Braid Plain Research Articles

Integrated analysis for constraining palaeoclimatic and volcanic influences on clay–mineral assemblages in orogenic basins (Palaeogene Andean foreland, Northwestern Argentina)

Variations in clay–mineral assemblages in ancient continental deposits are frequently used to reconstruct past climate changes. In active settings, volcanic events can supply highly labile volcaniclastic material, which can easily be transformed into smectite via diagenesis, which can produce a noticeable footprint in clay–mineral assemblages. Southern Central Andean foreland deposits are appropriate case studies to ascertain whether the climatic signal was preserved in the clay assemblages of their fine-grained sediments as tectonic uplift, volcanism, and sedimentation have been interacting since the Cretaceous. We have studied a 1400-m-thick coarsening-upward Palaeogene succession of the Tin Tin basin (northern Calchaquí Valley, Argentina), applying X-ray diffraction (XRD), electron microscopy, and detailed sedimentary facies analysis with the aim of comparing tendencies in the vertical fluctuations of clay minerals with evidence from sedimentological facies. Illite–muscovite plus smectite account for 78% to 100% of the clay minerals in the fine fraction, with kaolinite and chlorite in subordinate amounts. The vertical variation of sedimentary settings from an overbank/lacustrine domain to fluvial braided plains and an aeolian dune field suggests a gradual increase in aridity upsection. However, smectite abundances do not show a gradual decreasing trend compatible with progressively lower hydrolyzing conditions; their relative abundances vary widely throughout the section, depicting pulse-like, abrupt fluctuations. Despite the absence of field evidence for volcanic influence, several indications of volcanic and volcaniclastic material have been found under scanning electron microscopy (SEM) in levels with high smectite abundances from the middle to the top of the succession. They include quartz crystals showing embayments and skeletal forms, with smectite filling the voids, microcrystalline silica, as well as heulandite crystals in close association with authigenic smectite. The XRD analyses of these levels evidence well-crystallized smectite, which is characteristic of a volcaniclastic origin. Therefore, the increase in smectite abundance in these beds reflects a significant volcaniclastic contribution, which is also evidenced by a centimetre-thick ash layer topward in the sequence. The only smectite-rich level near the base of the Tin Tin section also contains well-crystallized smectite associated with heulandite, thus probably evidencing volcaniclastic input. We infer that most of the smectite in these sediments formed during early diagenesis, probably through the dissolution of labile tuffaceous material. Textural and morphological analysis by SEM is essential to determine whether clay–mineral assemblages could be interpreted in terms of palaeoclimate.

Stratigraphic analysis and paleoenvironmental implications of the Wijchen Member in the lower Rhine-Meuse Valley of the Netherlands

AbstractThe Late Pleistocene Wijchen Member (WM) and its informal stratigraphic precursors have been recognized for decades in the Rhine-Meuse Valley of the Netherlands. Although the WM marks the top of the Kreftenheye Formation (KF) at the boundary between Pleistocene and Holocene lithofacies and provides a confining bed for the regional alluvial aquifer, significant issues remain regarding WM depositional environment and processes of sedimentation. Regional WM chronology suggests a time-transgressive, millennium scale response of the Rhine River to Lateglacial climate oscillations. This paper compares interpretations of sedimentation process, stratigraphic pattern, and paleoenvironmental significance to prevailing viewpoints on the WM mode of origin.A flood basin in the Over Betuwe between the channel belts of the Neder Rijn and River Waal is investigated to characterize WM stratigraphy. The KF braided stream deposits (Kb) form a regionally extensive sandy to gravelly lithofacies. As Kb aggradation ceased, fluvial channels incised into local braid plain swales. The WM was deposited during episodes of fluvial activity as a suspended load mud drape across segments of the abandoned braid plain. The WM is a gray silty lithofacies that also contains local admixtures of sand. Explanations for the origin of the sand admixed into the mud include variability in hydrodynamic load across the flood plain, eolian mixing, and/or biogenic mixing. In the study area, eolian deposition of sand onto a wet flood plain surface is the most probable cause for the admixed sand fraction. Pedogenesis of the WM in the study area is limited to gleying under reduced wetland conditions and the development of organic rich vegetation horizons that formed on top of relatively unaltered fluvial strata. Similar reduced soil properties and limited pedogenic development occur downdip to the present coast, but updip of the study area, the WM is the parent material for poorly drained to well drained and oxidized profiles that range from Entisols to weakly expressed Alfisols.The presence of pumice granules in Kb deposits of the study area indicate that channel belt deposition continued after the Laacher See volcanic eruption in Germany at ~12,900 cal yr. Deposition of the WM occurred episodically throughout the Lateglacial and terminated by the early Holocene. The time interval between the end of WM deposition and subsequent burial by flood basin peat reflects a duration of exposure of at least 3500 yrs. Since regional water table rise affected the area ~5000 cal yrs ago, the early Holocene water table must have been maintained by spring fed ground water sources from nearby ice pushed ridges.Deposition of the WM is associated with transitional braided to meandering fluvial channels during times when the Rhine-Meuse Valley experienced a sensitive response to rapid climate change. The WM is regionally time transgressive and probably formed during flood plain transitions between permafrost and base-flow driven hydrologic regimes. Regional landscape dynamics suggest that WM deposition and subsequent preservation was driven by fluctuations of the southern limit of permafrost during Northern Hemisphere deglaciation.

Application of a 3D laser scanner in the assessment of erosion and deposition volumes and channel change in a proglacial river

AbstractPrevious studies of channel morphological change have often relied upon sparse spatial and temporal data‐sets, resulting in a degree of uncertainty in their conclusions. Furthermore, previous methodologies have been unable to quantify small‐scale changes in channels composed of sand and fine gravel due to inadequate data density. This study reports the use of a high‐resolution 3D laser scanner (LMS‐Z210) in the assessment of erosion and deposition volumes in the proglacial zone of Glacier du Ferpècle and Mont Miné, Switzerland. High‐resolution (>500 points/m2) data obtained across a 5881 m2 area of braid plain during the early part of the melt‐water season permitted digital elevation models (DEMs) of bar surfaces and channels to be produced. The approach negates the need for complex surface interpolation algorithms required in surveys with lower point density, e.g. those obtained from global positioning systems or total stations. Furthermore, the technique has the advantage of being able to cover a large area over a comparatively short time period. Laser scanner returns revealed high vertical precision, ±0·02 m, for dry bar surfaces; however, submerged areas (<0·2 m depth) returned lower precision, with a range of −0·15 to +0·06 m. To account for this, different levels of detection (LoD) were applied to the dry and wet parts of the study reach during DEM subtraction. Subtraction of successive DEMs revealed two short depositional episodes over the ten‐day study, with erosion dominating in between. Three episodes of avulsion were identified, two of which appeared to be triggered by constriction avulsion, whilst the other was initiated by bar‐edge erosion. Transient lobes and sediment sheets could also be identified at the downstream end of the reach, as could lobe progradation. Relatively minor, but common, changes in channel morphology could also be detected, such as bar edge accretion, bank erosion and chute development. In an analysis of the effects of survey frequency upon volumes calculated following DEM subtraction, daily surveys were found to increase erosion volumes by 67 per cent and deposition volumes by 14 per cent when compared with an 8‐day survey interval. Copyright © 2007 John Wiley & Sons, Ltd.

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures. Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.

Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene boundary

A prominent increase in atmospheric CO, at the Paleocene-Eocene boundary, ca. 55 Ma, led to the warmest Earth of the Cenozoic for similar to 100 k.y. High-resolution studies of continental flood-plain sediment records across this boundary can provide crucial information on how the hydrological cycle responds to rapidly changing CO2. Here we show from continental records across the Paleocene-Eocene boundary in the Spanish Pyrenees, a subtropical paleosetting, that during the early, most intense phase of CO2 rise, normal, semiarid coastal plains with few river channels of 10-200 m width were abruptly replaced by a vast conglomeratic braid plain, covering at least 500 km(2) and most likely more than 2000 km(2). This braid plain is interpreted as the proximal parts of a megafan. Carbonate nodules in the megafan deposits attest to seasonally dry periods and together with megafan development imply a dramatic increase in seasonal rain and an increased intra-annual humidity gradient. The megafan formed over a few thousand years to similar to 10 k.y. directly after the Paleocene-Eocene boundary. Only repeated severe floods and rainstorms could have contributed the water energy required to transport the enormous amounts of large boulders and gravel of the megafan during this short time span. The findings represent evidence for considerable changes in regional hydrological cycles following greenhouse gas emissions. (Less)

Sedimentological response to neotectonics and sea-level change in a delta-fed, complex graben: Gulf of Amvrakikos, western Greece

The Gulf of Amvrakikos is one of several E–W grabens developed in the Middle Quaternary almost orthogonal to the tectonic grain of the external Hellenides in western Greece and is critically located at the termination of the Kefallinia transform fault linking the Hellenic subduction zone to the Adriatic-Dinaride collision lineament. The tectonic evolution of the Gulf and the response of sedimentation have been investigated based on 3.5 kHz and sparker profiles. The Holocene stratigraphy results from rising eustatic sea-level breaching the tectonic sill north of Preveza and inundating the braid plain of the Arachthos river, which then prograded across the northern Gulf following the formation of the maximum flooding surface. Older major transgressive surfaces are recognised in sparker profiles. Interpretation of these transgressions in terms of eustatic sea-level changes allows a chronology to be established and estimates to be made of neotectonic rates of subsidence. Two sets of neotectonic faults are present: reactivated NNE-trending mid-Tertiary thrusts of the Ionian zone and more active E–W trending faults. The sedimentary succession in the Gulf of Amvrakikos is the result of a complex interplay between eustatic sea level changes, the neotectonic creation of accommodation, and autocyclic delta distributary switching. The details of this process are resolved for the Holocene and the general sedimentation pattern established back to marine isotope stage (MIS) 8 (245–300 ka). In general, MIS 8 shows greater evidence of terrestrial deposition, compared with MIS 6 (130–190 ka) and especially MIS 2–4 (10–80 ka), when there was lacustrine deposition and some marine incursions at intermediate eustatic sea levels. This implies that regional subsidence has been taking place at least since MIS 8. In addition, changing sediment supply plays a role in overall sediment architecture, but is only well resolved for the Holocene, with higher rates of fluvial supply recognised in the mid Holocene.

Sedimentology and stratigraphy of a transgressive, muddy gravel beach: Waterside Beach, Bay of Fundy, Canada

AbstractSediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal‐creek braid‐plain and delta; and, (3) wave‐formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward‐dipping terrace of sandy middle and lower intertidal, and muddy sub‐tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal‐creek braid plain and delta. Wave‐formed sand and gravel bars and associated deposits include: sediment sourced from low‐amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward‐migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud‐laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast‐ and matrix‐supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer‐grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal‐creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross‐bedded sand and gravel.

A complex subglacial clastic dyke swarm, Sólheimajökull, southern Iceland

A complex network of clastic dykes dissects loosely consolidated Holocene sediments along the east flank of the proglacial braid plain of Sólheimajökull, southern Iceland. The dykes comprise downward-bifurcating intrusions up to 0.5 m thick and several metres in length and are intruded into glaciogenic deposits (sandy gravel, gravelly sand, interlaminated silt and sand, and diamicton). The dykes were sourced from a clast-poor sandy diamicton, interpreted as a subglacial till, and were intruded downwards beneath Sólheimajökull glacier during a previous phase of advance. As the glacier advanced southwards, it loaded the sediment column resulting in the intrusion of dykes with a consistent south–southwest dip (with rare northward-dipping examples). The dyke fills are characterised by laminated sediment, with laminae oriented parallel to the dyke margins and comprise interlaminated clay, silty clay, silt, sand, sandy gravel and diamicton. In some dykes, high concentrations of pebble- to boulder-sized clasts occur in association with rotated pods of the laminated sediment. The laminae are thought to have evolved by a slow, long-lived intrusion process that involved the repeated fracture and expansion of the host sediments followed by viscous smearing-on of subglacial material onto the dyke walls, rather than rapid injection of fluidised sediment.

Late Pleistocene braided rivers of the Atlantic Coastal Plain, USA

Infrared Landsat imagery (band 4) clearly reveals braided river patterns on late Pleistocene terraces of unglaciated rivers in the Atlantic Coastal Plain of the southeastern United States, a region that presently exhibits meandering patterns that have existed throughout the Holocene. These Pleistocene braided patterns provide a unique global example of river responses to late Quaternary climate changes in an unglaciated humid subtropical region at 30–35° north latitude. Detailed morphological and chronological results are given for the Oconee-Altamaha River valley in Georgia and for the Pee Dee River valley in South Carolina, including 15 optically stimulated luminescence (OSL) dates and four radiocarbon dates. Correlative examples are drawn from additional small to large rivers in South- and North Carolina. OSL and radiocarbon ( 14C) dates indicate distinct braiding at 17–30 ka, within oxygen isotope stage 2 (OIS 2), and braiding probably existed at least during parts of OIS 3 and possibly OIS 4 back to ca 70 ka. The chronology suggests that braiding is the more common pattern for the late Quaternary in the southeastern United States. Braided terraces appear to have been graded to lower sea-levels and are onlapped by Holocene floodplain deposits up to 10–60 km from the coast. The braiding probably reflects the response of discharge and sediment yield to generally cooler and drier paleoclimates, which may have had a pronounced runoff season. Sedimentation of eolian dunes on the braid plains is coeval with braiding and supports the conclusion of dry soils and thin vegetation cover during the late Pleistocene. Our chronological data contribute to a body of literature indicating that reliable OSL age estimates can be obtained from quartz-rich bed load sand from braided rivers, based on good correlations with both radiocarbon dates from braided fluvial sediment and OSL dates from stratigraphically correlative eolian sand.

Tide-influenced sedimentation in a rift basin—Cretaceous Qishn Formation, Masila Block, Yemen: A billion barrel oil field

The Lower Cretaceous Qishn Clastics Member in Masila Block 14, Hadhramaut region, Republic of Yemen, has estimated reserves of 1.1 billion bbl recoverable oil and has produced 600 million bbl of oil. Sedimentation took place in an elongate paleogulf of the Say'un-al Masila basin, with open-marine carbonates to the east. The Qishn Clastics Member unconformably overlies mixed carbonates and clastics of the Sa'af Member. Lower Qishn onlap resulted in deposition of brackish and tidal (likely macrotidal) estuarine to open-bay or gulf deposits. The middle portion of the lower Qishn Clastics Member shows evidence of arid nonmarine sedimentation, including debris-flow deposits, red beds, and shale-clast conglomerates, in turn, overlain by interfingering coastal and nonmarine deposits. The lower Qishn Clastics Member is truncated by a sequence boundary overlain by an extensive sandstone deposited in a low-accommodation braid plain close to the shoreline. A flooding surface is present at the top of the sandstone, over which progradational, tide-influenced deltaic strata were deposited. Delta progradation culminated in shallow-water clastic dolomitic deposits on the coastal plain. With subsequent transgression, a rising water table caused a nonmarine flooding surface with lakes and lacustrine deltas, overlain by tidal-flat deposits. Ongoing transgression resulted in wave ravinement overlain by shallow-shelf clastics and then deposition of slightly deeper shelf carbonates. The uppermost unit comprises bioturbated, clastic-shelf deposits related to a drop in relative sea level. Throughout much of the Qishn Clastics interval, accommodation was moderate, except for low accommodation associated with regional sheet sandstone at the base of the upper Qishn Clastics Member. Virtually all marine and brackish deposits show evidence of tidal sedimentation. Climate was arid and seasonally wet.

Three‐dimensional analogue modelling of an alluvial basin margin affected by hydrological cycles: processes and resulting depositional sequences

ABSTRACTThe dynamics between sediment erosion and accumulation at an alluvial basin margin affected by changes in the surface hydrology are explored using scaled analogue models produced in a flume. The presented results differ from previous counterparts in that accumulation or erosion has not been forced at a spreading outlet, but occurred at a slope change produced by previously accumulated sediment. Cyclical upstream incision produced by increased stream discharge generated incised valleys, and these were subsequently filled by sediment carried by less efficient streams generated during the low discharge period. High resolution mapping using 2.5 mm contour maps allowed the study of sediment accumulation and terrain modelling. The results of three selected experiments are analysed. The only variable explored was discharge. The basin margin was simulated by a ramp inserted in a low sloping flume, consisting of two segments of different slopes selected to emulate high and low efficiency flume fans produced elsewhere. Water and fine‐medium sand entered the ramp along a narrow (0.1 m) channel and flow expanded but without occupying the complete 1.2 m flume width. Flows were highly concentrated and noncohesive.Fan‐like accumulation (slope: 0.11) began during low discharge (LD) periods at the ramp slope break, and proceeded upstream, onlapping quickly at first, but shifting to mostly progradation at the end of the period. High discharges (HD) usually generated two or three incised channels at the beginning of the period, but one of them prevailed and rapidly eroded parts of the LD fan and moved the sediment to a more distal low‐sloping fan (slope: 0.045). Both LD and HD fans passed downstream into a system of small parallel channels resembling a braided alluvial plain ending in sediment lobes. The mapping of the accumulated sediment during the various periods allowed calculation of sediment budgets for the entire flume. The stratal architecture of the deposits was investigated along five parallel trenches cut after experiment termination. The regression analysis of depositional profiles at fan‐like features (expanding flow) and at braided plains (parallel flow) indicated that these fan‐like systems are linear and dependent on applied discharge, while the latter showed an exponential decrease of slope downstream, with a starting value set up by the fan slope.Two main types of stratigraphic units were generated, the LDST and HDST (system tracts). The LDST has a nonerosive base over ‘bedrock’ and the previous HDST, filling proximal erosional topography and prograding as well, generating an onlap–downlap array. Its geometry is highly variable and dependent on pre‐existing topography. The HDST base is an important erosive surface comparable to sequence boundaries. However, there are places without erosion due to a marginal position with respect to the main stream. Indeed, the results suggest that the three‐dimensional variability of erosion and depositional processes might produce very different architectures along the same basin margin.

Braided channels: Self‐similar or self‐affine?

The dimensions of islands in seven braided river reaches are analyzed as an indicator of the scaling behaviour of braided channels at small scales. We examined the relationship, W∼LH, where L and W are island length and width which are measured along the major and minor axes respectively and H is Hurst's exponent, which is a measure of scaling anisotropy. Islands are shown to exhibit near isotropic scaling (H = 0.88–1.02), or self‐similarity, at length scales up to 1.5 times the braid plain width, with reasonable consistency occurring between all reaches studied despite large differences in scaling and hydrologic/geomorphologic characteristics. We suggest that previous conclusions that braided channels show more strongly self‐affine scaling behavior have been influenced by the inappropriate application of simple linear regression and by basing morphological feature dimensions on their projection parallel and orthogonal to the braid plain axis rather than in terms of their absolute (major axis) dimensions.

Changing Fluvial Environments and Vertebrate Taphonomy in Response to Climatic Drying in a Mid-Triassic Rift Valley Fill: The Omingonde Formation (Karoo Supergroup) of Central Namibia

Abstract The Omingonde Formation of Central Namibia is a redbed succession infilling a half-graben that developed on the upland plateau of southern Gondwana in the early mid-Triassic. Field studies of the rocks and vertebrate fossils of these strata are used to reconstruct the changes that occurred in the rift valley landscapes over approximately 10 My, and show how these changes affected the life habits and preservation of terrestrial reptiles that inhabited the valley at that time. The early rift basin contained extensive lakes that were filled rapidly with conglomeratic sands deposited on alluvial fans prograding from the active boundary fault. These fans subsequently drained into an axial braided river system. With cessation of downfaulting, the fault scarp retreated and gradients in the basin became gentle enough to convert the braid plain to one or more meandering rivers confined by extensive floodplains. The climate changed from sub-humid at the beginning of rifting, through semi-arid for most of t...

A model of alluvial megafan sedimentation: Ganga Megafan

Abstract Ganga Megafan is one of several megafans, developed in the northern and central part of the Ganga Plain, that formed during the middle Late Pleistocene under conditions of higher sediment-water discharge and higher regional slopes than today. Incision by the Ganga River exposes 15–30 m thick megafan deposits in the valley walls and cliffs. Ganga Megafan deposits can be separated into four zones: Zone I — Gravelly braided streams; Zone II — Sandy braid plain; Zone III — Anastomosing channel plain; and Zone IV — Meandering channels with broad interfluve areas. Deposits of each zone show distinct facies associations. Palaeocurrents fan from SW to SE and E, while the grain size decreases from proximal to distal fan. At present, fan-building activity is highly subdued and is restricted only to the northernmost 5–10 km wide belt of Piedmont plain.

Sandy submarine braid plains: Potential deep-water reservoirs

Sandy submarine braid plains, like their fluvial counterparts on land, are sand-rich depositional environments that may display excellent reservoir characteristics in terms of sediment volume, porosity, and permeability. The submarine examples may be laterally associated with potential source rocks such as the fine-grained levee deposits of deep-sea channels. A side-scan sonar study of the central Labrador Sea revealed the existence of a more than 700 km long and up to 120 km wide submarine sand and gravel plain that has been supplied with sediment by high-density turbidity currents, possibly resulting from subglacial lake outburst flooding in the Hudson Strait. The side-scan imagery of parts of the plain displays a conspicuous streaky pattern of alternating high and low backscatter intensity. High-resolution 3.5 kHz seismic profiles and 12 kHz bathymetric profiles show that the pattern represents a furrow-and-ridge (erosional) or channel-and-bar (depositional) topography, similar to a braided alluvial plain. The furrows or channels have low acoustic backscatter, are less than 10 m deep, and are separated by ridges or bars having high backscatter. Some channels terminate in depositional lobes. Individual channels and bars (or furrows and ridges) are less than 100 m wide and can be followed up to 40 km downcurrent. On sleeve-gun seismic profiles, the total sand thickness appears to be between 200 m (proximal) and 100 m (distal). Piston cores from the plain recovered massive sand layers up to 4 m thick, buried under 1 m of Holocene hemipelagic ooze. Texturally, the sands and gravelly sands display a trend of improving sorting with increasing mean grain size. Some very coarse grained samples are moderately well sorted and almost matrix free. The flooding events that deposited the sands might be the submarine counterpart of Heinrich events but need not be restricted to such events. Radiocarbon ages of about 10 k.y. from the base of the ooze overlying the youngest sand gave a minimum age for the sand that is similar to the age of Heinrich event 0. Estimates for the discharge volume of individual events are poorly constrained and range from 103 to 105 km3. Braided channel patterns in deep-water (Begin page 1500) sandy depositional environments are not restricted to high latitudes but also have been identified in various submarine fan settings in the lower latitudes, for example, the Orinoco, Var, and Monterey deep-sea fans and in the Santa Monica Basin. The largest examples, however, are known from high latitudes, suggesting that melt-water discharge from continental ice sheets may favor the formation of this habitat of giant sands in the deep sea. The occurrence of sandy braided deep-water environments having favorable reservoir characteristics in a variety of tectonic settings makes this type of environment a potentially interesting deep-water target.

Scale invariances in the morphology and evolution of Braided Rivers

This paper presents an overview and synthesis of an extensive research effort to characterize and quantify scale invariances in the morphology and evolution of braided rivers. Braided rivers were shown to exhibit anisotropic spatial scaling (self-affinity) in their morphology, implying a statistical scale invariance under appropriate rescaling of the axes along and perpendicular to the main direction of flow. The scaling exponents were found similar in rivers of diverse flow regimes, slopes, types of bed material and braid plain widths, indicating the presence of universal features in the underlying mechanisms responsible for the formation of their spatial structure. In regions where predominant geologic controls or predominant flow paths were present, no spatial scaling was found. Regarding their spatiotemporal evolution, braided rivers were found to exhibit dynamic scaling, implying that a smaller part of a braided river evolves identically to a larger one provided that space and time are appropriately normalized. Based on these findings, and some additional analysis of experimental rivers as they approach equilibrium, it was concluded that the mechanism bringing braided rivers to a state where they show spatial and temporal scaling is self-organized criticality and inferences about the physical mechanisms of self-organization were suggested.

Tectonic evolution of the Niksar and Tasova–Erbaa pull-apart basins, North Anatolian Fault Zone: their significance for the motion of the Anatolian block

The Tasova–Erbaa and Niksar basins are two adjacent pull-apart basins along the North Anatolian Fault Zone (NAFZ). Within the Tasova–Erbaa basin, sedimentary lithofacies of the Upper Pontus Formation (Plio-Pleistocene) are asymmetrically distributed, with laterally derived alluvial fans, coarse braid plain deposits, and axial braided stream deposits dominating the northern and western parts of the basin. The basin has gently dipping sediments locally affected by pervasive extensional (T) faulting. Isolated compressional structures are associated with master faults (deformation zone width ≤500 m). Pervasive extensional faults (T and R?) and late-stage PDZ-parallel compressional faults are also present. An additional set of extensional faults trends perpendicular to the PDZ, accommodating secondary pull-apart stretching within the basin. The distribution of sedimentary facies and structural styles is consistent with that predicted by models for pull-apart basins. The adjacent Niksar basin is an active pull-apart basin, possibly as young as 0.5–1 Ma. Its modern drainage and sedimentary facies are symmetrically arranged. Adding the lengths of the Tasova–Erbaa and Niksar basin suggests ∼80 km total displacement along the North Anatolian fault, which is consistent with Seymen (ITU Ph.D. Thesis, 1975), Sengor [J. Geol. Soc. 136 (1979) 269–282] and Armijo et al. [Tectonophysics 243 (1999) 135–154], who observed about 85±5 km total displacement along the North Anatolian fault. The change in course of the North Anatolian fault, marked by the formation of the Niksar–Ezinepazari segment, and this new geometry suggest that the Anatolian block is rotating anticlockwise by having two separate poles: one located near Damascus and the other occurring north of the Sinai peninsula. This change may also mark the transfer of the Anatolian triple junction from Erzincan to Karliova about 1 Ma ago. Furthermore, it appears that while the fault shortcuts the older basin, a new generation of pull-apart basins is formed due to changes of boundary conditions along the fault zone.

Upper Triassic – Jurassic sequence stratigraphy and its structural controls in the western Ordos Basin, China

Upper Triassic, Lower–Middle Jurassic and Upper Jurassic strata in the western Ordos Basin of North China are interpreted as three unconformity‐bounded basin phases, BP‐4, BP‐5 and BP‐6, respectively. The three basin phases were deposited in three kinds of predominantly continental basin: (1) a Late Triassic composite basin with a south‐western foreland subbasin and a north‐western rift subbasin, (2) an Early–Middle Jurassic sag basin and (3) a Late Jurassic foreland molasse wedge. Within the Late Triassic composite basin BP‐4 includes three sequences, S4‐1, S4‐2 and S4‐3. In the south‐western foreland subbasin, the three sequences are the depositional response to three episodes of thrust load subsidence, and are mainly composed of alluvial fan, steep‐sloped lacustrine delta and fluvial systems in front of a thrust fault‐bounded basin flank. In the north‐western rift subbasin, the three sequences are the depositional response to three episodes of rift subsidence, and consist of alluvial fan – braid plain and fan delta systems basinward of a normal fault‐bounded basin margin. In the sag basin BP‐5 includes four sequences, S5‐1, S5‐2, S5‐3 and S5‐4, which reflect four episodes of intracratonic sagging events and mainly consist of fluvial, gentle‐gradient lacustrine delta and lacustrine systems sourced from peripheral uplifted flanks. BP‐6, deposited in the foreland‐type basin, includes one sequence, S6‐1, which is the depositional response to thrust load subsidence and is composed of alluvial fan systems. The formation and development of these three kinds of basins was controlled by Late Triassic and Jurassic multi‐episode tectonism of basin‐bounding orogenic belts, which were mainly driven by collision of the North China and South China blocks and subduction of the western Pacific plate.

Ground penetrating radar images of selected fluvial deposits in the Netherlands

Ground penetrating radar (GPR) surveys have been carried out in order to characterise reflection patterns and to assess the method's potential for imaging palaeofluvial sediments in the Mass-Rhine former confluence area in the southern Netherlands. The results show that the deposits of meandering, braided and transitional river types produce characteristic radar facies. Representative examples of each of these river types were selected where the GPR data could be directly correlated with sedimentary information derived from exposures and detailed drilling, and geomorphological data could be supplied. Individual channels may be distinguished by the radar facies of their fills. The floodplains of the different river types also show a characteristic radar facies. In GPR data from the meander floodplain, parallel, dipping reflections represent point-bar structures, while irregular, intersecting small channel patterns that alternate with parallel continuous reflections are more typical in the braid plain. A transitional river type shows characteristics of both of these types. Typical examples of GPR sections recorded in each of the different fluvial palaeoenvironments are presented in an interpretative radar facies chart.

Characteristics of Coal Accumulation of Transform‐Extensional Basins in Linear Fracture Zones: A Case Study of the Meihekou Basin

Abstract This paper discusses the characteristics of coal accumulation of transfor‐extensional basins in linear fracture zones on the basis of an analysis of tectonic evolution and sequence stratigraphy in the Meihekou basin. Coal accumulation of the basin occurred mainly in the accelerated rifting period of the transtentional regime and in the late period of tectonic transformation of the transpressional regime. In the former period, swamps developed in shallow lakes. The thickness of coal seams was controlled by the activity of fault block. Thick coal was formed in a downfaulted trough. In the latter period, swamps developed in a braided fan delta plain. The coal seams are thin and have poor lateral continuity. The thickness of coal seams was controlled by the migration of water channels in the braided plain.