Sedimentological and ichnological signatures of changes in wave,river and tidal influence along a Neogene tropical deltaic shoreline

Analysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.     

NORTH ATLANTIC OSCILLATION SIGNATURES IN WESTERN IBERIAN TREE-RINGS

Exactly dated ring-width chronologies derived from Pyrenean oak and sweet chestnut trees growing in northern Extremadura, Spain, were evaluated for their potential as proxies for regional precipitation and North Atlantic Oscillation variability. The relationships among tree-rings, instrumental climatic records, and three versions of the NAO index were computed for different time subperiods over the last century. The results indicate that tree-ring records reflect, with variable intensity, both short-term and long-term variations in climate. Multiple correlation and regression analysis revealed that summer precipitation appeared to be the major factor affecting tree growth at inter-annual timescales. Moreover, since fluctuations in accumulated variability in annual rainfall over southwest Iberia are controlled by winter precipitation, the accumulated rainfall (August of the year n -1 to July of year n ) and winter NAO indices are also strongly correlated with tree-ring records at interdecadal timescales. This relationship appears to be especially strong during the second half of the 20th century, which is consistent with an increase of the NAO signal in the annual precipitation during the later part of the century. These results indicate that tree-rings from western Iberia are potential proxies of the NAO variability, useful to be included in palaeoclimatic model studies.     

内蒙古东部霍林河盆地霍林河组的年龄:来自锆石U-Pb测年和孢粉组合的证据

内蒙古东部霍林河盆地霍林河组是我国最重要的下白垩统含煤地层之一.该组产出大量不同种类且保存完好的植物化石,但因为缺乏海相沉积和火山岩夹层,我们对这套地层的精确年龄认知不足.我们最近在扎鲁特旗扎哈淖尔露天煤矿霍林河组发现了火山灰层,基于钻孔数据的地层分析显示该火山灰层位于霍林河组下含煤段的底部.二次离子质谱(SIMS)锆石U-Pb年代学研究得出的该火山灰层沉积年龄为125.6±1.0Ma(巴雷姆期晚期—阿普特期最早期),这一结果与激光剥蚀-等离子体质谱(LA-ICP-MS)对同一样品分析得出的年龄一致.火山灰层保存的孢粉化石组合以裸子植物花粉和蕨类及苔藓的孢子为主,被子植物花粉非常稀少,且仅有棒纹粉Clavatipollenites 一种.这一孢粉组合支持火山灰层的年龄为巴雷姆期晚期至阿普特期最早期.本研究为认识整个霍林河组的年龄及火山灰层上覆地层保存的重要植物化石的最大年龄提供了重要数据.