Abstract

The new 1:500,000 geological map of the Indochinese Peninsula reveals widely distributed Devonian rocks. Lack of deep drilling, however, causes information to be derived almost wholly from outcrops.

Lower Devonian is little known, and appears to be largely in continental Old Red Sandstone facies. Transgression of the older Indosinian continent began in the Middle Devonian and reached its maximum in the Couvinian, leaving several older massifs, of general northwesterly trend, emergent as islands.

Shallow epicontinental seas remained during Givetian and Frasnian time. Regression occurred in the Famennian, leaving only a shallow gulf southwest of Vinh, in which sedimentation was continuous from Silurian to the Dinantian. Elsewhere all known Devonian lies discordantly on older rocks, and is unconformably overlain by Upper Carboniferous and Permian. Identified Devonian fossils are mostly of Emsian and Couvinian ages, and brachiopods, pelecypods, and corals predominate.

Local series names have been proposed, but for the purposes of this paper the Devonian is divided between five regions:

1. Unfossiliferous Devonian of Cambodia and southern South Viet Nam: post-Caledonian and pre-Hercynian, including the only volcanics known in the region which could be of Devonian age.

2. Laotian facies, north of (1): fossiliferous limestones, shales, and sandstones, much covered by younger continental horizons.

3. Facies of central North Viet Nam: mostly shales, poorly fossiliferous.

4. Devonian of Pak Lay, in northwest Laos: folded and metamorphosed clastic rocks.

5. Devonian of northern North Viet Nam: the best known, very fossiliferous shales and limestones, mostly Emsian and Couvinian.