Sedimentary Facies Analysis Research Articles

Three-dimensional reservoir architecture modeling by geostatistical techniques in BD block, Jinhu depression, northern Jiangsu Basin, China

To reduce or avoid some of the ambiguities in a heterogeneous reservoir, a fine three-dimensional model would be inevitably established with the application of geostatistical techniques. BD oilfield is a low-permeability fractured reservoir with many fracture types, which composes a multi-azimuthal fracture system, causing strong anisotropy and heterogeneity. Moreover, fracture provides not only accumulation space for oil and gas but also channels for migration of hydrocarbons and water dashing and thus plays a leading role in controlling BD reservoir production. With the continuous exploitation of oil in reservoir for more than 30 years, BD oilfield has currently stepped into high water cut stage, exposing many problems such as low oil recovery, poor exploitation stability, rapid oil production decline, and one-way water intrusion. Therefore, a detailed reservoir characterization, constructed to describe reservoir behavior under strong water drive in a well-developed fractured reservoir, is urgently needed. In this study, combined with three-dimensional geostatistical techniques, an accurate and efficient reservoir parameter model that occurs in a strong heterogeneous fractured reservoir has been constructed through stratigraphic correlation and sedimentary facies analysis. Hence, all the study presented above will be available for well location, remaining oil potential-tapping and oil recovery improvement during the later study work.

Tectonosedimentary evolution model of an intracontinental flexural (foreland) basin for paleoclimatic research

Intracontinental flexural (foreland) basin sediments are now frequently used as archives for detailed paleoclimatic and sedimentary environmental reconstruction, fossil and stratigraphic correlation, and tectonic evolution and uplift of basin and orogen. However, sedimentologic characteristics vary considerably in time-space with the evolution of flexural basin, apt to cause misinterpretation of climatic change and stratigraphic correlation. Based on high resolution fossil mammal and magnetostratigraphic constraints and sedimentary facies analysis, here we took the Linxia Basin at the front of the NE Tibetan Plateau as a case to demonstrate and figure out a model how sedimentology and stratigraphy vary temporospatially with the evolution of such flexural basin. The results show that the Linxia Basin is a type intracontinental foreland basin subjected to two phases of flexural deformation exerted by the West Qin Ling (Mts.) and NE Tibetan Plateau to the south. Phase I began latest at the beginning of the Miocene (23.3Ma), indicated by a balanced fast flexural subsidence and mostly fine sediment infilling giving rise to the early underlying unconformity. It manifests as an obvious sediment wedge with high filling rate, thickening toward mountains and an occurrence of a mountains-parallel big river – shallow lake system along the foredeep, suggesting a less high mountain topography. In the late Phase I, from ~13Ma to 8Ma, the subsidence and thickening rates began to decrease, accompanied by faults and deformation propagating gradually into the basin, causing gradual basinward migration of the foredeep and its accompanying river-lake system. Since ~8Ma in Phase II, the West Qin Ling and NE Tibetan began to uplift rapidly and thrust/load onto the Linxia Basin, causing strong mountain erosion, thrust-fold belt propagation and basin overfilling. This forced the mountains-parallel river - lake system to turn to the mountains-perpendicular alluvial - braided river system, and finally to an outflow system by the Quaternary onset of the Yellow River in the basin. Concurrent are rapid rotation of the basin, occurrence of growth strata and late unconformities, widespread expansion of boulder conglomerates, great decreasing and increasing sedimentation rates above and before the hanging wall of the fault-fold system and new supplementary provenance from the thrust-fold system. This demonstrates that in stable climate, same lithologic units such as distinct lacustrine sediments and alluvial conglomerates will migrate basinwards with the foredeep moving into basin, causing a great diachroneity and often misleads to recognize the same lithologic unit in space as one unit in time. A dynamic model is presented that should help to avoid such pitfalls in tectonic basin evolution, especially concerning stratigraphic correlation and paleoclimatic change research.

Opening up a window into ecosystems with Ediacara-type organisms: preservation of molecular fossils in the Khatyspyt Lagerstätte (Arctic Siberia)

The Khatyspyt Formation in Arctic Siberia is one of only two carbonate settings with Ediacara-type fossils. As a potential hydrocarbon source rock, it contains abundant molecular fossils that may help to expand our understanding of these ecosystems. Unfortunately, however, the molecular fossil record in geological materials is commonly biased by secondary processes such as thermal maturation, migration of bitumen compounds or surface contamination. In this study, we evaluate the preservation of molecular fossils in a sample from the Khatyspyt Formation and elucidate their paleobiological meaning. Our results reveal that the organic matter is remarkably immature (oil window maturity) and shows little effect of biodegradation. Petrographic observations, exterior/interior experiments, and the similarity between free bitumen, mineral-occluded bitumen, and kerogen pyrolysate point to the syngeneity of the molecular fossils. Abundant hopanes, cyclohexylalkanes, and methyl-branched alkanes indicate a bacterial source of the organic matter, likely including cyanobacteria and anaerobic bacteria. At the same time, a carbonaceous compression fossil on top of the sample and abundant steranes indicate the presence of eukaryotes. The steranes show typical distributions for the Ediacaran (i.e., dominance of stigmastane). Given the exceptional preservation of the body fossils, trace fossils, and molecular fossils, the Khatyspyt Formation can be considered a fossil lagerstatte sensu Seilacher (1970: Begriff und Bedeutung der Fossil-Lagerstatten. Neues Jahrbuch fur Geologie und Palaontologie, Monatshefte: 34–39). The combined analysis of sedimentary facies, paleontology (body, trace, and molecular fossils), and biogeochemistry will provide a more complete understanding of ecosystems with Ediacara-type fossils.

Sedimentological and geochemical responses of Lake Żabińskie (north-eastern Poland) to erosion changes during the last millennium

Increased erosion triggered by land-use changes is a major process that influences lake sedimentation. We explored the record of erosion intensity in annually laminated sediments of Lake Żabinskie, northeast Poland. A 1000-year-long, annually resolved suite of sedimentological (varve thickness, sediment accumulation rate) and geochemical data (scanning XRF, loss on ignition, biogenic silica) was analyzed with multivariate statistics. PCA indicated erosion was a major process responsible for changes in the chemical composition of the sediments. Analysis of sedimentary facies enabled identification of major phases of erosion that influenced lake sedimentation. These phases are consistent with the history of land use, inferred from pollen analysis. From AD 1000 to 1610, conditions around and in Lake Żabinskie were relatively stable, with low erosion intensity in the catchment and a dominance of carbonate sedimentation. Between AD 1610 and 1740, higher lake productivity and increased delivery of minerogenic material were caused by development of settlements in the region and widespread deforestation. The most prominent changes were observed between AD 1740 and 1880, when further land clearance and increased agricultural activity caused intensified soil erosion and higher lake productivity. Landscape clearance also created better conditions for water column mixing, which led to changes in redox conditions in the hypolimnion. The most recent period (AD 1880–2010) was characterized by partial reforestation and a gradual decrease in the intensity of erosional processes.

Sedimentary facies and the rifting process during the late Cretaceous to early Oligocene in the northern continental margin, South China Sea

The late Cretaceous to early Oligocene strata in the northern continental margin of the South China Sea (SCS) are significant for understanding the contemporaneous continental rifting of the margin prior to the opening of the central SCS oceanic basin. Using new seismic and drilling data, combined with previous results, we have identified three episodes of rifting from the late Cretaceous to early Oligocene based on analyses of major unconformities, tectonostratigraphic units, and sedimentary facies. The first episode of rifting that occurred only in the Pearl River Mouth (PRM) basin during the late Cretaceous to Paleocene is observed. During the early to middle Eocene, littoral-shallow lacustrine and fan-delta facies were distributed in some faulted half-grabens in the Qiongdongnan (QDN) basin, while deep lacustrine deposits widely developed in the PRM basin. During the late Eocene to early Oligocene, marine transgression propagated from the southeast into the QDN, southern PRM, and Taixinan basins. We have inferred that late Cretaceous to the middle Eocene rifting is characterized by uniform lithospheric stretching related to the retreat of the paleo-Pacific subduction zone, whereas the late Eocene to the early Oligocene rifting controlled by multiple factors is characterized by depth-dependent lithospheric extension. It is the differential rifting process that led to the differentiation in the types and distribution of source rocks in the basins of northern SCS margin.

Facies analysis of the Balta Formation: Evidence for a large late Miocene fluvio-deltaic system in the East Carpathian Foreland

Deposits of the Balta Fm are preserved in a large arcuate sediment body that covers about 60,000km2 and is up to 350m thick. The Balta Fm spans ca. 5Ma as constrained by underlying Tortonian (Bessarabian) and overlying Messinian (early Pontian) Paratethys strata. It contains frequent terrestrial mammal fossils and fresh- as well as brackish-water (Paratethys) molluscs and ostracods. Over the past 140years our understanding of the sedimentary architecture of the formation and its origins has remained in its infancy, which has limited insight into the evolution of the East Carpathian Foreland. Here, we provide the first modern sedimentary facies analysis of the Balta Fm, which is integrated with an extensive review of previously published local literature. It is supported with micropalaeontological results and a wealth of historical borehole information. We show that the Balta Fm has a tripartite vertical division. Its lowermost part is clay dominated and consists of subordinate delta front sand bodies interspersed between muds. The middle unit contains separate delta plain channels or channel belts encased in thick muds. These are overlain by a unit with amalgamated delta plain channel deposits with only minor amounts of associated mud. The abundance of upper flow regime sedimentary structures in channel sands, the absence of peats (or coals) and the presence of calcareous nodules suggest a strongly seasonal and relatively dry climate with a flashy discharge regime. Deposition of the Balta Fm in an area previously characterized by distal shelf and prodelta environments indicates large-scale progradation triggered by high sediment volume from the uplifting Carpathian Orogen and enhanced by a general lowering of Paratethys sea-level. The tripartite internal architecture of the Balta Fm indicates that progradation continued during deposition. Its wedge-shaped geometry suggests that tectonic activity in the Carpathians generated a 300km wide foreland basin that allowed for significant delta-plain aggradation despite of the generally regressive trend in Paratethys sea-level.

Sedimentology, provenance and geochronology of the Miocene Qiuwu Formation: Implication for the uplift history of Southern Tibet

Located on the south of the Gangdese, the Qiuwu Formation has traditionally been considered as Eocene coal-bearing clastic sediments consisting of sandstone, mudstone and conglomerate, unconformably on top of Gangdese batholith. However, its precise age and depositional environment remain ambiguous. Here, we present a newly measured stratigraphic section near the Ngamring County, western Xigaze. Detrital zircon U–Pb ages were also applied to trace the provenance of sediments and to constrain the maximum depositional age of the Qiuwu Formation. Sedimentary facies analyses indicate subaqueous fan and alluvial fan depositional environments. Clast composition of the conglomerate is dominated by magmatic rocks at the lower part, while chert and mafic detritus occur in the upper part, suggesting a southern source. Sandstone modal analyses indicate that the compositions of quartz, feldspar and lithic grains changed from transitional arc to dissected arc, implying the unroofing of the Gangdese arc. Detrital zircon U–Pb ages of the Qiuwu Formation are compared with those from Gangdese magmatic rocks and Yarlung-Zangbo ophiolites, suggesting that the Gangdese arc is a main source of the Qiuwu detritus and that the southern source played a role during the later stage. The major peak of detrital zircon ages is at 45–55 Ma, which corresponds to Linzizong volcanic rocks in southern Gangdese arc. The weighted mean age of the five youngest zircons from the lower part of the section is 21.0 ± 2.2 Ma, suggesting that the Qiuwu Formation was deposited in early Miocene, coeval with other conglomerates exposed along the southern margin of Gangdese. Combining new observations with previously published data, we propose that the provenance of the Qiuwu Formation had shifted from a single northern source to double sources from both the north and the south. Activities of Great Counter Thrust were primarily responsible for the shift by making the south area a high elevation to provide sediments for the Qiuwu Formation.

Comprehensive assessment of source rocks in the Bohai Sea area, eastern China

The Bohai Sea area, offshore of the Bohai Bay Basin, is one of the most petroliferous regions in China, with proven original oil in place of approximately 2.4 × 109 m3 (150.94 × 108 bbl) and proven original gas in place of over 5 × 1012 m3 (1.76 × 1013 ft3). Cumulative oil production is over 50 million tons (3.5 × 108 bbl). In this study, using the limited data on source rock thickness, core samples, and Rock-Eval pyrolysis along with sedimentary facies analysis, source rock characteristics of different depositional settings were identified, and the thickness, richness, organic matter type, and thermal evolution of four sets of source rocks in the Bohai Sea area— the second member of Dongying Formation (E3d2), the third member of Dongying Formation (E3d3), the first and second members of Shahejie Formation (E2s1-2), and the third member of Shahejie Formation (E2s3)—were predicted and evaluated. Subsequently, the intensity and history of hydrocarbon expulsion for different sags was systematically compared and analyzed. The greatest thickness of the four sets of source rocks in the Bohai Sea area is 400–800 m (1300–2600 ft). The average richness of the organic matter of these source rocks is 1.74%–2.87%. The E2s3 set has the highest organic matter abundance; E2s1-2 has the lowest. The organic matter of these source rocks is mainly type I and type II, but their evolutions differ. The vitrinite reflectance of E3d2 is 0.5%–1.0%, that of E3d3 is 0.7%–1.25%, that of E2s1-2 is 0.75%–1.75%, and that of E2s3 is 0.75%–2.0%. The cumulative hydrocarbon expulsion of the four sets of rocks is 4.14 × 1010 t (2.90 × 1011 bbl). The E2s1-2 set has the greatest expulsion amount: 1.75 × 1010 t (1.22 × 1011 bbl). The peak stages of hydrocarbon expulsion of the four sets of source rocks were during Neogene Minghuazhen Formation (12.2–2.0 Ma) and Neogene Guantao Formation (16.6–12.0 Ma). The Bozhong sag expelled the most hydrocarbons, followed by the Liaozhong, Qikou, and Huanghekou sags.

Palaeoenvironment and provenance in the early Cape Basin of southwest Gondwana: sedimentology of the Lower Ordovician Piekenierskloof Formation, Cape Supergroup, South Africa

The lowermost unit of the Table Mountain Group (Cape Supergroup), the Ordovician Piekenierskloof Formation is a siliciclastic succession that formed during the initial stages of the Cape Basin development in southwest Gondwana. This contemporary sedimentological re-evaluation of the depositional history and provenance of the Piekenierskloof Formation provides new insights into the palaeoenvironment and tectonic setting of the early Cape Basin. Outcrop-based sedimentary facies analysis revealed distinctive depositional features, dominated by large trough cross-bedding, which suggest a shallow, perennial, sand-bed braided fluvial style with bedload transportation in roughly 1 to 2.5 m deep and relatively wide channels. This fluvial style was possibly influenced by the global absence of substrate stabilizing terrestrial flora in pre-Devonian rivers. Based on an integrated approach of clast count, framework petrography and palaeocurrent studies, the provenance analysis show that the source rocks of the Piekenierskloof Formation were likely situated in source areas characterized by two main rock types: (1) a dominant low metamorphic rank metasedimentary rock suite with parent rocks of variable grain-size, and (2) minor plutonic rocks. The data also suggest that the parent rocks were locally intruded by quartz veins and/or pegmatites. Consistent palaeoflow to the southeast and east further indicates that these source areas were located to the northwest and west of the clastic sediment depocentres in the Cape Basin of southwest Gondwana in the Early Ordovician. Accessible outcrops of the Piekenierskloof Formation are mostly laterally limited and sparsely distributed, which hinder stratigraphic correlations, and therefore inferences of tectonic activity during the sedimentation in the Early Ordovician cannot be convincingly made. For a better understanding of the Early Palaeozoic evolution of the Cape Basin in southwest Gondwana, systematic outcrop studies acquiring high-resolution digital imagery using unmanned aerial vehicles (UAVs) and terrestrial laser scanning, coupled with subsurface facies analysis should be undertaken. The Piekenierskloof Formation and the rest of the Table Mountain Group, with their exceptionally extensive, but rather inaccessible cliff faces, are excellent candidates for such high-resolution digital-based studies.

Taphofacies of Lower-Middle Pennsylvanian marine invertebrates from the Monte Alegre and Itaituba formations, part of the outcropped marine sequence of the Tapajós Group (Southern Amazonas Basin, Brazil) – regional palaeoecological models

One of the most relevant characteristics of the Pennsylvanian shallow-water carbonates of the Amazonas Basin is its diverse and well preserved invertebrate fossiliferous assemblages. In order to better understand the origin of these fossil concentrations, taphonomic data were obtained along well exposed areas of the uppermost part of the Monte Alegre Formation and basal part of the Itaituba Formation, which, based on conodonts, fusulinids and palynomorphs is of Atokan age. The taphonomic data focused on invertebrate organisms were supported by petrographic analysis. The understanding of the stacking pattern of the strata in the studied section allowed the identification of five type taphofacies, which contributed in the development of regional palaeoecological models, expressed as block-diagrams. These characterize the distribution of the environmental parameters, the composition of the faunal associations and the distribution and amplitude of the taphonomic processes that created the taphonomic signatures of the bioclastic elements throughout the supratidal to lower intertidal/deep subtidal depositional environments pertinent to the studied depositional environment. The regional palaeoecological models here presented are related to the particularities of the depositional environments of the studied rocks and are exclusive for the characterization of this intracratonic basin set influenced by high frequency climatic variations. Lithofacies, biofacies and taphofacies associations also reflect depositional conditions pertinent to the studied regional context, differing from the elements observed in modern intracratonic contexts analogous to the one studied, from different sedimentary basins around the world. Therefore, invertebrate taphonomy, supported by the analysis of sedimentary facies, fulfills the purposes recommended in this work, demonstrating its potential as a tool for palaeoecological analysis in the Pennsylvanian outcropping section in the southern platform of the Amazonas Basin.

Sedimentary facies analysis of a high-frequency, small-scale, peritidal carbonate sequence in the Lower Jurassic of the Tripolis carbonate unit (central western Crete, Greece): Long-lasting emergence and fossil laminar dolocretes horizons

The study examines a Lower Jurassic (late Liassic) cyclic lagoonal–peritidal stratigraphic unit outcropping in central western Crete (Tripolis unit), which corresponds to the eastern (internal) part of the mainland Gavrovo-Tripolis platform, the most significant external platform of the Hellenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner-carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces (elementary cycles). Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies (tidal flat progradation). The lithofacies association includes dolomitic intraclastic–peloidal–bioclastic wackestones–packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna (bivalves, gastropods, ostracods and seldom benthic foraminifers), representing a shallow subtidal to intertidal, moderately high-energy environment within an inner-platform setting (peritidal environment to restricted lagoon). This lithofacies association has been intermittently subaerially exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidal facies are capped by dolocretes controlled by root-activities (laminar dolocretes, peloidal–pisoid dolocretes and massive dolocretes), marking the end of each depositional cycle, and, thus, distinguishing the successive episodes of a prolonged subaerial exposure period and birth of paleosol horizons. Dolocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, “flower spar” to blocky sparry cement in primary pores, micritic coatings, crudely pelleted walls, alveolar-septal texture, infills of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy (allocyclic processes), in an inner- ramp-to-shelf carbonate setting with tidal flat and restricted lagoon depositional environment. However, a combination of allocyclic with autocyclic processes controlling accommodation space and sediment accumulation, should be also taken in consideration. The studied high-frequency cycles are interpreted to have been formed during the falling stage and correspond to lowstand systems tract (LST) sediments, which consists part of a regressive peak of the Lower/Middle Jurassic Transgressive–Regressive Facies Low Frequency Cycle (2nd order). Due to the absence of massive evaporites, dolomitization has been attributed to reflux of slightly increased-salinity marine fluids (i.e. penesaline), that are driven into the underlying lagoonal–peritidal carbonate sediments during periods of long term sea-level fall in a marginal marine setting.

Sedimentary environment and paleo-tidal evolution of the eastern Bohai Sea, China since the last glaciation

Numerical simulation suggests that the Holocene sediments re-suspension and distribution in the Bohai Sea was mainly controlled by tidal current regime, which was closely related with sea-level change. Study on sediments in the Bohai Sea thus can provide insights into the evolution of tidal-influenced sedimentary environment and its links with sea-level change. Our understanding of this issue remains incomplete, however, owing to the lack of comprehensive study on sediment core with high-resolution proxies to test such inference. In this study, analyses of sedimentary facies, proxies (grain size, total organic carbon and total nitrogen, X-ray fluorescence scanning Sulfur and Chlorine ratio) and accelerator mass spectrometry 14C dates of a sediment core recovered from the eastern Bohai Sea were carried out to clarify the Holocene sedimentary environment, tidal current change and its relation to the sea-level. The results indicate that the eastern Bohai Sea was dominated by fluvial-coastal environment prior to 12,400 cal a BP due to the sea-level lowstand and changed to tidal-influenced environment from 12,400 to 6700 cal a BP following the rapid sea-level rising. Thereafter shelf environment with minor tidal influence dominated the eastern Bohai Sea under the condition of a deceleration of sea-level rise. The significant change at ∼6700 cal a BP both in sedimentary environment and sediment proxies, indicating an environmental transition from strong tidal-influenced to less tidal-influenced setting. With the sea-level rising from the early Holocene to the mid-Holocene, tidal-current was much strong due to the low sea-level stand and became weak after the maximum transgression at ∼6700 cal a BP. These results are consistent with the numerical simulation, which suggested that less strong tidal current were the consequence of the most highstand sea-level since the mid-Holocene. Our study thus provides a sedimentary record to support the interpretation of numerical simulation-based tidal-influenced depositional process in the eastern Bohai Sea since the deglacial period.

Sedimentary response to the intracontinental orogenic process: insight from the anatomy of a small Mesozoic basin in western Yanshan, northern North China

ABSTRACTThe intra-continental orogeny and tectonic evolution of the Mesozoic Yanshan fold-thrust belt (YFTB) in the northern North China Craton (NCC) have been strongly debated. Here, we focus on the Shangyi basin, located in the centre of the YFTB. An integrated analysis of sedimentary facies, palaeocurrents, clast compositions, and detrital zircon dating of sediments was adopted to determine the palaeogeography, provenance, basin evolution, and intra-continental orogenic process. The Shangyi basin comprises the well-exposed Early–early Middle Jurassic Xiahuayuan Formation and the Longmen Formation, and the Late Jurassic–Early Cretaceous Tuchengzi Formation. Based on the 18 measured sections, five facies associations – including alluvial fan, fluvial, delta, lacustrine, and eolian facies – have been identified and described in detail. The onset of the Shangyi basin was filled with fluvial, deltaic, and lacustrine deposits controlled by the normal fault bounding the northern basin, corresponding to the pre-orogeny. In the Middle Jurassic, the cobble–boulder conglomerates of alluvial fan, as molasse deposits, were compatible with the syn-orogeny of the Yanshan movement, which played a critical role in northern North China and even East Asia. After the depositional break in the Middle–Late Jurassic, the Shangyi basin, controlled by the normal fault present in the north of the basin, re-subsided and quickly expanded southward with thick sedimentation, which is correlative with the post-orogeny. Combined with A-type granites, metamorphic core complexes, mafic dikes, and rift basins of the Late Jurassic–early Early Cretaceous present in the northern NCC and Mongolia, significant extension was widespread in the northern NCC and even in northeast Asia. Moreover, vertical changes of provenance indicate that the Taihang Mountain and the Inner Mongolia palaeo-uplift (IMPU) present at the west and north of the basin, respectively, experienced uplift twice in the Middle–Late Jurassic and Early Cretaceous, resulting in a regional depositional break.

Sedimentary facies analysis using borehole log data from turbidite successions

Sedimentary facies analysis using borehole log data from turbidite successions

Central Asian aridification during the late Eocene to early Miocene inferred from preliminary study of shallow marine-eolian sedimentary rocks from northeastern Tajik Basin

The tempo-spatial development of the Cenozoic Asian aridification across the Eocene-Oligocene and its controlling factors are important scientific topics in Earth Sciences, which are pertinent to regional and global tectonic and climatic events. However, sedimentary rocks preserving the record of aridification during this time from central Asia (ACA) are rare. Here we present a preliminary analysis of sedimentary facies of the lower Paleogene in the northeastern Tajik Basin, which reveals that: the lower part of the studied section is dominated by shallow marine deposits of the Paratethys, the middle part is typical of alternations of eolian dune and fluvial deposits, the upper part is represented by eolian loess-sandy loess (L&SL) like facies, and the top exhibits alternations of fluvial-lacustrine and loess like facies. Based on a chronological framework derived from preliminary magnetostratigraphy, published U-Pb dating of a volcanic ash, and regional litho-stratigraphic correlations, we determine that eolian and L&SL facies accumulated in the northeastern Tajik Basin during the Late Eocene and through most of the Oligocene. These sedimentary units indicate that semi-arid to arid environments of ACA had developed at least since the late Eocene. This initial aridification is closely linked to the westward retreat of the Paratethys that was likely driven by a combination of tectonic activity and sea level changes.

Sedimentology of granite boulder conglomerates and associated clastics in the onshore section of the late Mesozoic Pletmos Basin (Western Cape, South Africa)

Along the southern margin of South Africa, intermountain rift successions, which comprise unusually large, rounded granite boulders and other coarse clastics, reveal an important geological history about the mid-Mesozoic extensional tectonics that lead to the break-up of Gondwana. These strata, mapped as part of the Mid to Upper Jurassic Enon Formation, allow the assessment of the nature, intensity and mode of sediment transport in onshore section of the Pletmos Basin, which is one of the late Mesozoic basins in southern Africa. Based on sedimentary facies analysis, palaeocurrent measurements and semi-quantitative palaeohydraulic calculations, the results suggest that the abundant coarse sediment was deposited by debris-flows and stream-flow floods on a proximal alluvial fan with high gradient alluvial channels. The floods were intense with mean flow velocity of ∼6 m3/s and peak discharge of ∼450 m3/s. While the role of climate in the sedimentation dynamics remains unknown, syn-sedimentary rift tectonics were likely significant and caused, north of the major boundary fault, the unroofing and denudation of the uplifted mountainous source areas, including the Late Ediacaran–Cambrian Maalgaten Granite Suite and the Siluro-Ordovician Table Mountain Group (Cape Supergroup).

Stratigraphic variations in lacustrine sediment gravity-flow deposits intercalated in varved diatomite: An example from the Hiruzenbara Formation, Okayama Prefecture, southwest Japan

The Hiruzenbara Formation, which is distributed within the Hiruzenbara Basin of southwest Japan, includes a section of varved diatomite, representing ca. 8000 y of deposition in a dammed lake formed during the Middle Pleistocene. The varve, on average, ca. 1.5 mm thick, consists of light- and dark-green laminae sets, and is predominately composed of planktonic diatoms. Several facies types of sediment gravity-flow deposits are intercalated in the varved diatomite. In this study, we examined the sedimentary facies, lateral changes, and stratigraphic variations of these sediment gravity-flow deposits. In addition, we also analyzed a time series of varve thickness fluctuations.Based on their sedimentary facies, composition, and diatom species, sediment gravity-flow deposits in the varved diatomite can be classified as flood-generated or slope-failure-generated. Sedimentary facies analysis indicates that the slope-failure deposits were deposited by slope failure within the lake. Since no significant segregation of rip-up clasts is observed in the deposits, a proximal depositional setting at the base of the lake slope is suggested. The flood-generated deposits have eroded lower contacts, which indicate deposition by hyperpycnal flows. In contrast, deposits with no basal erosion indicate deposition by homopycnal or hypopycnal flows. The thickness time series of the varve shows a thickening-upward trend indicating an increase in diatom production or in diatom shells sizes. Running mean thicknesses show non-stationary periodicities of 200–500 y. The frequency of the deposit varies, but is approximately 10 events per 400 y. Flood deposits in the middle part of the section are dominated by volcaniclastic silt. This suggests that vegetation around the lake was reduced by ash fall, resulting in frequent flood events during this interval. The frequency and thickness of slope failure deposits increases in the lower part of the succession, together with the occurrence of slump scars. The frequency of the deposits in the upper part of the section is approximately 100–500 y, similar to the postulated interval at activity of the nearby Yamasaki Fault System. The results of this study improve our knowledge of paleoenvironmental changes in the study site, and also provide new criteria for classifying sediment gravity-flow deposits in lacustrine sedimentary cores.

Evolution of the Late Cretaceous Yongfeng-Chongren Basin in Jiangxi Province, southeast China: insights from sedimentary facies analysis and pebble counting

The Gan-Hang Belt in Southeast China is characterized by several igneous and siliciclastic basins associated with crustal extension during Late Mesozoic. The sedimentary evolution of the red basins is still poorly understood. In this study, sedimentary facies analysis and pebble counting were performed on outcrop sections of the Late Cretaceous Guifeng Group inthe Yongfeng-Chongren Basin in central Jiangxi Province. Thirty-five conglomerate outcrops were chosen to measure pebble lithology, size, roundness, weathering degree and preferred orientation. Results show that gravels are mostly fine to coarse pebbles and comprise dominantly quartzites, metamorphic rocks, granitoids and sandstones. Rose diagrams based on imbricated pebbles indicate variable paleocurrent directions. Combining with typical sedimentary structures and vertical successions, we suggest that the Guifeng Group were deposited in alluvial fan, river and playa lake depositional systems. The proposed depositional model indicates that the Hekou Formation represents the start-up stage of the faulted basin, accompanied by sedimentation in alluvial fan and braided river environments. Then this basin turned into a stable expansion stage during the deposition of the Tangbian Formation. Except for minor coarse sediments at the basin margin, the other area is covered with fine-grained sediments of lake and river environments. The Lianhe Formation, however, is once again featured by conglomerates, suggesting a probable tectonic event. Therefore, the study region possibly suffered two tectonic events represented by the conglomerates of the Hekou and Lianhe formations in the context of the crustal extension in Southeast China.

Bioevents and redox conditions around the Cenomanian–Turonian anoxic event in Central Mexico

The Xilitla section of central Mexico (western margin of the proto-North Atlantic) is characterized by pelagic sediments enriched in marine organic matter. Using biostratigraphic and radiometric data, it was dated at the latest Cenomanian–earliest Turonian transition. We identified an interval coeval with the faunal turnover associated with the Oceanic Anoxic Event 2 (OAE 2), recording the Heterohelix shift and the “filament event” for the first time in Mexico. An integral analysis of sedimentary facies, pyrite and geochemical proxies reveals vertically variable redox conditions, with prevailing anoxic to dysoxic bottom waters. Along with phosphorous and manganese depletion, the highest content of total organic carbon and of certain redox-sensitive trace elements (RSTEs) is found during part of the anoxic event, confirming more uniform and constant oxygen-depleted conditions. This interval is also characterized by a significant enrichment in biogenic barium and elevated TOC/NTOT ratios, suggesting a link between productivity and anoxia. Sulfur isotope fractionation has a maximum value within the anoxic event, favored by the increase in the flux of organic matter and intensified through sulfur recycling. Highly bioturbated beds representing short-lived episodes of oxic conditions are intermittent within the OAE 2 and become more frequent in the early Turonian. This study proposes a model similar to that of modern upwelling regions. High marine productivity controlled organic matter burial and oxygenation at the seafloor, varying between anoxic (laminated facies with small pyrite framboids) and dysoxic conditions (bedding-parallel bioturbated facies with inoceramid bivalves and large pyrite framboids), interrupted by short-term well-oxygenated episodes (thoroughly bioturbated facies with common benthic foraminifera). General low-oxygen conditions led to the formation of glauconite and pyrite (bacterially mediated); the enrichment of redox-sensitive trace elements in sediments (Cd, Zn, V and Cr scavenged by organic matter and Ni, Mo, Pb, Co and Re by pyrite) and resulted in Mn and P depletion.

Hydroclimatic variability in the Levant during the early last glacial (∼ 117–75 ka) derived from micro-facies analyses of deep Dead Sea sediments

Abstract. The new sediment record from the deep Dead Sea basin (ICDP core 5017-1) provides a unique archive for hydroclimatic variability in the Levant. Here, we present high-resolution sediment facies analysis and elemental composition by micro-X-ray fluorescence (µXRF) scanning of core 5017-1 to trace lake levels and responses of the regional hydroclimatology during the time interval from ca. 117 to 75 ka, i.e. the transition between the last interglacial and the onset of the last glaciation. We distinguished six major micro-facies types and interpreted these and their alterations in the core in terms of relative lake level changes. The two end-member facies for highest and lowest lake levels are (a) up to several metres thick, greenish sediments of alternating aragonite and detrital marl laminae (aad) and (b) thick halite facies, respectively. Intermediate lake levels are characterised by detrital marls with varying amounts of aragonite, gypsum or halite, reflecting lower-amplitude, shorter-term variability. Two intervals of pronounced lake level drops occurred at ∼ 110–108 ± 5 and ∼ 93–87 ± 7 ka. They likely coincide with stadial conditions in the central Mediterranean (Melisey I and II pollen zones in Monticchio) and low global sea levels during Marine Isotope Stage (MIS) 5d and 5b. However, our data do not support the current hypothesis of an almost complete desiccation of the Dead Sea during the earlier of these lake level low stands based on a recovered gravel layer. Based on new petrographic analyses, we propose that, although it was a low stand, this well-sorted gravel layer may be a vestige of a thick turbidite that has been washed out during drilling rather than an in situ beach deposit. Two intervals of higher lake stands at ∼ 108–93 ± 6 and ∼ 87–75 ± 7 ka correspond to interstadial conditions in the central Mediterranean, i.e. pollen zones St. Germain I and II in Monticchio, and Greenland interstadials (GI) 24+23 and 21 in Greenland, as well as to sapropels S4 and S3 in the Mediterranean Sea. These apparent correlations suggest a close link of the climate in the Levant to North Atlantic and Mediterranean climates during the time of the build-up of Northern Hemisphere ice shields in the early last glacial period.