有关陕北盆地西部边缘的一些构造问题

SOME TECTONIC PROBLEMS CONCERNING THE WESTERN LIMIT OF THE NORTH-SHENSI BASIN

一.陕北盆地的西缘不是六盘山以往的地质学家一向以为陕北盆地的西部是以六盘山褶曲带为界,正如陕北盆地的西北部是以贺兰山为界一样。而以为以灰绿色钙质砂泥岩为主的六盘山系地层和以红色的砂岩为主的保安系地层。

1. The Liupanshan not representing the western limit of the North-Shensi Basin The recent discovery, in the course of the geological survey and pro-specting work, of the Paleo-Liup'anshan-an Old folded north-south runningridge made of the Sinian and the Cambrian-Ordovician limestones-at aplace east of the present Liup'anshan, has disproved the conventional view-point held by geologists in the past. The western limit of the North-ShensiBasin and the Ordos Platform are not composed of the Liup'anshan: theCretaceous Pao-an formation in the North-Shensi Basin and the Liup'anshanseries making up the Liup'anshan are the sediments of two, mutually inde-pendent and isolated basins. Connected with the northern end of the Paleo-Liup'anshan are the Talo-shan, the Hsiaoloshan and the Niushoushan, made of middle Proterozoicsandstones. They constitute the northern limit of the western border of theplatform. To the west of such there have been found some Silurian-Devonianformations which were never revealed in the Sino-Korean Massive. 2. The succession of folding along the backbone of the Chilien-Luliang-Holan ∈-type of structure There lies, between the Paleo-Liup'anshan and the present Liup'anshan,a north-northwest running folded zone of the Yenshan cycle. The Paleo-Liup'anshan constitutes the real backbone of the ∈-type of structure of theChilien-Luliang-Holan System and the folding movement is estimated to have taken place at the earliest phase of the Hercynian Cycle, the LiukiangMovement: while the present Liup'anshan and the folded zone should obvious-ly belong to the system of the Lungsi vortex structure. The above-mentioned three folded zones upon reaching southward theapex of the ∈-type of structure, superimpose on each other to form a radialsystem. In the process of orogenesis, reckoned from east to west as progressingfrom older to younger systems of folding, they belong respectively to threedifferent orogenic cycles; thus it is believed that the orogenic force camefrom the west. In view of the above, it would be better to designate the Chilien-Luliang-Holan ∈-type of structure as the Chilien-Luliang-Paleo-Liup'anshan ∈-typeof structure so as to represent its real significance. 3. The nature of the relation between two flanks of the ancient ridge There are two different views on the relation between the two flanks ofthis ancient ridge. Some geologists consider that there exists to the east ofthe ridge a north-south major normal fault to have caused the subsidenceof the formations in the basin; others are of the opinion that such a relationis one of overlapping. Judging from the obtained data and inferring fromthe standpoint, of mechanics, the first supposition is hardly possible. In con-sequence of the constant and regular westward shift of the centre of thebasin, its western margin is most likely of an overlapping nature. As the depth of the sedimentary basin of the Liup'anshan Series is con-siderable, the sudden drop of the value of gravity on the western side of theridge is not necessarily the outcome of a fault, at the same time the viewpointthat there are even older strata underlying the Liup'anshan is thus provengroundless. 4. The Shihkouyi marginal basin, the North-Shensi Basin and the northwestern limit of the Ordos platform The Holanshan and the Yitzeshan constitute the northwestern limit of theOrdos platform. Affected by the drastic subsidence of the Holan para-geosyn- cline beginning from the Upper Carboniferous Period, the northwestern partof the platform was transformed into a transitional ares --the Shihkouyimarginal basin. The Yenshan Movement at the end of the Jurassic Period caused thepara-geosyncline of the Holanshan to have folded up into mountains, andmeanwhile gave rise to the series of structure lying south of the Chotzeshanin the marginal basin. This series of structure already assumed its incipientform before the Carboniferous Period. The scope of sedimentation of the Cre-taceous Pao-an formation was only limited to the area east of this structure, There exists a graben underlying the Ninghsia plain between the eastand west folded zones.