Early Cretaceous Vocanic-Sedimentary Strata Sequence And Tectono Palegeography, In Northern Daxinganling, China

Early cretaceous vocanic-sedimentary strata are wide distributed in north DaXinganling Mountain and adjacent area. The basic geological study is very poor in North DaXinganling area, compared with the areas (North Hebei-West Liaoning, Song liao Basin, Hailar Basin) where have the smilar sequence. Nearly no biostratigraphy had been done until recently. At least three kinds of lithostratigraphical division systems are used in this area.According to the portrait viriation and transverse distribution characters, this paper attributes those deposits into three Strata Regions: Eastern Slope of DaXingan Mountain Strata Region, Western slope of northern Daxingan Mountain Strata Region, Hailaer Basin Strata Region.At the eastern slope of DaXingan Mountain, these series of stratum is divided as Longjiang Fm(=Formation), Jiufengshan Fm, Ganhe Fm.Longjiang Fm is onconformitily over the middle Jurassic Nanping Fm, the lower part of Longjiang Fm is consisted of a series intermediate mafic volcanics, the upper part is consisted of volcanicclasts and normal sediments. Jiufengshan Fm overlaied the Longjiang Fm conformitily, it's a series vocanic rock intercalated with coal-bearing sediments, the lower part is charactered with lake face sediments and the upper part intercalated with several coal layers. Ganhe Fm is mainly composed of basalt, some times it's interveined with Jiufengshan Fm.At the western slope of north DaXingan Mountain, this kind of deposit consists of the Tamulangou Fm, Shangkuli Fm, Yiliekede Fm. Tamulangou Fm is mainly composed of basalt, some times intercalated with phyolite and vocanicclasts. Shangkuli Fm is wide distributed and very thick, it is mainly composed of intermediate-phyolite rocks and vocanicclasts, lake face deposits yelids thick oil shales are well devoloped in some areas. Yiliekede Fm iscomposed of basalt and intercalated with vocanicclast, coal and normal sediments. Yiliekede Fm is under the coal-bearing strata Damoguaihe Fm, it and Ganhe Fm represent the latest early cretaceous vocanic activement.Tamulangou Fm and Shangkuli Fm as a whole is correlated with Longjiang Fm, Yiliekede Fm is correlated with Jiufengshan Fm and Ganhe Fm.Longjiang Fm, Jiufengshan Fm, Shangkuli Fm, Yiliekede Fm yield doubtless Jehol Biota fossils. Longjiang Fm yields fossils Nestoria, Eose- stheria, Ephemeropsis trisetalis and so on. The type assemble Eosestheria middendorfii-Ephemeropsis trisetalis-Lycoptera davidi is has long been found in Jiufengshan Fm at Naketa Village, Arongqi County, Inner Mongolia, this strata has the similar age as the Jiufotang Fm in West Liaoning.The non-marine bivalves Arguniella which is familiar in Jehol Biota is newly discovered in the type section of Shangkuli Fm. Fossils are found in Yiliekede Fm for the first time at the area of Yanjianglinchang, Tahe County, Heilongjiang Province. The megaplant are: Ruffordia goepperti,Anomo- zamites sp.,Ginkgoites sibirca,Czekanowskia sp.,Czekanowskia rigida,Sphenobaiera sp.,Solenites sp.,Pityocladus sp.,Pagiophyllum sp.,Elatocl- adus sp.,Podozamites sp.,Brachyphyllum sp.,Strobilites sp.,Schizodepis sp.,Conites sp.,Carpolithus spp.. Invertebrate fossils are some Conchostraca and Blattaria. Vertebrate Acipenseriformes are also found, this fossil's characters are similar with Yanosteus longidorsalis which is often seen in Jehol Biota, and these fossils are the northest foundings in China. This flora is composed mainly of Gymnosperm, followed with Gingkgo, about 15 genus and over 10 kinds of fossil seed are reported in here, they are all the often seen fossils in Jehol Biota. Palynological assemble is charactered of the occurance of Fixisporites and a plenty of Protoconiferus, this assemble is similar with the assemble yields from Nantun Fm to Damoguaihe Fm in Hailar Basin. In conclusion, fossil assemble of Yiliekede Fm is approximately corresponded with the medium and late term fossil assemble (yield in Yixian Fm andJiufotang Fm in West Liaoning) of Jelol Biota.Early Cretaceous vocanic-sedimentary sequences in north Daxingan Mountain have some common ground with Jeol Group in west Liaoning, they have the same fossil assembles, identical isotope ages, similar sequences. Period of 130Ma-120Ma is the time when vocanic and tectonic actives are most intensity in north Daxingan Mountain and adjacent areas, Jehol Biota are also blooming at this time.The tectonic background of the early Cretaceous vocanic-sediment sequences in north Daxingan Mountain and adjacent area is very complex. It might be affected with the evolution of Mongolia-Okhotsk orogenic belt and the subduction of Pacific oceanic plate during the Mesozoic. It also might be one represent of the Mid-Cretaceous abnormal geological events that ever affected the west margin of Pacific Ocean.