Journal of Tropical Oceanography >
Geomorphological records of turbidity current activity in the northeastern margin of the South China Sea and analysis of triggering mechanism
Received date: 2020-02-26
Request revised date: 2020-05-24
Online published: 2020-06-11
Supported by
Pioneer Hundred Talents Program of the Chinese Academy of Sciences(Y8YB011001)
Copyright
Cyclic steps caused by supercritical turbidity currents are distributed widely along the West Penghu Canyon in the Taixinan Basin of the northeastern South China Sea. Turbidity currents occur frequently in this area due to high tectonic activities, marine factors such as typhoon and delivery of large sediments from rivers. Using high-resolution bathymetric data, we conduct quantitative analysis on the morphology of 23 net-erosional cyclic steps and 10 net-depositional cyclic steps along the West Penghu Canyon, and compute flow velocity of turbidity currents flowing through these cyclic steps. We find that the flow velocity of turbidity currents has an abrupt change in the transition of net-erosional cyclic steps, while the velocity decreases significantly in the transition from net-erosional to net-depositional cyclic steps. The former is mainly caused by the slope break in the canyon, while the latter is caused by the change from confined to unconfined environment. In addition, the net-depositional cyclic steps are located closer to the southwestern flank of the West Penghu Canyon; and we propose that this phenomenon should be mainly caused by turbidity currents affected by the Coriolis force. Investigating the evolution and controlling factors of cyclic steps in this area can help us better understand the interaction of submarine bedforms and turbidity currents, which plays a significant role in the geomorphological evolution along submarine canyons.
LI Shuang , LI Wei , ZHAN Wenhuan . Geomorphological records of turbidity current activity in the northeastern margin of the South China Sea and analysis of triggering mechanism[J]. Journal of Tropical Oceanography, 2021 , 40(1) : 111 -121 . DOI: 10.11978/2020022
图3 净侵蚀周期阶坎地貌图(a)与坡度图(b、c)Fig. 3 Geomorphologic (a) and slope gradient (b, c) maps of net-erosional cyclic steps |
表1 净侵蚀周期阶坎形态特征统计结果Tab. 1 Morphological characteristics of net-erosional cyclic steps |
H1/m | W1/m | H3/m | W3/m | α/° | β/° | θ/° | Lstep/m | Hstep/m | Ay | |
---|---|---|---|---|---|---|---|---|---|---|
S1 | 463 | 6980 | 445 | 6737 | 1.24 | 1.26 | 0.89 | 2611 | 48 | 0.18 |
S2 | 362 | 7022 | 354 | 6573 | 1.18 | 3.85 | 2.71 | 1729 | 90 | 0.27 |
S3 | 421 | 6508 | 323 | 6871 | 2.36 | 2.69 | 1.07 | 2730 | 87 | 0.45 |
S4 | 359 | 7070 | 334 | 6732 | 1.28 | 6.41 | 3.14 | 2267 | 146 | 0.49 |
S5 | 376 | 7031 | 332 | 6997 | 2.64 | 3.13 | 1.43 | 1825 | 70 | 0.43 |
S6 | 378 | 7225 | 310 | 7609 | 1.78 | 1.88 | 0.35 | 1565 | 24 | 1.29 |
S7 | 325 | 7179 | 300 | 7161 | 0.85 | 3.40 | 2.43 | 2350 | 108 | 0.28 |
S8 | 315 | 8031 | 297 | 8202 | 0.64 | 4.58 | 1.60 | 1347 | 47 | 1.17 |
S9 | 355 | 7884 | 316 | 8191 | 1.17 | 1.30 | 0.34 | 2309 | 32 | 0.67 |
S10 | 325 | 8357 | 283 | 8692 | 1.25 | 6.05 | 2.06 | 1514 | 75 | 1.04 |
S11 | 350 | 8855 | 321 | 9291 | 2.00 | 1.82 | 0.60 | 3311 | 70 | 0.44 |
S12 | 290 | 9132 | 263 | 9135 | 0.24 | 2.21 | 1.61 | 1609 | 47 | 0.49 |
S13 | 308 | 9225 | 275 | 9370 | 1.55 | 1.11 | 0.38 | 2425 | 34 | 0.41 |
S14 | 299 | 9501 | 275 | 9519 | 0.95 | 1.19 | 0.56 | 2731 | 40 | 0.52 |
S15 | 267 | 9538 | 212 | 9978 | 1.90 | 2.69 | 0.99 | 2263 | 68 | 0.45 |
S16 | 219 | 7013 | 202 | 7369 | 0.43 | 1.78 | 0.87 | 2821 | 51 | 0.71 |
S17 | 200 | 7327 | 182 | 7523 | 0.65 | 1.87 | 0.91 | 2059 | 44 | 0.82 |
S18 | 219 | 7740 | 198 | 8161 | 0.42 | 1.15 | 0.38 | 1616 | 17 | 0.96 |
S19 | 110 | 6189 | 38 | 6349 | 0.90 | 2.89 | 0.63 | 3486 | 71 | 1.45 |
S20 | 93 | 5592 | 20 | 5529 | 1.17 | 1.98 | 0.46 | 4730 | 93 | 0.99 |
S21 | 98 | 5995 | 19 | 6317 | 1.34 | 1.62 | 0.33 | 3258 | 46 | 2.03 |
S22 | 47 | 4108 | 32 | 5077 | 1.09 | 3.16 | 1.36 | 2478 | 79 | 0.78 |
S23 | 101 | 4195 | 39 | 5007 | 1.53 | 2.46 | 2.30 | 1479 | 30 | 1.45 |
误差 | ±0.08 | ±0.02 | ±0.11 | ±0.02 | ±0.10 | ±0.12 | ±0.18 | ±0.05 | ±0.18 | ±0.10 |
注: H1为水跃前的峡谷有效高度; H3为水跃后的峡谷有效高度; W1为水跃前的峡谷有效宽度; W3为水跃后的峡谷有效宽度; α为周期阶坎的迎流面坡度; β为周期阶坎的背流面坡度; θ为周期阶坎的坡度; Lstep为周期阶坎的波长; Hstep为周期阶坎的波高; Ay为周期阶坎的对称指数 |
表2 净沉积周期阶坎形态特征统计结果Tab. 2 Morphological characteristics of net-depositional cyclic steps |
α/° | β/° | θ/° | Lstoss/m | Llee/m | Lstep/m | Hstep/m | Ay | |
---|---|---|---|---|---|---|---|---|
W1 | 1.01 | 1.74 | 0.61 | 876 | 603 | 2248 | 36 | 1.06 |
W2 | 0.99 | 2.74 | 0.82 | 1158 | 1090 | 2228 | 43 | 0.82 |
W3 | 0.72 | 1.62 | 0.72 | 1002 | 1226 | 2647 | 55 | 0.85 |
W4 | 2.19 | 2.83 | 0.36 | 1216 | 1431 | 1905 | 65 | 0.75 |
W5 | 0.97 | 1.32 | 0.70 | 815 | 1090 | 2602 | 45 | 0.47 |
W6 | 1.56 | 2.24 | 0.95 | 826 | 1776 | 2962 | 78 | 0.81 |
W7 | 0.74 | 1.49 | 0.49 | 1330 | 1632 | 3387 | 58 | 0.79 |
W8 | 0.29 | 4.43 | 1.08 | 1490 | 1897 | 1951 | 73 | 0.80 |
W9 | 2.26 | 3.81 | 1.73 | 869 | 1082 | 3104 | 94 | 1.19 |
W10 | 1.58 | 3.80 | 1.03 | 1685 | 1419 | 2323 | 84 | 0.72 |
误差 | ±0.11 | ±0.08 | ±0.08 | ±0.03 | ±0.02 | ±0.02 | ±0.02 | ±0.04 |
注: α为周期阶坎的迎流面坡度; β为周期阶坎的背流面坡度; θ为周期阶坎的坡度; Lstoss为周期阶坎迎流面的长度; Llee为周期阶坎迎流面的长度; Lstep为周期阶坎的波长; Hstep为周期阶坎的波高; Ay为周期阶坎对称性 |
图7 浊流从破折带上部(a)至下部(b)的流体松弛现象[修改自Leeuw等(2016)和Pohl等(2019)]“+”代表侵蚀加剧 Fig. 7 Flow relaxation caused by turbidity currents flowing through the upper (a) and lower (b) reaches of the slope break (modified from Leeuw et al, 2016; Pohl et al, 2019).“+” denotes an increase of erosion |
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