摘要
桂西北右江地区广泛发育碳酸盐岩孤立台地,台地边缘发育大量沉积岩脉、角砾岩体、台地边缘同沉积正断层、及局部地层缺失造成的特殊界面,各要素之间的成因关系一直有争论。毗连不整合是指因同沉积断层作用使平坦地形变成强烈起伏的陡坡地形,粗碎屑物质快速沉积于地形凹处,新沉积的地层与早先坡下的较老基岩横向相接,因此造成新老地层之间的不整合,多形成于构造不稳定地区。国外地质学家最初将其识别为普通的角度不整合、陆架边缘不整合或者台地边缘不整合等等,国内地质学家也注意到了这类特殊界面,但在一些野外露头上观察其为断层面,而在另一些露头上观察其为侵蚀间断(不整合)面,由此产生争论:这类特殊界面是断层还是不整合?并产生很多名称:同构造沉积不整合、碳酸盐岩台地边缘围限断裂带、角度不整合接触带、陡崖式假不整合、事件沉积不整合、超覆不整合、构造窗和飞来峰等等,迄今为止没有统一。笔者等在研究广西晚古生代孤立碳酸盐岩台地边缘沉积岩脉和角砾岩体的过程中,发现特殊界面与沉积岩脉和角砾岩体关系密切,并发现如果将这些特殊界面归类于传统不整合面,均有与地质事实相矛盾的地方,因而认为其应重新归类。随着研究的深入:笔者等在凤山剖面的同一露头上观察到关键地质现象:特殊界面下部具断层特征,上部具溶蚀间断特征,因此认为其是遭受溶蚀的同沉积断层面。其成因是:同沉积构造十分活跃,使同沉积断层面上部来不及被新沉积物掩埋而遭受水下溶蚀,从而形成巨大地形起伏,造成新老地层横向相接,与Davis毗连不整合概念吻合,据此将这类特殊界面统一命名为毗连不整合。随即发现桂西北晚古生代地层内部广泛发育毗连不整合,并且发现晚古生代地层与下伏寒武系、及上覆三叠系之间也是毗连不整合接触,这是桂西北晚古生代地层及相关寒武系和三叠系的基本特色,由于毗连不整合代表区域地质构造活动强烈,表明桂西北地质构造从古生代到中生代一直处于活动状态,地层—构造活动记录与古特提斯域特征相似,属古特提斯洋的东沿部分。从而结束特殊界面是断层还是不整合的争论,为桂西北地层与构造的深入研究及其大地构造归属提供了新证据。
Objectives:Isolated carbonate platforms are widely observed in Youjiang River area,northwestern Guangxi.Many neptunian dykes,carbonate debris,synsedimentary normal faults,and special surfaces showing as local lost strata are observed in margins around those isolated carbonate platforms.There were different ideas about the relationships among these elements.Buttress unconformity refer to that megabreccia rapidly deposit in topographic low when synsedimentary normal faulting change flat terrain to steep topography,and cause the younger beds laterally abuts the older basement strata,and result in the unconformity between the younger beds and the older basement.Buttress unconformity generally formed in unstable structural regions.Until Davis(1984) named it as buttress unconformity,foreign geologists named it as angular unconformity,shelf-margin unconformity,platformmargin unconformity etc.Chinese geologists also observe those special surfaces of unconformity which shown as fault surface in some crops and shown as unconformity interface in some others,and named them as syntectonic sedimentary unconformity,carbonate-platform-margin limited fault zone,angular unconformity contact zone,escarpment-style pseudounconformity,event-sediment unconformity,unconformability of overlap,even tectonic window or klippe etc.Consequently cause argument for they are fault or unconformity,and for their different names.Methods:When study Late Palaeozoic isolated carbonate platform margin neptunian dyke and carbonatebreccia-body,author observes the paragenetic special surface and their difference from the sorts of known reported surfaces in Guangxi.Suppose to classify the special surface to the known reported sorts,there are competing between the names and the geologic crop facts.Special surface should belong to different type.In a key field crop of Fengshan of Guangxi,the special surface is in coexistence with the neptunian dyke and carbonate-breccia-body,and shows its lower part as normal fault surface and its upper part as solution unconformity interface.It is solution fault surface.Results:Due to the frequently large amplitude synsedimentary normal faulting,lower part of the fault plane could be hidden by present deposits,but the upper part of the fault plane could expose to be eroded and hidden by later deposits,cause steep topography and the younger beds laterally abuts the older basement strata in buttress unconformity,coincide with Davis’ s concept of buttress unconformity.Conclusions:The special surface of Guangxi could be named buttress unconformity.The collected reports show the unconformity between the Late Palaeozoic strata and the subjacent Cambrian strata,as well as the overlaying Triassic strata,are buttress unconformity.Buttress unconformity could be observed widely in Late Palaeozoic strata of Guangxi and become its geologic feature,which means the frequent regional structural activity have being developed from Palaeozoic to Mesozoic in northwestern Guangxi.The characteristics of strata and tectonic activity record of Guangxi is similar to that of Tethys.Guangxi belongs to the part of east Tethys.Therefore,the argument about the special surface is ended,and approaching geologic materials could help to the study for the strata and the tectonic of northwestern Guangxi.
作者
彭阳
陆刚
PENG Yang;LU Gang(Institute of Geology,Chinest Academy of Gelogical Sciences,Beijing,100037;Institute of Regional Geological Survey of Guangxi,Guilin,Guangxi,541003)
出处
《地质论评》
CAS
CSCD
北大核心
2019年第4期794-809,共16页
Geological Review
基金
中国地质地调局项目(编号:DD20160345)的成果
关键词
沉积岩脉
角砾灰岩体
同沉积断层
毗连不整合
晚古生代
桂西北
neptunian dyke
carbonate breccia
syn-sedimentary fault
buttress unconformity
Late Palaeozoic
northwestern Guangxi