A Permo-Triassic volcanic arc system, the Sukhothai Arc, is recognised between the Indochina and Sibumasu
continental blocks. The Chanthaburi terrane is here interpreted as a fault-detached, highly disrupted
southern segment of the Sukhothai Arc, occupying part of southeastern Thailand and extending
into Cambodia. The Klaeng tectonic line is defined as the boundary between the Chanthaburi terrane
and Sibumasu block. The stratigraphy of the Chanthaburi terrane is compared with that of the Sukhothai
terrane in Northern Thailand. The Late Palaeozoic–Mesozoic sequences of these two volcanic arc terranes
in the Sukhothai Zone share important similarities, but show marked contrasts to those of the Sibumasu
and Indochina blocks, where the Late Permian–Triassic is largely absent due to the Indosinian I unconformity
(western Indochina) or is dominantly carbonates with little terrigenous clastic input (Sibumasu).
There is no clear evidence of pre-Carboniferous sedimentary rocks for either the Sukhothai or Chanthaburi
terranes. Late Permian lyttoniid brachiopod shale near Klaeng in the Chanthaburi terrane was revisited.
The brachiopod, previously reported as Leptodus, is re-identified to Oldhamina, the genus previously
known, elsewhere in Southeast Asia, only in the Huai Tak Formation of the Sukhothai terrane. Oldhamina
in Thailand is confined to the Sukhothai Arc. The marine stratigraphy of the Sukhothai Arc is represented
by a Permian–Triassic lithological succession of mixed carbonates and siliciclastics, with common volcanic
material. The Late Permian and Triassic litho- and biostratigraphy of the Chanthaburi terrane are comparable
with the upper Ngao and Lampang groups of the Sukhothai terrane; in particular, they share
similar successions from Oldhamina brachiopod bearing shale to Palaeofusulina–Colaniella foraminifer
bearing limestone in the latest Permian. Marine depositional conditions were terminated on the Sukhothai
Arc by end-Triassic times, later than on the Indochina block (Late Permian) but earlier than on the
Sibumasu block (Jurassic/Cretaceous).