2. 中国科学院大学 北京 100049
华南板块的古陆面积在晚奥陶世之后逐渐扩增,其南侧的原黔中古陆隆起区成为与华夏古陆相连的西区部分,亦称为滇黔桂古陆(陈旭等, 1996,2001;Rong et al., 2003)。滇黔桂古陆在很大程度上控制了上扬子区奥陶—志留系的岩相—生物相分异(田海芹等,2006;牛新生等,2007;刘伟等,2011),全球志留纪古地理图上均显示这时的华南板块位于环冈瓦纳区之北,主体部位在低纬度区(Scotese and McKerrow, 1990;Boucot et al., 1995;Copper,2002;Rong et al., 2003;李江海等,2013)。上扬子区晚奥陶世—志留纪初岩相古地理格局明显控制生物相和岩相,位于整个扬子区的兰多维列统龙马溪组页岩沉积结束时间不等,上扬子区志留纪最早期的灰岩多见于黔东北的滨岸带,灰岩—碎屑岩时空分布与距离黔中古陆海岸线的远近有关(Chen et al., 2004)。
石阡县城北约7km的香树园—雷家屯剖面为香树园组建组剖面(中国科学院南京地质古生物研究所,1974)。上扬子区埃隆期古地理图中划定石阡香树园—雷家屯剖面的南部为黔中古陆(葛治洲等,1979;Rong et al., 2003),以北为宽阔的陆表浅海区。 黔东北有多条出露香树园组的剖面,按在兰多维列世鲁丹中期—埃隆中期与黔中古陆海岸线距离以及岩相—生物相特征,可分为近滨白沙型和远滨印江型。白沙型生物—沉积相以香树园—雷家屯剖面为代表,是壳相后生动物群的主要分布区,频繁出现生物礁—介壳滩灰岩,其中,壳相大化石或生屑、内碎屑颗粒丰富;同期异相的印江型限于印江、务川远滨带,所分布的香树园组则常见黄色灰泥质泥岩夹瘤状泥灰岩、泥质灰岩,单体四射珊瑚居多,还见小型复体珊瑚、三叶虫和笔石等,较高的海水浑浊度对适宜于清澈水体生长的生物礁群落有明显抑制作用,因而这个时期不发育生物礁。Li and Kershaw(2003)、李越(2004)和倪超等(2016)均以香树园—雷家屯剖面为范例,分析埃隆期近滨相带各类生态单元在纵向上从简单到复杂的发展序列。该剖面在志留系最底部鲁丹阶的黄灰色粉砂岩、泥岩中就开始出现薄层含生物碎屑灰岩(葛治洲等(1979)称其龙马溪组;戎嘉余等(2004)称为“龙马溪组”,以区别于扬子区其它剖面百余米厚典型黑色笔石页岩型的龙马溪组);之上与香树园组呈整合接触(图 1a;葛治洲等,1979)。Rong et al.(2003)和戎嘉余等(2004)限定香树园组顶部生物地层学对比为兰多维列世的埃隆中期。Li and Kershaw(2003)、李越(2004)均提及香树园组上部数十米厚的泥岩沉积结束后,稳定的清澈海水环境有利于扬子区志留纪最早的生物礁复苏,并简单描述了该层状礁的生长过程。作者近年数次野外勘察中剖析该生物礁亚相单元的空间展布,进一步勾勒出清晰的礁核边界,确认这是一个点礁。本文描述此礁灰岩的岩石学特征,以此勾勒礁体形态学以及化石组合分异,讨论当时海区的延展方向与礁灰岩沉积方式的相关性,并诠释其生物—环境的协同演化。
2 生物礁的形态学构建和礁灰岩微相分布于香树园组顶部生物礁层位相当于中国科学院南京地质古生物研究所(1974)建组剖面柱示图,厚度为16.41m,也相当于戎嘉余等(2004)所描记的香树园组38m地层的上部,下部为钙质泥岩所余下的灰岩部分。李越(2004)对含大量壳相化石的灰岩进行了研究,厚度为18.5m,其中珊瑚、层孔虫等大量格架生物繁盛,昭示生物礁复苏的时间节点。正因为这段灰岩地层属于生物礁相,生物礁的主要特征就是平面上厚度有变化,所以各文献中所记录的剖面线厚度有些差异。
黔东北的志留系产状多平缓,没有经历过剧烈的构造变形。目前的地层展布显示为南陆北海的方向,此据可确认礁体的南侧为当时的礁后亚相,而北延约30m处为礁前亚相,礁前和礁后之间约150m的区间内为礁核生长带(图 1b);以泥岩和粉砂岩出现作为香树园组之上并呈整合接触在雷家屯组底界。礁体地层略向北呈5°~7°倾斜,野外从礁核的东侧可观测到除礁基外的礁后、礁核、礁前以及礁顶亚相的大致全貌(图 2a)并复原各亚相的岩相学特征(图 2b)。
香树园组经历了早期生物滩的演化阶段,香树园组上段泥岩—粉砂岩沉积结束后出现礁基相棘屑滩,在棘屑滩上开始点礁生长过程(李越,2004)(图 1b),在粗颗粒的棘屑中混杂有约10%的珊瑚碎片(图 3a)。
礁后亚相与礁核亚相的接触界线可根据灰岩层的厚度和化石颗粒的埋葬学特征勾画出来(图 2a、图 2b)。礁后亚相的薄层灰岩(图 3b)与礁核亚相的块状灰岩突变界线清晰且呈指状穿插,可识别出礁核生长过程中呈3次向礁后拓展的趋势,拓展宽度约数米。礁后多沉积棘屑,而珊瑚等动物格架碎片含量偏少,说明海百合死亡后皮膜部分降解、大量茎板散落成较细的棘屑,容易被来自外海的水流向南搬运到礁后低洼地带堆积,而珊瑚—层孔虫等造礁生物格架岩抗浪能力强,体积大,不易被搬运。
2.3 礁核亚相礁核的北边界上部的灰岩已经被现代喀斯特作用剥蚀,其完整的原始轮廓为 图 2a和图 2b中虚线显示。大量复体四射珊瑚、床板珊瑚障积岩和层孔虫盖覆岩原地密集叠覆生长成块状礁核灰岩,鲜见陆源碎屑堆积。在礁核形成的整个过程中,局部造礁动物格架能互相固着生长,其抗浪作用是明显的,可形成局部正向地貌(图 3c、图 3d、图 3f),表现在这些格架岩之间的低洼带有利于无分选性(从砾级到砂级不等)的生屑堆积(图 3e)。
李越(2004)在礁核灰岩中识别出保存较完好的壳相附礁生物大化石,为少量三叶虫、腕足类、鹦鹉螺等。通过对珊瑚—层孔虫大型动物格架岩之间所堆积的较细生屑灰岩部分取样进行微相分析(图 4,图 5),可获知充填于其间的细颗粒生屑类型,藉此了解造礁群落中包括小型附礁生物的生物多样性。
从礁核相灰岩微相中可识别出Halysis(钙藻)、苔藓虫、棘屑、腕足类、腹足类以及其它软体动物、海绵、海绵骨针、介形类、三叶虫,还包括少量核形石;礁核灰岩中混有极少量石英砂。栖礁生长的腕足类丰富,但壳体偏薄,多以充填于珊瑚—层孔虫格架岩的碎片颗粒形式存在。
珊瑚格架岩在丰度上优于层孔虫盖覆岩,床板珊瑚在量上多于四射珊瑚,但在多样性上则次于四射珊瑚,其余晚奥陶世和志留纪生物礁中常见的的苔藓虫格架含量很低,钙藻或钙质微生物岩很少见,内碎屑中也不见凝块岩或叠层石,故可以确认主要造礁者是床板珊瑚—四射珊瑚—层孔虫。
2.4 礁前亚相礁前亚相仅有约3m的残余厚度,为珊瑚、层孔虫和棘屑角砾堆积的薄层生屑灰岩夹含少量的粉砂岩(图 2d),与泥页岩略呈指状交叉,且略向北倾斜,礁前薄层灰岩夹粉砂,与礁核很纯的块状灰岩差别明显,说明礁核向上生长时的礁平台部位水体清澈度高,也具有高于礁前部位的正向地貌隆起。
2.5 礁顶亚相礁顶亚相与礁核亚相的界线是渐变的,即粉砂质和泥质成分渐增,从礁核相很纯的灰岩变成灰岩夹含泥沙的钙质泥岩,礁核部位常见的大型珊瑚块状珊瑚—层孔虫叠覆生长格架岩已经消失,所见到少量的小型皮壳状珊瑚和零星的单体四射珊瑚多呈原位状保存于灰岩或粉砂岩中,密度也逐渐变得稀少(图 2c),且常见多节海百合茎茎板愈合保存,指示了低水动能的生长和埋葬环境,之上逐渐过渡为雷家屯组底部页岩。
综上所述,香树园组上部的珊瑚—层孔虫礁生长于棘屑滩之上,由多样性生物,特别是在丰度上占优势的后生动物组成生态群落,造礁的珊瑚和层孔虫形成原地格架岩,其它附礁生物类型包括腹足类、双壳类、棘皮动物、腕足类、三叶虫等,但钙藻类和钙质微生物含量极低。礁核的直径和高度比例为1 ︰ 8,属正向地貌隆起角度偏平缓的礁体。
3 生物礁的成因机制奥陶纪末冈瓦纳冰盖的形成导致全球海平面下降,海水温度、生态系统分布区改变导致海洋生物大灭绝(Berry and Boucot, 1973;Brenchley and Cullen, 1984;Brenchley,1988;Brenchley et al., 1994,2003;Berry,1996;Sheehan et al., 1996;Hallam and Wignall, 1997;Copper, 1997,2002)。奥陶纪末全球冰期影响到低纬度海区,导致纬向温梯度增强引发的多幕式生物集群绝灭事件,使生物礁群落受到不同程度的影响(Brenchley and Cullen, 1984;Webby,1984),仅在局部地区如加拿大Anticosti岛和Hudson湾盆地西北的Melville半岛,奥陶—志留界线附近还存在珊瑚造礁(Trettin,1978;McCracken and Barnes, 1981)。Copper and Brunton(1991)认为O—S界线之间热带生态系统没有显著的生物集群绝灭现象。随着志留纪开始全球暖期的到来,生物礁进入了Llandovery世的演化阶段。生物礁复苏在各大板块早晚不一,兰多维列世全球较大规模的生物礁主要集中于当时低纬度稳定浅海碳酸盐岩台地建造的北美地区,到埃隆期和特列奇期在才开始扩展到低纬度区(Copper and Brunton, 1991)。波罗的海板块鲁丹阶Hilliste 组的小型珊瑚—层孔虫礁(Nestor,1995)是目前已知奥陶纪末生物大灭绝事件之后生物礁最早复苏的例证;埃隆期发育较好生物礁主要见于劳伦板块(Dixon and Graf, 1992;DeFreitas and Dixon, 1995;Brunton et al., 1998;DeFreitas et al., 1999)和华南板块(万云等,1995;李越,2004)。Droser and Sheehan(1997)和Droser et al.(2000)认为尽管晚奥陶世的生物大灭绝事件造成了生物多样性的骤减,但海洋生态群落的构建方式上并无实质改变。
奥陶纪末的生物灭绝事件在华南板块的化石以及地球化学记录中得以体现(Wang et al., 1992;Rong et al., 2002;Chen et al., 2004,2005),赫南特期(Hirnantian)冰盖持续过程中,广布于上扬子区的观音桥组灰岩可次分为远岸冷水型和近岸区(如石阡、凤冈、仁怀、毕节)暖水型(Li et al., 2005;李越等,2008),后者为保存部分暖水型壳相动物生态单元提供了避难所。目前仅在上扬子区记录有志留纪生物礁的复苏过程,Li and Kershaw(2003)和李越(2004)从生物礁宏观演化的角度分析了华南板块晚奥陶世灭绝事件之前的生物礁与志留纪复苏的生物礁差异性;李越等(2007)推测上扬子区陆表海古水温回暖偏迟,这可能是控制生物礁复苏时间节点滞后的原因之一。
扬子区埃隆中期生物礁仅限于黔中古陆以北的陆表浅海区分布,白沙型香树园组是大灭绝事件后后生动物群落复苏过程的见证者。石阡香树园组上部的礁体宏观形态学上属于Copper(1984)和Copper and Brunton(1991)定义的小型点礁,造礁群落属全球志留纪生物礁最常见的类型,与华南板块晚奥陶世冰川—生物灭绝事件到来之前分布于江西玉山—浙江常山地区的下镇组—三衢山组生物礁组合群落结构(Li et al., 2004)具有一定相似性,其中的珊瑚含量特别丰富,均达到了早古生代浅海底栖群落演化的最多样性阶段;差异性在于后者海百合碎屑含量较弱;在礁灰岩厚度上则呈数量级差别,因为三衢山组沉积于斜坡位置,沉降和沉积补偿速度大,在厚度上可达千米。兰多维列世上扬子区陆表海缺乏厚层、大型灰泥丘生长的基本条件。
Brett(1984)认为志留纪棘皮动物,特别是海百合茎碎片是海相钙质生物颗粒最主要的成分之一。香树园组礁灰岩中礁基和礁后两个亚相带棘屑密度最高,在礁核部位含量中等,Kidwell(1985,1986)指出的壳相化石密集形成不同的海洋底质条件对栖居海底古生态群落变化的埋葬反馈过程,在香树园组礁基亚相中得到了很好的体现。香树园组礁核微相中显示化石颗粒之间的充填基质为灰泥或粉砂屑,不见亮晶方解石胶结。礁核生长过程中水动力强度中等,能有效保持珊瑚—层孔虫密集生长需要的清洁度高而富氧的水体环境,只能达到将较细的生屑颗粒带到礁后堆积的能量指标,不能达到击碎珊瑚—层孔虫格架并完全淘洗掉灰泥沉积物的强风暴浪级别,也不能搬运粗砾级的颗粒。礁核部位形态学上呈平缓状,在礁体北侧向外海的方向多被现代喀斯特作用剥蚀掉,推测可能存在由成岩固结砾块堆积礁前塌积岩。目前残余礁前堆积的含粉砂质、泥质灰岩薄层与礁核的岩性可能是逐渐过渡的。李越(2004)从生屑颗粒埋葬学角度认为可能是一次海侵事件使香树园组顶部礁平台的高能带变成潮下低能带。海水深度增加导致水流荡涤作用减弱和陆源粉砂、泥质的沉积速率增强,海水浑浊度逐渐增高窒息了珊瑚—层孔虫造礁群落正常生长。礁体朝北外海的迎风带以及南侧的背风带水动力条件的强弱是形成礁前、礁后岩相分异的重要因素,从南靠黔中古陆海岸一侧的礁后、块状灰岩礁核、以及向北的礁前相带在百米距离内均可辨析出来,礁后为棘屑颗粒为主的堆积区,大量原地生长的珊瑚—层孔虫格架形成抗浪结构和局部正向地貌,有效障积部分生屑颗粒,有利于礁核宏观上形成略高于周缘滩相的隆起地形。
4 结 论奥陶纪末生物大灭绝事件后的黔东北局部近岸浅海区在埃隆期中期维持了清澈度高且温暖富氧的水体,为香树园组上部后生动物造礁群落生长形成了理想环境,礁体规模小。南陆北海的地貌和水动力条件的差别造成了点礁的亚相分异;珊瑚—层孔虫等具有抗浪能力并以障积生屑的方式构成正向隆起地貌;雷家屯组开始时高浑浊度的海水是结束礁生长的直接原因。这一实例突显生物演化事件与古地理背景对生物礁复苏时间、生长规模、群落结构的控制。
致谢 感谢中国科学院地质与地球物理研究所吴亚生博士对文稿修改提出的建议。
[1] | 陈旭, Mitchell C E. 1996. 塔康运动与广西运动的地层学依据. 地层学杂志, 20 (4): 305-313 . |
[2] | Chen Xu and Mitchell C E. 1996. Stratigraphic evidences on Taconican and Guangxian orogeny. Journal of Stratigraphy, 20 (4): 305-313. |
[3] | 陈旭, 戎嘉余, 周志毅等. 2001. 上扬子区奥陶—志留纪之交的黔中隆起和宜昌上升. 科学通报, 46 (12): 1052-1056 . |
[4] | Chen Xu, Rong Jiayu, Zhou Zhiyi et al. 2001. The central Guizhou and Yichang uplifts, Upper Yangtze region, between Ordovician and Silurian. Chinese Science Bulletin, 46 (18): 1580-1584. |
[5] | 葛治洲, 戎嘉余. 1979. 西南地区的志留系. 见: 西南地区碳酸盐生物地层. 北京: 科学出版社. 155-220. |
[6] | Ge Zhizhou and Rong Jiayu. 1979. Silurian system of southwestern China. In: Carbonate Biostratigraphy of Southwestern China. Beijing: Science Press. 155-220. |
[7] | 李江海, 杨静懿, 马丽亚等. 2013. 显生宙烃源岩分布的古板块再造研究. 中国地质, 40 (6): 1683-1698 . |
[8] | Li Jianghai, Yang Jingyi, Ma Liya et al. 2013. A study of the distribution of source rocks in Phanerozoic based on paleoplate reconstruction. Geology in China, 40 (6): 1683-1698. |
[9] | 李越. 2004. 华南晚奥陶世至早志留世生物礁的演化历程. 见: 戎嘉余, 方宗杰主编. 生物大灭绝与复苏——来自华南古生代和三叠纪的证据. 合肥: 中国科学技术大学出版社. 187-222. |
[10] | Li Yue. 2004. Reef evolution during the Late Ordovician to Early Silurian in South China. In: Rong Jiayu and Fang Zongjie(Eds.). Mass Extinction and Recovery: Evidences from the Palaeozoic and Triassic of South China. Hefei: University of Science and Technology of China Press. 187-222. |
[11] | 李越, 戎嘉余. 2007. 黔北志留纪早期枝线贝类介壳层的时空分布和风暴沉积特征. 科学通报, 52 (10): 1158-1167 . |
[12] | Li Yue and Rong Jiayu. 2007. Shell concentrations of Early Silurian virgianid brachiopods in northern Guizhou: Temporal and spatial distribution and tempestite formation. Chinese Science Bulletin, 52 (12): 1680-1691. |
[13] | 李越, 王建坡, 张园园等. 2008. 华南奥陶—志留纪之交的碳酸盐岩对古气候事件的诠释. 自然科学进展, 18 (11): 1264-1270 . |
[14] | Li Yue, Wang Jianpo, Zhang Yuanyuan et al. 2008. Perspective of carbonates during the Ordovician-Silurian transition in South China: Implications of their palaeoclimate response. Progression on Natural Sciences, 18 (11): 1264-1270. |
[15] | 刘伟, 许效松, 余谦. 2011. 探讨黔中古隆起形成机制及演化. 沉积学报, 29 (4): 658-664 . |
[16] | Liu Wei, Xu Xiaosong and Yu Qian. 2011. Discussion on forming mechanism and evolution of the central Guizhou palaeouplift. Acta Sedimentologica Sinica, 29 (4): 658-664. |
[17] | 中国科学院南京地质古生物研究所. 1974. 西南地区地层古生物手册. 北京: 科学出版社. 1-454. |
[18] | Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. 1974. Palaeontological Atlas of Southwest China. Beijing: Science Press. 1-454. |
[19] | 倪超, 李越, 邓小杰. 2016. 黔东北石阡香树园剖面志留系香树园组生屑滩微相. 微体古生物学报, 33 (1): 94-104 . |
[20] | Ni Chao, Li Yue and Deng Xiaojie. 2016. Microfacies of bioclastic banks of the Silurian Xiangshuyuan Formation at the Xiangshuyuan section, Shiqian, NE Guizhou. Acta Micropalaeontologica Sinica, 33 (1): 94-104. |
[21] | 牛新生, 冯常茂, 刘进. 2007. 黔中隆起的形成时间及形成机制探讨. 海相油气地质, 12 (2): 46-50 . |
[22] | Niu Xinsheng, Feng Changmao and Liu Jin. 2007. Formation mechanism and time of Qianzhong uplift. Marine Origin Petroleum Geology, 12 (2): 46-50. |
[23] | 戎嘉余, 詹仁斌. 2004. 华南志留纪早期腕足动物的残存与复苏. 见: 戎嘉余, 方宗杰主编. 生物大灭绝与复苏——来自华南古生代和三叠纪的证据. 合肥: 中国科学技术大学出版社. 97-126. |
[24] | Rong Jiayu and Zhan Renbin. 2004. Survival and recovery of brachiopods in Early Silurian of South China. In: Rong Jiayua and Fang Zongjie(Eds.). Mass Extinction and Recovery: Evidences from the Palaeozoic and Triassic of South China. Hefei: University of Science and Technology of China Press. 97-126. |
[25] | 田海芹, 郭彤楼, 胡东风等. 2006. 黔中隆起及其周缘地区海相下组合与油气勘探前景. 古地理学报, 8 (4): 509-518 . |
[26] | Tian Haiqin, Guo Tonglou, Hu Dongfeng et al. 2006. Marine lower assemblage and exploration prospect of central Guizhou uplift and its adjacent areas. Journal of Palaeogeography, 8 (4): 509-518. |
[27] | 万云, 张廷山, 兰光志等. 1997. 川东南—黔北地区志留纪生物礁与古环境演化. 沉积学报, 15 (增刊): 106-113 . |
[28] | Wan Yun, Zhang Tingshan, Lan Guangzhi et al. 1997. Silurian reef and plaeoenvironment evolution in Chuandongnan-Qianbei, China. Acta Sedimentologica Sinica, 15 (suppl.): 106-113. |
[29] | Berry W B N and Boucot A J. 1973. Glacio-eustatic control of Late Ordovician-Early Silurian platform sedimentation and faunal changes. Geological Society of America Bulletin, 84 (1): 275-284. |
[30] | Berry W B N. 1996. Recovery of post-Late Ordovician extinction graptolites: A western North American perspective. Geology Society, London, Special Publication, 102 (1): 119-126. |
[31] | Boucot A J, Chen X and Scotese C R. 1995. Ibexian and Post-Ibeian paleogeography based on climatically sensitive sediments and biogeographic data. In: Cooper J D, Droser M L and Finney S C(Eds.). Ordovician Odyssey: Short Papers for the Seventh International Symposium on the Ordovician System. SEPM Pacific Section. 291-295. |
[32] | Brenchley P J and Cullen B. 1984. The environmental distribution of associations belonging to the Hirnantia fauna: Evidence from North Wales and Norway. Aspects of the Ordovician System, 295 : 113-125. |
[33] | Brenchley P J, Carden G A F, Hint L et al. 2003. High-resolution stable isotope stratigraphy of Upper Ordovician sequences: Constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation. GSA Bulletin, 115 (1): 89-104. |
[34] | Brenchley P J, Marshall J D, Carden G A F et al. 1994. Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period. Geology, 22 (4): 295-298. |
[35] | Brenchley P J. 1988. Environmental changes close to the Ordovician-Silurian boundary. Bulletin of the British Museum, Natural History.Geology, 43 :377-385. |
[36] | Brett C E. 1984. Autecology of Silurian pelmatozoan echinoderms. Special Papers in Palaeontology, 32 : 87-120. |
[37] | Brunton F R, Smith L, Dixon O A et al. 1998. Silurian reef episodes, changing seascapes and paleobiogeography. In: Landing E and Johnson M(Eds.). Silurian Cycles. Linkages of Dynamic Stratigraphy with Atmospheric, Oceanic, and Tectonic Changes(New York State Museum No. 491). Albany: University of Sate of New York, State Education Department.265-282. |
[38] | Chen X, Rong J Y, Li Y et al. 2004. Facies patterns and geography of the Yangtze region, South China, through the Ordovician and Silurian transition. Palaeogeography, Palaeoclimatology, Palaeoecology, 204 (3-4): 353-372. |
[39] | Copper P and Brunton F H. 1991. A global review of Silurian reefs. Special Papers in Palaeontology, 44 : 225-259. |
[40] | Copper P. 1984. A late Llandoverian patch reef complex of eastern Anticosti Island, Quebec. Geologic Association of Canada, Programs with Abstracts, 9 (4). |
[41] | Copper P. 1997. Reefs and carbonate productivity: Cambrian through Devonian. In: Proceedings of the 8th International Coral Reef Symposium, 2.1623-1630. |
[42] | Copper P. 2001. Reefs during the multiple crises towards the Ordovician-Silurian boundary: Anticosti Island, eastern Canada, and worldwide. Canadian Journal of Earth Sciences, 38 (2): 153-171. |
[43] | Copper P. 2002. Silurian and Devonian reefs: 80 million years of global greenhouse between two ice ages. In: Kiessling W, Flügel E and Golonka J(Eds.). Phanerozoic Reef Patterns. SEPM Special Publication, 72.181-238. |
[44] | DeFreitas T A and Dixon O A. 1995. Silurian microbial buildups of the Canadian Arctic. Special Publications of the International Association of Sedimentologists, 23 :151-169. |
[45] | DeFreitas T A, Trettin H P, Dixon O A et al. 1999. Silurian system of the Canadian Arctic archipelago. Bulletin of Canadian Petroleum Geology, 47 (2): 136-193. |
[46] | Dixon O A and Graf G C. 1992. Upper Silurian reef mounds on a shallowing carbonate ramp, Devon Island, Arctic Canada. Bulletin of Canadian Petroleum Geology, 40 (1): 1-23. |
[47] | Droser M L and Sheehan P M. 1997. Palaeoecology of the ordovicianradiation; resolution of large-scale patterns with individual clade histories, palaeogeography and environments. Geobios, 30 : 221-229. |
[48] | Droser M L, Bottjer D J, Sheehan P M et al. 2000. Decoupling of taxonomic and ecological severity of Phenerozoic marine mass extinctions. Geology, 28 (8): 675-678. |
[49] | Hallam A and Wignall P B. 1997. Mass Extinctions and Their Aftermath. Oxford University Press. 1-320. |
[50] | Kidwell S M. 1985. Palaeobiological and sedimentological implications of fossil concentrations. Nature, 318 (6045): 457-460. |
[51] | Kidwell S M. 1986. Models for fossil concentrations: Paleobiologic implications. Paleobiology, 12 (1): 6-24. |
[52] | Li Y and Kershaw S. 2003. Reef reconstruction after extinction event of latest Ordovician in Yangtze platform, South China. Facies, 48 (1): 269-284. |
[53] | Li Y, Kershaw S and Mu X N. 2004. Ordovician reef systems and settings in South China before the Late Ordovician mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, 205 (3-4): 235-254. |
[54] | Li Y, Matsumoto R and Kershaw S. 2005. Sedimentary and biotic evidence of a warm-water enclave in the cooler oceans of the latest Ordovician glacial phase, Yangtze platform, South China. Island Arc, 14 (4): 623-635. |
[55] | McCracken A D and Barnes C R. 1981. Conodont biostratigraphy and paleoecology of the Ellis Bay Formation, Anticosti Island, Québec, with special reference to Late Ordovician-Early Silurian chronostratigraphy and the systemic boundary. Geological Survey of Canada Bulletin, 329 : 51-134. |
[56] | Nestor H. 1995. Ordovician and Silurian reefs in the Baltic area. Publications du Service Géologique de Luxembourg, 29 : 39-47. |
[57] | Rong J Y, Chen X and Harper D A T. 2002. The latest Ordovician Hirnantia Fauna(Brachiopoda)in time and space. Lethaia, 35 (3): 231-249. |
[58] | Rong J Y, Chen X, Su Y Z et al. 2003. Silurian paleogeography of China. In: Landing E and Johnson M E(Eds.). Silurian Lands and Seas: Paleogeography Outside of Laurentia(New York State Museum No. 493). Albany: University of State of New York, State Education Department.243-298. |
[59] | Scotese C R and McKerrow W S. 1990. Revised world maps and introduction. Geological Society, London, Memoirs, 12 : 1-21. |
[60] | Sheehan P M, Coorough P J and Fastovsky D E. 1996. Biotic selectivity during the K/T and Late Ordovician extinction events. Geological Society of America Special Paper, 307 : 477-489. |
[61] | Trettin H P. 1978. Devonian Stratigraphy, West-Central Ellesmere Island, Arctic Archipelago(GSC Bulletin 302). Geological Survey of Canada. 1-119. |
[62] | Wang K, Chatterton B D E and Orth C J. 1992. Iridium abundance maxima at the latest Ordovician mass extinction horizon, Yangtze Basin, China: Terrestrial or extraterrestrial? Geology, 20 (1): 39-42. |
[63] | Webby B D. 1984. Ordovician reefs and climate: A review. Aspects of the Ordovician System, 295 : 89-100 . |
2. University of Chinese Academy of Sciences, Beijing 100049