Yen Chau Fm

The type section is located in the Yen Chau - Moc Chau distribution band, from To Bang to So Luon villages (N = 20°57’; E = 104°2r). Nguyen Xuan Bao 1969 . Continental red beds containing gypsum occur largely in the West Bac Bo Region, and were described as Yen Chau Fm during geological mapping at 1:200,000. It is present as discontinuous large bands with the trend not completely NW-SE is that of older formations in the region, such as the bands in Pu Sam Cap, from Nam Cum Village to Nam So Stream, from Quynh Nhai to Ta Chan, from Chi eng Dong Pass through Yen Chau to Moc Chau then extending to Laotian territory, and from Moc Chau to Mai Chau. In the initial description, Nguyen Xuan Bao did not designate a stratotype. Later, in geological mapping at 1:200,000 of the Van Yen Sheet Nguyen Xuan Bao described a detailed section extending from To Bang to So Luon villages situated in the Yen Chau Valley. This scction described in the following sequence is chosen by us as the lectostratotype section of the Yen Chau Fm.

Synonym: - Điệp Yên Châu: Nguyễn Xuân Bao, Từ Lê 1964; Nguyễn Xuân Bao 1970, (ìn Phan Cự Tiến et al.) 1977; Nguyễn Xuân Bao et al. 1978; Vũ Khúc (in Trần Văn Trị et al.) 1977; Vũ Khúc, Bùi Phú Mỹ et al. 1990; Nguyễn Vĩnh et al. 1978; Bùi Phú Mỹ et al.1978. - Hệ tầng Yên Châu: Vũ Khúc, Lê Thị Nghinh 1996; Trần Nghĩa et al. 1996; Vũ Khúc et al. 2000. - Indosinias supérieur: Fromaget J. 1934; Saurin E. 1956. - Jura không phân chia: Dovjikov A.E. et al. 1965. - Hệ Creta (?): Dovjikov A.E. et al. 1965.

Continental red beds. The lectostratotype section of the Yen Chau Fm (total of 1620 m): (1). Thick-bedded conglomerate and gritstone with pebbles of quartz, chert, quartz-rich sandstone and some mafic volcanic; cement is chocolate-colored silty sandstone, 20 m thick. This basal bed unconformably rests upon Devonian sediments. (2). Pink-brownish, thick-bedded sandstone, chocolate-colored siltstone and claystone with some interbeds of conglomerate and gritstone in the lower part, 400 m thick. (3). Brownish-pink, thick-bedded conglomerate and sandstone, 50 m thick; pebbles of the conglomerate consist mainly of mafic volcanics of the Vien Nam Fm (P₃ vn) present near the section and some quartz-rich sandstone, poorly rounded and sorted; cemented by chocolate-colored silty sandstone. (4). Brownish-pink, thick-bedded sandstone, chocolate-colored siltstone and some claystone, 300 m thick. (5). Thick-bedded conglomerate with some interbeds of brownish coarse-grained sandstone, 200 m thick; pebbles in the conglomerate consist of grey limestone, mafic volcanics and some quartz-rich sandstone, poorly rounded and up to some tens of centimeters in size. (6). Brownish, thick-bedded conglomerate and sandstone, 250 m thick; pebbles consist of quartz-rich sandstone, and some mafic volcanics, but without limestone. (7). Brownish, thick-beddcd, medium- to coarse-grained sandstone, chocolate-colored thick- to medium-bedded siltstone with some interbeds of conglomerate whose pebbles consist mainly of sandstone, 400 m thick.

Bivalve Unio sp. and leaf impressions of Ficus aff. beauveriei, Ulmus longifolia, u. cf. carpinoides, Betula c. cuspidens, and Laurus cf. similis. this flora assemblage was assigned to the cenophytic flora. It includes species well known in the Tertiary sediments in Viet Nam.

It is interpreted as continental red beds.