藏南定日地区Cenomanian/Turonian界线附近的生物古海洋事件

藏南定日地区在白垩纪中期发育一套浅灰色深灰色的以钙质页岩、泥灰岩及微晶灰岩为主的浅海陆棚相沉积,岩石中除含较高的粘土矿物及陆源石英矿物颗粒以外,还含有十分丰富的有孔虫化石。依据浮游有孔虫Helvetoglobotruncana praehelvetica的首次出现将C/T界线置于样品9922及9923之间,该界线位于Whiteinella archaeocretacea化石带之中。通过定量分析,该区有孔虫的丰度、分异度以及浮游与底栖有孔虫比率、具旋脊与不具旋脊有孔虫的比率等指标在剖面纵向上表现出3个明显的演化阶段,即Rotalipora cushmani带上部、W.archaeocretacea带及H.helvetica带下部,有孔虫动物群的变化特征完整地记录了C/T界线附近古海洋事件的全过程。该次事件中,碳稳定同位素δ¹³C值存在着明显的异常变化——正向偏移,元素地球化学U、Th及K的丰度也表现出明显的异常变化,其丰度值均比标准平均值偏高。所有这些特征均是在全球洋脊迅速扩张这一背景下海平面发生剧烈变化的结果。     

白垩纪黑色页岩与海水含氧量变化——以西藏南部为例

黑色页岩具有特殊的沉积学、古生物学和有机地球化学特征，主要表现在生物组合和丰度的变化、有机碳含量的增高与碳稳定同位素的偏移等方面。具有丰富有机质来源的高生产力表层水是黑色页岩中富有机碳沉积物形成与埋藏的先期条件。白垩纪中期黑色页岩在全球广泛分布，是Cenomanian/Turonian期界线事件的最重要标志。在西藏南部的C／T界线处，浮游有孔虫种一级的绝灭速率达34％，底栖有孔虫的绝灭速度达50％；有机碳含量偏高；δ^13C出现2.74‰的正向偏移；生物扰动几乎消失，微小钻孔黄铁矿化；有孔史房室充填有黄铁矿微球丛；诸多其他地球化学指标亦存在不同程度的异常，这些特征说明当时海洋不同深度水层处于不同强度的贫氧至缺氧状态，同时沉积的黑色页岩是缺氧环境中有机碳大量保存的结果。该现象发生的原因是白垩纪中期的海平面上升和气候变化的影响。西藏特提斯海古海洋条件的变化明显受全球和区域性海平面上升的影响，由此形成的白垩纪中期黑色页岩具有全球一致的特征。     

高分辨率碳氧同位素应用及西藏岗巴地区白垩纪中期Cenomanian-Turonian期碳同位素偏移

介绍了高分辨率碳氧同位素的应用成果;指出半远洋、远洋碳酸盐全岩高分辨率碳同位素高值正偏可作为甄别大洋缺氧事件的指示剂和分析古气候变化的指针,碳同位素偏移型式可为长期海平面变化和短期海平面波动提供参照对比的依据。西藏南部岗巴地区的高分辨率偏移曲线显示,碳同位素Cenomanian Tu ronian界线时期的正偏幅度达2.90‰,Turonian期长期持续负偏,并在M. sigali带中、下部呈现两个负偏凹陷区。这表明,该区白垩纪中期Cenomanian Turonian期存在可以在特提斯甚至全球类比的碳同位素变化趋势,表现为Cenomanian Turonian界线时期的缺氧事件、Turonian期的长期海平面下降趋势和Turonian晚期短暂气候变冷的响应。     

利用有孔虫氧同位素重建古海平面变化的研究进展

海平面变化重建结果是一个重要的气候变化指标,可以被用来与其他气候指示要素以及基于计算机模型的重建结果进行比较,从而可以更好地理解海平面变化以及与其气候系统其他要素之间的关系。从深海沉积物中的钙质有孔虫得到的氧同位素记录包含着全球大陆冰量的变化过程,因此可以用来重建地质历史时期海平面演化的历史。尽管诸多研究者利用海洋沉积物中有孔虫氧同位素比值作为基础数据重建了长时间尺度上连续的海平面变化记录,但是,在未来的海平面重建过程中,还需要与其他学科领域结合,并对海平面重建结果和其他气候指标进行对比和相互验证,才能获得可靠的海平面变化记录。     

上扬子石炭-二叠纪海相碳酸盐的锶同位素演化与全球海平面变化

地质历史中海水的锶同位素组成是时间的函数,全球海平面变化是其最主要的控制因素,上扬子地区石炭-二叠纪海相碳酸盐的锶同位素演化曲线与海平面变化曲线有着很好的一致性。锶同位素演化曲线说明:1)早石炭世是一个海水逐渐加深的全球海平面上升时期,锶同位素最小值所显示的最大海泛面的年龄为 34 2Ma,位于杜内阶和韦宪阶的界线上 ;2 )晚石炭世是一个全球海平面下降时期 ;3)整个二叠纪都是全球海平面上升时期,晚二叠世的海平面上升不仅幅度大,而且海水在短时间内迅速加深 ;4)晚二叠世具有古生代海相碳酸盐的锶同位素最小值,显示晚二叠世末的全球淹没事件,最大海泛面的年龄为 2 5 0Ma,正好在二叠 /三叠纪界线附近 ;5 )二叠 /三叠纪之交的全球生物绝灭事件可能与二叠世末的全球淹没事件有关。     

桂东北D/C界线沉积特征及对台盆相间演化的启示

台盆相间是桂东北泥盆纪和石炭纪沉积的重要特色,而D/C界线又是台盆相间沉积最突出的一个时段之一。文章回顾并评述了桂东北D/C界线的研究历程、相序划分、主要代表剖面,认为桂东北D/C界线可划分为碎屑岩相序和碳酸盐岩相序以及它们的过渡类型。通过对20条典型剖面的岩性组合特征对比,恢复了桂东北D/C界线海平面相对下降期的岩相古地理,结果显示桂东北在该时期主要存在柳州、桂林两大台地以及多个小型台地,台地间普遍发育快速相变的台沟、盆地沉积,石炭纪初期海水变浅,台地分布范围显著扩大,江南古陆供给增强,台盆相间格局迅速发展。为了进一步探讨桂东北D/C界线的演化过程和背后的控制因素,文章分析了广西整体的台盆相间演化的阶段性、同沉积构造作用以及海平面变化的影响,梳理了台盆相间演化过程中沉积环境分异与构造运动- 海平面的关系,认为在滇黔桂裂谷盆地在持续断陷发展的前提下,桂东北D/C界线沉积分异的主要原因可能是受冈瓦纳大陆冰川形成与消融导致的海平面异常波动,以及柳江运动引发的局部同沉积断陷强度差异变动影响,且构造和沉积要素在D/C界线的耦合作用达到最强,最终促进桂东北台盆相间格局在D/C界线达到鼎盛。在此模式下,可以将台盆相间的演化划分为4个阶段,早泥盆世中期—晚期为台盆相间格局的孕育期,早泥盆世晚期—中泥盆世晚期沉积开始分异,为台盆相间格局的发展期,晩泥盆世—早石炭世台- 盆出现强烈分割,海平面异常下降和柳江运动同时作用于沉积,为台盆相间格局的鼎盛期,早石炭世之后,台盆相间沉积维持了一定时期的强度,晚石炭世开始海水逐步恢复正常后台盆相间逐渐减弱,为台盆相间格局的衰退期。     

上扬子石炭—二叠纪海相碳酸盐的锶 同位素演化与全球海平面变化

地质历史中海水的锶同位素组成是时间的函数，全球海平面变化是其最主要的控制因素，上扬子地区石炭—二叠纪海相碳酸盐的锶同位素演化曲线与海平面变化曲线有着很好的一致性。锶同位素演化曲线说明:1）早石炭世是一个海水逐渐加深的全球海平面上升时期，锶同位素最小值所显示的最大海泛面的年龄为342 Ma，位于杜内阶和韦宪阶的界线上；2）晚石炭世是一个全球海平面下降时期；3）整个二叠纪都是全球海平面上升时期，晚二叠世的海平面上升不仅幅度大，而且海水在短时间内迅速加深；4）晚二叠世具有古生代海相碳酸盐的锶同位素最小值，显示晚二叠世末的全球淹没事件，最大海泛面的年龄为250 Ma，正好在二叠/ 三叠纪界线附近；5）二叠/ 三叠纪之交的全球生物绝灭事件可能与二叠世末的全球淹没事件有关。     

中国东北地区白垩纪温室时期陆相生物群与气候环境演化

地球历史中生命与环境不断发生更替,先后形成寒武纪生态系统、古生代生态系统和现代生态系统。地域上,又分为海洋生态系统和陆地生态系统。中生代以来,中国以陆地生态系统为特色。东北地区发育以松辽、嘉荫盆地为主的大型白垩纪沉积盆地,以及冀北、辽西和内蒙古东部盛产热河生物群的火山-沉积盆地。白垩纪先后发育有热河生物群、阜新生物群、松花江生物群和嘉荫生物群。该地区白垩系是建立中国陆相地层格架的标准,依据不同生物群发育及气候演化阶段,自下而上被划分为下白垩统冀北阶、热河阶和辽西阶,上白垩统农安阶、松花江阶和绥化阶。基于松科1井多重地层学研究,完善了包括磁性地层、天文地层、同位素年代地层、生物地层和定量地层学为主的中国晚白垩世陆相年代地层的标准,还使其达到与国际地层表的对比。通过生物群的宏观演化,探讨了中国东北地区白垩纪环境与气候的演化规律,明确了晚白垩世的2次重大湖泊事件。其中,湖泊咸化问题得到深入研究,从古生物学和地球化学角度为松辽湖盆的海侵活动提供了可靠证据。化石与地化记录揭示,当时全球高海平面的影响可能波及松辽盆地,海水短暂地侵入湖盆,带入海相生物,引发海水咸化、δ~(13) Corg异常和有机质大量埋藏。这些记录可与全球高海平面地层对比。嫩江组下部海侵印记最为明显,其背景与第3期大洋缺氧事件(OAE 3)极为相近。松辽盆地的环境演化既受制于区域性构造作用,也受控于全球气候的影响,更代表了白垩纪温室时期的东亚陆地表现。     

西藏岗巴晚白垩世—古近纪早期锶同位素地层

在地质历史时期中,海相碳酸盐的锶同位素组成具有全球基本一致的演化规律,并且海水锶同位素组成受锶的来源控制,其比值与海平面的变化有内在的联系。西藏岗巴地区晚白垩世—古近纪早期地层发育良好,笔者对宗山、宗浦两个剖面中未经蚀变的26个海相碳酸盐样品进行锶同位素测试,根据得出的数据绘制演化曲线与全球锶同位素曲线有较好的可对比性。曲线的负偏极值与地层界线较为一致,宗山剖面曲线的负向峰值（0.707612）与上/下白垩统界线相对应,宗浦剖面中,锶同位素曲线的负偏值（0.707761）与古新统/始新统界线相对应,晚白垩世—古近纪锶同位素演化曲线也与海平面变化相一致。另外,还讨论了岗巴地区锶同位素变化的原因。     

西藏岗巴晚白垩世有孔虫动物群与海平面变化

西藏南部岗巴地区上白垩统的有孔虫动物群十分丰富，经鉴定共有52属130种。根据岩性，该地区上白垩统可以划分出三个最基本的岩石地层单位：冷青热组、岗巴村口组及宗山组。依据其中有孔虫的形态结构、生态环境、系统演化及地史分布等特征，可以划分出五个主要的有孔虫动物群，Rotalipora动物群，Whiteinella-Helvetoglobotruncana动物群，Dicarinella-Marginotruncana动物群，Globotruncana-Globotuncanita动物群及Orbitoides-Omphalocyclus动物群。这五个有孔虫动物在晚白垩世相继绝灭，其绝灭与古水深有着十分密切的关系，主要是受海平面变化的影响。     

Variation on Foraminiferal Composition in Cretaceous Black-Gray-Red Bed Sequence of Southern Tibet, China

An Upper Cretaceous black-gray-red bed sequence was deposited in the Tethys-Himalayan Sea where abundant foraminifera, especially planktons, were yielded. In the shallow shelf to the upper slope on the north margin of Indian plate was recorded an extinction-recovery-radiation cycle of foraminiferal fauna highly sensitive to paleoceanographical changes. The black unit, consisting of the Late Cenomanian-earliest Turonian beds, displays a major extinction, with keeled planktonic and many benthic species as the principal victims at the end of the Cenomanian when existed only low diversity, sin-face water-dwelling foraminifera. The gray unit spans a long-term recovery interval from the Turonian to the early Santonian with keeled planktonic foraminifera returning stepwise to the water colunm. The planktonic biota in the red unit, extremely abundant, indicate a biotic radiation during the Late Santonian and the Early Campanian, implying that the high oxygen levels had returned to all the oceanic depth levels,and that the water stratification disappeared, followed by the radiation of all depth-dwellers. The variation on foraminiferal faunas from the whole sequence refers to the extreme warm climate that appeared in the Middle Cretaceous and to the declined temperature toward the late epoch. Substantial deposits for this warming and cooling zones represent the black shales in the Middle Cretaceous and the red beds in the later period of the southern Tibet. The change in the foraminiferal composition corresponded to the formation of dysaerobic facies and to the development of high-oxidized circumstances.     

西藏特提斯喜马拉雅海相白垩纪—古近纪生物地层与重大地质事件研究进展

地质历史时期曾发过许多对生命的演化进程造成过重大影响与制约的全球性地质事件,白垩纪—古近纪就是一个重大地质事件频发的时期。随着冈瓦纳大陆在中生代时期的解体,全球海陆格局发生了根本的变化,地球的表层和岩石圈层均发生了重大的改变,由此引发了构造运动空前活跃,发生过诸如大洋缺氧事件(OAE)、大洋富氧事件(CORBs)、白垩纪/古近纪(K/Pg)生物大灭绝事件、古新世—始新世极热(PETM)事件、印度-亚洲板块碰撞、新特提斯洋的演化及最终消亡等一系列的全球性重大地质事件。对这些重大地质事件的研究,有助于加深我们对古海洋、古地理、古环境的认识。尝试追踪和捕捉这些重大地质事件,恢复和重建古地理,其基础是建立精确的年代地层格架。西藏南部保存了中国最为完整的海相白垩纪—古近纪沉积,完整地记录了上述的全球性重大地质事件,通过对札达、岗巴、定日、亚东、江孜、萨嘎和吉隆等地区高分辨率浮游和底栖有孔虫、介形虫、钙质超微化石和放射虫生物地层学研究,可直接约束全球性重大地质事件发生的时间,并为重建新特提斯洋古海洋环境和古地理提供证据。此外,在重大地质环境突变期间生物的演化过程,也可为探明极端环境变化发生时期气候-环境-生物之间的协同演化关系提供证据。本文系统总结了课题组为主的近年来对藏南白垩纪—古近纪海相地层中微体古生物学的研究成果及重要进展,并对未来研究提出展望。     

Integrated foraminifera and δ<Superscript>13</Superscript>C stratigraphy across the Cenomanian–Turonian event interval in the eastern Baltic (Lithuania)

The Cenomanian–Turonian transition marks one of the most important extinction episodes of the Mesozoic era. This extinction event was associated with the development of widespread oceanic anoxia and pronounced stable carbon isotopic excursion. Despite its importance, the effects of the perturbation on higher latitude biotas, and from the Baltic region in particular, are currently underexplored. Therefore, in this contribution we present the fossil record of a foraminifera succession integrated with δ¹³C trends from two deep cores: Bliūdsukiai-19 from western Lithuania and Balta?i?k?-267 from southern Lithuania. Two foraminiferal zones were distinguished: Rotalipora cushmani from the upper Cenomanian and Whiteinella archaeocretacea from the boundary strata between the Cenomanian and Turonian in the Balta?i?k?-267 core section, and a W. archaeocretacea Zone in the Bliūdsukiai-19 core section. A chemostratigraphical analysis of the stable carbon isotopes revealed a positive Cenomanian–Turonian δ¹³C anomaly, with maximum values reaching 3.57‰ in the upper part of the Bliūdsukiai-19 core section. A non-metric multidimensional scaling analysis of the foraminifera communities revealed that the major changes in their assemblages were strongly temporally organized and associated with the changes in the stable carbon isotopic ratios. This fact points to the significant effects of the C–T extinction event on the northern Neotethys paleocommunities.     

Lithostratigraphy and depositional environments of the upper cretaceous deposits in the central Taurides (S Turkey)

The study area is located in the Central Taurides (southern Turkey), which is bounded by the K?rkkavak fault to the west and Ecemi? fault to the east. The sequences are studied in detail based on measured sections composed of the rocks deposited during the Cenomanian–Maastrichtian and located within different tectonic units previously described in the Taurides. The study materials include 217 thin section data from seven Cenomanian–Maastrichtian sequences of outcropping in different parts of the Central Taurides. The sediments deposited during the Cenomanian–Maastrichtian period in the Central Taurides are subdivided into eight units based on their lithological, paleontological, and textural properties. The lower boundaries of the upper Santonian and Campanian are unconformable contacts. The Upper Cretaceous sequence starts with the middle Cenomanian and represents a continuation of the Lower Cretaceous tidal flat and shelf lagoon sequence. Upper Turonian–Coniacian sediments are not observed due to the eustatic sea level drop. The second main transgression period of the Upper Cretaceous platform took place in the Santonian. This unit is represented by limestones composed of wackestones/packstones containing benthic foraminifera and rudist fragments, which are deposited in tidal flats and subtidal environments. The late Campanian starts with a transgression, and the environment transformed transitions into slope facies from inner platform facies, as a result of the thrust of ophiolitic rocks. In the following period, slope front and basin plain environments were dominant due to the increasing slope. Slumped pelagic limestones were deposited on the slope. Planktonic foraminiferal pelagic limestones were unconformably deposited on plaque limestone in the slope front environment depending on the increase in slope gradient and local faulting. As a result of decreasing tectonic activity, the sediments were deposited onto a stable basin plain. They were initially fed from the nearby carbonate platform and then by siliciclastic turbidites derived from the thrusted ophiolitic rocks. In this study, the lithostratigraphic properties of the Cenomanian–Maastrichtian units outcropping in various parts of the Central Taurides are described. The sedimentary deposits described here suggest different basinal conditions in the region.     

Yaxcopoil-1 and the Chicxulub impact

CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the Chicxulub crater. An organic-rich marly limestone near the base of the hole (1,495 to 1,452 m) was deposited in an open marine shelf environment during the latest Cenomanian (uppermost Rotalipora cushmani zone). The overlying sequence of limestones, dolomites and anhydrites (1,495 to 894 m) indicates deposition in various carbonate platform environments (e.g., sabkhas, lagoons). A 100-m-thick suevite breccia (894–794 m) identifies the Chicxulub impact event. Above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of Plummerita hantkeninoides zone CF1, which spans the last 300 ky of the Maastrichtian. An erosional surface 50 cm above the breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones above this contact contain the first Tertiary planktic foraminifera indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm upsection separates zone P1a(2) and hemipelagic limestones of planktic foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc to P3b age are present from a depth of 794.04 up to 775 m. The Cretaceous carbonate sequence appears to be autochthonous, with a stratigraphic sequence comparable to late Cretaceous sediments known from outside the Chicxulub crater in northern and southern Yucatan, including the late Cenomanian organic-rich marly limestone. There is no evidence that these sediments represent crater infill due to megablocks sliding into the crater, such as major disruption of sediments, chaotic changes in lithology, overturned or deep dipping megablocks, major mechanical fragmentation, shock or thermal alteration, or ductile deformation. Breccia units that are intercalated in the carbonate platform sequence are intraformational in origin (e.g., dissolution of evaporites) and dykes are rare. Major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center. Yaxcopoil-1 may be located outside the collapsed transient crater cavity, either on the upper end of an elevated and tilted horst of the terrace zone, or even outside the annular crater cavity. The Chicxulub site thus records a large impact that predates the K/T boundary impact and mass extinction.     

弱海相性有孔虫群的特征及其代表的沉积环境

本文针对我国第四纪内陆有孔虫化石群的出现及其存在问题,提出有没有非海相有孔虫以及第四纪内陆有孔虫化石群是否都与海水入侵有关等问题。作者从我国现代海岸带有孔虫群调查着手,结合国外资料,说明弱海相性有孔虫群的分布及特征,分析我国第四纪内陆有孔虫化石群的生态环境,指出弱海相性有孔虫群具有一定的指相意义,是一类非正常的海相有孔虫群,不仅存在于海岸带,也可出现在内陆环境,出现有孔虫群不一定证明有海水入侵,只要有适宜的水化学条件也可出现。推论第四纪内陆有孔虫群有的与海水有关,有的可能无关。     

Broeckina gassoensis sp. nov., a larger foraminiferal index fossil for the middle Coniacian shallow-water deposits of the Pyrenean Basin (NE Spain)

The Upper Cretaceous shallow-water carbonates of the Pyrenean Basin (NE Spain) host rich and diverse larger foraminiferal associations which witness the recovery of this group of protozoans after the dramatic extinction of the Cenomanian–Turonian boundary interval. In this paper a new, large discoidal porcelaneous foraminifer, Broeckina gassoensis sp. nov., is described from the middle Coniacian shallow-water deposits of the Collada Gassó Formation, in the Bóixols Thrust Sheet. This is the first complex porcelaneous larger foraminifer of the Late Cretaceous global community maturation cycle recorded in the Pyrenean bioprovince. It differs from the late Santonian–early Campanian B. dufrenoyi for its smaller size in A and B generations and the less developed endoskeleton, which shows short septula. Broeckina gassoensis sp. nov. has been widely employed as a stratigraphic marker in the regional geological literature, under the name of “Broeckina”, but its age was so far controversial. Its middle Coniacian age (lowermost part of the Peroniceras tridorsatum ammonite zone), established in this paper by strontium isotope stratigraphy, indicates that it took about 5 My after the Cenomanian–Turonian boundary crisis to re-evolve the complex test architecture of larger foraminifera, which is functional to their relation with photosymbiotic algae and K-strategy.     

Repeated anoxia–extinction episodes progressing from slope to shelf during the latest Cenomanian

Oceanic Anoxic Event 2 (OAE 2) during the Cenomanian–Turonian (C/T) transition caused stepwise marine extinctions. Using organic compounds, stable carbon and oxygen isotopes, and foraminifera from three depth-transect sections in northern Spain, this study revealed repeated anoxic/euxinic events coinciding with warming and stepwise extinctions of planktonic and/or benthic foraminifera within intermediate to surface waters in the proto-North Atlantic during the C/T transition. Those short-duration euxinic events occurred four times: at 93.95 Ma, marked by the extinction of Rotalipora greenhornensis; at 93.90 Ma, marked by the extinction of Rotalipora cushmani; at the mid-time maximum of the plateau of the δ¹³C of carbonates (93.70 Ma); and at the time of the C/T boundary (93.55 Ma). Furthermore, the main benthic foraminiferal extinctions occurred during the first and second euxinic events in the upper slope, during the second and third euxinic events in the outer to middle shelf, and during the third and fourth events in the middle shelf. The main euxinic events in each section also showed a progression to the shallow shelf. The main anoxia–extinction events occurred in the upper slope and outer shelf then moved to the middle shelf. The shallowest section had relatively weak anoxia and a proportionally low extinction rate. These new findings indicate that foraminiferal extinctions started from the intermediate water and the continental slope and then moved to the continental shelf. This was the result of the repeated progression of euxinic–anoxic water from the upper slope to the middle shelf on the eastern continental margin of the proto-North Atlantic four times during a 400 kyr period, to the end of the Cenomanian.     

Stratigraphy of the Cenomanian–Turonian Oceanic Anoxic Event OAE2 in shallow shelf sequences of NE Egypt

Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ¹³C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ¹³C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ¹³C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.