Au Lau Fm
Type Locality and Naming
Upper Red River Valley. The type section is located exposed in the Au Lau Bridge area, Yen Bai Town (N = 21° 42’; E = 104°50’). It was named by C Nguyen Xuan Huyen.
Synonym: An Lau Fm: Thanh and Khuc (editors, 2011);. Tập sh3+Tập sh4+Tập sh5: Nguyên Xuân Huycn và nnk., 2004.
Lithology and Thickness
Coarse-grained sandstone in 3 units: (1). Boulder-bearing conglomerate, gritstone and sandstone with a predominance of conglomerate and gritstone in the lower part and of sandstone in the upper part; from 100-130 m thick in the Pho Moi Bridge, Trai Hut, etc. areas to 350 m in the Au Lau Bridge area. These sediments are commonly white-grey, greenish-grey to brown-grey colored, coarscly cross-bedded of current type. Pebbles of conglomerate consist mainly of quartz (75-85%), besides, feldspar and rock fragments; pebbles and boulders are well rounded, that is different in comparison with pebbles of the Van Yen Fm. Sandstone sometimes contains nests and small lenses of lignite 40-50 cm in length. The lower boundary of this member has not been observed. (2). Light-grey, medium- to thick-bedded, fine- to coarse-grained sandstone interbedded with brown-grey, commonly thin-bedded siltstonc, sometimes interbedded with silty claystone and carboniferous shale, 120-150 m thick. These sediments are arranged in rhythms, beginning with sandstone, then siltstone and silty claystone with the thickness of each rhythm being 8-12 m. (3). Grey, thick-bedded sandstone and siltstone, brown-grey to black-grey, thin-bedded siltstone and claystone commonly interbedded with carboniferous shale and brown-coal seams; 60- 120 m thick. This member has a good prospect in brown coal in the Tertiaiy section of the Red River Trough. Sandstone is commonly composed of quartz and quartz-chert; siltstone and claystone contain abundant floral remains, siderite concretions or pyrite crystals. The total thickness of the formation in this section is 310-620 m.
Relationships and Distribution
Lower contact
Unconformably upon the Co Phuc Fm
Upper contact
The lower boundary of the Au Lau Fm has not been observed, but based on the presence of boulders in conglomerate of member 1, one can suppose that this is the basal member of the formation.
Regional extent
widespread in the Au Lau Bridge, Bat Xat, Duyen Hai, Pho Moi Bridge, Trai Hut, Yen Bai Town, Co Tiet, Thanh Ba and Truftg Ha areas.
GeoJSON
Fossils
Age
Depositional setting
It is interpreted that the Au Lau Fm consists of fluvio-proluvial sediments in its lower part and fluvio-lacustrine oneS in its upper part,
Additional Information