西沙群岛琛科二井中新世和更新世的一些珊瑚_余克服.pdf

收稿日期:2022-05-29;改回日期:2022-08-18;录用日期:2022-10-13*国家自然科学基金(42090041,42030502)资助。*通讯作者:廖卫华,研究员,e-mail: 古生物学报,61(4):541557(2022年12月)Acta Palaeontologica Sinica,61(4):541557(December 2022)封面论文 DOI:10.19800/ki.aps.2022023 西沙群岛琛科二井中新世和更新世的一些珊瑚*余克服1 廖卫华1,2*廖芝衡1 李银强1 王文欢1 边立曾1,3 1 广西大学海洋学院,广西大学珊瑚礁研究中心,广西南海珊瑚礁研究重点实验室,南宁 530004;2 中国科学院南京地质古生物研究所,南京 210008;3 南京大学地球科学与工程学院,南京 210093 提要 西沙群岛琛航岛琛科二井(CK-2)有878.3 m厚的新生代珊瑚礁灰岩(Cenozoic reef limestone)。新生代珊瑚礁灰岩自上而下包括了第四系的全新统和更新统以及新近系的上新统和中新统。其中,中新统厚514.3 m,产19属石珊瑚(Scleractinia corals)。更新统厚215.6 m,产21属石珊瑚(Scleractinia)和1属八射珊瑚(Octocorallia)。珊瑚是营底栖固着生活的动物,在地层断代方面它没有能像一些浮游动物(例如浮游有孔虫等)那么“精准”。但也有一些珊瑚的地质历程相对比较短暂的,它们在地层划分和对比方面还是能起到重要作用。本研究在琛科二井井深364878.3 m的中新世地层中鉴定出有若干只见于中新世或者常见于中新世的珊瑚,如:星日珊瑚(Astrhelia)、安的列斯珊瑚(Antillophyllia)和大安的列斯珊瑚(Antillia)等。另外在井深21.4237 m的更新世地层中也发现了数只限于第 四纪的珊瑚,如:叶状珊瑚(Lobophyllia)、合叶珊瑚(Symphyllia)、轮沙珊 瑚(Trochopsammia)和苍珊瑚(Heliopora)等。上述珊瑚的时代分布与有孔虫生物地层鉴定的结果以及古地磁和同位素年代地层测定的数据完全吻合。珊瑚除了在地层的划分对比方面能够起到一些关键作用外,更重要的是它在恢复和解释古地理、古气候和古生态等方面也都具有非常重要的意义。关键词 西沙群岛 琛航岛 中新世 更新世 石珊瑚 八射珊瑚 中文引用 余克服,廖卫华,廖芝衡,李银强,王文欢,边立曾,2022.西沙群岛琛科二井中新世和更新世的一些珊瑚.古生物学报,61(4):541557.DOI:10.19800/ki.aps.2022023 英文引用 Yu Ke-fu,Liao Wei-hua,Liao Zhi-heng,Li Yin-qiang,Wang Wen-huan,Bian Li-zeng,2022.Some Mio-cene and Pleistocene corals from Well Drilling CK-2 in Xisha Islands,South China Sea.Acta Palaeontologica Sinica,61(4):541557.DOI:10.19800/ki.aps.2022023 Some Miocene and Pleistocene corals from Well Drilling CK-2 in Xisha Islands,South China Sea YU Ke-fu1,LIAO Wei-hua1,2,LIAO Zhi-heng1,LI Yin-qiang1,WANG Wen-huan1,BIAN Li-zeng1,3 1 Guangxi Laboratory on the Study of Coral Reefs in the South China Sea,Coral Reef Research Center of Guangxi University,School of Marine Sciences,Guangxi University,Nanning 530004,China;2 Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences,Nanjing 210008,China;3 School of Earth Sciences and Engineering,Nanjing University,Nanjing 210093,China 542 古 生 物 学 报 61 卷 Abstract Scleractinia are the major members of the Mesozoic,Cenozoic,and extant corals.Scleractinian corals are distinguished by their calcareous external skeletons.Scleractinian corals may be divided into two ecological groups:hermatypic(reef-building)and ahermatypic corals.Hermatypic corals are characterized by the presence of vast num-bers of zooxanthellae in their endodermal tissues.They are the most common in warm and shallow marine waters of the tropics.Strong sunlight is essential for the vigorous growth of hermatypic corals.Ahermatypic corals lack zooxan-thellae and are environmentally less restricted than reef-building corals.It is now generally recognized that,since the Miocene,there are two first-level reef-coral biogeographic pro-vinces,i.e.,the Caribbean and Indo-Pacific,in the world.The South China Sea,including Xisha Islands,is attributed to the Indo-Pacific Province.During the Neogene and Quaternary,the Indo-Pacific coral faunas were already rich than those of the Caribbean.The oldest coral-bearing beds are from the early Miocene in Xisha Islands,South China Sea.About 19 genera of Miocene scleractinian corals are recognized from the 364878.3 m interval in the Well CK-2 core from Chenhang Island,Xisha Islands,South China Sea.They are Acropora,Astreopora,Antillophyllia,Antillia,Astrhelia,Caryophyllia,Cyphastrea,Diploastrea,Echinophyllia,Favia,Favites,Fungia,Galexea,Goniopora,Hydnophora,Montipora,Platygyra,Porites and Turbinaria.Among them,the geological ranges of the genera Echinophyllia,Fungia and Galaxea are from the Miocene to the Quaternary,indicating that the age of the strata that yield these three genera is not older than the Miocene.While the genus Antillia has been recorded from the Eocene to the Miocene and the genus Antillophyllia has been reported from the Oligocene to the Miocene,indicating that the age of the strata that contain these two genera is not younger than Miocene.Moreover,the genus Astrhelia has hitherto been known only from the Miocene.All the above-mentioned evidence suggests a Miocene age.About 21 genera of the Pleistocene scleractinian corals and one genus of Octocorallia are identified from the 21.4215.6 m interval in the Well CK-2 core.Among them,the geological ranges of Acanthophyllia,Enallopsammia,Fungia,and Galaxea are from the Miocene to the Quaternary.The genera Acoropora,Coenocythus,Cyphastrea,Euphyllia,Montipora,Pavona,Porites,Favites,Goniastrea,Platygyra and Turbinaria have been recorded from the Palaeogene to the Quaternary and the genera Astreopora,Goniopora and Leptoria are the characteristic corals from the Cretaceous to the Quaternary.A few genera,however,afford evidence of considerable weight for the determination of the age of the strata.The three scleractinian corals,i.e.,Lobophyllia,Symphyllia,and Trochopsammia,and the octocoral genus Heliopora have hitherto been known only from the Quaternary.The above evidence suggests that the age of the coral assemblage is Quaternary(Pleistocene).Since very few Miocene and Pleistocene corals have been reported previously from the Xisha Islands,a brief description of these fossils is warranted in the paper.Key words Xisha Islands;Chenhang Island;Miocene;Pleistocene;Scleractinia;Octocorallia Systematic Descriptions Phylum Coelenterate Frey and Leuckart,1847 Subphylum Cnidaria Hatschek,1888 Class Anthozoa Ehrenberg,1834 Order Scleractinia Bourne,1900 Family Acroporidae Verrill,1902 Genus Acropora Oken,1815 Acropora sp.(Fig.2-A)Description Acropora is an important genus of hermatypic corals.Ramose colonies.Branch 10 mm thick.Corallites raised tube in form,1.52.0 mm in di-ameter.Tube 2 mm long.Corallites separated by dis-tance of 1 mm only.Coenosteum united by reticulate.Columella absent.Depth of borehole 45.6 m.Strata Chenhang Formation(Pleistocene).Genus Astreopora Blainville,1830 Astreopora sp.(Fig.2-B)Description Massive colonies and cylindrical corallites.Corallites 2.02.5 mm in diameter.Corallites separated by distance ranging from 0.14.0 mm.Septa 12,in two cycles.Septa short,about 0.20.3 mm only.Coenosteum reticular with spinose surface.Depth of borehole 138.6 m.Strata Chenhang Formation(Pleistocene).Family Agariciidae Gray,1847 Genus Pavona Lamarck,1801 Pavona varians Verrill,1864(Figs.2-C,2-D)1937 Pavona varians,Ma,p.153,pl.78,fig.1.1975 Pavona varians,Zou et al.,p.25,pl.4,fig.7.1991 Pavona varians,Nie et al.,p.21,pl.31,figs.58.2000 Pavona varians,Veron,vol.2,p.186,figs.18.2001 Pavona varians,Zou,p.117,pl.23,fig.4,fig.20.2009 Pavona varians,Dai and Hong,p.136.Description Foliaceous colonies.Caly deep or shallow.Corallite walls absent,united by 12 septa.Columella styliform.4期 余克服等:西沙群岛CK-2中新世、更新世珊瑚;Yu et al.:Miocene and Pleistocene corals from Xisha Islands 543 Depth of borehole 131 m.Strata Chenhang Formation(Pleistocene).Family Poritidae Gray,1842 Genus Porites Link,1807 Porites lutea Milne-Edwards and Haime,1851(Figs.3-A,3-B)1851 Porite lutea,Milne-Edwards and Haime,p.80.1975 Porites lutea,Zou et al.,p.34,pl.4,fig.1.1991 Porites lutea,Nie et al.,p.32,pl.6,fig.1;pl.44,figs.14.2000 Porites lutea,Veron,p.283,figs.46.2001 Porites lutea,Zou,p.131,pl.27,fig.3,fig.21.2009 Porites lutea,Dai and Hong,p.116.Description Porites is one of the most important hermatypic coral genera,second only to Acropora.Massive colonies.Corallites hexagonal in outline,12 mm in diameter.Twelve septa in two cycles;58 paliform lobes present along the inner septal margins.Columellar trabecula.Depth of borehole 125.45 m.Strata Chenhang Formation(Pleistocene).Family Faviidae Gregory,1900 Genus Favites Link,1807 Favites sp.(Figs.3-C,3-D)Description Massive colonies.Corallites pen-tagonal or hexagonal in outline,510 mm in diameter.Twenty-six to thirty septa in three cycles.Septa of first cycle long,nearly reaching the columella;septa of second cycle shorter;septa of third cycle very short.Septa frequently thickened peripherally but becoming attenuated at the axial ends.Septa trabeculae,with lateral dentations.Depth of borehole 63.8 m.Strata Chenhang Formation(Pleistocene).Genus Platygyra Ehrenberg,1834 Platygyra sp.(Figs.4-A,4-B)Description Meandroid colonies.Collines thin,1520 mm long and 1011 mm wide.Twelve to thirteen septa per 10 mm.Long and short septa alternately arranged.Upper margin of septa with dentations.Depth of borehole 97.25 m.Strata Chenhang Formation(Pleistocene).Leptoria phrygia(Ellis and Solander),1786(Figs.4-C,4-D)Description Meandroid colonies.Collines straight or curved,2 mm wide.Six or seven septa per 5 mm.Columella thin,lamellar.Depth of borehole 138.1 m.Strata Chenhang Formation(Pleistocene).Genus Antillophyllia Vaughan,1932 Antillophyllia cf.sawkinsi(Vaughan),1926(Fig.5-A)cf.1943 Antillophyllia sawkinsi(Vaughan),Vaughan,p.170,pl.28,figs.6,6a.cf.1956 Antillophyllia sawkinsi(Vaughan),Wells,p.407,figs.305-3a,b.Description Trochoid solitary,30 mm in dia-meter.Septa in four cycles.First cycle with about 28 septa,long,nearly reaching the axis;septa of second cycle about half length of septa of first cycle;septa of third cycle about a quarter length of septa of first cycle;septa of fourth cycle very short.Columella spongy.Depth of borehole 391.67 m.Strata Yongle Formation(Upper Miocene).Antillophyllia sp.(Figs.5-C,5-D)Description Turbinate solitary and 32 mm in diameter.One hundred septa in four cycles.Septa long but poorly preserved.Columella spongy.Depth of borehole 736.8 m.Strata Xisha Formation(Lower Miocene).Family Oculinidae Gray,1847 Genus Astrhelia Milne-Edwards and Haime,1849 Astrhelia palmata(Goldfuss),1826(Fig.5-B)1943 Astrhelia palmata(Goldfuss),Vaughan,p.182,183.1956 Astrhelia palmata(Goldfuss),Wells,p.411,fig.310-1a.Description Subramose colonies.Septa denta-tion lacerate.Pali absent.Columella trabecular.Coenos-teum smooth,slightly raised.Depth of borehole 594.81 m.Strata Xisha Formation(Lower Miocene).Family Mussidae Ortmann,1890 Genus Antillia Dubcan,1863 Antillia dentata Duncan,1863(Fig.5-C)Description Turbinate solitary.Epitheca poor preserved.Septa in three cycles;12 septa in first cy-cle long and dilated,nearly reaching the axis;12 septa in second cycle,shorter;septa in third cycle very short.Depth of borehole 697.57 m.Strata Xisha Formation(Lower Miocene).544 古 生 物 学 报 61 卷 1 前 言 由于印度板块迅速向北漂移并向欧亚板块俯冲,使得青藏高原快速隆起,从古近纪的始新世开始海水逐渐从西藏退出,西藏目前确定的最高海相地层是始新世中期的沉积(中国科学院西藏科学考察队,1974;Wen et al.,1981)。地壳运动此起彼伏相互关联,当青藏高原迅速隆起时,南海盆地也开始下沉,从晚渐新世早中新世开始发育形成珊瑚礁(中国科学院南沙综合科学考察队,1992,1997)。南海大多数的珊瑚礁碳酸盐岩都是从中新世开始一直持续发育到第四纪,部分地区珊瑚礁的厚度超过千米,因此可以借助珊瑚礁钻井岩芯的研究,重建南海完整的地质发育史(刘新宇等,2015;李银强等,2017;余克服,2018;李诗颖等,2019;覃业曼等,2019;Wang et al.,2018,2021,2022;Zhang et al.,2018,2020;Jiang et al.,2019;Xu et al.,2019;Fan et al.,2020;Li et al.,2021a,2021b;Ma et al.,2021;Yang et al.,2022)。中国的南海海疆辽阔、资源丰富。西沙群岛就像是镶嵌在南海海域上一串耀眼的明珠。西沙群岛主要由永乐和宣德两大环礁(Atoll)组成。为了探索和揭示南海珊瑚礁沉积发育的历史过程,笔者在西沙群岛永乐环礁的琛航岛上实施了钻探工作(CK-2)并提取岩芯(图1)。琛科二井(CK-2)钻井的孔深为980 m,其中包括878.3 m厚的新生代珊瑚礁灰岩,珊瑚礁灰岩之下是火山岩(厚度超过53 m)。根据有孔虫、珊瑚、珊瑚藻、古地磁和同位素的综合研究,现将琛科二井的珊瑚礁灰岩的地层划分成如下(表1)。图 1 西沙群岛琛航岛琛科二井地理位置图 Fig.1 Location map of Well CK-2 in the Xisha Islands A.南海海域地图;B.西沙群岛地图。A.Map of the South China Sea;B.Map of the Xisha Islands in the South China Sea.表 1 琛科二井地层表 Table 1 Stratigraphic subdivision of Well CK-2 on the Chenhang Island,Xisha Islands 地层地层 厚度厚度(m)第四系 全新统 东岛组 21.4 更新统 琛航组 215.6 新近系 上新统 永兴组 127.0 中新统(上部)永乐组 158.0 中新统(中部)宣德组 89.0 中新统(下部)西沙组 267.3 4期 余克服等:西沙群岛CK-2中新世、更新世珊瑚;Yu et al.:Miocene and Pleistocene corals from Xisha Islands 545 琛科二井最下部的地层是中新统(Miocene)。从井深364 m起往下一直到878.3 m止都属于中新统,中新统的厚度为514.3 m。它自上而下又可划分成中新统上部的永乐组(厚158 m)、中部的宣德组(厚89 m)和下部的西沙组(厚267.3 m)三个部分。本研究在中新世地层中一共发现了19属石珊瑚,它 们 是:鹿 角 珊 瑚(Acropo ra)、星 孔 珊 瑚(Astreopora)、安的列斯珊瑚(Antillophyllia)、大安的列斯珊瑚(Antillia)、星日珊瑚(Astrhelia)、丁香珊瑚(Caryophyllia)、刺星珊瑚(Cyphastrea)、双星珊瑚(Diploastrea)、刺叶珊瑚(Echinophyllia)、蜂房珊瑚(Favia)、角蜂巢珊瑚(Favites)、石芝珊瑚(Fungia)、盔珊瑚(Galexea)、角孔珊瑚(Goniopora)、刺柄珊瑚(Hydnophora)、蔷薇珊瑚(Montipora)、扁脑珊瑚(Platygyra)、滨珊瑚(Porites)和陀螺珊瑚(Turbinaria)等。上述石珊瑚的地质历程各有不同、长短不一,其中有一些属是从新近纪的中新世才开始出现,并一直生活到现代,如:刺叶珊瑚(Echinophyllia)、石芝珊瑚(Fungia)和盔珊瑚(Galaxea)等属;也有不少的属出现得稍早一些,它们从古近纪的始新世或者渐新世就已出现,经过新近纪,然后延续到现代的,如鹿角珊瑚(Acropora)、蔷薇珊瑚(Montipora)、滨珊瑚(Porites)、角蜂巢珊瑚(Favites)、扁脑珊瑚(Platygyra)、刺星珊瑚(Cyphasyrea)和陀螺珊瑚(Turbinaria)等属。另外还有一些属出现得比较早一些,它们从中生代晚期的白垩纪就开始出现,经过古近纪和新近纪,一直延续到第四纪,如:星孔珊瑚(Astrepora)、蜂房珊瑚(Favia)、角孔珊瑚(Goniopora)、刺柄珊瑚(Hydnophora)、双星珊瑚(Diploastrea)等属;甚至于还有个别属如丁香珊瑚(Caryophyllia)出现得更早,它从侏罗纪就已经出现,而且一直生活到现在。上述这些石珊瑚的地质历程都比较长,在准确判断地层时代上无法起到决定性的作用,但是庆幸的是,笔者分别在井深391.67 m处发现了安的列斯珊瑚(Antillophyllia)、在井深594.81 m处发现了星日珊瑚(Astrhelia)、在井深697.57 m处发现了大安的列斯珊瑚(Antillia)和在井深736.8 m处又发现了安的列斯珊瑚(Antillophyllia)等属。这几个属的地质历 程 都 比 较 短 暂。它 们 的 发 现 对 本 研 究 划 分琛科二井的地层时代大有帮助。根据有关资料查证:Antillia 属的地质历程比较短,只从始新世到中新世(EoceneMiocene);Antillophyllia 属比它更短,从 渐 新 世 到 中 新 世(OligoceneMiocene);而Astrhelia 属的地质历程最短,只生活于中新世(Miocene)。笔者根据珊瑚的鉴定,再结合对有孔虫、珊瑚藻等生物门类的鉴定,以及古地磁和同位素数据的综合分析,认为从井深 364 m 以下一直到878.3 m的珊瑚礁碳酸盐地层都应归划为新近系的中新统。并将中新统进一步自上而下详细划分成:中新统上部的永乐组(井深 364522 m)、中部的宣德组(井深 522611 m)和下部的西沙组(井深611878.3 m)。琛科二井第四系的厚度是237 m。它又可以进一步划分为全新统的东岛组(厚21.4 m)和更新统的琛航组(厚215.6 m)(表 1)。在更新统中笔者一共发现 了 2 1 个 属 的 石 珊 瑚,它 们 是 鹿 角 珊 瑚(Acropora)、星孔珊瑚(Astreopora)、蔷薇珊瑚(Montipora)、牡丹珊瑚(Pavona)、石芝珊瑚(Fungia)、角孔珊瑚(Goniopora)、滨珊瑚(Porites)、角蜂巢珊瑚(Favites)、菊花珊瑚(Goniastrea)、扁脑珊瑚(Platygyra)、肠珊瑚(Leptoria)、刺星珊瑚(Cyphastrea)、盔形珊瑚(Galaxea)、针叶珊瑚(Acanthophyllia)、叶状珊瑚(Lobophyllia)、合叶珊瑚(Symphyllia)、共杯珊瑚(Coenocyathus)、真叶珊瑚(Euphyllia)、轮沙珊瑚(Trochopsammia)、变沙珊瑚(Enallopsammia)、陀螺珊瑚(Turbinaria)和1个八射珊瑚苍珊瑚(Heliopora)。其中,Acropora、Coenocyathus、Cyphastrea、Euphyllia、Favites、Goniastrea、Montipora、Pavona、Platygyra、Porites和Turbinaria这11个属是从古近纪的始新世或渐新世开始出现并延续到第四纪。而Astreopora、Goniopora和Leptoria这3个属则有更长的地质发展历程,它们可以从中生代晚期的白垩纪就已经开始出现并一直延续至第四纪的。但Lobophyllia、Symphyllia、Trochopsammia和Heliopora等4个属的地质历程却非常短暂,它们仅见于第四纪(邹仁林等,1975;聂宝符等,1991;邹仁林,2001;余克服,2018;Vaughan and Wells,1943;Wells,1956;Veron,2000)。它们是第四纪地层划分对比的重要依据。546 古 生 物 学 报 61 卷 叶状珊瑚(Lobophyllia)在井深61.2 m和163.83 m这二处层位都有发现,而合叶珊瑚(Symphyllia)这个属则是在井深167.06 m处找到的,轮沙珊瑚属(Trochopsammia)是在井深85.5 m处发现,而苍珊瑚(Heliopora)则是在井深43.25 m处找到。总之,上述4个属都是在井深237 m之上的岩芯中找到的。根据有孔虫的鉴定结果以及古地磁和同位素测定的数据,琛科二井第四系的底界是在井深237 m处,而这几块珊瑚最低的层位是在井深167.06 m处,所以它们都应该是在第四系的范围之内。珊瑚的地层鉴定结果与有孔虫的地层鉴定结果以及古地磁和同位素年代地层测定的结果都是完全吻合的(Jiang et al.,2019;Xu et al.,2019;Fan et al.,2020;Li et al.,2021)。2 关于西沙群岛新生代最早的珊瑚礁碳酸盐沉积的时代 关于南海西沙群岛海域最早的新生代珊瑚礁碳酸盐是从什么时代开始沉积的这个科学问题历来都是中外地质学家争论的焦点。本文引用西沙群岛最新的二口千米深钻的珊瑚和其他相关资料进行深入分析。第一口井的位置是在宣德环礁永兴岛旁边的石岛上的西科一井(XK-1),第二口井的位置则是在永乐环礁琛航岛上的琛科二井(CK-2)。西科一井(XK-1)完钻的井深为1268.02 m,其中珊瑚礁碳酸盐沉积厚1257.52 m,取芯率达85%以上。该钻井最下部(井深374.951257.52 m)是中新世地层,该口钻井的地层古生物资料已经公开发表(祝幼华等,2016)。西科一井的中新统自上而下又可划分为：上部的黄流组、中部的梅山组和下部的三亚组三个部分。三个组总共发现有19属珊瑚:Acropora、Antillophyllia、Astreopora、Caryophyllia、Coenangia、Cricocyathus、Cyphastrea、Diploastrea、Enallopsammia、Favia、Favites、Fungia、Goniastrea、Leptastrea、Meandrina、Montipora、Phyllangia、Porites和Turbinaria等(刘新宇等,2015,2019)。琛科二井(CK-2)完钻的井深为980 m,其中珊瑚礁碳酸盐沉积厚878.3 m,取芯率约70%。该井岩芯的研究工作是在广西大学海洋学院、珊瑚礁研究中心主持下进行的,参加单位还有中国科学院南京地质古生物研究所和南京大学等单位并得到了国家基金委员会的国家重点基金项目的资助。该项目的研究工作已经基本结束,并取得了一系列丰硕成果(Wang et al.,2018;Zhang et al.,2018;Jiang et al.,2019;Xu et al.,2019;Fan et al.,2020;Zhang et al.,2020;Li et al.,2021;Wang et al.,2021;Yang et a