PROVENANCE OF FLUVIAL-LACUSTRINE SANDSTONES OF LOWER PERMIAN LUCAOGOU LOW-ORDER CYCLE, BOGDA MOUNTAINS, NW CHINA

Compositional and textural maturity of sandstones can reflect provenance lithology and sediment transport distance, useful paleogeographic reconstruction. 18 sandstones from Taodonggou, SW Tarlong (TL), S TL and N TL sections were studied to investigate the fluvial-lacustrine sandstone provenance in lower Permian Lucaogou (LCG) low-order cycle, Bogda Mountains, NW China. The sections are 83, 84, 112 and 106 m thick, respectively, in a half graben covering 88 km². 400 points were counted in each thin section. The sandstones include 6 lithic arenites, 7 lithic subarenites and 5 lithic wackes with an average composition of Q₃F₁₁L₈₆, Q₄F₇L₈₉ and Q₆F₁₁L₈₃. Volcanic lithics (Lv) dominate with minor sedimentary lithics (Ls) with an average Lv/Ls ratio of 5:1. Abundant basaltic lithics indicate an uplifted oceanic crust source, probably the N Tianshan suture zone to the south. Upsection trend in compositional and textural maturity varies among sections. Taodonggou and N TL sections have an average composition of Q₄F₆L₉₀ and Q₁F₈L₉₁, respectively, with higher compositional maturity upsection. The similarity in composition and trend indicates a similar source. However, samples from SW TL have an average composition of Q₅F₈L₈₇, a 1:1 Lv/Ls ratio, and an upsection decrease in compositional maturity. Abundance of Ls indicates a local sedimentary source. Samples of S TL have an average composition of Q₈F₁₄L₇₈ and the highest compositional maturity in the middle part, suggesting source variation during deposition. Texturally, sandstones of N TL are poorly-moderately well sorted with 4.6% matrix on average, while those in the other sections have variable matrix content of 12-16% on average. The higher textural maturity of N TL sandstones implies longer transport distance. Grain size trend varies in each section: fining upward in lower LCG in all sections. However, in upper LCG, grain size fines upward in Taodonggou and SW TL sections, but coarsens up in N and S TL sections. This suggests that transport distance increased during deposition of lower LCG, and kept increasing for Taodonggou and SW TL sections but became shorter for N and S TL sections during deposition of upper LCG. Grain size trend may also be affected by depositional environments of sandstones.