四川南部埃迪卡拉纪-寒武纪过渡时期碳同位素地层学再研究

A RESTUDY OF CARBON ISOTOPE CHEMOSTRATIGRAPHY OF THE EDIACARAN-CAMBRIAN TRANSITION IN SOUTHERN SICHUAN PROVINCE

扬子台地作为中国埃迪卡拉纪-寒武纪过渡时期地层研究的重要区域,已经建立起一套较为完整的生物地层序列。然而,扬子台地寒武系下部地层的划分和对比目前依然存在一些困难和争议。在生物地层尚未得到很好限定的情况下,使用非生物地层标准,特别是碳同位素异常事件作为辅助标准来划分和对比寒武系下部地层值得尝试。本文选择四川南部乌斯河大桥剖面与范店剖面的灯影组和麦地坪组开展碳同位素地层学研究,结果显示乌斯河大桥剖面碳同位素值自下而上在稳定正值后,在麦地坪组底部出现了一次显著的负异常事件,由0左右快速降至-3.86‰;而范店剖面碳同位素值自下而上在稳定负值后,在麦地坪组上段出现了一次正异常事件,由-1.12‰左右快速升至3.43‰。在小壳化石生物地层学框架下,通过与扬子台地内其他地区寒武系下部典型剖面的碳同位素变化曲线进行对比后发现:乌斯河大桥剖面麦地坪组底部的碳同位素负异常事件为BASE事件;范店剖面麦地坪组上部碳同位素正异常的底部与Watsonella crosbyi首现点一致,是ZHUCE事件的可靠记录。本研究不仅补充了该地区碳同位素化学地层信息,也为认识扬子台地不同地区间寒武系下部地层发育差异与地层划分对比提供了有价值的证据。此外,通过综合分析华南、西伯利亚、蒙古和摩洛哥四个地区的经典剖面的同期地层碳同位素曲线与相关化石记录及同位素年龄的关系,本文认为碳同位素事件BACE与ZHUCE具有全球可对比性。虽然全球小壳化石的首现与BACE事件底部负异常的开始点不完全一致,但在有一定生物地层限定的情况下,结合同位素年龄, BACE事件可以作为寒武系底界对比的标志。而在大部分地区, ZHUCE事件底部与Watsonella crosbyi首现点一致,具有作为寒武系纽芬兰统第二阶地层划分对比辅助标准的潜力。

Yangtze Platform is a key region for the studies of the Ediacaran-Cambrian transition in China, where a series of successive biozones have been established. However, the subdivision and correlation of the traditional lower Cambrian strata on Yangtze Platform still remain some uncertainties. While no robust biostratigraphic constraint is available, it is still worth trying to use other auxiliary stratigraphic tools, especially carbon isotope chemostratigraphy,to divide and correlate the traditional lower Cambrian successions. Herein, we conduct a study of the carbon isotope(δ13Ccarb) chemostrigraphy of the Dengying and Maidiping formations from the Wusihe-Daqiao section in Hanyuan,and the Fandian section in Leshan, southern Sichuan Province. A negative carbon isotope excursion is identified at the base of the Maidiping Formation in the Wusihe-Daqiao section, and a positive carbon isotope excursion with its onset corresponding to the FAD of Watsonella crosbyi is recognized at the upper Maidiping Formation in the Fandian section. By comparing with other coeval carbon isotope profiles from some classic lower Cambrian sections on the Yangtze Platform under the control of small shelly fossil biostratigraphy, we suggest that the negative excursion in the Wusihe-Daqiao section is corresponding to the BASE(Basal Cambrian Carbon isotope Excursion), and the positive excursion in the Fandian section is correlative to ZHUCE(Zhujiaqing carbon isotope excursion). This study not only reports the new carbon isotope data from southern Sichuan Province, but also provides valuable information for understanding the stratigraphic heterogeneities as well as the subdivision and correlation of the lower Cambrian strata within the Yangtze Platform. Based on a correlation between the present results and those published classic Terreneuvian stratigraphic studies in the world, we confirm that the BACE and ZHUCE are of global and synchronous occurrences and can serve to define respectively the lower boundaries of the Fortunian and the Stage 2, along with constraints of biostratigraphy and isotopic geochronology.