Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco

With a multi-proxy approach, an attempt was made to constrain productivity and bottom-water redox conditions and their effects on the phosphorus accumulation rate at the Mohammed Plage section on the Tarfaya coast, Morocco, during the Cenomanian-Turonian Anoxic Event (OAE 2). A distinct δ¹³C_org isotope excursion of +2.5‰ occurs close to the top of the section. The unusually abrupt shift of the isotope excursion and disappearance of several planktonic foraminiferal species (e.g. Rotalipora cushmani and Rotalipora greenhornensis) in this level suggests a hiatus of between 40–60 kyrs at the excursion onset. Nevertheless, it was possible to determine both the long-term environmental history as well as the processes that took place immediately prior to and during OAE 2. TOC% values increase gradually from the base of the section to the top (from 2.5% to 10%). This is interpreted as the consequence of a long-term eustatic sea-level rise and subsidence causing the encroachment of less oxic waters into the Tarfaya Basin. Similarly a reduction in the mineralogically constructed ‘detrital index’ can be explained by the decrease in the continental flux of terrigenous material due to a relative sea-level rise. A speciation of phosphorus in the upper part of the section, which spans the start and mid-stages of OAE 2, shows overall higher abundances of P_reactive mass accumulation rates before the isotope excursion onset and lower values during the plateau. Due to the probable short hiatus, the onset of the decrease in phosphorus content relative to the isotope excursion is uncertain, although the excursion plateau already contains lower concentrations. The C_org/P_total and V/Al ratios suggest that this reduction was mostly likely caused by a decrease in the available bottom oxygen content (probably as a result of higher productivity) and a corresponding fall in the phosphorus retention ability of the sediment. Productivity appears to have remained high during the isotope plateau possibly due to a combination of ocean-surface fertilisation via increased aridity (increased K/Al and Ti/Al ratios) and/or higher dissolved inorganic phosphorus content in the water column as a result of the decrease in sediment P retention. The evidence for decreased P-burial has been observed in many other palaeoenvironments during OAE 2. Tarfaya's unique upwelling paleosituation provides strong evidence that the nutrient recycling was a global phenomenon and therefore a critical factor in starting and sustaining OAE 2.     

再论各向异性介质转换波地震学,第一部分:理论

我们业已研发了计算各向异性、非均质介质中P- SV转换波(C-波)的转换点和旅行时的新理论。据此 可以利用诸如相似性分析、迪克斯模型建模、克契 霍夫求和等常规方法来完成各向异性的处理和各向 异性处理,并使各向异性的处理成为可能。这里将 我们的新发展分作两部分来介绍。第一部分为理 论,第二部分为对速度分析和参数计算的应用。第 一部分理论包括转换点的计算和动校正的分析。     

Oceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco

Profound biotic changes accompanied the late Cenomanian δ¹³C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid δ¹³C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding δ¹³C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in diversity to just two species, the dominant (80–90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.     

Cenozoic sedimentary dynamics of the Ouarzazate foreland basin (Central High Atlas Mountains, Morocco)

Cenozoic continental sedimentary deposits of the Southern Atlas named "Imerhane Group" crop out (a) in the Ouarzazate foreland basin between the Precambrian basement of the Anti Atlas and the uplifted limestone dominated High Atlas, and (b) in the Aït Kandoula and Aït Seddrat nappes where Jurassic strata detached from the basement have been thrust southwards over the Ouarzazate Basin. New biostratigraphic and geochronological data constraining the final Eocene marine regression, the characterization of the new "Aït Ouglif Detrital Formation" presumed to be of Oligocene age, and the new stratigraphic division proposed for the Continental Imerhane Group clarify the major tectonogenetic alpidic movements of the Central High Atlas Range. Four continental formations are identified at regional scale. Their emplacement was governed principally by tectonic but also by eustatic controls. The Hadida and Aït Arbi formations (Upper Eocene) record the major Paleogene regression. They are composed of margino-littoral facies (coastal sabkhas and fluviatile systems) and reflect incipient erosion of the underlying strata and renewed fluvial drainage. The Aït Ouglif Formation (presumed Oligocene) had not been characterized before. It frequently overlies all earlier formations with an angular unconformity. It includes siliciclastic alluvial deposits and is composed predominantly of numerous thin fining-upward cycles. The Aït Kandoula Formation (Miocene-Pliocene) is discordant, extensive, and represents a thick coarsening-upward megasequence. It is composed of palustro-lacustrine deposits in a context of alluvial plain with localized sabkhas, giving way to alluvial fans and fluviatile environments. The Upper Conglomeratic Formation (Quaternary) is the trace of a vast conglomeratic pediment, forming an alluvial plain and terraces. The second and third formations correspond to two megasequences engendered by the uplift of the Central High Atlas in two major compressive phases during late Oligocene and Miocene-Pliocene times. These two geodynamic events were separated by a tectonically calm phase, materialized by palustro-lacustrine sedimentation (Görler et al. 1988). Tectono-sedimentary analysis of the two megasequences shows that the basin structure and depositional processes were controlled by the compressive tectonic context generated by the collision of North Africa and Iberia in Tertiary times (Jacobshagen et al. 1988). The Quaternary Formation was apparently controlled by a tectonic continuum and by climatic variations.     

Géodynamique et évolution thermique de la matière organique: exemple du bassin de Qasbat-Tadla, Maroc central

Seismic data analysis of the Qasbat-Tadla Basin allows the deciphering of the main tectonic and sedimentary events that characterised the Hercynian orogen and its role in the basin's structural development. The global tectono-sedimentary framework involves structural evolution of an orogenic foreland basin and was the source of rising geotherms in an epizonal metamorphic environment. The complementary effects of these parameters has led to different source rock maturity levels, ranging from oil producing to graphite domains. Different maturity levels result from three distinct structural domains within the basin, each of which exhibit characteristic geodynamic features (tectonic contraints, rate of subsidence, etc.).     

徐家围子断陷构造地质特征研究新进展

在徐家围子断陷深层连片三维地震精细解释的基础上,结合前人的研究成果,系统的刻画了深大断裂的性质、产状和时空分布规律。从区域构造应力场着手,详细剖析了徐家围子断陷的构造地质特征,重新构建了深大断裂体系。依据剖面特征和断陷的地质结构,首次在徐家围子断陷内解释出两条深大走滑断裂带,发现深大走滑断裂控制了营城组火山岩的形成和分布。明确了徐家围子断陷"两凹夹一隆、东西分带、南北分块"构造格局的动态演化过程及其控制因素。详细剖析了断陷期地层的沉积发育过程、后期改造过程以及现今赋存特征。为庆深气田火山岩储层分布规律预测奠定了基础。     

徐家围子断陷深层结构形成与演化的探讨

徐家围子断陷是松辽盆地北部深层最重要的含气断陷。本文根据最新地震和钻井资料综合研究成果,结合前人研究认识,探讨了徐家围子断陷的形成与演化,认为徐家围子断陷在区域构造位置上处于东北地区松辽盆地的北部,处于松辽盆地区域莫霍面隆起的西斜坡上,特定的位置决定了其形成演化的各个阶段受东北地区区域板块构造运动的控制,与松辽盆地的形成与演化具有一定的关系。徐家围子断陷形成与演化可以划分为5个阶段,是太平洋构造域板块间的相互作用和深部热力作用发育演化的结果,这种过程造就了现今南北分块、东西分带、凹隆相间、构造复杂和沉积岩与火山岩并存的地质结构。徐家围子断陷深层现今构造格局受控于近NW向的徐中断裂、近SN向的徐西断裂、近NW向的徐东断裂带和四组近NE向的断裂。本次研究,对以往无法解释的很多地质现象给予了很好的解释,得出了一些崭新的认识,也从宏观上为寻找天然气资源给予了有力指导。