塔中西北部上奥陶统丘状地震反射异常体的成因

塔中地区上奥陶统良里塔格组富含礁滩沉积体,由于礁体具有丘状外形,因此人们普遍将其北西部台缘外带的丘状地震反射体解释为生物礁。本文通过地震地质综合分析,认为上奥陶统生物礁个体较小,其厚度小于1/8波长,宽度小于1个菲涅尔带半径,在常规地震资料中不能直接辨别,真正的礁滩沉积体总是隐伏于代表大套灰岩层顶面的极强反射波(T₇⁴)及其之下的1~2个弱反射波之中。那些位于极强反射波(T₇⁴)之上,内部为低频强振幅层状反射波结构的丘状体,其地层结构表现为灰岩(或泥质灰岩)与泥岩互层,为低能与高能过渡带的沉积环境的产物,内含礁滩沉积体的概率也很低。精细解释表明,此类丘状体是低倾角逆断层构造、斜坡滑塌堆积及浊积扇等非礁滩沉积体的地震响应。

The Origin of Domal Seismic Reflection Anomaly Bodies of Upper Ordovician in Northwestern Tazhong Area

The Lianglitage formation of Upper Ordovician is rich in reef, which led the domal seismic anomaly bodies in Tazhong area of Tarim basin to be interpreted as reef in recent years. According to the integrated study of seismic and geological analysis, it is indicated that the reef or multiple circled complex reef-banks can not be recognized directly in conventional seismic profiles because the thickness of the reef is less than one eighth of wavelength, the width of it is less than one radius of Fresnel zone. Reefs or complex reef-banks are always invisible and intermixed in the strong reflection (T74), the top boundary of successive carbonate overlain by interbedded limestone and mud or/and the under 1~2 feeble and discontinuous reflections. Those domal seismic anomaly bodies overlying the strong reflection of T74, with inner bedding configuration composed of mid-strong reflections, reflect the interbedded configuration of limestone or argillaceous limestone with mudstone, which are the products of sedimentary environment with low- and high-energy transition zones, and the low probability of reef-bank deposits within them. The accurate interpretation shows that such domal bodies are the seismic reflections of non-reef-bank deposits of low angle thrust faults, slope slump accumulative zones and turbidite fans.