Xom Nha Fm

Period:

Devonian

Age Interval:

Fammenian, D₃j cn, (18)

Province:
North Trung Bo

Type Locality and Naming

Quang Binh Province. Xom Nha (Cay Da Hamlet) Section, Quy Dat area, Quang Binh Province (N = 17°48'; E = 105°37'). The name of the formation is taken from Xom Nha on the geographic map at I: 100 000 (Thanh Lang Sheet), which at present is Cay Da Hamlet 2.7 km west Quy Dat Townlet. The Xom Nha Fm was first described by Nguyen Huu Hung (in Lc Hung et a1. 1981).

Synonym: Calcaires du Dévonien moyen (part.): Fromaget J. 1927. Trầm tích Carbon: Hoàng Thanh Cảnh 1964. Các bậc Civet - Frasrti (part.): Marcichev A.M. và Trần Đức Lương (Dovjikov A.E. et al,1965). Trầm tích Permi thượng chứa than: Lc Hùng 1975. Hệ Permi, thống thượng (part.): Lê Hùng (Trần Văn Trị 1977). Đá vói Frasni - Famen (Devon thượng): Nguyễn Hữu I lùng và nnk 1980. Hệ tầng Xồm Nha: Nguyền Hữu Hùng (Lê Hùng 1981), Nguyễn Đức Khoa (Vù Khúc 1984), Tống Duy Thanh vá nnk 1986, 1988, Nguyễn Hữu Hùng và nnk 1995; Nguyễn Hữu Hùng 1996, Phạm Huy Thông và nnk 1998, (D₃), Phạm Muy Thòng 2001 (D₁ - C₁). Diệp Cát Dằng: Nguyễn Quang Trung và nnk 1983; Nguyễn Văn Hoành, Phạm Huy Thông 1984 (Djfm). Xóm Nha Fm-. Tong-Dzuy Thanh 1993; Ta Hoa Phuong, Doan Nhat Truong 1995

[Figure: Xóm Nha Fm: Type section of the Xóm Nha Fm (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Lithology and Thickness

Limestone: Characterized by dark grey, light grey stylolitic limestone containing abundant corals, stromatoporoids and conodonts. The fullest section is met with in the core of the Xom Nha Syncline: (1). Light grey, medium bedded limestone, 70 m thick, (2). Black grey limestone, 10 m thick. (3) Light-grey limestone with suture structures, 120 m thick. The total thickness of the section is about 200 m.

Lithology Pattern:

Limestone

Relationships and Distribution

Lower contact

Conformably rests upon the chert and carbonaccous shale of the Ngoc Lam Fm.

Upper contact

Unconformably under the overlying Lower Carboniferous La Khe Fm. Regionally, next younger unit (or partly coeval) is the Phong Nha Fm

Regional extent

Apart from the Xom Nha area the limestone of the Xom Nha Fm occurs also in Dong Le, Xom Sung, Tang Hoa (Cat Dang Sach), Thanh Lang areas, Quang Binh Province, and in the Chuc A area, Ha Tinh Province.

GeoJSON

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Fossils

Contains abundant stromatoporoids Stachyodes cosiulata, S. angulata, S. paralleloporoides, S. lagowiensis; corals Scoliopora aff. denticulata, Alveolites sp., Thamnopora polyforata, Temnophyllum cf. lantenoisi, T. isetense. Lower unit conodonts include Palmatolepis gigas; Middle unit conodonts include Palmatolepis triangularis (base of Famennian); and the Upper unit conodonts include Palmatolepis glabra. Conodont assemblages span the Lower and Upper Palmatolepis rhenana, Palmatolepis linguiformis, Palmatolepis triangularis, Palmatolepis Marginifera and Palmatolepis crepida Zones (latest Frasnian through Famennian).

[Figure: Xóm Nha Fm: Conodonts (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Xóm Nha Fm: F/F boundary section, Minh Hoa Distr., Quang Bình Prov. (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Age

Well dated by conodonts as crossing the Frasnian-Famennian boundary through middle Famennian.

Age Span:

Beginning stage:

Frasnian

Fraction up in beginning stage:

0.9

Beginning date (Ma):

371.88

Ending stage:

Famennian

Fraction up in the ending stage:

0.6

Ending date (Ma):

364.02

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).