北山地区碎石山剖面菊石动物群的发现及其地质意义

Discovery of the Ammonite Fauna from Suishishan Section,Beishan Area and Its Geological Significance

北山地区碎石山剖面为一套碎屑岩为主的地层,前人认为地质时代属于早石炭世杜内期,划归绿条山组,其主要分布于中蒙边界,呈带状近东西向展布。在碎石山剖面采获了丰富的、保存完好的菊石化石,共鉴定出11属16种(含未定种)。菊石以棱菊石目中的腹菊石科最为丰富,就属而言,以Gastrioceras、Branneroceras和Reticuloceras为主,该菊石动物群可以与华北、华南、西亚、东欧、北非和北美等地区对比,属于Gastrioceras-Branneroceras带,所以将碎石山剖面的地层时代重新厘定为晚石炭世滑石板期,岩石地层划归石板山组。说明杜内期的地层在碎石山地区并不存在。古亚洲洋闭合以后,在额济纳旗—银根地区伸展背景下形成的最早沉积物为白山组和石板山组。白山组主要分布于北山西部,岩性主要为火山岩夹灰岩组合,盆地性质为裂谷盆地;到滑石板期,盆地向东迁移,沉积形成石板山组,岩性以碎屑岩为主,火山岩不发育,盆地性质以裂陷盆地为主。

The strata of Suishishan section were assigned to the Tournaisian （Early Carboniferous）, and were classified into the Ltitiaoshan Formation. These strata are mainly distributed along the Sino-Mongoli- an border, extending from east to west like a belt. The authors collected abundant, well-preserved am- monoid fauna from the so-called ＂Ltitiaoshan Formation＂ at Suishishan area, and identified 16 species in 11 genera （including undetermined species）. This ammonoid fauna is dominated by the family Gastrioceratid- ae, including genera, Gastrioceras, Branneroceras and Reticuloceras. The genera Gastrioceras and Bran neroceras are index fossils of the Gastrioceras-Branneroceras Zone, typical for the age of the Upper Car- boniferous Huashibanian. The ammonite fauna can be compared with those faunas from the northern Chi- na, southern China, western Asia, eastern Europe, North Africa and North America. Therefore, the so- called ＂Ltitiaoshan Formation＂ in this paper should be assigned to the Upper Carboniferous Huashibanian in age and to the Shibanshan Formation in lithostratigraphy. It can be further indicated that the Ejinaqi- Yingen extensional basin was first formed in western Beishan in Visean following the closure of the Paleoa- sian Ocean during the Frasnian （Late Devonian）, with the deposition of the Baishan Formation dominated by volcanic rocks intercalated with limestones. This rift basin extended eastward during the Huashibanian, without the deposition of volanic rocks.