Devonian lacustrine shore zone architecture: Giving perspective to cliff exposures with ground penetrating radar

Lake margin sedimentary systems have been the subject of only limited study. The cyclic Middle Devonian lacustrine succession of Northern Scotland contains repeated developments of shore zone sandstones and thus provides an ideal location for the study of these units. The cycles comprise deep lake, shallow lake, playa and shore zone facies. Detailed field observations are presented alongside ground penetrating radar data which has aided large‐scale and three‐dimensional characterization of the shore zone sand bodies. Loading and discrete channel forms are recognized in thin‐bedded sandstones within the lower portion of the lake shore zone successions. Up‐section, the sandstone beds appear to become amalgamated, forming subtle low angle accretionary bar complexes. Where imaged on the radar profiles, the repeated development of shoreward migrating features succeeded by more shallow angled lakeward accreting surfaces is recognized; these are ascribed to washover and swash–backwash processes, respectively. The orientation of these features is similar to palaeocurrent measurements from oscillation ripples, suggesting an alignment of the shore zone bars perpendicular to the prevailing wind direction. Further loaded sandstone beds and sand‐filled shallow channel features overlie the bar forms. The context of the shore zone facies allows the controls on its formation to be examined. The shore zone sandstones overlie playa facies which contain abundant desiccation horizons, reflecting the most arid phase in the climatically controlled lacustrine cycle. As climatic conditions ameliorated, the rejuvenation of fluvial systems resulted in the transport of sand out into the basin. Initial deposition was limited to intermittent events where sediment was laid down on a water‐saturated substrate. High resolution fluctuations in lake level resulted in periodic short‐lived reworking events along the lake margins which produced amalgamated sands, forming low relief bars. Shore zone reworking is likely to have occurred over a wide  area as the lake margin migrated back and forth, and gradually transgressed.     

西藏尼玛北部新生代盆地沉积记录及控盆机理

通过野外地质调查和地层对比,将尼玛北部盆地新生代陆相地层定为牛堡组。根据岩石组合和沉积特征分析,尼玛北部盆地牛堡组可划分为扇三角洲相、湖泊相和冲积扇相。扇三角洲相可进一步划分为扇三角洲前缘和前扇三角洲2种亚相;湖泊相可划分为半深湖—深湖和滨湖—浅湖2种亚相。盆地的演化特征可分为盆地形成初始期、盆地扩张期和盆地萎缩期,3期的演化可分别对应牛堡组的一段、二段和三段。尼玛北盆地发育的各个阶段都跟古气候变化和构造活动有很大的联系,两者共同影响着盆地发育的各个阶段。结合前人的研究资料,认为尼玛盆地的发育时代为早白垩世末期—晚白垩世初。根据盆地边缘相与半深湖—深湖沉积相伴生、牛堡组底部发现火山岩夹层等沉积特征,可以推断尼玛盆地是一个具有走滑拉张性质的盆地。     

墨西哥湾GC238区冷泉碳酸盐岩的微结构与石化微生物特征

采于墨西哥湾GC238海底天然气渗漏区浅表层的冷泉碳酸盐岩呈结核状产出,由方解石微晶和胶结物及少量的黄铁矿构成。胶结物由直径为0.1～0.5 m的方解石化的球体、卵形体、棒状体组成,充填于方解石晶体之间。冷泉碳酸盐岩结核下表面发育有由方解石化的球体、卵形体、棒状体组成的薄层,其中的一些球状集合体（约5 m）断面显示发育有核和外壳的层圈结构。黄铁矿呈草莓状,也具有相似的层圈结构。这种层状结构与活体古细菌被硫酸盐还原细菌包裹的层圈结构相似。样品中所保存的球体、卵形体、棒状体及其所组成的层圈结构可能是石化的甲烷氧化古细菌和硫酸盐还原细菌。     

鄂尔多斯盆地及周缘地区下二叠统沉积特征

通过大量的野外剖面观察和盆地内钻井岩心的详细描述,综合测井资料,从岩石类型、结构、沉积构造、古生物化石及测井曲线响应特征等方面对鄂尔多斯盆地及周缘地区下二叠统太原组和山西组沉积相特征进行了全面分析研究。结果表明,太原组主要发育陆棚、海岸、冲积扇和三角洲等沉积相,海岸沉积相包括障壁岛、潟湖和潮坪沉积,三角洲沉积相可划分为三角洲平原、三角洲前缘和前三角洲沉积。山西组主要发育冲积扇、河流、曲流河三角洲、湖泊和海岸沉积相,其中河流沉积相包括辫状河和曲流河沉积,曲流河三角洲沉积相可划分为曲流河三角洲平原、曲流河三角洲前缘和前三角洲沉积,而湖泊沉积相以浅湖沉积为主,海岸沉积主要为潟湖沉积。太原期,海相沉积占主导,主要分布于银川-榆林北部一线以南广大地区,并且从东西两侧至中部地区由浅海陆棚沉积和滨浅海过渡为潟湖沉积和潮坪沉积,其间发育障壁岛。盆地西北缘发育冲积扇和扇三角洲沉积,北部广大地区以三角洲沉积为主,自北向南依次为三角洲平原和三角洲前缘沉积。山西期,海水从盆地东南部退却,整体演变为海陆过渡相沉积,盆地北部乌达-杭锦旗-鄂尔多斯一线发育冲积扇沉积,向南至靖边一带依次发育辫状河和曲流河沉积,靖边以南至延安以北地区以三角洲平原沉积为主,向南至同心-庆阳一线发育三角洲前缘沉积,盆地南部彭阳-泾阳地区主要为浅湖沉积,再向南发育物源来自南部的三角洲沉积,在东南部武乡-义马一带为潟湖沉积。     

青藏高原晚三叠世构造-古地理综述

遵循刘宝珺院士提出的“构造控盆、盆地控相”指导思想，在系统厘定地层格架和构造单元划分基础上，确定青藏高原巨型造山带晚三叠世构造-古地理从北往南依次发育：羌塘-三江多岛海、班公湖-双湖-怒江洋、冈底斯-喜马拉雅多岛海和若干次级构造-古地理单元。班公湖-双湖-怒江洋是分隔冈瓦纳大陆和欧亚大陆的特提斯大洋，南羌塘地块是漂浮在特提斯大洋中的块体。本次重点对北羌塘前陆盆地和北喜马拉雅被动大陆边缘盆地的沉积相带展布和古地理进行了研究。造成两个盆地沉积序列及古气候差别的主要因素是构造地质事件。构造事件决定了盆地性质，盆地性质又控制了沉积相带的空间展布。北喜马拉雅盆地位于冈瓦纳构造域，晚三叠世盆地基底南浅北深，继承了古生代构造离散型被动大陆边缘沉积，印支造山作用不发育；北羌塘盆地位于泛华夏构造域，晚三叠世发育印支挤压造山作用及其前陆盆地沉积记录。盆地分析研究表明，北羌塘南部江爱达日那和热觉茶卡等地下三叠统康鲁组底部均发现灰紫色中厚层复成分砾岩、含砾粗砂岩、细砂岩组成向上变细的海侵型地层结构，沉积相为滨岸三角洲；上三叠统土门格拉群沉积相为含煤盆地边缘三角洲。从沉积相展布型式和北东向古水流方向分析，三叠纪北羌塘沉积盆地的物源主要来自羌塘中部双湖造山剥蚀区或“中央隆起带”。     

Evolution of an organic‐rich lake basin – stratigraphy,climate and tectonics: Piceance Creek basin,Eocene Green River Formation

The Piceance Creek basin formed as a continental foreland basin ca 53 to 48 Ma in the early to middle Eocene. On a global basis, the basin contains one of the richest oil shale resources known, where the profundal oil shale deposits, kerogen‐rich mudstones (clay and carbonate), exist over most of the basin. Despite its economic importance, the evolution of the Piceance Creek basin is still somewhat unclear. Based on facies association analysis, depositional trends, and gamma ray and Fischer assay data, six evolutionary lake stages are recognized: (i) fresh lake; (ii) transitional lake; (iii) highly fluctuating lake; (iv) rising lake; (v) high lake; and (vi) closing lake. Lake stages are composed of depositional units and characterize large‐scale changes in sedimentological patterns, depositional trends and fluctuations in the oil shale richness related to changes in climate and tectonics. Lake stage evolution is also consistent with the global Eocene climate trend. Stage 1 formed prior to the Eocene climate optimum. At the beginning of the Eocene climate optimum, a saline‐restricted lake formed (Stage 2) and evolved into the highly fluctuating lake (Stage 3) indicating rapid climate changes during the peak of the Eocene climate optimum. This stage was followed by the rising and high lakes (Stages 4 and 5) after the climate optimum and during a change to a more humid climate. The closing of the lake (Stage 6) was caused by increased sand input from the north, indicating the influence of both tectonics and climate. Based on depositional trends and climate evolution, it is suggested that, during the arid climate, laterally heterogeneous highly cyclic depositional units dominate, whereas, during the humid climate, depositional units form laterally continuous sediments that can be traced over long distances.     

鄂尔多斯盆地及周缘地区下二叠统沉积特征*

通过大量的野外剖面观察和盆地内钻井岩心的详细描述,综合测井资料,从岩石类型、结构、沉积构造、古生物化石及测井曲线响应特征等方面对鄂尔多斯盆地及周缘地区下二叠统太原组和山西组沉积相特征进行了全面分析研究。结果表明,太原组主要发育陆棚、海岸、冲积扇和三角洲等沉积相,海岸沉积相包括障壁岛、潟湖和潮坪沉积,三角洲沉积相可划分为三角洲平原、三角洲前缘和前三角洲沉积。山西组主要发育冲积扇、河流、曲流河三角洲、湖泊和海岸沉积相,其中河流沉积相包括辫状河和曲流河沉积,曲流河三角洲沉积相可划分为曲流河三角洲平原、曲流河三角洲前缘和前三角洲沉积,而湖泊沉积相以浅湖沉积为主,海岸沉积主要为潟湖沉积。太原期,海相沉积占主导,主要分布于银川—榆林北部一线以南广大地区,并且从东西两侧至中部地区由浅海陆棚沉积和滨浅海过渡为潟湖沉积和潮坪沉积,其间发育障壁岛。盆地西北缘发育冲积扇和扇三角洲沉积,北部广大地区以三角洲沉积为主,自北向南依次为三角洲平原和三角洲前缘沉积。山西期,海水从盆地东南部退却,整体演变为海陆过渡相沉积,盆地北部乌达—杭锦旗—鄂尔多斯一线发育冲积扇沉积,向南至靖边一带依次发育辫状河和曲流河沉积,靖边以南至延安以北地区以三角洲平原沉积为主,向南至同心—庆阳一线发育三角洲前缘沉积,盆地南部彭阳—泾阳地区主要为浅湖沉积,再向南发育物源来自南部的三角洲沉积,在东南部武乡—义马一带为潟湖沉积。     

Chronology and stratigraphy of Late Quaternary sediments in the Konya Basin,Turkey: Results from the KOPAL Project

The Late Quaternary environmental history of the Konya plain, in south central Turkey, is used to examine sediment facies changes in a shallow non-outlet basin which has experienced major climatically driven changes in lake extent. Two principal types of sedimentary archive are used to reconstruct a palaeoenvironmental record, namely alluvial sequences on the Çarşamba alluvial fan and sediments from residual lakes. The latter have been used to investigate broader climatic and vegetational histories via palaeolimnological techniques including pollen, diatom and stable isotope analysis. These changes are dated here by radiometric techniques including radiocarbon (AMS and conventional), OSL, and U–Th. Chronological agreement is generally good between the different dating techniques, although typically there is greatly reduced precision beyond ca. 25 ka. Lake sediment cores investigated have basal ages beyond the range of ¹⁴C dating, but contain hiatuses as a result of subsequent alternation between phases of lacustrine sedimentation and aeolian deflation. In contrast to most deepwater non-outlet lake systems, the Konya basin may have been occupied by a single extensive lake for as little as 10% of Late Quaternary time, mainly around the time of the LGM. This lake highstand was followed by an important arid interval. In the absence of unbroken chronostratigraphic sequences, palaeohydrological investigation of shallow non-outlet lakes may require analysis of basin-wide changes in sedimentation rather than reliance on single core records. Stratigraphic continuity in such sedimentary environments cannot be assumed, and requires independent chronological control through radiometric dating.     

若尔盖-松潘前陆盆地的沉积与构造特点

若尔盖-松潘盆地中、上三叠统沉积剖面研究表明以陆相沉积为主,只在东北部青山峪有浅海相碳酸盐岩沉积。盆地北部出现河流、浅湖相沉积,中部以浅湖相沉积为主,局部出现深湖相沉积,南部主要为深湖相沉积。中、上三叠统地层的变形构造以冲褶席(duplex)构造为特点,表现出从北向南应变强度减弱。深部构造反映了盆地位于扬子板块大陆壳基底之上,叠覆在扬子板块北缘的前陆褶皱冲断带上。大地构造位置、沉积相与构造特点,共同说明若尔盖-松潘盆地是三叠纪碰撞造山作用时期扬子板块北部北缘的前陆盆地。     

柴达木盆地西部地区渐新世下干柴沟组上段盐湖沉积特征

利用大量地质及地球化学资料,开展了柴达木盆地西部地区渐新世下干柴沟组上段盐湖沉积特征研究,重点识别岩石类型,恢复古沉积环境,建立沉积模式,探讨控制因素。研究认为：盐湖环境主要发育两大类、五小类沉积相组合,盐湖边缘沉积相包括滨岸斜坡带相组合、缓坡带相组合、陡坡带相组合,盆内沉积相包括水下隆起带相组合和盐湖深水区相组合;盐湖可划分早、中、晚3个沉积演化阶段,分别对应半咸水湖泊、咸水湖泊和盐湖;形成碳酸盐岩与钙质砂岩、富含石膏的碳酸盐岩与泥岩、厚层石盐与薄层碳酸盐岩等主要岩石组合;盐湖沉积主控因素为气候、古地貌与构造运动。     

A highstand oolitic sequence and associated facies from a Late Triassic lake basin, south-west England

High energy, lake‐shoreline carbonate sequences are rarely documented in the geological record. However, one example occurs in the Upper Triassic Mercia Mudstone Group (MMG) of southern Britain. The MMG is one of a number of thick, non‐fossiliferous mudstone deposits associated with North Atlantic Mesozoic rift basins. The origin of the MMG mudstones is the subject of current debate, with marine, playa‐lacustrine and alluvial–aeolian models having been proposed. Shoreline features have been documented from the northern margin of the basin, but the rarity of such features elsewhere in the MMG has led many workers to doubt a lacustrine origin for the mudstones. Wave‐dominated, lake‐shoreline deposits have been recognized in several sections from the southern basin margin in the Clevedon area of the Bristol Channel in south‐west England. These deposits provide evidence for the development of a sizeable perennial to semi‐perennial hypersaline lake in which the MMG mudstones accumulated. Shoreline sediments overlie alluvial stream and sheet‐flood deposits, and pass from transgressive gravel–conglomerate beach units with bioclasts, influenced by shore‐normal waves (deposited under semi‐humid conditions), to lower gradient, highstand oolitic sands affected by more varied wave approach (deposited under progressively more arid conditions), which culminated in lowstand, oolitic strand‐plain deposits overlain by a playa‐mudflat unit. Shoreline deposits record a simple shallowing‐upward transgressive–highstand–lowstand sequence. However, a change from a reflective (transgressive) to dissipative (highstand) shoreline is believed to represent a climatically induced change in prevailing wind direction. Shoreline features recognized in the MMG are similar to those of recent playa‐lacustrine basins of the western United States. Ooids display a variety of size, fracture and dissolution features in addition to beachrock fabrics, suggesting that they were originally composed of radial aragonite, similar to modern ooids from the Great Salt Lake, Utah.     

辽东苏子河盆地聂尔库组沉积相与沉积环境研究

辽东苏子河盆地是早白垩世形成的小型断陷盆地,盆地内发育了相对连续的陆相沉积,是研究白垩纪陆地环境及气候演化的理想地区。早白垩世晚期沉积的聂尔库组地层出露连续,沉积现象典型。通过分析聂尔库组沉积岩类型、沉积构造、古生物化石及垂向沉积序列,可识别出扇三角洲相和湖泊相,其中扇三角洲相包括扇三角洲平原亚相、扇三角洲前缘亚相和前扇三角洲亚相,湖泊相主要为滨浅湖泥砂沉积。研究表明,聂尔库组形成于断陷盆地边缘陡坡带,属于间歇性洪水控制的扇三角洲—湖泊沉积体系,经历了扇三角洲—湖泊—扇三角洲的沉积环境变化,代表了断陷湖盆由深陷扩张期—抬升收缩早期的沉积充填过程。根据沉积岩与沉积相特征,结合古生物、特殊沉积、黏土矿物及地球化学资料,认为早白垩世晚期辽东地区总体与当时全球"温室气候"的大背景相一致,处于高温干旱/潮湿的气候条件,但这种高温气候具有不均一性,存在气候波动。     

Palaeoenvironmental reconstruction of a middle Miocene alluvial fan to cyclic shallow lacustrine depositional system in the Calatayud Basin (NE Spain)

ABSTRACT The middle Miocene sedimentary fill of the Calatayud Basin in north‐eastern Spain consists of proximal to distal alluvial fan‐floodplain and shallow lacustrine deposits. Four main facies groups characteristic of different sedimentary environments are recognized: (1) proximal and medial alluvial fan facies that comprise clast‐supported gravel and subordinate sandstone and mudstone, the latter exhibiting incipient pedogenic features; (2) distal alluvial fan facies, formed mainly of massive mudstone, carbonate‐rich palaeosols and local carbonate pond deposits; (3) lake margin facies, which show two distinct lithofacies associations depending on their distribution relative to the alluvial fan system, i.e. front (lithofacies A), comprising massive siliciclastic mudstone and tabular carbonates, or lateral (lithofacies B) showing laminated and/or massive siliciclastic mudstone alternating with tabular and/or laminated carbonate beds; and (4) mudflat–shallow lake facies showing a remarkable cyclical alternation of green‐grey and/or red siliciclastic mudstone units and white dolomitic carbonate beds. The cyclic mudflat–shallow lake succession, as exposed in the Orera composite section (OCS), is dominantly composed of small‐scale mudstone–carbonate/dolomite cycles. The mudstone intervals of the sedimentary cycles are interpreted as a result of sedimentation from suspension by distal sheet floods, the deposits evolving either under subaerial exposure or water‐saturated conditions, depending on their location on the lacustrine mudflat and on climate. The dolomite intervals accumulated during lake‐level highstands with Mg‐rich waters becoming increasingly concentrated. Lowstand to highstand lake‐level changes indicated by the mudstone/dolomite units of the small‐scale cycles reflect a climate control (from dry to wet conditions) on the sedimentation in the area. The spatial distribution of the different lithofacies implies that deposition of the small‐scale cycles took place in a low‐gradient, shallow lake basin located in an interfan zone. The development of the basin was constrained by gradual alluvial fan aggradation. Additional support for the palaeoenvironmental interpretation is derived from the isotopic compositions of carbonates from the various lithofacies that show a wide range of δ¹⁸O and δ¹³C values varying from ?7·9 to 3·0‰ PDB and from ?9·2 to ?1·7‰ PDB respectively. More negative δ¹⁸O and δ¹³C values are from carbonate‐rich palaeosols and lake‐margin carbonates, which extended in front of the alluvial fan systems, whereas more positive values correspond to dolomite beds deposited in the shallow lacustrine environment. The results show a clear trend of δ¹⁸O enrichment in the carbonates from lake margin to the centre of the shallow lake basin, thereby also demonstrating that the lake evolved under hydrologically closed conditions.     

New evidence for an extended occupation of the Provo shoreline and implications for regional climate change, Pleistocene Lake Bonneville, Utah, USA

Lake Bonneville was a climatically sensitive, closed-basin lake that occupied the eastern Great Basin during the late Pleistocene. Ongoing efforts to refine the record of lake level history are important for deciphering climate conditions in the Bonneville basin and for facilitating correlations with regional and global records of climate change. Radiocarbon data from this and other studies suggest that the lake oscillated at or near the Provo level much longer than depicted by current models of lake level change. Radiocarbon data also suggest that the lake dropped from threshold control much more rapidly than previously supposed. These revisions to the Lake Bonneville hydrograph, coupled with independent evidence of climate change from vegetation and glacial records, have important implications for conditions in the Bonneville basin and during the Pleistocene to Holocene transition.     

柴达木盆地北缘东部侏罗系发育特征

侏罗系是柴达木盆地主力生油层,主要分布于盆地北缘。通过对柴北缘侏罗系标志层、岩性特征和沉积体系的综合研究,明确了主要露头剖面侏罗纪不同时期的沉积相类型。本区侏罗系主要发育 5 种类型沉积相,包括冲积扇、辫状河、扇三角洲、辫状河三角洲和湖泊,相带的展布和古地理演化均与区域构造运动密切相关。根据侏罗系内部及其与上下地层的接触关系和沉积旋回演化,柴达木盆地北缘东部经历了早--中侏罗世断陷湖盆沉积到晚侏罗世挤压坳陷沉积两大沉积演化阶段。     

汾河裂谷晋中盆地内第四纪沉积序列与沉积环境恢复

晋中盆地是位于汾渭地堑系中部的新生代陆内断陷沉积盆地,研究晋中盆地第四纪的沉积序列对于进一步认识该盆地区域构造和区域气候环境变化具有重要意义。为查明晋中盆地第四纪沉积序列结构、研究揭示晋中盆地第四纪沉积环境及演化,通过构造岩相学方法,对晋中盆地地表和钻孔内第四纪沉积物和沉积环境进行了研究。结果表明:晋中盆地清徐地区地表第四纪沉积序列结构为盆地内部沱阳组河床相和河漫滩相-盆地边缘汾河组河流相—盆山过渡带峙峪组河流相、汾河组冲积扇相及马兰组风积相-基岩山地（盆地外围）二叠系浅海相。钻孔岩芯内第四纪沉积序列结构为下更新统浅湖相和滨湖相—中更新统浅湖相和河流相—上更新统河流相和冲积扇相—全新统冲积扇相。研究认为晋中新生代陆内断陷盆地内沉积序列和演化结构为早更新世陆相湖盆沉积环境—中更新世萎缩湖泊环境—晚更新世强烈萎缩的湖泊和河流环境—全新世再度沉降的陆相湖盆;陆相湖盆从中心向山地沉积环境分带为湖泊沉积环境—河流和湖泊沉积环境—冲积扇沉积环境（盆地边缘出山口区）。盆山过渡带地区上更新世沉积物错位现象与同沉积活动断裂有关,断裂活动性揭示了晋中盆地阶梯式断陷成盆的动力学机制。这些研究成果为区域气候环境变化和晋中盆地区域构造研究提供了新证据,也为太原市城市群建设中工程场址的选择提了供参考。      

早白垩世义县盆地义县组顶部金刚山层沉积相及其古环境意义

辽西义县盆地内近东西走向的马神庙-刀把地-三百垄-金刚山一带为义县组标准地层剖面出露地,义县组建阶标准地层剖面枣茨山金刚山层为一套古湖盆相沉积组合。野外详细的沉积学和地层学研究将金刚山层自下而上划分为湖缘碎屑浊流相、浅水湖坪相和半深湖相3个沉积亚相。金刚山层下部湖缘碎屑浊流相包括3个粗-细粒序递变沉积韵律。粗粒岩石单元为灰绿色含长石岩屑凝灰质砂砾岩、杂色砾岩、灰白色含砾凝灰岩;细粒岩石单元由灰白色含砾凝灰岩、灰绿色致密凝灰岩、质纯膨润土组成。韵律沉积底部粗粒岩石单元为浊流头部沉积产物,其中常形成弥散式正粒序递变层理,细粒岩石单元为浊流体部、尾部逐渐稀释的沉积结果,其中常发育水平层理。中部浅水湖坪相沉积由5个膨润土化粉砂岩（质纯膨润土或页岩）-泥灰岩（含方解石细脉灰岩）沉积韵律构成。每一湖坪相的碳酸盐岩与下伏火山灰质沉积厚度之比值变化范围为:0.11~0.47,指示湖盆围缘经历了多期次湖坪沉积时间间隔相等的地质规律。金刚山层上部半深湖相沉积为灰白色纸片状页岩-粉砂质页岩组合。金刚山层下部湖缘火山碎屑浊流沉积指示火山喷发物质近源性、火山喷发多期作用的地质演化规律。中部湖坪相的5个膨润土化粉砂岩（质纯膨润土或页岩）-泥灰岩（含方解石细脉灰岩）沉积韵律指示古湖盆水体至少经历了5期变浅过程,在区域性干旱气候影响下,形成湖坪相泥灰岩沉积。湖盆中心相对深水沉积区接受大量细粒悬浮质沉积,形成静水半深湖相沉积。     

Variable style of transition between Palaeogene fluvial fan and lacustrine systems, southern Pyrenean foreland, NE Spain

Two Palaeogene fluvial fan systems linked to the south‐Pyrenean margin are recognized in the eastern Ebro Basin: the Cardona–Súria and Solsona–Sanaüja fans. These had radii of 40 and 35 km and were 800 and 600 km² in area respectively. During the Priabonian to the Middle Rupelian, the fluvial fans built into a hydrologically closed foreland basin, and shallow lacustrine systems persisted in the basin centre. In the studied area, both fans are part of the same upward‐coarsening megasequence (up to 800 m thick), driven by hinterland drainage expansion and foreland propagation of Pyrenean thrusts. Fourteen sedimentary facies have been grouped into seven facies associations corresponding to medial fluvial fan, channelized terminal lobe, non‐channelized terminal lobe, mudflat, deltaic, evaporitic playa‐lake and carbonate‐rich, shallow lacustrine environments. Lateral correlations define two styles of alluvial‐lacustrine transition. During low lake‐level stages, terminal lobes developed, whereas during lake highstands, fluvial‐dominated deltas and interdistributary bays were formed. Terminal lobe deposits are characterized by extensive (100–600 m wide) sheet‐like fine sandstone beds formed by sub‐aqueous, quasi‐steady, hyperpycnal turbidity currents. Sedimentary structures and trace fossils indicate rapid desiccation and sub‐aerial exposure of the lobe deposits. These deposits are arranged in coarsening–fining sequences (metres to tens of metres in thickness) controlled by a combination of tectonics, climatic oscillations and autocyclic sedimentary processes. The presence of anomalously deeply incised distributary channels associated with distal terminal lobe or mudflat deposits indicates rapid lake‐level falls. Deltaic deposits form progradational coarsening‐upward sequences (several metres thick) characterized by channel and friction‐dominated mouth‐bar facies overlying white‐grey offshore lacustrine facies. Deltaic bar deposits are less extensive (50–300 m wide) than the terminal lobes and were also deposited by hyperpycnal currents, although they lack evidence of emergence. Sandy deltaic deposits accumulated locally at the mouths of main feeder distal fan streams and were separated by muddy interdistributary bays; whereas the terminal lobe sheets expand from a series of mid‐fan intersection points and coalesced to form a more continuous sandy fan fringe.     

中扬子晚二叠世沉积特征及古地理演化

通过剖面实测、岩相及沉积相分析,中扬子区晚二叠世主要发育碳酸盐岩开阔台地沉积、陆棚沉积及盆地沉积,吴家坪期以浅海相沉积为主,深水盆地范围较小,长兴期发育台地相及盆地相,深水盆地范围变大,研究区晚二叠世的沉积主要受南秦岭海活动控制和盆地演化影响,沉积分异在晚二叠世较发育.古地理演化表明,吴家坪期中扬子北缘为深水盆地沉积,...     

Facies analysis and depositional systems of Cenozoic sediments in the Hoh Xil basin, northern Tibet

A sedimentary succession more than 5800 m thick, including the Lower Eocene to Lower Oligocene Fenghuoshan Group, the Lower Oligocene Yaxicuo Group, and the Lower Miocene Wudaoliang Group, is widely distributed in the Hoh Xil piggyback basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan plateau. The strata of the Fenghuoshan and Yaxicuo groups have undergone strong deformation, whereas only minor tilting has occurred in the Wudaoliang Group. We analyze their sedimentary facies and depositional systems to help characterize continental collision and early uplift of the Tibetan plateau. The results indicate fluvial, lacustrine, and fan-delta facies for the Fenghuoshan Group, fluvial and lacustrine facies for the Yaxicuo Group, and lacustrine facies for the Wudaoliang Group. Development of the Hoh Xil basin underwent three stages: (1) the Fenghuoshan Group was deposited mainly in the Fenghuoshan-Hantaishan sub-basin between 56.0 and 31.8 Ma ago; (2) the Yaxicuo Group was deposited mainly in the Wudaoliang and Zhuolai Lake sub-basins between 31.8 and 30.0 Ma ago; and (3) the Wudaoliang Group was deposited throughout the entire Hoh Xil basin during the Early Miocene. The Fenghuoshan and Yaxicuo groups were deposited in piggyback basins during the Early Eocene to Early Oligocene, whereas the Wudaoliang Group was deposited in a relatively stable large lake. The Hoh Xil basin underwent two periods of strong north–south shortening, which could have been produced by the collision between India and Asia and the early uplift of the Tibetan plateau. The study suggests the Hoh Xil region could reach a high elevation during the Late Oligocene and the diachronous uplift history for the Tibetan plateau from east to west.