陕西汉中梁山实习基地大南沟剖面寒武系灰岩层化石组合及地层对比

Small skeletal fossils from the lower Cambrian carbonates of the topmost Dengying and the Xiannvdong Formations at the Danangou Section in Liangshan Area,Hanzhong of Southern Shaanxi,and their stratigraphic correlations

小壳化石是前寒武纪—寒武纪地层界限的主要生物标志,代表着地球生物矿化发生的起始,通常以磷酸盐化的骨骼碎片和微管保存,在寒武纪早期地层的划分与对比方面具有十分重要意义。该文首次系统描述了陕西汉中梁山地质实习基地大南沟剖面灯影组顶部杨家沟段以及寒武系仙女洞组灰岩中的微型壳体化石。研究发现,杨家沟段富含似软舌螺类、开腔骨类、织金壳类等小壳化石;仙女洞组保存有大量的三叶虫碎片(小寻甸虫未定种Syndianella sp.、宜良虫未定种Yiliangella sp.、瘤壳马龙虫Malungia granulosa)、腕足动物乳孔贝和博茨傅徒贝、软舌螺类、似软舌螺、开腔骨类、高肌虫、棘皮动物和软体动物等壳体化石。其中,以乳孔贝和博茨傅徒贝为代表的磷酸钙质腕足动物,在滇东马龙地区主要出现在玉案山组顶部到乌龙箐组下部,但在鄂西三峡地区和镇巴房县地区主要从水井沱组的中下部大量出现。因此,通过与滇东和镇巴地区寒武纪早期壳体化石生物组合对比,认为梁山地区灯影组杨家沟段地层应属于寒武纪前三叶虫化石带,隶属纽芬兰世第二期;仙女洞组生物地层属于滇东地区沧浪铺阶,在国际地层对比中属于寒武纪第二世第三期晚期到第四期早期。

Small shelly(skeletal)fossils are the main index biomarkers of the Precambrian-Cambrian stratigraphic boundary,representing the onset of biomineralization of multicellular metazoans on Earth.They are usually preserved as phosphatized fragments and microtubules,which are of great significance in the subdivision and correlation of early Cambrian stratigraphic strata across different Chinese regions and continents.Here,we present the first description of the small shelly fossils in details,acid-etched from the lower Cambrian carbonates at the Danangou section,near Liangshan,Hanzhong of southern Shaanxi Province.Some of them were recovered from the bioclastic limestone of Yangjiagou Member in the upper-most Dengying Formation,and the other majority from the archaeocyathids-bearing carbonate of Xiannvdong Formation.After a series of acid macerations of carbonates samples,the small shelly(skeletal)fossil assemblages of hyolithelminths,chancelloriids and zhijinitids were recognized in Yangjiagou Member;Xiannvdong Formation yielded a large number of fossils,including trilobites fragments and abundant shelly lingulate brachiopods consisting of acrotretids and botsfordiids,as well as hyoliths,hyolithelminths,chancelloriids,bradoriid,echinoderm,carinachitids and molluscs.Skeletal components comprising the brachiopod acrotretids and botsfordiids mainly occurred from the upper of Yuanshan Formation to the lower Wulongqing Formation in Malong County of eastern Yunnan.At the meantime,they made their rapid appearance in the middle and lower of Shuijingtuo Formation in Three Gorges and Zhenba-Fangxian areas.In the lights of shelly fossil assemblages of early Cambrian in eastern Yunnan and Zhenba areas,the Yangjiagou Member of Dengying Formation in Liangshan area could be correlated with the pre-triolobite Cambrian Stage 2 with abundant occurrences of hyolithelminths,chancelloriids.By contrast,fossil assemblages recovered from the Xiannvdong Formation in the Danangou section contains rich fragments of trilobites(Syndianella sp.,Yiliangella sp.,Malungia granulosa),and abundant lingulate brachiopod shell vavles of acrotretids and botsfordiids.All of the trilobite and brachiopod assembalges indicates an age of the Canglangpuan Stage in eastern Yunnan,commonly correlated with the late Stage 3 to early Stage 4 of Cambrian Series 2 in the newly revised International Chronostratigraphic Chart.