上扬子地区早三叠世特殊沉积：“无煤时期”的煤及“无后生动物礁时期”的海绵丘？

The Early Triassic coal gap and metazoan reef gap are often interpreted as the outcome of combined persistent or intermittent harsh environmental conditions and exacerbated biotic competitive pressures due to the devastating Permian–Triassic mass extinction about 252 million years ago. However, the special sediments of coal and sponge mound limestone were sensed from the Lower Triassic Feixianguan Formation in Upper Yangtze Region. Two sections at Yudong, Chaotian and Baimiaozi, Beibei will be selected as the major research sections to study these special Early Triassic sediments. Detailed investigation, measurement and sampling are planned for the 2 outcrop sections and the date of the Early Triassic coal and sponge limestone will be determined by means of the identification of the conodonts, sporopollen, etc. The lithological and mineralogical features of the coal and sponge- microbialite will be revealed according to the analyses as microscopic thin section, SEM, EPMA, etc. The information of sedimentary environment and paleoocean conditions represented from the sponge limestone will be studied in detail with field investigation, fossil identification, measurements of trace element and carbon isotopes, etc., while the sedimentary conditions represented from the Early Triassic (?) coal at Yudong, Chaotian will be researched with the measurements of pollen assemblages, organic carbon isotopes, trace element, etc. Based on these studies, the evolution of plants and metazoan sponges will be discussed from the latest Permian devastating mass extinction.

早三叠世通常被认为是“无煤时期”和“无后生动物礁时期”，申请者在上扬子地区朝天鱼洞剖面发现了疑为早三叠世的煤沉积，并在北碚白庙子剖面发现Griesbachian亚阶微生物丘灰岩中含钙质海绵。本次研究拟以朝天鱼洞、北碚白庙子剖面为主干剖面，选择飞仙关组薄煤层、煤线沉积（朝天鱼洞剖面）及海绵-微生物丘灰岩（北碚白庙子剖面）为主要研究对象，详细观察、测制、描述野外露头剖面，通过孢粉、牙形石等微古生物鉴定确定其时代，通过显微薄片、扫描电镜、电子探针等手段了解其岩石学、矿物学特征，综合野外和室内鉴定资料、微量元素、有机碳同位素测定资料研究北碚白庙子剖面海绵-微生物丘灰岩的沉积环境及其代表的古海洋特征，了解P-T生物大灭绝后古海洋条件与海绵灰岩发育之间的相互关系。应用孢粉组合、碳同位素、微量元素测试等判断早三叠世（？）煤层的沉积环境信息，探讨早三叠世煤层沉积的可能性及其反映的植物演化特点。

通过该项目资助，详细研究了北碚白庙子剖面发现Griesbachian亚阶微生物丘及球状粒，在该剖面中可见飞仙关组一段上部发育10余个穹窿状微生物丘，其生长在在厚约17m的浅灰色鲕粒灰岩之上，丘基为浅灰-灰紫色微生物岩，丘主体由三套微生物岩组成且微生物发育，丘盖为浅灰色生屑灰岩，微生物丘中可见由微生物（钙质蓝藻）、小型腹足类及介形虫组成的简单生态系统，并在一次快速海平面上升事件中停止发育。作为一种特殊的碳酸盐颗粒，下三叠统地层中沉积的球状粒鲜有发现和研究。在北碚地区，早格里斯巴赫期具泥晶结构的球状粒完好保存在下三叠统飞仙关组底部地层之中，直径一般为3.9~10.8mm，产于大套鲕粒灰岩中的灰泥岩夹层中。鲕粒灰岩顶界面具波痕构造，球状粒即沉积于波谷之中，镜下可见泥晶套、亮晶方解石、粘土矿物、有机质和黄铁矿。球状粒中发育球状微生物，平均直径为10.0~25.0μm，推测通过吸附海水中碳酸钙来促使球状粒的形成。球状粒之间的暗色基质主要由微晶方解石、粘土矿物、有机质和少量的后生生物化石组成。球状粒的形态结构通常为球形和椭球形，表明其经历了快速增长和岩化的过程。北碚地区的球状粒沉积作为一种特殊的碳酸盐岩沉积模式，记录了继生物大灭绝不久之后的缺氧和碳酸钙过饱和的古海洋条件。由于朝天鱼洞剖面晚三叠世煤线时代无法确定，原设想的煤线沉积研究成果未能发表相关论文。