基于反射地震资料探讨泥河湾盆地的新生代和中生代地层

摘要

查明泥河湾盆地新生界及中生界地层序列及基底的分布与赋存形态是研究泥河湾盆地形成机制的基础性工作。本研究在泥河湾盆地桑干河与壶流河交接处附近采集了四条测线的反射地震数据,通过对精细处理后的地震剖面进行详细分析,在剖面上厘定了三个具有明显强反射标志的地层,即古近系的玄武岩、侏罗系上统的含煤地层和侏罗系下统的含煤地层；其中,新近系沉积层与古近系玄武岩界面在地震剖面上出现的双程旅行时为0.3~0.6 s,深度范围为300~500 m,而第四系泥河湾组底界由南西到北东逐渐加深,其界面深度为200~300 m；侏罗系上统含煤地层在地震剖面上出现的双程旅行时在0.8 s 上下波动,其深度约为1200 m,探测出侏罗系上统顶界面的深度范围为800~1100 m；侏罗系下统含煤地层在地震剖面上出现的双程旅行时为1.0~1.4 s,深度范围为1800~2000 m,由此推测出中生界侏罗系与古生界寒武系地层分界线的深度为2000 m 左右；根据地震反射波阻抗差别推测出新元古界和太古界地层的分界面在地震剖面上出现的时间约为2.5 s,其深度为4000 m 左右。本研究通过四条反射地震测线的数据基本上查明了泥河湾盆地位于探测区处第四系泥河湾组底界面的深度分布范围及起伏形态、侏罗系上统顶界面和下统底界面的大致埋深,为查明泥河湾盆地新生界及中生界地层序列提供了基础性的参考信息。%An investigation into the stratigraphic sequences of the Cenozoic and Mesozoic Eras and the depth distribution of the bottom boundary of the Quaternary strata in the Nihewan Basin was implemented.In this paper we obtained the seismic data of four seismic survey lines close to the intersection point between the Sangan River and the Huliu River.After processing the seismic data and analyzing the four seismic profiles in detail,we found three seismic reflection interfaces with strong amplitudes,which may rearesent three different sets of strata,that is,the basalt strata of the Paleogene system,the coal-bearing strata of the upper Jurassic system and the coal-bearing strata of the lower Jurassic system.We deduced the interface between the bottom boundary of the deposit layer of the Neogene period and the basalt strata of the Paleogene system in the Nihewan Basin.The two-way-travel-time of the seismic reflection interfaces in the seismic profile is located in the range from 0.3 to 0.6 second and the depth of it changes from 300 to 500 meters.The bottom boundary of the Nihewan formation varies from 200 to 300 meters and it becomes deeper and deeper from the southwest to northeast along the seismic profiles.We inferred that the two-way-travel-time of the coal-bearing strata of the upper Jurassic system in the seismic profiles was around 0.8 second and their depth is located around 1200 meters.Hence,the depth of the top boundary of the upper Jurassic system varied from 800 to 1 100 meters.The two-way-travel-time of the coal-bearing strata of the lower Jurassic system in the profiles were in the range from 1.0 to 1.4 seconds and their depths vary from 1800 to 2000 meters.Therefore,the interface between the Jurassic and Cambrian systems is located around 2000 meters in depth.According to the differences of seismic wave impedances between the strata of the Proterozoic Era and those of the Archaeozoic Era,we found their interface is located around 2.5 seconds in the two-way-travel-time axis and around 4000 meters in depth.In this paper we found basically the depth distribution of the bottom boundary of the Quaternary System,the location of bottom boundaries of the upper and lower Jurassic System along the seismic profiles in the Nihewan Basin based on the seismic data of four survey lines.It provides some important information for the further study of the stratigraphic sequences of the Cenozoic and Mesozoic Eras in the Nihewan Basin.