the margin of a foreland thrust wedge, outboard of a major Early to Middle Jurassic, crustal-scale thrust system that carried oceanic Cache Creek terrane rocks in its hanging wall. Coarse-grained sediment, derived either from northwards erosional back-stepping of King Salmon Fault or from older Bowser Basin sediment, was cannibalized and redeposited in the Gilbert deltas and associated lithofacies. Either case provides a mechanism to account for the reworking necessary to produce the high clast sphericity and the monomict chert composition.

Pleistocene rapid uplift of the Himalayan frontal ranges recorded in the Kathmandu and Siwalik basins

Core-drilling project carried out in the southern margin of the Kathmandu Basin revealed that muddy debris flow deposits dammed up the Proto-Bagmati river to form the Paleo-Kathmandu Lake during the Jaramillo subchron from 1.07 to 0.97 Ma. Subsequent deposition of the alluvial fanglomerate, derived from the uplifting Mahabharat Range to the south, raised the dam deepening the lake-water. After 1 Ma, in the southern part of the basin, palaeo-current directions changed from southward to northward and deposition of gneissose and granitic detritus are replaced by meta-sediments derived from the Mahabharat Range. During the same time, at about 1 Ma, the boulder conglomerates were deposited on top of the Siwalik Group as piggy-back basins in front of an intra-basinal high, along the Main Dung Thrust in Nepal and NW India. Onset of movement of the Main Dung Thrust is dated back to 3 to 2.4 Ma [Mugnier, J.L., Huyge, P., Leturmy, P., Jouanne, F., 2003, Episodicity and rates of thrust-sheet motion in the Himalayas, Western Nepal. Am. Assoc. Petrol. Geol., Mem. 82, 1–24]. The Main Frontal Thrust is most active at present suggesting that imbricated structure of the Siwalik Group was formed by convergence of the Indian plate during the last 3 myr. The accretionary wedge of the Siwalik Group, stacked beneath the Main Boundary Thrust, might have started to jack up the frontal range of the Lesser Himalaya since 1 Ma. Coeval uplift and erosion of the frontal range of the Lesser Himalaya and the intra-basinal high in the Siwalik since 1 Ma are possible causes of an abrupt increase in both sedimentation rate and grain size of detrital quartz, and changes in composition of clay minerals, recorded in the sediments of the Bengal Deep Sea Fan at 0.9 Ma.

Carbonate deposition and hydrocarbon reservoir development at the Precambrian–Cambrian boundary: The Ara Group in South Oman

The Ediacaran–Early Cambrian Ara Group in the South Oman Salt Basin consists of six evaporite–carbonate cycles (A0/A1 to A6) that record tectono-eustatic sea-level changes. The A4 cycle developed at the Precambrian–Cambrian boundary (∼ 542 Ma). It forms a single depositional sequence with evaporites deposited in a lowstand systems tract (LST). Overlying carbonates represent an open-marine shallowing-upward ramp succession that developed in transgressive (TST) and highstand systems tracts (HST). The low-energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high. The TST facies include sulfates, evaporite–carbonate laminites, and organic-rich carbonate laminites that record initial flooding, deepening of the basin, and establishment of an outer ramp depositional environment. Carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity. Subsequent HST facies comprise mostly fine-grained clastic carbonates and stromatolites that formed in middle and inner ramp settings. These facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp. Sediment was redistributed in suspension, by muddy turbidity currents, muddy debris flows, storm and shallow-water currents. During the late HST, carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying LST evaporite unit. A combination of strong tectonic subsidence and transient flooding caused significant shallow-water evaporite deposition to occur not only down dip, but also on top of the former carbonate platform, where several hundreds of meters of evaporites accumulated. The transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit. The presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties. Distribution of organic material and carbonate mud can be linked to specific environmental conditions (low physical and biogenic disturbance of sediment and stratified water mass) and the sequence stratigraphic position (low flux of fine-grained carbonate during TST). In contrast, diagenetic evaporite formation has largely degraded reservoir quality in porous shallow-water facies near the top of the A4C.

Experiments on Reworking by Successive Unconfined Subaqueous and Subaerial Muddy Debris Flows

Debris flows occur under water, e.g., in lakes and the ocean, as well as under air. These two types of debris flow have many similarities, but also differ in striking ways. The focus of the research reported here is on the ability of an unconfined debris flow to remobilize or rework an antecedent deposit over which it runs. Experiments were performed in a tank in order to quantify reworking. The debris slurry was premixed and consisted of a mixture of water, sand, and kaolinite clay. Three experiments were performed, each consisting of four individual runs. In each run the slurry was released impulsively from a head tank. The first run of each experiment formed a deposit over the initial inerodible bed. The second, third, and fourth runs progressed over and reworked to a greater or lesser extent the antecedent deposit(s). Two experiments of four runs each were performed in the subaqueous configuration, and one experiment of four runs was performed in the subaerial configuration. In the subaerial case, reworking was immediate and extensive. In the subaqueous case, reworking of the deposit of the first run by the second run was suppressed. Part of the reason for this appears to be hydroplaning of the head of the debris flow. As the deposit built up, however, considerable reworking was eventually observed in the subaqueous case as well.