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

doi:10.19657/j.geoscience.1000-8527.2020.02.10

Abstract

Abstract:

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.

Key words: beach-bar, Qinghai Lake, lithofacies, wave zone, sedimentary model, sedimentary sequence, identification mark

CLC Number:

YUAN Kun, JIN Zhenkui, PENG Biao, ZHU Xiao’er, TIAN Tian, LI Rui. Sedimentary Model and Identification Marks of Modern Lacustrine Beach-bar on Southeast Coast of Qinghai Lake[J]. Geoscience, 2020, 34(02): 309-320.

Figures/Tables 13

Fig.1 Satellite map of Qinghai Lake and location map of beach-bar profile near Erhai

Fig.2 Caishiken profile of Erhai in Qinghai Lake (shore parallel)

Fig.3 Caishiken profile of Erhai in Qinghai Lake (shore perpendicular)

Fig.4 Probability cumulative grain-size curves of beach-bars in southeast Qinghai Lake

Fig.5 Division of wave zones in Qinghai Lake (normal weather conditions)

Fig.6 Sedimentary environments of Buha River and Erlangjian in Qinghai Lake

Fig.7 Developing beach-bar in Qinghai Lake (oblique top view)

Fig.8 Beach-bar Sedimentary model of Qinghai Lake

Fig.9 Normal sandy beach-bar (a) and beach-bar made of modified bedrock (b)

Fig.10 Beach-bar formed by longshore current in Erlangjian, Qinghai Lake

Fig.11 Columns of beach-bar profile in Erhai, southeastern Qinghai Lake

Table 1 Lacustrine beach-bar sequences on the southeast coast of Qinghai Lake from different perspectives

方向	平面观察	剖面观察
平行岸线	单相稳定延伸, 偶有断续	自底至顶显ABC、ABAB、BCBC序列,纹层均水平展布
垂直岸线	自湖向陆显 ABC序列	ABC、ABAB、BCBC,各小层纹层倾向差异明显,易识别越岸沉积

Fig.12 Differences between beach-bar, mouth bar and distributary channel

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