贵州威宁麻窝山岩溶盆地沉积物中粘土矿物特征及其古气候指示意义

利用X射线衍射等分析方法,对贵州威宁麻窝山岩溶盆地沉积物中粘土矿物进行研究,获得了粘土矿物的组成、相对含量、结晶度指数、化学指数以及相对含量比值等方面的信息,探讨该盆地的沉积环境演化特征。研究表明,粘土矿物以伊/蒙混层矿物(30%~75%)和绿泥石(10%~45%)为主,高岭石(5%~20%)和伊利石(10%)次之,基本不含蒙脱石。伊/蒙混层矿物和绿泥石含量呈负相关性。高岭石含量在剖面底部较稳定,在484cm上部出现较大的波动。伊利石结晶度和化学指数与伊/蒙混层矿物含量具有较好的正相关性,且在第Ⅰ阶段相对于其它指标较敏感,出现波动。通过综合对比分析,将研究区剖面划分为4个阶段,其是3个冷暖干湿的大循环和若干个小循环气候环境的物质记录。     

临夏盆地晚更新世沉积物粘土矿物的特征及其古气候指示

利用X射线衍射、扫描电子分析方法,对临夏地区塔湾黄土沉积物的粘土矿物学特征及其古气候指示作用进行了深入研究。结果表明：晚更新世的黄土沉积物风成黄土或经过表生风化改造形成的古土壤,其粘土矿物组成主要为伊利石、绿泥石、伊/蒙混层粘土矿物、高岭石和少量的坡缕石,沉积物中高岭石、伊利石/蒙脱石、伊利石和绿泥石矿物组合,指示物源区经历了气候从湿润变为冷干的转变。沉积物中伊利石结晶度的测量结果显示,其（001）衍射峰的半高宽在0.23°~0.86°之间,不同岩性层中伊利石结晶度存在明显的差别,伊利石结晶度存在着两个明显的峰值区,这表明临夏地区晚更新世的气候变化经历了从湿润→冷干→湿润→冷干的两个周期性的演化,而且晚期与早期相比更加湿润同时又更为冷干。沉积物中既有自生成因的坡缕石,又有碎屑成因的坡缕石。沉积物中自生成因坡缕石和碎屑成因坡缕石均稳定存在的现象表明,本区晚更新世的古气候是以干旱为特征,年均降雨量在300 mm左右。     

Land snails from the last glacial maximum on Andikithira,southern Greece and their palaeoclimatic implications

An assemblage of land snails from an aeolianite deposit on the coast of the southern Greek island of Andikithira is shown to date to 16 000 yr BP and thus represents the period of the last glacial maximum (LGM; Oxygen Isotope Stage 2). The assemblage has no modern analogue. Five of the ten species are extinct on the island and some of these now live only at high elevations (> 950 m). Significantly cooler temperatures, some 5-8°C below present, and slightly drier moisture conditions (lower rainfall, partially offset by reduced evapotranspiration at the lower temperature) are inferred. The large temperature depression at the LGM, well documented in northern and central Europe, extended also to the Mediterranean climate of southern Europe. Late Quaternary climatic changes had a considerable impact on the fauna of this isolated island.     

南黄海南部坳陷下第三系阜宁组、戴南组泥岩的粘土矿物与热成熟度的关系

粘土矿物在埋藏成岩过程中的重要变化是膨胀型粘土逐渐减少(如蒙脱石),伊利石、绿泥石结晶度逐渐增加(半高宽减小)。通过对具有代表性的常6—1—1井粘土矿物X衍射分析,系统研究了伊利石10A峰、绿泥石7A峰的半高宽、A/H值(背影峰面积/峰高)与镜质体反射率R_0以及有机质热变指数LOM的对应关系。研究结果表明伊利石(特别经乙二醇处理过)的10A峰半高度及A/H值可以估算有机质成熟与否;可以作为预测源岩生油门限以及油气保存阶段的指标。     

红寺堡盆地晚更新世沉积特征及古气候背景

萨拉乌苏组、水洞沟组和马兰黄土是青藏高原东北缘弧形构造带乃至鄂尔多斯高原西部晚更新世发育的3套重要的沉积层系,但三者之间的沉积接触关系一直存在着争议。以红寺堡盆地为研究对象,在路线地质调查的基础上,结合砂岩石英颗粒表面扫描电镜分析及黏土矿物含量分析,查明3套地层之间的纵横向变化关系,确定红寺堡盆地晚更新世接受黄土沉积的最早时限及所处的古气候背景。研究认为：红寺堡盆地萨拉乌苏组与水洞沟组和马兰黄土之间是上下叠置关系,湖盆中心萨拉乌苏组上覆水洞沟组,湖盆边缘萨拉乌苏组上覆马兰黄土;水洞沟组与马兰黄土之间为同时异相的相变关系,在湖盆中心为湖相沉积,在湖盆边缘地形相对较高的部位,则为同时期的风成黄土沉积;红寺堡盆地接受黄土沉积的最早时限为萨拉乌苏组沉积末期,萨拉乌苏组顶部局部可见风成沉积的柱状节理,砂岩石英颗粒表面具有水成和风成兼顾的特征。红寺堡盆地晚更新世经历了温暖湿润-寒冷干旱-温暖湿润-寒冷干旱的气候变化过程,在萨拉乌苏组二段与水洞沟组中部,伊利石和绿泥石含量表现为相对低值,而蒙脱石和高岭石含量表现为相对高值,蒙脱石含量明显增加,但同时又有高岭石矿物的出现,总体上反映了温暖湿润的古气候特征,两期温暖湿润的环境分别对应于深海氧同位素曲线MIS3与MIS5阶段,具有全球对比意义。研究成果为鄂尔多斯高原西部生态环境的演化及综合治理提供了基础地质依据。     

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.     

辽南晚更新世风成沉积序列与古气候意义

作为中国东部重要的第四纪风成堆积地层,辽南风成沉积反映的气候振荡记录对于揭示区域古气候变化、东亚季风变迁等有着不言而喻的重要性。本研究利用辽南老虎村5.44 m厚的风成沉积剖面作为研究对象,采用光释光（OSL）测年技术建立了研究区沉积地层的年代框架,通过岩石学和代用指标重建了晚更新世期间的风成沉积历史。结果表明：风成活动发生在71.7~61.7 ka,49.4~42.4 ka和约23.0 ka,分别对应深海氧同位素阶段（MIS）5a~4、3b和2,寒冷干燥的气候条件与增强的东亚冬季风促使风成活动加剧;而MIS 3a和MIS 3c的沉积中断可能受到了暖湿气候的影响。研究区的粒度数据表明在MIS5/4转变时期有一个短暂的冬季风波动,同时,中值粒径增大可能与海因里希事件（H6、H5、H2）有关,未来仍需更多的区域气候纪录来支持这一观点。

     

山西太谷断裂南段晚更新世以来的活动特征

太谷断裂为太原盆地的边界断裂,是晋中盆地研究程度较低的一条断裂。根据野外地质调查,断裂展布在黄土台地前缘,断错了盆地的洪积扇及河流阶地,是一条全新世活动的正断层。在上曹麻和北甲开挖了3个探槽,根据地层基本单元和年代学测试结果,探槽揭示了3次古地震事件,分别为8ka之后、11ka之后和17ka年之后。全新世垂直滑动速率为0.09mm/a;晚更新世以来的平均滑动速率为0.16~0.19mm/a。完善了太谷断裂的研究,为晋中盆地防震减灾提供可靠的科学依据。     

Depositional system and palaeoclimatic interpretations of Middle to Late Pleistocene travertines: Kocabaş, Denizli,south‐west Turkey

Travertine deposits in western Turkey are very well‐exposed in the area of Kocaba?, in the eastern part of the Denizli Basin. The palaeoclimatic significance of these travertines is discussed using U/Th dates, stable isotope data and palynological evidence. The Kocaba? travertine occurrences are characterized by successions of depositional terraces associated with palaeosols and karstic features. The travertines have been classified into eight lithotypes and one erosional horizon, namely: laminated, coated bubble, reed, paper‐thin raft, intraclasts, micritic travertine with gastropods, extra‐formational pebbles and a palaeosol layer. The analysed travertines mostly formed between 181 ka and 80 ka (Middle to Late Pleistocene) during a series of climatic changes including glacial and interglacial intervals; their δ¹³C and δ¹⁸O values indicate that the depositional waters were mainly of basinal thermal origin, occasionally mixed with surficial meteoric water. Palynological results obtained from the palaeosols showed an abundance of non‐arboreal percentage and xerophytic plants (Oleaceae and Quercus evergreen type) indicating that a drought occurred. Marine Isotope Stage 6 is represented by grassland species but Marine Isotope Stage 5 is represented by Pinaceae–Pinus and Abies, Quercus and Oleaceae. Uranium/thorium analyses of the Kocaba? travertines show that deposition began in Marine Isotope Stage 6 (glacial) and continued to Marine Isotope Stage 4 (glacial), but mostly occurred in Marine Isotope Stage 5 (interglacial). The travertine deposition continued to ca 80 ka in the south‐west of the study area, in one particular depression depositional system. Palaeoenvironmental indicators suggest that the travertine depositional evolution was probably controlled by fault‐related movements that influenced groundwater flow. Good correlation of the stable isotope values and dates of deposition of the travertines and palynological data of palaeosols in the Kocaba? travertines serve as a starting point for further palaeoclimate studies in south‐west Turkey. Additionally, the study can be compared with other regional palaeoclimate archives.     

川东华蓥山二叠系/三叠系界线附近粘土层中粘土矿物的类型及成因

对川东华蓥山二叠系/三叠系界线附近粘土层中的粘土矿物进行野外采样和室内X射线衍射分析,结果表明,这些粘土矿物主要由规则混层伊利石-蒙脱石组成,其中1个样品可能含有少量不规则混层绿泥石-蒙脱石等粘土矿物。总地看来,该界线粘土层中的规则混层伊利石－蒙脱石中伊利石层的含量自下而上呈现减少的趋势,平均含量约83%,所经历的最大古地温在125～150℃之间。这些粘土矿物的构成和特征与前人在华南其他地区相同层位取得的成果总体上具有较好的一致性,显示了该粘土层成因的相同性及所具有的等时意义。不同地区粘土矿物构成的差异性（如高岭石的存在与含量）可能与二叠系/三叠系界线不整合的存在与否或间断时间有关,规则混层伊利石－蒙脱石的混层比则与埋藏历史和沉积—沉积期后的流体介质有关。     

青藏高原东北缘循化盆地中中新世—早上新世黏土矿物及其古气候意义

青藏高原东北缘古气候可能受控于全球变冷、青藏高原隆升及局地地形变化的影响。为解析气候演化过程及驱动因素,本文以青藏高原东北缘循化盆地西沟剖面作为研究对象,在已有古地磁年龄约束基础上,分析了中中新世—早上新世沉积物中黏土矿物的组成和微观形貌特征。结果表明,西沟剖面沉积物中黏土矿物主要由伊利石、蒙脱石、绿泥石和高岭石组成,其中伊利石含量最高,平均为59. 3％;蒙脱石次之,平均为18. 2％,绿泥石平均含量为12. 3％,高岭石平均含量为10. 2％。根据剖面中黏土矿物含量和比值的变化特征,结合循化盆地西沟剖面的沉积速率、孢粉记录、有机质碳同位素和沉积岩地球化学比值,并与深海氧同位素值（δ18O）变化曲线对比,将循化盆地14. 6~5. 0 Ma气候环境演化划分为3个阶段：14. 6~12. 7 Ma,气候干冷期,与北半球冰盖扩展引发的全球性降温事件有关;12. 7~8. 0 Ma,气候相对温暖湿润期,可能与循化盆地周围山体隆升有关,即积石山在~12. 7 Ma隆升至临界高度,成为西风带输送水汽的地形屏障,使得循化盆地内的降水增强;8. 0~5. 0 Ma,气候再次转向干冷期,该阶段气候的干旱化对应于青藏高原在8 Ma左右的快速隆升,高原进一步的隆升阻碍东亚季风西风带的暖湿气流向内陆的输送,从而引起区域干旱化。     

The Late Pleistocene clastic deposits in the Romito Cave,southern Italy: a proxy record of environmental changes and human presence

Clastic sediments deposited in caves and rock shelters bear peculiar sedimentological characteristics and have seldom been considered as a high‐resolution proxy record of climatic or environmental changes. The Romito Cave has its entrance at 275 m above sea level, about 25 km from the Tyrrhenian coast of Calabria, southern Italy. New archaeological excavation performed since 2000 has revealed a sedimentary succession spanning the record of Gravettian to Late Epigravettian cultures (Late Pleistocene). The present study focuses on the lower part (2.5 m thick) of the succession, where three main unconformity‐bounded stratigraphic units have been recognised (labelled RM1–3). Each unit consists of water‐lain deposits indicating high‐ to low‐competence flow, capped with anthropogenic deposits. The gradual deactivation and reactivation of the water drainage between 23 475 ± 190 and 16 250 ± 500 cal. a BP is correlated with regional precipitation changes due to the onset of dry climatic conditions of the Last Glacial Maximum. However, the deactivation of cave drainage after the deposition of unit RM3, around 15 400 ± 500 cal. a BP, deviates from the regional hydrological trend of progressively increasing water discharges and is attributed to the drainage cut‐off by probable cave wall collapses. Copyright © 2008 John Wiley & Sons, Ltd.     

Geology,mineralogy, and geochemistry of low grade Ni-lateritic soil (Oman Mountains,Oman)

The laterite profiles investigated in the present study developed after the emplacement of a slab of oceanic crust and upper mantle sequence (the Semail ophiolite) onto the East Arabian margin during the late Cretaceous. The laterites formed as a result of prolonged weathering of the ophiolite assemblage under tropical to subtropical conditions.Nine laterite profiles have been examined for their Ni potential along a NW-SE segment of the Oman Mountains. The preserved laterite profile shows variations in thickness, mineralogy, and chemical composition. The profiles show a vertical succession from bedrock protolith through saprolite, oxide laterite, to clay laterite. The laterite profiles are unconformably capped either by clastics rocks of the Late Cretaceous Qahlah Formation or by Palaeogene carbonates of the Jafnayn or Abat Formations.The protolith corresponds either to a fine-grained, blackish to greenish serpentinized peridotite or to a coarse-grained dark green altered layered gabbro.The bulk geochemistry of the studied profiles indicates a typical low Ni-laterite pattern in which magnesium (Mg) and silica (Si) become depleted towards the top of the profile, whereas iron (Fe) and aluminum (Al) increase. A significant enrichment in Ni and Co occurred as a result of the laterization process. Ni concentrations average 0.63% (Ibra), 0.72% (East Ibra), 0.67% (Al-Russayl), and 0.33% (Tiwi). Other elements such as Cr, V, Pb, TiO₂, Zr, Ba, and Zn were also remobilized across the profiles during the laterization processes.     

东海外陆架晚更新世沉积物中的有用重矿物及其资源潜力

对东海外陆架中部表层沉积物34个样品,南部两个岩心75个样品的沉积物特征和重矿物特征进行了分析,计算了其中有用重矿物砂的品位。东海外陆架晚更新世残留沉积中有用的重矿物砂来源于中国大陆,其含量与细砂含量、重矿物总量、沉积物的粒度和分选性呈正相关关系。重矿物砂赋存在海退层位中,形成于滨岸带,并经过后期海侵过程的改造和进一步富集,主要分布在水深100～200 m范围内。外陆架中部有用重矿物砂高品位异常区面积达1.2×10.4 km锆石和石榴石品位达到Ⅰ级异常。南部岩心中异常品位的有用重矿物砂的厚度1.5～2 m,锆石品位达到工业边界品位,研究区钛铁矿普遍出现Ⅱ级和Ⅱ级以上的异常品位。东海外陆架区浅海砂矿成矿条件较好,具有较大的资源潜力,值得进一步调查研究。     

西藏阿里札达盆地上新世-早更新世的古植被、古环境与古气候演化

本文将西藏札达盆地河湖相地层重新划分为第四系下更新统香孜组(Qp1-1x)、新近系上新统古格组(N22g)和上新统托林组(N21t)。河湖相地层的古地磁法和ESR法测年结果表明,札达盆地内河湖相沉积地层的形成时代为新近纪上新世—第四纪早更新世。根据该套河湖相地层沉积演化和其中的孢粉组合特征、河湖相沉积中所发现的各种古动植物化石等的综合分析,笔者对札达盆地上新世—早更新世的古植被、古环境与古气候演变进行了探讨。结果表明,札达地区上新世—早更新世气候经历了从湿热—温暖潮湿—偏冷潮湿—寒冷干旱的变化,以及植被从森林—灌木—草原的逐渐演化。可将札达盆地上新世—早更新世环境演化划分为7个大的阶段,其总体特征是15.4~4.4Ma,札达地区处于亚热带湿热气候环境;24.4~3.95Ma,为暖温带温暖潮湿气候;33.95~3.5Ma,为偏凉潮湿阶段,气候开始转冷;43.5~3.2Ma,为温暖潮湿阶段;53.2~2.9Ma,气候转为偏冷潮湿阶段;62.9~2.57Ma,该阶段气候偏冷而干旱,整体较为干冷;72.57~1.36Ma,气候寒冷而干旱。表明自上新世—早更新世,该区的古气候环境在逐渐变干、变冷的总趋势上,经历了多次明显的冷暖与干湿波动。     

金沙江支流硕曲河中更新世红土层的发现及地质意义

金沙江支流硕曲河V级及以上阶地阶面和两级带状剥夷面上发现中更新世红土层。根据ESR、14C测年资料,结合地层层位、冰期和深海氧同位素阶段对比,将红土层形成时代确定为150～185ka,与康滇地区中更新世末期网纹化红土可以对比。拟建为吾斯顶红土层。红土层形成于海拔2000m以下湿热的河流宽谷环境,现今海拔为2890m以上,因此该区中更新世末期以来处于平均速率为5.9mm/a的快速隆升状态,气候与环境向着日趋恶化的方向演化     

Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ¹³C and δ¹⁸O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO₂ at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ¹³C and δ¹⁸O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ¹⁸O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ¹⁸O and highly variable δ¹³C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.     

长江中下游宣城水东地区早白垩世酸性火山岩年代学、地球化学及岩石成因

位于长江中下游的宣城水东地区发育一套酸性火山岩,主要由流纹质角砾岩、流纹岩和珍珠岩组成。本文对该套火山岩进行了详细的锆石U-Pb年代学、主量元素、微量元素以及Nd-Hf同位素研究。LA-ICP MS锆石U-Pb定年结果显示3种岩性的火山岩年龄分别为133.2±0.8、133.4±0.8和131.5±0.9 Ma。主量元素组成上,这套酸性火山岩具高硅(72.51%~81.79%)、富钾(K_2O/Na_2O=2.04~14.93,平均6.72)、贫钙镁(Ca O=0.19%~1.57%,Mg O=0.06%~0.29%)的特征,属于弱过铝质(A/CNK=1.02~1.24)的高钾钙碱性-钾玄岩系列岩石。微量元素方面,轻重稀土元素分馏明显[(La/Yb)_N=5.43~9.17],具明显的负铕异常(Eu/Eu~*=0.44~0.60),富集大离子亲石元素(LILE)Rb、Th、K和Pb等,亏损Ba、Sr、Nb、P和Ti等元素,表现出壳源的特征。全岩Sr-Nd和锆石Hf同位素组成变化范围相对较小,(~(87)Sr/~(86)Sr)_i为0.707 3~0.708 8,εNd(t)值为-7.05~-5.56,εHf(t)为-8.6~-1.3。结合区域地质研究成果,认为宣城水东地区酸性火山岩可能是在约135 Ma古太平洋板块俯冲作用之后的伸展-拉伸环境下,由新元古代早期新生地壳重熔而成。     

鲁中山地北麓黄土-古土壤粘土矿物组成及古环境意义

鲁中山地北麓的黄土沉积是我国东部平原区第四纪冰期-间冰期古环境变化的重要记录。目前,对这套沉积经历的风化成壤过程还没有开展系统的研究,限制了轨道尺度古环境变化信息的挖掘。本文采用X射线衍射分析方法,对鲁中山地北麓黑山剖面末次间冰期以来代表性的黄土L₁和古土壤S₁样品进行了粘土矿物测试分析。结果表明,黑山黄土沉积的粘土矿物组成以伊利石和蛭石（含层间羟基矿物,简称HIM）为主,二者含量在70.9%~82.2%之间,而高岭石、绿泥石和蒙脱石含量相对较少;与冰期黄土L₁相比,间冰期古土壤S₁含有较高的蛭石（+HIM）和高岭石,而伊利石、绿泥石和蒙脱石的含量相对较低,并且伊利石结晶度KI值和绿泥石风化指数（0.47/0.71 nm）显著增高。粘土矿物组合以及伊利石和绿泥石的化学风化特征表明古土壤经历了中等风化成壤作用,指示了暖温带湿润-半湿润的气候环境,夏季风显著增强。末次冰期黄土L₁中含有较多碎屑成因的蛭石（+HIM）,与已有黄土高原末次冰期黄土研究结果对比分析后,排除了鲁中山地北麓黄土与黄土高原黄土一样主要来自西北内陆荒漠区的可能,这进一步支持了鲁中山地北麓黄土主要来源于近源的华北平原各类沉积物的结论。

     

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin,Llanbedr (Mochras Farm) borehole,Wales

Early Jurassic climate is characterized by alternating cold and warm periods highlighted by studies based notably on oxygen isotopes measured on belemnite guards and other marine invertebrate shells. These climatic changes include changes in the hydrological cycle, and consequently weathering and runoff conditions. In order to clarify the erosion and weathering conditions during the Pliensbachian, this study determined the mineralogical composition of the clay fraction of 132 samples taken from the entire stage drilled in the Llanbedr (Mochras Farm) borehole (Cardigan Bay Basin). The clay mineral assemblages are composed of various proportions of chlorite, illite, illite/smectite mixed‐layers (R1 I–S), smectite and kaolinite, with possibly occasional traces of berthierine. The occurrence of abundant smectite indicates that the maximum burial temperature never exceeded 70°C. Consequently, clay minerals are considered mainly detrital, and their fluctuations likely reflect environmental changes. The variations in the proportions of smectite and kaolinite are opposite to each other. Kaolinite is particularly abundant at the base of the jamesoni Zone, in part coinciding with the δ¹³C negative excursion corresponding to the Sinemurian/Pliensbachian Boundary Event, and through the davoei Zone, whilst smectite is abundant in the upper part of jamesoni and base of ibex zones and through the subnodosus/gibbosus subzones of the margaritatus Zone. The kaolinite‐rich intervals reflect an intensification of hydrolysis and an acceleration of the hydrological cycle, while the smectite‐rich intervals indicate a more arid climate. The spinatum Zone is characterized by a distinct clay assemblage with abundant primary minerals, R1 I–S, kaolinite reworked from previously deposited sediments or from Palaeozoic rocks, and probably berthierine originating from contemporaneous ironstone‐generating environments of shallower waters. This mineralogical change by the end of the Pliensbachian likely reflects a transition from a dominant chemical weathering to a deeper physical erosion of the continent, probably related to a significant sea‐level fall consistent with a glacio‐eustatic origin.