Lutxia Fm

Period:

Ordovician

Age Interval:

Early Ordovician; O₁lx, (1, 2)

Province:
Viet Bac

Type Locality and Naming

Upper formation in Ham Rong Gr version of East Bac Bo. Along the track from the Chang Pung border post to Dong Van Townlet, from Seo Then Pá Village to Bac Bun Village (N = 23°16’; E = 105°24’)- Deprat J. 1915. During Geological Mapping at 1:200,000 of the Bao Lac Map sheet, Hoang Xuan Tinh (1976) established the Seo Then Pa “Suite” that is a synonym of the Lutxia Fm. During Geological Mapping at 1:200,000 of the Bac Kan Map sheet, Nguyen Kinh Quoc et al. (1974) established the Ban Loa Fm in the upper course of the Song Gam River. Lithologically, this “new” unit is very similar to the Lutxia Fm and should be its synonym

Synonym: Séríe de Loutẹia: Deprat J. 1915 (Ordovicien inferiéur); Hệ tầng Lutxia: Phạm Kim Ngân, Lương Hồng Hược 1995 (O₁₁); Tống Duy Thanh in vo Khúc el al. 2000; Điệp Lutxia: Phạm Kim Ngân 1986 (O₁); VQ Khúc, Bùi Phú Mỹ et al. 1990 (O₁); Hệ Ordovic, thống họ: Dovjikov A.E. et al. 1965, Phạm Kim Ngân, 1970,1975,1976, 1980; Phạm Kim Ngân, Lương Hồng Hược 1977; Hệ tầng Bản Lầu: Nguyễn Kinh Quốc et al. 1974 (O₁); Điệp Seo Thèn Pá: Hoàng Xuân Tình et al. 1976 (O₁)

Lithology and Thickness

Limestone and minor Claystone: Grey-banded limestone, yellow laminated claystone, chocolate and yellow laminated marl, yellowish-brown limestone, marly shale and sandy shale. In its distribution areas, according to Deprat J. (1915), this stratotype is represented by the following sequence: 1) yellow laminated claystone, 45 m thick; 2) grey banded limestone, 50 m thick; 3) chocolate and yellow laminated marl, 45 m thick; 4) yellowish-brown limestone, 20 m thick; 5) marly shale and sandy shale, 25 m thick; 6) greenish-grey limestone, 35 m thick; 7) laminated marl, 60 m; 8) lenses of brecciated limestone; 9) grey oolitic limestone, 20 m; 10) pinkish laminated marl, 4 m thick; 11) pinkish coarse sandstone, 6 m thick; 12) grey regular-bedded limestone, 90 m thick. The total thickness of this section reaches 400 m; and is 200 m in some other areas.

[Figure: Lutxia Fm: Outcrop of Lower Ordovician limestone and shale of the Lut Xia Fm in Xi Mần Kha village, Đồng Văn Distr. (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Lutxia Fm: Outcrop of Lower Ordovician clayey limestone and shale of the Lut Xia Fm in Xi Mần Kha village, Đồng Văn Distr. (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Lithology Pattern:

Shallow-marine marl

Relationships and Distribution

Lower contact

The Lutxia Fm rests conformably upon the Chang Pung Fm

Upper contact

It is covered unconformably by the basal conglomerate bed of the Si Ka Fm (early Devonian; D_lsk). In NW corner of Ha Giang Province, the next younger unit (also early Devonian) is the unique Khao Loc Fm of carbonate.

Regional extent

According to the described sections, the beds of the Lutxia Fm in the Dong Van and Vi Xuyen area are almost metamorphosed or very weakly metamorphosed; as in the Ban Loa area (Linh Ho, Tuyen Quang Province), they are phyllitic and dolomitic. Particularly in the Ban Loa area, upper course of the Song Gam River (Tuyen Quang Province), the Lutxia Fm is exposed as a tectonic wedge between Devonian sediments.

GeoJSON

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Fossils

Isotelus stenocephalus, Hysierolenus sp., Pseudokainella sp. Leiobienvillia sp., Bienvillia sp.,

Age

Early Ordovician (drawn as early-Early Ordovician). Trilobites found in the Seó Then Pa section consist of characteristic Early Ordovician forms, such as Isotelus stenocephalus, Hysierolenus sp. Apart from these; there are Bienvillia sp. and Leiobienvillia which have been met with in the Dong Son Fm (O1 ds). It is notable that the trilobite found in Ban Loa (Niobella sp.) was later determined as Niobella aurora which is close to a Late Cambrian species

Age Span:

Beginning stage:

Tremadocian

Fraction up in beginning stage:

0.0

Beginning date (Ma):

486.85

Ending stage:

Tremadocian

Fraction up in the ending stage:

1.0

Ending date (Ma):

477.08

Depositional setting

The lithological composition of the Lutxia Fm consists mainly of carbonate interbedded with some shallow-marine facies terrigenous sediments, that belong to the Lower Paleozoic stable depositional series (Ha Giang - Chang Pung - Lutxia formations) in North Bac Bo.

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).