Tay Trang Fm

Period:

Silurian, Devonian

Age Interval:

Pridoli-Early Devonian, S₄-D_2,tt, (14, 15, 16)

Province:
North Trung Bo, Viet-Laos NW

Type Locality and Naming

Viet-Laos NW. Holostratotype - along the car road from Dien Bien to Tay Trang Pass and along the trail from Ban Mac to Ban Bcng villages, cross-cutting Pu Sung Mt (N = 21°14'; E = 102°55'). Type section didn't expose either upper or lower limits, therefore the hypostratotype: is along Huoi Nhi Stream (in the scction from Ta Nam Village to 5 km south of Phac Horn Village) (E = 19°30'; E = 104°18') and along Nam Tan Stream (in the section from the mouth of Nhun Stream to that of Pha Nach Stream), Ky Son District, Nghe An Province

Synonym: Hoi Nhi Fm (Nguyen Van Hoanh et al., 1978) has long been considered as the synonym, because the Hoi Nhi section in west Nghe An Province is the hypostratotype. Hệ tầng Tây Chang: Dovjikov A.E. etal. 1965 (in Russian), 1971 (in Vietnamese) (P₂ - T 1). Hệ tầng Tây Chang: Phan Sơn et al. 1975, 1978; Phan Cự Tien 1973 (part.); Tổng Duy Thanh et al. 1986. Phức hệ Tây Trang: Tống Duy Thanh et al. 1995 (Devonian). Hệ tầng Iluồi Nhị: Ngiiycn Văn Hoành, 1978; Nguyễn Dức Khoa, Vũ Khúc 1984: Nguyỗn Văn Hoành, Phạm Huy Thông 1984; Tống Duy 'lìianh et al.. 1986, 1988 (Upper Silurian - Lower Devonian); Vũ Khúc & Bùi Phú Mỹ et al 1990 (Upper Silurian - Lower Devonian); Tống Duy Thanh 1993 (Fm: Upper Silurian - Lower Devonian). Les complex de grcs schimeiix aiitrucolitique: Fromaget J., 1927. Trầm tích Paleoĩoi trung và tằng Sông C(i: Dovjikov A.E. et al. 1965 (Middle Paleozoic, Palcozoic). Hệ ừmgSông Cá (part.): Le Duy Bách 1968, Trần Văn Tri 1 9 7 7 (Silurian - Devonian). Hệ tầng Nậm Tầm: Nguyễn Văn Hoành 1978. Phức hệ líuổi A7ỉị: Nguyền 1 Iữu Hùng 1996 (Upper Silurian - Lower Carboniferous). Hệ tầng Thầm Hốc: Nguyền Văn Phúc 1998

Lithology and Thickness

Flysch; Hypostratotype is 1200 m of: (1) Ash-grey sandstone interbedded with siltstone, black shale and sericite shale, 500-750 m thick; with poorly preserved coniconch, spores of Late Silurian age. (2) Grey, thick-bedded sandsone, shale interbedded with siltstone and coaly shale containing graptolite, coniconch, crinoid and flora fossils.

Lithology Pattern:

Clayey sandstone

Relationships and Distribution

Lower contact

In western Nghe An Province the Tay Trang Fm (=“Huoi Nhi Fm”) rests unconformably upon the Song Ca Fm (O-S sc). In the Dien Bien area the lower boundaries of the Tay Trang Fm have not been observed.

Upper contact

In western Nghe An Province the Tay Trang Fm (=“Huoi Nhi Fm”) is conformably under the Huoi Loi Fm. In the Dien Bien area the upper boundaries of the Tay Trang Fm have not been observed; but regionally, the next younger unit is the Bac Son Fm carbonate platform of Carboniferous.

Regional extent

Dien Bien, Lai Chau and western Ngh An provinces. In this area the formation occurs in a large band extending along the Vietnamese-Laotian border west of Dien Bien Town to Muong Lan Village. In western Nghe An Province, during Geological Mapping at 1:200.000 scale in the Song Ca area.

GeoJSON

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Fossils

Erbenoceras sp. (Cf. E. advolvens), Anetoceras sp., Nowakia sp. (cf. N. acuaria), Styliolina sp., Chonetes ex gr. tenuicostata, date the host rocks as Early Devonian. Those found in the sections of the Huoi Nhi area are more complicated. Besides the Early Devonian graptolites, such as Monograptus aequabilis notoaequabilis, M. hercynicus, M. praehercynicus, M. yukonensis yukonensis, M. yukonensis fangensis, M. yukonensis langunensis (Nguyen Van Phuc 1998), and tentaculitids of the same age: Nowakia acuaria, N. cancellata, Styliolina intermedia, there arc also Middle Devonian tentaculitids, such as: Nowakia holynensis, N. cf. Nucleata, Homoctenus sp., Vinatellina cf. hollardi, Distriatostlus cf. akkaensis.

From Tong-Dzuy Thanh and Vu Khuc (Editors; Stratigraphic Units of Viet Nam, 2^nd edition; 2011): "The presence of these two assemblages in a unit lying conformably under the Lower-Middle Devonian Huoi Loi Fm [in west Ngh An Province] has caused embarassment to geologists, and forccd them to explain the mixture of different age fossils by tectonic movement, or by mistakes made during the collecting of the fossils. This abnormal situation leads to the dating of the formation as Early-Middle(?) Devonian in spite of the fact that in the Tay Trang stratotype the colic - fossils consist only of Early Devonian forms." But, this descripancy is resolved (?) by assigning the base of the Huoi Loi Fm to the Emsian by Tran Van Tri and Vu Khuc (Editors; Geology and Earth Resources of Viet Nam, 2011).

[Figure: Tây Trang Fm: Nowakia acuaria (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Age

Based on the paleontological collections and the geological relations, he formation is dated as latest Silurian (Pridoli) through Early Devonian.

Age Span:

Beginning stage:

Pridoli

Fraction up in beginning stage:

0.0

Beginning date (Ma):

422.73

Ending stage:

Pragian

Fraction up in the ending stage:

1.0

Ending date (Ma):

410.51

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).