大兴安岭南部下三叠统幸福之路组的建立及其地质意义

幸福之路组广泛分布于大兴安岭南部地区,该组可分为三个岩性段。下段为杂色砾岩,厚173～342m,平行不整合于上二叠统林西组之上;中段为紫红色杂砂岩及粉砂岩,一般厚520m;上段为细粒碎屑岩,厚860m以上,其顶部被上侏罗统地层所不整合覆盖。在上段地层中含丰富的双壳类、叶肢介、植物等化石,具有晚二叠世—早三叠世生物相混生特点。但从区域地层对比来看,该组在岩性及构造变形方面,与晚二叠世地层存在明显差异,其时代应归属于早三叠世沉积更为妥当。

THE ESTABLISHMENT OF THE LOWER TRIASSIC XINGFUZHILU FORMATION IN THE SOUTHERN SECTOR OF THE GREATER KHINGAN MOUNTAINS AND ITS GEOLOGICAL IMPLICATIONS

The Xingfuzhilu Formation is widely distributed in the southern part of the Greater Khingan Mountains. This formation may be divided into three lithologic members. The lower member, 173—342m thick, is represented by mottled conglomerate, which overlies disconformably the Upper Permian Linxi Formation; the middle member, 520 m thick, consists of purplish red graywacke and siltstone; the upper member, over 260m thick, is marked by fine clastic rocks, which is unconformably overlain by the Upper Jurassic. The upper member contains abundant animal and plant fossils. Of these the bivalves comprise the common elements of the Palaenodonta-Palaeomutela assemblage. In the past, it was considered that they only occurred in the Late Permian, but in recent years they have been found in the CIS to range into the Early Triassic. Cornia and Diaplaxa of conchostracans are Early Triassic and Triassic common elements respectively. The genera such as Neoclamites, Eguisetites, Cladophlebis and Pozozamites in the fossil flora are all Early Mesozoic common elements; while their accompanying genera such as Pecopteris, Noeggerthiosis and Paracalamites were considered previously to be restricted in the Late Paleozoic, but they are now found in Lower Triassic strata of the CIS. To sum up, the biotas in the formation show the distinctive features of mixture of Late Permian-Early Triassic elements. But from the correlation of regional stiatigraphic sequences, there are notable differences between this formation and the Late Permian strata in respect to lithology and structural deformation.It seems to be appropriate to assign its age to Early Triassic.