Lateral Accretion Research Articles

GIS-based geomorphologic study of Presque Isle Peninsula, a compound lacustrine barrier-spit system along the south-central Lake Erie margin

Abstract The Presque Isle peninsula is a sandy barrier-spit system situated along the south-central Lake Erie coast. This investigation focuses on landform succession along its eastern terminus, where nautical charts and aerial images chronicle morphologic change over the last century. A conceptual framework of this recent terrain evolution provides a blueprint for understanding the process–landform interactions responsible for constructing the compound peninsula over the late Holocene. Presque Isle is compartmentalized into four distinct en echelon strandplains, each comprised of multiple beach ridges of similar orientations, heights and spacing. These strandplain segments are separated by topographically prominent ridgelines that extend across the width of the peninsula and obliquely truncate landward paleoshorelines. The processes responsible for this landform juxtaposition are exemplified by recent spit growth and decay along Presque Isle's eastern terminus, where net lateral accretion has extended the peninsula by ~ 0.8 km 2 since the early 1900s. Truncation of newly formed ridge morphologies here since the 1990s provides a reference frame for understanding broader peninsular evolution, as reflected in Presque Isle's morphological segmentation. Episodes of spit accretion and low-elevation beach-ridge formation sequentially construct strandplains with little internal heterogeneity in way of beach-ridge spacing and height. Net-depositional episodes are subsequently punctuated by extensive erosion and the formation of high-elevation ridgelines at oblique angles to paleoshorelines. While this process is demonstrated over the recent ~ 100 years as a function of human-altered coastal hydrodynamics and sediment-supply reductions, potential linkages between older erosive landforms and major lake-level transgressions are inferred and could potentially be elucidated upon continued research.

Influence of hydrodynamic energy on Holocene reef flat accretion, Great Barrier Reef

The response of platform reefs to sea-level stabilization over the past 6ka is well established for the Great Barrier Reef (GBR), with reefs typically accreting laterally from windward to leeward. However, these observations are based on few cores spread across reef zones and may not accurately reflect a reef's true accretional response to the Holocene stillstand. We present a new record of reef accretion based on 49 U/Th ages from Heron and One Tree reefs in conjunction with re-analyzed data from 14 reefs across the GBR. We demonstrate that hydrodynamic energy is the main driver of accretional direction; exposed reefs accreted primarily lagoon-ward while protected reefs accreted seawards, contrary to the traditional growth model in the GBR. Lateral accretion rates varied from 86.3m/ka–42.4m/ka on the exposed One Tree windward reef and 68.35m/ka–15.7m/ka on the protected leeward Heron reef, suggesting that wind/wave energy is not a dominant control on lateral accretion rates. This represents the most comprehensive statement of lateral accretion direction and rates from the mid-outer platform reefs of the GBR, confirming great variability in reef flat growth both within and between reef margins over the last 6ka, and highlighting the need for closely-spaced transects.

Sediment flux from the morphodynamics of elongating linear dunes

Although dunes are very common bedforms in terrestrial sand seas, the description of linear dune growth, either by extension or lateral accretion, is still hindered by our limited understanding of the underlying mechanisms. Therefore, sand flux estimates from remote imagery rely essentially on the migration speed of barchan dunes, but not on the dynamics of linear dunes. Here we use ∼50 yr of high-resolution aerial and satellite imagery of the Tenere desert (Niger), the world's largest source of mineral aerosols, to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration. As they elongate from topographic obstacles in a zone of low sediment availability with multimodal winds, these elongating lee dunes are ideal to isolate and quantify linear dune growth only by extension. Using similar conditions in a numerical model, we show how deposition downstream of low hills may result in nucleation and development of bedforms. From elongation we derive the local sand flux parallel to the linear dune crests. This study shows that the morphodynamics of linear dunes under complex wind regimes can also be used for assessing sediment flux and wind conditions, comparably to the more-established method of using sand flux estimates perpendicular to the barchan dune crests in zones of unidirectional wind.

Using LiDAR data to characterize and distinguish among different types of raised terraces in a fjord-valley setting

Depositional terraces of diverse origin are common in Norwegian fjord valleys. They differ subtly morphologically and may be difficult to distinguish from one another without detailed field and stratigraphic investigation. We used LiDAR elevational data to describe previously identified glaciofluvial, glaciodeltaic, fluvial, and fluviodeltaic terraces in the Målselv valley, northern Norway, and show how they differ morphologically. Fluvial terraces are divided into two subtypes based on their difference in morphology. One type shows an undulating surface, often with a ridge-and-swale topography (scroll bars) associated with lateral accretion of moderate-to-high-sinuosity (meandering) channels, while the other shows remains of channels and braid bars associated with low-sinuosity channels. Fluviodeltaic terraces are generally flat and featureless, probably as a result of wave and tidal reworking of the terrace surface as it was raised above sea level. Glaciofluvial terraces show braided channel forms and occasional eolian dunes, while glaciodeltaic terraces are generally flat and featureless, in some cases having a moraine ridge on top and small beach ridges on the proximal, ice-contact slope. Terraces also show a distinct distribution pattern, with glaciofluvial and glaciodeltaic terraces graded to the marine limit occurring at higher elevations than fluviodeltaic and fluvial terraces. Based on the results presented here we show that a distinction can be made between different terraces based on morphological criteria revealed by LiDAR data. Such data should be acquired where possible to improve the quality of geomorphological mapping.

Stratigraphic Expression of Intra-Point-Bar Erosion and Rotation

Abstract Meander belts are characterized by a complex amalgam of point bars and associated depositional elements, such as oxbow-lake fills. Point-bar deposits are composite elements often characterized by complex scroll-bar patterns formed in response to processes of expansion, rotation, and translation of a meander bend. Intra-point-bar erosion is commonly observed in modern river systems, yet its product is rarely described from the ancient record. Late Cretaceous meander-belt deposits, consisting of a transition from a point-bar to counter-point-bar deposit, as well as an associated abandoned channel fill, crop out in south central Alberta, Canada. The strata are characterized by widespread evidence for intra-point bar deposit erosion and punctuated rotation. In the stratigraphic record, intra-point bar deposit erosion and punctuated rotation is expressed by steeply dipping discordant surfaces (up to 20°), which truncate previously deposited lateral-accretion packages. Across these erosion surfaces the direction of bar migration often rotates significantly, up to 50°. Subsequent accretion surfaces onlap intra-point-bar-deposit erosion surfaces. The analysis reveals that point-bar surfaces are formed through a complex interplay of erosional and depositional processes along their length, resulting in stratigraphic surfaces that may be highly composite. This study departs from more simplistic process models of point bars that consider deposits to be the product of continuous lateral accretion, and as such has important implications for paleoenvironmental reconstructions and subsurface reservoir characterization.

Millennial‐scale stratigraphy of a tide‐dominated incised valley during the last 14 kyr: Spatial and quantitative reconstruction in the Tokyo Lowland, central Japan

AbstractSpatial and quantitative analysis of infilling processes of the tide‐dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post‐Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea‐level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet‐like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea‐level rise of 4 to 7 mm yr−1. Before migration of the Tone River at 5 ka, the tide‐dominated bay in the Nakagawa Valley was filled by upward‐fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide‐dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward‐coarsening deltaic sediments.

Analysis of Intermediate-scale Reservoir Heterogeneity Based on Well Exposed Outcrop Analogue within the Maastrichtian Enagi Formation, Bida Basin, North Western Nigeria

Analysis of intermediate-scale reservoir heterogeneity in a well exposed outcrop analogue of a point bar deposit within the Enagi Formation was carried out in order to reveal its reservoir dimensions. The study area is located in northern part of Bida basin, Nigeria. Three lithofacies were identified: (1) conglomerate facies interpreted as channel lag deposits; (2) epsilon and trough cross-bedded, fine to medium grained, sandstone facies interpreted as products of lateral accretion and 3D migration of subaqueous dunes in a mixed-load channel system; and (3) mudstone facies recording overbank deposits. Outcrop measurements reveal reservoir dimensions as follows: 1) bankfull channel depth of 3.9 m; 2) bankfull channel width of 31.5 m; 3) reservoir width (meander amplitude) of 132 m; and 4) reservoir length (meander wavelength) of 911 m. Analysis of internal lithologic variability within the point bar deposit allowed the detection of three main internal reservoir heterogeneities. They include: a) vertical reservoir heterogeneity resulting from overall fining-up trend within the point bar deposits; b) small scale vertical heterogeneity owing to fining-up grain size within each lateral accretion unit; c) lateral reservoir compartmentalization due to mud-drapes observed on some of the lateral accretion surfaces. Information provided in this work has implications for improved computation of Stock Tank Oil in Place (STOIP) at the initial stage of exploration and in planning of well spacing during development and production stages. This work, also suggests that field measurements on outcrop analogues combined with empirical computation of reservoir dimensions provides valuable information useful at all stages of exploration and development of an oil field.

Recognition and importance of amalgamated sandy meander belts in the continental rock record

Meandering fluvial channels and their meander belts are common in modern continental sedimentary basins, yet compose a minor constituent of the reported fluvial rock record. Here we document exhumed amalgamated meander belt deposits from the upper Jurassic Morrison Formation, Utah (United States). The size of the amalgamated meander belt (9000 km2) is significantly larger than any documented previously and comparable in size to those from modern sedimentary basins. We describe a representative outcrop of sandy point bar deposits that shows features considered characteristic of both braided and meandering fluvial systems. Lateral accretion sets compose <5% of the outcrop area, yet point bar morphology is clearly visible in plan view. We suggest that difficulties in the identification of sandy, amalgamated meander belt deposits indicate that they have gone largely unrecognized in the rock record. Their recognition has important implications for basin-scale reconstructions of fluvial systems and interpretation of tectonic setting.

Zircon U–Pb geochronology and Lu–Hf isotopes from the Kolar greenstone belt, Dharwar Craton, India: Implications for crustal evolution in an ocean-trench-continent transect

The Dharwar Craton in southern India is composed of Neoarchean greenstone successions in association with banded iron formations (BIFs), felsic volcanics and voluminous calc-alkaline to potassic plutons intruding tonalite–trondhjemite–granodiorite (TTG) basement rocks. In this study, we investigate a suite of BIFs (pelagic sediments), amphibolite (oceanic basalts), quartz mica schist (trench sediments from continental source), and porphyritic granite with mafic magmatic enclave (magma mixing and mingling in continental magmatic arc), together representing an ocean-trench-continent transect within a Neoarchean subduction–accretion–collision zone in the Kolar greenstone belt of the Eastern Dharwar Craton. The morphology, internal structure and high Th/U values of zircon grains from these rocks suggest magmatic crystallization, closely followed by metamorphism. The magmatic zircons in the BIFs show upper intercept ages of 2719±31Ma to 2698±50Ma and weighted mean 207Pb/206Pb mean ages of 2718±26Ma to 2696±35Ma marking the time of deposition of the pelagic sediments with which the ocean floor basalts were intercalated, and from which the magmatic zircons where incorporated within the pelagic sediments. The ca. 2.5Ga ages from the younger group of zircons in the BIFs correspond to the timing of metamorphism. Zircons from the amphibolite shows upper intercept age of 2581±31Ma representing the crystallization age of the protolith basalts. Zircons from the porphyritic granite yield upper intercept age of 2576±10Ma and weighted mean 207Pb/206Pb mean age of 2572±13Ma suggesting the timing of emplacement of this rock, which is comparable to the ages of magmatic zircons in the amphibolite. Magmatic zircons in the dioritic enclave show upper intercept age of 2564±13Ma and weighted mean 207Pb/206Pb age of 2560±10Ma. The younger population of zircons in this rock show upper intercept age of 2492±33Ma. Inherited zircons in some of these rocks show older 207Pb/206Pb ages of 2707±21Ma and 2666±22Ma. The zircon εHf(t) values are dominantly positive (up to 5.1), although some grains show negative values. The crustal residence ages suggest Mesoarchean to Neoarchean juvenile and reworked sources consistent with vertical and lateral accretion in a continental arc setting. Our data trace crustal growth during Neoarchean at 2.72–2.56Ga in Eastern Dharwar along an active convergent margin. The multiple events of crustal growth recorded in our study are comparable with similar features in some of the other cratonic nuclei elsewhere on the globe, and suggest that the late Archean marks an important period of continent building.

Nature, origin and evolution of a Late Pleistocene incised valley‐fill, Sunda Shelf, Southeast Asia

AbstractUnderstanding the stratigraphic fill and reconstructing the palaeo‐hydrology of incised valleys can help to constrain those factors that controlled their origin, evolution and regional significance. This condition is addressed through the analysis of a large (up to 18 km wide by 80 m deep) and exceptionally well‐imaged Late Pleistocene incised valley from the Sunda Shelf (South China Sea) based on shallow three‐dimensional seismic data from a large (11 500 km2), ‘merge’ survey, supplemented with site survey data (boreholes and seismic). This approach has enabled the characterization of the planform geometry, cross‐sectional area and internal stratigraphic architecture, which together allow reconstruction of the palaeo‐hydrology. The valley‐fill displays five notable stratigraphic features: (i) it is considerably larger than other seismically resolvable channel forms and can be traced for at least 180 km along its length; (ii) it is located in the axial part of the Malay Basin; (iii) the youngest part of the valley‐fill is dominated by a large (600 m wide and 23 m deep), high‐sinuosity channel, with well‐developed lateral accretion surfaces; (iv) the immediately adjacent interfluves contain much smaller, dendritic channel systems, which resemble tributaries that drained into the larger incised valley system; and (v) a ca 16 m thick, shell‐bearing, Holocene clay caps the valley‐fill. The dimension, basin location and palaeo‐hydrology of this incised valley leads to the conclusion that it represents the trunk river, which flowed along the length of the Malay Basin; it connected the Gulf of Thailand in the north with the South China Sea in the south‐east. The length of the river system (&gt;1200 km long) enables examination of the upstream to downstream controls on the evolution of the incised valley, including sea‐level, climate and tectonics. The valley size, orientation and palaeo‐hydrology suggest close interaction between the regional tectonic framework, low‐angle shelf physiography and a humid‐tropical climatic setting.

Late Holocene fluvial activity and correlations with dendrochronology of subfossil trunks: Case studies of northeastern Romania

The main objective of this paper is to describe the late Holocene behaviour of rivers using an interdisciplinary approach combining fluvial geomorphology and subfossil trunk dendrochronology. The subfossil wood material collected along the rivers was investigated for dendrometric and dendrochronologic parameters. The research methods in these fields helped us to understand the effect of the fluvial environment on riparian trees and their records and helped in reconstructing the riparian palaeoenvironment.The study area consists of two rivers with different typologies but comparable sizes: the Moldova River, which features a braided to wandering channel in its lower reach, and the Siret River, which features a sinuous-meandering channel. Along the 100-km-long floodplain of the former and the 144-km-long floodplain of the latter, we found and sampled 77 subfossil trunks, of which 26 were subjected to 14C dating. The resulting data consist of floodplain facies mapping data, electric resistivity measurements, absolute dates, and dendrometric and dendrochronologic data. The results indicate that during a 100-year period, the two rivers were sensitive to climate change and anthropogenic effects, particularly a narrowing of the active channel by 76% in the braided channel and 38% in the sinuous-meandering channel. During the past 3300–3000YBP, the Moldova River maintained its braided style, whereas the sinuous-meandering style has been characteristic of the Siret River for the previous 6800–4600YBP. The two distinct fluvial environments are recorded in the dendrometric structure of the trunks buried in the channel-fill sediments. The braided fluvial environment was more effective in uprooting riparian trees and incorporating them in the floodplain deposits, whereas the sinuous-meandering style of stream effectively buried tree trunks in lateral accretion lobes. Absolute and dendrochronologic dating allowed for the reconstruction of timelines of the felling of the trees and estimates of the magnitude of the responsible hydrological event. The flood events on the Siret River with a recurrence interval of 200years (Qmax~2500–2800m3/s) were those most effective in destroying riparian forests, and the events on the Moldova floods with a 100-year recurrence interval (Qmax~1200–1400m3/s) were the most effective. Dendrochronology allowed for identification of wet phases (i.e., 3500–2900YBP, 2200–2075YBP, and 1000–800YBP) and dry phases (e.g., 3200–3150 or 2775–2700YBP, 1400YBP). Finally, we draw attention to the potential for creating a highly replicable dendrochronological series spanning at least 7000YBP.

Testing Applicability of Models Of Distributive Fluvial Systems Or Trunk Rivers In Ephemeral Systems: Reconstructing 3-D Fluvial Architecture In the Beaufort Group, South Africa

Abstract Multi-kilometer-scale strike and dip exposures of fluvial deposits from the Permo-Triassic Beaufort Group enable detailed analysis of spatial and temporal changes in fluvial style and stacking patterns. A study succession 145 m thick in the Abrahamskraal Formation comprises a hierarchy of channel-related deposits, from stories through channel belts that are stacked into nine channel-belt complexes in four complex sets. Channel belts show evidence for both downstream and lateral accretion and include common upper-phase plane bedding. Floodplain deposits comprise crevasse-splay sandstones and siltstone packages showing upward fining from green-gray siltstone into distinctive purple claystone, interpreted as a drying-upward trend in shallow ephemeral lakes. The lower stratigraphy is dominated by splay complexes, overlain by increasingly incised and amalgamated channelized systems with little preserved floodplain material. Paleocurrents are consistently to the northeast. While aspects of the dataset fit a basin-axial trunk-river-dominated system, they better (but not entirely) reflect a prograding distributive fluvial system (DFS). Lines of evidence include the consistent paleocurrents in early (frontal) splays and younger channel-belt complexes, and the presence of splay complexes only in the lower stratigraphy, interpreted as precursors to the prograding fluvial system. Deposition took place under conditions of flashy discharge influenced by high-frequency climate cycles, also expressed in the mudrock color changes and distribution of paleosols. These cycles overprint the expected gradual drying-upward trend in overbank deposits proposed in DFS models. The abrupt increase in channel belt incision, amalgamation, and lack of floodplain preservation associated with Complex 6 is interpreted to reflect sequence-boundary formation and a basinward facies shift that forced progradation beyond the rate predicted in gradualistic DFS models.

Brasiliano crustal structure and the tectonic setting of the Parnaíba basin of NE Brazil: Results of a deep seismic reflection profile

A 1430 km, deep crustal, seismic reflection profile of the Parnaíba basin shows an asymmetric, structured western margin and a gently dipping eastern margin. The ~3 km thick, Phanerozoic sedimentary section overlies a pronounced, planar, regional unconformity that crosses three Precambrian blocks with differing seismic facies: the Amazonian/Araguaia block, the Parnaíba block, and the Borborema block. The blocks are separated by steep crustal-scale boundaries across which seismic facies change abruptly. In the west, the ophiolitic metasedimentary rocks of the Araguaia Group overlie the Amazonian craton. Both craton and metasediments terminate eastward against a steep, east dipping fault zone defining the Amazonian/Araguaia block and Parnaíba block boundary. Reactivation of this Neoproterozoic margin in the Late Triassic and Late Jurassic/Early Cretaceous, folded and elevated basement and basin over 2 km. A second crustal boundary defines the eastern margin of the Parnaíba block with the Neoproterozoic Borborema block. This boundary is interpreted as the extension of the Transbrasiliano shear zone. These data demonstrate that the basement of the Parnaíba basin was formed during Brasiliano orogenesis by west directed collision-related thrusting, succeeded by lateral accretion along steep, crustal-scale boundaries. After formation of a post-Brasiliano peneplain, the Parnaíba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline. No extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation, but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin.

Progradational patterns at the head of single units of base-of-slope, submarine granular flow deposits (“Conglomérats des Gâs”, Coniacian, SE France)

Upper Cretaceous conglomerates, sandstones and calcarenites outcropping in the Glandage syncline (Les Gâs Gorge) belong to two superimposed “turbidite” systems, the first being the shortest (about two hundred metres thick, five kilometres wide, and a mere ten kilometres long). Both onlap a SW–NE oriented palaeoslope created by faulting in the latest Turonian on the southern edge of the Vercors carbonate platform. The first system is Coniacian in age, the second probably Campanian.This study deals only with the first system which comprises, in stratigraphic order, classical sandstone turbidites, clast-supported conglomerates, and coarse-grained, cross-bedded, sandy calcarenites. The transition between the last two lithologies is progressive, allowing the mechanisms, responsible for apparent lateral accretions found pervasively in both the conglomerates and the calcarenites, to be understood. Here, cross-bedding is not the result of changes in the direction of accretion in “lateral bars” or in oblique infillings of sinuous channels, as is often suggested in other similar deep-water carbonate systems. Each event bed comprises a prograding conglomeratic head plug, blanketed in continuous sedimentation by forward spreading bioclastic calcarenites. The superimposition of such mixed beds gives rise to a low-angle cross-stratification, an unusual feature for a system interpreted as a base-of-slope apron. Conglomerates were probably deposited through a granular flow mechanism at a strong hydraulic jump at the base of the slope. Associated calcarenites show supercritical flow features. The undulating transverse architecture of its large head onlapping the paleoslope suggests it is a multi-apex apron. The sandstone-conglomerate-calcarenite succession of the depositional sequence is interpreted as overall transgressive after the late Turonian tectonic phase that led to the exposure of the Vercors carbonate platform updip.

Point bar configuration and residual oil analysis based on core and dense well pattern

Abstract Based on the dense well pattern and the core data of straight wells and horizontal wells of PI332 unit of Putaohua oil layer, Cretaceous Yaojia Formation in Xingbei area of Daqing Oilfield, according to the model of modern sediments and ancient outcrop, the configuration parameters of lateral interbeds configuration are acquired by rock-electricity combination, which are used to analyze the three-dimensional configuration characteristics of the point bar lateral accretion interbeds located in the meandering distributary channel of the delta distributary plain and the residual oil distribution. The results show that the lateral accretion interbeds of point bar are thin, slow, and dense in PI332 unit of Xingbei area. The thickness of interbeds is mainly 2−25 cm. The upper and lower dip angle of interbeds is smaller at 2°−5°. The middle of interbeds is steeper at 7°−10°. The plane distribution density is 0.11/m. The three-dimensional spatial configuration consists of the plane and profile features. The lateral accretion interbeds are in crescent arrangement on the plane, and in imbricate and echelon distribution on the profile. Under the influence of lateral accretion interbeds architecture and the physical difference of lateral accretions, the middle and lower parts of point bars are seriously flooded, and the residual oil is mainly distributed in the middle and upper parts. The horizontal section of a horizontal well should be designed in the middle and upper parts of point bars, and it should drill out the middle and upper parts of lateral accretion bodies to increase the supply unit and enhance oil recovery.

Palaeoenvironmental reconstruction of the early Anisian from sedimentology and plant remains in the SE Iberian Range (E Spain)

The end of the Permian period is characterized by the largest mass extinction ever recorded in marine and terrestrial sedimentary rocks. The oldest plant macro-remains after the P–T boundary recorded in the Castilian Branch of the Iberian Range after this mass extinction belong to the Eslida Formation of Anisian age. These associations have been recovered at four localities and include sphenophytes (Equisetites, perhaps Neocalamites or Schizoneura), conifers (Volzia, Albertia, Pelourdea), lycophytes (Pleuromeia) and a possible seed fern (Peltaspermum).Facies associations and sedimentological analysis of the Eslida Formation have allowed for distinguishing four architectural elements: CH (channel deposits), LA (lateral accretion), CS (crevasse splay) and FF (overbank fines). The largest fossil remains are associated with CH and LA architectural elements and high-energy events, and they form part of the basal lag or reactivation surfaces. Smaller and well-preserved fragments are associated with the CS architectural elements.A sedimentologic, taphonomic and palaeoclimatic analysis reveals gradation from riparian hygrophytic vegetation to semi-arid xerophitic vegetation across the alluvial complexes and a progressive aridification through time. A comparison with other areas shows that this association is similar to coeval floras of Iberia and of the “Grès à Voltzia” Formation of northeastern France.

The base-of-slope carbonate breccia system of Céüse (Tithonian, S-E France): Occurrence of progradational stratification in the head plug of coarse granular flow deposits

Once interpreted as a deep-water canyon infilling and later as some kind of tempestite deposit, the Tithonian breccias of Ceüse are here reinterpreted as a small lobe deposit, emplaced at the mouth of a gully cutting into a slope between two deep-water terraces in the eastern part of the subalpine basin. The breccias belong to two basic categories: mostly grain-supported and occasionally mud-supported. Spectacular large-scale oblique geometries (apparent lateral accretions) are observed in the grain-supported breccias, both within the infilling of the lower feeder channel and in the updip part of the lobe. Detailed analyses of these geometries show that they correspond to progradational features. Both laterally and in a more distal position on the lobe, these progradational bodies evolve into a pile of unsorted massive breccias, sometimes showing a crude, even stratification. We thus propose a depositional model for the grain-supported breccias, linking the two facies as follows: the deposition of a gravity flow begins at the toe-of-slope (mouth of the tributary gully) by a progradational head which evolves downslope and laterally to a splay of massive or crudely stratified breccia. The internal oblique stratification flattens distally over a distance of a few kilometres. Elementary beds pile up in morphological compensation to build the 100m thick breccia body of Céüse. Because the breccias are derived from the gravity-reworking of deep-water nodular carbonates, there is no sand-sized material available in the depositional system, or only a little probably produced through an erosive clast grain diminution process during transport. This explains why the thick breccia beds pass quickly eastward to thin-bedded mud turbidites.

Depositional controls on tidally influenced fluvial successions, Neslen Formation, Utah, USA

The stratigraphic architecture of marginal marine successions records the interplay of autogenic and allogenic processes, and discerning their relative role in governing the morphology of the palaeoenvironment and the architecture of the preserved sedimentary succession is not straightforward. The Campanian Neslen Formation, Mesaverde Group, Utah, is a tidally influenced fluvial succession sourced from the Sevier Orogen, which prograded eastwards into the Western Interior Seaway. Detailed mapping in three dimensions of architectural relationships between sandstone bodies has enabled documentation of lateral and vertical changes in the style of channel-body stacking and analysis of the distribution of sedimentary evidence for tidal influence. Upwards, through the succession, sandstone channel bodies become larger and more amalgamated. Laterally, the dominant style of channel bodies changes such that ribbon channel-fills are restricted to the east of the study area whereas lateral accretion deposits dominate to the west.Combined allogenic and autogenic controls gave rise to the observed stratigraphy. A temporal decrease in the rate of accommodation generation resulted in an upward increase in amalgamation of sand-bodies. Autogenic processes likely played a significant role in moderating the preserved succession: up-succession changes in the style of stacking of channelized bodies could have arisen either from progradation of a distributive fluvial system or from an upstream nodal avulsion of a major trunk channel; accumulation of tide influenced, wave dominated units likely record episodes of delta-lobe abandonment, subsidence and submergence to allow accumulation of near shore sand bars with associated washover complexes.

Zircon U–Pb–Hf isotopes and geochemistry of Neoarchean dioritic–trondhjemitic gneisses, Eastern Hebei, North China Craton: Constraints on petrogenesis and tectonic implications

Dioritic–trondhjemitic gneisses of Neoarchean age are widely distributed throughout the Gualanyu–Shangying area in Eastern Hebei, North China Craton. These orthogneisses consist of dioritic, trondhjemitic, and quartz-rich trondhjemitic gneisses, which were subjected to amphibolite- to granulite-facies metamorphism. LA-ICP-MS zircon U–Pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535±23 to 2513±8Ma, reflecting an important Neoarchean dioritic–trondhjemitic magmatic event followed by sequential Proterozoic tectonothermal events between 2472±60 and 2307±50Ma.Dioritic and trondhjemitic gneisses exhibit similar zircon ɛHf(t) values of +1.5 to +5.2 and +2.9 to +5.3, respectively, indicating their depleted mantle affinity. Geochemical analysis reveals that the dioritic gneisses have low SiO2 contents (55.4–63.0%), various MgO contents (2.07–4.78%) and Mg# (36.07–63.56), and high (La/Yb)N ratios (5.45–15.19). In contrast, the trondhjemitic gneisses exhibit higher SiO2 contents (67.4–73.8%) and (La/Yb)N ratios (12.86–76.73), and lower MgO contents (0.66–2.67%), although their Mg# (36.29–61.41) are similar to those of dioritic gneisses. These geochemical and zircon Lu–Hf isotopic features, together with geochemical modeling, suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab-derived melts or fluids, and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas. Quartz-rich trondhjemitic gneisses exhibit extremely high SiO2 contents (77.4–84.6%) with zircon ɛHf(t) values ranging from −3.4 to +4.1, suggesting that their precursors formed after the partial melting of a mixed magmatic source. It is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids, and a felsic member derived from the partial melting of ancient crustal material. These new data, combined with previous studies of contemporaneous metavolcanic rocks in Gualanyu–Shangying area and surrounding areas, suggest that the magmatic precursors of these Neoarchean dioritic–trondhjemitic gneisses formed in a subduction-related tectonic environment at an E-W strike convergent plate margin. Furthermore, the Eastern Block of the North China Craton had a major lateral accretion of crustal growth in the north margin in the Neoarchean, whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the Eastern Block.

Architectural Styles and Sedimentology of the Fluvial Lower Beaufort Group, Karoo Basin, South Africa

Abstract The Permo-Triassic Beaufort Group comprises in excess of 3000 m of sandstone and mudstone-rich fluvial deposits that vary both laterally and vertically in abundance, architecture, and clustering styles. The lower 1370 m has been studied in detail. Detailed stratigraphic logging and photographic interpretation have been carried out on river cliffs and hillside sections, and sedimentary structures have been mapped by close inspection of clean outcrops. Seven fluvial architectural styles have been identified, encompassing sand body and sand body cluster scales: 1) flat-topped sheets dominated by lateral accretion, 2) concave-up topped sheets dominated by lateral accretion, 3) heterolithic ribbons dominated by lateral accretion, 4) flat-topped ribbons with attached splays dominated by downstream accretion, 5) unconfined moderately amalgamated concave-up topped complexes, 6) unconfined strongly amalgamated sheet complexes, and 7) valley-confined strongly amalgamated ribbon complexes. The internal architecture of each indicates mixed lateral and downstream accretion modes on several scales with local variability. Lateral-accretion deposits differ from typically published facies models in that trough cross-bedding is rare, possibly the result of an inability to generate dunes due to the dominant very fine sandstone grain size. The observed architecture and sedimentary structures are the product of: 1) a semiarid climate, producing fluvial and debris flows, and cementation of sediment packages at the surface (which exerts a strong control on the magnitude of channel scour, the nature of channel fill, and compaction history); 2) local variability in subsidence rate causing variation in the vertical spacing of channel belts, channel slope, and plan form, and variation in preservation of topographically higher features (e.g., levees); 3) relative position in a fluvial megafan influencing suspended load to bedload partitioning from proximal to distal locations, and formation of channel belt complexes; and 4) relative sea-level changes causing incised-valley formation and fill only in the most distal parts of the system.