Provenance and Paleoclimate of the Triassic to Middle Jurassic Adigrat Sandstone, Blue Nile Basin, Central Ethiopia

Mineralogical and geochemical studies have been undertaken on the Triassic to Lower Jurassic Adigrat Sandstone of the Blue Nile Basin of central Ethiopia to infer its source rock type, paleoweathering, and paleoclimatic history. The Adigrat Sandstone occurs at the basal section of the Mesozoic sedimentary formation and unconformably overlays the Neoproterozoic–Paleozoic crystalline rocks, or locally, the Karroo sediments in the northern Blue Nile Basin. A mineralogical study reveals that quartz (Q), feldspars (F), and lithic fragments (L) are the framework grains of the sandstone. On the QFL diagram, the plot of the modal composition of the sandstone mainly falls within the feldspathic arenite and quartzose arenite fields. The geochemical data of the lower section of the sandstone mainly falls within the arkose and subarkose fields, whereas the upper section data falls within the quartzose and sublithic arenite fields. Mineralogical and geochemical weathering indices indicate that the provenances of the Adigrat Sandstone were exposed to pronounced weathering intensity, where the lower part of the sandstone was controlled by arid to semi-arid conditions, whereas the upper section was linked to humid to semi-humid (tropical to subtropical) climatic conditions. Mineralogical and geochemical data also indicate that mafic to intermediate basement rocks were the primary source rocks of the sediment. Perhaps the sediment was assumed to have been reworked by multi-cyclic sedimentary processes. The discriminant function diagram, the REE pattern, La/Th vs. La/Yb, and the Th–Hf–Co plot are consistent. A comparison of provenance studies for the Adigrat Sandstone in the Blue Nile Basin and the Mekele outlier of northern Ethiopia indicates that the sediment of the former is highly sorted, experienced higher weathering intensity, and compositionally derived from mafic to intermediate crystalline rocks. On the other hand, the sediment of the latter is essentially a weathering product of felsic rocks.     

Geotechnical Aspects and Stability of Road Cuts in the Blue Nile Basin,Ethiopia

Road construction in the Blue Nile basin is largely determined by geotechnical factors. The area is characterized by steep slopes and has a history of landsliding. The geological formations range from Mesozoic sedimentary to Tertiary volcanic rocks making the stratigraphic makeup sensitive to deformation and failure. The heterogeneity of these rocks also means it is difficult to depend on results of stability analyses alone for road design and construction. As an alternative, ratios of cut-slope lengths to cut-slope heights have been developed in this study based on the performances of unsupported natural and artificial cuts and some stability analyses. Hence, road cuts on cliffs of hard rocks need a horizontal to vertical ratio of 0.25:1. Slopes made up of weak rocks can remain stable at a threshold angle of 45°. For heterogeneous slopes, it is advisable to use different road cuts depending on material makeup, and the degree of weathering and consolidation.     

The role of geodiversity on the groundwater resource potential in the upper Blue Nile River Basin, Ethiopia

Groundwater has been the main source of water supply for large cities and towns over the last few decades in the upper Blue Nile River Basin, Ethiopia. However, provision is often unsuccessful because of poor well productivity, difficult drilling conditions, poor well positioning, or sometimes due to poor water quality. The growing pressure of urban population and industrial development is focusing unprecedented attention on the groundwater potential of the basin. The purpose of this work is to spatially characterize the groundwater potential of the upper Blue Nile River Basin with respect to variable recharge and geodiversity. The study shows that from the annual recharge obtained using the base flow separation method, the renewable groundwater potential in the basin was estimated to be in the range of 1.2 and 2 billion m³/year. The aquifers in the area are divided into three categories: low to moderate productivity (≈3.5 l/s) which includes crystalline basement rocks, acidic lava flows and domes, and very fine alluvial sediments; moderate to high productivity (≈5.5 l/s) that includes Mesozoic sedimentary rocks (sandstone, limestone, gypsum, dolomite); and high to very high productivity (≈20 l/s) which includes basic lava flows of the Trap series, Quaternary lava flows and alluvial sediments.     

Landslide susceptibility evaluation in the Chemoga watershed,upper Blue Nile,Ethiopia

Landslide susceptibility consists of an essential component in the day-to-day activity of human beings. Landslide incidents are typically happening at a low rate of recurrence when compared and in contrast to other events. This might be generated into main natural catastrophes relating to widespread and undesirable sound effects. Landslide hotspot area identification and mapping are used for the regional community to secure from this disaster. Therefore, this research aims to identify the hotspot areas of landslide and to generate maps using GIS, AHP, and multi-criteria decision analysis (MCDA). MCDA techniques are applied under such circumstances to categorize and class decisions for successive comprehensive estimation or else to state possible from impossible potentiality with various landslides. Analytical hierarchy process (AHP) constructively applies for conveying influence to different criteria within multi-criteria decision analysis. The causative landslide identifying factors utilized in this research were elevation, slope, aspect, soil type, lithology, distance to stream, land use/land cover, rainfall, and drainage density achieved from various sources. Subsequently, to explain the significance of each constraint into landslide susceptibility, all factors were found using the AHP technique. Generally, landslide susceptibility map factors were multiplied by their weights to acquire with the AHP technique. The result showed that the AHP methods are comparatively good quality estimators of landslide susceptibility identification in the Chemoga watershed. As the result, the Chemoga watershed landslide susceptibility map classes were classified as 46.52%, 13.83%.18.71%, 15.39%, and 5.55% of the occurred landslide fall to very low, low, moderate, high, and very high susceptibility zones, respectively. Performance and accuracy of modeled maps have been established using GPS field data and Google earth data landslide map and area under curve (AUC) of the receiver operating characteristic curve (ROC). As the result, validation depends on the ROC specifies the accuracy of the map formed with the AHP merged through weighted overly method illustrated very good accuracy of AUC value 81.45%. In general, the research outcomes inveterate the very good test consistency of the generated maps.

     

Sedimentology and depositional environments of the Ordovician Umm Sahm Sandstone Formation in southern Jordan

The Ordovician Umm Sahm Sandstone Formation of Jordan comprises approximately 200-m-thick succession of fluvial quartzarenites with subordinate claystone and siltstone lithologies of shallow marine conditions. The Umm Sahm Formation is characterized by its dark brown color, frequent jointing, and steep scarps. The Umm Sahm Formation is bounded by the marine claystones of Hiswah Formation at the top and the fluvial sandstones of the Disi Formation at the bottom. The Umm Sahm Formation is composed of two main facies: fluvial facies and tidal facies. The fluvial facies constitutes about 93% of the total thickness. The lower few meters of the succession passes upward from the Disi Sandstone Formation into similar massive white sandstone facies exhibiting similar white color, fine- to coarse-grained sandstone, with round-shaped pebbles. Trough and planar cross-bedding show a northwest unidirectional palaeocurrent trend. Light brown colored quartzarenites similar to those of the Cambrian Umm Ishrin Sandstone Formation are most common in the upper part of the succession. The tidal facies occupies three intervals in the middle part of the succession. It is composed of laminated and thin-bedded sandstones, siltstones, and claystones. They are rippled and varicolored with abundant trace fossils (Cruziana, Harlania, ruzophycus). The presence of hummocky cross stratification indicates the earliest short-lived tempestite conditions during the Paleozoic erathem of Jordan. The first appearance of Graptolites in the Ordovician rocks of Jordan was recorded during this study in the tidal facies of the Umm Sahm Formation. The vertical arrangement of both fluvial and tidal facies indicates three successive short-lived transgressions and regressions. These marine incursions indicate the successive shoreline advances of the Tethys Ocean, which was located northward, and inundated the southern braid plain. The three short-lived transgressive events took place, and the Tethys marine margin was displaced southward, giving rise to deposition of tidal facies in an open coast tidal flat. Following the transgressive events, renewed progradation and strandline retreat took place, fed by large amounts of siliciclastics derived from the continent and transported by braided streams across the intertidal zone.     

Basin hydrogeological characterization using remote sensing,hydrogeochemical and isotope methods (the case of Baro-Akobo,Eastern Nile,Ethiopia)

Hydrogeochemical and isotopic signatures of the waters of the Baro-Akobo River Basin show deviation from signatures in other Ethiopian river basins. In this study, hydrogeochemical and isotope methods were employed to determine regional and local hydrogeology and characteristics of the basin. Optical, thermal and radar remote sensing products were used to update geological and structural maps of the basin and determine sampling points using the judgment sampling method. A total of 363 samples from wells, springs, rivers, lakes, swamps and rain were collected for this study, and an additional 270 water quality data sets were added from previous studies. These data were analyzed for their hydrogeochemical characteristics and isotope signatures. Analysis of the oxygen, deuterium and tritium isotopes shows the groundwater of the basin is modern water. Among all basins in Ethiopia, the Baro-Akobo Basin shows the highest enrichment. This indicates the proximity of the rainfall sources, which presumably are the Sudd and other wetlands in South Sudan. The hydrochemical properties of the waters show evapotranspiration is the dominant hydrologic process in the basin and explains the large amount of water that is lost in the lowland plain. Analysis of radon-222 shows no significant groundwater flux over the wetlands, which are part of Machar Marshes. This shows evaporation to be dominant hydrologic process in this zone. Results from all analyses help explain the limited holding capacity of the aquifers in the recharge zone and their vulnerability to anthropogenic impacts and climate variability. There is a trend of decreasing surface flow and rainfall and increasing water soil erosion.     

Differentiating fluvio-marine depositional environments in the Nile Delta using textural and compositional components

Textural and coarse fraction compositional components are evaluated to distinguish samples from modern environments of the Nile Delta of Egypt. These environments include: river, coastal dune, accretion ridges, beach, neashore, lagoon and prodelta. In this study petrological variables (12 textural and 18 mineralogical, faunal and floral) were considered for each sample. Discrimination was achieved by using simple bar graphs of the raw data for each environment and Q-mode factor analysis. The factor analysis yielded four compositional assemblages: Factor 1 is dominated by terrigenous fine sand; factor 2 consists of biogenic mud; factor 3 contains terrigenous coarse and medium sands; and factor 4 comprises composite silty sand. Discrimination of the seven environments is generally good, with a lower resolution for beach, coastal dune and river sands.Having discriminating the examined environments using Q-mode factor analysis, a graphic model was constructed to help determine the origin of ‘unknown’ samples. As a test, this model is satisfactory in identifying and interpreting the origin of sediments of Holocene age from two additional cores recovered off the delta coast.     

Lithostratigraphy and depositional environment of the Permian Nowra Sandstone in the southwestern Sydney Basin,Australia

Lithofacies in the mid‐Permian Nowra Sandstone indicate a middle/upper shoreface to foreshore environment of deposition under the influence of storm‐generated waves and north‐northeasterly directed longshore currents. Palaeogeographic reconstruction for the Nowra Sandstone portrays a sand‐dominated high energy shelf and offshore shoal forming a sequence thickening seaward away from the western shore of the Sydney Basin. The shoal‐crest at the outer edge of the shelf trends north‐northeast. It is characterized by fine‐ to medium‐grained sandstone with upper flow regime structures and a high proportion of conglomerate, whereas coarser sandstone with lower energy bedforms occurs along the seaward side of the shoal. In the deeper water to the east, the lower Nowra Sandstone becomes rapidly thinner as it passes seaward, via bioturbated storm redeposited sandstone beds, into the shelf deposits of the Wandrawandian Siltstone. This sequence accumulated during a regressive event and the base of the formation becomes progressively younger eastward. The sand may have been supplied by rivers along the western coast but the major source was south of the study area. The lower Nowra Sandstone is separated from the upper part of the formation by an extensive ravinement surface overlain by the Purnoo Conglomerate Member. In contrast to the lower unit, the upper Nowra Sandstone forms a westward thickening wedge that represents a backstepping nearshore sand facies that accumulated during a transgression. The upper Nowra Sandstone passes vertically and laterally eastward into the Berry Siltstone. Thus both boundaries of the Nowra Sandstone are diachronous, first younging eastward and then westward as a response to a regressive‐transgressive episode.     

Sedimentology and depositional environments of the Maastrichtian Patti Formation, southeastern Bida Basin, Nigeria

The Maastrichtian Patti Formation, which consists of shale - claystone and sandstone members, constitutes one of the three Upper Cretaceous lithostratigraphic units of the intracratonic southeastern Bida Basin, in central Nigeria. Well exposed outcrops of this formation were investigated at various locations around the confluence of the Niger and Benue Rivers. The lithostratigraphic sections were measured and their peculiar sedimentological features such as textures, physical and biogenic sedimentary structures, facies variations and associations were documented and used to interpret the depositional environments and develop a paleogeographic model. Some selected representative samples of the sedimentary depositional facies were also subjected to grain size analysis.Three shoreline sedimentary depositional facies composed of shoreface, tidal channel, and tidal marsh to coastal swamp facies were recognized in the study area. Continental sedimentary depositional facies such as fluvial channel, swamp, and overbank were also documented. The sandstones of the shoreface and tidal channel facies are medium- to coarse-grained, moderately sorted (standard deviation ranges from 0.45–1.28 averaging 0.72), and quartzarenitic. The fluvial channel sandstone facies are coarse- to very coarse-grained, mostly poorly sorted (standard deviation ranges from 0.6–1.56 averaging 1.17), and subarkosic. Typical sedimentary structures displayed by the shoreface and tidal channel facies include burrows, clay drapes, hummocky and herringbone cross stratifications, whereas the fluvial channel sandstone facies are dominated by massive and planar cross beddings. The tidal marsh to coastal swamp shales and ferruginised siltstone facies are fossiliferous and bioturbated, whereas the nonmarine swamp siltstones contain vegetal imprints and lignite interbeds. The overbank claystone facies are massive and kaolinitic.In the study area, a regressive to transgressive model is proposed for the Patti Formation. This model correlates with stratigraphically equivalent sediments of the Ajali and Mamu Formations in the adjacent Anambra Basin to a great extent.     

Stratigraphic and structural evolution of the Blue Nile Basin,Northwestern Ethiopian Plateau

The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains ∼1400 m thick Mesozoic sedimentary section underlain by Neoproterozoic basement rocks and overlain by Early–Late Oligocene and Quaternary volcanic rocks. This study outlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along the Gorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre‐sedimentation phase, include pre‐rift peneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic to Early Cretaceous, including: (a) Triassic–Early Jurassic fluvial sedimentation (Lower Sandstone, ∼300 m thick); (b) Early Jurassic marine transgression (glauconitic sandy mudstone, ∼30 m thick); (c) Early–Middle Jurassic deepening of the basin (Lower Limestone, ∼450 m thick); (d) desiccation of the basin and deposition of Early–Middle Jurassic gypsum; (e) Middle–Late Jurassic marine transgression (Upper Limestone, ∼400 m thick); (f) Late Jurassic–Early Cretaceous basin‐uplift and marine regression (alluvial/fluvial Upper Sandstone, ∼280 m thick); (3) the post‐sedimentation phase, including Early–Late Oligocene eruption of 500–2000 m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of ∼300 m thick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed to Triassic–Cretaceous NE–SW‐directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formed as a NW‐trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed by Late Miocene NW–SE‐directed extension related to the Main Ethiopian Rift that formed NE‐trending faults, affecting Lower volcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E–W and NNE–SSW‐directed extensions related to oblique opening of the Main Ethiopian Rift and development of E‐trending transverse faults, as well as NE–SW‐directed extension in southern Afar (related to northeastward separation of the Arabian Plate from the African Plate) and E–W‐directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). These Quaternary stress regimes resulted in the development of N‐, ESE‐ and NW‐trending extensional structures within the Blue Nile Basin. Copyright © 2008 John Wiley & Sons, Ltd.     

Facies analysis and depositional environments of the OligoceneeMiocene Asmari Formation,Zagros Basin,Iran

The Asmari Formation(a giant hydrocarbon reservoir)is a thick carbonate sequence of the Oligocenee Miocene in the Zagros Basin,southwest of Iran.This formation is exposed at Tang-e-Lendeh in the Fars interior zone with a thickness of 190 m comprising medium and thick to massive bedded carbonates.The age of the Asmari Formation in the study area is the late Oligocene(Chattian)eearly Miocene(Burdigalian).Ten microfacies are defned,characterizing a gradual shallowing upward trend;the related environments are as follows:open marine(MF 8e10),restricted lagoon(MF 6e7),shoal(MF 3e5),lagoon(MF 2),and tidal fat(MF 1).Based on the environmental interpretations,a homoclinal ramp consisting of inner and middle parts prevails.MF 3e7 are characterized by the occurrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp,infuenced by wave and tidal processes.MF 8e10,with large particles of coral and algae,represent a deeper fair weather wave base of a middle ramp setting.     

From Tethys to Eastern Paratethys: Oligocene depositional environments, paleoecology and paleobiogeography of the Thrace Basin (NW Turkey)

The Oligocene depositional history of the Thrace Basin documents a unique paleogeographic position at a junction between the Western Tethys and the Eastern Paratethys. As part of the Tethys, shallow marine carbonate platforms prevailed during the Eocene. Subsequently, a three-staged process of isolation started with the Oligocene. During the Early Rupelian, the Thrace Basin was still part of the Western Tethys, indicated by typical Western Tethyan marine assemblages. The isolation from the Tethys during the Early Oligocene is reflected by oolite formation and endemic Eastern Paratethyan faunas of the Solenovian stage. The third phase reflects an increasing continentalisation of the Thrace Basin with widespread coastal swamps during the Late Solenovian. The mollusc assemblages are predominated by mangrove dwelling taxa and the mangrove plant Avicennia is recorded in the pollen spectra. The final continentalisation is indicated by the replacement of the coastal swamps by pure freshwater swamps and fluvial plains during the Late Oligocene (mammal zone MP 26). This paleogeographic affiliation of the Thrace Basin with the Eastern Paratethys after ~32 Ma contrasts all currently used reconstructions which treat the basin as embayment of the Eastern Mediterranean basin.     

Groundwater recharge,circulation and geochemical evolution in the source region of the Blue Nile River,Ethiopia

Geochemical and environmental isotope data were used to gain the first regional picture of groundwater recharge, circulation and its hydrochemical evolution in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster analysis (HCA) was used to classify water into objective groups and to conduct inverse geochemical modeling among the groups. Two major structurally deformed regions with distinct groundwater circulation and evolution history were identified. These are the Lake Tana Graben (LTG) and the Yerer Tullu Wellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis accompanied by CO₂ influx from deeper sources plays a major role in groundwater chemical evolution of the high TDS Na–HCO₃ type thermal groundwaters of these two regions. In the basaltic plateau outside these two zones, groundwater recharge takes place rapidly through fractured basalts, groundwater flow paths are short and they are characterized by low TDS and are Ca–Mg–HCO₃ type waters. Despite the high altitude (mean altitude ∼2500 masl) and the relatively low mean annual air temperature (18 °C) of the region compared to Sahelian Africa, there is no commensurate depletion in δ¹⁸O compositions of groundwaters of the Ethiopian Plateau. Generally the highland areas north and east of the basin are characterized by relatively depleted δ¹⁸O groundwaters. Altitudinal depletion of δ¹⁸O is 0.1‰/100 m. The meteoric waters of the Blue Nile River basin have higher d-excess compared to the meteoric waters of the Ethiopian Rift and that of its White Nile sister basin which emerges from the equatorial lakes region. The geochemically evolved groundwaters of the YTVL and LTG are relatively isotopically depleted when compared to the present day meteoric waters reflecting recharge under colder climate and their high altitude.     

Sedimentary facies, depositional environments and palaeogeographic evolution of the Neogene Denizli Basin, SW Anatolia, Turkey

The Denizli Basin (southwestern Anatolia, Turkey) contains a record of environmental changes dating since the Early Miocene. Detailed facies analysis of the Neogene formations in this half-graben enables us to document successive depositional regimes and palaeogeographic settings. Sedimentation commenced in the Early Miocene with the deposition of alluvial-fan and fluvial facies (K?z?lburun Formation). At this stage, alluvial fans sourced from elevated areas to the south prograded towards the basin centre. The Middle Miocene time saw the establishment of marginal lacustrine and wetland environments followed by the development of a shallow lake (Sazak Formation). The uppermost part of this unit consists of evaporitic saline lake and saline mudflat facies that grade upward into brackish lacustrine deposits of Late Miocene-Pliocene age (Kolankaya Formation). The lake became shallower at the end of the Pliocene time, as is indicated by expansion shoreface/foreshore facies. In the Early Quaternary, the Denizli Basin was transformed into a graben by the activation of ESE-trending normal faults. Alluvial fans were active at the basin margins, whereas a meandering river system occupied the basin central part.Oxygen isotope data from carbonates in the successive formations show an alternation of wetter climatic periods, when fresh water settings predominated, and very arid periods, when the basin hosted brackish to hypersaline lakes. The Neogene sedimentation was controlled by an active, ESE-trending major normal fault along the basin's southern margin and by climatically induced lake-level changes. The deposition was more or less continuous from the Early Miocene to Late Pliocene time, with local unconformities developed only in the uppermost part of the basin-fill succession. The unconformable base of the overlying Quaternary deposits reflects the basin's transformation from a half-graben into a graben system.     

松辽盆地深层侏罗系-下白垩统生物组合和沉积环境

本文描述了松辽盆地探井揭露的深层地层序列及其所产的5个牛物门类化石组合特征.9个孢粉组合是Monosulcites-Cyathidites组合、Pinaceae-Cyathidites-Osmundaeidites组合、Classopollis-Piceites组合、Lophotriletes-Cicatricosisporites组合、Paleoconiferus-Lygodiumsporites组合、Cyathidites-Leiotriletes-Clavatipollenites组合、Cicatricosisporites-Leiotriletes-Polyporopollenites组合、Leiotriletes-Schizaeoisporites-Classopollis组合和Trilobosporites-Cyathidites-Tricolpollenites组合.3个藻类组合是Vesperopsis-Australisphaera组合、Balmula granorugosa组合和Vesperopsis zhaodongensis组合.2个介形类组合是Cypridea unicostata-Limnocypridesadscondida组合和Mongolocypris limpida-Paracandona planiuscula组合.4个植物组合是Elatocladus submanchurica-Ginkgoites orientalis组合、Acanthopteris gothani-Pterophyllum组合、Sphenopteris johnstrupii-Coniopteris nympharum组合和Platanus septentrionalis-Protophyllum undulaturn组合.3个大孢子组合是Minerisporites sp.组合、Ricinospora leavigata-Galamospora sp.组合和Arcellites组合.还介绍了叶肢介、轮藻、双壳类和鱼类化石的产出情况.结合同位素资料认为：大庆群时代为中侏罗世,火石岭组可能为贝里阿斯期,沙河子组为凡兰吟期-欧特里夫期,营城组为欧特里夫期-巴列姆期,登娄库组为巴列姆期-阿普第期早期,泉头组一-二段为阿普第-阿尔卑期,并对各组沉积环境进行了初步分析.     

Provenance and depositional environments of Quaternary sediments from the western North Sea Basin

As the majority of the data on Quaternary sediments from the North Sea Basin are seismostratigraphical, we analysed the Elsterian Swarte Bank Formation, the Late Saalian Fisher Formation and the Late Weichselian (Dimlington Stadial) Bolders Bank Formation in order to determine genesis and provenance. The Swarte Bank Formation is a subglacial till containing palynomorphs from the Moray Forth and the northeastern North Sea, and metamorphic heavy minerals from the Scottish Highlands. The Fisher Formation was sampled from the northern and central North Sea. In the north, it is interpreted as a subglacial till, with glaciomarine sediments cropping out further south. These sediments exhibit a provenance signature consistent with the Midland Valley of Scotland, the Eocene of the North Sea Basin, the Grampian Highlands and northeast Scotland. The Bolders Bank Formation is a subglacial till containing palynomorphs from the Midland Valley of Scotland, northern Britain, and a metamorphic heavy‐mineral suite indicative of the Grampian Highlands, Southern Uplands and northeast Scotland. These data demonstrate that there was repeated glaciation of the North Sea Basin during the Middle and Late Pleistocene, with ice sheets originating in northern Scotland. There was no evidence for a Scandinavian ice sheet in the western North Sea basin. Copyright © 2011 John Wiley & Sons, Ltd.     

Evolution of the depositional environments of the Jurassic Walloon Coal Measures,Surat Basin,Queensland, Australia

The Jurassic Walloon Coal Measures of the Surat Basin in eastern Australia host the continent's most significant coal bed methane resources. Previous studies have interpreted the Walloon Coal Measures within a single depositional facies model encompassing a wholly terrestrial setting. Using a multidisciplinary approach (facies analysis, palynology and wireline logs), the evolution of the Walloon Coal Measures is described within a new chronostratigraphic framework defined by accurate and precise U–Pb tuff dates. Analysis of sedimentary facies indicates that the majority of the Walloon Coal Measures was deposited by relatively small (<300 m wide), low gradient rivers on a poorly‐drained floodplain with numerous small lakes and mires. However, this study also identified some marine‐influenced facies with brackish palynomorphs (notably dinoflagellate cysts) and tidal sedimentary structures. These facies appear to have been deposited in estuaries during times of transgression. The evidence for base level shifts suggests that the coals may not have coevally accumulated with at least some of the thicker sandstones. Palaeogeographic maps for eleven time intervals suggest that rivers drained towards to the south/south‐west and south‐east, as indicated by sandstone percentage and gross unit isopach maps, presumably into proximal estuarine complexes. Marine incursions into the continent probably came from the north and east during times of high eustatic sea level and as precursors to those of the more persistent and extensive transgressions of the Early Cretaceous. A similar multidisciplinary approach should help to elucidate the evolution of other fluviolacustrine systems in other basins and aid in resource prediction.     

Sedimentation, depositional environment and diagenesis of Eocene biosiliceous deposits in Gafsa basin (southern Tunisia)

The sedimentary rocks of the Metlaoui Formation in the Gafsa basin (southern Tunisia), which may be grouped in three units: the basal (Thèlja), middle (Chouabine) and upper unit (Kef Eddour), provide a record of preserved sedimentary, authigenic and biological processes. This paper presents the findings of sedimentological investigations of the biosiliceous deposits of the middle unit. This unit contains either well-preserved (Opal-A) or diagenetically altered (Opal-CT, clinoptilolite, quartz and even clays) diatom frustules. Such diagenetic changes are commonly described in marine and lacustrine biosiliceous deposits. The fossil content of theses diatomaceous layers implies shallow-marine conditions.The opal-rich sediments, and the associated facies record the transgressive transitions associated with high organic productivity, probably enhanced by seasonal input of nutrients, and high sea level stands, and a close association with stratified water column conditions. The formation of bedded diatomaceous sediments is known to require either high organic productivity or anoxic conditions in bottom/intermediate water, and eventually both processes. The initial organic content of the biogenic deposits was impoverished in early stages of sedimentation and diagenesis. A large part of the organic matter could have been destroyed during early diagenetic processes and from further oxidation in outcrops.     

The Mid-Cenomanian Event in shallow marine environments: Influence on carbonate producers and depositional sequences (northern Aquitaine Basin,France)

The Mid-Cenomanian Event was a positive carbon-isotope (δ¹³C) excursion recorded in hemipelagic basins of the western Tethyan Sea, North to Tropical Atlantic Ocean, and Japan. It is thought of as a prelude to the Oceanic Anoxic Event 2. However, the Mid-Cenomanian Event has never been studied in detail in shallow marine platform deposits and it is not known how it relates to carbonate production and stratigraphic geometry. To better understand how this carbon cycle disruption influenced the neritic biological communities in shallow carbonates during the Cenomanian, a facies, geochemical, diagenetic, and sequence stratigraphic study of the northern Aquitaine platform has been conducted. Seventy-six δ¹³C and δ¹⁸O measurements have been made on micrite, rudists, and diagenetic cements. Fifteen sedimentary facies have been arranged into four depositional environments. Three third-order sequences (C_B, C_C, C_D) are defined from late early Cenomanian to early late Cenomanian and are well correlated with eustatic cycles in European basins. Two peaks of the Mid-Cenomanian Event (MCE1a, +1.2‰, and MCE1b, +1.7‰) have been identified for the first time in shallow marine carbonates. Analysis of diagenetic blocky calcite cements suggests that diagenesis did not affect the δ¹³C of micrite, which can be discussed in terms of the initial signal. The Mid-Cenomanian Event was synchronous with a turnover in neritic carbonate producers marking a transition from photozoan to heterozoan facies. This facies change resulted from the establishment of mesotrophic to eutrophic conditions at the early/mid-Cenomanian transition, reflecting a clear connection between the Mid-Cenomanian Event and neritic biological communities. Depositional geometry and carbonate production varied with δ¹³C during the Mid-Cenomanian Event on the Aquitaine platform. When δ¹³C values were between 2.5‰ and 3‰, the geometry was a flat platform with a high carbonate sedimentation rate leading to the formation of sandbars and rudist bioherms (Accommodation/Sedimentation ratio less than 1, A/S < 1). When the δ¹³C value exceeded 3‰, a carbonate demise occurred and clays and marls were deposited in the lower offshore environment (A/S >> 1). The general carbonate demise affecting the northern Aquitaine platform during the mid-Cenomanian can be explained by both a eustatic sea-level rise and the establishment of eutrophic conditions. The coincidence of the Mid-Cenomanian Event with both (1) the occurrence of mesotrophic to eutrophic conditions marked by carbonate producer turnover from photozoan to heterozoan facies and (2) the transgressive cycles, suggests that eustatic sea-level rise leading to high trophic conditions could explain this positive δ¹³C excursion in the Atlantic and western Tethyan domain. During the mid-Cenomanian, carbon cycle perturbations largely controlled the neritic biological communities on shallow carbonate platforms in a part of the western Tethyan domain.     

Dasycladalean algae and their depositional environments in the Sylhet Limestone Formation (lower-middle Eocene), Bengal Basin

A very rich and diversified dasycladalean algal assemblage has been discovered from the Sylhet Limestone Formation (lower-middle Eocene) of the Bengal Basin of India for the first time. The depositional environments of the Sylhet Limestone Formation have been discussed based on the presence of the 11 species of the dasycladalean algae belonging to the three families Dasycladaceae (Cymopolia inflataramosa Segonzac, C. mayaenese Johnson and Kaska, C. paronai Raineri, Cymopolia sp.), Triploporaceae (Dissocladella lunata Segonzac, Dissocladella sp., Jodotella sloveniaensis Deloffre and Radoicic) and Acetabulariaceae (Clypeina socanensis Deloffre and Radoicic, Clypeina sp., Terquemella sp., Neomeris sp.). The lower Eocene Sylhet Limestone Formation revealed predominance of dasycladalean algal assemblage with the halimedacean and udoteacean algae and rare occurrence of coralline algae. This suggests their luxuriant growth in the open lagoonal to shelf environment at the depth of 5–6 m in the warm waters. There is a gradual decrease in the dasycladalean species and genera in the middle Eocene Sylhet Limestone Formation. The predominance of coralline algae associated with the Sporolithon indicates that the limestone of middle Eocene Sylhet Limestone Formation have been deposited at the littoral to shallow, high energy open shelf marine environments at a depth of about 40–60 m in warm tropical waters.