Unconformity of red sandstones in north Vietnam: field evidence for Indosinian orogeny in northern Indochina?

Abstract

We re‐examine several unconformity sites in northwestern Vietnam where the Indosinian orogenesis has been defined. Field observations in the Black‐River (Song Da) region, near Son La, demonstrate that red sandtones and conglomerates, reportedly of Cretaceous age, occur unconformably above schistosed epimetamorphic middle‐Triassic sediments. A mesozoic unconformity thus really exists in this region, though the age of deformation is poorly contrained (upper Triassic to upper Cretaceous). In most places, however, the unconformity is strongly deformed, folded Triassic limestones are often thrusted onto the Cretaceous rocks. This implies that Tertiary deformations probably due to the India–Asia collision have been strong in the northern part of the Indochina block.