【作者】 何卫红；

【导师】 汪啸风；

【作者基本信息】 中国地质科学院 ， 古生物学与地层学， 2002， 博士

【摘要】 本文通过生物地层、化学地层和层序地层的研究，对晚奥陶世晚期至志留纪初期的海平面变化以及奥陶／志留系界线的划分进行了探讨。笔石、腕足类、放射虫的形态、分异度和丰度的研究表明：扬子海盆晚奥陶世晚期至志留纪初期，总体上，海平面经历了上升→下降→上升的变化，海平面上升的最大阶段发生在Tangyagraptus typicus亚带下部，到观音桥层顶部，海平面下降到最低点。在苟家垭和大塘口剖面五峰组-龙马溪组底部，Ni，V，Sc，Rb，Sr和Ba等微量元素丰度以及稀土元素Ce异常的变化具有相似的规律性，可划分成5次旋回，分别对应于D.complexus带下部，D.complexus带上部，P.pacificus带下部，P.pacificus带上部至N.ojsuensis带，以及N.persculptus带至P.acuminatus带。在这2个剖面上，放射虫丰度变化也表现出相似的规律，与笔石和腕足类的形态、分异度和丰度变化所反应的海平面升降大体一致，和北美、非洲同期海平面的5次波动可以对比，据此指出，以上5次海平面旋回代表了晚奥陶世晚期至志留纪初期全球性海平面变化。在前4次旋回中，每次波动的周期为0.75～1Ma，与冰川活动引起的Ⅳ级海平面变化周期(0.1～1Ma)相近，以此进一步推断晚奥陶世晚期(从D.complexus带到N.ojsuensis带)发生过4次Ⅳ级海平面变化。根据放射虫、笔石、腕足类、海绵骨针以及岩性等对古环境的指示意义，认为五峰组底部灰绿色泥岩段产出时水深约60-100m，观音桥层水深约50-80m，在晚奥陶世最大海侵期(对应于T.typicus亚带下部)，水深约400m，五峰组含放射虫硅质岩沉积时最小水深约200m，结合以上海平面变化的5次旋回，从而推算出各旋回海平面的变化幅度：在第①旋回，海平面变化范围大致为20～120m；在第②、第③和第④旋回中，海平面变化分别为80～130 m、大约150 m和50～250m；第⑤旋回的海平面升降幅度大于200 m。按“国际地层指南”的要求所划分的年代地层界线的地质标志不明显，不利于识别，并且与地球演化节律不完全吻合。从震旦／寒武系之交，到奥陶／志留系、弗拉斯期／法门期、二叠／三叠系、三叠／侏罗系以及白垩／第三系之交的各地史转折期，发生的地质事件存在如下规律：(1) C、O同位素异常事件所跨越的地史时间，长于或基本等于界线粘土层或生物绝灭事件所跨越的时间；(2)海侵事件一般紧随其它地质事件之后发生，并与生物绝灭之后的复苏基本一致；(3)海侵之前为事件发生的高峰期，之后为较平静的地史期；(4)以上相邻地史转折期的时间跨度分别为105Ma，66Ma，122Ma，45Ma和140Ma，可见地质事件以长、短周期相间出现，相邻2个长、短周期之和基本相等(66Ma+122Ma=188Ma，45Ma+140Ma=185Ma)；(5)在二叠／三叠系、白垩／第三系等断代界线附近，地质事件的规模更大，不仅存在生物集群绝灭，还出现过全球性的化学异常事件。这些规律表明：地球演化存在节律性；全球性海侵事件可能为地球由非平衡状态走向平衡状态的转折点，或为地史转折期的转折点。因而指出，划分年代地层界线时要充分考虑体现地球演化节律的自然事件，特别是海侵事件的重要意义。根据生物演化的不可逆性、地球演化的节律性以及各地史转折期地质事件之间的规律，建议划分年代地层单元界线时，以生物地层划分为基础，密切结合自然事件，并以此为辅助标志。在划分奥陶／志留系界线时，主张以笔石N.persculptus的首现为生物标志，以基本同时发生的地质事件为其辅助标志，以利于全球广泛对比。鉴于不同地区留下的地质记录不完全相同，奥陶／志留系界线对比方法不同：在界线层型剖面上，以N.persculptus的首现为生物标志，以与之基本一致的海侵事件为辅助的物理标志，以生物集群绝灭事件和化学异常事件为辅助的生物和化学标志；在笔石极不发育的地区，则以海侵事件、生物绝灭事件或化学异常事件为辅助标志，以与其相距最近、并且具有区域对比性的其它门类化石(如牙形石)作为界线对比的生物标志；对于由于海退事件所造成地层缺失很多的地区，则以海退／海侵事件作为划分界线的物理标志，并以相映的海侵事件的开始作为奥陶／志留系的分界。更多还原

【Abstract】 This paper probed into the sea-level changes from the latest Ordovician to the earliest Silurian and thesubdivision of the Ordovician-Silurian boundary based on the study of biostratigraphy, chemical stratigraphyand sequence stratigraphy.Analysis of the morphological feature, diversity and abundance of graptolite, brachiopod and radiolariaindicated that the sea-level in the Yangtze basin underwent the change from rising to falling, then to risingagain during the latest Ordovician to the earliest Silurian interval, and the maxium rising episode appeared inthe lower Tangyagraptus typicus Subzone and the maxium falling at the top of the Guanyinqiao bed.The variation of the microelement Ni, V, Sc, Rb, Ba and Sr was analogous to the change of the Ceanomaly from the Wufeng Formation to the basal Longmaxi Formation at the Goujiaya and Datangkousections, and 5 cycles, separately corresponding to the lower and upper part of the Dicellograptus complexusZone, the lower Pacificograptus pacificus Zone, the upper part of P. pacificus Zone to the Normalograptusojsuensis Zone, and N. persculptus to Parakidograptus acuminatus Zones, could be recognized during theinterval. In addition, the abundance changes of the radiolaria at the Goujiaya and Datangkou sections werecoincided with the sea-level changes, and correlatable with the synchorous 5 fluctuations of the sea-level inNorth America and Africa. So above-mentioned 5 cycles could represent the eustasies during the latestOrdovician to the earliest Silurian. Every fluctuation period of the earlier 4 cycles covered 0.75 to 1 Ma beingsimilar to that of the fourth-order glacial-eustasy with the interval of 0.1 to 1 Ma, imagining there probablyoccurred 4 fourth-order eustasies during the latest Ordovician from the D. complexus Zone to N. ojsuensisZone.In the light of the paleoenvironmental significance reflected by the radiolaria, graptolite, brachiopod,sponge spicule and the rock character, it’s suggested that the greyish green mudstone at the basal WufengFormation of the Goujiaya section would be probably deposited in 60 to 100 meters deep water, theGuanyinqiao bed in 50 to 80 meters deep, and the radiolaria-bearing silicalite from Wufeng Formation inwater with 200 to 400 meters deep. Combining the above-mentioned water depth with the sea-level changecycles, the change range of every one of the 5 cycles could be further conjectured. They were in ascendingorder: 20～120 m, 80～130 m, about 150 m, 50～250 m, and in excess of 200 m.The boundary marks of the chronostratigraphic units which is determined on the InternationalStratigraphic Guide, weren’t convenient for being identified, and all of them didn’t coincide with thegeoevolutional rhythm. However, the studies of the Sinian-Cambrian, Ordovician-Silurian,Frasnian-Famennian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Tertiary boundaries indicated that,the geological events at these boundary intervals generally comply with the following laws: (1) the age rangeof the carbon and oxygen isotope events was equal to or longer than that of the boundary clay or massextinction events. (2) The transgression events closely followed the other geological events and basicallycoincided with the organism recovery after extinction. (3) The peak of the geological events happened before transgression and the quiet time afar the transgression. (4) Two adjacent geological events above-mentionedcovered respectively 105 Ma, 66Ma, 122Ma, 45M and 140Ma. It can see that, the geological events withlonger period alternated with those events spanning shorter period, and the sum of a longer period and itsadjacent shorter period was identical (66Ma+122Ma=188Ma; 45Ma+140Ma=185Ma). (5) The geologicalevent scales across the Permian-Triassic and the Cretaceous-Tertiary intervals were bigger, occurred not onlymass extinctions but also global chemical anomaly events. These laws above-mentioned indicated the earthevolutional rhythm, and the global transgression event probably represents the turning point of the earthevolution from unbalanced to balanced condition, or the turnover of geohistory. Therefore present author holdsthat the natural events (catastrophic events), characterizing the geoevolution rhythm, especially thetransgression event above-mentioned, should be fully considered in the subdivision of the chronostratigraphicboundaries.On the basis of the irreversibility of the bioevolution, geoevolution rhythm and above-mentioned laws ofthe geological events at the turning point of the geohistory, it’s suggested that the subdivision of thechronostratigraphic boundaries should be in line with the biostratigraphic classification at first, in associationwith the natural events and regard the latter as the supplementary indicators. Therefore the Ordovician-Silurianboundary should he defined by the FAD of N. persculptus displaying the geohistorical rhythm as the biomark,and by the not fully identical geological events as the supplementary marks for the global correlation. In viewof geological records preserved from different regions of the world are not identical, it’s suggested that, whencorrelating the Ordovician-Silurian boundary, at the global boundary stratotype section, the FAD of N.persculptus be selected as the biomark, the transgression event identical to the FAD of N. persculptus as theauxiliary physical mark and the mass extinction and chemical anomaly events respectively as the auxiliarybiomark and chemomark. In some regions with few graptolite, the Ordovician-Silurian boundary should beindicated by the auxiliary marks, such as the transgression event, mass extinction or chemical anomaly event,with the other fossil such as conodont nearest to other geological events as the provincial biomark forcorrelation. In other regions with much stratigraphic hiatus originated from the regression, theOrdovician-Silurian boundary should he marked by the regression-transgression event nearly relevant to the N.persculptus Zone.更多还原