Abstract

A continuous outcrop of the neritic sediments ranging from the Cretaceous/Paleogene to Eocene/Oligocene boundary is exposed along the Kheu R., Kabardino-Balkaria Republic, southern Russia. High calcareous nannofossil abundance and species diversity in Kheu section make possible the application of various standard calcareous nannofossil zonations for the investigated interval. This provides a direct correlation of the biohorizons used as zonal markers in these zonations and recognition of the stage boundaries in this complete succession, which represents the reference Paleogene section of the NE Peri-Tethys. The carbon and oxygen isotope records revealed a series of excursions correlated to peculiar Paleogene paleoecological events on the basis of nannofossil biostratigraphy. The continuous Maastrichtian/Danian transition displays ca. 1 m thick interval containing only scarce Cretaceous survivors between the highest occurrence (HO) of the most Cretaceous nannofossil taxa and the lowest occurrence (LO) of the Paleogene taxa; this interval is characterized by prominent positive δ¹³C and δ¹⁸O excursions. The Late Danian Event (LDE) is identified as minor δ¹³C and δ¹⁸O excursions in the upper Danian. The base of Selandian stage is defined by the 2nd radiation of fasciculiths, followed by negative δ¹³C and δ¹⁸O shifts corresponding to the Danian/Selandian Transition Event. Mid-Paleocene Biotic Event (MPBE), A, B1/B2, C1/C2 and D1/D2 events are pronounced in the upper Paleocene C and O isotope record. Wide expansion of Discoasterales (Fasciculithus, Heliotrochus, Heliolithus, Discoaster), served as a basis for nannofossil zonations, is featured for this time span. The base of Ypresian (lower Eocene) corresponds to the onset of the carbon isotope excursion (CIE) documented at the base of the TOC-rich bed. This 0.6 m thick sapropelitic bed corresponds to the Paleocene–Eocene Thermal Maximum (PETM) and contains specific nannofossil assemblage known as “excursion taxa”. The high sedimentation rate and completeness of the early Eocene sedimentary succession allowed to record the evolution of Tribrachiatus bramlettei–T. digitalis–T. contortus–T. orthostylus lineage spanning the interval of NP10–NP12 zones. Multiple δ¹³C and δ¹⁸O excursions, occurred in the lower Ypresian sediments, correspond to the early Eocene hyperthermals E1/E2, F1, F2, G, ETM2, I1/I2 and J. The intercalations of a series of TOC-rich layers in the interval ranging the upper NP12 to lower NP13 zones correspond to ETM3 and individual episodes (L-T) of the Early Eocene Climatic optimum (EECO). Long-term cooling trend in the Caucasian basin begins in the uppermost Ypresian (upper NP13 zone). The middle Eocene δ¹⁸O record demonstrates a succession of negative excursions occurred against the background of relatively high values. The hyperthermal C21r–H6 is recognized at the base of Lutetian stage defined by the LO of Blackites inflatus. Two later excursions of this time interval corresponding to the Late Lutetian Event (LLE) and Middle Eocene Climatic Optimum (MECO) occur in the enriched in TOC Kuma Fm. (late Lutetian to Bartonian) and fall at the levels with highest TOC concentrations. Nannoplankton assemblages are widely dominated by eurytopic reticulofenestrids during both events, while Chiasmolithus and Discoaster become oppressed that can indicate lower salinity caused by enhanced humidity. The base of Priabonian is defined by the LO of Chiasmolithus oamaruensis and the base of Cribrocentrum erbae acme. These bioevents are preceded by the negative δ¹³C excursion and very positive δ¹⁸O values, possibly correlated to the Priabonian Oxygen Maximum (PrOM). The relatively warm Priabonian time, characterized by recovery of Chiasmolithus, Discoaster and Sphenolithus, gives way to a new cooling phase known as Late Eocene Event (LEE). The base of Clausicoccus subdistichus acme, approximated to the base of Oligocene, falls at the level ca. 0.5 m below the facial change corresponding to the large-scale sea-level fall and, thus, slightly precedes the accumulation of low-calcareous oil-bearing Oligocene-lower Miocene Maikop Fm. The prominent positive δ¹³C and δ¹⁸O excursions occur at the base of this formation.

中文翻译：

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俄罗斯北高加索古新世-始新世综合纳米化石和稳定同位素地层学

摘要

从白垩纪/古近纪到始新世/渐新世边界的浅海沉积物连续露头，沿俄罗斯南部卡巴尔达-巴尔卡尔共和国的 Kheu R. 暴露。Kheu 剖面的高钙质超微化石丰度和物种多样性使得在调查区间应用各种标准钙质超微化石带成为可能。这提供了在这些分区中用作分区标记的生物水平面的直接相关性，并在这个完整的演替中识别阶段边界，这代表了 NE Peri-Tethys 的参考古近纪部分。基于纳米化石生物地层学的碳和氧同位素记录揭示了一系列与特殊的古近纪古生态事件相关的偏移。连续的马斯特里赫特/达尼安过渡显示约。1 m 厚的间隔，在白垩纪最多的超微化石类群的最高出现 (HO) 和古近纪分类群的最低出现 (LO) 之间仅包含稀有的白垩纪幸存者；这个区间的特点是显着的正 δ¹³ C 和 δ ¹⁸ O 偏移。晚大年事件 (LDE) 被确定为上大年的次要 δ ¹³ C 和 δ ¹⁸ O 偏移。Selandian阶段的底部由束状体的第二次辐射定义，随后是与Danian / Selandian过渡事件相对应的负δ ¹³ C和δ ^{18 O偏移。}中古新世生物事件 (MPBE)、A、B1/B2、C1/C2 和 D1/D2 事件在上古新世 C 和 O 同位素记录中很明显。Discoasterales 的广泛扩张（Fasciculithus , Heliotrochus , Heliolithus , Discoaster)，作为纳米化石分区的基础，在这个时间跨度内具有特色。Ypresian（始新世下）的底部对应于在富含 TOC 的床底部记录的碳同位素偏移 (CIE) 的开始。这个 0.6 m 厚的腐泥岩床对应于古新世 - 始新世热最大值（PETM），并包含称为“偏移分类群”的特定纳米化石组合。早始新世沉积序列的高沉积速率和完整性允许记录Tribrachiatus bramlettei-T 的演化。洋地黄-T。扭曲-T。跨越 NP10-NP12 区域区间的 orthostylus 谱系。多重 δ ¹³ C 和 δ ¹⁸O 偏移，发生在下伊普勒斯沉积物中，对应于早始新世超高温期 E1/E2、F1、F2、G、ETM2、I1/I2 和 J。 NP12 到较低的 NP13 区域对应于 ETM3 和早始新世气候最佳 (EECO) 的个别事件 (LT)。高加索盆地的长期降温趋势始于伊普勒斯最上部（NP13 上部）。中始新世 δ ^​​18 O 记录表明在相对较高值的背景下发生了一系列负偏移。超高温 C21r-H6 在由Blackites inflatus的 LO 定义的 Lutetian 阶段底部被识别。该时间间隔的两个后期偏移对应于晚鲁特斯事件（LLE）和中始新世气候最佳（MECO）发生在富含TOC的Kuma组中。（Lutetian 晚期到 Bartonian）并下降到 TOC 浓度最高的水平。在这两个事件中，微浮游生物群落都以广域网状生物群居多，而Chiasmolithus和Discoaster受到压迫，这表明湿度增加导致盐度降低。Priabonian 的基部由 Chiasmolithus oamaruensis 的 LO和Cribrocentrum erbae acme的基部定义。这些生物事件之前是负 δ ¹³ C 偏移和非常正的 δ ¹⁸O 值，可能与普里雅本期氧最大值 (PrOM) 相关。以Chiasmolithus、Discoaster和Sphenolithus恢复为特征的相对温暖的普里亚波尼亚时期让位于一个新的冷却阶段，即晚始新世事件（LEE）。Clausicoccus subdistichus acme 的基部，近似于渐新世的基部，落在约 20 米的水平。大尺度海平面下降所对应的面变以下0.5 m，因此略早于低钙质含油渐新统-下中新统Maikop组的堆积。显着的正 δ ¹³ C 和 δ ¹⁸ O 偏移发生在该地层的底部。