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Na Mo Formation
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Na Mo Fm base reconstruction

Na Mo Fm

Period: 
Ordovician

Age Interval: 
Middle-Late Ordovician; O2-3 nm, (4)


Province: 
East Bac Bo

Type Locality and Naming

The type section is located near Na Mo Village, Trang Xa Commune, Vo Nhai District, Thai Nguyen Province (N= 21°44'; E = 106° 03'). . It was named by Pham Binh Long et al. (1981).

Synonym: Hệ tầng Nà Mô: Phạm Đinh Long et al 1981 (Ordovic trung - Ordovic muộn); Phạm Kim Ngân, Lương Hồng Hược, Phạm Đình Long 1986 (Ordovic trung - Ordovic muộn; Vũ Khúc et al. 2000 (Ọrdovic trung- muộn); Vũ Khúc, Bùi Phủ Mỹ et al. 1990 (Ordovic). Schisle de Nam Ho à Caỉymene: Mansuy H. 1908 (Silurien), Patte E. 1927 (Ordovicien); Saurin E. 1956 (Série de - Orđovicien, Caradocien inférieur?). Hệ Ordovic: Dovjikov A.E. et al 1965 (Ordovic trúng - ? Ordovic muộn). Các trầm tích Ordovic thượng: Trần Văn Trị el al. 19771977 (Ordovic muộn)


Lithology and Thickness

Sandstone to siltstone. Stratotype is ca. 450 m thick: (1). Light gey, thick-bedded quartz sandstone; 150 m thick. (2). Blue to violet silty shale interbedded with light grey, pinkish-grey, fine- to coarse-grained feldspar-rich sandstone; 200 m. (3). Grey siltstone, 100 m, containing trilobites.


Lithology Pattern: 
Siltstone


Relationships and Distribution

Lower contact

The Na Mo Fm has tectonic contact with the underlying Than Sa Fm.

Upper contact

The Na Mo Fm has tectonic contact with other formations. Regionally, next younger unit is the Late Ordovician-Early Silurian Phu Ngu Fm.

Regional extent

Thai Nguyen Province. (East Bac Bo Zone in strat Fig. 3.2; region 1b in Fig.3.1). The Na Mo Fm occurs limitedly in the adjoining area of Na Mo Village in the form of a tectonic wedge lying between the Than Sa and Bac Bun formations. In the main distribution area, the Na Mo Fm is observed only at Na Mo Village (Vo Nhai District, Thai Nguyen Province) in the tectonic wedge between the Than Sa Fm and the Song Cau Gr. Besides this distribution area, the following Ordovician deposits also belong to the Na Mo Fm (Pham Kim Ngan, Luong Hong Huoc, Pham Dinh Long 1986).


GeoJSON

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Fossils

Contains the Middle Ordovician trilobites Synhomalonotus birmanicus, Lonchodomas aff. yohi, Ceraurinus sp., Plaisicomia? sp.; Middle Ordovician - Silurian plectambonitid brachiopods and in a limestone lens at Na Mo Village the Ordovician, possibly Late Ordovician, coral Plasmoporella sp. Vasilevskaia E.D. did not show the thickness of this member, but according to the description of Pham Dinh Long et al. (1981), it could be 100 m.


Age 

Fossils enable dating as Middle-Late Ordovician. [For graphic purposes to allow display of a Late Ordov unit of Phu Ngu Fm in the Lang Son region, the span is shown as only Middle Ordovician.]

Age Span: 

    Beginning stage: 
Dapingian

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
471.26

    Ending stage: 
Darriwilian

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
458.18

Depositional setting

Unclear


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).