No.2
Location (Geographic coordinates): E102° 46' 4.998", N30° 59' 5.978"
Geological period: Late Triassic Carnian
Type of geological interests: Stratigraphy and sedimentology
Level of significance: International
Geological description: Located in the Yin-Yang Valley at the mouth of Shuangqiao Valley, the strata belong to the Upper Triassic Juwan Formation (T3zh). The flysch exhibits distinct rhythmic features and comprises a sequence of shallowly metamorphosed sandstone and slate interlayers (with typical “zebra stripes” characteristics). The lithological units are divided into upper and lower segments:
The upper segment consists of deep gray and grayish middle-thick-thick metamorphosed lithic arenite, quartz sandstone, lithic quartz sandstone, siltstone, and interbedded or rhythmic layers of mudstone, siltstone, and sericite slate, with some local reverse grain-size sequences.
The lower segment includes deep gray and gray thick-bedded, massive metamorphosed lithic arenite interbedded with a small amount of siltstone and thin to medium-bedded metamorphosed feldspar quartz sandstone with silt (mud) stone layers.
Additionally, the Juwan Formation flysch profile in Shuangqiao Valley features large-scale intense folding and thrust or detachment faults. The folds are predominantly complex chevron types, with common upright cleavage, fan-shaped cleavage, and refracted cleavage, and occasional wrinkle lineations. This section represents the product of complex internal stresses within the Songpan-Garzê Orogenic Belt and is a significant component of the renowned “Xikang-type” folds.
“Xikang Group” turbidite flysch formation Deposition Thickness Map
“Xikang Group” turbidite flysch formation (Yin-yang Valley)
In this stratigraphic sequence, the Bouma Sequences is relatively complete, commonly including the Tbe segment, with some local occurrences of the Tbcde segment. The B segment consists of fine-grained sandstones with parallel lamination; the C segment features siltstones with oblique and convolute laminations; the D segment contains siltstones with horizontal lamination; the E segment comprises massive mudstones.
Bouma sequence
Justification for international significance: The “Xikang Group” turbidite flysch formation in the Songpan-Garzê Orogenic Belt represents one of the largest flysch depositional areas on Earth, covering an area of over 20×104 km² with a thickness of 5-15 km, and occupying a prominent triangular position on the East Asian tectonic map (Cheng et al. 2002; China Geological Survey-GSC & Chengdu Institute of Geology and Mineral Resources-CIGMR, 2004). The “Xikang Group” flysch provides a reliable record of Middle to Late Triassic sedimentation and is key evidence for reconstructing the Triassic paleogeography of the Songpan-Garzê Basin, the closure history of the Paleo-Tethys Ocean subduction, and the orogenic process of the Songpan-Garzê Belt, possessing global scientific significance.
Representative literature：
a) Bruguier, O., Lancelot, J. R., & Malavieille, J. (1997). U–Pb dating on single detrital zircon grains from the Triassic Songpan–Ganze flysch (Central China): provenance and tectonic correlations. Earth and Planetary Science Letters, 152(1-4), 217-231.
b) She, Z., Ma, C., Mason, R., Li, J., Wang, G., & Lei, Y. (2006). Provenance of the Triassic Songpan–Ganzi flysch, west China. Chemical Geology, 231(1-2), 159-175.
c) Yan, Q., Li, J., Li, X., Liu, Y., Li, P., & Xiong, X. (2020). Source of the Zhawulong granitic pegmatite-type lithium deposit in the Songpan-Ganzê orogenic belt, Western Sichuan, China: Constrants from Sr-Nd-Hf isotopes and geochemistry. Lithos, 378, 105828.
d) Dan-Ping Yan, Mei-Fu Zhou, Honglin Song, Zhaoren Fu, Structural style and tectonic significance of the Jianglang dome in the eastern margin of the Tibetan Plateau, China, Journal of Structural Geology, Volume 25, Issue 5, 2003, Pages 765-779.
e) Li X, Dai H, Liu S, Wang D, Huang F, Qin J, Sun Y, Zhu H. Genesis of the Ke’eryin Two-Mica Monzogranite in the Ke’eryin Pegmatite-Type Lithium Ore Field, Songpan–Garze Orogenic Belt: Evidence from Lithium Isotopes. Minerals. 2024; 14(7):687.
f) Gong, D. X., Wu, C. H., Zou, H., Zhou, X., Zhou, Y., Tan, H. Q., & Yue, X. Y. (2021). Provenance analysis of Late Triassic turbidites in the eastern Songpan–Ganzi Flysch Complex: Sedimentary record of tectonic evolution of the eastern Paleo-Tethys Ocean. Marine and Petroleum Geology, 126, 104927.
g) Wang, B. Q., Wang, W., & Zhou, M. F. (2013). Provenance and tectonic setting of the Triassic Yidun Group, the Yidun terrane, Tibet. Geoscience Frontiers, 4(6), 765-777.
h) Zhu, Z.-x., Wang, L.-z., Zhao, Y.-n., & Pan, Y.-t. (1993). A new stratigraphic sequence of the Xikang Group in Sichuan. Geological Bulletin of China, (1), 18–27.