Sedimentological Characteristics Of The Baitushan Formation In Songnen Plain And Its Indication For Stratigraphic Divisions

The Baitushan Formation is the lowest stratum of the Early Pleistocene in the Songnen Plain.In-depth analysis of its stratigraphic attributes is highly significant for Quaternary stratigraphic division.However,the lack of other studies of the Baitushan Formation limit the deep understanding of relevant problems,such as stratigraphic division of the Songnen Plain and the evolution of regional tectonism,geomorphological factors and drainage.Therefore,the Yaobaitu,Ping’anzhen,Chuo’erhe and Wangfutun profiles were selected for analysis of their sedimentological and heavy mineral properties.The results showed that:(1)The Yaobaitu and the Ping’anzhen profiles are the standard Baitushan Formation strata,with lithology consisting mainly of grayish-white sand and gravels interspersed with clay and fine-grained clastic lenses,with oblique bedding(dipping to the southeast)seen in the lenses,and the gravels are mainly medium and coarse gravels,poorly sorted,sub-angular to sub-rounded and moderately weathered gravels.Gravel lithology is mostly composed of tuff,quartz,rhyolite and clastic rock.(2)The sedimentary characteristics of Chuo’erhe profile are similar to those of Yaobaitu and Ping’anzhen prifiles,and a small amount of diabase gravel is found.Wangfutun profile is obviously different from the other three profiles,without gray clay lens,oblique bedding(dipping to the northwest)is widely developed.The gravel diameter(10～40mm)is small and weakly weathered gravels,with gravel composition almost completely dominated by quartz(87.5%).(3)From the perspective of mineralogy,the heavy mineral composition of the Yaobaitu and the Ping’anzhen profiles are highly simila,which is dominated by ilmenite,white titanite and epidote;But apatite-sphene-garnet are the diagnostic minerals in the Chuo’erhe profile;Metamorphic minerals(e.g.,kyanite crosstalk,tremolite and monazite)are contained only in the Wangfutun profile.This profile does not contain epidote,and extremely stable minerals are greatly enriched.(4)The Yaobaitu,Ping’anzhen and Chuo’erhe profiles have similar provenance and sedimentary environment,the lithology of the gravel parent rocks of these profiles are intermediate-acid magmatic rocks.The sediments come from the Great Xing’an Range as the products of running water transport in an alluvial-proluvial process.The Wangfutun profile indicates a river terrace origin;its sediments are derived from the southeastern mountainous area of the Songnen Plain.(5)Compared with the classic Baitushan Formation strata(Yaobaitu and Ping’anzhen profiles),Chuo’erhe profile is slightly different in sedimentology and heavy mineral characteristics,such as the appearance of diabase gravel,the existence of apatite-sphene-garnet and the less obvious bedding structure.Therefore,whether it can be divided into Baitushan Formation strata needs further chronology study.(6)Through the comparative study of sedimentology,mineralogy and geomorphology of various profiles,we think that it is inappropriate to regard Wangfutun profile as Baitushan Formation strata.For the stratigraphic division and correlation of Baitushan Formation,further chronological attribute work needs to be carried out,and new chronological data will be accumulated to improve the stratigraphic division of Baitushan Formation in the western Songnen Plain.