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China and Southeast Asia have complex tectonic histories, resulting from multiple stages of terrane accretion and overprinting associated with the Himalayan Orogeny. The timing of accretion events remains disputed, largely due conflicting age indicators. The South China Craton is separated from the Siamo-Sibumasu and Indochina blocks on its southern margin by the Ailaoshan and Songma sutures. Two general models exist for these sutures, one with accretion occurring in the Late Devonian to Early Carboniferous, and another with accretion in the Triassic.
The Nanpanjiang Basin lies on the southern margin of the South China Craton, adjacent to the sutures. The Nanpanjiang Basin is surrounded by the Yangtze Platform and contains several isolated carbonate platforms and over 200,000 square kilometers of Triassic turbidites. The basin is bordered by the Precambrian Khamdian, Jiangnan, and Yunkai Massifs on the west, northeast, and southeast respectively. This study uses provenance analysis of turbidite sandstones in order to evaluate the hypothesis of active tectonic convergence along the Ailaoshan and Songma sutures.
Turbidite units include the Lower Triassic Shipao Fm. and Middle Triassic Baifeng and Lanmu formations in the southern part of the basin, and the Middle Triassic Xinyuan and Bianyang formations in the northern part of the basin. The Middle Triassic turbidites are composed of very fine grained sandstone, siltstone, and mudstone. Sedimentary structures include flute casts, groove casts, load casts, dewatering structures, asymmetric ripples, convolute lamination, and plane lamination. Size grading and complete Bouma sequences are lacking in the northern basin, but with the addition of coarser sand size, up to medium sand, grading and complete bouma sequences are common in the southern basin.
Sandstones of the Lower Triassic Shipao Fm. have volcaniclastic composition including embayed quartz and glass shards. Middle Triassic sandstones are moderately mature, matrix-rich, lithic wackes. The average QFL ratio from all point count samples is 54.1/18.1/27.8% and the QmFLt ratio is 37.8/ 18.1/ 44.1%. Lithic fragments are dominantly claystone and siltstone clasts and metasedimentary clasts such as quartz mica tectonite. Volcanic lithics are rare. Most samples fall in the recycled orogen field of QmFLt plots, indicating a relatively quartz and lithic rich composition consistent with derivation from Precambrian massifs such as the Jiangnan, Khamdian, or Yunkai. A few samples from the southwest part of the basin fall into the dissected arc field, indicating a somewhat more feldspar-rich composition.
Integrating the provenance results from the Middle Triassic sandstones with paleocurrent data indicates that there were two major turbidite source areas, the Jiangnan Massif to the northeast, and the Archean-Proterozoic Yunkai Massif bordering the southeast. However, the presence of volcanic contribution is supported by the composition of the Lower Triassic Shipao Formation. These observations lead to the hypothesis that convergence along the Ailaoshan and/ or Songma sutures began in the Early Triassic, and by the Middle Triassic resulted in rejuvenated uplift of the Yunkai massif that supplied a large flux of relatively mature sediment into the southern basin.
Goers, Alexa, "Provenance analysis of Triassic turbidites within the Nanpanjiang Basin, South China" (2012). Geosciences Student Honors Theses. 8.
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