On some significant unconformities in the Paleozoic and Mesozoic stratigraphy of North Viet Nam

Abstract

A lot of unconformities and stratigraphic gaps have been discovered in the Paleozoic and Mesozoic stratigraphy of Bac Bo region (North Viet Nam), but their role in the regional geology was not accessed yet in detail. This paper is the first attempt to describe the most significant unconformities and stratigraphic gaps and discussing their role in regional geology. The most important is the angular unconformity of Norian-Rhaetian formations upon various formations of different formations; it marked the change tectonic regime of the region resulting from the indosian orogeny in Viet Nam and in Southeast Asia in common. The gap of Lower Devonian red beds formations (Si Ka Formation, Song Cau Group) upon Lower Paleozoic formations (Upper Cambrian - Than Sa Formation, Lower Ordovician - Lutxia and Na Mo formations) is a regional unconformity, which had been taken place in the epicaledonian terranne of South China and North East of Viet Nam. The gap between the Da Mai Formation and the Dong Dang Formation was a result of a crust rising, but was not through the orogeny activity, although after this gap the structural plan was more differentiated. Other unconformities and stratigraphic gaps seems to be local unconformities, for instance, the one of the Tan Lap Formation (D2g-D3 fr tl) upon the Ban Pap Formation (D1p-D3 fr bp) and the unconformity between the Sinh Vinh (O3-S sv) and the Ben Khe (ε-O1 bk) formations.