Abstract: Evaporite Deposits of Northeast Thailand

Thawat Japakasetr, David R. Workman

The Khorat Plateau of northeast Thailand is an area of about 130,000 sq km, covered by gently folded continental and paralic deposits of Mesozoic age known as the Khorat Group. The upper part of the Khorat Group contains thick evaporites. This evaporite-bearing Cretaceous formation occurs in two structural basins with a combined area of about 55,000 sq km.

A recent drilling program has established three main evaporite sequences in the upper Khorat, separated and overlain mostly by claystones. The lowest evaporite sequence begins with a thin anhydrite layer followed by massive halite (locally more than 300 m thick) and then, over much of the central and western part of both basins, a zone containing potassium minerals associated with halite and, in many places, tachyhydrite. The commonest potassium mineral is carnallite. Sylvite is present locally. The carnallite zone is up to 95 m thick in some areas. High-grade sylvinite is present as small lenticular bodies up to 19 m thick. Zones with potassium minerals usually are capped by a few meters of pure halite. The thickest recorded section of the lowest evaporite sequence is 435 m, with 0.8 m of anhydrite, 351.9 m of halite, and 82.3 m of halite and carnallite.

The middle evaporite sequence is generally thinner than the lower one, in places only a few meters thick, but commonly in the range 60 to 120 m. It consists essentially of halite. Thin anhydrite layers are present in most places, and traces of carnallite and sylvite are found in one or two places. The uppermost evaporite sequence is thinner still, averaging about 22 m of halite, generally with a little anhydrite but no potassium minerals.

The thickness of the claystones dividing the evaporite sequences is commonly in the range 10 to 50 m. Massive halite is present within as little as 55 m of the surface, and the carnallite zone has been intersected at a depth of only 90 m.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii