Fluvial Depositional Settings Research Articles

Comprehensive Facies Analysis and Depositional Environments of the Kikim Formation, Garba Mountain, South Palembang Subbasin, Indonesia

This study provides an exhaustive facies analysis of the Kikim Formation within the Garba Mountain area of the South Palembang Subbasin, emphasizing the stratigraphic significance and depositional environments during the Paleogene period. Utilizing field data collected from the Bumi Agung and Batu Belang measurement lines, this research categorizes various lithofacies using Miall’s (1978) classification scheme, identifying eight distinct lithofacies and four facies associations indicative of a predominantly fluvial depositional setting. The results depict a detailed sedimentological framework, showing that the Kikim Formation was characterized by a braided river system with significant gravel and sandstone deposits. This analysis not only sheds light on the geological history and processes shaping the South Sumatra Basin but also enhances the understanding of its potential hydrocarbon reservoirs. By detailing the lithological diversity and facies distribution, this paper contributes to the broader geological knowledge required for effective exploration and management of the region's natural resources.

Depositional Environment and Reservoir Prospect Analysis of Barail Sandstone Exposed in the Northeastern Part of Bangladesh

The study characterized the Oligocene Barail sandstone in the Surma Basin from a reservoir perspective. Data were collected by direct field observations; rock samples were collected from outcrops for detailed sedimentological, petrographic, and geochemical analysis. Collected sedimentary logs are analyzed to delineate the facies associations, sandbody architecture, and depositional environment. A total of ten facies were identified, which were further grouped into three facies associations: FA1: Tidal-influenced multistorey channel-fill deposits, FA2: Fluvial multistorey channel-fill deposits, and FA3: Overbank deposits. Facies analysis suggests that the Barail sandstone was deposited in the tidal-influenced paralic to fluvial depositional settings, and channel body dimensions increase towards the top. A petrographic study shows that these sandstones are mostly litharenite (Q70F4L26) in type, and sediments were derived from quartzose recycled orogens. The average porosity of the sandstones is 16.9%; the pore spaces are of both primary and secondary origin and are mostly interconnected. Porosity distribution in the fluvial multistory channel-fill deposits (FA2)—the upper part of the sandstone succession, is less heterogeneous, while that in the tidal-influenced channel-fill deposits (FA1), occurs in the lower part is more heterogeneous. Both the depositional process and diagenesis influenced the porosity of Barail sandstones. Geochemical analysis suggests that the sandstones are chemically stable and moderately matured. The study demonstrates that the Barail sandstones may be a good petroleum reservoir. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(1), 2023, P 69-88

Multi-scale sedimentary facies controls on fluvial sandstone-type uranium accumulation process in the Songliao Basin, NE China

The relationship between uranium mineralisation and sedimentary facies in a fluvial depositional setting remains a challenging topic. However, the specific effect of muti-scale sedimentary facies control on meandering sandstone-type uranium enrichment is poorly understood. Based on sedimentological, geochemical, mineralogical, and petrophysical analyses, three third-order sequences (SQ1, SQ2, and SQ3) composed of four systems tracts in meandering deposits were identified in the Sifangtai Formation of the northern Songliao Basin. Four subfacies (channel, levee, flood, and abandoned channels) and seven microfacies were identified in meandering rivers, exhibiting spatio-temporal migration of sedimentary fillings. Here, uranium deposits were mainly enriched in semi-abandoned channel deposits during the lowstand systems tract (LST) period of SQ1. Stable-distributed and thick-layered industrial-grade uranium deposits occurred in a positive rhythmic rock assemblages, exhibiting high-frequency variation from thicker medium–fine sandstone to thin charcoal-rich siltstone–fine sandstone. Rapid tectonic uplift and warm-semi humid climate during early Sifangtai period controlled sequence growth and sedimentary filling. Furthermore, sequence and sedimentation during the LST period mediated the formation of uranium-rich rock assemblage and fluid migration pathway, reacting on the abundance, location, and distribution of uranium enrichment. Ultimately, a conceptual model of the sequence and sedimentary control on uranium accumulation was established in the context of meandering rivers. This model revealed that semi humid-warm climate and tectonic inversion (74.9–72.2 Ma) control meandering river, sedimentary hiatus, and charcoal-rich uranium-hosted rock series formation in the LST. Enhanced Cenozoic tectonic uplift (38.0–10.0 Ma) and continental weathering prompted uranium-rich fluids along steep paleotopography, migrated to geochemical barriers of charcoal-rich rock series, and reductive mineralisation around semi-abandoned channel deposits. Our results illustrate the importance of examining multi-scale sedimentary facies to interpret the uranium accumulation process of meandering deposits, ultimately contributing to uranium exploration in similar settings.

Impact of weathering processes on n-alkane pattern in badlands

The aim of this study is to investigate the influence of weathering processes on the distribution of n-alkanes in unweathered and weathered samples of badland material. Organic matter (OM) of both the weathered and unweathered zones in badland environments is derived from the same sources such any differences between the n-alkane pattern of the two zones can be attributed to the impact of weathering processes under different climate conditions. Organic geochemical and mineralogical characterizations were employed to investigate twenty-one samples from weathered badland environments in Italy, Spain, and Canada that originally derive from marine, distal alluvial, and fluvial depositional settings, respectively.According to the distribution of n-alkanes and diterpanes, unweathered samples from Italy have an original mixed aquatic-terrestrial origin of OM, with a variable (low to moderate) contribution of terrestrial plants as precursors, while unweathered samples from Spain predominantly contain OM originating from submerged/floating macrophytes deposited under reducing conditions. Unweathered samples from Canada predominantly consist of terrigenous OM.Intensity of changes in n-alkanes distribution when subjected to weathering depends on the type of precursor biomass and quantity of smectite. Predominantly terrigenous OM under semi-arid conditions shows the highest affinity for adsorption is in contact with smectite surfaces. Mid- and long-chain odd n-alkanes have a tendency to build colloidal particles with polar organic compounds such as fatty acids by strong binding and thus their preservation from degradation is possible.

Obliquity‐driven mountain permafrost‐related fluvial magnetic susceptibility cycles in the Quaternary mid‐latitude long‐term (2.5 Ma) fluvial Maros Fan in the Pannonian Basin

Magnetic susceptibility (MS) of the Quaternary long‐term mid‐latitude Maros fluvial fan (Pannonian Basin) was recorded to understand the stratigraphical features of source‐proximal fluvial depositional settings. Three fully cored 500‐m‐deep boreholes were sampled at 0.5‐m intervals; low‐field and frequency dependent MS were measured, and complementary hysteresis and SEM‐EDAX investigations were performed on selected samples. Logged susceptibility data were also used to log correlations established by a simultaneous comparison of wireline log and laboratory measurements. Time‐series analyses of the susceptibility records reveal a ~41‐ka and ~100‐ka cyclicity. Towards the source‐distal sections the intensity of the ~41‐ka cycles decreases, while that of the ~100‐ka cycles remains strong. Stratigraphical and spectral similarities were observed between the Maros fluvial fan and Chinese loess records; however, based on complementary magnetic data, the magnetic phase of the Maros Fan sections is related to the detrital magnetite that originates from the catchment during early postglacial permafrost degradations. The amplification of the ~41‐ka cycles can be attributed to the very high susceptibility values in source‐proximal settings and to the special stratigraphical feature of the distributive fluvial systems. This comprises the increased avulsion frequency on the fluvial fans in ‘glacial recession periods’, in concert with the ‘early postglacial’ occurrence of the permafrost‐related magnetite originating from the catchment. As a local phenomenon, this is significant since it records the obliquity‐driven variations in permafrost development in a catchment. However, fluvial and alluvial fans are widespread depositional landforms within the Eurasian mountain range and were possibly the same during the Quaternary deglaciations. Thus, obliquity‐driven magnetic susceptibility variations in source‐proximal fan deposits attached or adjacent to regions of loess deposition should also be considered when scanning for potential source material of aeolian deposits.

Barform deposits of the Carolyn Shoemaker formation, Gale crater, Mars

ABSTRACT The early environmental history of Mars is encoded in the planet's record of sedimentary rocks. Since 2012, the Curiosity rover has been ascending Mount Sharp, Gale crater's central mound, making detailed observations of sedimentary strata exposed there. The primary depositional setting represented by the rocks examined thus far has been a perennial lake, represented by the mudstones and sandstone lenses of the Murray formation. Here, we report on the sedimentology of outcrops examined in the Carolyn Shoemaker formation, which sits stratigraphically above the Murray formation. We interpret strata exposed in the Glasgow and Mercou members of the Carolyn Shoemaker formation to represent river bars in ancient alluvial and shoreline settings based on sedimentary structures, stratal geometries measured from photogrammetric data, and erosional morphology. The transition from a lacustrine to a fluvial depositional setting records the aggradation and progradation of coastal rivers into what was previously the extent of the Gale lake system. This may have occurred due to the shrinking of the lake over time due to climate-driven changes in the basin water balance, or local three-dimensionality in shoreline evolution, such as the formation of a new sedimentary lobe following a channel switch.

CONTROLLED METHOD OF DETERMINE GOLD MINERALIZATION POTENTIALS IN AN UNEXPLOITED AREA; A CASE STUDY OF ITAGUNMODI AND OSU, SOUTHWESTERN, NIGERIA

The research was conducted to open up further unexploited areas for gold mineral exploitation in part of Ilesha schist belt using geological and geophysical approach. Itagunmodi was used as a control in order to determine the mineralization potentials of Osu as a case study. Both towns lie within Ilesha schist Belt, Southwestern Nigeria. The residual magnetic anomaly map of the study area revealed magnetic low in the two towns indicating the presence of geological structure serving as a conduit for mineralizing fluid. Airborne radiometry interpretation showed that both Itagunmodi and Osu are affected by hydrothermal alteration which is an indication of gold mineralization. Overall interpretation of aeromagnetic and airborne radiometry datasets show that hydrothermal alteration generally is associated with fault and shear zones in the study area. The isolated altered zones were subjected to further investigation by using six samples of stream sediment collected along Imo (Itagunmodi) and Olomumu (Osu) stream channels for the grain size and geochemical analyses. The results from the grain size analysis revealed that sediments in both areas are fine to medium grained, poorly sorted, fine to moderately fine skewed, which implied fluvial depositional settings, and revealed that the occurrences of gold mineralization in both areas are of secondary deposits (alluvial). The concentration of identified trace elements as related to their threshold values revealed similar contents of low to high concentrations in both areas. The results of the analyses observed in Itagunmodi are similar to that of Osu, which implies that Osu is also mineralized will gold deposit.

Seismic stratigraphic analysis for hydrocarbon exploration in the Beta Field, Coastal Swamp Depobelt, Niger Delta

Hydrocarbon exploration analysis from a seismic stratigraphic approach was carried out within the Beta Field, Coastal Swamp Depobelt, Niger Delta, to identify system tracts and sequence boundaries that could be associated with potential hydrocarbon accumulation. 3D seismic volume and data from four wells were analyzed. Depositional sequences, system tracts, sequence boundaries, and candidate maximum flooding surfaces were picked from logs, while reflection patterns and terminations were interpreted from seismic sections. Log shapes from gamma rays in combination with seismic facies analysis were interpreted in order to delineate lithology, depositional environments and depositional sequences. Seismic attributes were extracted and were draped on gridded surfaces from the interpreted seismic horizons. These were integrated with structure maps to obtain structural and stratigraphic trends, and the possible presence of reservoir sand. Five depositional sequences and nine seismic facies were identified within the field. The depositional sequences were designated Sequences S1 to S5 based on the depth of the occurrences and stacking patterns. This study reveals a progression from fluvial depositional settings to the shelf. The main reservoirs identified are the sand units of the highstand and lowstand within three depositional sequences (S1, S4 and S5) although interbedded sands within the TST of S4 and S5 are also suspected of being potential reservoirs. The channel sand deposits within the study area are suspected to be hydrocarbon bearing as they occurred within the complex fault trapping system popular in the Niger Delta. The application of seismic stratigraphy, as shown in this study, serves to encourage exploration in the Niger Delta where it could be effectively employed for reducing risk in hydrocarbon exploration.

Meandering rivers in modern desert basins: Implications for channel planform controls and prevegetation rivers

The influence of biotic processes in controlling the development of meandering channels in fluvial systems is controversial. The majority of the depositional history of the Earth's continents was devoid of significant biogeomorphic interactions, particularly those between vegetation and sedimentation processes. The prevailing perspective has been that prevegetation meandering channels rarely developed and that rivers with braided planforms dominated. However, recently acquired data demonstrate that meandering channel planforms are more widely preserved in prevegetation fluvial successions than previously thought. Understanding the role of prevailing fluvial dynamics in non- and poorly vegetated environments must rely on actualistic models derived from presently active rivers developed in sedimentary basins subject to desert-climate settings, the sparsest vegetated regions experiencing active sedimentation on Earth. These systems have fluvial depositional settings that most closely resemble those present in prevegetation (and extra-terrestrial) environments. Here, we present an analysis based on satellite imagery which reveals that rivers with meandering channel planforms are common in modern sedimentary basins in desert settings. Morphometric analysis of meandering fluvial channel behaviour, where vegetation is absent or highly restricted, shows that modern sparsely and non-vegetated meandering rivers occur across a range of slope gradients and basin settings, and possess a broad range of channel and meander-belt dimensions. The importance of meandering rivers in modern desert settings suggests that their abundance is likely underestimated in the prevegetation rock record, and models for recognition of their deposits need to be improved.

SEDIMENTOLOGY AND PETROGRAPHY ATTRIBUTES OF OUTCROPPING SEDIMENTS OF COASTAL PLAIN SANDS FROM ABUDU AND ITS ENVIRONS, SOUTHERN NIGERIA

Relying on field and laboratory evidences, outcropping sediments at Abudu and its environs were studied for their sedimentological and petrographic attributes. Based on field and sedimentological evidences, two lithofacies, sandstone and claystone were identified in the study area. The claystone facies include two (2) sub-lithofacies of silty clay and sandy clay; while the sand facies include medium to coarse-grained friable sands and, fine to medium-grained sands. Particle size analysis shows sorting values ranging from 0.94 to 2.10 indicating moderately to poorly sorted; skewness values of -0.13 to 0.2 indicating near symmetrical to positively skewed and kurtosis values in the range of 0.77 to 1.34 suggestive of platykurtosis to leptokurtosis. The petrographic analysis reveals the major framework minerals to include quartz (76.9 to 97.5%), rock fragment (0 to 12.9%) and feldspar (0 to 15.4%) and mineral maturity index (MMI) range from 3.8 to 39.0. Poor to moderate porosity was inferred for the sediments. Using field evidences and scatter plots of Skewness versus sorting and mean versus sorting, a fluvial depositional setting was interpreted for the sediments. The sediments range from sub-mature to super-mature; subarkosic to quartz arenites and sediments have undergone intense chemical weathering and/or erosion as evident in the predominance of quartz over feldspar. A craton interior source with contributions from recycled orogen is suggested for the sediments of the study area.

Lithofacies characters and depositional processes of a Middle Miocene Lower Siwalik fluvial system of the Himalayan foreland basin, India

The Lower Siwalik sequence in south-eastern Uttarakhand state of India records fluvial sedimentation during middle Miocene in the Himalayan foreland basin. Three distinct lithofacies associations (‘A’, ‘B’ and ‘C’) have been identified, each one of which comprises one or more of the eight major lithofacies and points to specific sub-environments of fluvial depositional setting. The lithofacies association ‘A’ is sandstone dominated and suggests deposition mainly by 0.09–2.0m/s flowing traction currents in braided streams. The lithofacies association ‘B’ is sandstone–mudstone dominated and suggests deposition by 0.1–1.0m/s flowing traction currents as well as suspension fallout in meandering streams and adjoining overbank areas. The facies association ‘C’ is mudstone dominated, characteristically pedogenically altered and disassociated with any major channel sandstone body. It represents reworking of pre-deposited sediments by ephemeral sheet and shallow channelized flows in the upland interfluve regions. The palaeoflow pattern reveals regional radial outward sediment dispersal by a distributary fluvial system, indicating convex-up transverse surface profile. The dominance of fine- to medium-grained sand and general absence of coarser extrabasinal clasts suggests that this distributive fluvial system of meandering and braided streams was located in the middle-distal part of a megafan. The upland interfluve surface developed following the cessation of megafan surface aggradation due to incision of major river channels.

OSL signal resetting in young deposits determined with a pulsed photon-stimulated luminescence (PPSL) unit

Optical Stimulated Luminescence (OSL) is a technique that can be used for dating geological materials deposited within the last half-million years, including sediments transported by air, water or gravity, as well as rocks heated at high temperatures. Recently, several studies have shown that OSL can also provide information on sediment transport. The pulsed photon-stimulated luminescence (PPSL) unit (also known as a portable OSL reader) developed by the Scottish Universities Environmental Research Centre is an instrument designed to read luminescence signals from bulk (untreated) sediment samples comprising poly-mineral and poly-grain fractions. In this contribution, we evaluate the potential of the PPSL unit to assess the degree of OSL signal resetting in 27 young deposits (<2 ka) transported by different geomorphic agents in volcanic, coastal and fluvial depositional settings located in Mexico. Our results are in agreement with previous findings that used the Riso TL/OSL reader, confirming that sediments transported by debrisflows contain the highest inherited luminesce signals. Infrared stimulation (IRSL) values in volcanic ash, lavas, and sand beach and dune deposits exhibit low scatter. However, with blue stimulation (BLSL) these samples reveal a large degree of scattering, attributed to charge transfer in the case of the coastal deposits and to the low sensitivity of quartz in the case of volcanic material. The luminescence signals of fluvial sediments exhibit a highly scattered distribution in both IRSL and BLSL. We conclude that the use of a PPSL unit is a simple approach to assess the degree of OSL signal resetting in deposits sourced from different geological environments. This research contributes to previous studies that have investigated new applications of the PPSL unit to assist in OSL dating of geological materials.

What does the ichnological content of the Middle McMurray Formation tell us?

Abstract Despite the abundant well-log and core data available for the McMurray Formation, the succession has remained difficult to interpret. Presently, a fundamental aspect of understanding the depositional nature of the McMurray Formation is whether or not the formation is dominantly estuarine or fluvial. Focussing on the informal middle McMurray, our detailed analysis of the McMurray Formation within the Kearl Oil Sands area critically evaluates the utility of collecting high-resolution ichnological and sedimentological data as a means to assess the depositional environments and to evaluate the evidence for an estuarine versus a tidally influenced fluvial depositional setting. This study asserts that the middle McMurray Formation within the Kearl Oil Sands area was deposited on estuarine point-bars. The examined dataset encompasses what are interpreted to be inner to middle estuarine depositional locales within a mesotidal regime. Integration of detailed ichnological and sedimentological data permits recognition of intertidal flat deposits which exist at sea level, and the identification of which provides a locally useful stratigraphic datum. Identification of successions of stacked tidally influenced channels of varying dimensions help identify stratigraphic levels of amalgamated estuarine channel bars and gives an approximation of relative changes in sea level.

Taphonomy and paleoecological implications of fossorial microvertebrates at the Middle Paleolithic open-air site of Nesher Ramla, Israel

Variation in taphonomic modes, which describe the characteristics of fossil depositional environments, is an important but much understudied aspect of the formation of microvertebrate assemblages. This study examined an assemblage of microvertebrate remains which was recovered from archaeological deposits in a deep karst depression at an open-air setting – a unique and so far undocumented type of depositional environment in the southern Levant. Taxonomic and taphonomic characteristics of the assemblage reveal a striking difference from other known assemblages in the region from cave and fluvial depositional settings. This includes especially low density, taxonomic diversity and skeletal element survivorship. Three of the four taxa are fossorial or semi-fossorial and their remains dominate the assemblage indicating strongly an accumulation history which involves underground burrow deaths of fossorial animals. This is supported by additional evidence from skeletal modifications showing rare evidence of weathering, extensive in situ fragmentation where the fragments remain fused together and abundant evidence for light abrasion. These taphonomic patterns suggest extended underground protection of the remains, followed by limited dispersal within a sedimentary matrix likely due to movement from the surrounding soil mantle into the karst depression through colluviation processes. This taphonomic reconstruction has significant implications for understanding the long record of hominid occupation at the Middle Paleolithic site of Nesher Ramla. Phases of high density of the remains along the section correspond with glacial phases of Marine Isotope Stages (MIS) 6 and 5a-4 indicating an association between enhanced fossorial activity and environmental conditions during glacial stages. Intense deposition of cultural remains was recorded only during MIS 6, however, and this leads us to suggest that continued low intensity hominid presence in the upper layers of the site is related either to change in the usefulness of the site for hominid subsistence activities as it was being filled with sediments or to demographic changes linked to the arrival of new populations during MIS 5a-4. Detailed paleoenvironmental sequences which are recovered directly from hominid sites are critical for testing such hypotheses.

Record of tide-wave influence on the coal-bearing Permian Barakar Formation, Raniganj Basin, India

The coal-bearing Barakar Formation (early Permian) of the Raniganj Gondwana basin was previously interpreted as continental fluvial sediments with little evidence for tide-wave influences. The present contribution details the sedimentary attributes of marginal marine tidal and wave activities preserved within an overall braided-meandering fluvial depositional setting from the Barakar Formation. Tide-dominated sedimentary features include (i) tidal-bundle sequences showing sandstone foresets with mud drapes, (ii) periodic variation in foreset thickness, (iii) lateral and vertical accretion of cross-strata bundles with a downcurrent change to sigmoidal cross-strata, (iv) mutually opposite cross-strata sets separated by sub-horizontal, plane laminated sandstone–mudstone, and (v) alternate sand- and mud-dominated rhythmites. Draping of the tidal stratifications by wavy laminated muddy-siltstone suggests immediate invasion by low energy, silt/mud depositing waves that helped preserve the tidal features during subsequent reworking. Open marine storm wave activities are evident from hummocky cross-stratification and various types of wave ripples, preserved separately within coarse- to fine-grained sandstone in the middle–upper part of the succession. Recognition of tide-storm influenced marine sedimentation significantly changes the existing paleogeographic model to a fluvial–estuarine depositional environment.

Seismic geomorphology and sedimentology of a tidally influenced river deposit, Lower Cretaceous Athabasca oil sands, Alberta, Canada

The bitumen of the Lower Cretaceous McMurray Formation in Alberta arguably represents one of the most important hydrocarbon accumulations in the world. In-situ development relies on heat transfer through the reservoir via horizontal steam injection wells placed 4 to 6 m (13–20 ft) above horizontal producers near the base of the sandstone reservoirs. Given this technology, understanding the distribution of the resource is paramount for a successful development program. Sedimentary facies provide a direct control on bitumen distribution and recovery. Most facies models developed to describe and predict sedimentary units of the McMurray Formation consider fluvial, estuarine, and/or deltaic depositional settings. In-situ development, however, requires a particularly high-resolution sedimentologic interpretation. High-quality three-dimensional seismic reflection data and extensive drill cores from acreage located approximately 50 km (31 mi) south of Fort McMurray provide important insights into the sedimentologic organization of reservoir and nonreservoir deposits in the upper one third (40 m [131 ft]) of the reservoir interval. Geomorphologic characteristics of the strata observed in seismic time slices reveal that a fluvial depositional setting was prevalent. Ichnologic and palynologic data, as well as sedimentary structures suggestive of tidal processes, indicate a marine influence in the upper reaches of a fluvial system characterized by channels that were 390 to 640 m (1280–2100 ft) wide and 28 to 36 m (92–118 ft) deep. The complex stratigraphic architecture consists of a mosaic of large-scale depositional elements, including abandoned channels or oxbow lake fills, point bars associated with lateral accretion, point bars associated with downstream accretion, counter point bars, and sandstone-filled channels. Reservoir deposits are primarily associated with point bars and sandstone-filled channels.

Mechanism of calcrete formation in the Lower Cretaceous (Neocomian) fluvial deposits, northeastern Iran based on petrographic, geochemical data

The Kopet-Dagh basin of northeastern Iran was formed during the Middle Triassic orogeny. From Jurassic through Miocene time, sedimentation was relatively continuous in this basin. The Shurijeh Formation (Neocomian), which consists of red bed siliciclastic sediments that were deposited in fluvial depositional settings, crops out in the southeastern part of the Kopet-Dagh basin. In addition to clastic lithofacies, non-clastic facies in the form of calcrete paleosols, were identified in this formation. The calcrete host rocks are mainly sandstone, pebbly sandstone. The calcrete in middle unit in the Shurijeh Formation consists of, from bottom to top: incipient calcrete, nodular calcrete, massive calcrete horizons. The maturity pattern of these calcrete gradationally increases from bottom to top in this unit. Lack of organo-sedimentary structure (mainly plant roots), diversity of calcite fabric, suggest that the studied calcretes have a multi-phase development: a short vadose phase followed by a long phreatic phase. These calcretes are neither pedogenic nor groundwater calcretes. Petrographic studies show that they are composed of micritic textures with a variety of calcite fabrics, microsparitic/sparitic veins, displacive, replacive fabrics, quartz, hematite grains. Cathodoluminescence images, trace elemental analysis (Fe, Mn increased, Na, Sr decreased) of calcrete samples show the effects of meteoric waters during the calcrete formation when water tables were variable. In this study, we conclude that evaporation, degassing of carbon dioxide are the two main factors in the formation of non-pedogenic or groundwater calcrete. The sources of carbonate were probably parent materials, surface waters, ground waters, eolian dusts, numerous outcrops of limestones that have been exposed in the source area during Neocomian time.

Late Neogene continental stages in NW Europe revisited

Pollen records from fluvial deposits in The Netherlands have traditionally been used to establish continental stages for the Neogene in NW Europe. These stages are still widely used across Europe for stratigraphic correlations and palaeoclimatic studies. Despite their extensive use, the validity of this continental correlation scheme for the Plio- and Pleistocene has never been tested in the area where they were originally proposed: the Roer Valley Graben (RVG). The fluvial and deltaic depositional setting in this area is controlled by a complex pattern of tectonic and climatic processes, which strongly complicates the definition of stages and sub-stages of regional or supra-regional significance. Here, the original palynological data used to define the Praetiglian, Reuverian, Brunssumian, Susterian, and ‘Linne’ (sub-)stages are assessed and compared to new borehole data. With a multivariate (Correspondence Analysis) approach we show that most of these stages cannot be distinguished by numerical analysis of the pollen assemblages. A tripartition of pollen assemblages is possible, which results in a cooler Pleistocene cluster, warm-temperate to sub-tropical Pliocene cluster, and a warm and relatively dry Late Miocene to Early Pliocene cluster. However, no further stages, sub-stages or clear climatic phases can be consistently recognized in these data. A detailed lithostratigraphic framework of the study area allows a correlation of the sections independent of the pollen data. Results confirm the numerical analyses and furthermore point to a strong lithological (depositional) bias on the pollen assemblages. Variations in the abundance of Sequoia and Taxodium, originally used to define the Brunssumian/Reuverian transition and Brunssumian A–C sub-stages, are shown to represent mostly local changes. The number and absolute percentage of the fluctuations in Sequoia and Taxodium abundances depend largely on the sample resolution and the site location within the RVG. Our data assessment indicates that long-distance chronostratigraphic correlations based on the original continental Neogene stages likely are invalid. Furthermore, the palaeoclimatic reconstructions that have been based on the RVG successions are largely over-interpreted. The present palaeoclimate reconstructions from the RVG reflect variations in the highly-dynamic fluvial depositional setting, taphonomic effects and environmental conditions, including climate. A sequence-stratigraphic approach combined with the analysis of interfingering marine sediments is potentially better suited to establish absolute ages and associated climatic conditions of the Neogene deposits within the RVG.

Geologic and Hydrologie Characterization of Coalbed-Methane Reservoirs in the San Juan Basin

Summary Fruitland coals are best developed in the north-central part of the San Juan basin. Coal distribution is controlled by shoreline and fluvial depositional settings. Hydraulic gradient, pressure regime, and hydrochemistry reflect regional permeability contrasts. The most productive (&amp;gt;1 MMcf/D) coalbed wells occur along a structural hinge line in association with a regional permeability barrier (no-flow boundary) at the basin center.

Depositional models for two Tertiary coal-bearing sequences in the Powder River Basin, Wyoming, USA

Depositional controls on peat-forming environments which produce thick (&gt;10m) coal beds can be inferred from relationships between coal bed geometry, maceral composition and associated lithologies. Study of these relationships within sedimentary sequences associated with the Wyodak-Anderson (Palaeocene) and the Felix (Eocene) sub-bituminous coal beds in the Powder River Basin, Wyoming, USA suggests two modes of fluvially controlled peat accumulation. The Wyodak-Anderson peat is interpreted to have formed in restricted parts of the floodplain that were separated by deposits of contemporaneous, anastomosed channels. The channels and associated sediments maintained their position through time because they were confined by thick deposits of raised Wyodak-Anderson peat. In contrast, the Felix coal bed is interpreted to have formed as a raised but widespread peat on an abandoned platform of meander-belt sands. The purpose of this paper is to compare and contrast two different fluvial depositional settings that produced anomalously thick (&gt;10m) coal deposits in the intermontane Powder River Basin of Wyoming, USA. These models may be useful as predictive tools for coal exploration and production.