Hon Ngang Fm

Period:

Triassic

Age Interval:

Anisian (T_2a, hn) (28)

Province:
West Nam Bo

Type Locality and Naming

Kien Giang Province, and Gulf of Thailand. Lectostratotype - from Bai Ot Beach to the To Chau of the Ha Tien area, Kien Giang Province (N = 10° 15'; E = 104°32'). The formation bears the name of an islet in the Gulf of Thailand belonging to Kien Giang Province.

Synonym: - Formation de Hon Ngang: Fontaine H. 1969, 1970 (truớc Fermi); Nguyễn Ngọc Hoa et al. 1991 (Hệ tầng - Trias sớm); Nguyễn Xu.n Bao et al. 1994,2000 (Hệ tầng - Anisi). - Hệ tầng Nam Du (part.): Nguyễn Kinh Quốc {in Vũ Kh.c, B.i Ph. Mỹ et ai. 1990) (PZ|_2). - Hệ tầng Núi Cọp: Trương C.ng Đượng et al. 1997 (Anisi); Vũ Kh.c (in VQ Kh.c et aỉ. 2000 - Anisi)

Lithology and Thickness

Volcaniclastics: (1). Tuffaceous conglomerate, boulderstone and gritstone grading upwards into greyish porphyritic rhyolite and its different tuffs, about 350 m thick. In general, the coarse-grained basal bed has a varying thickness, up to some tens of metres. Its grains consist of limestone, sandstone and chert with a size range from some mm to some cm, sometimes there are limestone boulders about cubic-meter in size. The cement consists of tuffaceous sandstone and gritstone. (2). Rhyolite and its tuff form a rather homogeneous cover; ca. 350 m thick. (3). Grey, thin- to medium-bedded tuffaceous gritstone and sandstone, tuffaceous silty sandstone, rhyolitic tuff interbedded with dark grey cherty shale, sandstone, siltstone and clayey shale, greyish felsite and porphyritic felsite; about 100 m thick. For the first time relatively well preserved plant remains have been found in the beds of siltstone, such as: Cordaites sp., Taeniopteris (Anomozamites) sp., and Pecopteris sp.. The total thickness of the formation in this section is about 550 m.

Lithology Pattern:

Volcanic ash

Relationships and Distribution

Lower contact

The lower boundary of the Hon Ngang Fm is still unknown. Only in borehole N2 823 drilled in the Ha Tien area does the volcano-sedimentary beds of the Hon Ngang Fm lie unconformably upon limestone of the Permian Ha Tien Fm. Regionally in Gulf of Thailand, the next younger unit is the Hon Duoc Fm of Early Triassic.

Upper contact

Chcrty shale of the Hon Ngang Fm is covered unconformably by basal conglomerate beds of the Upper Jurassic Deo Bao Loc Fm on Hon Mau Islet. Regionally, the next younger unit is the Ta Pa Fm of Early Jurassic.

Regional extent

Kien Giang Province, and Gulf of Thailand. The Hon Ngang Formation occurs largely in the mainland from Ha Tien to Bai Ot Bcach and in the Nam Du, Ba Lua and Hai Tac archipelagoes in the Gulf of Thailand as well.

GeoJSON

{"type":"Feature","geometry":{"type":"MultiPolygon","coordinates":[[[[104.12,10.56],[104.2,10.61],[104.58,10.61],[104.72,10.5],[104.75,10.32],[104.72,10.11],[104.66,9.98],[104.62,9.82],[104.44,9.61],[104.2,9.85],[104.22,10.18],[104.12,10.45],[104.12,10.56]]]]}}

Fossils

Ammonites and bivalves.

Age

The Anisian age of the formation has been reliably determined on the basis of the ammonoids found in the Mui Nai - Ta Pang Mt scction. In a given level the Hon Ngang Fm can be correlated with many mainly felsic volcano-sedimentary formations containing Anisian marine faunas, such as: Khon Lang Fm in East Bac Bo, Dong Trau Fm in North Trung Bo, Mang Yang Fm in South Trung Bo and Chau Thoi Fm in East Nam Bo. Possibly, in Viet Nam and adjacent countries, volcanic activities happened actively and largely in the Anisian in separated intracontinental rift structures. [Note: Volcanism in Vietnam is mainly only Late Anisian; with the Early Anisian being a Gap.]

Age Span:

Beginning stage:

Anisian

Fraction up in beginning stage:

0.5

Beginning date (Ma):

244.08

Ending stage:

Anisian

Fraction up in the ending stage:

1.0

Ending date (Ma):

241.46

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).