Fine-grained Floodplain Deposits Research Articles

Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina

The early-middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The lower levels of the CBF consist of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork provided fossil vertebrates from these levels with an estimated age between ~16.5 Ma and 15.1 Ma
 (late Burdigalian-early Langhian). The studied section temporally overlaps with the middle or upper sections of the Santa Cruz Formation (SCF) in the Austral-Magallanes Basin of southern Patagonia, the Río Frías Formation in Chile, and the lower Collón Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality, including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato. We also recorded a testudine, which constitutes the southernmost record of tortoises in South America and worldwide. Faunal dissimilarities between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías
 and Collón Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones. The co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both forested and open environments for the area occupied by the CBF rocks. However, it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other. Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification during the Miocene in southern Patagonia, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.

Implications of palaeosols in low net-to-gross fluvial architecture reconstruction: Reservoir analogues from Patagonia and Spain

The changes in the geometry, density and distribution of the fluvial reservoir sand-bodies have been modelled both in the laboratory and from outcrop analogues. On the other hand, fine-grained floodplain deposits, often called “background” sedimentation, have not been in the spotlight. However, they provide a very important database for fluvial architecture reconstruction and therefore constitute a key tool in characterizing, correlating and modelling heterogeneous fluvial reservoirs. The methodology used in this work combines classical sedimentological and palaeopedological data. Palaeosols were identified in outcrops of the Cretaceous of Patagonia and Triassic red beds of Spain based on the main pedofeatures such as colour, soil structures, mottles, nodules, clay illuviation cutans, slickensides and rhizoliths. Palaeosol horizons, thickness and types of contact were described. The palaeosols classification was made through the comparison with the Soil Taxonomy. For the characterization of lateral palaeosols variability, several palaeopedological logs were constructed following the same stratigraphic level at both margins of the main channels. Small-scale or high-frequency palaeosol variations, identified in both fluvial succession are represented by the lateral and vertical superimposition of Inceptisols/Entisols, Vertisols and hydromorphic Vertisols/swamp deposits. We recognized a decrease in the drainage conditions within the palaeosols catena moving away from the main channel bodies. The most waterlogged palaeosol is located at 200 to 300 m from the main channels. Lateral changes of palaeosols are interpreted as the result of intrinsic factors to the depositional systems, such as the relative position within the floodplain and the distance from the main channels, that condition the particle size of parent material, the sedimentation rate and the palaeotopographic position which state the drainage conditions of palaeosols. Vertical stacking of different palaeosols is linked to avulsion processes and the relatively abrupt change in the distance to main channels as the system aggraded. The study of these two environmentally similar examples (low-gradient meandering fluvial systems subjected to a high rate of aggradation), although located in different tectonic and chronostratigraphic contexts, shows that the type of fluvial system acted as the main control of sedimentary and pedogenic processes; as well as the scale at which they occur. Thus, when choosing an outcrop fluvial reservoir analogue, the type of fluvial system and climate should be more important than the age or the tectonic framework of the succession to be studied.

Palaeohydraulic reconstruction and depositional model of the episodic flooding channels developed in the modern arid alluvial fan: Implications for the exploration target of the heterogeneous alluvial fan reservoirs

Alluvial fan reservoirs associated with episodic flood flows are generally heterogeneous, both laterally and vertically, owing to the variable depositional conditions and mechanisms. The complexity of sedimentary processes results in distinct challenges in terms of reservoir characterization and subsurface modelling. The ability to discern fan deposits in the subsurface directly impacts the assessment of the distribution, physical properties, and hydrocarbon recovery of reservoirs. Taking the Quaternary Poplar Fan as an example, this study aims to determine the characteristics, depositional model, and controlling factors of the episodic flooding channels developed in an alluvial fan through palaeohydraulic reconstruction. These research topics have rarely been studied and are poorly understood. The Poplar Fan is dominated by coarse conglomerates from apex to toe. In particular, lenticular gravelly and sandy flooding channel deposits can be identified throughout the fan and are easily distinguishable from the surrounding sheet-like conglomerates of the incised channel flood, sheetflood, and unconfined streamflood deposits in outcrops. Such flooding channels are characterized by the absence of fine-grained floodplain deposits or overbank deposits and the lack of in-channel mudstone drapes or plugs. This indicates the short-lived and episodic nature of the sedimentation processes. Individual channels are ribbon-shaped, with width/thickness ratios of less than 15:1, and show limited amalgamated characteristics. The estimated palaeoslopes of these flooding channels are all above the maximum gradient for modern typical rivers. In particular, the width, thickness, maximum particle size, calculated bankfull discharge, and stream power of these preserved channels all exhibit a decreasing downstream trend. Additionally, the channel width/thickness ratios increase initially and then decrease from the proximal to the distal fan. Accordingly, these episodic flooding channels predominately exhibit a distributary pattern in the plan view. Moreover, 70% of the estimated channel length is less than the maximum radial length of the fan, which suggests that these episodic flooding channels may gradually disappear downstream. Based on these outcrop data, a depositional model, together with an evolution pattern of the in-fan episodic flooding channels, was established. The variations in the water and sediment concentrations of the flood flow could be the major cause of the cyclic transformations between the episodic sheet-like and channelised flooding processes. Notably, compared with the sheet-like flood flow deposits, these episodic flooding channel deposits are much more organized in gravel arrangement, well-sorted, and more homogeneous, consequently having better physical properties and constituting a favourable exploration target. Moreover, the proposed cyclic evolution model of the episodic flooding channel can provide effective guidelines for stratigraphic correlation in characterizing heterogeneous alluvial fan reservoirs. The lithofacies and reconstructed palaeohydraulic parameters from the outcrops can also provide reasonable parameters and references for underground reservoir modelling and core analysis.

Applicability of OSL Dating to Fine-Grained Fluvial Deposits in the Mekong River Floodplain, Cambodia

Abstract Optically stimulated luminescence (OSL) dating of fine-grained (4–11 μm) fluvial sediments is rarely attempted but is crucial for constraining the evolution of mud-dominated floodplains. This study investigated the applicability of OSL dating to fine-grained deposits in the Mekong River, Cambodia based on a very young (<600 yr) point-bar to riverbank succession and modern flood deposits. In succession, fine-grained quartz OSL provided the youngest ages, whereas age estimates by multi-grain sand-sized quartz OSL, and feldspar and polymineral infrared-stimulated luminescence are >200 years older than the fine-grained quartz OSL age estimates. Ages of fine-grained quartz OSL are concordant with the minimum ages obtained from the single-grain quartz OSL. These results are supported by the generally small residual doses (<0.1 Gy) measured in modern fine-grained flood-plain deposits. This indicates that fine-grained sediments in the Mekong River (Cambodia) are sufficiently bleached at deposition and can yield reliable quartz OSL ages for establishing the chronology of the floodplain. The sufficient bleaching of fine-grained quartz partly results from the long transport distance and may also occur in other large river systems.

The first aspinothoracid arthrodire from the Late Devonian continental strata are found in western Junggar, Xinjiang

Arthrodira is the most widely distributed and diversified subgroup of Placodermi, which itself is the most diverse major group of the Devonian jawed vertebrates. Arthrodires were traditionally divided into the basal, paraphyletic “Dolichothoraci” and the highly nested monophyletic Brachythoraci, the latter of which displays reduction of body armor. Among brachythoracids, the Coccosteomorphi and Pachyosteomorphi further form a nested monophyletic Eubrachythoracidi. The sister group relationship between Coccosteomorphs and Pachyosteomorphs has been supported by most phylogenetic analyses, but the basal members and their interrelationships in these two groups are still unstable. Coccosteomorphs and pachyosteomorphs are the most diverse arthrodires in the Middle and Late Devonian. Among them, coccosteomorphs are found in strata of the lacustrine and littoral-neritic facies, while the pachyosteomorphs are mostly marine. Selenosteidae, one of the most well-known families in Pachyosteomorphi, are found in marine strata of Europe, North America, and North Africa. Junqing Wang found the nuchal plate of suspected Selenosteidae in the Xiejingsi Formation (early Famennian, Late Devonian) in Hunan Province, China, but due to the lack of identification features of the material, its exact systematic position awaits further determination. The Zhulumute Formation is a lithological unit in western Junggar, Xinjiang of northwest China and is mainly composed of conglomerates, pebbly sandstones and medium- to coarse-grained sandstones, with typical braided stream lithofacies. Channel-fill deposits are thick, with well-developed parallel bedding, scour structure, and large-scale trough, wedge and tabular cross-beddings. This part contains a large number of plant stem fossils in fragment. The fine-grained floodplain deposits are thin, with horizontal beddings and small sand ripple beddings; fine fragments of plant stems or leaves can be seen in the siltstones. These characteristics indicate that the Zhulumute Formation in the Bulongguoer area was formed in the braided stream environment. At the same time, according to the preserved plant fossil Leptophloeum rhombicum , its age is limited to the early Famennian. Here we report on a new selenosteid genus Bulongosteus , collected from the upper part of the Zhulumute Formation. The diagnostic characters of the new genus include: (1) A triangular nuchal plate with convex front edge, concave back edge and bilateral micro concave, and the side edge is overlaid with paranuchal plate; (2) the posterior margin of the paranuchal plate longer than that of the nuchal plate, and two sensory tubes were developed; (3) the preorbital plate enlarged to form the anterolateral margin of the orbit, with dense tumor spots on the surface; (4) the anterior edge of the marginal plate thickened to form the posterior margin of the orbit, and the three sensory tubes in the shape of “Y”; (5) a slender posterior ventrolateral plate. Pachyosteus , Rhinosteus , Melanosteus , and Walterosteu are similar to Bulongosteu s in Selenosteidae. In Rhinosteus , there are a few tumor spots on the surface of the bone and the distribution of the sensory tubes of the paranuchal plate is different from Bulongosteus . The posterior margin of Walterosteu is wider than Bulongosteus , and the distribution of the sensory tubes of the paranuchal plate is different. The characteristics of Pachyosteus and Melanosteus are the most similar to Bulongosteus , but the distribution of the sensory tubes in their paranuchal plate is different from Bulongosteus . And Pachyosteus has less tumor spots in the nuchal plate and has a trapezoid rostral plate. Melanosteus has some tumor spots in the preorbital and suborbital plate. Bulongosteu s is the first arthrodire reported from Xinjiang of northwest China, and also the first relatively complete aspinothoracid reported in China. Strikingly, Bulongosteu s is the first aspinothoracid arthrodire found in the continental strata in the world. The discovery of the new material significantly expands the geographical and ecological distribution of aspinothoracid arthrodires, and also shows that aspinothoracids, like many fishes in the Late Devonian, are euryhaline.

Climate and landscape reconstruction of the Arroyo Chijuillita Member of the Nacimiento Formation, San Juan Basin, New Mexico: Providing environmental context to early Paleocene mammal evolution

The sedimentologic, stratigraphic, and paleopedologic attributes of a laterally continuous outcrop exposure of the basal portion of the lower Paleocene Arroyo Chijuillita Member of the Nacimiento Formation located in the Bisti/De-Na-Zin Wilderness Area was examined to provide paleoenvironmental context for the Puercan North American Land Mammal Age (NALMA) terrestrial ecosystems of the San Juan Basin, New Mexico. The studied outcrop spans the middle and late Puercan (Pu2-Pu3) NALMA, and thus is an ideal location to assess the potential causes of faunal turnover between Pu2 and Pu3. The stratal succession consists of fluvial channel and laterally continuous sandstone and associated fine-grained floodplain deposits, including paleosols that document variability in drainage and maturity. Quantitative and qualitative reconstructions of paleoclimate from paleosols, supplemented by analyses of fossil floras, indicate no change in climate through the studied interval. Further, there is no evidence that regional tectonics relate to changes in depositional environments. Given the lack of evidence for climate change or regional tectonics, and the characteristics of the fluvial facies and the paleosols across the study locality, we interpret the spatial and temporal variability in paleopedologic and sedimentologic characteristics to be the result of autocyclic channel avulsion and migration. Additionally, the stable climatic conditions and lack of tectonic influence between the Pu2 and Pu3 biozones suggest that mammalian turnover is unlikely to have been caused by extrinsic environmental changes (i.e. climate or tectonics) and is instead probably the result of rapid evolution following the K-Pg boundary and/or competition from the migration of new mammalian species into the basin.

Sedimentology and alluvial architecture of the Bajo Barreal Formation (Upper Cretaceous) in the Golfo San Jorge Basin: Outcrop analogues of the richest oil-bearing fluvial succession in Argentina

This study addresses the sedimentology, architecture and dimensions of fluvial deposits in the Bajo Barreal Formation (Upper Cretaceous) in the Codo del Senguerr anticline, Golfo San Jorge Basin, Argentina. The stratigraphic framework of the 450–650 m thick alluvial succession was carried out through description and interpretation using 18 detailed sedimentary logs along a 4.5 km wide exposure, where 314 fluvial channels were analysed. The Bajo Barreal Formation is a high-accommodation fluvial succession, dominated by fine-grained floodplain deposits, with isolated-to-vertically stacked, sheet-like, low-sinuosity or braided fluvial channels of limited lateral mobility clustered in several coeval channel belts, in which fluvial channels were relocated by avulsion. The Lower Member consists of reworked ash-fall materials preserved in floodplain areas (78%), floodplain sandstones (4%) and fluvial channels (18%). The mean thickness of sandbodies is 2.96 m (n = 118) and mean true width is 112 m (mean W:T = 43), with thicker and wider sandbodies in upper levels of the Member. The Upper Member consists of grey siltstones and mudstones preserved in extensive lowland areas (78%) and fluvial channels (19%), with scarce preservation of sandstones in the proximal floodplain (3%). Fluvial channels are narrow low-sinuosity sheets with comparable thickness (mean = 3.21 m, n = 196), but greater width than those of the Lower Member (mean width = 147 m, mean W:T = 50). Paleoflow data from 298 fluvial sandbodies indicates a paleoflow direction toward the SE (Az. 112°). Rivers flowed parallel to inherited early Cretaceous normal faults and are oriented to high-angle (>70°) to the current axis of the Codo del Senguerr anticline, suggesting that the uplift of the anticline occurred after the deposition of the formation. The variation in geometry of fluvial channels in both Members of the Bajo Barreal Formation could help in planning and developing primary and/or enhanced oil recovery projects in nearby oilfields, and provide data necessary for modelling the subsurface connectivity of hydrocarbon reservoirs.

Facies And Architectural Analysis To Interpret Avulsion Style and Variability: Upper Cretaceous Blackhawk Formation, Wasatch Plateau, Central Utah, U.S.A

Abstract Sequence stratigraphic models emphasize the role of external, allogenic controls such as relative sea level, tectonic subsidence, and climate on alluvial to coastal plain stratigraphic architecture, at the expense of internal, autogenic behaviors such as avulsion. These models are tested via sedimentologic analysis of fine-grained floodplain deposits and their relationship to channelized fluvial sandbodies in a well exposed alluvial to coastal plain succession developed under progressively increasing distance from the coeval shoreline and an accompanying decrease in the rate of accommodation creation (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, U.S.A). The studied alluvial to coastal plain succession contains fourteen lithofacies, which are grouped into four associations and eight architectural elements. Facies associations consist of: 1) channelized fluvial sandstone bodies; 2) nonchannelized fluvial sandstone and siltstone bodies; 3) fine-grained floodplain deposits; and 4) brackish floodplain deposits. Paleosols are immature, and are classified as entisols, inceptisols, and histosols. Integrated analysis of paleosol character, facies analysis, and architectural-element analysis indicates deposition on a rapidly aggrading floodplain that was subject to frequent crevassing and channel-belt switching. The character, relative abundance, and vertical stacking of the paleosol types, architectural elements, and the first three facies associations exhibit little variation stratigraphically, from base to top of the Blackhawk Formation (c. 250 m), or paleogeographically between the northern and southern limits of the outcrop belt (c. 100 km). Local stratigraphic architectures record three styles of avulsion, representing avulsion by reoccupation, avulsion by progradation, and avulsion by incision. Avulsion style displays no consistent pattern with stratigraphic position or paleogeographic location. In combination, these results suggest that floodplain sedimentation and local stratigraphic architecture are dominated by autogenic avulsion. The three observed avulsion mechanisms operated in apparent uniformity over a range of tectonic subsidence rates (c. 80–700 m/Ma) and distances from the coeval shoreline (c. 0–100 km).

The effect of fluvial environments on sediment bleaching and Holocene luminescence ages — A case study from the German Alpine Foreland

This study investigates the potential of luminescence to date deposits from different fluvial sedimentary environments; namely point bar deposits, sandy and silty channel fills and floodplain sediments. Samples were taken from Holocene (<5 ka) terraces of the Lech and Danube rivers, for which independent age constraint is available through 14C ages, archaeological data and historical maps. OSL-ages were obtained using small aliquots of coarse grain quartz for the majority of samples. Two further samples were dated by the IRSL-signals of polymineral fine grain extracts, as no sufficient number of coarse grains could be extracted from these sediments. In order to detect and ac-count for incomplete bleaching, we used the decision process suggested by Bailey and Arnold [Statistical modelling of single grain quartz De distributions and an assessment of procedures for estimating burial dose. Quaternary Science Reviews 25, 2475–2502, 2006]. Although their model was designed for single grains of quartz, our study shows that it is also applicable to multiple grains of quartz, pro-vided that a low number of luminescent grains is present on one aliquot. Luminescence ages of point bar deposits and a sandy channel fill correspond most closely to the independent age control. In the floodplain, sand-striped floodplain channel deposits were incompletely bleached to a moderate degree, yielding ages with acceptable overestimations, while fine-grained floodplain deposits were worst bleached. One crevasse splay deposit was so severely incompletely bleached that none of the age models was able to yield accurate ages.

Stratigraphic architecture of fluvial deposits of the Cretaceous McMurray Formation, Athabasca oil sands, Alberta, Canada

In-situ thermal recovery of bitumen requires a detailed understanding of the subsurface stratal architecture of oil-sands deposits. In the present study, 455 well-logs and 10 drilled cores acquired at Christina Lake (Township 76–77, Range 6W4), northeastern Alberta, Canada are examined to reveal the stratigraphic architecture of the oil-sands deposits of the Early Cretaceous McMurray Formation, focusing on the lower fluvial succession overlying unconformably Paleozoic limestone. The lower McMurray succession comprises 3 major lithologic units of thick sand, interbedded sand/mud, and thick mud, which represent main fluvial channels, proximal floodplains with crevasse splays close to the main channels, and distal floodplains, respectively. Spatial correlation of well-log data shows the stratigraphic architecture of the succession and the paleotopography of the underlying unconformity surface. Thick channel sand bodies forming good bitumen reservoirs occur mainly along the axis of a paleovalley with minor intercalation of fine-grained floodplain deposits, which is ascribed to frequent interconnection of channel bodies resulting from slow increase of accommodation space during lowstand and the limitation of channel locations to the narrow axial part of the paleovalley during the early stage of valley-filling. The channel bodies tend to decrease in thickness and proportion toward the valley margin and upsection at the expanse of increase in proportion of fine-grained deposits. This overall stratigraphic architecture is in agreement with the sequence stratigraphic model. The succession, however, locally displays significantly different architectures from the model, controlled by the paleotopography. The recessed areas between topographic highs on the valley margins are dominated by fine-grained floodplain deposits, because those areas were not suitable for being occupied by channels.

Differential transportation and deposition of terrestrial biomarkers in middle Eocene fluvial to estuarine environments, Hokkaido, Japan

Abstract Higher plants constitute a significant portion of sedimentary organic matter in middle Eocene fluvial–floodplain–estuarine sedimentary rocks exposed in the Bibai region of central Hokkaido, Japan. The differential transportation and deposition of terrestrial biomarkers were studied in this area. The distribution of higher plant and fungal biomarkers in the Bibai non-marine sedimentary rocks was constrained by sedimentary facies and lithology. Claystones and siltstones lacking coaly fragments tended to be rich in angiosperm biomarkers, whereas coal and coaly mudstones were comparatively rich in gymnosperm biomarkers. Perylene, a possible fungal biomarker, was detected in fine-grained floodplain deposits. Facies and lithologic controls on terrestrial biomarker distribution can be attributed to the differential transportation and deposition of organic debris and degradation products derived from higher plants and fungi. Redox potential of the depositional environment is another important factor for the formation and preservation of terrestrial aromatic biomarkers. Reconstruction of land-based paleovegetation from terrestrial biomarkers in non-marine sediments requires careful consideration of the differential transportation and preservation of precursory terrestrial organic compounds.

Fingerprinting historical fluvial sediment fluxes

In order to better understand the human impact on fluvial sediment dynamics at various timescales, information on the changing connectivity between the various geomorphic units in a landscape is crucial. Quantitative sediment budgets for various time periods provide a first assessment of changes in coupling between slopes and river systems. However, the application of sediment fingerprinting can yield additional spatial information on the sediment pathways. Furthermore, the fingerprint approach can also be used in cases where detailed sediment budgets are difficult to obtain. Provenance studies have been executed in various sedimentary environments, covering a wide variety of spatial and temporal scales. Here, an overview is provided of the tracer properties that can be used to distinguish between the various sediment sources in a river catchment. From this general compilation, those tracers useful to fingerprint fine-grained floodplain deposits on a historical timescale (decadal to millennial) are identified. Geochemical and mineralogical compositions, mineral magnetic signals and isotope ratios can be considered the most suitable fingerprinting properties. The impact of source area weathering, grain size selectivity of erosion and sedimentary processes and post-depositional alteration on tracer properties from source to sink are considered. Finally, a synopsis is given of the qualitative to fully quantitative approaches that can be used to discriminate the sources of fine-grained alluvial sediments.

USING LASER ABLATION-INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (LA-ICP-MS) TO EXPLORE GEOCHEMICAL TAPHONOMY OF VERTEBRATE FOSSILS IN THE UPPER CRETACEOUS TWO MEDICINE AND JUDITH RIVER FORMATIONS OF MONTANA

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to determine rare earth element (REE) content of 76 fossil bones collected from the Upper Cretaceous (Campanian) Two Medicine (TMF) and Judith River (JRF) Formations of Montana. REE content is distinctive at the formation scale, with TMF samples exhibiting generally higher overall REE content and greater variability in REE enrichment than JRF samples. Moreover, JRF bones exhibit relative enrichment in heavy REE, whereas TMF bones span heavy and light enrichment fields in roughly equal proportions. TMF bones are also characterized by more negative Ce anomalies and greater U enrichment than JRF bones, which is consistent with more oxidizing diagenetic conditions in the TMF. Bonebeds in both formations show general consistency in REE content, with no indication of spatial or temporal mixing within sites. Previous studies, however, suggest that the bonebeds in question are attritional assemblages that accumulated over considerable time spans. The absence of geochemical evidence for mixing is consistent with diagenesis transpiring in settings that remained chemically and hydrologically stable during recrystallization. Lithology-related patterns in REE content were also compared, and TMF bones recovered from fluvial sandstones show relative enrichment in heavy REE when compared with bones recovered from fine-grained floodplain deposits. In contrast, JRF bones, regardless of lithologic context (sandstone versus mudstone), exhibit similar patterns of REE uptake. This result is consistent with previous reconstructions that suggest that channel-hosted microfossil bonebeds of the JRF developed via the reworking of preexisting concentrations embedded in the interfluve. Geochemical data further indicate that reworked elements were potentially delivered to channels in a recrystallized condition, which is consistent with rapid adsorption of REE postmortem.

Exhumed channel sandstone networks within fluvial fan deposits from the Oligo-Miocene Caspe Formation, South-east Ebro Basin (North-east Spain)

The Oligo-Miocene Caspe Formation corresponds to the middle fluvial facies of the wider Guadalope-Matarranya fluvial fan, located in the South-east Ebro foreland basin (North-east Spain). At the time of the Caspe Formation deposition, this sector of the Ebro basin underwent a very continuous, moderate sedimentation rate. Lithofacies comprise deposits from channellized and unchannellized flows. Channellized flow lithofacies form multi-storey ribbon-like sandstone bodies that crop out as extensive sandstone ridges belonging to exhumed channel networks. Width/thickness ratios of these channel-fill bodies average close to six. Sinuosity is usually low (most common values around 1·1), although it can be high locally (up to 2). Thicknesses range from a few metres to 15 m. Unchannellized flow lithofacies form tabular bodies that can be ascribed to overbank deposits (levees, crevasse splays and fine-grained floodplain deposits) and also to frontal lobes, although recognition of this last case requires exceptional outcrop conditions or geophysical subsurface studies. The unchannellized flow lithofacies proportion ranges from 75% to 97·8%. Methods applied to this study include detailed three-dimensional architectural analysis in addition to sedimentological analysis. The architecture is characterized by an intricate network of highly interconnected ribbon-like sandstone bodies. Such bodies are connected by three kinds of connections: convergences, divergences and cross-cuttings. Although the Caspe Formation lithofacies and architecture resemble anastomosed channels (low topographic gradient, high preservation potential, moderate aggradation rate, high lateral stability of the channels, dominance of the ribbon-like morphologies and high proportion of floodplain to channel-fill sediments), an unambiguous interpretation of the channel networks as anastomosed or single threaded cannot be established. Instead, the observed architecture could be considered as the product of the complex evolution of a fluvial fan segment, where different network morphologies could develop. A facies model for aggrading ephemeral fluvial systems in tectonically active, endorheic basins is proposed.

Facies and architectural element analysis of a meandering fluvial succession: The Permian Warchha Sandstone, Salt Range, Pakistan

The 30 to 155 m thick Early Permian (Artinskian) Warchha Sandstone of the Salt Range, Pakistan is a conglomerate, sandstone and claystone succession within which seven lithofacies types (Gt, St, Sp, Sr, Sh, Fl and Fm) occur in a predictable order as repeated fining-upward cycles. Common sedimentary structures in the conglomerates and sandstones include planar and trough cross-bedding, planar lamination, soft sediment-deformed bedding, compound cosets of strata with low-angle inclined bounding surfaces and lags of imbricated pebbles. Structures in the finer-grained facies include desiccation cracks, raindrop imprints, caliche nodules and bioturbation. Groups of associated facies are arranged into nine distinct architectural elements (channels, gravel bars, sandy bedforms, downstream and laterally accreting barforms, sand sheets, crevasse splays, levees, floodplain units and shallow lakes), which is consistent with a fluvial origin for the succession. The types of architectural elements present and their relationship to each other demonstrate that the Warchha Sandstone preserves a record of a meandering river system that drained the northern margin of Gondwanaland. The dominance of fine-grained (floodplain) facies over gravel-grade (channel-base) facies and the widespread occurrence of large-scale lateral accretion elements supports the interpretation of a high-sinuosity, meandering fluvial system in which channel bodies accumulated via the lateral accretion of point bars but in which the active channels covered only a small part of a broad floodplain at any time instant. Although the regional and temporal distribution of these deposits is complex, in broad terms the lower part is dominated by stacked, multistorey channel bodies, whereas single-storey channel elements isolated in abundant fine-grained floodplain deposits dominate the middle and upper parts of the formation.

Depositional facies, architecture and environments of the Sihwa Formation (Lower Cretaceous), mid-west Korea with special reference to dinosaur eggs

This paper presents detailed facies and architectural analyses and palaeoenvironmental reconstruction of the Sihwa Formation (Lower Cretaceous), mid-west Korea, which comprises an about 3-km-thick non-marine succession containing abundant dinosaur eggshells. Based on constituent facies, bedset geometry, stacking pattern, and bounding surface characteristics, the entire succession can be classified into three architectural elements. Element I occurs along the basin margin and shows a monotonous stacking of tabular or crudely stratified conglomeratic units. It represents alluvial-fan deposits of debris-flow-dominated fan and sheetflood-dominated fan. Element II is characterized by multi-storey, sheet or upward-widening, conglomeratic channel-fills whose maximum thickness exceeds 1.5 m. Each channel-fill unit is encased within red–brown silty sandstones of Element III with sharp erosional bases but diffuse gradational upper boundaries. It consists generally of (1) cut-and-fill deposit (trough cross-stratified or openwork conglomerate) in the lower part and (2) composite low-relief bar deposit (lenticular conglomerate and stratified gravelly sandstone) in the upper part. Shallowness of each cut-and-fill unit, absence of fining-upward lateral accretion package and the predominance of simple-bar deposit collectively suggest deposition from ephemeral braided streams. Element III typically shows a fining-upward stacking of (1) single- or multi-storey small-scale (<1.5 m thick) channel fills with limited lateral extent of less than 15 m, (2) poorly sorted, graded and diffusely bounded silty conglomerates or gravelly siltstones with discontinuous gravel sheets and pockets and (3) homogeneous or graded, red–brown fine-grained deposits with calcretes and burrows in ascending order. Element III is interpreted as channel-margin to floodplain, including crevasse channel fill, crevasse splay and floodplain fines. The entire sequence of the Sihwa Formation can be divided into the alluvial-fan and terminal-fan successions. The alluvial-fan succession displays a progradational stacking pattern and indicates a low rate of basin subsidence in the initial phase of rifting. The upper terminal-fan succession consists of proximal braided stream and distal floodplain deposits in the western and central parts of the basin and is characterized by an axial dispersal pattern and an aggradational stacking. It suggests rapid subsidence of the basin floor during the main phase of rifting. The asymmetrical cross-basin distribution of each architectural element reflects a half-graben structure of the basin with steep-gradient fault-bounded eastern margin (footwall block) and gently sloped, flexural western margin (hangingwall block). The predominance of ephemeral braided-stream deposits along with red–brown fine-grained floodplain deposits with common calcretes indicates arid to semi-arid palaeoclimates. Approximately 140 dinosaur eggs (Faveoloolithidae and Dendroolithidae) were identified mainly from the (gravelly) siltstones and small-scale channel fills of Element III deposits and partly from the cut-and-fill conglomerates of Element II deposits. The eggs commonly retain their original oval shape but are invariably breached and stuffed with the substrate of gravels and silt. They are either isolated or clustered, forming a circular concentration in plan view. The abundant yield of eggs, more than 20 eggs in 5 separate nests from a single depositional unit, suggests a dense population of the parental dinosaurs. The repetitive occurrence in many stratigraphic horizons reflects site preference as a nesting habitat of the near-channel or abandoned channel areas.

An Overview of Younger Kathmandu Lake, Nepal, during the Late Quaternary, with Special Reference to Ferruginous Structures in Carbonaceous Sediments

The Kathmandu Basin in Central Nepal is located in the Lesser Himalayas. Older Kathmandu Lake evolved during the Pliocene and Early Pleistocene; Younger Kathmandu Lake with its widespread carbonaceous rocks and ferruginous biogenic structures silted up during the Late Quaternary. Sedimentological analyses supplemented with palynological and chemical studies of carbonaceous sediments formed the basis of the present paleogeographic and paleoclimatic studies. The basin was covered by a perennial freshwater lake before 30,000 yr. B.P. It was silted up with alluvial and fluvial sediments. Diatomaceous earths, and carbonaceous mud- and siltstones were deposited in the center of the lake and small relict ponds. Toward the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. The most proximal parts of this alluvial-fluvial sedimentary wedge also have debris flows interfingering with fine-grained floodplain deposits. Three lake highstands (> 30,000 yr. B.P., 28,000-19,000 yr. B.P., 11,000-4000 yr. B.P.(?)) have been recognized in the sedimentary record of Younger Kathmandu Lake. Second-order waterlevel fluctuations were triggered by damming as a result of tectonically induced landslides. Firstorder water-level fluctuations were due to climatic changes. During the last highstand, favorable conditions occurred for vertical to subvertical tubular ferricretes to develop. The tubular structures are likely to have been created by bottom dwellers. Three different types of mineralization have evolved during diagenesis, each of which is characterized by different Fe-bearing marker minerals. The overall silting-up process in Kathmandu Basin is manifested on a microscopic scale in the textural and mineralogical variations of Fe mineralization in these biogenic sedimentary structures.

Sedimentology of the Upper Cretaceous mammal- and dinosaur-bearing sites along the Râul Mare and Barbat rivers, Ha [formula omitted]eg Basin, Romania

Mammal and dinosaur localities in the Ha t ̧ eg Basin belonging to the Upper Cretaceous Sânpetru and Densuş-Ciula formations have been known since the beginning of the last century. Recently, two new exposures with a comparable fauna have been discovered in the Râul Mare valley. The sediments at these new sites, Toteşti-baraj and Năla t ̧ -Vad, have been compared with the sediments at the previously known Pui site. At all three sites the deposits reflect a fluvial environment with coarse-grained channel deposits channelized in fine-grained floodplain deposits with calcrete palaeosols. However, the nature of the calcrete palaeosols is different at the three sites. At Toteşti-baraj and Năla t ̧ -Vad, hydromorphic calcic vertisols have been observed, and locally these grade into groundwater calcretes. At Pui, the calcretes are associated with redbed deposits. Considering the similar palaeogeographical situation and age of the deposits at the three sites, the difference in soil type should not be interpreted as a climatic difference but rather as a difference in the height of the palaeogroundwater table. The soils at Pui are indicative of a general semi-arid climate. Those at Toteşti-baraj and Năla t ̧ -Vad do not directly reflect the climate, but rather the high seasonal groundwater table. Differences in palaeohydrology between the sites are reflected in the fossil content with humidity-loving species only occurring at the Râul Mare sites.

Differential subsidence in the Ness Formation (Bajocian), Oseberg area, northern North Sea: Facies variation, accommodation space development and sequence stratigraphy in a deltaic distributary system

The Ness Formation (Bajocian) consists of the alluvial distributary plain deposits of the Middle Jurassic Brent delta. The formation comprises fluvial channel sandstones intercalated with fine-grained floodplain deposits. The sandstone bodies are significant hydrocarbon reservoirs in the Oseberg field and surrounding smaller structures in the Norwegian North Sea, to which the present study pertains. The alluvial succession shows significant thickening across normal faults, reflecting syndepositional differential subsidence. The thickness proportion of fluvial sandstones varies with the succession thickness. Where the succession is relatively thin, it is characterized by a large variation in the content of fluvial sandstones. Where thicker, the succession shows less variation in the proportion of fluvial sandstones. The proportion of fluvial sandstones tends to stabilize or even decrease in the thickest profiles of the formation. These findings are in contrast to theoretical alluvial stratigraphy models, which predict greater sandstone body proportions in areas of greater subsidence. The stacking pattern of fluvial sandbodies shows widespread temporal changes that can be correlated throughout the study area, and are independent of variations in thickness of the formation. These facies changes are approximately chronostratigraphic and allow for the definition of two sequences, each comprising a low-accommodation systems tract, succeeded by a high-accommodation systems tract.

Sedimentologic Aspects, Mannville Group, Southwestern Alberta: ABSTRACT

Due to the relative paucity of data, the Mannville sediments located at the western margin of the plains (T10-20, R27W4 to W5) have not been subjected to detailed study. However, recent exploration activity in this area has resulted in the acquisition of much core and drilling data. Using these data, together with those obtained from outcrop studies in the Foothills and from all the available core toward Range 20, a regional appreciation of the stratigraphy, sedimentology, and diagenesis of southern Alberta is now possible. Three major phases of sedimentation are recognized. The first phase of Mannville sedimentation commenced with the deposition of coarse conglomerates in the Foothills (Cadomin) and chert-rich, often pebbly, sandstones in the plains (Basal Quartz, Cutbank, and Sunburst equivalents). These sediments were deposited as shallow braided or meandering stream deposits, often restricted to broad valleys on the Pre-Cretaceous unconformity. Fine-grained flood-plain deposits with well-developed pedogenic horizons are recognized within this interval. The second, a transgressive phase of sedimentation, followed, with the deposition of bentonitic shales and fossiliferous limestones (Ostracod Member) and quartzose (Glauconitic) sandstones. The areally extensive sediments, which were primarily deposited in a complex of shallow restricted marine(?) environments, have a distinctive mineralogy compared to the underlying and overlying units. Changes in sediment provenance can account for this mineralogic diversity. Thirdly, continental conditions returned to much of the area during post-Glauconitic time. This interval is represented by silts and shales intercalated with feldspathic and volcanic channel and crevasse sandstones. The textural and mineralogic immaturity of these sandstones is typical of the upper Mannville sediments of Alberta. Complexities arise to the east of Range 22 where deep-channels originating during Ostracod, ?Glauconitic, and Upper Mannville times have cut through the underlying sediments. Recognition of the initiation point of these channels is important because reservoir quality is largely controlled by sediment composition. The most prospective reservoirs are quartz-rich sandstones which were not subjected to extensive silica cementation, a consequence of deep burial. End_of_Article - Last_Page 585------------