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Na Khuat Formation
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Na Khuat Fm base reconstruction

Na Khuat Fm

Period: 
Triassic

Age Interval: 
Ladinian, T2a, (4)


Province: 
East Bac Bo

Type Locality and Naming

The type section is located in the dirt road from Mai Pha to Na Khuat, 5 km SE of Lang Son City (N= 21°48’; E= 106°45’), It was named by Jamoida A.I. and Pham Van Quang (1965)

Synonym: Hệ tầng h'à Khuất: Jamoida A.I., Phạm Văn Quang (in Dovjikov A.E. et al 1965); Vũ Khúc (in Vũ Khúc et al. 1965 - Lađin-Cami); Hoàng Ngọc Kỳ et al. 1978 - Hệ tầng; Vũ Khúc 1980, 1989 - Hệ tầng; Vũ Khúc (in Vũ Khúc, Bùi Phú Mỹ et al. 1990- Hệ tầng), Vũ Khúc et al. 2000- Trias trung - Hộ tầng); Đặng Trần Huyên (in Nguyễn Kinh Quốc et al 1991 - Hệ tầng) (Trias trung). Schistes à Margarítes samneuaensis: Pattc E. 1927 (Cami). Diệp Sơn Dương, Điệp Dương Hưu, Điệp Trại Siru: Phạm Văn Quang 1966. Điệp Đa PhúcĐi': Nguy ễn Quang Hạp 1967. Điệp Đèo Nhe: Phạm Đình Long et al. 1974. Bậc Laditr:Vũ Khúc, Trịnh Thọ (in Trần Vãn Trị et al 1977)


Lithology and Thickness

Sandstone and Claystone: Characteristic section near Lang Son City has 950 m of: (1) Dark-grey, thin-bedded calcareous siltstone; 90 m, containing Anisian bivalves. (2) Greenish-grey, fine- to medium-grained sandstone; 45 m. (3) Greenish-grey to violetish, medium-bedded siltstone; 160 m; containing Ladinian bivalves. (4) Pinkish, thick-bedded sandstone interbedded with some siltstone; 40 m. (5) Violetish siltstone with some interbeds of sandstone in the lower part; 300 m. (6) Pinkish, thick-bedded sandstone; 130 m. (7) Violetish, thick-bedded siltstone; 180 m.


Lithology Pattern: 
Siltstone


Relationships and Distribution

Lower contact

Conformably (but diachronous) upon Anisian sediments belonging to the Diem He Fm and Khon Lang Fm

Upper contact

Conformably underlies Carnian beds of the Mau Son Fm.

Regional extent

Occurs in many narrow bands, among them the largest extents from the east of Lang Son City southwestwards to Thai Nguyen City.


GeoJSON

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Fossils

The formation is characterized by 2 bivalve assemblages: (1) Pteria - small sized Costatoria Assemblage consisting of Anisian bivalves including: Pteria sturỉ, Costatorỉa proharpa, c. pahangensis, Velopecten albertii, Neoschizodus orbicularis; (2) Costatoria-Trigonodus Assemblage with the well known Ladinian bivalves: Costatoria gold/ussi, c. ỉnaequỉcostata, Trigonodus sandbergeri, T. trapezoidalis


Age 

In general, it is dated as Ladinian; although the basal unit seems to extend down into Anisian based on the bivalves.

Age Span: 

    Beginning stage: 
Ladinian

    Fraction up in beginning stage: 
0.2

    Beginning date (Ma): 
240.57

    Ending stage: 
Ladinian

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
237.00

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, 2nd edition; 2011; English edited by A. Boucot).