Deo Bao Loc Fm
Type Locality and Naming
The type section is located along the Bi O River, in the Gia Ray area (N = 11°12’; E = 108°06’), Nguyen Duc Thang et al. 1999.The formation, including continental red beds that grade upwards into thick members of intermediate volcanics largely ocurring in South Trung Bo and Nam Bo regions was described as the Deo Bao Loc Fm (Nguyen Xuan Bao 1977). It was named by when the formation was described its stratotype was not designated, and although the formation bearst he name of the pass situated on Road No.20, the most complete section of the formation lies along the Bi o River, which was described in detail (see Vu Kliuc, Bui Phu My et al. 1990). However, the Deo Bao Loc Fm is a well-known term in geology of Viet Nam, and to avoid unnecessary changes it should be preserved with its stratotype section situated on the Bi O River.
Synonym: - Hệ tầng Đèo Bảo Lộc: Nguyễn Xuân Bao 1977; Nguyễn Kinh Quốẹ (in Vũ Khúc, Bùi Phú Mỹ et al. 1990); Nguyễn Ngọc Hoa et al. 1996; Trần Tính et al.1998; Nguyền Đức Thánge et al. 1999; Vũ Khúc et al. 2000. - Hệ tầng Ca Tô: Belousov A.p. et al. 1984. - Hệ tầng Long Bình: Bùi Phú Mỹ, Dương Văn Càu 1991; Nguyễn Ngọc Hoa et al. 1996; Vũ Khủc et al. 2000; Ma Công Cọ et al.2001. - Hệ lồng Xa Lon: Trưcmg Công Đượng et al. 1997. - Hệ tầng Hòn Mấu: Trịnh Dánh et al. 1998.
Lithology and Thickness
Succession of volcano-sedimentary beds; variable among regions. Type section situated in the source of the Bi O River (Dong Nai Prov.): (1). Greyish, light violet - brown, irregularly bedded conglomerate, tuffaceous gritstone with some interbeds of brownish, chocolate-colored sandstone and siltstone, andesite, andesite-dacite and their tuffs, 350 m thick; siltstone yields the palynomorph Lygodium sp. The basal conglomerate of this member unconformably rests upon thin-banded siltstone of the Ma Da Fm (= lower part of current La Nga Fm), which occurs in the source of the Dak Rium Stream source. (2). Andesite, andesitoporphyrite, andesitobasalt, andesitodacite, dacite, rhyodacite and thek tuffs with some thin interbeds of sandstone containing volcanic materials, 400 m thick. The total thickness of the formation in this section reaches about 750 m.
In other areas, the section of the Fm has similar features. Especially in the Long Binh area, the sedimentary beds is thicker and the upper members of the lower part belong to the marshy-lacustrine facies with beds of black coaly shale, cherty coaly shale, grey shale. In the Bảy Nui area, situated in the north of Ha Tien Townlet, the Đeo Bảo Lộc Fm is exposed in the slope of the Xa Lon Mt with a thickness of about 900 m, including basal conglomerate, pebble-bearing sandstone interbedded with red-brown siltstone (100 m thick) grading upwards into greenish-grey andesite and rhyolite.
On Hon Mấu Island in the Thailand Gulf, the Fm begins by basal conglomerate overlying unconformably Anisian rhyolite, and grading upwards into andesitobasalt and its tuffs with the observed thickness of about 95 m..
Relationships and Distribution
Lower contact
In fully exposed sections, the Đeo Bảo Lộc Fm rests unconformably upon Middle Triassic or Middle Jurassic sediments. such as upon the Middle Jurassic Ma Da Fm (= lower part of current La Nga Fm) in the Ma Noi area, upon the Lower Jurassic Dak Bung Fm near Die Binh An Quarry, upon the Middle Jurassic Ea Sup Fm in the Loc Ninh area, and the Ta Pa Fm.
Upper contact
Covered by the Dak Rhum Fm of Early Cretaceous in South Trung Bo. In the Gulf of Thailand, the younger unit (no contact exposed) is the Phu Quoc Fm of Early Cretaceous; and in West Nam Bo (again, no contact exposed), it is a discontinuity to the Ca Coi Fm of Eocene age.
Regional extent
It is exposed over large areas in Bao Loc Pass, in the Bi o and Song Len River sources, Gia Ray area, and in the Ca Tot River source, NW Phan Thiet Town; there are also small exposures of the formation around Di Linh, in cast Gia Bac, south of Ta Dung Mt, west of Cau Pass and Giac Lan Mt near Cam Ranh, in the Long Binh area near Ho Chi Minh City and in the Loc Ninh area; recently small exposures of the formation were found in the Xa Lon Mt, Bay Nui area and in Hon Mau Islet in the Gulf of Thailand.
GeoJSON
Fossils
In the Long Binh area, the upper members of the lower part contain fish fossils of Pholidophorus(?) sp. and Actinopterygii of Late Jurassic [Filleul A., Vũ Kh.c, 2001] and flora Zamites aff. haufmani, Pagiophyllum sp. and Dicksonia sp..
Age
Depositional setting
It is interpreted as volcano-sedimentary beds of mainly intermediate composition formed in the continental environment.
Additional Information