青海湖东南岸现代滩坝沉积模式与识别标志

青海湖是研究湖泊滩坝的有利区域。通过对青海湖现代滩坝的实地考察和精细解剖,对滩坝的沉积特征、形成机制、沉积序列、沉积模式、识别标志等方面进行了系统研究。青海湖滩坝发育3种岩相,即块状砾质砂岩相、块状砾岩相和递变砂砾岩相。结合岩相分布和现代湖岸水动力观测,将青海湖波浪带划分为对称浪带、歪浪带、破浪带和冲洗浪带,并据此建立了青海湖滩坝沉积模式。提出滩坝相较于河口坝的4点关键识别标志:(1)岩性底界面接触关系差异,滩坝与下伏半深湖泥质沉积总是突变接触,有明显的突变面,而河口坝则是渐变接触;(2)滩坝垂向上发育典型的“ABC”序列,沉积物自底至顶呈细-粗-细的变化;(3)滩坝普遍发育冲洗层理,以及向陆倾斜的漫越层,而河口坝主要发育槽状交错层理;(4)砂体走向差异,滩坝砂体走向总体平行于湖岸,而河口坝砂体走向总体垂直于湖岸。

Sedimentary Model and Identification Marks of Modern Lacustrine Beach-bar on Southeast Coast of Qinghai Lake

Qinghai Lake is a favorable target for lacustrine beach-bar research.In this study,the sedimentary sequence and characteristics,formation mechanism,sedimentary model and identification marks of beach-bars are systematically examined through field work and micro-analysis on modern beach-bars in Qinghai Lake.There are three lithofacies developed in the beach-bar on the southeast coast of Qinghai Lake,i.e.massive pebbly sandstone,massive conglomerate,and normal-graded sandy conglomerate.Combining lithofacies and hydrodynamic observation on modern lake shore,the wave zone of study area is divided into the wave symmetric,breaker and swash zones.The beach-bar sedimentary model of Qinghai Lake is established,and four identification marks of beach-bar(c.f.mouth-bar)are proposed:(1)beach-bar deposits always change sharply to the underlying semi-deep lacustrine mud deposits with a clear discontinuous surface,whereas in mouth-bar deposits the transition is gradual;(2)beach-bars show typical“ABC”vertical sequence,and the sediments coarsen upwards then return to fine grain;(3)swash cross-bedding and wash-over deposits(tilted to land)can be found in beach-bars,whereas mouth-bars are featured by trough cross-bedding;(4)beach-bar deposits are always shore parallel,whereas mouth-bar deposits are usually shore perpendicular.