Deltaic Deposits Research Articles

Gravel roundness quantitative analysis for sedimentary microfacies of fan delta deposition, Baikouquan Formation, Mahu Depression, Northwestern China

AbstractGravel roundness is one of the simple but crucial parameters in particle shape, which is related to the transportation and deposition of sediments. Based on the digital images of underground drilling cores, this study attempted to characterize and distinguish sedimentary microfacies by quantitatively test gravel roundness trends in the fan delta deposition of the Early Triassic Baikouquan Formation in Mahu Depression, Juggar Basin of China. By the new proposed de-flat roundness measurement, two major parameters are recorded to manifest the rounding degree, which is de-flat roundness value (Rdn) and the corresponding variance value. In the case study, roundness characteristics of four microfacies in the fan delta deposits are displayed and compared. Results show that braided channels and submerged distributary channels are characterized by better rounding (Rdn0.379 to 0.603, and 0.366 to 0.591, respectively) and smaller variance, while debris flow and submerged debris flow are characterized by worse rounding (Rdn0.333 to 0.405 and 0.256 to 0.391, respectively) and larger variance. In vertical of the roundness curves, channels deposits and gravity flow deposits reveal notable distinction in terms of the numeric ranges, fluctuation amplitude, and fluctuate frequency. It is interpreted to represent the different sedimentary processes during particle transport and deposit, as well as new clastics mixed in from other sources. Therefore, roundness could be a useful complementary data to provide quantitative sedimentological evidence and theoretical support for the study of coarse clastic depositional system.

Boulder Fall Activity in the Jezero Crater, Mars

AbstractJezero crater is the landing site for the Mars 2020 Perseverance rover. We report 63 boulder fall tracks within the crater using High‐Resolution Imaging Science Experiment (HiRISE) images. The boulder tracks have both fresh and faded morphologies similar to those reported elsewhere on Mars, but reported for the first time in Jezero crater. We combine observations from 16 boulder‐tracks on the western delta deposit with 47 in the surrounding regions to infer possible process(es) of boulder destabilization, which can be tested with rover observations. This newly found hazard should be taken into account for rover operations. Boulders associated with tracks are geologically “recent falls,” so it is possible that the surfaces of these boulders may provide an opportunity to sample material less exposed to radiation than other rocks at the martian surface and could be ideal targets to analyze for organics.

Brachiopods from the Palaeogene clastic deposits of north-western Italy

Four brachiopod species, i. e. Erymnaria bolcensis ( Massalongo , 1850), Gryphus cf. minor (Philippi, 1836), Terebratulina tenuistriata (Leymerie, 1846), and Megathiris detruncata (Gmelin, 1791) have been identified in the Oligocene deposits of NW Italy. The species G. cf. minor and T. tenuistriata dominate in the studied material. This is the first record of M. detruncata from the Oligocene of Italy, although it has been noted both from the Eocene and Miocene of Italy. The stratigraphic ranges of E. bolcensis and T. tenuistriata have been extended. The studied assemblage differs considerably from the majority of the Palaeogene and Neogene assemblages of Europe dominated by micromorphic megathyridids. Geological context and sedimentary features point to mass-flow deposits of fan deltas, indicating that the assemblage has been transported within the sediment down a slope.

Relevant data-based approach upon reliable safety factor for pile axial capacity

Pile foundations are one of the significant marine substructure systems for a variety of offshore and onshore facilities. Thus, optimum and reliable design during their lifetime and under inherent uncertainties is noteworthy. Among various methods for predicting axial pile bearing capacity, cone penetration test (CPT)-based methods enhance geotechnical design to a more satisfying level due to continuous and reliable CPT records and their suitable performance in offshore environments. This article aims to reveal the importance of CPT and pile databases, especially in deltaic deposits. A new relevant CPT and pile data-based approach is introduced by assessing a few elaborated works on typical CPT and pile case studies. The proposed procedure assists in comparing the characteristics and conditions of a given pile with similar database cases through screening and, consequently, deciding on the design methods' priority. It is depicted that superior methods result in a more reliable design and higher resistance factors and efficiency ratios in load and resistance factor design (LRFD). Accordingly, it is engaged with lowered safety factors in the allowable stress design (ASD) approach. Eventually, upon various assessment criteria, the optimum site-specific methods are proposed leading to a performance-based design that can be realized in geotechnical practice.

Study of Heavy Minerals from the Vjosa and Mati river delta sediments in Albania

This research is focused on the determination of the heavy minerals (HM) load of the Vjosa and Mati river delta deposits along the Albanian coastline and it is based on X-ray Diffractometry. The Albanian coastline consists of sandy beaches at the north (Adriatic coastline) and rocky escarpments at the south (Ionian coastline). Several layers of heavy mineral deposits, up to 50 cm thick, with heavy mineral fraction up to 95% and 88% of total sample for Vjosa and Mati sediments respectively, are identified. The layers enriched in heavy minerals from Vjosa delta deposits are almost black in colour, while at Mati delta these layers are dark green coloured. Separation of the heavy from the light fraction was performed, in order to compare the different fractions between the two studied delta areas. The accumulation of HM occurs mainly in the fraction of 125-250 µm in the sediments of both deltas. The HM dominate in the magnetic field of 0.4-0.8 A/m. XRD analysis results show a great variety of minerals present in the delta samples which can be attributed to the wide variety of geological zones and lithologies that are intersected by the Vjosa and Mati rivers, respectively. In the 63-425 μm fraction rich in heavy minerals of both delta systems considerable amounts of magnetite (up to 39.4% in Vjosa samples), chromite (up to 20.2% in Vjosa samples), garnet (up to 13.6% in Vjosa samples), ilmenite (up to 8.3% in Mati samples), rutile (up to 4.7% in Mati samples), hematite (up to 2.2% in Mati samples), and zircon (up to 2.1% in Vjosa samples) are observed. Rock forming minerals such as pyroxene, amphibole, and epidote compose significant percentages of this fraction. In addition, the presence of gold grains in Vjosa delta sediments is remarkable. Both catchment areas consist to a great extent of similar formations such as the Mirdita Ophiolite Zone and the Pindos Ophiolite complex, providing thus a similar HM fingerprint at both delta areas. Minerals that occur in higher abundances reflect the extensive presence in the drained areas of related parent rocks which are rich in these minerals and which are often more vulnerable to weathering. The samples of Vjosa river delta show high percentage of carbonate constituents, which is related to the presence of carbonate rocks of the Ionian and Kruja tectonic zones within which the hydrographic network of the Vjosa River has been developed. The samples of Mati river delta show lower abundance of carbonate minerals, reflecting the limited presence of carbonate rocks at the Kruja Zone, which occur in the catchment area of the river near its mouth.

Paleoenvironmental interpretation and palynoflora of Devonian–Carboniferous subsurface sections from the eastern part of the Moesian Platform (Romania)

Palynomorph assemblages and palynofacies analyses have been performed on several core samples from the Devonian–Carboniferous deposits identified in five wells located within the eastern region of the Moesian Platform. The investigated sections include, in ascending stratigraphic order, the Ţăndărei, Smirna, Călăraşi and Vlaşin formations. Based on the stratigraphic distribution of key taxa identified (miospores, chitinozoans, acritarchs), seven biozone intervals (four for the Devonian and three for the Carboniferous) have been recognized. The oldest samples were dated as being part of the micrornatus–newportense (MN) – lower part of breconensis–zavallatus (BZ) Assemblage Zones (Early Devonian), while the younger ones are assigned to the kosankei–varioreticulatus (KV) – nobilis–junior (NJ) Assemblage Zones (Pennsylvanian). Palynofacies observations suggest a fairly distal depositional environment during the period between the Lochkovian and Pragian times, followed by some proximal/fluvio-deltaic conditions in Emsian–Early Eifelian (top of the Ţăndărei Formation). The upper Tournaisian to Serpukhovian sedimentary rocks of the Călăraşi Formation and lower part of Vlaşin Formation were deposited in inner neritic environments. Mud-dominated dysoxic/anoxic conditions of the Bashkirian, abruptly transitioned to deltaic deposition and oxidizing environments that persisted to the Moscovian.The Early Devonian terrestrial palynoflora is dominated by trilete spores which belong to lowland vegetation (vascular plants), the latter preferring areas with open connections via rivers to marine sedimentary basins. Carboniferous samples yielded only terrestrial miospores of various types of arborescent and herbaceous lycopsids and ferns, suggesting different habitats such as non-flooded wetlands or swamps within coastal plain and continental interiors. These assemblages of Carboniferous miospores are an indication of neutral-humid climatic conditions which existed at the time of deposition.

Sediment transport and riverbed evolution of sinking streams in a dammed karst river

Little knowledge has been gained about the mechanics of the sediment transport and riverbed evolution of sinking streams in a karst river, because of inadequate data of field investigation and laboratory experiments. Numerical simulation of mixed flow and sediment transport on moveable riverbed in sinking streams is seldom reported, due to difficulties in modeling irregular closed cross-sections (CS). In this study, a unified hydrodynamic and sediment transport model is developed for the free-surface and the pressurized mixed reaches of a karst river with sinking streams, where a technique of symmetric shadow subgrid is proposed to describe and account for an irregular closed CS and its riverbed evolution. Mechanics of riverbed evolution of sinking streams in a dammed karst river are studied using a real karst-river reservoir (Jiayan reservoir), which is still conducted in a data-scarce environment. Under damming effects, a delta deposition of sediment develops at the end of reservoir backwater and steps forward. It is found that the deposition delta is able to go through the sinking streams while a narrow passage is finally reserved in sinking streams and keeps conveying flow. The riverbed elevation of a closed CS, mainly determined by the lower one of CS ceiling and reservoir water level, is dynamically stable. In a dammed karst river, a fully developed (shrunk) sinking stream has periodic morphological dynamics, one cycle of which can be described as a successive process of “siltation → reduction of conveyance area → flow lag upstream of sinking streams → large hydraulic gradient → increase of velocity → large sediment transport capacity → erosion of closed CS → increase of conveyance area”. Sensitivity studies of the parameters of the hydrodynamic and the sediment transport models are performed, whose results indicate that the periodic morphological dynamics of a fully developed sinking stream in a dammed karst river is not changed. The sinking stream in a dammed karst river will not be silted completely, when the condition for forming a large gradient of piezometric head along a sinking stream is not broken by special geomorphologies or anthropogenic influences. This fact is supported by both the numerical simulations and our field investigation of sinking streams in an existing karst-river reservoir.

Experimental Study at the Reservoir Head of Run-of-River Hydropower Plants in Gravel Bed Rivers. Part II: Effects of Reservoir Flushing on Delta Degradation

Run-of-river hydropower plants (RoR HPPs) are capable of interrupting the sediment connectivity of many alpine rivers. Still, there is a lack of systematical investigations of possible sediment management strategies for small and medium sized RoR HPPs. This study deals with the headwater section of an impoundment and the approach of sediment remobilization during drawdown operations. Therefore, a typical medium sized gravel bed river having a width of 20 m, a mean bed slope of 0.005, a mean flow rate of 22 m3/s, and a 1-year flood flow of 104 m3/s is recreated by a 1:20 scaled physical model. Heterogenous sediment mixtures were used under mobile-bed conditions, representing a range of 14–120 mm in nature. During the experiments, the flow rate was set to be 70% of the 1-year flood (HQ1) regarding on the ability to mobilize all sediment fractions. The possibility to remobilize delta depositions by (partial) drawdown flushing within a reasonable period (≈9 h in 1:1 scale) was shown by the results. The erosion of existing headwater delta deposition was found to be retrogressive and twice as fast as the preceding delta formation process. A spatiotemporal erosion scheme points out these findings. This supports the strategy of a reservoir drawdown at flood events of high reoccurrence rate.

Sedimentary architecture and provenance analysis of a sublacustrine fan system in a half-graben rift depression of the South China Sea

Research on the sedimentary architecture and provenance of sublacustrine fans is crucial not only to better understand the gravity flow evolution in lacustrine basins but to successfully locate hydrocarbon reservoirs. This study targets at a sublacustrine fan with a large petroleum reserve that has been revealed by drilling within an Eocene lacustrine succession in the Fushan Depression, northern South China Sea. The sedimentary architecture of the sublacustrine fan was investigated through the integration of core, well logs and 3D seismic data, and its provenance was identified by detrital zircon UPb dating. The examination of 117 m long continuous drilling core reveals four major lithofacies in the sublacustrine fan deposits, which are interpreted as deposits of two types of gravity flows (cohesive debris flows and turbidity currents). The cohesive debris flows developed in the lower section are mainly composed of massive gravelly mudstones with various floating clasts and high clay content, whereas the turbidity currents in the upper section are characterized by parallel to ripple bedding and normal graded bedding. These observations reflect a transformation from cohesive debrites to turbidites from deep to shallow water environments. In addition, provenance analysis of 648 detrital zircon UPb ages yield a well-defined relationship between potential provenance areas and sublacustrine fan deposits in the deep lake. The UPb age pattern of the fan is similar to that of northern fan delta deposits, but different from sediments from the western and southern deltas, suggesting that the sublacustrine fan was sourced from the north. Therefore, a deep-lacustrine depositional model is proposed for the sublacustrine fan, showing a gravity-driven evolution process from fan delta front in the northern slope into cohesive debrites and then turbidites in the lake center. This was mainly controlled by fault-related topography and sediment supply. This study highlights the effective use of the detrital zircon UPb dating method to identify the provenance direction of sublacustrine fans, and the proposed depositional model may be applicable to reservoir prediction in other lacustrine basins.

Shelf-edge delta and reef development on a mixed siliciclastic–carbonate margin, central Great Barrier Reef

ABSTRACT The Great Barrier Reef (GBR) is the world's largest extant mixed silicilastic–carbonate margin. Previous research on the Great Barrier Reef has suggested that the extensive barrier reef system may act as an impermeable barrier and limit the development of delta systems during lowstands, but sufficient geophysical data to support this hypothesis are lacking. We use dense sparker seismic and sub-bottom profiler data to better understand the structure of a large lobe-shaped feature (∼ 10 km × 10 km) on the shelf edge of the central GBR and the interactions between siliciclastic and carbonate sedimentary systems. Interpreted sparker seismic contains prograding clinoforms and suggest that the lobe-shaped feature was a river-dominated shelf-edge delta. A delta on the shelf edge implies that the presence of an exposed barrier reef was not a major impediment to deposition and that other adjacent lobe-shaped features are also deltaic deposits. The shelf-edge deltas were deposited onto a broad upper-slope terrace that allowed continued progradation and limited incision when sea level fell below the shelf edge. Delta foresets are commonly colonized by coral reefs, but the spatial and temporal relationship between reefs and some deltaic units remains unclear. The presence of multiple shelf-edge deltas that link to previously mapped Burdekin River paleo-channels indicates a complex history of sedimentation, with the Burdekin River delta migrating up to 100 km along the GBR margin during the late Quaternary. Regional bathymetric data suggest that large modern or recent shelf-edge deltas are rare on the GBR and that there was a broad range of sedimentary processes operating along the margin of the GBR during periods of low sea level.

Unraveling the timing of the Caviahue depression, Andean Southern Volcanic Zone: insights from the sedimentary infill

The Caviahue–Copahue volcanic complex in the Southern Volcanic Zone of the Andes is composed of the Pliocene–Pleistocene Caviahue depression and the active Copahue volcano. This volcanic complex is located in a zone of profuse volcanic and tectonic activity, and thus, the origin of the depression includes both tectonic and volcanic hypothesis (e.g., formation of a collapse caldera and a pull-apart basin). According to current knowledge, the oldest rocks filling the depression are lavas and ignimbrites of the Pleistocene Las Mellizas Formation. However, clastic sedimentary deposits underlying this unit have been recognized. Here, we document the characteristics of these deposits at the south shore of the Caviahue lake using field data and digital outcrop models (DOM). We have concluded that the deposits are deltaic and alluvial in origin and made up by a prodelta/distal deltaic front, delta front deposits, delta plain sediments and alluvial deposits. An unconformity separates the deltaic deposits from overlying alluvial deposits, indicating the decrease in the stratigraphic base level probably associated with a sudden coarse-grained clastic input and a drop in the lake level. Our study has implications on the timing and evolution of the Caviahue depression and indicates that the depression precedes the Las Mellizas Formation. Thus, Las Mellizas ignimbrite should not be interpreted as the main unit related to the Caviahue caldera as it has been proposed by some authors. If the depression had indeed been formed by a collapse caldera, this event would be previous to the described sediments.

Multi-attribute Seismic application for Modeling Static Reservoir In “Athran” Field South Sumatera Basin

“Athran” field is an active field for hydrocarbon exploration with trending northeast-southwest and focus on the reservoir zone is the Talang Akar Formation with A-C layer intervals. The method used in this research is electrofacies, delta deposition system, formation evaluation, static reservoir modeling, and multi-attribute linear regression. The distribution of reservoir modeling is done by using the stochastic variogram method to make modeling lithofacies and depositional facies. Identification of lithofacies in the study area obtained two units, namely sandstone and shale facies. The depositional facies analysis was carried out using the electrofacies approach, showing that the research target was in a transitional environment with a delta deposition system, namely the distributary channel, delta front and prodelta. The deposition process. Sandstones and shales affect the distribution of porosity which functions as a reservoir property. The data integration of the distribution of lithofacies, gamma ray and porosity was carried out using the multi-attribute linear regression method to predict data both vertically and horizontally. The results of this study are used to help determine the distribution direction and direction of sediment deposition as a good reservoir zone for exploration activities.

The role of fault reactivation in the development of tropical montane lakes

AbstractThis work details the role of fault reactivation in the development of tropical montane lakes by using basin morpho‐structural analysis and seismostratigraphic studies. The upland lakes are severely faulted sinkholes, whose faults penetrate the Quaternary sedimentary units. Four main stages are related to the lake formation: (i) an Early Proterozoic tectonic deformation of the rocks along the southern border of the Carajás Structure, where the lake is placed; (ii) differential erosion by – and building of – the formation of the South Carajás Hill; (iii) Fe‐rich crust formation by weathering and gravitational collapse faults following the E–W plateau border and the start of Violão Lake formation during the Pliocene–Pleistocene; and (iv) episodic fault‐fracture reactivation by gravitational collapse causing pulses of subsidence in the lake and outlining its faulted borders. Dissolution of the lateritic crust and erosion by runoff drainage under wet climate conditions were coeval with fault activities, which allowed the deposition of relatively thick clastic deposits organized in three main seismostratigraphic units associated with major lake‐level fluctuations. Initial fault reactivation under low‐level water started lacustrine basin development with deposition of prograding fan deltas related to the main drainage. A second fault reactivation by gravitational collapse increased the lake accommodation space and resulted in the deposition of fine‐grained sediments from dilute interflows or overflows until 36 000 cal year BP. At about 31 000 cal year BP, rapid decreases in the lake water level under redox conditions at the sediment/water interface allowed widespread siderite formation. A third gravitational collapse episode was responsible for the increase in the lake area and depth and the returning of clastic/organic deposition up to the present. This tropical montane lake can be seen as a representative example for understanding the formation of other upland lakes controlled by fault reactivation. © 2020 John Wiley & Sons, Ltd.

Integrated Geoenvironmental and Geotechnical Risk Assessment of East Port Said Region, Egypt for Regional Development

The present paper discusses the use of integrated topographical, hydrogeological, and geotechnical data as a tool to assess the suitability of the East Port Said Region (EPR) for the establishment of new major projects. The method comprises geo-environmental and geotechnical characterization of the surficial soil layer and subsurface rock layers and identification of soil behavior along the proposed area. We analyze the digital elevation model to identify the topographic patterns across the project area. The geology of EPR comprises a normal to slightly consolidated fluvio-marine deltaic deposits of Pleistocene to Holocene age with more than 1000 m thick. We carry out a statistical analysis of geotechnical parameters to group the risky effective parameters for regional development. The study correlated the depth-averaged values of the design geotechnical parameters over the entire EPR. It also integrates the geotechnical properties with the topographical and hydrogeological parameters in the Geographic Information System (GIS) platform aiming to identify the spatial distribution and classes of the integrated risks across the EPR. For each geotechnical–environmental domain, it links the averaged values of geotechnical parameters to the groundwater levels and the topographic gradient that control groundwater flow. The methodological approach enters the hydrogeological model of EPR to the GIS environment and crossing with the results of the geotechnical analysis to identify zones of higher water saturation and possible geotechnical risk. The developed integrated geoenvironmental and geotechnical model assesses the prevailed hazards among the entire EPR. The integrated model uses to identify and delineate geotechnical hazards zones and to shape the spatial distribution of the geotechnical hazards classes, based on the dominant geotechnical and geo-environmental specifications. The achieved model shows that the risky zone spread widely along with the middle and southwestern sectors, while the marginal zones spread along with the southeastern and northwestern sectors. The study recommends applying the prefabricated vertical drains or deep mixing technique to stabilize the soft soil in the middle and northern risky zones of EPR. It also recommends the use of gravel drains and a well-compacted sub-base gravel layer for shallow foundations to enhance the properties of unstable clay formation at the southern risk and marginal zones of EPR.

Investigating the Potential Impact of Louisiana Coastal Restoration on the Trace Metal Geochemistry of Constructed Marshlands

Coastal restoration through diversion of suspended sediments from the Lower Mississippi River (LMR) into hydrologically isolated marshlands of Mid-Barataria Bay and Mid-Breton Sounds in southern Louisiana has the potential to mobilize lead (Pb), and other trace elements. We investigate the potential impact(s) of the diversion on marsh porewater through analysis of modern riverbank and suspended sediments, compared to sediments from pre-industrial deltaic deposits of LMR. Sequential extraction methods were used to evaluate Pb, cobalt (Co), copper (Cu), nickel (Ni), and zinc (Zn) in the sediments. Our results show that metal contents are higher (e.g., 8- to 10-fold for Pb) in the modern sediments relative to pre-industrial deposits. Also, the reducible fraction, presumably iron/manganese (Fe/Mn) oxides/oxyhydroxides, is the chief reservoir of environmentally available metals. The substantially higher trace metal contents of the modern relative to pre-industrial sediments suggest that the modern sediments contain a sizeable amount of anthropogenic contributions. Furthermore, the concentration of the trace metals in the reducible fraction suggests bioavailability to marsh organisms upon reductive dissolution within the planned, constructed coastal marshes. Still, additional sediment samples from the marshlands during the diversion implementation phase will be necessary to support the preliminary findings in this contribution as it affects coastal marshes and vital local fisheries.

Lake level oscillations recorded in Buracica Stage (Barremian, Early Cretaceous) of Recôncavo Basin, Bahia State (N.E. Brazil)

This work studies the Buracica Stage (Barremian, Early Cretaceous) in the Reconcavo Basin, Bahia State (N.E. Brazil), comprising the Sao Sebastiao and Pojuca formations, from the analysis of 143 well logs and outcrop records. The Reconcavo Basin is a rift basin associated with the beginning of the South Atlantic Ocean. The basin started with very slow subsidence rates, leading to the formation of shallow lakes and the deposition of shales and sandstones, which compose the Brotas Group; later, subsidence accelerated and gave rise to the formation of a deep lake. Then the deep lake basin was filled throughout six depositional episodes making up the so-called Reconcavo tectonic sequence. Previous studies divided the last depositional event of Reconcavo Basin into seven fourth-order sequences concerning the Marker Bed “1” to the Marker Bed “F” stratigraphic interval. Sequence boundaries were marked by sharp flooding surfaces related to the transgression of lacustrine system due to rapid tectonic subsidence and generation of accommodation. In turn, the sequences are divided into three systems tracts: the lake lowstand systems tract, the transgressive systems tract and the lake highstand systems tract. The maximum flood surface of the sequences generally coincides with the stratigraphic marker “B” to “F”. Depositional systems like braided-fluvial plus eolian dunes, meandering-fluvial, deltaic, or moving-lake and near-shore deposits were identified in this work. Isopach, isolith and sand percentage maps indicate depocentre and source area distribution like the previous Aratu Stage. The primary sources of the sediments are located in the west and north areas; only from the third sequence, an eastern source could be described conspicuously. The fourth-order cyclicity is controlled by lake level changes, related to climatic factors (Milankovitch cycles). The results from the studies of cyclicity confirm the proposed stratigraphic model. They indicate 43,000 years for each fourth-order sequence and 150,000 years for the third-order sequences, which provide an accumulation rate of 1200 mm per 1000 years for the studied interval.

A multi-proxy approach for reconstructing environmental dynamics since the mid Holocene in Lake Ismarida (Thrace, N. Greece)

The paleoenvironmental evolution of Lake Ismarida in Thrace (Northern Greece) is revealed by the combined lithological, micropaleontological (benthic foraminifera, pollen and NPPS), molluscan analyses, magnetic susceptibility measurement and radiocarbon dating of a 5.8-m long sediment core. The mid Holocene evolution of the lake area is evidenced by the documentation of four sedimentary Units in the core ISMR-2, corresponding to four distinct evolutionary stages: (1) during ∼5500-3500 cal yr BP the lake area was a shallow marine environment characterized by an Ammonia beccarii, small rotaliids, miliolids, Bittium reticulatum and Veneridae spp. assemblage, marine dinoflagellate cysts, and low magnetic susceptibility values; (2) during ∼3500-3000 cal BP the environment is gradually tending to more isolated conditions forming an open lagoon, characterized by marine and euryhaline fauna and low magnetic susceptibility values; (3) during 3000 cal yr BP, the open lagoon presented a transition to an oligohaline inner lagoon, characterized by an Ammonia tepida, Haynesina germanica, Aubignyna perlucida, Pirenella conica, Cerastoderma glaucum and Abra spp. assemblage, sedges and aquatic vegetation. This restricted, entirely isolated from the sea inner lagoon could be definitely used as the landmark of the Lake Ismaris from Heorodotus, while describing the march of Xerxes through Thrace in 480 B.C.; (4) since ∼2000 cal yr BP to the present, the Lake Ismarida is formed, characterized by fresh-water indicators and aquatic pollen, Pseudoschizaea and high magnetic susceptibility values. Finally, the progradation of the Filiouris River deltaic deposits resulted to a 4 km wide deltaic plain between Lake Ismarida and the nowadays coastline. Pollen assemblages record the dominance of a rather rich deciduous forest in the area with traces of human presence in the lower part of the sequence, whereas the opening of the plant landscape under the increasing human pressure is evidenced after ∼ 3000 cal yr BP. Finally, an open vegetation pattern, contemporaneous with the retreat of forest vegetation, is evidenced in the area already before 2000 cal yr BP.

Sequence stratigraphy in post‐rift river‐dominated lacustrine delta deposits: A case study from the Upper Cretaceous Qingshankou Formation, northern Songliao Basin, northeastern China

The Upper Cretaceous Qingshankou Formation in the northern Songliao Basin was deposited in river‐dominated lacustrine delta systems. Three third‐order sequences and two types of sequence architecture were recognized using core observations, logging curve interpretations, and three‐dimensional (3‐D) seismic profiles in Qingshankou Formation. One type of sequence architecture was developed in a syndepositional flexural slope break, and the other was deposited on a gentle slope. The studied sequences were deposited in a high accommodation setting, which was due to the large rate of subsidence of the basin basement, and a tropical to semi‐tropical and a humid to semi‐humid climate. The episodic tectonic/thermal subsidence of the basin controlled the development of the sequence architecture. Turbidite bodies within transgressive systems tracts and early highstand systems tracts were deposited predominantly in the basinward part of the syndepositional flexural slope breaks, and they were favourable exploration targets, particularly for stratigraphic traps.

The Phycosiphon Ichnofacies and the Rosselia Ichnofacies: Two new ichnofacies for marine deltaic environments

ABSTRACTSeilacherian Ichnofacies have been established, to date, for characterizing relatively stable depositional settings. Environments characterized by temporally and spatially varying physico-chemical stresses, however, have languished and been described ichnologically in the context of their “departures” from the archetypal expressions of otherwise ambient environments. Correspondingly, discrimination between shoreface and marine deltaic deposits have been addressed mainly by identifying variations in the individual trace-fossil suites without an over-arching ichnological model. Based on trace-fossil suites reported from globally distributed strata throughout the Phanerozoic, the case can now be made for erecting two new temporally and geographically recurring Seilacherian Ichnofacies for marine deltaic successions—the Phycosiphon Ichnofacies for muddy prodelta environments and the Rosselia Ichnofacies for sandy delta-front settings.The Phycosiphon Ichnofacies is characterized by variable bioturbation intensities (BI 0–5), although many sandstone and mudstone beds may have very low bioturbation intensities (BI 0–1), pointing to their rapid, event-style deposition. Most biogenic structures record grazing or deposit-feeding behaviors, with subordinate horizontal dwellings that reflect deposit feeding and/or carnivory. Meiofaunal cryptic bioturbation is locally present in tempestites. Episodic deposition is accompanied by common escape structures and locally, sediment-swimming structures. The Phycosiphon Ichnofacies typically shows beds characterized by diverse, fully marine trace-fossil suites intercalated with discrete beds dominated by low-diversity, facies-crossing traces. Such bed-scale juxtaposition points to short-term but recurring alternations between stable marine conditions and physico-chemically stressed conditions.The Rosselia Ichnofacies is also characterized by variable (BI 0–5; though typically BI 0–3) bioturbation intensities and sporadically distributed burrows. Most ichnogenera record deposit-feeding behaviors, many associated with vertically and horizontally oriented permanent dwelling structures. Dwelling structures commonly display re-equilibration and escape structures are typically abundant, both characteristic of elevated sedimentation rates and sporadic deposition. Most spreitenated structures are markedly retrusive, also attesting to elevated rates of sedimentation. Cryptic bioturbation is only locally abundant, particularly associated with erosionally amalgamated tempestites lacking mudstone drapes. While many of the trace fossils constitute facies-crossing elements, fully marine (ambient) ichnogenera also occur in some suites. Associated mudstone interbeds display low BI values, sediment-swimming structures, and top-down bioturbation, consistent with their rapid deposition as fluid mud.The two ichnofacies may pass gradationally into one another vertically, associated with lobe progradation or autogenic lobe abandonment. Further, the two ichnofacies may pass along depositional strike into their non-deltaic counterparts—the Phycosiphon Ichnofacies grading into the Cruziana Ichnofacies in distal positions, and the Rosselia Ichnofacies passing into the Skolithos Ichnofacies in shallow-water locales.The erection of these new ichnofacies will enhance the reliable identification of deltaic successions, particularly in wave-dominated settings, and their differentiation from classical strandplain shoreface deposits. As well, the two ichnofacies better explain animal–substrate relations in the context of the detailed sedimentological characteristics of delta deposits, refining the overall facies characterization of shallow-marine environments.

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

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