High-energy Depositional Environment Research Articles

Magnetic polarity stratigraphy of quaternary sediments from Ramganga paleolake, NW Himalaya, India and its paleoenvironmental implications

Paleomagnetic records from the sediments provide the foundation for studying variations in the geomagnetic field across the geological time sacle. Here, we report a new paleomagnetic data from Quaternary sediments of Ramganga Paleolake, NW Himalaya, India, to provide more insight into the global geomagnetic excursions and paleoenvironmental implications. Paleomagnetic and rock magnetic analyses were performed on a 5 m thick vertical profile of unconsolidated sediments from the Ramganga paleolake (29°46′867″N; 79°14′043″E), located in the Lesser Himalaya at a height of 798 m. One hundred fifty oriented samples from 50 sites (10 cm intervals) were obtained from the vertical section. Extrapolating the optically stimulated luminescence (OSL) dates from literature indicates that the entire section was probably deposited between 38 and 0.8 ka. The rock magnetic data shows that the remanence is carried by a low coercivity mineral and a significant amount of a soft magnetic mineral, most likely magnetite. Virtual Geomagnetic Pole (VGP) latitudes were calculated. Plot of the VGP latitudes against the sample locations from the litho-column allows us for the construction of Magnetic Polarity Stratigraphy (MPS) column. The MPS of the Ramganga paleolake has detected three geomagnetic excursions that are correlated with the global events: the Mono Lake Event (∼28.4-25.8 ka), the Gulf of Mexico Event (∼12.5–17 ka), and the Bagwalipokar Excursion Events (15.5-14.7 ka and 8-2.85 ka) respectively. The lower and middle portion of the paleolake represents the low and high energy depositional environment during the sediment deposition due to wet and arid climatic conditions.

Texture and major element geochemistry of channel sediments in the Orsang and Hiren River Basins, Gujarat, India: Implications for provenance and weathering

Size, shape, degree of sorting, and composition of sediments in the river channels are controlled by climate, lithology, weathering, sorting, and medium of transportation. The present investigation is focused on the grain-size and geochemical analysis of the channel sediments of the Orsang and Hiren river basins. Major outcrops in the study area are Archaean granites, granitic gneisses, Upper Cretaceous to lower Eocene Deccan Volcanic Basalts (DVB), Quaternary sediments and minor proportion of Proterozoic low grade metamorphic rocks. The sediments are poorly to moderately sorted, very finely skewed, suggesting its derivation from heterogenous sources, while the kurtosis value indicates a high-energy depositional environment. The sediments are with gravelly sand texture and the mean grain size is varying from 581.9µm to 1284.2µm. The DVB provenance of the Hiren river basin and granitic provenance of the Orsang river basin is clearly reflected in the texture and geochemical composition of sediments. The TiO2 and Fe2O3 contents of sediments from the Hiren river basin are distinctly higher and are comparable to the basalts of the Saurashtra region of the Deccan Province. Sediments collected after Orsang and Hiren rivers confluence and from Narmada river show higher concentration of felsic sources, indicating that Orsang river's sediment supply significantly outweighs Hiren rivers. The arkosic-litharenite nature points towards less transportation and moderate chemical weathering for the Orsang river sediments. The low Chemical Index of Alteration (CIA) values (Avg. 48.45 and 56.99 for Orsang and Hiren rivers, respectively) and A-CN-K plot also suggest the supply of sediments from minimally weathered detritus under a semi-arid condition.

Characteristics of quartz grains in the middle and lower reaches of the Hutubi River, NW China, and its paleo-environmental significance

Quartz sand characteristics in the downstream floodplain of the Hutubi River, located near the oasis–desert transition zone at the southern edge of the Gurbantunggut Desert, were examined to understand the sedimentary dynamics and processes. By analyzing three fluvial–lacustrine–eolian profiles and collecting surface sediment samples from different reaches of the Hutubi River, the characteristics of quartz sand were examined using optically stimulated luminescence (OSL) dating and scanning electron microscopy (SEM). The qualitative and quantitative results reveal spatial variations in surface sedimentary quartz sand, reflecting a transition from glacial–hydraulic interaction to alluvial dominance, followed by a balance between eolian and alluvial processes, and ultimately eolian dominance. The quartz sand within the profiles exhibits sub-rounded and rounded shapes, with significant alluvial-pluvial characteristics, indicating a predominance of fluvial processes within the lower sections of the profiles, while the upper sections exhibit a higher occurrence of eolian microstructures. Overall, the study area experienced a shift from a relatively high-energy and stable depositional environment during the mid-Holocene to a relatively low-energy and unstable condition in the Late Holocene. This was evidenced by stable fluvial–lacustrine deposition between 8 and 6ka and subsequent transitions between high and low-energy hydraulic processes, accompanied by wind erosion and frequent changes in sedimentary environments since 4.2ka.

A multi-proxy reconstruction of the late Holocene vegetation dynamics in Krabi mangroves, Thailand Andaman Sea

Pollen, charcoal, loss on ignition, geochemistry, and geophysical analyses were used to reveal the palaeoenvironment, vegetation, and sedimentary dynamics of the Krabi mangroves and the Andaman coast during the late Holocene. Two sediment cores, radiocarbon dated to ∼4400 cal BP, were collected from the Nai Nang mangroves in Krabi Province, the Thailand Andaman coast. This analysis documented how the area evolved from a tidal channel to a tidal area dominated by mangrove ecosystems. During the tidal channel phase (∼4400–2700 cal yr BP), the study site was characterized by a high-energy depositional environment, where sediment accumulated along tidal channels. Mangrove development began around 4400 cal yr BP and was influenced by varying contributions from tidal and freshwater inputs. Sea level during this period showed a slight rise until ∼4200 cal yr BP, followed by a subsequent fall. From ∼2700 to 1050 cal yr BP, the tidal channel was filled with finer sediments, forming intertidal flats that supported mangroves, back mangroves, and freshwater forest taxa, indicating a transition to mixed coastal vegetation. Sea levels remained relatively low during this period. From around 1050 cal yr BP to the present, mangroves replaced the previous mixed coastal vegetation, most likely driven by rising sea levels, particularly during the last 200 years. The observed sedimentation rates indicate that mangroves in Krabi will need to undergo continued future landward migration to adapt to changing coastal conditions, given the current and projected global sea-level rise.

Origin of Silicate Spherules and Geochemistry of Re and Platinum-Group Elements Within Microfossil-Bearing Archean Chert from the 3.4 Ga Strelley Pool Formation, Western Australia.

Silicate spherules have been identified from the ca. 3.4 Ga-old Strelley Pool Formation (SPF) in the Pilbara Craton, Western Australia. Their origins and geochemical characteristics, including the Re and platinum-group elements of their host clastic layer and the overlying and underlying microfossil-bearing finely laminated carbonaceous cherts, were examined. The spherules have various morphologies (completely spherical to angular), sizes (∼20 to >500 μm), textures (layered, non-layered, and fibrous), mineralogy (various proportions of microcrystalline quartz, sericite, anatase and Fe-oxides), and chemistry (enriched in Ni and/or Cr), commonly with thin anatase-rich walls. Their host clastic layer is characterized by rip-up clasts, suggesting a suddenly occurring high-energy depositional environment, such as tsunamis. Although various origins other than asteroid impact were considered, none could unequivocally explain the features of the spherules. In contrast, non-layered spherical spherules that occur as individual framework grains or collectively comprise angular-shaped rock fragments appear to be more consistent with the asteroid impact origin. The calculated Re-Os age of the cherts (3331 ± 220 Ma) was consistent with the established age of the SPF (3426-3350 Ma), suggesting that the Re-Os system was not significantly disturbed by later metamorphic and weathering events.

Sedimentary evolution of lagoons along the Namibian coast reveals fluctuation in Holocene biogeographic faunal provinces, upwelling intensity and sea level

Within the wave-dominated and high-energy depositional environment of the hyper-arid Namibian coast, lagoons and related saltpans represent one of the few regional settings in which sediments originating from both marine and terrestrial sources can accumulate under sheltered conditions. This allows for an approximately continuous depositional record of mid to late Holocene coastal evolution. For this paleoenvironmental reconstruction, 26 sediment cores from six coastal (paleo-)lagoons were taken and investigated along a 430 km-long latitudinal gradient. Based on 56 age determinations of sediments and shell material, the initial formation of the studied lagoons can be dated back to 6.0–5.3 cal kyr BP. The sediment cores present different types of lithoclastic sediments which can be assigned to five sedimentary facies ranging from sand spit sediments rich in shell material to eolian dune sands and evaporites. From these cores, 221 samples of macrobenthic faunal material have been collected and determined to possess 46 shallow marine species. Biogeographic analyses have resulted in the identification of 10 (sub-)tropical warm water species that are not part of the regional benthic fauna in the present upwelling system. Age determinations of the shell material revealed four phases of biogeographic range expansion/shift into the study area, at 5.3 cal kyr BP, 2.8 cal kyr BP, 1.2–0.9 cal kyr BP, and 0.36–0.12 cal kyr BP from both the northern tropical Angola Current as well as from the southern subtropical Agulhas Current. The combination of datasets from this study with published datasets of regional sea-level fluctuations and upwelling intensities presents an apparent correlation between both processes and presumably an additional linkage to the ENSO and Benguela Niño variability.

An Early Cretaceous (Berriasian) fossil-bearing locality from the Rocky Mountains of Alberta, yielding the oldest dinosaur skeletal remains from western Canada

Western North America preserves iconic dinosaur faunas from the Upper Jurassic and Upper Cretaceous, but this record is interrupted by an approximately 20 Myr gap with essentially no terrestrial vertebrate fossil localities. This poorly sampled interval is nonetheless important because it is thought to include a possible mass extinction, the origin of orogenic controls on dinosaur spatial distribution, and the origin of important Upper Cretaceous dinosaur taxa. Therefore, dinosaur-bearing rocks from this interval are of particular interest to vertebrate palaeontologists. In this study, we report on one such locality from Highwood Pass, Alberta. This locality has yielded a multitaxic assemblage, with the most diagnostic material identified so far including ankylosaurian osteoderms and a turtle plastron element. The fossil horizon lies within the upper part of the Pocaterra Creek Member of the Cadomin Formation (Blairmore Group). The fossils are assigned as Berriasian (earliest Cretaceous) in age, based on previous palynomorph analyses of the Pocaterra Creek Member and underlying and overlying strata. The fossils lie within numerous cross-bedded sandstone beds separated by pebble lenses. These sediments are indicative of a relatively high-energy depositional environment, and the distribution of these fossils over multiple beds indicates that they accumulated over multiple events, possibly flash floods. The fossils exhibit a range of surface weathering, having intact to heavily weathered cortices. The presence of definitive dinosaur material from near the Jurassic–Cretaceous boundary of Alberta establishes the oldest record of dinosaur body fossils in western Canada and provides a unique opportunity to study the Early Cretaceous dinosaur faunas of western North America.

A rock magnetic perspective of gas hydrate occurrences in a high-energy depositional system in the Krishna-Godavari basin, Bay of Bengal

We conducted a detailed rock magnetic study complemented by sedimentological and mineralogical methods on a 177.2-m-long sediment core of Hole NGHP-01-14A to constrain the influence of high-energy depositional environment on the magnetic mineral diagenesis and formation of gas hydrates in the Krishna-Godavari (K-G) basin, Bay of Bengal. Five sediment magnetic zones were identified based on the downcore variation in rock magnetic parameters. The magnetic mineralogies comprised of detrital titanomagnetite, diagenetically formed magnetic iron sulfides and their mixture. A distinct magnetically enhanced zone (Z-3) close to a bottom simulating reflector (BSR) is dominated by higher magnetite content, sediment bulk density, and grain size and probably represents several sand-rich sediment layers formed as a result of short-duration intense sedimentation events and got rapidly buried and provided conducive environment for the formation of gas hydrate deposits at the studied site. The concurrence of gas hydrates and high gas saturations within the sand-rich layers (Z-3) suggests that the high-energy depositional environment had a larger control over hydrate formation at Hole NGHP-01-14A. Two magnetic iron sulfide bearing sediment intervals in Z-1b and Z-1c below sulfate-methane transition zone (SMTZ) suggest that their formation is controlled by microbially mediated diagenetic reactions fuelled by presence of gas hydrates and probably represents the fossil gas hydrate zones. We propose that the rock magnetic variations in the studied sediment core at Hole NGHP-01-14A is controlled by both differential loading of detrital magnetic minerals as well as hydrate-induced late diagenetic magnetic iron sulfide formation.

Strukturalne, litologiczne oraz tektoniczne uwarunkowania rozwoju i ewolucji świętokrzyskich skałek piaskowcowych

Structural, lithological and tectonic constraints on the development and evolution of sandstone tors in the Świetokrzyskie (Holy Cross) Mountains. A b s t r a c t. Majority of ca. 90 sites of sandstone crag groups and individual crags, occurring in the Świetokrzyskie (Holy Cross) Mts. region, represent the following crag-forming lithostratigraphic units: Cambrian Wiśniowka Formation, Devonian Barcza Fm and Zagorze Fm, Triassic Zagnansk Fm and Krynki Beds, as well as Jurassic Skloby Fm and Ostrowiec Fm. Specific features of these rocks are the occurrence of sandstone series, up to 20 m thick, above more plastic, clayey or heterolithic series, high-energy depositional environments, and siliceous composition. The crag-forming sandstones differ in the amount of siliceous cement: from strongly cemented Paleozoic quartzitic sandstones to porous Mesozoic sandstones with poor cement, which determines diverse mechanical properties. Strongly cemented Paleozoic rocks display high rock strength and abrasion resistance, while porous and theoretically friable Mesozoic sandstones are characterised by high grain packing due to compaction. Regarding the principal role of gravitational disintegration of rock massifs under the periglacial conditions in the Pleistocene, other factors constraining the crag formation and shaping are the tectonic situation of rocks (orientation of strata and joints), adequate joint spacing, and bed thickness. The interrelations between lithological and structural features of crag-forming sandstones and tectonics, conditioning erosion and weathering rates are specific for particular types of these sandstones.

New species of Cyclopsiella (Acritarcha) from the Jurassic of northern Nordland, Norway

New species of the genus Cyclopsiella Drugg & Loeblich 1967 are recovered in Middle and Upper Jurassic strata on the island Andøya and offshore Nordland, Northern Norway. One species is formally described as new: Cyclopsiella nordlandensis sp. nov. Observations from the Middle and Upper Jurassic of the Norwegian Sea and North Sea suggest that acmes of Cyclopsiella morphotypes indicate shallow marine/nearshore, relatively high energy depositional environments.

Alveolina-dominated assemblages in the early Eocene carbonates of Jaintia Hills, NE India: Biostratigraphic and palaeoenvironmental implications

Larger benthic foraminifera (LBF) are significant proxies in biostratigraphy and also act as excellent indicators of shallow-marine carbonate environments in fossil series. The Palaeogene LBF recorded from Meghalaya, NE India (eastern part of the relic eastern Tethys/Neo-Tethys) have high potential for dating shallow-marine sediments and documenting the multiple episodes of carbonate sedimentation that have contributed to the development of the Sylhet Limestone Group. Early Eocene witnessed the proliferation of LBF species worldwide, the phenomenon better known as the Larger Foraminiferal Turnover (LFT). Genera like Alveolina, Nummulites and Orbitolites with broad species complexes thrived as the dominant LBF amidst numerous other taxa on the verge of extinction or only surviving as stable forms. The current study emphasizes on the biostratigraphic and palaeoenvironmental account of the early Eocene Umlatdoh Limestone successions outcropping in the Jaintia Hills of Meghalaya, primarily based on the recorded species of Alveolina and other larger benthic foraminifera. Five species of Alveolina — A. oblonga, A. schwageri, A. cf. ruetimeyeri, A. aff. haymanensis and A. aff. varians are recorded in the evaluated sections that indicate an early Eocene age corresponding to the Shallow Benthic Zone 10. Major carbonate facies types in the present assessment include oolitic-smaller benthic foraminiferal -green algal grainstone–packstone, smaller miliolid-Alveolina grainstone, green algal-benthic foraminiferal grainstone, larger porcellaneous (Alveolina) grainstone-packstone, Alveolina-nummulitid grainstone-rudstone, and nummulitid grainstone-rudstone, which indicate a shallow marine, high-energy depositional environment ranging from shoal-sandy bars to a distal inner ramp setting.

Key factors controlling hydrocarbon enrichment in a deep petroleum system in a terrestrial rift basin—A case study of the uppermost member of the upper Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, NE China

Abstract Mineralogical, petrographical, geochemical analyses, and fluid inclusions petrography and microthermometry were combined to investigate the basic properties of the source rock (organic matter content, kerogen type, and thermal maturity) and sandstone reservoirs (composition, pore systems, porosity, and permeability), the hydrocarbon origin, and the formation mechanisms of the deep (>3.5 km) petroleum system in the Upper Paleogene Es1 (the uppermost member of the Shahejie Formation) in the No. 3 Structural Belt in the Nanpu Sag, Bohai Bay Basin. The results indicate that the Es1 source rock is primarily composed of dark gray mudstone and presents a set of high-quality source rocks (>500 m, average total organic carbon (TOC) 1.38%, in a mature stage (maximum pyrolysis yield of 430 °C–460 °C)). The Es1 sandstone is composed of lithic arkose formed in a braided river delta and has low porosity (average 13.4%) but moderate-high permeability (average 184.9 mD). The hydrocarbons originated from the Es1 source rocks in the Caofeidian subsag during two charging periods: 10 Ma and 3 Ma. The key factors controlling the formation of the deep petroleum system are as follows: (a) High-quality hydrocarbon supply, (b) Medium-high permeability reservoirs, (c) A favorable configuration of the source rock and reservoirs (self-generated and self-accumulated), and (d) Late hydrocarbon charging; the second factor is the most important. The medium-high permeability reservoirs are attributed to a high-energy depositional environment (distributary channels of braid delta front), a high content of rigid detrital grains (mainly quartz and metamorphic rock fragments) provided by the Archean granite provenance, and the secondary pores formed by feldspar dissolution. This study provides a new example for revealing the formation mechanisms of deep (>3.5 km) petroleum systems in the terrestrial rift basin across the Bohai Bay Basin.

Chemostratigraphy and palaeo-environmental characterisation of the Cambrian stratigraphy in the Amadeus Basin, Australia

The sedimentary succession of the central Australian Amadeus Basin consists of Neoproterozoic to Carboniferous sedimentary rocks and contains shallow marine, subtidal carbonates of Middle to Late (Series 2 to Furongian) Cambrian age. A combination of sequence stratigraphy, geochemistry and mineralogy shows a transgressive 2nd order cyclicity deposited between ~511–490Ma and a change from arid, low energy to humid, high energy depositional environments. This is reflected in an initially evaporitic sequence with upward decreasing halite and anhydrite abundance and transition from oxygenated to anoxic conditions, reflected by the Fe mineral species change from hematite to pyrite during transgression. Sequence boundaries of several 3rd order cycles consisting of HST carbonate rocks and LST siltstones, correlate with globally recognised sequence boundaries linked to the inferred eustatic sea level record for the upper two series of the Cambrian System. The carbon isotope record for this ~1400m thick succession in combination with biostratigraphic age correlation allowed the identification of the globally recognised Steptoean Positive Carbon Isotope Excursion (SPICE), Drumian Carbon Isotope Excursion (DICE) and Redlichiid-Olenellid Extinction Carbon Isotope Excursion (ROECE).

The geological constraints of the development of sandstone landforms in Central Europe, a case study of the Świętokrzyskie (Holy Cross) Mountains, Poland

Eighty sites of crag groups and individual crags occurring in the Świętokrzyskie (Holy Cross) Mts., upland situated in central Poland, were described in detail in order to examine the lithological, structural, mechanic properties and the tectonic features of sandstones and quartzites representing six crag-forming lithostratigraphic units: Cambrian quartzites, Devonian quartzitic sandstones, Triassic and Jurassic sandstones. Specific features of these rocks are: their occurrence within the sequences consisting of different rock series, high energy depositional environments and siliceous composition. The cragforming rocks differ in the amount of cement (from strongly cemented quartzites to very porous sandstones with poor cement), which determines the diverse mechanical properties (from very strong to friable rocks). The crucial feature enabling formation of crags built of porous and friable sandstones is very dense grain packing due to chemical and mechanical compaction. Regarding the principal role of the gravitational disintegration of rock massifs under the periglacial conditions in the Late Pleistocene, other factors constraining the crag formation and shaping are: the tectonic situation of rocks (the orientation of strata and joints) as well as adequate joint spacing and bed thickness. Petrographic, structural and tectonic features are interrelated.

Long term evolution and internal architecture of a high-energy banner ridge from seismic survey of Banc du Four (Western Brittany, France)

The recent completion of a coupled seismic and swath bathymetric survey, conducted across the sand ridge system of the Banc du Four located on the Atlantic continental shelf of Brittany (Mer d’Iroise, France), provided new data for the study of the long term evolution of deep tidal sand ridges. Five seismic units are distinguished within the ridge, separated by pronounced major bounding surfaces. The basal unit is interpreted to be shoreface deposits forming the core of the ridge. It is overlaid by a succession of marine sand dunes fields forming the upper units. Sandwave climbing, which combines progradation and accretion, is the major process controlling the growth of the ridge. The elevation of the preserved dune foresets reaches values of about 20–30m within the ridge. The foresets indicate a combination of giant dunes characterized by numerous steep (up to 20°) clinoforms corresponding to a high-energy depositional environment. Moreover, the presence of scour pits linked to the 3D geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines. All of the radiocarbon ages of the biogenic surficial deposits of the Banc du Four range from 10,036 to 2748calyears B.P. and suggest the Banc du Four has grown during the last sea-level rise. The apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non-conservation of recent deposits. In contrast, the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge. Moreover, the long-term evolution of the ridge appears strongly controlled by the morphology of the igneous basement. The multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy, consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement.Therefore, the Banc du Four should be thought of as a representative example of a large-scale high-energy banner bank.

Paleocurrents and Paleohydraulics Studies of the Proterozoic Kolhan Siliciclastic Unit (India): A Case Study from the Chaibasa-Noamundi Basin, Jharkhand, India

Detailed palaeocurrent and palaeohydraulic analyses of the fluvio-deltaic Proterozoic Kolhan siliciclastic unit has provided new insights into the sedimentological knowledge of the fan delta deposition, allowing for the reconstruction of paleogeographic and paleoclimatic conditions prevalent in the Chaibasa-Noamundi intracratonic basin of India. The azimuth reading of cross-bed stratifications of fluvial deposits and the preferential grain orientation of different lithofacies have indicated at least two major paleo-flow systems, NW and NE trending. The local occurrence of sedimentary structures of high energy depositional environments has indicated the superimposition of retrogradational strata pertaining to an older progradational fan complex. The fluvio-deltaic Proterozoic sedimentation has followed the emplacement of volcanic dykes, basic in composition, indicating a thermal rejuvenation of the continental crust. Finally, the partial development of limestones and dolomites in the Kolhan succession has pointed out the peneplanation of the cratonic areas and the development of carbonate sedimentation on stable carbonate platform.

Geology of the Brega area and the ichnofauna of the ‘Brega Sandstone Bed’ (Pliocene), Libya

The sandstone outcrops in the Brega Area Two are designated as ‘Brega Sandstone Bed’ on the basis of the lithological similarities and stratigraphy of the region surrounding Brega. The ‘Brega Sandstone Bed’ is considered to be equivalent to the lower part of the Member V of the Sahabi Formation of the Pliocene age. The thinly bedded greyish to greenish sandstone interbedded with gravel and pebble beds exposed at the ‘Sea cliff section’ represents the upper part of Member V of the Sahabi Formation. Outcrops of Pleistocene-cemented marine dunes belong to the Ajdabiya Formation and are seen as patches of marine dunes of various shapes, lengths and heights that run parallel to the modern day Mediterranean Sea coastline. The Holocene sediments around Brega include coastal and inland sand dunes and sand sheets mixed with millions of 'beach balls'. The ichnofaunal assemblage of the ‘Brega Sandstone Bed’ is dominated by various types of vertical, oblique and horizontal burrows that are branched and unbranched. They are dominated by ichnogenera Ophiomorpha and Skolithos. The ichnocoenose dominated by these two ichnogenera belong to the marine soft ground Skolithos ichnofacies. Lithology of the ‘Brega Sandstone Bed’ and its ichnocoenose indicate a shallow marine, intertidal to shallow subtidal, moderate to high energy depositional environments.

Sedimentology, stratigraphy, and reservoir properties of an unconventional, shallow, frozen petroleum reservoir in the Cretaceous Nanushuk Formation at Umiat field, North Slope, Alaska

Numerous oil and gas accumulations exist in the Brooks Range foothills of the National Petroleum Reserve in Alaska (NPRA). We use cores and well logs from 12 abandoned legacy wells at Umiat field, near the southeastern boundary of the NPRA, to characterize the sedimentology and stratigraphy of unconventional shallow frozen reservoirs in sandstones of the Cretaceous (Albian–Cenomanian) Nanushuk Formation. The Nanushuk Formation at Umiat has five facies associations: offshore and prodelta, lower shoreface, upper shoreface, delta front, and delta plain.Three stratigraphically distinct, regionally extensive Nanushuk Formation depositional systems at Umiat contain several potential petroleum reservoirs. The lower Nanushuk Formation, including a reservoir interval known informally as the lower Grandstand, primarily consists of marine mudstone and shoreface sandstones. The middle Nanushuk Formation is dominantly deltaic and contains a second major reservoir interval in the informal upper Grandstand sandstone. Both the upper Grandstand and lower Grandstand are regressive. The transgressive upper Nanushuk Formation contains an additional potential reservoir interval in shoreface sandstones of the informal Ninuluk interval. The primary reservoir intervals at Umiat field are upper shoreface and delta-front sandstones in the upper Grandstand and lower Grandstand, where increased sorting and decreased bioturbation in high-energy depositional environments affect overall permeability and permeability anisotropy.

Lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows: From cyclic steps to humpback dunes

The preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record, although these bedforms are known from a variety of depositional environments. This field-based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps, chutes-and-pools, antidunes and humpback dunes from three-dimensional outcrops. The well exposed Middle Pleistocene successions from northern Germany comprise glacilacustrine ice-contact subaqueous fan and glacial lake-outburst flood deposits. The studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high-energy depositional environments such as fluvial settings, coarse-grained deltas or turbidite systems.Deposits of cyclic steps occur within the glacial lake-outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets. Cyclic steps formed due to acceleration and flow thinning when the glacial lake-outburst flood spilled over a push-moraine ridge. These bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes-and-pools and antidunes. The subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms, which are interpreted as deposits of aggrading stationary antidunes, which require quasi-steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation. Humpback dunes are characterised by downflow divergent cross-stratification, displaying differentiation into topsets, foresets and bottomsets, and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa. Gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common.The absence of planar-parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions. The large-scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows, which was strongly affected by the occurrence of hydraulic jumps. Small-scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge, bed topography and pulsating unstable flows.

Southeast Bayuda volcano-sedimentary sequences (Kurmut terrane, Sudan): juvenile island arc series within the mega-shear zone marking the eastern boundary of the Saharan Metacraton

This study was conducted to investigate the volcano-sedimentary sequence of Kurmut terrane in southeast Bayuda Desert and its continuation east of the Nile. The sequence is situated at the inferred transition between the Neoproterozoic juvenile Arabian–Nubian Shield and the old reworked Saharan Metacraton. Lithologically, the rocks are composed of intermediate to basic metavolcanics and volcanoclastics intercalated with tuffeous material and metapelite sediments that formed in cyclic graded bedding, indicating a turbid environment in the active orogenic zone. For the first time in the study area, intraformational conglomerates with cigar-shaped volcanic clasts within the low-grade metasediments suggest a high-energy depositional environment. Minerals and textural examination of rock assemblage indicate an anticlockwise P/T condition from amphibolite facies to retrograde greenschist facies conditions in a large high strain zone of distributed deformation. Geochemical results of metavolcanics indicate derivation from different mantle sources (island arc tholeite, calc-alkaline basalt, mid-oceanic ridge, and back-arc basalt). Within this sequence, field relation and microstructural analysis clearly show the best evidence of superimposition of sinistral shear of the NNE–SSW-trending Keraf Shear Zone on the early dextral NE–SW Nakasib Suture Zone. Consequently, based on lithological and structural observations with support of available geochemical data, the volcano-sedimentary sequences SE of Bayuda Desert and east of Nile are similar to their equivalent assemblage of juvenile Neoproterozoic arc lavas association of the Arabian–Nubian Shield.