Sandy Braided River Research Articles

Sedimentary architecture of a sandy braided river with seasonal hydrodynamic variations: insights from the Permian Lower Shihezi Formation, Ordos Basin, China

Sedimentary architecture of a sandy braided river with seasonal hydrodynamic variations: insights from the Permian Lower Shihezi Formation, Ordos Basin, China

Sedimentary architecture of a sandy braided river: insights from a flume experiment

Sedimentary architecture of a sandy braided river: insights from a flume experiment

Sedimentary Characteristics and Internal Architecture of a Sandy Braided River: Insights from a Numerical Model

Sandy braided river deposits are widely observed in ancient stratigraphic records and modern sedimentary basins. Frequent channel migration shapes intricate internal architecture, while research on its influence on fine sedimentary architecture remains limited. This study utilizes DELFT3D to simulate and analyze the sedimentary processes and architecture of the channel bar in a sandy braided river. The results show that: (1) The channel bar consists of downstream accretion (DA), lateral accretion (LA), and aggradational accretion (AA). (2) Channel bars can be divided into three types: unit bars, compound bars, and complex compound bars. Unit bars are lobate-shaped and consist of multiple accretions, including DA in the core, AA layered above, and LA deposited on the sides. Compound bars result from the migration and amalgamation of multiple unit bars, characterized by a greater number of accretions. Compound bars can further be classified as DA-dominated or LA-dominated, depending on the dominant accretion type. Complex compound bars form from the merging of multiple unit bars and compound bars. In this paper, insights from a braided river on its sedimentary processes and evolution may help to improve river management and water resources research. The in-depth understanding of sedimentary architecture on channel bars can be utilized for detailed subsurface oil and gas reservoir characterization.

Sedimentary characteristics of sandy braided river deposits and factors controlling their deposition: A case study of the lower Shihezi Formation in the northern ordos basin, China

Fluvial sandstones have been identified as significant reservoirs due to their importance in global hydrocarbon exploration. The current study investigated the sedimentary characteristics and temporal evolution of sandy braided river systems as well as the factors controlling their deposition in the Lower Shihezi Formation (H1-1 to H1-4 sub-member) of the Xinzhao area in the northern Ordos Basin using well logs, core intervals, whole rock geochemical data, and other measured reservoir parameters. Within the braided river deposits, five architectural elements were identified: multistory braided channel, single-story braided channel, sandy braid bar, overbank, and floodplain. The evolution of architectural elements in the Lower Shihezi Formation is closely related to paleoclimatic conditions and tectonic activity. Climate influences sediment accumulation through its effect on terrestrial detrital input, and tectonism influences stacking patterns and architectural style by influencing subsidence rates and accommodation changes. High sediment deliveries and high-energy hydrodynamic conditions in warm and humid climates, combined with low subsidence rates and low accommodation/sediment supply (A/S) ratios, can result in the formation of highly amalgamated braided channels and braid bars as well as complex vertical stacking patterns and aggradational processes in the H1-2 and H1-3 sub-members. Conversely, the dry and hot climates, high subsidence rates and associated high A/S ratios are responsible for the low detrital influx input, stacking of isolated sand bodies, and high preservation of overbank and floodplain deposits in the H1-4 sub-member. Considering the accommodation-based systems tracts, we contend that climate and tectonism both have a significant impact on the evolution and architectural styles of sandy braided river deposits. It is suggested that climates control the stacking pattern of fluvial sand bodies during high accommodation systems tracts, whereas both climates and accommodation spaces influence architectural styles and the degree of sand body amalgamation during low accommodation systems tracts. Considering the thick, highly amalgamated sand bodies with better reservoir properties and high vertical and lateral connectivity in the H1-2 and H1-3 sub-members, we posit that these stacked multistory sand bodies should be the preferential targets for hydrocarbon exploration. The findings of this study can further elucidate the architectural styles and factors that influence the deposition of sandy braided river systems under complex tectonic and climatic conditions.

Reservoir Modeling of Braided River Reservoirs Based on Geological Knowledge Database: A Case Study of P1x Formation of the Daniudi Gas Field, Ordos Basin, China

Abstract The sandy braided river depositional system developed in the Lower Shihezi Formation of the Daniudi gas field, Ordos Basin. It has the characteristics of frequent migration and oscillation of braided channels and large well spacing, making it challenging to portray the braided river sand body in this area, bringing uncertainty to the 3D geological modeling of the reservoir. This study takes the primary gas reservoir H1 member as an example. It establishes a quantitative geological knowledge database for the reservoir by statistically fitting the correlation equations between the braided channel and channel bar in the planes and profiles to reduce the uncertainty of reservoir modeling. This study combines the multisource and multiscale information from modern sedimentation, field outcrops, and tank simulation experiments of the braided river. From the data, the distribution intervals for the thickness and width of the braided channel sand body in the H1 member are 1–22 m and 7–320 m, respectively, and the thickness, width, and length distribution intervals of the sand body in the channel bar are 3–30 m, 80–1500 m, and 240–4200 m, respectively. A 3D training image is established using the object-based simulation method based on the H1 member’s well data and combining the quantitative parameters of various microfacies in the geological knowledge database. The multiple-point geostatistical modeling method is applied to establish a sedimentary microfacies model. The model’s uncertainty is reduced through multi-information fusion constraint modeling, providing a reliable basis for guiding the prediction of the remaining gas in the Daniudi gas field.

Analysis of the main controlling factors and the distribution rules of effective reservoirs in sandy braided river delta plain — A case study of Ct3 area, Ordos Basin, China

To clarify the main controlling factors of effective reservoirs in the sandy braided river delta plain, the eighth member of Xiashihezi Formation in the Ct3 area, Ordos Basin, is taken as an example. We describe and analyze approximately 600 cores in detail and correspond the cores on the logging curve. We summarize the logging facies mode suitable for the Ct3 area, and finally investigate the types and spatial distribution characteristics of sedimentary microfacies. The P2h8 member of Ct3 area in Ordos Basin is braided river delta plain subfacies. Sedimentary microfacies are divided into three types: distributary channel, channel bar, and distributary interchannel. The sand body in the h84 of the P2h8 member is the thickest, and the number of channel bar is the largest. All gas reservoirs (effective reservoirs) are developed on the sand body of the channel bar, which is the most favorable microfacies type. Poor gas reservoirs are mostly developed in distributary river channel sand body, whereas gas-bearing reservoirs (gas reservoirs and poor gas reservoirs) are undeveloped in distributary interchannel. According to the hydrodynamic conditions and sedimentary mechanism, the channel bar is divided into the bar head, the bar tail, the bar main body, and the bar wing. It is found that 63% of the gas reservoirs in the channel bar are developed in the bar main body, followed by the bar tail. The number of gas reservoirs developed at the bar head and the bar wing is the least. The results found that the most effective reservoirs are developed in the bar main body. Sedimentary microfacies, sand-body thickness, and grain size are the main controlling factors for effective reservoirs. The research results can guide the gas field exploration and later well location deployment in the sedimentary environment similar to the Ct3 area.

3D Sedimentary Architecture of Sandy Braided River, Based on Outcrop, Unmanned Aerial Vehicle and Ground Penetrating Radar Data

Ground Penetrating Radar (GPR) is a geophysical method that uses antennas to transmit and receive high-frequency electromagnetic waves to detect the properties and distribution of materials in media. In this paper, geological observation, UAV detection and GPR technology are combined to study the recent sediments of the Yungang braided river study area in Datong. The application of the GPR technique to the description of fluvial facies and reservoir architecture and the development of geological models are discussed. The process of GPR detection technology and application includes three parts: GPR data acquisition, data processing and integrated interpretation of GPR data. The geological surface at different depths and scales can be identified by using different combinations of frequencies and antenna configurations during acquisition. Based on outcrop observation and lithofacies analysis, the Yandong Member of the Middle Jurassic Yungang Formation in the Datong Basin has been identified as a typical sandy braided river sedimentary system. The sandy braided river sandbody changes rapidly laterally, and the spatial distribution and internal structure of the reservoir are very complex, which has a very important impact on the migration and distribution of oil and gas as a reservoir. It is very important to make clear the characteristics of each architectural unit of the fluvial sand body and quantitatively characterize them. The architectural elements of the braided river sedimentary reservoir in the Datong-Yungang area can be divided into three types: Channel unit, bar unit and overbank assemblages. The geological radar response characteristics of different types of sedimentary units are summarized and their interfaces are identified. The channel sediments form a lens-shaped wave reflection with a flat at the top and convex-down at the bottom in the radar profile, and the angles of the radar reflection directional axes are different on both sides of the sedimentary interface. In the radar profile, the deposit of the unit bar is an upward convex reflection structure. The overbank siltation shows a weak amplitude parallel reflection structure. The flood plain sediments are distributed continuously and stably in the radar profile, showing weak reflection characteristics. Different sedimentary units are identified by GPR data and combined with Unmanned Aerial Vehicle (UAV) detection data, and the establishment of the field outcrop geological model is completed. The development pattern of the diara is clarified, and the swing and migration of the channel in different stages are identified.

Study on the Distribution of Interlayer in Thick Sandy Braided River under Large Well Spacing—A Case of C Oilfield in Bohai Sea

Study on the Distribution of Interlayer in Thick Sandy Braided River under Large Well Spacing—A Case of C Oilfield in Bohai Sea

Fluvial architecture of the Upper Cretaceous Nubia Sandstones: An ancient example of sandy braided rivers in central Eastern Desert, Egypt

The present paper focuses on the fluvial architecture and depositional facies model of the Late Cretaceous large sandy braided-river system of the Nubia Formation in the central Eastern Desert of Egypt. Six lithofacies types and two paleosol pedotypes (entisol and vertisol) are arranged in channel and overbank architectural elements that display first- to fifth-order bounding surfaces. Two distinctive fluvial architectural styles are recognized. The Fluvial architecture of the lower member identifies a low sinuous and energetic sandy braided river, in which the channels cover the entire of fluvial landforms with limited overbank areas. The suggested depositional model resembles the classic Platte-type braided river with its broad, low sinuosity streams, rich sand bedload and minor overbank sediments. Recurrent autogenic fining-upward cycles, rarely preserved overbank deposits, dominant entisol pedotype, prevailing trough cross-bedded sandstones, sporadic to absent planar cross-bedded sandstone, and dominant downstream accretion also imply perennial, sandy braided rivers. By contrast, the fluvial architecture of the upper member is characterized by fining-upward cycles, downstream and lateral accretion elements, and commonly preserved overbank deposits (overprinted by the mature vertisol pedotype). This architecture style is ascribed to a perennial sandy braided river characterized by sinuous energetic streams and laterally developed overbank areas, similar to the modern South Saskatchewan River in Canada. The fluvial depositional system and sequences in the Nubia Formation are strongly controlled by Late Cretaceous base-level fluctuations and tectonic pulses of the Laramide Orogeny.

The Architectural Surfaces Characteristics of Sandy Braided River Reservoirs, Case Study in Gudong Oil Field, China

The architecture analysis of the different orders sedimentary bodies is of great significance to the efficient development of oil and gas fields. In order to investigate the effects of the architectural interfaces on reservoir quality and heterogeneity, this study takes the Gudong oil field as a case to investigate the architectural characteristics of hierarchical bounding surfaces using detailed descriptions of core and wireline logs. Architectural models from the 7th-order to the 3rd-order are analyzed, and the developmental characteristics of the 5th-order braided river, 4th-order single sandstone, and 3rd-order accretion are summarized. The interlayer between two braided rivers is floodplain mud deposition, with poor physical properties, stable thickness, and strong blocking capacity. Two models of interlayers are found in the 4th-order deposition. The first interlayer is between the braided filling channel and midchannel bar, which is composed of generally fine-grained sediments with calcium cementation and poor physical properties. The second interlayer is a transformation belt between two midchannel bars and is generally composed of gravel-scoured deposition with penetration capability. The 3rd-order surfaces are defined as the surfaces of accretions within midchannel bars. Two models of interlayers are also found in the 3rd-order surfaces of accretions. The paleocurrent of the sandy braided river is reconstructed by synthesizing the core data, well logging data, and production performance data. A total of 1 fluvial system (7th-order), 2 compound braided rivers (6th-order), 11 braided rivers (5th-order), 41 midchannel bars (4th-order), and 96 accretions (3rd-order) are developed in the study area.

New Understanding of Bar Top Hollows in Dryland Sandy Braided Rivers from Outcrops with Unmanned Aerial Vehicle and Ground Penetrating Radar Surveys

Bar top hollows (BTHs) are morphological elements recognized in modern braided rivers; however, information regarding their depositional features and types of filling units in ancient strata is unclear. This is an important reason behind why it is difficult to identify BTH units in subsurface reservoirs. A Middle Jurassic dryland sandy braided river outcrop in northwestern China is characterized in this study through the application of an unmanned aerial vehicle (UAV) surveying and mapping, and ground penetrating radar (GPR). A workflow of UAV data processing has been established, and a 3D digital outcrop model has been built. By observation and measurement of the outcrop model and GPR profiles, two types of BTH filled units were found: (a) sandstone-filled, and (b) mudstone-filled hollows. Both of these units were located between two adjacent bar units in an area that is limited to a compound bar, and were developed in the upper part of a braided bar depositional sequence. The ellipse-shaped, sandstone-filled unit measures 10 m × 27 m in map view and reaches a maximum thickness of 5 m. The transversal cross-section across the BTHs displays a concave upward basal surface, while the angle of the inclined structures infilling the BTHs decreases up-section. The GPR data show that, in the longitudinal profile, the basal surface is relatively flat, and low-angle, inclined layers can be observed in the lower- and middle part of the sandstone-filled BTHs. In contrast, no obvious depositional structures were observed in the mudstone-filled BTH in outcrop. The new understanding of BTH has a wide application, including the optimization of CO2 storage sites, fresh water aquifers exploration, and oil and gas reservoir characterization.

Identification of architectural elements based on SVM with PCA: A case study of sandy braided river reservoir in the Lamadian Oilfield, Songliao Basin, NE China

To improve the oil recovery in the middle and late stages of oilfield development, it is very necessary to study the subsurface reservoir architecture. However, how to effectively improve the efficiency of architecture identification is also a big problem for the logging big data in dense well pattern area. In this study, we apply support vector machine (SVM), a supervised machine learning algorithm, and principal component analysis (PCA) data dimension reduction to three types of logging data (spontaneous potential (SP), gamma ray (GR), and acoustic (AC)) in order to automate the identification of braided river architectural elements on the PI2 braided river sand group of the Lamadian Oilfield in the north of Daqing Placanticline in the Songliao Basin, China. Manual qualitative identification shows that the architectural elements of the second sand group of the first member of Putaohua braided river reservoir (PI2) in the Lamadian area consist of mid-channel bars, braided channels, and floodplains. The presence of three types of sediments (medium-to fine-grain sand, fine-grain silty sand, and mudstone) indicates that changes occur in the water body energy within the PI2 braided river. The SVM algorithm development process includes determining the input and output variables, selecting the kernel functions, optimizing the parameters, training the algorithm with known samples, and testing the algorithm. With the twelve feature parameters (median, relative barycenter, variance, and the root of variational variance of SP, GR and AC data, respectively) as input variables and the braided river architecture categories as output variables, we use Gaussian radial basis function (RBF) and grid search to determine the optimal parameters values for penalty factor C and parameter gamma (γ). Without dimension reduction, the identification accuracy of the algorithm is 85.15%. To eliminate data redundancy, we used PCA to reduce the input data dimensionality. With dimension reduction, the identification accuracy of the algorithm increased by 6.93 percentage points to 92.08%. Because the logging response features are not distinctly different in the transition zone of mid-channel bar and braided channel, it is common for these two architectures to be misclassified. Even with this persistent misclassification, the error of our SVM algorithm identification process is only 7.92%, this method is robust enough to provide accurate results when applied to specific repetitive and complex geological big data problems.

Reservoir variations and heterogeneities under valley‐monadnock palaeo‐geomorphology in hinterland area of the Junggar Basin, northwestern China: Sedimentary characteristics and diagenetic alterations

Palaeo‐geomorphology can exert significant effects on reservoir quality via controlling the distribution of depositional facies and diagenetic alterations. However, little attention has been paid to the studies of the reservoir variations and heterogeneities under distinctive valley‐monadnock palaeo‐geomorphology. Taking the Lower Cretaceous Qingshuihe Formation (K1q) in the Junggar Basin as an example, this study improves the current understanding of reservoir potential within topography of this kind by examining the link between depositional facies, diagenesis and their impact on reservoir quality. In total, nine types of lithofacies can be identified in the K1q reservoirs, which can be further grouped into lowstand systems tract (LST) gravelly braided river and debris flow deposits and transgressive systems tract (TST) sandy braided river delta deposits. These reservoirs are rich in rock fragments and characterized by moderate porosity and moderate to low permeability with development of intergranular and dissolution pores. Generally, detrital composition has a greater impact on reservoir quality of the LST conglomerates than on that of the TST sandstones. However, texture maturity only shows positive correlation with physical properties of the sandstone reservoirs. The mechanical compaction was responsible for the major destruction of the initial porosity. However, the large heterogeneities in calcite cements account for the extremely heterogeneous reservoir quality. Calcite cements formed during eodiagenesis usually show poikilotopic fabric and preferentially concentrated inthe basal coarser units/facies within inidividual fining‐upward successions. Consequently, the upper finer units/facies became more favourable for the secondary dissolution and oil charging processes during mesodiagenesis. In contrast, the amounts of authigenic quartz and clay minerals are relatively low. Nevertheless, the contents of different authigenic clay minerals show a variety of relationships with the reservoir quality. The valley‐monadnock topography spatially controls the scales of the LST and TST reservoirs. The simultaneously evolutionary processes between the back‐filling of the valleys and weathering of the monadnocks resulted in the prolonged residence time and large area of depositional water contacting with the weathering crust. This may give rise to the formation of alkaline pore fluids and facilitate the nucleation and growth of eogenetic carbonate cements and grain‐coating chlorite. Furthermore, the reservoir quality generally displays an increasing trend from the monadnock to the valley thalweg due to the increase of hydrodynamic strength and the less influence of mélange debris flow deposits.

Gradual Evolution from Fluvial Dominated to Tide Dominated Deltas and Channel Type Transformation: A Case Study of MPE3 Block in the Orinoco Heavy Oil Belt of the Eastern Venezuelan Basin

Based on the data of core description and sporopollen analysis, the gradual evolution of deltas in vertical direction and transition of channel types in the MPE3 block of the eastern Venezuela Basin have been surveyed by seismic phase and well logging facies interpretation. The results show that due to the great sea level rise, the sedimentary system of the Miocene Oficina Formation in the MPE3 block shifted from the distal‐source sandy braided river delta to tide‐affected delta, and eventually to tide‐dominated delta. Vertically, during the early stage of sedimentation of Oficina Formation, the distributary channels of the delta were dominated by braided river channels. While in the later stage, as the tidal effect was gradually intensified, the channel changed from braided channel to meandering channel. On plane, as a result of differential transgression, sedimentary framework and distribution of sand bodies vary across the study area. Compared with the eastern part, the western part has more braided channels, larger channel bars, less developments of distributary bay and higher ratio of sand to mud. Whereas the braided channels in the south are larger than those in the north. It is the first time we pointed out the impact of marine transgression differences on the sedimentary facies distribution and river type transition in the study area. Factors like the structural and paleogeomorphological change, sea level variation, supply of sediments have strong influence on the evolution of sedimentary system and distribution of sandbodies. It is predicted that the major sandbody is more developed in the central south, which can guide the subsequent horizontal well development.

Bar dynamics in a sandy braided river: Insights from sediment numerical simulations

Bar dynamics in sandy braided rivers are keys to better understand the morphological processes of braided rivers and sediment distributions. Limited by insufficient quantitative information and satellite images that only show the larger-scale morphology and do not reveal the sand bars in the deeper channels, most previous studies have qualitatively assessed bar dynamics, but the ways in which the hydraulic parameters of sandy braided rivers control bar dynamics remains poorly understood. Here, we present sediment numerical simulations on sandy braided rivers to investigate the quantitative controls of river parameters on bar dynamics. Three bar dynamic forms (downstream migration, lateral variations and bar tail elongation) are observed. Unit bars are prone to downstream migration following formation, and the migration rate shows a deceasing trend during braided river evolution. This gradually decreasing migration rate is proven to be strongly connected to the decreasing flow velocity of braided channels that confine bar growth. The lateral variation in bar dynamics exhibits narrowing, migration and widening of sectional morphologies. We suggest that braided channel curvature dominantly controls lateral sediment transportation around the bar and thus results in several lateral variation forms of bars. The bar tail elongation difference between the two sides of the bar is associated with bar asymmetry. Strong bar asymmetry can make apparent differences in bar tail elongation.

Case study of sandbody architecture and quantitative parameters of the far-source sandy braided river: Saertu Oilfield, Daqing, China

Aimed at the braided river sandbodies deposited in the pure oil area of western Saertu Oilfield in the Songliao Basin, Daqing, Placanticle, the sedimentary physical simulation experiments were designed. The characteristics of channel bar microfacies were identified and analyzed by four rounds of experiments. Based on the measurement results, the correlation parameter model of the braided river reservoir architecture was established by describing the geometric occurrence of the structures of the single-phase braided river sandbodies and channel bar. Combined with prototype sedimentation simulation experiment, the internal quantitative architecture of far-source sandy braided river was researched by identifying and analyzing the interface of individual well. The interfaces of the 5th-order, 4th-order and 3rd-order architectures were emphatically analyzed and the inter-well quantitative architecture model was established. The research results showed that the interlayer in the center of channel bar was approximately horizontal, with dip angle of about 1°. In short-axis toward end direction, the dip angle of the architecture interface of accretionary body was 2–4°. Combined with the internal dip angle and occurrence of channel bar sandbodies, the interface of the accretionary body in the central part of channel bar was parallel, with dip angle of no more than 2°. The dip angle of the accretionary body in water flow direction was large, which was 2°–5°. On this basis, the size of braided river, channel bar and accretionary body was calculated quantitatively in the target area, and different orders of braided river sandstone reservoirs were described.

Quantification of bedform dynamics and bedload sediment flux in sandy braided rivers from airborne and satellite imagery

AbstractImages from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure‐from‐Motion (SfM) techniques and application of a depth‐brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near‐bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth‐brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low‐turbidity and shallow (< 2 m deep) water.The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18 km reach and three ~1 km long reaches respectively. Dune and unit bar migration rates are highly variable in response to local variations in planform morphology. Sediment transport rates for dunes and unit bars, obtained by integrating migration rates (from UAV) with the volume of sediment moved (from DSMs using MAV imagery) show near‐equivalence in sediment flux. Hence, reach‐based sediment transport rate estimates can be derived from unit bar data alone. Moreover, it is shown that reasonable estimates of sediment transport rate can be made using just unit bar migration rates as measured from 2D imagery, including from satellite images, so long as informed assumptions are made regarding average bar shape and height. With recent availability of frequent, repeat satellite imagery, and the ease of undertaking repeat MAV and UAV surveys, for the first time, it may be possible to provide global estimates of bedload sediment flux for large or inaccessible low‐turbidity rivers that currently have sparse information on bedload sediment transport rates. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

An Early Jurassic braided river system from Mehadia, South Carpathians, Romania

The Lower Jurassic (Hettangian–Sinemurian) continental deposits of the Mehadia area (Caras–Severin County) belong to the Presacina Basin, Upper Danubian Units of the South Carpathians, Romania. This un-named succession is exposed along large areas within the Greaţca Quarry where deposits of a complex braided river system can be seen. Nine clastic facies are identified within the Mehadia section and grouped into two facies associations reflecting two main sub-environments. (A) a fine-grained association, up to 6.5 m thick, composed of greyish black and dark grey massive and horizontally laminated mudstones interbedded with fine- to medium-grained sandstones showing ripple lamination, tabular low-angle large-scale cross-bedding and in some places massive sandstones. Within this association coarsening and fining-upward sequences are visible, while subordinate pensymmetrical-like sequences were observed. The sediments of the fine-grained association are interpreted as having been deposited along an alluvial plain from flood waters. The sandstone interbeds are interpreted as crevasse channel-fills and the thinner sandstone bodies as proximal crevasse splays. (B) a coarse-grained facies association represented by sandstones and conglomerates, both clast- and matrix-supported, from granule to pebble clast sizes. Almost all basal members of this association are underlain by erosional surfaces, while internal erosional surfaces occur at some levels. The coarse-grained facies association is interpreted as having been deposited within active river channels of a sandy braided river system. The occurrence of numerous erosional surfaces, the recurrence of the facies within this association (channel bodies), as well as the abundant mudstone and sandstone intraclasts all point to frequent conditions of erosion (including redeposition) of previously deposited sediments. The relatively low fine-grained sediment content (<25%) can be explained by two factors: (1) a creation rate for accommodation space was not enough for the accumulation of a large amount of overbank deposits; (2) a lack of larger quantities of fine-grained material being transported in the river system. Both factors may have operated independently or interacted.

Tectonic controls on the stratigraphic development of the rifted Taipei Basin: A late quaternary marine-influenced inland half graben

We establish the three-dimensional stratigraphic architecture of the Taipei Basin and its spatiotemporal palaeoenvironmental development during the past 50 kyr by analysing 36 borehole cores and 177 age dates. We calculate the rates of basin subsidence from the borehole data at depths where radiocarbon age dates are available. Our results indicate that, during the eustatic sea level falling period (35–20 ka), low rates of sediment supply and/or rapid basin subsidence controlled sedimentation, leading to a change in the depositional environment from gravelly braided rivers, through sandy braided rivers, to meandering rivers with falling eustatic sea level. During the early stage of eustatic sea level rise (∼20–10.2 ka), balanced rates of sediment supply, eustasy and basin subsidence maintained the meandering river environment. Rapid sea level rise led to the initial appearance of estuarine facies at ∼10.2 ka and widened the distribution of the estuarine environment after 8.5 ka; however, the coeval phases of rapid basin subsidence ∼10.6–10.2 ka and 9–8.5 ka promoted the first appearance and widening of the estuary, respectively. After 7 ka, when the eustatic sea level and rate of basin subsidence remained relatively stable, sufficient sediment supply gradually infilled the estuary. The established stratigraphy improves our understanding of the factors controlling stratigraphic development in marine-influenced inland half grabens, specifically in the Taipei Basin with high rates of sediment supply. Accommodation space in the Taipei Basin was governed by eustatic fluctuations with modulations by basin subsidence and sediment supply since the last glacial period.

Facies analysis, depositional environments and paleoclimate of the Cretaceous Bima Formation in the Gongola Sub – Basin, Northern Benue Trough, NE Nigeria

Facies analysis of the Cretaceous Bima Formation in the Gongola Sub –basin of the Northern Benue Trough northeastern Nigeria indicated that the Lower Bima Member is composed of alluvial fan and braided river facies associations. The alluvial fan depositional environment dominantly consists of debris flow facies that commonly occur as matrix supported conglomerate. This facies is locally associated with grain supported conglomerate and mudstone facies, representing sieve channel and mud flow deposits respectively, and these deposits may account for the proximal alluvial fan region of the Lower Bima Member. The distal fan facies were represented by gravel-bed braided river system of probably Scot – type model. This grade into sandy braided river systems with well developed floodplains facies, forming probably at the lowermost portion of the alluvial fan depositional gradient, where it inter-fingers with basinal facies. In the Middle Bima Member, the facies architecture is dominantly suggestive of deep perennial sand-bed braided river system with thickly developed amalgamated trough crossbedded sandstone facies fining to mudstone. Couplets of shallow channels are also locally common, attesting to the varying topography of the basin. The Upper Bima Member is characterized by shallow perennial sand-bed braided river system composed of successive succession of planar and trough crossbedded sandstone facies associations, and shallower channels of the flashy ephemeral sheetflood sand – bed river systems defined by interbedded succession of small scale trough crossbedded sandstone facies and parallel laminated sandstone facies. The overall stacking pattern of the facies succession of the Bima Formation in the Gongola Sub – basin is generally thinning and fining upwards cycles, indicating scarp retreat and deposition in a relatively passive margin setting. Dominance of kaolinite in the clay mineral fraction of the Bima Formation points to predominance of humid sub – tropical to tropical climatic conditions. This favors pedogenic activities which are manifested in the several occurrences of paleosols. Pronounced periods of arid climatic conditions are also notable from the subordinate smectite mineralization. Chlorite mineralization at some localities is indicative of elevation of the provenance area, and this is synonymous with deposition of the Bima Formation, because of its syn – depositional tectonics. The absences of lacustrine shales in the syn – rift stratigraphic architecture of the Bima Formation indicates that the lower Cretaceous petroleum system that are common in the West and Central African Rift basins are generally barren in the Gongola Sub – basin of the Northern Benue Trough.