Mudstone Cap Research Articles

CBM sealing system and its relationship with CBM enrichment

It is of great significance to study the controlling effect of sealing systems on CBM enrichment in coalbed methane (CBM) exploration and development. In this paper, the relationships between CBM enrichment and geological elements were analyzed. The geological elements include sealing layers (e.g. regional cap rock, regional floor, and immediate roof and floor), later structural adjustment and strata production status. It is shown that CBM tends to enrich in the areas where regional mudstone cap rocks and floors are distributed stably, structures are uplifted and inversed slightly after the hydrocarbon accumulation period and the strata is gentle in a balanced state. Then, the concept of sealing system was put forward based on the worldwide CBM exploration and development practices over the years. A sealing system refers to a geological unit composed of a lateral stable zone and cap rock which prevents gas from migrating upward and downward. In a sealing system, CBM can get enriched and coal-measure gas can also be accumulated. Finally, three gas reservoir types (i.e., sandwich-type CBM reservoir, associated CBM-sandstone gas reservoir and coal-derived sandstone gas reservoir) were identified based on the configuration relationships between elements of the CBM (or coal-measure gas) sealing system. It is recommended to change the exploration ideas from simple CBM exploration to 3D CBM and coal-measure gas exploration. In addition, an evaluation index system of CBM (or coal-measure gas) geological selection was established. It is pointed out that good application effects may be realized if the stereoscopic CBM and coal-measure gas exploration is applied in the Junggar Basin and the eastern margin of the Ordos Basin.

Bleached mudstone, iron concretions, and calcite veins: a natural analogue for the effects of reducing CO2‐bearing fluids on migration and mineralization of iron, sealing properties, and composition of mudstone cap rocks

AbstractThis study investigates the Wangfu Depression of the Songliao Basin, China, as a natural analogue site for Fe migration (bleaching) and mineralization (formation of iron concretions) caused by reducing CO2‐bearing fluids that leak along fractures after carbon capture, utilization, and storage. We also examined the origin of fracture‐filling calcite veins, the properties of self‐sealing fluids, the influence of fluids on the compositions of mudstone and established a bleaching model for the study area. Our results show that iron concretions are the oxidative products of precursor minerals (pyrite and siderite) during uplift and are linked to H2S and CO2 present in early stage fluids. The precipitation of calcite veins is the result of CO2 degassing and is related to CO2, CH4, and minor heavy hydrocarbons in the main bleaching fluids. In our model, fluids preferentially enter high‐permeability fracture systems and result in the bleaching of surrounding rocks and precipitation of calcite veins. The infilling of calcite veins significantly decreases the permeability of fractures and forces the fluids to slowly enter and bleach the mudstone rocks. The Fe2+ released during bleaching migrates to elsewhere with the solutions or is reprecipitated in the calcite veins and iron concretions. The formation of calcite veins reduces the fracture space and effectively prevents fluid flow. The fluids have an insignificant effect on minerals within the mudstone. In terms of the chemistry of the mudstone, only the contents of Fe2O3, U, and Mo change significantly, with the content of U increasing in the mudstone and the contents of Fe2O3 and Mo decreasing during bleaching.

Hybrid Event Beds Generated By Local Substrate Delamination On A Confined-Basin Floor

Abstract The outer parts of deep-water fans, and the basin plains into which they pass, are often described as areas where erosion is negligible and turbidite systems have net aggradation. Nevertheless, sedimentological and stratigraphic analysis of outer-fan-lobe and confined-basin-plain deposits in Cretaceous–Paleocene Gottero Sandstone (NW Italy) has revealed extensive but cryptic bedding-parallel substrate-delamination features at the base of many sheet-like event beds. These comprise a variety of shallow but wide scour structures showing evidence of lateral expansion by sand injection. The scours commonly occur at the base of beds made up of a basal clean sandstone overlain by argillaceous sandstone containing abundant mudstone clasts and locally large substrate rafts (up to 20 meters long). These strata are interpreted as a type of hybrid event bed. Field observations suggest that mud-clast entrainment occurred by delamination at the base of dense sandy flows. The large rafts, in some cases only partly detached, were incorporated in the flows locally and then carried for short distances (hundreds of meters to a few kilometers) before partly disaggregating and undergoing deformation due to internal shearing. The development of such features may be common in flat and/or confined basin settings where high-volume flows interact with a cohesive and well layered substrate (e.g., muddy outer fans or confined or ponded basins with thick mudstone caps). Delamination is therefore suggested as an alternative mechanism leading to the formation of hybrid event beds following local substrate entrainment on the basin floor as opposed to on more remote slopes and at channel–lobe transition zones.

Hydrocarbon migration pathways in the Neogene of Laixi structural belt, southern Bohai Sea, China

Characteristics, distribution, temporal and spatial configuration, and effectiveness of hydrocarbon migration pathways have been investigated through geochemical and formation pressure analysis etc, to further reveal hydrocarbon accumulation patterns in the Neogene of the Laixi structural belt, southern Bohai Sea of China. Firstly, three factors, faults, carrier rocks (beds) and mud caprocks of the transporting framework in the Neogene were examined to find out its feature and space configuration and made static state evaluation. It is found that besides the Guantao Formation – lower Minghuazhen Formation No. V oil layers, the No. I – III oil layers in the lower Minghuazhen Formation are also the dominant oil and gas carrier beds. Cutted by the adjustment faults and sealed by mudstone capcocks, the two carrier layers are combined into a “layered cake” type hydrocarbon migration framework. Then the hydrocarbon passage pathways were tracked dynamically and described using nitrogen compounds concentration, formation pressure, fluorescence logging and sand bodies description, which shows that the “T03-T02-T01 mudstone cap layers” separate the Neogene hydrocarbon migration pathways into upper and lower units. The massive sandy conglomerate reservoirs in the Guantao Formation – lower Minghuazhen Formation No. V oil layers are the carrier bed of the lower hydrocarbon migration pathway, while the north-south trending overlapped and connected channel sandbodies in lower Minghuazhen Formation No. I – III oil layers are the carrier bed of the upper unit. Hydrocarbon migrated laterally along the two passage ways, accompanying with differential accumulation.

Permeability characteristics of mudstone cap rock and interlayers in bedded salt formations and tightness assessment for underground gas storage caverns

Permeability of nonsaline cap rock and interlayers is a key parameter for the assessment of the tightness of gas storage caverns in bedded salt formations. X-ray Diffraction, permeability tests, Scanning Electron Microscope studies and theoretical analyses have been performed for the mudstone cap rock and interlayers of a potential cavern in a bedded salt formation. The results show that the permeability of cap rock and interlayers is in the range of 10−18–10−20m2, whereas the interface in between salt and interlayer behaves as if impervious. Applied confinement conditions significantly affect the permeability. The higher the applied hydrostatic pressure, the lower the permeability. Permeability decreases more than one order of magnitude with hydrostatic pressure increases, up to a certain “compression threshold pressure”. Permeability remains virtually constant, at an extremely low magnitude, once the hydrostatic pressure exceeds this “compression threshold pressure”. The intrinsic reasons for the low permeability have been revealed by SEM studies, and are as follows: (1) the grains making up the bulk of the mudstone are very small and extremely tightly cemented; secondary minute clay minerals completely fill the pores and fissures between grains of quarts and feldspar, etc., resulting in very little residual void space and reducing connectivity for fluid penetration; and (2) the boundaries between quartz, feldspar and other grains are mainly plate-shape cracks that are poorly interconnected while the finer matrix is very tight and crack-free. The mechanical compaction investigation shows that the plate-shape cracks are much easier to be compacted than sphere-shape pores, which contributes significantly to the decrease in permeability. A capillary tube model suggests that permeability decreases very rapidly in the initial stages of compaction, but decreases extremely slowly in subsequent stages. So the permeability obviously behaves differently before and after the “compression threshold pressure”. By comparison with previous studies, the research we launched demonstrates that the cap rock and interlayers are characterized by extremely low permeability in compression regions. Hence the requirements of tightness (except for the possible presence of Excavation Disturbed Zones) are basically guaranteed. Also, a recommendation is expressed as: to ensure higher tightness and safety, reasonable design and operating programs should be adopted to reduce the EDZs as much as possible.

Formation conditions and exploration prospects of Sinian large gas fields, Sichuan Basin

Abstract Exploration potential of natural gas in the Sinian, Sichuan Basin, is evaluated based on research on the evolution of the Leshan-Longnusi palaeouplift and reservoir forming conditions such as sedimentary facies, source rocks, reservoirs and caprocks. The Sinian has a potential to form large gas fields in the Sichuan Basin: (1) Large-scale inherited palaeo-uplift provided conditions for oil/gas formation and accumulation. (2) A stable sedimentary environment guaranteed the extensive development of source rocks and reservoirs. (3) Deng-2, Deng-3 and Deng-4 members overlap each other and are widely developed. (4) Multiple source rocks overlay each other vertically and are distributed widely, superimposed between reservoirs like “sandwiches”. (5) The Sinian has good preservation conditions, regional mudstone cap rocks are thick and faults are not developed. Considering these favorable reservoir forming conditions, four favorable prospective areas were selected: Leshan-Longnusi Palaeouplift, Southeast Sichuan, East Sichuan, and Northwest Sichuan. Of them, the Leshan-Longnusi palaeo-uplift is the most favorable prospect, where the litho-stratigraphic gas reservoirs in the slope deserve further exploration.

Studies on Main Factor Coupling and Its Control on Coalbed Methane Accumulation in the Qinshui Basin

There are many factors controlling coalbed methane (CBM) accumulation. So, it is fundamental for CBM exploration and development to identify these factors and investigate their control mechanism. Through statistical analysis of experimental and field test data, the basin tectonic evolution, sedimentary environments and hydrodynamic conditions are comprehensively analyzed, and the gas control mechanism of three main factors (structure, sedimentation and hydrodynamic force) is discussed. Results show that the tectonic evolutions complicated the process of CBM reservoir formation. Multi-episode tectonic stress fields with different features reform the physical properties of coal seams that make areas with high structural principal stress difference convert to high permeability reservoir areas. The mudstone cap rock develops well in shallow water delta system. The coal seams are thick and distributed stably in deltaic plain swamp, and their abundant vitrinite, micropores and strong adsorption capacity are favorable for CBM accumulation. Appropriate hydrodynamic condition may carry secondary biological gas into the gas reservoir to supply gas source, and it is also the main force of methane carbon-isotope fractionation during CBM drainage and production. The methane carbon-isotope fractionation is available to identify the yield stability of CBM well.

Multiphase tectonic movements, cap formations and evolution of the Majiang paleo-reservoir

The Majiang paleo-reservoir is a typical destroyed hydrocarbon reservoir, buried in carbonate strata of China’s southern marine-facies. Field geological explorations, interpretations of seismic profiles and balanced cross-section restorations around this paleo-reservoir reveal that its formation and evolution have been restricted by multiphase tectonic movements of different intensities. A regional tectonic mechanism and model have been suggested for the formation and evolution of the Majiang paleoreservoir. Geological field exploration has been carried out along three typical Silurian cross-sections and rock samples were tested in combination with water-rock interaction. Based on the result of cap tests, the planar distribution, the residual thickness, the erosion thickness and the preservation conditions, the Silurian mudstone cap is discussed around the Majiang paleo-reservoir. Combining the hy drodynamic conditions of its formation and evolution and its tectonic movements, we determined the fact that the thicker the cap is, the more resistant it is to hydrodynamic destruction. The multi-phase formation and destructive geological model of the paleo-reservoir is established through an overall analysis of multiphase tectonic evolutions, cap developments, hydrodynamic conditions and solid mineral metallogenic ages measured by Rb-Sr, Pb and Sm-Nd isotope techniques.

Regional Differences in Geological Conditions Related to Reservoir-forming in the Foreland Fold and Thrust Belt of Southwest Tarim Basin

The foreland fold and thrust belt of southwest Tarim basin can be divided into four structural zones, and different tectonic sectors have quite different geological conditions related to reservoir-forming. Most favorable locations are Pusha–Keliyang zone and Sugaite–Qimugen zone, not only with many fault-propagation folds, triangle-zone structures, and three sets of excellent source rocks including Lower–Middle Cambrian carbonate, Lower Carboniferous–Lower Permian, and Jurassic dark mudstone but also with a series of good petroleum reservoir and seal combinations such as Lower-Cretaceous sandstone reservoirs associated with Paleogene cap rocks, Paleocene carbonate reservoirs, and Miocene sandstone reservoirs associated with their corresponding mudstone cap rocks, respectively. The second favorable location is Hotan thrust nappe zone with some good tectonic traps including traps of par-autochthonous system and in-situ system, and Lower-Permian mudstone as a set of high quality regional cap rocks, but their source and reservoir are relatively bad, that is, Lower-Permian carbonate source rocks just meet the criteria of gas source rocks, and Lower-Permian sandstone or carbonate reservoirs and Lower-Ordovician carbonate reservoirs are all low-porous and low-permeable in spite of Lower-Permian Pusige mudstone as excellent regional cap rocks. The last, Pamir thrust zone might not be favorable for reservoir-forming due to its large-scale nappes and distinctly-exposed thrust faults which result in the lack of good tectonic traps although there are some advantageous petroleum source-reservoir-seal combinations.

Rapid hydrocarbon accumulation mechanism in later period in Kelasu thrust belt in Kuqa depression

Based on the analyses of generation, migration and accumulation of oil and gas in the structures of Kela 1, Kela 2 and Kela 3 in Kasangtuokai anticlinal belt using a series of geological and geochemical evidence, this paper proposes that the rapid rate of hydrocarbon generation, main drain path for overpressured fluid flow and converging conduit system are indispensable conditions for the rapid, late-stage gas accumulation in the Kelasu thrust belt in the Kuqa depression. Due to structural overlapping and the resultant rapid burial, the maturity of the source rocks had been increased rapidly from 1.3 to 2.5% Ro within 2.3 Ma, with an average rate of Ro increase up to 0.539% Ro/Ma. The rapid maturation of the source rocks had provided sufficient gases for late-stage gas accumulation. The kelasu structural belt has a variety of faults, but only the fault that related with fault propagation fold and cut through the gypsiferous mudstone cap could act as the main path for overpressured fluid release and then for fast gas accumulation in low fluid potential area. All the evidence from surface structure map, seismic profile explanation, authigenic kaolinite and reservoir property demonstrates that the main drain path related with faults for overpressured fluid and the converging conduit system are the key point for the formation of the giant Kela 2 gas field. By contrast, the Kela 1 and Kela 3 structures located on both sides of Kela 2 structure, are not favourable for gas accumulation due to lacking converging conduit system.

Controlling factors for large gas field formation in thrust belt of Kuqa coal derived hydrocarbon foreland basin

Kuqa depression is a foreland basin developed with Mesozoic-Triassic-Jurassic coal-bearing formation. The research results of the coal-derived hydrocarbon foreland basins in Kuqa depression indicated that the coal-bearing formation can be the rich sources for generating gas because of their thickness and rich source rocks with gas-generating predominant kerogen. Although the foreland thrust belt mainly acting in compression is very complicated, integral large structural traps can be formed. Moreover, the thrust belt can act as the passage for communication with deep source rocks. The high quality gypsolish and gypseous mudstone cap rock developed in the upper formation is the key for the formation of the large gas field. The late formation of reservoirs in the large gas fields depended on the hydrocarbon-generating history controlled by the foreland basin and the developing process of foreland thrust belt.

ABSTRACT: Controls on the Variability of the Turbidite Systems in the Ainsa Slope Complex (South-Central Pyrenees, Spain)

The Ainsa slope complex developed during the lower and middle Eocene in a foredeep to piggy-back setting in south-central Pyrenees. It is preserved as a 40 Km long, 30 Km wide and 4000 m thick, mudstone-dominated clastic wedge, which includes coarser grained turbidite lithosomes. The latter correspond mostly to channel fills and associated overbank and splay deposits. Our research in the Ainsa slope complex has produced a stratigraphic and structural dataset resolving from the basin fill to the reservoir scale. From the analysis of this dataset we offer a scheme of stratigraphic subdivision which covers the entire range of scales mentioned above plus tabulated parameters and related graphs for each of the elements in the scheme. The basin fill is first divided into four major depositional cycles (2-5 m.y. each) which are bounded by unconformities and occur stepped several kilometers towards the foreland due to the propagation of the thrust front. Within a major depositional cycle, several turbidite systems occur which are bounded below by unconformities and above by a mudstone cap. From system to system, in a younging direction, there is an increase in N/G and reductions on stratigraphic thickness, amount of forelandwards migration of system axis, volume of bypass facies and maximum sediment caliber. These changes can all be explained by an overall decrease in gradient from base to top within a major depositional cycle and fit into a model characterized by progressively reduced rates of forelimb rotation along thrustrelated frontal anticlines.

Contained turbidites used to track sea bed deformation and basin migration, Sorbas Basin, south‐east Spain

ABSTRACT The mechanisms driving subsidence in late orogenic basins are often not easily resolved on account of later fault reactivation and a rapidly changing stress field. Contained turbidites in such basins provide a unique opportunity of monitoring sea bed deformation and evolving bathymetry and hence patterns of subsidence during basin filling. A variety of interpretations have been proposed to explain subsidence in Neogene basins in SE Spain, including extensional, strike‐slip and thrust top mechanisms. Ponded turbidite sheets on the floor of the Neogene Sorbas Basin (SE Spain) were deposited by sand‐bearing currents which ran into enclosed bathymetric deeps where they underwent rapid suspension collapse. The structure and distribution of these sheets (and the thick mudstone caps which overlie them) act as a proxy for the containing sea bed bathymetry at the time of deposition. An analysis of the sheet architecture helps identify a trough‐axial zone of syndepositional faulting and reveals a westwards stepping of the ponding depocentre with time. Fault breaks at the sea bed influenced the position of flow arrest and the distribution of sandstone beds on the basin floor. Westward stepping of the deeper bathymetry was episodic and probably controlled by transverse faults. Re‐locations of the depocentre were accompanied by the destabilization of carbonate sand stores on the margins of the basin, resulting in the repeated emplacement of large‐volume carbonate megabeds and calciturbidites. The fill to the Sorbas Basin was shingled by the onset of compression in the east attributed to transfer of slip between intersecting strike‐slip fault strands. A sinistral fault (a splay of the Carboneras Fault System) propagated through the evolving basin fill from the east as the eastern part of the basin became inverted and the locus of subsidence migrated into the Tabernas area 20 km area to the west. The sedimentological analysis of the basin fill helps see through a late dextral overprint which ultimately juxtaposed basement rocks to the south against the inverted and upended basin, along a late slip‐modified unconformity. Conventional palaeostress analysis of fractures along the basin margin fails to see past this late dextral shearing event. Basin migration parallel to the E–W‐orientated basin axis, slip‐reversal (sinistral to dextral) and the active involvement of strike‐slip faults are now identified as important aspects of the evolution of the Sorbas Basin during the latestTortonian.

Evolving turbidite systems on a deforming basin floor, Tabernas, SE Spain

The Tabernas‐Sorbas basin was a narrow, east‐west trending, marine trough of Late Miocene age. Sediment gravity flow deposits dominate the basin fill and provide a record of changing bathymetry in response to tectonically induced sea bed deformation. A reanalysis of the western end of the basin in the vicinity of Tabernas establishes an upward evolution involving: (1) sand‐starved marls that were incised by axial channels recording a period of bypass, during which sand deposition took place in a depocentre further to the east; (2) punctuated infilling of the incisions, locally by high‐sinuosity embedded channels. Channel filling is related to a gradient reduction, which presaged collapse of the axial slope as the depocentre began to migrate westwards into the Tabernas area; (3) draping of the earlier incision fills by laterally extensive sheet turbidites, which were initially contained in structurally controlled depressions. These ‘deeps’ opened up as active faults propagated through the former axial slope. Flow containment is inferred on account of the unusual structure of the sheet sandstone beds, complex palaeoflow relationships and thick mudstone caps; (4) fault‐controlled topography was subsequently healed, and further sheet turbidites showing evidence of longer range containment and progressive slope onlap were emplaced. These record mixed supply from both seismically trigged ‘axial’ failures and a reactivated, fault‐controlled slope building out from the northern margin of the basin. Flows traversing the trough floor were strongly reflected off slopes marking the southern limit of the basin. The studied succession is capped by (5) the Gordo megabed event, a large, probably seismically triggered, failure which blanketed the basin floor, demonstrating an enlarged but still contained basin now devoid of significant intrabasinal fault topography. Tectonics played a key role in driving the evolution of the turbidite systems in this basin. Deformation of the basin floor had an important impact on gradients, slope stability, bathymetry and the ability of flows to bypass along the trough axis. Westward migration of the depocentre into the Tabernas area led to a change from incision and bypass to conduit backfilling to flow containment, as fault‐induced subsidence generated a ‘sump’, which trapped flows moving along the basin axis.

Palaeokarst development in a lower Frasnian (Devonian) platform succession, Canning Basin, northwestern Australia

Recognition of palaeokarst in the oldest exposed Devonian (Givetian ‐ lower Frasnian) platform successions of the Canning Basin reef complexes has eluded investigators for over forty years. The first evidence for palaeokarst, developed on microbial mud‐mounds in a single stratigraphic horizon, is documented and records an episode of exposure during early carbonate platform development. Surface palaeokarst features are scalloped surfaces, solution pits and a pipe, underlain by fenestral limestone with sediment‐filled fossil moulds and vugs. The platform succession has variably developed metre‐scale cycles which are composed predominantly of shallowing‐upward subtidal facies, with some cycles having fenestral peloidal mudstone caps. Changes in facies type and stratigraphic arrangement up the succession define two deepening‐upward units (∼70 and 180 m thick), with the palaeokarst surface representing emergence following rapid shallowing at the top of the lower unit. The stratigraphic position of the palaeokarst between these two units suggests it may represent a sequence boundary. This may have been caused by a low‐magnitude eustatic fall or footwall‐uplift event superimposed on a rapidly subsiding basin margin.

Statistical analysis of facies clustering in submarine-fan turbidite successions

ABSTRACT The Hurst statistic, converted easily into a fractal dimension, quantifies the degree of clustering of high and low values in spatial or temporal series of data. In an effort to better understand the stratigraphic organization of submarine-fan deposits, the Hurst statistic was calculated for nineteen turbidite sections that span a wide range of geological time, tectonic settings, facies characteristics, and depositional environments. Three bed-by-bed variables were considered: coarse-division thickness (i.e., net thickness, in a single bed, of conglomerate + sandstone + siltstone), grain size, and coarse-division thickness percentage (relative to thickness of overlying mudstone caps). Sixteen of the 19 turbidite sections analyzed (84.2%) show the Hurst phenomenon, i.e., irregular, long-term clustering of high and low values of the three bed-by-bed variables. This clustering is related to stratigraphic changes of sedimentary facies caused by lateral shifting of sub-environments on a submarine-fan surface. There is no evidence that the clustering is controlled by sea-level fluctuations. The strength of the clustering, inferred from Hurst K values, holds promise as an index to distinguish submarine-fan sub-environments. Preliminary results suggest that channel-levee deposits tend to have strong clustering; lobe-interlobe deposits tend to display moderate clustering; and basin-floor sheet-sand systems tend to have weak clustering. Combining facies characteristics with these statistical results provides more reliable criteria for the identification of submarine-fan sub-environments than are available from recently discredited models based on asymmetric cycles of bed thickness or grain size.

Deposits of deflected and ponded turbidity currents, Sorbas Basin, Southeast Spain

ABSTRACT ?Serravalian-Tortonian sediments of the Sorbas Basin, Southeast Spain, preserve an oblique slice through a narrow east-west-trending turbidite basin. Close to Lucainena, unusual sandstone sheets with thick mudstone caps can be traced obliquely away from the faulted southern margin of the trough in the vicinity of where north-to-south transverse paleoflows are replaced eastward by west-to-east axial flows. Three distinctive types of bed profile are recognized. Lateral correlations over distances up to 5 km establish that Types I, II, and III bed profiles are laterally equivalent and must have been emplaced together as part of the same event. Beds with Types I and II profiles are present to the WSW, where they thin and pinch out toward the contact with the Alhamilla basement to the south. As beds are followed along strike to the ENE, they thicken and develop Type III profiles before thinning again and passing laterally into bioturbated marls. The structure and lateral relationships are thought to have been produced by deposition from turbidity currents derived from the north and northwest but deflected against a raised Alhamilla block and forced to flow eastward. The composite structure of the beds close to the Alhamilla basement is related to deceleration and subsequent collapse of flows that partly rode up the southern flank of the basin as they were turned to travel eastward down the basin axis. The thick mudstone caps represent fallout from a suspension cloud produced by axial ponding of the muddy tail of the deflected currents (suggesting that the basin was closed to the east); internal seiches set up by ponding of the flows may account for some of the features of Type II profiles.

Turbidite sedimentation during Alpine thrusting: the Taveyannaz sandstones of eastern Switzerland

ABSTRACTThe Taveyannaz sandstones of eastern Switzerland are a succession of turbidites found within the Tertiary North Helvetic Flysch system; they represent a portion of the early, underfilled stage of the North Alpine Foreland Basin. The Taveyannaz sandstones were deposited in two sub‐basins (Inner and Outer basins) separated by a topographic high trending ENE‐WSW (parallel to the subsequent structural strike of the region), interpreted as an emergent thrust tip that propagated into the basin. The southerly Inner basin is therefore considered as a ‘piggy‐back’basin comprising a 140 m thick succession dominated by approximately 12 very thick bedded sandstones with thick mudstone caps; these very thick bedded sandstone‐mudstone couplets are interpreted as having resulted from the ponding of megaturbidite flows in the topographically confined Inner basin. Intercalated with the very thick bedded sandstones are thin to medium bedded sandstones. The Outer (northerly) basin comprises at least 240 m of turbidites characterized by sandstone packets (5–50 m thick) with extensive amalgamation of beds and a dominantly symmetrical vertical bed thickness and grain size profile. Intercalated between the sandstone packets are laminated graded siltstones and mudstones.The Inner basin sediments underwent localized deformation on the sea floor, generating an irregular surface topography which was then capped by a mud sheet emplaced by superficial sliding. During the emplacement of the mud sheet, large sandstone blocks (up to 130 m across) were incorporated from the underlying succession. The resultant geometry of the upper surface of the Inner basin sandstones exhibits vertical walls which truncate, and are perpendicular to, the underlying beds.The depositional style and structural control of the Taveyannaz sandstones, in association with the emplacement of superficial mud sheets, reflect processes that are highly analogous to those occurring in modern accretionary wedge environments.The sandstone packets of the Outer basin reflect a cyclical pattern of sedimentation alternating between deposition of sandstones and mudstones. The autocyclical or allocyclical controls on these high frequency alternations are difficult to interpret; likely mechanisms include lobe switching, climatic variations, eustatic sea level fluctuations and changes of horizontal in‐plane deviatoric stress on the lithosphere. In this example, an alternative mechanism is speculated upon. This is based on the analogy with accretionary wedge processes. In this hypothesis, it is proposed that high frequency fluctuations in the accommodation space available on the shelf may result from fluctuations in the topographic slope of an accretionary wedge around its critical taper. Hence, during periods of accelerated frontal accretion, the taper angle of the thrust wedge becomes subcritical resulting in a broad, low angle topographic slope and increased shelfal accommodation. Consequently, sediment becomes trapped in a relatively landward position. The necessary rejuvenation of the surface slope of the thrust wedge to a critical taper is achieved through internal reactivation resulting in tectonic uplift and hence a relative fall in sea level; this leads to the reworking of sediment to the base of slope or outer trench. Repeated alternations of relative sea level between a subcritical highstand and a supercritical lowstand are considered to be sufficient to generate the observed alternations between sandstone and mudstone packages in the turbidite basin.

Contained (reflected) turbidity currents from the Middle Ordovician Cloridorme Formation, Quebec, Canada: an alternative to the antidune hypothesis

ABSTRACTThe parautochthonous Cloridorme Formation is a syn‐orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle‐fan, outer‐fan, and basin‐floor deposits. The basin‐floor deposits (about 1.5 km thick) are confined to members β1, β2 and γ1, and are characterized by graded, thick (1–10 m) mud‐rich calcareous greywacke beds previously interpreted as deposits of concentrated, muddy, unidirectional turbidity currents that locally generated backset (antidune) lamination in internally stratified flows.The dominant flow directions were from east to west, but west to east transport also occurred, as seen in the orientation of ripples, climbing ripples, flutes, consistently overturned flames, and grain imbrication. We believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180° one or more times during deposition of the lower sandy part of the beds. Flow reversals are consistent with the sharp grain‐size breaks and mud partings within sandy divisions. Measurement of grain fabric relative to stratification in the most celebrated ‘antidune’ bedforms indicates flow from west to east; thus, the bedforms were produced by west‐to‐east migration of megaripples, not by the upcurrent migration of antidunes.The thick muddy beds were deposited by large‐volume, muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor, much like the ‘Contessa’ and similar beds of the Italian Apennines. Large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed. Because of their mode of emplacement, we propose that these beds be called contained turbidites.

The Origin of Red Beds: A Study of the Conditions of Origin of the Permo-Carboniferous and Triassic Red Beds of the Western United States

Previous articleNext article FreeThe Origin of Red Beds: A Study of the Conditions of Origin of the Permo-Carboniferous and Triassic Red Beds of the Western United StatesC. W. TomlinsonC. W. Tomlinson Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 24, Number 3Apr. - May, 1916 Article DOIhttps://doi.org/10.1086/622326 Views: 34Total views on this site Citations: 3Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:X. R. Ming, L. Liu, M. Yu, H. G. Bai, L. Yu, X. L. Peng, T. H. Yang Bleached mudstone, iron concretions, and calcite veins: a natural analogue for the effects of reducing CO 2 -bearing fluids on migration and mineralization of iron, sealing properties, and composition of mudstone cap rocks, Geofluids 16, no.55 (Nov 2016): 1017–1042.https://doi.org/10.1111/gfl.12203Jarmila Slánská A red-bed formation in the South Bohemian Basins, Czechoslovakia, Sedimentary Geology 15, no.22 (Feb 1976): 135–164.https://doi.org/10.1016/0037-0738(76)90041-5E. Bölau Rote Tone im Rhät-Lias Schonens, Geologiska Föreningen i Stockholm Förhandlingar 76, no.22 (Jan 2010): 215–233.https://doi.org/10.1080/11035895409453538