Li Basin Gr

Li Basin (NW Thailand, Inthanon Zone), Lamphun province. The Chiang Mai Basin and its en echelon extension of the Li Basin is the largest rift basin in Northern Thailand; over 100 km long and up to 35 km wide, with maximum fills of 2 to 2.5 km. The Chiang Mai Basin succession is the Mae Rim alluvial fan deposits, Mae Sod Fm claystone, Mae Fang Fm semi-consolidated sand, sandy clay and gravel (up to 760 m), and Pleistocene gravels (150 m); but their ages are poorly known. (Morley and Racey, 2011). The Li Basin succession is well-dated in exposures mined for coals, and the "Li Basin Gr" is used here as a convenient informal name for the upward succession in the Li Basin of the Ban Pa Kha Fm, Mae Long Fm and Mae Taeng Fm, capped by Pleistocene conglomerates.

Synonym: หมวดหินล, Li Fm (The "Li Fm" usage was only to the lower two formations: Ban Pa Kha Fm and Mae Long Fm.)

[Figure: Map showing the Tertiary basins of Thailand, with Phetchabun basin highlighted. The solid black lines onshore are the principal rivers draining Northern and Central Thailand and the Khorat Plateau (Morley and Racey, 2011, page 224).]

Mudstone, shale, greenish-grey to greyish-brown; coal; oil shale; conglomerate and sandstone, white to light grey. The section in the Li Basin is strongly deformed by several episodes of normal faulting and compression.

Filling of rift basin from lower alluvial, through lacustrine with coals, to fluvial.