Don Duong Fm
Type Locality and Naming
Da Lat Basin. The type section is located on the road from Phim Non to Nha Trang (N = 11°46’; E = 108°28’). Nguyen Due Thang et al. 1988. The formation is mainly felsic volcanogeno-sedimentary rocks formed under continental conditions, covering the Lower-Middle Jurassic Bân Don Gr in the Da Lat Depression, and was described as Don Duong Fm in geological mapping at 1:500,000 of South Viet Nam (1975- 1978). It occurs largely in the Don Duong, Ta Dung Mt, upper course of Luy, Krong Kno and To Hap rivers, North Da Lat, Phan Dung, At Mt (West Cau Pass), Rat Mt (NE Tan My Bridge) areas, but scattered in NW Cam Ranh, sw Ninh Hoa, North Vung Tau and the Long Hai areas.
Synonym: - Hệ tầng Đơn Dương: Nguyễn Kinh Quốc 1979, (in Vũ Khúc, Bùi Phú Mỹ et al. 1990); Vữ Như Hùng 1987; Trần Tinh et al. 1998; Nguyễn Đức Thắng et al. 1999; Vũ Khúc et al.2000,; Nguyễn Xuân Bao, Vũ Như Hùng et al. 2002. - Hệ tầng Đa Pren: Belousov A.p. et al. 1984. - Hệ tầng Lạc Lâm: Belousov A.p. et al. 1984.
Lithology and Thickness
Volcano-sedimentary beds of acidic-intermediate composition formed in continental conditions. The formation stratotype exposed on the road from Phim Non to Nha Trang (Don Duong area), was described in detail by Nguyen Dục Thang in the following succession. (1). Polymictic conglomerate and gritstone, feldspar-rich gritstone grading upwards into tuffaceous gritstone, tufflte, sometimes interbedded with chocolate-colored claystone and siltstone, rhyolitc and porphyritic felsite, 250 m thick; this member unconformably covers granitoids of the Late Jurassic Dinh Quan and Cretaceous Deo Ca Complexes. (2). Ash-grey, greenish-grey porphyritic dacite, some interbeds of grey to light grey rhyolite, tuffaceous sandstone and siltstone, irregularly grained gritstone and some interbeds or lenses of andesite, andesitic tuff, 350-450 m thick. (3). Chocolate-colored to brownish, medium-bedded tuffaceous sandstone and siltstone, 150 m thick; tuffaceous sandstone contains palynomorphs Schizosporites sp., Lygodium sp., Picea sp., Cedrus sp. (4). Greenish-grey porphyritic daeite, dacitic tuff, grey to light grey porphyritic rhyodacite and rhyolỉte, 150 m thick; dacitic tuff is of fragmental texture, massive structure, with the fragments occupying 19-34%, the groundmass of volcanic glass. (5). Porphyritic rhyolite rich in pink potash feldspar phenocrysts and its tuff, 200 m thick. The total thickness of the formation in this section is about 1250-1350 m.
Relationships and Distribution
Lower contact
Unconformably rests upon the Middle Jurassic Ma Da Fm (= lower part of current La Nga Fm) occurring in the Ta Dung Mt and Phan Dung areas, upon the Upper Jurassic Deo Bao Loc Fm - in the Phim Non Fork, and upon the Lower Cretaceous Dak Rium Fm - in the Thanh Tri Pren Pass foot areas. Partly coeval with the Nha Trang Fm volcanics in the Da Lat Basin (into the Middle Trung Bo region).
Upper contact
Regionally, the next younger unit in Lam Dong region (central) is the late Miocene deposits of Bac Loc Fm.
Regional extent
It has a relatively large distributive area in the Lam Dong Province. The amount of sedimentary and volcanogeno-sedimentary beds in the formation is variable, possibly depending oh the volcanic activity in each area. They are developed in the Don Duong, Due Trong, Ta Dung Mt, Dang Mt areas, but occur in the form of insignificant interbeds in some other areas, such as in Pren Pass, upper course of Song Luy River, Phan Dung and West Cam Ranh areas. Therefore, some authors, when studying the section of the formation in the Don Duong - Lac Lam area, subdivided the section into two formations: one including only volcanics (Da Pren Fm), the other - mainly of sedimentary rocks (Lac Lam Fm), but the study over all of South Viet Nam shows that this scheme is not useful
GeoJSON
Fossils
Schizosporites sp., Lygodium sp., Picea sp., Cedrus sp.,
Age
Depositional setting
Additional Information