Luminescence sensitivities of quartz grains from eolian deposits in northern China and their implications for provenance

The thermoluminescence (TL) and optically stimulated luminescence (OSL) sensitivities of quartz grains from deserts and loess–red clay sequences are used to trace eolian provenances in northern China. Our results indicate that the 110°C TL peak and OSL sensitivities of quartz grains show differences among Chinese deserts, which can be subdivided into four groups according to the spatial variations of luminescence sensitivities. Such differences are related mostly to the regional difference in rock types of mountains surrounding or adjacent to the deserts. We also examine the possible provenance changes between the Quaternary loess and the Tertiary eolian red clay, and the results indicate that the luminescence sensitivity of Tertiary red clay is higher than that of Quaternary loess (L1, L15, and L33), implying source materials of the eolian deposits changed relative to those of the Quaternary.     

Climatic implications of loess deposits from the Beijing region

Variations in magnetostratigraphy, pedostratigraphy, grain size and magnetic susceptibility of the loess deposits near Beijing have been studied at two sections. The sections are about 400 km east of the main loess deposits in China, have a maximum thickness of 100 m and extend back to 1.1 Ma. The sequence consists of 14 loess–palaeosol couplets (S0‐S14), which correlate well with sequences in the Loess Plateau. Susceptibility records from the sites near Beijing are comparable to the Xifeng, Luochuan and Baoji sections located in the middle part of the Loess Plateau; however, the down‐core variations in the grain size in the Upper Lishi Formation exhibit some differences. The median grain size increases by about 25–30 µm from L4 to L2, with the sandy grains (>63 µm) increasing from 10–20 wt% to 40–50 wt% . This implies that the depositional environment of the dust sources in the Beijing loess section is different in some aspects from the Loess Plateau. The Beijing loess may have had a different dust source than the Loess Plateau. Copyright © 2001 John Wiley & Sons, Ltd.     

Multiple and single aliquot luminescence dating techniques applied to quartz extracted from Middle and Upper Weichselian loess,Zemechy, Czech Republic

This paper reports the application of optically stimulated luminescence (OSL) dating to relatively coarse grains (100–200 μm) of quartz extracted from loess. Two approaches were tested – a multiple aliquot technique and a new single aliquot approach, which is especially suited to the dating of small quantities of such grains found in some loess. The results are compared with infra‐red stimulated luminescence and thermoluminescence (TL) measurements on the finer‐grained (4–11 μm) polymineral fraction and TL measurements on the relatively coarse grains of quartz. The advantages of the new single aliquot (OSL) approach are described and the iterating, computer‐controlled measurement procedure is given. The automated iterative single aliquot regeneration (AISAR) method corrects for changes in luminescence sensitivity during the measurement sequence by monitoring the response to a test dose. Some initial measurements are presented for the loess section at Zemechy, Czech Republic, and these show that the AISAR method gives more precise results than those using a multiple aliquot method. Copyright © 2000 John Wiley & Sons, Ltd.     

The loess sequence of Dolní Věstonice,Czech Republic: A new OSL‐based chronology of the Last Climatic Cycle

The Dolní Věstonice loess section in the Czech Republic is well known for its high‐resolution loess–palaeosol sequence of the last interglacial–glacial climatic cycle (Upper Pleistocene). The loess section is situated in a climatic transition zone between oceanic and continental climates and is therefore of great value in reconstructing past regional climate conditions and their interaction with climate systems, in particular that of the North Atlantic. Based on a combination of optically stimulated luminescence (OSL) ages, stratigraphic field observations and magnetic susceptibility measurements, a chrono‐climatic interpretation of the Dolní Věstonice loess section is presented. To establish a reliable Upper Pleistocene chronology, a quartz OSL approach was applied for equivalent dose (D_e) determination. Monomineralic quartz extracts of three distinct grain sizes, fine (4–11 μm), middle (38–63 μm) and coarse (90–200 μm), were used and compared. Within error limits, the calculated OSL ages are the same for the different grain sizes, and the OSL ages are in stratigraphic order. The established OSL chronology is in agreement with a Weichselian litho‐ and pedostratigraphy. The Dolní Věstonice loess section is characterized by four pedosedimentary subsequences. At the base of the profile, subsequence I is characterized by a distinct Early Glacial soil complex, OSL‐dated to c. 110 to 70 ka, representing one of the most complete records of environmental change in the European loess belt. Subsequence II is allocated to the Lower Pleniglacial and is characterized by laminated sandy loess. Middle Pleniglacial subsequence III is represented by a brown soil complex, and is followed by the uppermost subsequence IV, characterized by a thick body of laminated sandy loess, indicating strong wind activity and a high sedimentation rate of more than ～1 mm a^?1 during the Upper Pleniglacial. According to the OSL chronology, as well as to the sedimentological and palaeopedological investigations, it is likely that the sequence at Dolní Věstonice has recorded most of the climatic events expressed in the NGRIP δ¹⁸O reference record between 110 and 70 ka.     

Quartz OSL chronology and dust accumulation rate changes since the Last Glacial at Weinan on the southeastern Chinese Loess Plateau

The fine‐grained (4–11 μm) quartz Optically Stimulated Luminescence (OSL) dating technique was applied to the Weinan section on the southeastern Chinese Loess Plateau (CLP) with a high luminescence sampling resolution (10‐ and 20‐cm intervals). Fifty‐eight OSL ages, spanning c. 1–74 ka, were obtained for the 10‐m loess–palaeosol sequences. The reliability of the OSL dating and the constructed chronology was confirmed by comparing the OSL ages with independent dates from other studies and by correlation with palaeoclimatic time series. The closely spaced OSL ages at Weinan indicate that the mean dust accumulation rate (DAR) of L_1–1 (MIS 2, 7.0±0.9 cm ka^?1) is lower than those of L_1–3 (MIS 4, 19.1±6.1 cm ka^?1) and L_1–2 (MIS 3, 16.0±0.7 cm ka^?1) and that the mean DAR c. 30–20 ka ago (11.4±2.4 cm ka^?1) is higher than that c. 20–10 ka ago (3.0±0.1 cm ka^?1) in L_1–1. The subsequent calculation of the mass accumulation rate (MAR) indicates that the MAR of L_1–1 (107 g m^?2 a^?1) is much lower than those of L_1–2 (247 g m^?2 a^?1) and L_1–3 (307 g m^?2 a^?1). By comparing the mean DAR results during the Last Glacial at Weinan with those at a further six sites from other studies, we observed that the mean DAR of L_1–2 is higher (lower) than that of L_1–1 on the eastern (western) CLP, and that the mean DAR during MIS 2 has an evident transition from high to low at c. 20 ka on the entire CLP. Possible mechanisms for the above mean DAR changes at orbital and sub‐orbital time scales are presented.     

辽河东部凹陷驾掌寺—小龙湾地区沙三段上亚段沉积古地理及聚煤作用分析

利用钻探、测井及地震资料对辽河盆地东部凹陷驾掌寺—小龙湾地区沙河街组三段上亚段进行了沉积环境和聚煤作用分析,编制了各种单因素图件及古地理图,识别出冲积扇、三角洲平原、三角洲前缘和前三角洲以及滨浅湖等古地理单元。分析认为,研究区物源来自东侧东部斜坡带和西侧中央凸起,并在区内沉积中心—驾掌寺—小龙湾凹陷处汇集,在适宜的气候、缓和的构造背景、适中的沉降幅度、较少的碎屑输入条件下,有一定水体深度的三角洲平原分流间湾沼泽区是成煤的有利地带,巨厚煤层分布于小龙湾地区的小14井、小5井以及小31井一带。     

A luminescence‐based chronology for the Harletz loess sequence,Bulgaria

The Harletz loess‐palaeosol sequence is located in northwestern Bulgaria and represents an important link between well‐studied loess sequences in eastern Romania and further sites to the west of the Carpathians (e.g. Serbia and Hungary). The aim of this study was to establish a chronostratigraphy of the deposits, using various methods of luminescence dating, together with basic stratigraphical field observations as well as magnetic properties. Luminescence dating was carried out using the quartz fine grain fraction and a SAR protocol, and the feldspar coarse grain fraction, applying the MET‐pIRIR protocol. Due to underestimation of the quartz fine grain fraction in the lower parts of the sequence, the resulting chronology is mainly based on the feldspar ages, which are derived from the stimulation temperature at 150 °C. A comparison with nearby sequences from Serbia, Hungary and Romania, and interpretations obtained through the stratigraphical and sedimentological signature of the sequence, supports the established chronology. Our data suggest that the prominent palaeosol (soil complex) in the upper quarter of the sequence was formed during MIS 5. It would follow that large parts of the Last Glacial loess overlying this palaeosol were probably eroded, and that the thick loess accumulation underlying this soil complex can be allocated to the penultimate glacial (MIS 6). A prominent MIS 6 tephra, which has been reported from other sequences in the area, is also present at Harletz.     

High‐resolution quantification of earthworm calcite granules from western European loess sequences reveals stadial–interstadial climatic variability during the Last Glacial

High concentrations of calcite fossil granules produced by earthworms (ECG) have been identified in most of the stratigraphical units along the loess‐palaeosol reference sequence of Nussloch (Germany). They are particularly abundant in interstadial brown soils and in tundra gley horizons, the latter reflecting short‐term phases of aggradation then degradation of permafrost. These granules are characterized by a radial crystalline structure produced in the earthworms by specific bio‐mineralization processes. In our study, we used this biological indicator combined with ¹⁴C and OSL dating, and sedimentological parameters to characterize millennial‐time scale climatic variations recorded in loess sequences. The approach is based on high‐resolution counts of ECG throughout a 17‐m‐thick loess sequence (332 samples). Strong increases in granule and mollusc concentrations suggest warmer climate conditions during palaeosol formation phases, associated with increasing biodiversity, biological activity and vegetation cover. Decreased granule concentrations occur within primary loess deposits, indicating a strong correlation with palaeoenvironmental conditions and demonstrating the reliability of ECG concentration variations as a new palaeoenvironmental proxy. Finally, this pattern is also recorded in loess sequences located about 600 km westward in northern France demonstrating the large‐scale validity of this new palaeoclimatic proxy.     

Particle size contributions to bulk magnetic susceptibility in Chinese loess and paleosol

The magnetic enhancement of paleosol is observed in the Chinese loess. The origin of this magnetic enhancement is still very uncertain. It is, however, a key problem in correctly understanding the paleoclimatic significance of changes of magnetic susceptibility and in transferring the magnetic signals to paleoclimatic parameters. Two main models have been proposed to explain the mechanism of magnetic enhancement in paleosols: a depositional model and a pedogenic model. Together with composition and concentration, grain size distribution of magnetic minerals also plays an important role to the magnetic enhancement of paleosols. Systematic susceptibility measurements were carried out on the samples of the upper part (S₀ to top of L₂) of three loess sections from Jixian, Xifeng, and Xining, along an east–west transect in the loess plateau, China. The samples with the highest value of magnetic susceptibility in S₁ and the lowest value in L₂ of each section were selected as the representatives. These representatives were separated into different grain size fractions based on Stokes’ law for coarse grains and by centrifuge for fine grains. Measurements of magnetic susceptibility and mass have been carried out on these fractions. Results show that for the loesses magnetic susceptibility changes little in the fractions with different grain size and for the paleosols it increases with decreasing grain size. The magnitude of changes is bigger in the east (Jixian and Xifeng) than that in the west (Xining). The fraction with the finest particle size in paleosols does not show very high magnetic susceptibility values. A new approach is used to estimate the contribution of each fraction to total magnetic susceptibility. The contribution comes mainly from coarse grains (>10 μm) for loess samples. It reaches about 90%. The main contribution comes, however, from the particles with medium size (10–0.2 μm). The very fine grained particle (<0.3 μm), which is considered to be with the pedogenic origin, contributes little to the bulk magnetic susceptibility, no more than 3% because of their very little amount. This approach provides a sounder basis for the study of the origin of the magnetic susceptibility enhancement in paleosols and of the paleoclimatic significance of magnetic susceptibility of loess and paleosols.     

Testing polymineral post-IR IRSL and quartz SAR-OSL protocols on Middle to Late Pleistocene loess at Batajnica,Serbia

The loess–palaeosol sequence of Batajnica (Vojvodina region, Serbia) is considered as one of the most complete and thickest terrestrial palaeoclimate archives for the Middle and Late Pleistocene. In order to achieve a numerical chronology for this profile, four sets of ages were obtained on 18 individual samples. Equivalent doses were determined using the SAR protocol on fine (4–11 μm) and coarse (63–90 μm) quartz fractions, as well as on polymineral fine grains by using two elevated temperature infrared stimulation methods, pIRIR₂₉₀ and pIRIR₂₂₅. We show that the upper age limit of coarse quartz OSL and polymineral pIRIR₂₉₀ and pIRIR₂₂₅ techniques is restricted to the Last Glacial/Interglacial cycle due to the field saturation of the natural signals. Luminescence ages on coarse quartz, pIRIR₂₂₅ and pIRIR₂₉₀ polymineral fine grains are in general agreement. Fine quartz ages are systematically lower than the coarse quartz and pIRIR ages, the degree of underestimation increasing with age. Comparison between natural and laboratory dose response curves indicate the age range over which each protocol provides reliable ages. For fine and coarse quartz, the natural and laboratory dose response curves overlap up to ~150 and ~250 Gy, respectively, suggesting that the SAR protocol provides reliable ages up to c. 50 ka on fine quartz and c. 100 ka on coarse quartz. Using the pIRIR₂₂₅ and pIRIR₂₉₀ protocols, equivalent doses up to ~400 Gy can be determined, beyond which in the case of the former the natural dose response curve slightly overestimates the laboratory dose response curve. Our results suggest that the choice of the mineral and luminescence technique to be used for dating loess sediments should take into consideration the reported limited reliability.     

Sand dunes as a major proximal dust source for late Pleistocene loess in the Negev Desert, Israel

Grain size analyses of three hilltop, primary eolian loess sequences in the Negev desert, southern Israel, show a bimodal grain-size distribution at 50-60 μm and 3-8 μm. Using analyses of mineralogy and OSL ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time, suggesting an enhancement of a time-transgressive proximal dust source compared to a distal, Saharan fine-grain dust. The only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into Sinai and the Negev concurrently with the loess accretion during the late Pleistocene as a result of the exposure of the Mediterranean shelf. We therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea. This relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as Northern Africa, China and Australia.     

Late Pleistocene loess‐palaeosol sequences in the Vojvodina region,north Serbia

Late Pleistocene loess‐palaeosol sequences are widespread in the Vojvodina region, with thicknesses reaching a maximum of about 20 m. Our investigations include more than 40 of these loess sections. Geochronology of the last glacial loess‐palaeosol sequences, based on luminescence dating and amino acid racemisation, provides correlations between Upper Pleistocene loess‐palaeosol sediments in Vojvodina and comparable deposits at other European localities. Sedimentary logs of magnetic susceptibility, grain‐size measurements and carbonate content, combined with malacological evidence, indicate two main relatively cold and arid phases during the last glacial period, related to intensive accumulation of loess units L1L1 and L1L2, as well as many brief episodes of dry and windy climatic conditions, suggesting a possible relationship with cold events recorded in the North Atlantic region. Generally, late Pleistocene climate in the region was dry and relatively warm, compared with glacial period sites in central Europe, and was characterised by sharp differences between glacial and interglacial modes. New data and interpretations presented in this study emphasise the significance of loess‐palaeosol sequences in Vojvodina for the reconstruction of the temporal and spatial evolution of late Pleistocene palaeoclimate in this part of Europe. Copyright © 2007 John Wiley & Sons, Ltd.     

Establishing the Late Pleistocene-Holocene sedimentation boundary in the southern North Sea using OSL dating of shallow continental shelf sediments

This paper provides insight into the fate of Late Weichselian and Early Holocene sediments accumulated in the German sector of the southern North Sea. A combination of optically stimulated luminescence (OSL) dating and radiocarbon dating was applied to set up the chronology. Seven cores were studied to obtain ten quartz OSL samples and ten radiocarbon samples. The core locations were chosen along a southeast to northwest transect along the western side of the Elbe palaeovalley, giving a good coverage of the entire German North Sea area. All samples for OSL dating showed a significant scatter in the equivalent dose (D_e) distribution of quartz due to heterogeneous bleaching. The Minimum Age Model (MAM-3) was found to be the most suitable to extract true burial ages. It was inferred from the study that sedimentation did still occur during the late deglaciation period in many areas. These are mainly Late Weichselian glaciofluvial or glaciolacustrine sediments directly overlain by early Holocene fluvial and/or transgressive deposits and followed by modern marine sands. However, considerable late Weichselian erosion or a possible period of non-deposition was observed in the highland area to the northeast of the Dogger Bank and a small discontinuity in the near-shore region was noticed, probably due to early Holocene fluvial erosion. Relicts of a palaeo-river bank or terrace were identified in core 14VC to the east of the Dogger Bank. A possible interpretation of the Pleistocene-Holocene interface along the core transect is provided based on lithology and measured OSL and radiocarbon ages.     

山西平陆黄河阶地与古三门湖消亡、黄河贯通三门峡时代问题的探讨

山西平陆黄底沟入三门峡水库北侧发育了一级湖滨阶地(T₃)和两级河流阶地(T₂和T₁)。阶地上覆黄土地层野外划分和室内磁化率测试结果与蓝田段家坡黄土地层的对比结果表明,T₃上覆黄土地层最底部为古土壤层S₂;T₂最底部为黄土层L₂中弱发育古土壤层L_2-2;T₁最底部为马兰黄土L₁中弱发育古土壤层L_1-4。根据黄土-古土壤年代序列及光释光(OSL)年龄测试结果,T₃,T₂和T₁阶地分别于245kaB.P. ,149kaB.P.和50kaB.P. 前形成。其中,T₃和T₂阶地的形成时间,分别代表了山西平陆古三门湖开始消亡和最老黄河阶地形成的年代。古三门湖消亡与黄河贯通三门峡的时代是否一致,有待进一步研究。     

应用石英释光灵敏度溯源的研究现状和展望

应用石英释光灵敏度进行物源研究是释光技术发展的新方向。回顾了石英释光灵敏度在测年研究中的发展现状,对其产生的机制、实验中的影响因素及其在测年中对灵敏度的监测、矫正等进行了介绍,详细介绍了空间尺度上不同沉积物(冰碛物、风成沉积物、水成沉积物)石英释光灵敏度的差异和时间序列上风成沉积物释光灵敏度的变化特征。据现有的研究成果分析,沉积历史和母岩来源对沉积物中石英的释光灵敏度的高低有重要的影响,因此可用它来追溯石英的物源。探讨该技术在物源研究领域存在的问题,对其发展前景进行了展望。     

Sedimentological characteristics and morphology of the aeolian sand dunes in the eastern part of the UAE,a case study from Ar Rub' Al Khali

The Umm Az Zimul–Al Wijan area is located in the southeastern part of the UAE and covers about 1600 km². It is bounded to the east by the Oman Mountains and to the south and west by the extensive dune area of Ar Rub' Al Khali. Field investigation revealed the common occurrence of parallel to sub-parallel sand ridges with a general east–west orientation. Also present, but less frequent, are barchan, barchanoid and star dunes. Interdune areas are classified into four types that include: dolomite, mixed ancient sabkha and ophiolitic rock fragments, mixed sand sheet and ophiolitic rock fragments, and recent sabkha. The textural characteristics of the sand forming the dunes are unimodal with a modal class in fine sand size, moderately to very well sorted, positively skewed and mesokurtic. Quartz grains are mainly subrounded to subangular. The similarity in roundness properties in different dune types in the eastern province of the UAE seems not to be influenced by dune form. Based on the heavy mineral suites and the angularity of quartz grains, it is believed that the sand sediments are mostly derived from the ultrabasic rocks forming the ophiolitic sequence of the Oman Mountains, acidic rocks of Iran and from the coastal area.     

Advances and Issues in Luminescence Dating of Loess Deposits in Arid Central Asia

High-resolution loess deposits are widely distributed in Arid Central Asia (ACA) and provide important records associated with dust transportation, paleoenvironmental and paleoclimatic evolution. The chronology is the foundation of the research into loess deposits as an environmental archive. In recent decades, the gradually developed optical dating method has been increasingly matured and become an important approach to establishing the loess-paleosol sequences. Here, we summarized and discussed previous work on loess chronology mainly based on optical dating approach in ACA. The following understandings have been listed: ① In comparison with optical dating method, the suitable material for ¹⁴C dating is uncommon in ACA. However, the dating range of luminescence dating is more extensive, and the dating materials are accessible. Thus, the optical dating is widely applicable in the establishment of loess framework in this area. ② Until now, the quartz Single Aliquot Regeneration (SAR) method can be applied to the establishment of loess-paleosol sequence since last glacial period. But several issues remain unaddressed. For example, the Optically Stimulated Luminescence (OSL) signal sensitivity of quartz grains are low in some areas. Furthermore, the results of OSL dating of different grain sizes within a single sample are inconsistent in some areas. The solution of these problems still requires more methodological research. ③ The post-IR IRSL (pIRIR) and multiple elevated temperature stimulation (MET-pIRIR) protoCols of feldspar have basically overcome the anomalous fading issue in the traditional IRSL dating process. In ACA, the framework since MIS 7 can be established with K-feldspar luminescence dating method. Compared with quartz luminescence characteristics, the K-feldspar luminescence signals are more sensitive and exhibit a high saturation level. In specific applications, it is necessary to establish the age frame according to the luminescence sensitivity, the age of samples or other factors.     

Regional lithofacies and pedofacies variations along a north to south climatic gradient during the Last Glacial period in the central Loess Plateau,China

Five lateral sand–loess–palaeosol continua occur within the last glacial sediments of the central Loess Plateau of China along a 500 km north to south climatic gradient. The continua shift southward or northward in concert with desert expansion or contraction, respectively. Lateral lithofacies (desert sand to loess) variations are evident at the north end of the gradient and follow Walther's Law of the correlation of facies. Lateral pedofacies (loess to palaeosol) variations are present near the south end of the gradient, where the climate was warmer and wetter. The lateral stratigraphic changes from sand to loess or loess to soil are driven by variations in the rate of sedimentation along a climatic gradient.Vertical stratigraphic profiles at the north end of the gradient reveal alternating sand and loess beds. In contrast, alternating loess and palaeosols occur within the same stratigraphic interval in the southern Loess Plateau, where dust accretion rates were lower. However, in high resolution studies of climate change vertical profiles of alternating loess and palaeosols (especially weak palaeosols) may not reflect regional or global climate change. Alternating loess and weak palaeosols may reflect local variations in the balance between the rates of dust accretion and pedogenesis. Local fluctuations in either of these rates could result in the presence of time equivalent loess and palaeosols at high resolutions. Thus, some of the high resolution loess-palaeosol alternations may reflect local climatic variation rather than global or hemispherical climate change.     

Late Pleistocene baldcypress (Taxodium distichum) forest deposit on the continental shelf of the northern Gulf of Mexico

Approximately 13 km south of Gulf Shores, Alabama (United States), divers found in situ baldcypress (Taxodium distichum) stumps 18 m below the ocean surface. These trees could have only lived when sea level fell during the Pleistocene subaerially exposing the tectonically stable continental shelf. Here we investigate the geophysical properties along with microfossil and stratigraphical analyses of sediment cores to understand the factors that lead to this wood’s preservation. The stumps are exposed in an elongated depression (~100 m long, ~1 m deep) nested in a trough of the northwest–southeast trending Holocene sand ridges and troughs with 2–5 m vertical relief and ~0.5 km wavelength. Radiocarbon ages of the wood were infinite thus optically stimulated luminescence (OSL) dating was used to constrain the site’s age. Below the Holocene sands (~0.1–4 m thick), separated by a regional erosional unconformity, are Late Pleistocene mud-peat (72±8 ka OSL), mud-sand (63±5, 73±6 ka OSL), and palaeosol (56±5 ka OSL) facies that grade laterally from west to east, respectively. Foraminiferal analysis reveals the location of the terrestrial-marine transitional layer above the Pleistocene facies in an interbedded sand and mud facies (3940±30 (1σ) ¹⁴C a BP), which is part of a lower shoreface or marine-dominated estuarine environment. The occurrence of palaeosol and swamp facies of broadly similar ages and elevation suggests the glacial landscape possessed topographic relief that allowed wood, mud and peats to be preserved for ~50 ka of subaerial exposure before transitioning to the modern marine environment. We hypothesize that rapid sea-level rise occurring ~60 or ~40 ka ago provided opportunities for local flood-plain aggradation to bury the swamp thus preserving the stumps and that other sites may exist in the northern Gulf of Mexico shelf.     

Facies of slurry-flow deposits, Britannia Formation (Lower Cretaceous), North Sea: implications for flow evolution and deposit geometry

ABSTRACT Mud‐rich sandstone beds in the Lower Cretaceous Britannia Formation, UK North Sea, were deposited by sediment flows transitional between debris flows and turbidity currents, termed slurry flows. Much of the mud in these flows was transported as sand‐ and silt‐sized grains that were approximately hydraulically equivalent to suspended quartz and feldspar. In the eastern Britannia Field, individual slurry beds are continuous over long distances, and abundant core makes it possible to document facies changes across the field. Most beds display regular areal grain‐size changes. In this study, fining trends, especially in the size of the largest grains, are used to estimate palaeoflow and palaeoslope directions. In the middle part of the Britannia Formation, stratigraphic zones 40 and 45, slurry flows moved from south‐west and south towards the north‐east and north. Most zone 45 beds lens out before reaching the northern edge of the field, apparently by wedging out against the northern basin slope. Zone 40 and 45 beds show downflow facies transitions from low‐mud‐content, dish‐structured and wispy‐laminated sandstone to high‐mud‐content banded units. In zone 50, at the top of the formation, flows moved from north to south or north‐west to south‐east, and their deposits show transitions from proximal mud‐rich banded and mixed slurried beds to more distal lower‐mud‐content banded and wispy‐laminated units. The contrasting facies trends in zones 40 and 45 and zone 50 may reflect differing grain‐size relationships between quartz and feldspar grains and mud particles in the depositing flows. In zones 40 and 45, quartz grains average 0·30–0·32 mm in diameter, ≈ 0·10 mm coarser than in zone 50. The medium‐grained quartz in zones 40 and 45 flows may have been slightly coarser than the associated mud grains, resulting in the preferential deposition of quartz in proximal areas and downslope enrichment of the flows in mud. In zone 50 flows, mud was probably slightly coarser than the associated fine‐grained quartz, resulting in early mud sedimentation and enrichment of the distal flows in fine‐grained quartz and feldspar. Mud particles in all flows may have had an effective grain size of ≈ 0·25 mm. Both mud content and suspended‐load fallout rate played key roles in the sedimentation of Britannia slurry flows and structuring of the resulting deposits. During deposition of zones 40 and 45, the area of the eastern Britannia Field in block 16/26 may have been a locally enclosed subbasin within which the depositing slurry flows were locally ponded. Slurry beds in the eastern Britannia Field are ‘lumpy’ sheet‐like bodies that show facies changes but little additional complexity. There is no thin‐bedded facies that might represent waning flows analogous to low‐density turbidity currents. The dominance of laminar, cohesion‐dominated shear layers during sedimentation prevented most bed erosion, and the deposystem lacked channel, levee and overbank facies that commonly make up turbidity current‐dominated systems. Britannia slurry flows, although turbulent and capable of size‐fractionating even fine‐grained sediments, left sand bodies with geometries and facies more like those deposited by poorly differentiated laminar debris flows.