Ta Khoa Fm

Period:

Devonian

Age Interval:

Early Devonian, D₁tk, (9)

Province:
West Bac Bo

Type Locality and Naming

Nguyen Huu Hung surveyed a 7-km section along Song Da River, from the Ta Khoa Bridge downstream to Ban Mong Village. [Ta Khoa nickel mine is in Son La Province on the NW to SE zone of the Song Da Rift.]

Synonym: Ta Khoa Complex of metamorphic rocks (Deprat, 1914). Le complexe des schistes cristallins de Ta Khoa: Deprat J. 1914; Dussaulc L. 1929. Tang Devon hạ và các tầng cồ hon: Lê Thạc Xinh, Hoàng Trí Nghị 1964. Phức hệ biển chất Tợ Khoa: Nguyền Xuân Tùng 1964. Hệ tầng Nậm Sập: Dovjikov A.E. et al. 1965 (Triassic). Tầng Sà Piệt: Nguyễn Xuân Bao 1969. Trầm tích Devon hạ - Eifdi: Trần Văn Trị et al. 1977. Các trầm tích già thiết tuồi Devon: Nguyễn Vĩnh 1977. Hệ lẩng Tạ Khoa: Phan Cự Tiến et al. 1977; Vũ Khúc & Bùi Phú Mỹ et al. 1990.

Lithology and Thickness

Sandstone and chlorite schist. Nguyen Xuan Bao (1978) described the Ta Khoa Fm in two distinguishable parts in ascending order. (1). Lower part along the Sa Piet Stream consists of dark grey, thin-bedded sandstone and quartz-rich sandstone intercalated with grey clayey scricite schist, sericite - biotitc schist, and a member of green schist (chlorite-actinolite schist) or mafic volcanic by Dovjikov A.E. et al. (1965), 1000 m thick. (2). Upper part along the Nam Sap Stream, a tributary of the Song Da River, composed of chlorite schist, which was gradually weakly sericitized upward, intercalated with trace ripple and knotted schist, 750 m thick.

Lithology Pattern:

Clayey sandstone

Relationships and Distribution

Lower contact

The present materials cannot show clearly the stratigraphic relation of the Ta Khoa Fm with the adjacent units. The contact of the TaKhoa with the underlying formation was not found. . [Regionally, the next younger unit is the Ban Pap Gr of mid-Devonian?]

Upper contact

Its relation with the overlying formation, according to some geologists conformable, but for other geologists there is an unconformity. . [Regionally, the next older unit is the Ket Hay Fm of Silurian?]

Regional extent

Mainly in the Song Da Rift, from Ban Koa to Ban Mong villages.

GeoJSON

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Fossils

Contains abundant brachiopods Howellella cf. crispa, Howellella sp., Dicoelostrophia sp., Atrypa sp.; Bivalves Modiomorpha oblonga, and Modiolopsis cf. wuxuanensis, Bisoptera? sp.; Trilobites Ancanthopyge cf. yiminensis, and Phacopids; Crinoids Hexacrinites (?) trangxaensis, Hexacrinites sp., and Bryozoans Fenestella sp.

[Figure: Tạ Khoa Fm: Brachiopod and Ammonoid fossils (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Age

Early Devonian implied by the fossils. Considered correlative with the stratigraphic levels from the Song Mua Fm to the Ban Nguon Fm inclusive; which would imply spanning the Lochkovian.

Age Span:

Beginning stage:

Lochkovian

Fraction up in beginning stage:

0.0

Beginning date (Ma):

419.00

Ending stage:

Lochkovian

Fraction up in the ending stage:

1.0

Ending date (Ma):

412.40

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).