Deltaic Sequence Research Articles

Sediment supply variation control on Lower Eocene delta sequences (Tremp Basin, Spain)

Sediment supply variations are often overlooked when interpreting depositional sequences, with most studies instead emphasizing changes in accommodation space. Here, we investigated a temporally well-constrained shallow-marine succession in the Tremp Basin to test the control of sediment supply variations on the development of deltaic sequences during the early Eocene. We analyzed the paleoenvironmental record (sedimentary facies and δ13Corg values) of the Morillo Limestone and the Castigaleu Formation (52.2−50.6 Ma). The first progradation of the deltaic system is marked by the abrupt appearance of thick delta-front sandstones and associated with the first negative carbon isotopic excursions (CIEs) in the measured δ13Corg record. Subsequent phases of progradation align with subsequent negative CIEs. Conversely, positive CIEs correspond to finer-grained, more-distal prodelta deposits. A series of hyperthermal events occurred during the deposition, globally identified as negative CIEs on δ13Ccarb reference curves, which we tentatively correlate with our δ13Corg data. We therefore suggest that during deposition of this Lower Eocene shallow-marine succession, the primary trigger behind sequence generation was the high-frequency climate-induced variation in sediment supply, specifically the hyperthermal events, rather than changes in accommodation. This linkage underscores the complex interactions between climate dynamics and sedimentary responses that shape the stratigraphic architecture of shallow-marine settings.

Post-glacial small delta process uncovered by luminescence and radiocarbon chronology of core sediments from coastal South China Sea

Reliable chronology is crucial for reconstructing delta processes. The past decades witnessed a boost of dating works on the most economically influential large deltas (i.e., subaerial area >1000 km2), but chronology remains lacking on many small deltas (i.e., subaerial area <1000 km2) that are also densely populated and economically active. Luohe River Delta (LRD) in the coastal South China Sea is such a small delta, whose evolution concerns hundreds of thousands of people, and literally no reliable dating results have been reported to support research of its processes. Herein, dating work combining radiocarbon (14C) on mollusk shells and quartz Optically Stimulated Luminescence (OSL) methods were performed on core LFZK06 from LRD, to test applicability of both methods by age comparison and to establish a chronological framework, using Bayesian age-depth models, of LRD for the first time. 14C ages are systematically c. 0.5–1 ka younger than OSL ages from the same depths. Such young bias of 14C ages in LRD contrasts with previously observed overestimated 14C results in the nearby Pearl River Delta (PRD), likely due to the distinction in bedrock types between Luohe River (granites and sandstones only) and Pearl River (limestone prevalence causing hardwater effect) drainages. Ages of core LFZK06 span from 57 ± 7 ka to 3.42 ± 0.05 cal ka BP and contain a hiatus between 57±7–11.9 ± 1.7 ka. Holocene deltaic sequence of the core shows three-stage sedimentary processes: (1) rapid deposition of prodelta/delta front sediments at 10.2‐0.4+0.5 ka–7.7‐0.3+0.4 ka related to rapid sea level rise, (2) hiatus during 7.7‐0.3+0.4 ka–5.7‐0.8+0.5 ka likely due to reduced sediments input or river channel migration, (3) rapid accumulation of delta plain sediments during 5.7‐0.8+0.5 ka–3.42‐0.85+0.42 ka reflecting depocenter shift toward core location. Moreover, changes of quartz OSL sensitivity were detected, indicative of sediment provenance transition. A change in sediment source from nearby granite weathering towards fluvial long-transported materials occurred at a depth of 24.7 m, with low quartz OSL sensitivity in the gravelly layer below (11.9 ± 1.7 ka) and one magnitude higher quartz OSL sensitivity in prodelta deposits above (10.2‐0.4+0.5 ka).

Lithostratigraphy and Sedimentation Models of TMG-C4 Well Offshore Eastern Niger Delta

Analysis was carried out in TMG-C4 well by integrating well logs data that revealed three depositional sequences within the Agbada Formation of about 400ft thick. Three systems tracts identified include highstand systems tract, transgressive systems tract and lowstand systems tracts. The highstand systems tract is majorly stacked regressive shoreface sands that occurred in two parasequence sets, whereas the transgressive systems tract is a transgressive unit with both source rock and sealing potentials. The lowstand systems tract shows a boxcar log motif and is identified to be probably channel or barrier bar sand. Major shale units (maximum flooding surfaces) with regional significance mapped are designated MFS. This division is relevant to identifying genetic depositional units. Three genetic sequences occurred in the well. The transgressive shales represent interruptions in the overall regressive sequence that is related to sea-level rise. Three of these shales have been mapped and two of them correspond to the eleven genetic megasequences that occur delta wide. A predominantly marine and deltaic sequences strongly influenced by clastic output from the continent is inferred from the well logs. Paleo water depth is interpreted to fluctuate considerably and deposition occurred within a variety of littoral and neritic environments ranging from nearshore barrier sand complexes to fully marine outer shelf mudstones. The sediments are rapidly deposited within the shallow marine realm and reworked into longitudinal bars by wave action, strong longshore drift and tidal effects. The dominant depositional trend observed in the well shows progradation

Implication of the micro- and lithofacies types on the quality of a gas-bearing deltaic reservoir in the Nile Delta, Egypt

The Messinian Qawasim sequence, as one of the most important reservoirs in the Egyptian Nile Delta, represents a typical gas-bearing deltaic reservoir sequence. It aims at delineating the implication of the litho- and microfacies associations on the reservoir characteristics of the different stages of the fluvial deltas including the prodelta, proximal/distal delta front, and the delta plain depositional sequences. The studied reservoir sequence was divided into two units; upper and lower clastic units. The petrophysical properties of these two units were studied on the borehole scale using the gamma-ray, caliper, sonic, density, neutron, and resistivity logs to estimate the reservoir parameters including the total and effective porosities, water saturation, shale volume, and the net-pay thickness. For more details, they are also examined on the plug-scale using the core dataset including helium porosity, density, permeability, and fluids saturations, where the flow zone indicator, the reservoir potentiality index, the reservoir quality index, and the average reservoir pore radius were then estimated. The entire sequence is divided into five reservoir rock types (RRT1-RRT5) where, the best quality is assigned to the first RRT (the upper reservoir unit), and the lowest quality is assigned to the RRT5 of the lower reservoir unit. Based on core description and the petrographical studies five lithofacies and four microfacies have been identified. The lithofacies are (1) laminated mudstones/siltstones, (2) ripple laminated sandstones, (3) cross-laminated sandstones, (4) cross-bedded sandstones, and (5) pebbly massive sandstones. These lithofacies are primarily composed of four microfacies; sublithic arenites, subarkose arenite, glauconitic quartz wacke, and sandy mudstone/siltstone. Based on this study, the pebbly massive and the cross-bedded sandstones of the upper unit which is composed of the sublithic and subarkose arenites (RRT1-RRT2) have the best reservoir quality. On the other hand, the lowest quality is assigned to the RRT5 (sandy mudstone/siltstone microfacies) which is represented on the macro scale by the laminated mudstones/siltstones lithofacies. The integration between multi-scale datasets (core-well, petrography, well logs, and seismic) gives a precise picture of the deltaic Qawasim reservoir rock units in the Nile Delta. This workflow has never been applied to the deltaic system in the onshore Nile delta and North Africa. Thereby, this study is considered a standard case study for the deltaic sequences and its proposed workflow is applicable to the Nile Delta elsewhere and worldwide for similar reservoir sequences.

Late-Holocene hydrological variability from the NW Himalaya and southwestern Tibetan Plateau: Paleo-salinity records from Pangong Tso

Variations in lake-levels and hydrology are connected to climatic dynamics over the Trans-Himalaya and Tibetan Plateau (TP). Pangong Tso, a ~140 km long hypersaline lake, is sensitive to changes in air temperature, precipitation and snow-glacier melt over the southwestern TP. The incised tributaries entering the lake expose deltaic sequences constituting topset, foreset and bottomset. Sedimentology, chronology and diatom analysis of delta sediments; and stable isotopic and sclerochronological analysis of gastropods helped delineate the Late-Holocene variation in lake levels, surface temperature and salinity. The elevation of the topsets is considered as representing past lake high-stands from where molluscs were also recovered. Three phased lake level changes are observed during the past 3 ka. The first high-stand (+1.4–3.0 m) was at ~2.8–2.0 ka when lake surface salinity and temperature were 4.67–6.01 ppt (parts per thousand) and 5°C–7°C, respectively, against the modern average values of 7.7 ± 0.09 ppt and 16.1°C ± 2.0°C. Followed by a brief decline, another high-stand (+3.0–3.6 m) is observed at 1.1 ka when the salinity is reduced to 4.03–5.72 ppt and lake surface temperature to 5°C–8°C. A corresponding increase in freshwater diatom concentration is also observed here. The third phase over the past 1 ka witnessed a fall of ~3.6–6 m in lake level that is attributed to an abrupt rise in aridity over the TP. We demonstrate that lake level variation in the region is a function of the variability of the Indian Summer Monsoon (ISM) and the westerlies, however during the high-stand, the hydrology of the lake was dominated by ISM precipitation.

Appraisal of tectonically-influenced lowstand prograding clinoform sedimentary fairways of Early-Cretaceous Sember deltaic sequences, Pakistan using dynamical reservoir simulations: Implications for natural gas exploration

The Early Cretaceous reservoir system of the Indus basin has a suite of stratigraphic-based trapped petroleum plays such as the fluvial-dominated channelized systems, shallow marine incised valley systems, and lowstand prograding clinoforms. However, the seismically-imaged Lowstand prograding clinoform plays (LCP) are hot topics in the stratigraphic-based hydrocarbon traps-bearing traps fairways. These LCPs remain established throughout an extensively falling sea level with succeeding growth, which fills these LCPs with gas-saturated reservoirs. The LCP remains mud-dominated, which has created ambiguous expressions in full-spectrum seismic. Hence, they in turn complicate the prediction of the stacking patterns of the depositional parasequences, the source of sedimentary influx, the angle of stratigraphic traps, and their controls on the vertical/lateral migration of hydrocarbon-bearing fluids. The bandlimited seismic amplitude lacks the tuning frequency, which is one of the key parameters in the seismic sedimentological analysis given the above-mentioned constraints. This study connects the broadband spectral analysis with the bandlimited seismic amplitude-based attributes to quantify the LCP using wavelet transform-based impedance-decomposed porosity-gas-saturation of velocity and density simulations (PGVDS). The seismic amplitude and 23-Hz CWT attribute have allowed the detection of vertical thickness, lateral distribution, and angle of coarse-grained and fractured sigmoidal LCP facies of 22 m, 6 km, 7°, and 26 m, 12 km, 5°. The PGVDS have predicted the thickness, density, and velocity for porous gas-saturated LCP as 31–34 m, 2.1–2.15 g/c. c, 80–85%, 29–32%, 2139–2304 m/s, and non-porous water-saturated LCP as 18–28 m, 2.2–2.39 g/c. c., 60–75%, 17–25%, 2863–3108 m/s. The PGVDS have also imaged the highly fractured and laterally continuous fluvial-dominated onlapping channel-filled reserves that served as a source of generating a high impact of sedimentary influx within aggradational stacking patterns for gas-saturated (80–85%) sedimentary facies of LCP at an angle of zero degrees and porosity range of >30%. The lateral changes in the densely fractured network validate the facies-controlled migration of hydrocarbon-bearing reserves within the LCP, which implicates the development of regionally developed stratigraphic-based trapping plays throughout the basins of Pakistan. This study has categorically broadened the impact of connecting the spectral attributes of inverted reservoir simulations in inferring the geometry, extent, thickness, and distribution of hydrocarbons in prograding clinothems inside the Indus Basin. Hence, this research can serve as an analogue for the exploration of worldwide LCP with similar geological conditions.

Paleoenvironmental reconstructions along the southern margin of Eocene Lake Uinta during a period of climatic variability

The stratigraphy preserved in the lacustrine Eocene Green River Formation suggests a variety of depositional environments that were subject to pulses of sediment and lake level fluctuations in the Uinta Basin. This paper defines 13 facies within six facies associations observed in outcrop and core from the middle to upper Green River Formation in Gate Canyon, south-central Uinta Basin, and Willow Creek/Indian Canyon, western Uinta Basin, along the margin of the ancient lake system. The facies associations are defined by dominant lithology, relative siliciclastic sediment supply, relative slope, and lake zonation, and encompass a suite of facies revealing distinct environments across a relatively small region (~35 km apart). In the lower interval, a shift from a high energy, steeper ramp setting in the east, to a lower energy, shallower ramp setting to the west suggests a transition off the main depositional axis. Correlatable siliciclastic and carbonate dominated packages reflect the impact of early Eocene hyperthermal events that followed the Paleocene-Eocene Thermal Maxi- mum, and the diverse facies present at the two study areas suggest variable responses to these climatic events. In the overlying stratigraphy, the facies are consistent in both regions and consist of organic rich and poor carbonate mudstones and siltstones. The interval thickens slightly from east to west, and the overlying Horse Bench Sandstone transitions from a wave-dominated shoreface sequence to a river-dominated deltaic sequence, indicating an overall shift in the lake depocenter towards the west. This study offers a unique glimpse at the paleoenvironmental changes along the margin of a continental interior lake system during a time of climatic variability. It also highlights the need to use multiple depositional models to accurately characterize marginal deposits in different regions of large internally-drained lake systems which, unlike typical marine depositional models, are more sensitive to basin geometry, size, and sediment fairways.

Marginal fan thick lacustrine sedimentation: Implication for tectonics and/or climatic causes in Kathmandu basin

Very thick gravel deposits are widely distributed from southern marginal to central parts of the Kathmandu basin named as Itaiti Formation. This deposit is more than 120 m thick and deposited around 1 Ma, which is dominated by three types of facies and fans. The first stage fan is widely distributed near the mountain front where sediments aggradation is more active. The third stage fan is the youngest among three fans and is distributed on the present top surface and has covered lacustrine deltaic sequence of the Sunakothi Formation. Seven types of facies elements namely Gms, Gm, Gp, Sp, Sr, Sh, and Fl have been recognized among these three fans. Based on stratigraphic relationship among different geological formations, the first debris flow fans have been spreading over basement rocks and fluvial gravel facies before 1 Ma. This event has played a vital role in the initiation of an ancient lake in the Kathmandu basin. The second and third stage fans originated during the draining stages of the lake. The distal part of the second stage fan and third stage fan is interfingering with the lacustrine delta towards the center of the basin. The first stage debris flow fan was most probably generated by the recent uplift of the Mahabharat range. These gravelly fan deposits and the uppermost Siwalik conglomerate deposits may be synchronous to recent tectonic events throughout the Himalayas. The second and third stage fans have been initiated not only by the tectonics but also by climatic which is indicated by sedimentological and stratigraphic relations among formations.

Holocene evolution and depositional model of a bayhead delta, Lake Illawarra, Australia

AbstractExtensive research into the evolution of wave‐dominated estuaries has been documented since the 1960s. However, there has only been limited research on the prograding bay‐head deltas that are the primary drivers for the rate and stage of estuary evolution. This paper presents the findings of a high‐resolution spatial study into the evolution of the Macquarie Rivulet bayhead delta in the Lake Illawarra barrier estuary. The delta’s evolution has been established based on sedimentological analysis of 74 cores, two14C ages and 45 amino acid racemization ages. This study intersected two Pleistocene and 10 Holocene sedimentary facies associations representing both the retrogradational and progradational phases of delta development. The distribution of bayhead delta facies associations in shallow barrier estuaries is initially controlled by the antecedent morphology, but, in contrast to deeper estuaries, this influence decreases as sedimentation proceeds. Changing relative sea level also has a major control on facies distribution with transgressive facies deposited as relative sea‐level rises being replaced by prograding deltaic facies during the highstand and subsequent minor relative sea‐level fall. Fluvial sediment supply and river flood events affect the rate of delta progradation, and produce low and high flow sediment deposits within the overall deltaic sequence. The low tidal flux in barrier estuaries, the muddy cohesive nature of the sediments and the prevalence of bioturbation means that primary sedimentary structures are rarely preserved in these bayhead delta facies. The depositional model of bayhead delta evolution shown by the Macquarie Rivulet delta would be widely applicable to other similar barrier estuary settings.

Volcaniclastic lacustrine sedimentation in the Pleistocene Guayllabamba intermontane basin in the Ecuadorian Andes

Abstract In this work, we present a description of the sedimentary fill of a well-exposed lacustrine succession in the Ecuadorian Andes. The Guayllabamba basin is an intermontane basin located in the Andean range of Ecuador, and part of its sedimentary history is represented by a volcanically influenced c. 100 m-thick lacustrine unit of Pleistocene age. We create a stratigraphic cross-section from the eastern to western lake margins and identify 19 facies that were used to carry out a palaeoenvironmental reconstruction. The Guayllabamba palaeolake was developed in a tectonic depression surrounded by volcanoes and it was filled with sediments derived from the erosion of the volcanic edifices, the reworking of unconsolidated pyroclastic deposits and deposition of pyroclastic currents into the lake. The lake shows a deepening trend, passing from shallow deltaic sedimentation to varved diatomites with turbidites. Abundant ash-fall beds, monolithological pumiceous deltaic sequences and pumice-dominated thick ignimbrites show the impacts of volcanism on lacustrine sedimentation within this basin. Soft-sediment deformation and gravity flow deposits are common owing to the intrabasinal tectonic activity and to the intrusion of a lava body. Aulacoseira -rich diatomites dominate the background lake sedimentation. The outcrops of the Guayllabamba basin are outstanding examples of the interaction between volcaniclastic and lacustrine sedimentation.

Chronostratigraphic and palaeogeographic interpretation of Nakdong deltaic sequences in the south-eastern Korean Peninsula

We investigated the chronostratigraphy of six boreholes in the Nakdong River delta area in South Korea based on optically stimulated luminescence (OSL) and 14C dating. The standardised growth curves (SGCs) produced from single and multiple boreholes performed well in determining low equivalent dose (De; < 40 Gy). 14C dates from shell and wood fragments were mostly representative of depositional time. Based on 14C and OSL ages, the lowermost fluvial sediments showed relatively low sediment accumulation rates (0.1–0.8 m/kyr−1) prior to 12 ka. A hiatal discontinuity ranging from 12 to a maximum of 50 ka resulted in a break in sedimentation between the Last Glacial Maximum and the Holocene. A thick fluvio-estuarine unit in the lower part indicated relatively high accumulation rates (3.3–9.4 m/kyr−1) from 12 to 9 ka, which corresponded to rapid sea level rise. In the middle part, decreased accumulation rates (1.7–5.5 m/kyr−1) from approximately 9 to 6 ka were interpreted as transgressive deepening. Lower accumulation rates (0.9–3.2 m/kyr−1) during the middle to late Holocene were caused by relative sea level fall and a horizontally extended sedimentation area. In the upper part, the highest sediment accumulation rates (3.5–8.3 m/kyr−1) and rapid delta progradation during the last 2.0 ka might have been caused by widespread human influence and monsoonal climatic change. Isochrons of 1-kyr scale were identified based on the lithologic boundary and age results for each borehole, which represent palaeoshoreline positions during the transgressive cycle and boundaries of considerable delta progradation.

Holocene sedimentary evolution of a subaqueous delta off a typical tropical river, Hainan Island, South China

High-resolution seismic surveys were conducted off the eastern coast of Hainan Island to study the distribution and mass budget of Holocene subaqueous clinoform sediments. The Holocene subaqueous clinoform extends 45 km seaward across the shelf and more than 80 km northeastward along the shore. It covers an area of over 3,000 km2, which is almost equivalent to the expansion of large river systems. Overlying the transgressive surface, the deltaic sequence can be divided into a transgressive unit and a highstand unit separated by a downlap maximum flooding surface. The volume of sediments within the Holocene subaqueous clinoform is estimated to be 33.12×109 m3, equivalent to an annual accumulation of 3.61–3.31 Mt/yr, which is substantially higher than the global average. The high sediment yield presumably resulted from strong tropical weathering and active tectonics. The sediment flux during the transgression and sea-level highstand were 3.85–3.08 and 3.47 Mt/yr, respectively, suggesting no substantial changes in sedimentation. Compared to river systems originating from high mountain glaciers, the sedimentation variation of small tropical rivers throughout the Holocene is relatively minor. However, the sediment discharge of the Wanquan River has drastically decreased in recent decades because of increased human activity.

High resolution architecture of neotectonic fault zones and post-8-Ma deformations in western Hungary: Observations and neotectonic characteristics of the fault zone at the Eastern Lake Balaton

Lake Balaton (Hungary), the largest lake in Central Europe, formed by the interplay of tectonic and external forces. Its shallow water and young soft sediments together allow to carry out ultra-high resolution reflection seismic surveys to investigate shallow tectonic structures and near surface stratigraphy at depth of ca. 0–30 m. To document neotectonics of the eastern lake basin and its onshore surroundings to the East, we have integrated new ultra-high-resolution seismic-reflection data with deeper penetrating multichannel lake and land seismic profiles, regional geological, geophysical and seismicity data, and geomorphological observations. Combined use of these different data sets provided an opportunity to understand better, how these different types and scales of structural features are linked. In our study area, late middle to late Miocene formations compose a deepening than shallowing sedimentary cycle from terrestrial clastic through offshore marl and deltaic sequence filling up the basin between ca. 8.6 to 7.5 Ma. The deltaic sequence is unconformably overlain by erosional remnants of late Pleistocene fluvial deposits and a mantle of latest Pleistocene to Holocene lake mud. Post-early Miocene deformation history involved two phases; a latest middle to early late Miocene transtension and a dominantly strike-slip regime with locally transpressional or transtensional character. The latter neotectonic phase reactivated the earlier faults and resulted in the propagation of 4 major fault zones across the complete late Miocene sequence. The resulting young faults show segmented geometry, stepovers, and connecting splays. The deformation also induced the modest but penetrative folding of the highest preserved Miocene deltaic sequence. The change in deformation style could happen during the late-stage of delta formation, at ca. 8 Ma although a slightly younger timing is not excluded. Faults imaged offshore apparently do not offset the Holocene lacustrine mud by discrete fractures, but the improved distribution map of recent seismicity and morphotectonic indices along their onshore continuations suggests that several segments of the fault pattern are still active, and might be capable of generating earthquakes. Integration of these different data provided an opportunity to understand better, how these different types and scales of structural features are linked and evolved one after another. • 4 fault zones and 4 types of folds were mapped below Lake Balaton. • Post-8-Ma neotectonic faults are the reactivation of earlier Miocene structures. • The mapped fault pattern serves as an example for the complexity of segmented fault zone. • Faults below the lake continue onland form a coherent structural pattern. • Recent fault activity is suggested by the new seismicity map.

Multiphase deformation history of the Porcupine Basin, offshore west Ireland

AbstractThe Porcupine Basin is a large underexplored sedimentary basin located offshore west of Ireland within the structurally complex European North Atlantic Margin. The basin has evolved through multiple Jurassic–Recent phases of deformation and although the overall plate tectonic context of the margin is well‐documented, there are still uncertainties regarding the phases of tectonic activity, and their associated strain distribution and fault kinematics. Based on the analysis of large volumes of 2D and 3D seismic data from the Porcupine Basin, we provide an overview of the nature and origin of multiple fault systems, both tectonic and nontectonic, with links to regional tectonic events where possible. Three distinct basin‐wide phases of tectonically induced extensional faulting are recognized: (a) Late Jurassic N‐S to NE‐SW trending rift faults, (b) Late Cretaceous E‐W to ENE‐WSW trending normal faults and (c) Mid Eocene N‐S trending faults. The Jurassic faults were active over a period of 11.5 Myr between the Oxfordian and Kimmeridgian but with an intervening 4.5 Myr period of quiescence providing a two‐stage rift evolution. The Late Cretaceous faulting in the Porcupine Basin broadly correlates with extension in the Celtic Sea basins and is tentatively attributed to the rotational spreading of the Bay of Biscay in the south. The Mid Eocene phase of extension, which coincides with the onset of spreading between Europe and Greenland (Atlantic spreading), resulted in partial reactivation of the Jurassic faults. A series of non‐tectonic fault systems occur within specific stratigraphic intervals suggesting a compactional or gravitationally driven origin, including basinward dipping normal faults within a Palaeocene–early Eocene deltaic sequence, and multiple tiers of polygonal faults within Cretaceous, late Eocene and Neogene strata.

Salt‐induced crestal faults control the formation of Quaternary tunnel valleys in the southern North Sea

Tunnel valleys are major features of glaciated margins and they enable meltwater expulsion from underneath a thick ice cover. Their formation is related to the erosion of subglacial sediments by overpressured meltwater and direct glacial erosion. Yet, the impact of pre‐existing structures on their formation and morphology remains poorly known. High‐quality 3D seismic data allowed the mapping of a large tunnel valley that eroded underlying preglacial delta deposits in the southern North Sea. The valley follows the N–S strike of crestal faults related to a Zechstein salt wall. A change in downstream tunnel valley orientation towards the SE accompanies a change in the strike direction of salt‐induced faults. Fault offsets indicate important activity of crestal faults during the deposition of preglacial deltaic sediments. We propose that crestal faults facilitated tunnel valley erosion by acting as high‐permeability pathways and allowing subglacial meltwater to reach low‐permeability sediments in the underlying Neogene deltaic sequences, ultimately resulting in meltwater overpressure build‐up and tunnel valley excavation. Active faults probably also weakened the near‐surface sediment to allow a more efficient erosion of the glacial substrate. This control of substrate structures on tunnel valley morphology is considered as a primary factor in subglacial drainage pattern development in the study area.

Fracturing and Near-Surface Diagenesis of a Silicified Miocene Deltaic Sequence: The Montjuïc Hill (Barcelona)

Near-surface diagenesis has been studied in the Langhian siliciclastic rocks of the Montjuïc Hill (Barcelona Plain) by means of petrographical (optical and cathodoluminescence) and geochemical (electron microprobe, δ18O, δ13C, δ34S and 87Sr/86Sr) analyses. In the hill, these rocks are affected by strong silicification, but the same unit remains non-silicified at depth. The results reveal that fracturing took place after lithification and during uplift. Fracture cementation is clearly controlled by the previous diagenesis of the host rock. In non-silicified areas, cementation is dominated by calcite, which precipitated from meteoric waters. In silicified areas, fractures show multiepisodic cementation produced firstly by barite and secondly by silica, following the sequence opal, lussatite, chalcedony, and quartz. Barite precipitated only in fractures from the mixing of upflowing seawater and percolating meteoric fluids. The presence of silica stalactites, illuviation, and geopetal structures, and δ18O values indicate that silica precipitation occurred in the vadose regime from low-temperature percolating meteoric fluids, probably during a glacial period. Moreover, the presence of alunite suggests that silica cement formed under acidic conditions. Karst features (vugs and caverns), formed by arenisation, reveal that silica was derived from the dissolution of surrounding silicified host rocks.

LITHOSTRATIGRAPHY AND PALAEONTOLOGY OF THE ‘BLACK SHALE’ FACIES OF THE SEMANGGOL FORMATION (CHERT UNIT) AT BUKIT MERAH (PERAK, PENINSULAR MALAYSIA)

In the Semanggol formation cropping out in Northern Perak, a ‘black shale’ facies was identified at the lowermost level of the chert unit at Bukit Merah, containing trace and body fossils. Despite some recent contributions made in Northern Perak outcrops, some questions about the origin of the ‘black shale’ facies remain open, particularly its depositional palaeoenvironment, palaeo-oxygenation history and biochronological age. Moreover, the presence of trace and body fossils in the facies contradicts ‘classical’ views of the ‘black shales’ of the Semanggol formation. Therefore, this study aims to describe the lithostratigraphic and palaeontological contents of the facies, as well as to apply sedimentological, palaeoichnological, petrographical and geochemical analyses for ascertaining the origin of it. The results revealed a shallow marine, sublittoral setting based on the identification of the Cruziana ichnofacies. The coarsening- and the thickening-upward trend observed in the studied stratigraphic section is compatible with a prograding, distal deltaic sequence. Two sources of additional oxygen are identifying as the episodic storm events andthe presence of microbial mats, which frequently replenished the oxygen levels at the seafloor. Hence, the deposition occurred under relatively well-oxygenated conditions. Finally, the age of the succession is Early Triassic based on the presence of the bivalve genus Claraia.&#x0D; Keywords: Cruziana ichnofacies, palaeo-oxygenation, body fossils, sedimentological, petrographical, microbialmats

E&amp;P Notes (October 2019)

E&amp;P Notes (October 2019)

Age and context of mid-Pliocene hominin cranium from Woranso-Mille, Ethiopia.

A fossil hominin cranium was discovered in mid-Pliocene deltaic strata in the Godaya Valley of the northwestern Woranso-Mille study area in Ethiopia. Here we show that analyses of chemically correlated volcanic layers and the palaeomagnetic stratigraphy, combined with Bayesian modelling of dated tuffs, yield an age range of 3.804±0.013 to 3.777±0.014million years old (mean±1σ) for the deltaic strata and the fossils that they contain. We also document deposits of a perennial lake beneath the deltaic sequence. Mammalian fossils associated with the cranium represent taxa that were widespread at the time and data from botanical remains indicate that the vegetation in the lake and delta catchment was predominantly dry shrubland with varying proportions of grassland, wetland and riparian forest. In addition, we report high rates of sediment accumulation and depositional features that are typical of a steep topographic relief and differ from younger Woranso-Mille fossil localities, reflecting the influence of active rift processes on the palaeolandscape.

Facies Analysis and Depositoinal Environment of D-3 Reservoir Sands Vin Field, Eastern Niger Delta

Facies analysis and depositional environment identification of the Vin field was evaluated through the integration and comparison of results from wireline logs, core analysis, seismic data, ditch cutting samples and petrophysical parameters. Well log suites from 22 wells comprising gamma ray, resistivity, neutron, density, seismic data, and ditch cutting samples were obtained and analyzed. Prediction of depositional environment was made through the usage of wireline log shapes of facies combined with result from cores and ditch cuttings sample description. The aims of this study were to identify the facies and depositional environments of the D-3 reservoir sand in the Vin field. Two sets of correlations were made on the E-W trend to validate the reservoir top and base while the isopach map was used to establish the reservoir continuity. Facies analysis was carried out to identify the various depositional environments. The result showed that the reservoir is an elongate , four way dip closed roll over anticline associated with an E-W trending growth fault and contains two structural high separated by a saddle. The offshore bar unit is an elongate sand body with length: width ratio of &gt;3:1 and is aligned parallel to the coast-line. Analysis of the gamma ray logs indicated that four log facies were recognized in all the wells used for the study. These include: Funnel-shaped (coarsening upward sequences), bell-shaped or fining upward sequences, the bow shape and irregular shape. Based on these categories of facies, the depositional environments were interpreted as deltaic distributaries, regressive barrier bars, reworked offshore bars and shallow marine. Analysis of the wireline logs and their core/ditch cuttings description has led to the conclusion that the reservoir sandstones of the Agbada Formation in the Vin field of the eastern Niger Delta is predominantly marine deltaic sequence, strongly influenced by clastic output from the Niger Delta. Deposition occurred in a variety of littoral and neritic environment ranging from barrier sand complex to fully marine outer shelf mudstones.