Geologic structural model illustrated by the mesozoic stratigraphic units in the upper Southern Thailand, with special reference to Changwat Krabi and Surat Thani

A structural geologic study was performed in peninsular upper southern Thailand from Changwat Chumphon to Trang. The study includes a field reconnaissance on the stratigraphy of the Mesozoic units and a general structural geologic survey. A detailed study was done in eastern Changwat Krabi and Changwat Surat Thani along 3 traverse lines, which are 50, 24, and 29 Km. long respectively. The study was meant to understand the deformation pattern imprinted on to the Mesozoic stratigraphic units in this area. The study area is underlain by the marine Triassic Sai Bon Formation and non-marine Jurassic-Cretaceous Trang Group. Trang Group is further subdivided into Khlong Min, Lam Thap, Sam Chom, and Phun Phin Formations. These units angular-unconformably overly the Permian Ratburi Group and Permo-Carboniferous Kaeng Krachan Group, and are further unconformably overlain by Tertiary Krabi Group and the Quaternary sediments. The Mesozoic rocks were folded to form a huge first-ordered syncline, within which is composed of 4 major lower-ordered gentle synclines, Chumphon, Ta Pi, Krabi-East, and Trang synclines. These synclines are somewhat elongated in the north-south to northnortheast-southsoutheast direction, with asymmetric lower-ordered parasitic folds. The folds have moderately to steeply eastward dipping limbs and more gently westward dipping limbs. These folds were transected by fractures, at high angle by the strike-slip faults, and being perpendicular by the extension joints and normal faults. These geologic structures indicate the east-west pushing, perhaps also with the shear drag, and concurrently or immediately followed by the extensional mechanism along the synclinal trend. This occurred before the deposition of the tertiary sedimentary units. After then, no more major folding was recognized, except the formation of fault-controlled tertiary basins.