皖南地区埃迪卡拉系皮园村组微体和宏体化石--兼论埃迪卡拉纪-寒武纪界线

MICRO-AND MACROFOSSILS FROM THE PIYUANCUN FORMATION AND THEIR IMPLICATIONS FOR THE EDIACARAN-CAMBRIAN BOUNDARY IN SOUTHERN ANHUI

国际上寒武纪的开始以遗迹化石Treptichnus pedum的首次出现为标志,我国南方通常以小壳化石的出现或者疑源类Asteridium-Comasphaeridium-Heliosphaeridium化石组合的出现作为前寒武纪-寒武纪界线。由于化石保存的不完整性,仅仅依靠寒武纪化石的首现来确定埃迪卡拉纪-寒武纪界线可能导致其层位偏高,因此需要利用界线之下的埃迪卡拉纪化石对界线进行进一步限定。皖南地区的埃迪卡拉纪-寒武纪界线一直被厘定在皮园村组硅质岩和荷塘组底部磷块岩或黑色页岩交界处,这一划分主要是根据岩石地层的证据和邻区的生物地层资料,但是皮园村组本身缺乏直接的生物地层学资料。为了进一步确定皖南地区的埃迪卡拉纪-寒武纪界线和皮园村组的沉积年龄,系统研究了皖南休宁蓝田剖面皮园村组中的微体化石组合,在皮园村组中发现了埃迪卡拉纪化石Horodyskiacf.minor、Palaeopascichnus cf.jiumenensis、Palaeopascichnus sp.,以及未定名的纺锤状化石和球形化石。Palaeopascichnus在全球范围的埃迪卡拉纪地层中都有发现,且其仅仅局限在埃迪卡拉纪的晚期。在华北地台西缘的正目观组板岩和华南留茶坡组硅质岩曾报道有Horodyskia minor,H.moniliformis?(有可能为H.minor的同物异名),Palaeopascichnus minimus,P.meniscatus,和P.jiumenensis。在皖南Palaeopascichnus cf.jiumenensis和Palaeopascichnus sp.出现在皮园村组硅质岩的中上部,Horodyskiacf.minor一直延伸到皮园村组的顶部,说明由生物地层所划分的埃迪卡拉纪-寒武纪界线与皮园村组-荷塘组岩石地层的界线基本上是一致的。

The Ediacaran-Cambrian boundary is defined at the first appearance of the trace fossil Treptichnus pedum by the International Stratigraphy Commission. In South China, the boundary is traditionally placed at the appearance of the first small shelly fossil assemblage or the AsteridiurrrComasphaeridium- Heliosphaeridium acritarch assemblage. However, due to the incompleteness of the fossil record, it is possible that the first observed Cambrian fossils may be younger than the Ediacaran-Cambrian boundary. Therefore, it is important to further constrain the Ediacaran-Cambrian boundary using characteristic Edia caran fossils below the boundary. In southern Anhui Province, the Ediacaran-Cambrian boundary is tradi- tionally placed at the lithostratigraphic boundary between the Piyuancun Formation and the overlying Hetang Formation. This placement is mainly based on basal Cambrian fossils from the lowermost Hetang Formation in the neighboring western Zhejiang Province. However, no paleontological data have been reported from the Piyuancun Formation. To further constrain the age of the Piyuancun Formation and the Ediaearan-Cambrian boundary in southern Anhui, we systematically studied the microfossil record in the Piyuancun Formation at Lantian, southern Anhui. We identified several characteristic Ediacaran taxa, including Palaeopascichnus cf. jiumenensis, Horodyskia cf. minor, Palaeopascichnus sp. , as well as unnamed spindle-shaped and spherical microfossils. The genus Palaeopascichnus has a world-wide distribu- tion but it is restricted to late Ediacaran strata. Horodyskia minor and Palaeopascichnus jiumenensis have been reported from the upper Ediacaran Liuchapo Formation in Guizhou, and Horodyskia moniliformis？ （possibly synonymous to H. minor）, Palaeopascichnus minimus , and Palaeopascichnus meniscatus occur in the upper Ediacaran Zhengmuguan Formation in Ningxia. Thus, the occurrence of Palaeopascichnus cf. jiumenensis and Palaeopascichnus sp. in the middle-upper part of the Piyuancun Formation, and the occurrence of Horodyskia cf. minor up to the very top of the formation, indicate that the Ediacaran-Cambrian boundary in southern Anhui is indeed very close to the lithostratigraphic boundary between the Piyuancun and Hetang formations.