The effect of weathering on the grain-size distribution of red soils in south-eastern China and its climatic implications

Particle-size analysis is a useful way to determine the source and deposition of sediments. However, there are inconsistencies when this method is used to constrain the origin of the red soils in south-eastern China. To address this problem, we performed a detailed grain-size analysis of two red soil sequences in Xuancheng and Qiliting located along the lower reaches of the Yangtze River. By comparing their particle-size characteristics with those of the loess on the Loess Plateau in northern China, we found that the aeolian samples plot in a particular zone in the C–M (grain size of the cumulative 1% versus median grain size) plot and cluster nearer the lower left corner of the plot as their degree of weathering increases. The grain-size features suggest that the onset of large-scale aeolian deposition along the lower reaches of the Yangtze River occurred at approximately 0.8 Ma. Although both sedimentary sorting and post-depositional weathering control the grain-size variations in the deposits, the extremely strong weathering due to the humid, warm climate along the lower reaches of the Yangtze River primarily modified the grain-size distributions of the primary red soil deposits. Strong weathering increased the very fine silt (2–5 μm) fraction and decreased the coarse (>63 μm) fraction. We also found that certain grain-size parameters of the red soils varied with the weathering intensity, which can be used as indicators of palaeoclimate variations. The grain size variations in both the Qiliting and Xuancheng sequences suggest that the mid-Pleistocene climate transition (MPT) may have affected the lower reaches of the Yangtze River at 0.9 Ma.     

Comparison of particle size characteristics of the Tertiary 'red clay' and Pleistocene loess in the Chinese Loess Plateau: implications for origin and sources of the 'red clay'

The origin of the Tertiary ‘red clay’ underlying the Pleistocene loess in the Chinese Loess Plateau remains controversial, although several lines of evidence have suggested a wind‐blown origin. This study examines the particle‐size parameters of the late Miocene and Pliocene ‘red clay’ by comparing it with those of the late Pleistocene loess. The particle‐size distribution of a total of 15 339 loess and 6394 ‘red clay’ samples taken from 12 loess sections along a north–south transect and two ‘red clay’ sequences at Lingtai and Jingchuan was systematically analysed. The median grain size, skewness and kurtosis of the late Pleistocene loess all show a systematic southward change and are principally influenced by distance from source region. The spatial and temporal differentiation of dust deposits is expressed in a skewness–kurtosis–median grain size ternary diagram, from which the distance to the source region can be inferred. The particle‐size characteristics of the Tertiary ‘red clay’ sediments are very similar to those of the palaeosols within the late Pleistocene loess deposits, suggesting an aeolian origin for the ‘red clay’. Based on the comparison of ‘red clay’ and loess in the ternary diagrams, it is inferred that the source–sink distance was greater in the Neogene than in the last and penultimate interglacials, and that the dust source region in north‐western China underwent a progressive expansion during the period from at least 7·0 Ma to the present.     

黄土高原红粘土与黄土古土壤粒度特征对比--红粘土风成成因的新证据

对黄土高原具有代表意义四个剖面的晚第三纪红粘土样品进行了粒度分析,并将结果与典型风成黄土古土壤样品的粒度特征进行详细对比,得到红粘土是风成成因的沉积学证据。结果表明,黄土高原红粘土与其上覆第四纪古土壤形成环境相似,红粘土是在相对于黄土堆积搬运风动力变幅小的条件下沉积的,在沉积之后受到较强的风化成壤改造作用。     

长江中下游地区第四纪红土磁学特征及其环境意义

通过采集长江中下游湖南长沙(CS)、岳阳(HN)、江西南昌(NC)、新余(XY)和浙江金华高村(GC)5个剖面不同层位的红土样品,对其粒度和多种磁学参数进行测量和分析,探讨了红土剖面随深度变化的成土作用强弱以及多磁学参数在揭示我国南方红土第四纪气候演化信息的价值。结果表明:①粒度分析显示,红黄土交接带(约28°~31°N)的CS、HN、NC、GC 4个剖面的红土粒度与北方黄土和下蜀黄土有很好的可比性,具有明显的风成特性;位置更偏南的XY剖面红土颗粒较粗,10~50 μm粒组富集不明显,具有典型的冲积相特征。②各红土剖面的多磁学参数分析表明,成土过程中产生的磁铁矿、磁赤铁矿和赤铁矿是红土磁性的主要载体。③同一红土剖面内,不同沉积结构的磁性矿物含量和组成具有很大差异。下部网纹红土中赤铁矿的含量较高,磁赤铁矿的含量较上覆均质红土或黄棕色土低,揭示了网纹红土形成于中国南方一个极端湿润期,长期剧烈的水分活动导致均质红土中磁赤铁矿的溶解和铁质的流失,同时较强的成土作用使强磁性的磁赤铁矿向弱磁性的赤铁矿转化,从而导致网纹红土的磁化率比上覆均质红土或黄棕色土低了一个数量级。④具有冲积相特征的XY剖面磁学特征与其他风尘成因红土(特别是NC剖面)差异甚大,可能成为它具有不同物质来源的一个佐证。     

第四纪网纹红土的类型与网纹化作用

第四纪网纹红土在中国南方红土发育区内广泛分布,具有特殊的结构,是第四纪红粘土的重要组成部分,按照质地网纹红土可分为均质类网纹红土和非均质类网纹红土。测试结果显示:（1）均质类网纹红土的粒度组成以粉砂（5～50 μm）和粘粒（<5 μm）为优势粒级,砂（>50 μm）含量很小,不足3%;粒度频率曲线多数呈单峰正偏形态,众数峰值位于5.5～6.5之间,部分呈双峰形态,两个峰值分别位于6和8附近;CIA值较高（84.31%～89.26%）,ba值较低（0.19～0.30）;稀土元素平均含量和分布模式与风尘沉积相似,87Sr/86Sr值与佳县上新世红粘土接近;εNd（0）介于-10.54～-12.66之间,表明物质源区基本稳定;均质类网纹红土的理化特征体现了风成沉积的特性和风化成土作用的显著影响,可称之为加积型网纹红土。（2）非均质类网纹红土发育自冲洪积相、坡积相沉积物或基岩风化壳红土,土体粉砂含量明显减少,多小于50%,砂含量大大增加,>50 μm和>250 μm粒级的离散系数较高,指示了显著的非均质特征;CIA值多数大于84%,化学元素和风化指标的离散系数多小于10%,显示其经历了较为强烈和稳定的风化作用。第四纪网纹红土的形成先后经历了风化成土过程和网纹化过程,南方红土区在网纹红土发育期内具有整体湿润的气候条件。     

The origin and sources of loess‐like sediment in the Jinsha River Valley,SW China

The grain size of the loess‐like sediment in the Jinsha River Valley is essentially similar to that of typical loess, and it differs from that of the fluvial sediment in the Jinsha River area. We conclude that the loess‐like sediment is of aeolian origin, as is the loess of the Chinese Loess Plateau. The geochemical characteristics of the loess‐like sediment are similar to those of dammed lake and fluvial sediments in the same area, which is significantly different from those of the typical loess in NW China. The geochemical characteristics show that the loess‐like sediment originated from proximal materials. Combined with the grain‐size distribution pattern of the aeolian deposits, we suggest that the loess‐like sediment of the Jinsha River Valley originated from the dammed lake and fluvial sediments that accumulated in the valley. The local mountain‐valley wind circulation (especially the valley wind in winter and spring) provided the transport force for the loess‐like sediment. The presentation of the loess‐like sediment may indicate the appearance of the grassland and forest steppe and the beginning of the dry‐hot valley phenomenon.     

Geochemical and isotopic studies of aeolian sediments in China

The Sr and Nd isotopic, rare earth element (REE) and major element compositions, together with mineral and grain‐size proportions, are reported for aeolian loess deposits and desert sands from several Chinese localities. The study was carried out in order to examine regional variations in the isotopic and geochemical features of these aeolian sediments, and to constrain the provenance of Chinese loess. Samples include loesses from the Tarim and Junggar basins and desert sands from the Taklimakan desert in north‐west China, loess from the Ordos area and desert sands from the Tengger and Mu‐us deserts in north‐central China, as well as loess and desert sands from the Naiman area, north‐east China. REE distributions show minimal variation among the Chinese loess deposits, whereas those for the desert sands show regional variations. New isotopic data document a latitudinal variation in Sr and Nd isotopic features for the loesses and desert sands. The Naiman and Junggar loesses have distinctly lower ⁸⁷Sr/⁸⁶Sr ratios and higher ε_Nd(0) values than the loesses from the Tarim Basin, the Ordos area and the Loess Plateau. Among the desert sands, the Naiman samples have higher ε_Nd(0) values than the Taklimakan, Tengger and Mu‐us samples. Isotopic data suggest that loesses of the Loess Plateau were supplied from the Tarim Basin loesses and Taklimakan Desert sand, and that the Naiman loesses were supplied from the Junggar Basin loesses. The latitudinal variation in the loesses and desert sands may be partly explained by isotopic variations reported previously for moraines from the Tianshan and west Kunlun Mountains, which are possible sources for the loesses and desert sands. These inferences on the provenance of the loesses and desert sands are consistent with the dust transport pattern over East Asia.     

贵州普定碳酸盐岩上覆松散堆积物的沉积环境初探

通过对贵州省普定县马官地区不同地貌部位的4个剖面进行详细的粒度分析,把粒度参数以及频率曲线与各种沉积环境的特征进行对比,排除了河流沉积、风成沉积的可能性。研究区域各剖面的粒度分布特征表明其沉积环境属于碳酸盐岩溶蚀残余风化,同时还有坡面流水和地下水作用的参与。     

华南衡阳盆地上白垩统黄土类似物的初步研究

中国新生代黄土序列具有沉积速率高、连续性好等特点,记录了东亚地区持续的古气候动力学演化和重大地质事件信息。受区域构造-地貌和大气环流格局影响,中生代晚期华南发育了众多盆山型古沙漠系统且出露了巨厚的风成砂,但几乎未有学者提及黄土/黄土类似物的存在。本文通过粒度端元解析、野外露头的构型分析等手段,对衡阳盆地上白垩统红花套组进行了初步研究,识别出风成砂丘、砂席和泥滩等环境类型;并在泥滩环境中提取到黄土类似物的粒度组分信号,同时提出了黄土类似物的风成成因,总结了其形成-堆积过程。这项工作旨在探索白垩纪黄土类似物存在的可能性,为未来开展华南陆块中生代风尘记录研究提供契机。     

Geochemical characteristics of fine and coarse fractions of sediments in the Yarlung Zangbo River Basin (southern Tibet,China)

Tracing sources of aeolian sediment is key to reconstructing earth surface processes in arid areas and interpreting the paleoenvironmental significance of aeolian sequences. However, the difference of geochemical characteristics between different fractions of sediments in the Yarlung Zangbo River Basin (YZRB) is still unclear, so we want to identify which fraction of sediments is more suitable for tracing sediment sources. Considering the long distance between different wide valleys in the YZRB, we wonder whether there is any difference between these wide valleys in terms of geochemical characteristics of sediments. Forty-three surface sediment samples in the YZRB are collected, and the grain-size distributions and the major-element composition for 37 samples and the trace and rare earth element composition for the coarse (75–500 μm) and fine (<?75 μm) fractions of all samples are determined. The results reveal the following: (1) the fine fractions of the deposits contain more environmental information, suggesting that the fine fractions cannot be directly compared between different climate zones for provenance identification and that appropriate coarse fractions, based on the grain-size distribution of the targeted sediments, are more ideal for tracing sediment sources; and (2) geochemical characteristics of various sediment types show spatial heterogeneity. The coarse and fine fractions of the loose sediment samples can be divided into two regional groups based on geochemical characteristics: the Maquanhe zone in the upper reach, the Xigaze, Shannan and Mainling zone in the middle reach, which is consistent with the geological background of the YZRB; (3) aeolian deposits in the YZRB are a local origin and predominantly derived from the adjacent loose sediments, and fluvial sediments in the upper reach contribute little to the aeolian sands in the middle reach.     

Textural and Geochemical Characteristics of Proglacial Sediments: A Case Study in the Foreland of the Nelson Ice Cap, Antarctica

This paper presents a detailed study on the textural and geochemical characteristics of the proglacial sediments near the edge of modern Nelson Ice Cap, Antarctica. The grain size distributions of the proglacial sediments are characteristic of glacigenic deposits, but very different from those of aeolian and lacustrine sediments. Moreover, the grain size distributions of the proglacial sediments are fractal with a dimension of about 2.9, and the fractal dimensions can be used as another summary statistical parameter for quantifying the relative amounts of coarse and fine materials. Correlations between the absolute element abundances of the proglacial sediments are very weak due to mineral partitioning and other effects of glacial processes, but correlations between the element/Rb ratios are statistically significant. This finding indicates that element/Rb ratios can be used to reduce or eliminate the effects of glacial processes, evaluate geochemical data and determine the sediment provenance in the for     

中国东部中新世风尘沉积的发现及其研究意义

黄土高原红粘土古气候与古环境研究取得巨大进展。已知最老的红粘土时代达到22 Ma，在亚洲晚新生代环境演变研究，特别是在指示亚洲内陆干旱荒漠化、高原隆升的环境效应等方面有重要意义。结合野外实地调查和室内粒度、磁化率、扫描电镜和地球化学实验分析结果，对南京附近灵岩山剖面的沉积特征与风尘沉积证据进行了探讨。在灵岩山剖面玄武岩下覆的松散沉积层中有厚度超过4 m的风尘沉积层，时代在12 Ma以前，为迄今中国东部发现的新生代最老的风尘沉积物，是中新世季风气候出现之后的产物，推测与秦安红土剖面记录的15～13 Ma快速强烈粉尘沉积阶段一致，并和南极冰盖15 Ma开始扩大造成的全球降温事件有关联。     

青藏高原东缘盐源盆地古近纪风成沙丘及其古地理意义*

盐源盆地始新世丽江组沉积时期发育了一套具大型高角度交错层理的红色砂岩,其确切成因对于厘定青藏高原东缘古近纪古地理格局、恢复东亚新生代干旱带与古气候演化过程具有重要意义。文中通过对盐源盆地始新统丽江组红色砂岩的沉积特征进行系统分析,证明该套砂岩的岩性、粒度分布、石英颗粒表面特征与沉积构造等明显不同于水成沉积,而与典型风成沙丘相应特征完全一致,表明其为风成沙丘沉积。结合古近纪研究区的古气候条件、大气环流样式与大地构造背景,笔者进一步推断盐源盆地始新世风成沙丘可能沉积于中国古近纪干旱带内的沙漠环境。根据目前相关研究资料,确认青藏高原东缘存在一个厚度稳定的古近纪风成沙丘富集带。     

Geochemistry of Quaternary sediments of the Jeongokri archaeological site,Korea: implications for provenance and palaeoenvironments during the Late Pleistocene

The geochemical characteristics of an approximately 6‐m‐long sediment core collected from the Jeongokri archaeological site (Hantan River, Korea) were examined to determine the provenance of the sediments and to reconstruct the palaeoenvironment of the study area during the Late Pleistocene (200–127 ka). The core sediments were subdivided into two parts based on grain size: an upper (0–380 cm depth) and lower part (>380 cm depth). The lower part was deposited primarily by fluvial processes during an interglacial period [marine isotope stage (MIS) 7] with warm and humid conditions. Conversely, the upper part was mainly deposited by aeolian processes during a cold and dry glacial period (MIS 6). Geochemical characteristics, represented by major, trace and rare earth element (REE) compositions, and textures of the Jeongokri core sediments show distinct differences between the upper and lower parts. The mineralogy and major, trace and REE compositions indicate that the influence of felsic source rocks (e.g. granite) was dominant in the lower sediments, which were derived from the Hantan River. In contrast, increased quartz content and the very fine and homogeneous grain size in the upper sediments indicate an aeolian origin. REE ratios and distribution patterns, and geochemical signatures indicate that these sediments were derived predominantly from Chinese loess deposits and partially from local sediments from the Korean Peninsula; the possible source area of the upper sediments is the northern part of the Yellow Sea basin. The Yellow Sea basin, having mixed geochemical signatures between Chinese loess and Korean‐derived sediments, was exposed because of low sea levels during MIS 6. The exposed Yellow Sea basin was located in the path of winter‐monsoon winds, which may have carried a great deal of airborne sediments from the basin to the Jeongokri area. Copyright © 2011 John Wiley & Sons, Ltd.     

吕梁山西麓红粘土和水成堆积物之间的关系

通过对吕梁山前（西）山西保德剖面和柳林剖面的野外考察,结合前人研究成果,研究了新近纪地层的组成及空间分布。新近纪地层自下而上可以分为三段:下段为水成砂砾石层,上段为风尘堆积的红粘土,中部则为二者的过渡带。柳林剖面的详细研究揭示了水成堆积物与红粘土的细部特征:粗粒的砾岩和砂岩常切穿红粘土层,而粒度与红粘土相近的泥岩则常与红粘土平行产出。野外调查结合区域资料分析表明新近系各组成成分空间分布具有明显的规律性:靠近吕梁山（保德剖面）下部水成堆积物粒度较粗且厚度较大,红粘土厚度相对较薄;背离吕梁山下部水成堆积物逐渐变细且厚度减薄（柳林剖面、府谷剖面）直至尖灭（佳县剖面）,与之伴生的是红粘土厚度逐渐增大;中间过渡带中水成堆积物与红粘土的比例与此相似,即区域上吕梁山前的水成堆积物呈楔状“侵入”于红粘土的中下部。红粘土与水成堆积物的关系的确立为更好的理解红粘土的成因、吕梁山的新生代隆升历史及恢复新近纪晚期的古气候演变具有重要意义。     

Climate‐controlled aggradation and cyclicity of continental loessic siliciclastic sediments in Asselian–Sakmarian cyclothems,Permian, Hugoton embayment,USA

Siliciclastic intervals in Lower Permian carbonate–siliciclastic cyclothems in western Kansas record climate control on facies progression, deposition and preservation. The 26 000 km² study area comprises seven marine‐continental (carbonate–siliciclastic) cyclothems caused by glacioeustasy. Core data and a three‐dimensional geological model provide a detailed view of the sub‐surface on a gently sloping ramp. Siliciclastic intervals in the cyclothems are fine‐grained red beds with extensive pedogenic features, indicating a continental origin. Bed geometry (sheet‐like deposits that thin to the east), lateral grading, grain size (very fine‐grained sand to silt) and grain angularity (sub‐angular to angular) suggest that the sediment is loess sourced from the west, probably the Ancestral Rocky Mountains. There is a repeated record of glacial‐cycle‐scale, climate‐controlled cyclicity within siliciclastic intervals that has not been recognized previously. Aeolian silt grain size coarsens upward towards the middle, then fines upward in each siliciclastic interval. When sea‐level was high (interglacial) and carbonate production flourished, aeolian sedimentation nearly ceased, suggesting increased vegetation and rainfall at the source. As sea‐level fell, fine‐grained siliciclastic sediments were deposited under relatively dry, but seasonally wet conditions on an exposed ramp. Laterally graded coarser grained siliciclastic sediments with diagnostic fabrics indicate drier conditions with seasonal rainfall during a continued relative fall in sea‐level. The coarsest siliciclastic sediments were deposited during the lowest sea‐level and driest conditions, but still with sufficient seasonal moisture to allow vegetative cover and bioturbation. Subsequent upward fining is correlated with sedimentological indications of wetter conditions during relative sea‐level rise. Unlike common sequence stratigraphic models that relate siliciclastic sediment accumulation to base‐level rise, continental deposits were preserved because plants and pedogenesis stabilized aeolian sediment. The aggradational landscape formed by this process had several metres of positive relief that reduced accommodation for overlying marine carbonate strata. Thus, this mechanism for continental siliciclastic aggradation has a significant effect on sequence stratigraphic architecture.     

Late Neogene loess deposition in southern Tarim Basin: tectonic and palaeoenvironmental implications

Uplift of the Tibetan Plateau during the late Cainozoic resulted in a thick apron of molassic sediments along the northern piedmonts of the Kunlun and Altyn Mountains in the southern Tarim Basin. Early Neogene sediments are characterised by sandstone, siltstone and red mudstone, representing floodplain to distal alluvial fan environments. The Early Pliocene Artux Formation consists of medium-grained sandstone and sandy mudstone with thin layers of fine pebbly gritstone. The Late Pliocene to Early Pleistocene Xiyu Formation is dominated by pebble to boulder conglomerate typical of alluvial fan debris flow deposits. Sedimentological investigation, together with grain size and chemical analyses of siltstone bands intercalated with sandstone and conglomerate in the Xiyu and Artux Formations, point to an aeolian origin, suggesting desertic conditions in the Tarim Basin by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau inferred from the lithofacies change from fine-grained mudstone and sandstone to coarse clasts. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.     

Clay record of climate change since the mid‐Pleistocene in Jiujiang,south China

To obtain a better understanding of climate change in south China in the Quaternary, a clay mineralogical study was undertaken on the red earth profile at Jiujiang, using X‐ray diffraction (XRD) and particle‐size distribution analysis methods. The XRD results showed that the clay minerals of the Jiujiang red earth were mainly mixed‐layer illite–smectite (I/S), illite, kaolinite and vermiculite, with trace amounts of mixed‐layer kaolinite–smectite (K/S). Changes in clay mineral composition displayed a trend of three‐stage evolution. The higher mixed‐layer I/S clays and kaolinite contents in the lower portion suggest extremely warm and humid climates over the period c. 700 to c. 350 ka ago. A gradual decrease in I/S clays and kaolinite reveals a gradual climate change from warm/humid to cool/dry during the period c. 350 to c. 130 ka ago. The higher illite and vermiculite contents indicate a relatively cool and dry climate during the period since c. 130 ka ago. The particle‐size distribution pattern of the upper section was similar to that of the Xiashu loess, while that of the middle to lower section was similar to those of fluvially reworked red earth. A rapid increase in the abundance of large grain‐size components at 2.6 m depth indicates an intensification of the winter monsoon and a cool and dry climate during the period, in good agreement with results from the clay mineral composition and homogeneous structure. The red earth sequences in south China could probably be used to test the response of tropical to subtropical regions to global climate changes.     

丰宁黄土-古土壤剖面常量元素地球化学特征

丰宁滦河上游的河谷地带有多处黄土堆积,由于该地区黄土分布零星、厚度较小,在以往的研究中却很少引起人们的关注,然而该地区黄土堆积是河北坝上地区环境演化和气候变化的重要地质记录.为了探讨该地区黄土的成因、物源以及化学风化强度,选取代表性的黄土-古土壤剖面77个样品进行了常量元素分析,并与已知典型风成堆积物的地球化学元素特征进行对比.结果表明:(1) 丰宁剖面的主要常量元素(SiO₂、Al₂O₃和Fe₂O₃)之和以及UCC(Upper Continental Crust)标准化曲线均与典型风成堆积物具有较好的相似性, 表明丰宁黄土和古土壤为风成成因;(2) 丰宁黄土处于初等化学风化阶段,古土壤则进入了中等化学风化阶段.与典型风成堆积物相比,化学风化强度序列为:宣城风成红土＞＞西峰红黏土、镇江下蜀土＞丰宁古土壤、洛川古土壤＞洛川黄土＞丰宁黄土;(3) 常量元素迁移特征表明丰宁黄土和古土壤的化学风化已完成了早期去Na、Ca阶段,并进入到了中期去K阶段;(4) 丰宁黄土、古土壤的K₂O/Al₂O₃和Fe₂O₃/Al₂O₃比值与洛川黄土、洛川古土壤、镇江下蜀土、西峰红黏土较为接近,表明这些风成堆积物可能具有相似的源区.而TiO₂/Al₂O₃比值小于其他风成堆积物, 指示丰宁剖面具有其他物源.      

Carbonate aeolianites of western Saurashtra,India: experimental decipherment of the depositional mechanisms

Granular carbonate deposits of Late Pleistocene to Early Holocene age, commonly referred to as ‘miliolite limestone’, occur in a linear belt, parallel to the southern coast of Saurashtra, India. In the present study area these carbonate deposits are found in select valleys between ridges and mounds of pyroclastic material present in the Deccan trap plateau. Two different depositional histories have been proposed for these sediments. The presence of marine bioclasts led to the postulation of a marine origin for these deposits. The second school of thought propounded redeposition of the coastal sediments by aeolian processes. Although a few features could not be explained by the proposed aeolian model, critical comparison of these two views favoured the aeolian origin. The mode of occurrence, lithological and structural attributes, and microscopic evidence presented here, also support a possible aeolian origin for these deposits. Experimental observation indicates that these carbonate aeolianites represent backflow deposits, which accumulated because of the flow separation caused by the presence of topographic highs. The conspicuous concave‐up geometry of the deposit conformed to the shape of the separation bulb. In view of the inferred depositional mechanism, the disposition of the deposits and the signature of the palaeoflow direction suggest that the carbonate particles were derived from the north‐western coast of Saurashtra by strong south‐easterly winds. Massive granular carbonates with outsized basement clasts appear to be the product of avalanching of granular material from the higher contours because of oversteepening of the primary deposit.