燕辽生物群和燕山运动

YANLIAO BIOTA AND YANSHAN MOVEMENT

燕辽生物群和热河生物群是以我国燕辽地区为核心的两个中生代中晚期特异埋藏生物群,关系到哺乳动物的早期演化,最早的带毛恐龙,鸟类起源,被子植物起源,寄生昆虫等重大科学问题,引起科学界广泛关注。这两个生物群时代相差较远,代表分子差异很大。目前,研究者对燕辽生物群地层划分意见分歧很大,主要争论为内蒙宁城道虎沟化石层和辽宁建昌玲珑塔化石层的关系,将二者分别归入中侏罗统海房沟/九龙山组和中晚侏罗统髫髻山组,或均归入髫髻山组,这一分歧造成地层划分对比和生物演化序列的极大混乱,从未得到有效解决。本文结合近年来本地区同位素年代学、岩石地层学和生物地层学新进展,对上述地层划分提出新建议。结合前辈地质学家的基本思想对燕山运动的构造幕进行新的理解,并划分为序幕(即绪动期)约为168 Ma,第一幕(即火山幕)约为163.5 Ma,第二幕(即主动幕)约为135 Ma。其中序幕对燕辽生物群的形成至关重要,而第一幕导致其发生重大转变。道虎沟化石层的底砾岩角度不整合于前寒武系变质岩之上,这个界面代表燕山运动的绪动,在燕辽地区表现为砾石沉积,地壳垂直升降与小规模火山喷发,形成了一系列山间盆地,伴生了燕辽生物群的早期组合,如道虎沟生物群,以燕辽划蝽-"滦平真叶肢介"-费尔干蚌-初螈为代表,为中侏罗世中晚期。道虎沟化石层顶部的凝灰岩受到显著剥蚀,呈明显的角度不整合下伏于一套角砾岩之下,此不整合面代表燕山运动第一幕,造成一系列深度的北东向断裂,引发大规模岩浆侵入及髫髻山期火山喷发。燕辽生物群的早期群落随之瓦解并迅速扩散演化,且在燕辽地区发展形成燕辽生物群晚期组合,如玲珑塔生物群,以柴达木叶肢介-额尔古纳蚌-古鳕为代表,为晚侏罗世早期,与道虎沟生物群明显不同但存在广泛联系。京西九龙山-髫髻山组之下的不整合面和辽西海房沟组之下的不整合面不能对比,且髫髻山期大规模火山活动前存在两个重要的燕山期构造运动。过去的观点认为道虎沟化石层相当于九龙山组或在九龙山组之上,本文根据京西九龙山组是燕山运动第一幕之后的产物,而以道虎沟化石层为代表的海房沟组在其之前的基本观点,认为二者不能对比且是上下关系。燕山运动的第二幕发生于土城子组和张家口-大北沟组或义县组之间,从而造就了著名的热河生物群,它与燕辽生物群的过渡存在于土城子组。

The Yanliao and the Jehol biotas are two middle to late Mesozoic Lagerstatten represented mainly in northeastern China. They have received extensive attention since they contribute significantly to understanding multiple major scientific questions, including the origin and early evolution of mammals, earliest feathered dinosaurs, origin of birds, origin of angiosperms, origin of parasitic insects and so forth. The two biotas are of different ages and their representatives are barely related. Currently, the Yanliao biota-bearing strata are controversial particularly concerning the relationship between the Daohugou beds in Ningcheng Basin of Inner Mongolia and the Linglongta beds in Jianchang of Liaoning. They are assigned to the Middle Jurassic Haifanggou/ Jiulongshan Formation and Middle-Late Jurassic Tiaojishan Formation respectively, or both to the Tiaojishan Formation. The differences have caused great confusion about stratigraphic division and correlation between them, and the issue still remains to be solved. Integrating recent advances in isotope stratigraphy, lithostratigraphy and biostratigraphy, a new stratigraphic division framework is herein proposed. Based on fundamental hypothesis of earlier geologists, a better understanding of Yanshan Movement is made. Yanshan Movement is divided to the Initiation （ca. 168 Ma）, the First Stage （the volcanism, ca. 163. 5 Ma） and the Second Stage （ca. 135 Ma）. The Initiation is crucial to the formation of Yanliao biota, and the First Stage caused major transformation of this biota. The bottom conglomerates of Daohugou beds unconformably overlies the Precambrian metamorphic rocks, and the interface represents the Initiation of Yanshan Movement, which is characterized by conglomerate deposit in Yanliao area; significant vertical crustal movements and multiple small-scale volcanic eruptions also occurred, forming a series of inter-mountain basins and resulting in early assemblages of Yanliao biota, such as the Daohugou biota characterized by Yanliaocorixa- Qaidamestheria-Ferganoconcha-Chunerpeton , which belongs to middle-late Middle Jurassic. The tuff of the top layer of Daohugou beds was subject to remarkable erosion, and was an angular unconformity underlying beneath conglomerates; this unconformity represents the First Stage of Yanshan Movement, resulting in a series of deep northward faults and triggering massive magmatic intrusions and volcanic eruptions during the Tiaojishan period. Consequently, early assemblages of Yanliao biota began to collapse, and rapidly spread and evolve, and formed late assemblages of Yanliao biota, including the early Late Jurassic Linglongta biota characterized by Qaidamestheria-Arguniella-Palaeonisci f ormes , which is obviously different from and extensively related to Daohugou biota. The unconformities below the Jiuiongshan-Tiaojishan Formation of western Beijing and the Haifanggou Formation of western Liaoning are not corresponded, and there are two major tectonic movements before largescale volcanic activities during the Tiaojishan period. It was thought that the Daohugou beds were corresponding to Jiulongshan Formation or overlying it. According to the fact that Jiulongshan Formation in western Beijing formed after the First Stage of Yanshan Movement, while Haifanggou Formation in western formed before the First Stage and they are not correlated and are overlaying. The Second Stage of Yanshan Movement occurred between Tuchengzi and Zhangjiakou/Dabeigou Formation or Yixian Formation, which yielded the famous Jehol Biota. The transition between Yanliao and Jehol biotas should exist in the Tuchengzi Formation.