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Nha Trang Formation
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Nha Trang Fm base reconstruction

Nha Trang Fm

Period: 
Cretaceous

Age Interval: 
Late Cretaceous, (24).


Province: 
South Trung Bo, Middle Trung Bo

Type Locality and Naming

Da Lat Basin. The type section is located in the vicinity of Nha Trang City (N = 12°15’; E = 109°08’). Belousov A.p. et al. 1984. During the process of geological mapping at 1:500,000 scale of South Viet Nam (1975-1978) volcanics of the Nha Trang Fm were attributed to the same unit with underlying volcano-sedimentary beds under the name of Don Duong Fm. However, during the process of geological mapping at 1:200,000 scale they have been separated as an independent unit on the basis of the analysis of volcanic activities in the Da Lat Depression, and based on the presence of alkaline rocks in this formation, it has been attributed to the highest position in the series of these volcanic beds.

Synonym: - Hệ iầng Nha Trảng: Belousov A.P., Nguyễn Đức Thắng, Bùi Phú Mỹ, Vũ Như Hùng, 1984; Vũ Khúc (in Vũ Khúc và nnk.) 1998; Nguyễn Đức Thắng và nnk. 1999; Trần Tính và nnk. 199 - Hệ tầng Đơn Dương (part.): Nguyễn Kinh Quốc (in Vũ Khúc, Bùi Phú Mỹ và nnk.) 1990.


Lithology and Thickness

Lower part. Mainly of andesite, andesite-dacite, đacite and their tuffs, in some places there still are conglomerate, tuffaceous gritstone, tuffaceous agglomerate with some interbeds of chocolate-colored, medium-bedded sandstone and siltstonẹ, 250 m thick. Upper-part. Rhyolite, ữachyrhyolite, porphyritic felsite interbedded with some porphyritic rhyodácite and many interbeds of fine tuffaceous sandstone, tuffaceous siltstone and clayey shale, 250-350 m thick. The total thickness of the formation in this section is 500-600 m.

Along Highway Na ỈA, in the section extending from Phan Thiet Town to Vinh Hao the formation occurs with two members (Nguyen Due Thang et al. 1999). (1). Andesite, andesitic tuffs, and in some places, greenish-grey tuffaceous agglomerate of intermediate composition containing fragments of from some mm to some cm in size of andesite, dacite, rhyolite, felsite, etc. and fragments of feldspar crystals, with the cement from andesitic tuff or fine-grained andesite; 50-100 m thick. (2). Light grey, brownish-grey, massive or having flow structure, dacite, rhyodacitc, rhyolite, trachyrhyolite and their tuffs, 450-500 m thick.


Lithology Pattern: 
Volcanics


Relationships and Distribution

Lower contact

Rests unconformably on the Lower Jurassic Dray Linh Fm, on the Upper Jurassic Deo Bao Loc Fm volcanics. Regionally, it is partly coeval with the Lower Cretaceous (hence maybe slightly younger) Dak Rhum Fm of the Lam Dong Province.

Upper contact

Partly coeval with the Don Duang Fm of volcanics in the Lam Dong Province.

Regional extent

The volcano-sedimentary beds of the formation occur in the Da Lat Depression (coastal areas of the Middle Trung Bo), besides scattered exposures have been found in the Hon Chong area of West Nam Bo.


GeoJSON

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Fossils

Clayey shale in lower beds yields die Kainophytic sporomorphs Selaginella sp., Taxodium sp., Lygọdium sp. and Seitotylus sp..


Age 

When the formation has been established, it has been dated as Cretaceous because the agglomerate beds of the lower part of its basal part contain breccias and boulders from biotite-hornblende granodiorite of the Upper Jurassic Dinh Quan Complex (in Ta Luong Mt situated near Cam Ranh Townlet and in the Ninh Hoa Peninsula). Attributed to undifferentiated Cretaceous age, possibly Late Cretaceous.

Age Span: 

    Beginning stage: 
Aptian

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
121.40

    Ending stage: 
Turonian

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
89.39

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).