Long Dai Fm

Period:

Ordovician, Silurian

Age Interval:

Late Ordovician-middle Silurian, O-S2, (18, 20)

Province:
North Trung Bo

Type Locality and Naming

The Ban Ho - Vit Thu Lu section extending along Road No. 10, in the basin of the upper course of the Long Dai River, Le Thuy District, Quang Binh Province (N= 17°05', E=106°37). However, in the stratotype the lowest part of the formation does not occur, therefore the hypostratotype, the Le Ky section, is situated along die Le Ky Stream, southwest of Dong Hoi Townlet (N= 17°25’, E= 106°35'). The formation bears the name of the river, whose basin contains the stratotype (Dovjikov A.E. et al. 1965).

Synonym: -Hệ tầng Long Đợi'. Mareichev A.M. in Dovjikov A.E và nnk. 1965) (O₃-S₁); Trần Văn Trị và nnk. 1973, 1975, 1977 (O₃-S₁); Trần Đức Lươngvà nnk, 1975 (O3-Si); Phạm Đình Long 1975,1976 (O1-S1); Nguyễn Văn Phúc 1977, 1981, 1991, 1998 (O₃-S₁); Nguyễn Xuân Dương và nnk. 1977, 1984, 1996 (0 3 -S,); Lương Hồng Hược và nnk. 1980, 1982, 1984, 1985 (O₃-S₁); Phạm Kim Ngâ/1 và nnk. 1980, 1986 (O₃-S,), 1994, 1995 (Or Sj); Đinh Minh Mộng 1980 (O₃-S₁); Nguyễn Quang-Trung và nnk. 1981, 1982,1983 (O₃-S₁); Nguyễn Văn Hòanh vò nnk. 1984, 1985, 1987 （O₃-S₁), 1994 (O₃-S); Nguyễn Văn Trang và nnk. 1985 (0₁-S); Vũ Khúc, Bùi Phú Mỹ và nnk. 1988 (O₃-S); Vũ Mạnh Điển và nnk. 1993, 1997 (O₁*Si); Phạm Huy Thông và nnk. 1994,1997 (O- S₁); Đỗ Văn Long vá nnk. 1997,1999 (O1-S₁). Devon thượng (part): Fromaget J. 1927; Hoffet J.H. 1933,1935

Lithology and Thickness

Rhythmic and banded terrigenous sediments interbedded with intermediate to felsic volcanics. The combined stratotype and hpostratotype spans ca. 1800 m: (1) Intercalated sandstone, siltstone and shale with a basal conglomerate and with interbeds of andesite in the lower part; ca. 1000 m. The siltstone yields Late Ordovician trilobites. (2) Shale, siltstone and cherty shale containing Radiolarians, interbedded with sandstone and banded siltstone containing Wenlock graptolites; ca. 750 m thick.

Lithology Pattern:

Clayey sandstone

Relationships and Distribution

Lower contact

Rests unconformably upon the A Vuong Fm.

Upper contact

Unconformably below the Dai Giang Fm.

Regional extent

Quang Binh, Truang Tri and Thua Thien Hue provinces; from south of the Rao Nay Fault (Gianh River) to the upper course of the Vang River. The fine-grained terrigenous components gradually increase in the upper part of the section, and the thickness of the formation decreases north-northwestward in the distribution area of the formation.

GeoJSON

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Fossils

Containing the Late Ordovician trilobites Cycỉopyge sp., Microparia (?) sagaviaformis, ogygiocaris sp., Niieus sp., Agnostidae; brachiopod Orbiculoidea sp. and badly preserved bivalves and gastropods. Ordovician graptolite Diplograptus sp., and the Silurian graptolites Neodiversograptus nilssoni, Monograptus sp., Pristiograptus sp., brachiopods and plant remains. Demirastrites triangulatus, Diversograptus ramosus, Glyptograptus tamariscus, Prisiiograptus gregarius, Monograptus gemmatus.

[Figure: Long Dai Fm (O₃-S₂ ld): Ordovician: Radiolarian fossils (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Age

Based on fossils and stratigraphic relations, the Long Dai has been referred to Ordovician-Silurian, possibly Late Ordovician - Middle Silurian (Wenlock epoch).

Age Span:

Beginning stage:

Sandbian

Fraction up in beginning stage:

0.0

Beginning date (Ma):

458.18

Ending stage:

Homerian

Fraction up in the ending stage:

1.0

Ending date (Ma):

426.74

Depositional setting

Depositional pattern:

Additional Information

Compiler:

Tran Van Tri and Vu Khuc (Editors; Geology and Earth Resources of Viet Nam, General Dept. of Geology and Minerals of Viet Nam; completed in Dec., 2011), with additional details from Tong-Dzuy Thanh and Vu Khuc (Editors; Stratigraphic Units of Viet Nam, 2^nd edition; 2011; English edited by A. Boucot).