Dong Trau Fm

Period:

Triassic

Age Interval:

Late Anisian, T₂, (16, 17, 18)

Province:
North Trung Bo

Type Locality and Naming

The type section is located Muong Sai area situated near the Viet Nam - Laotian border, Quan Hoa District, western Thanh Hoa Province (N= 19°42’; E= 105°27’). It was from Dong Trail Village, Nhu Xuan District. Lectostratotype was selected in the Muong Sai area, Quan Hoa District: the Muong Sai - Ban Mot section.

Synonym: Hệ tầng Dong Trầu: Jamoida A.I., Mareichcv A.M. (in Dovjikov A.E. et a!. 1965); Vũ Khúc et al. 965; Vũ Khúc 1980, 1990 (Điệp); Vũ Khúc et al. 2000; Vũ Khúc (in Vù Khúc, Bùi Phú Mỹ et al. 1990 - Diệp); Vũ Khúc (ìn Tống Duy Thanh et al. 1995); Vũ Khúc, Trịnh Thọ 1969; Lê Duy Bách, Đặng Trần Qúân et al. 1996; Đặng Trần Quân, Lé Duy Bách, Nguyễn Văn Hoành et al. 1996: Nguyễn Quang Trung et al. 1996; Nguyền Văn Hoành et al. 1996; Trần Nghĩa et al. 1996; Trần Tính et.aỉ, 1996; Vũ Khúc et al. 1996 (Điệp). - Schistes íriasiques: Jacob c. 1921. -Bậc Anisi: Vũ Khúc (in Trần Văn Trị et al. 1977); (ìn Dương Xuân Hảồ eí ai. 1980). - Hệ iầng Dộng Hà (part.): Vũ Mạnh Điền et al. 1999

Lithology and Thickness

Volcanics and Volcaniclastics: Volcanogenic-sedimentaiy beds with interbeds of banded cherty shale. At the lectostratotype: (1). Thick- to medium-bedded conglomerate with pebbles of quartz, chert, well rounded quartzitic sandstone, cemented by rhyolitic tuff, chcrty shale, grading upwards into pebble-bearing tuffaceous sandstone, whitc-grcy silty sandstone, yellowish-grey sandstone, 190 m thick. (2). Greenish-grey, motley weathering, porphyritic rhyolite, 250 m thick. (3). Fine-grained quartz sandstone, greenish-grey, medium-bedded tuffaceous silty sandstone grading upwards into grecnish-grcy, locally slightly violet tuffaceous conglomerate interbedded with gritstone, 80 m thick. (4). Greenish-grey, massive rhyolite, some interbeds of greenish-grey rhyolitic tuffs, 295 m thick. (5). Yellowish-grey silty sandstone containing volcanic materials interbedded with polymictic sandstone, calcareous fine-grained sandstone, chocolate-coloured claystone and some grey calcareous siltstone, 170 m thick. The total thickness of the formation in this lectotype section is 985 m (Tran Nghia et al. 1996).

Lithology Pattern:

Volcanics

Relationships and Distribution

Lower contact

Rests unconformably upon Paleozoic basement of the Sam Noa Basin (e.g., Cam Lo Fm in middle-North Trung Bo of late Permian, or Song Da Fm or Bac Son Fm (Permian) in Nghe An Province region.

Upper contact

In Nighe An, it conformably underlies the Hoang Mai Fm or the Quy Lang Fm of Ladinian. In Quang Binh, the Hoang Mai Fm is absent; therefore the next younger unit is the late Jurassic Bai Dinh Fm

Regional extent

Southern Thanh Hoa – northern Nghe An provinces and extending northwest through Laotian territory to the Nam Ban area of the Son La province. However, there are scattered occurrences in southern Sam Nua Basin in the southern Ha Tinh – northern Quang Binh provinces. The facies of the Dong Tran Formation quickly changes along the strike of its beds; expressed in the change of volume of volcanics.

GeoJSON

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Fossils

Balatonites cf. baỉatonicus, Leiophyllites cf. laevis, Beyrichites sp., Cuccoceras sp., Acrochordiceras sp. and Gymnotoceras sp.

Age

Based on paleontological materials, especially on ammonoids, the Dong Trau Fm is assigned to the Anisian. [Note: Volcanism in Vietnam is mainly only Late Anisian; with the Early Anisian being a Gap. However, in Nghe An, the conformably overlying Hoang Mai Fm limestone is assigned as Late Anisian; therefore the Dong Trau Fm is graphically placed here as mid-Anisian.]

Age Span:

Beginning stage:

Anisian

Fraction up in beginning stage:

0.4

Beginning date (Ma):

244.60

Ending stage:

Anisian

Fraction up in the ending stage:

0.7

Ending date (Ma):

243.03

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).