Ta Pa Fm

Holostratotype - the Ta Pa Range situated in the Bay Nui area, Tri Ton District, An Giang Province (10°21', 105°00). The formation bears the name of the above range. The Ta Pa Formation was established recently for describing the continental, subcontinental sediments occurring in the Ta Pa and Nam Quy areas of An Giang Province and on Minh Hoa Island, Kien Giang Province, that were formerly attributed to different formations.

The Ta Pa Formation is best exposed in its stratotype area, and its stratotype begins from Choeng Phnoum Village following the trail towards the mount summit, supplemented by artificial exposures made by the quarry in the southeastern slope of the range. The beds of the formation are arranged as an isocline plunging northwestwards with the attitude changing from 330-335° <10-15°.

Synonym: - Hệ tầng Tà Pa: Vũ Khúc, Đặng Trần Huyên, Trương Công Đượng et aỉ. 1997); Vũ Khúc, Đặng Trần Huyên (in Trịnh Dánh et al. 1997); Vũ Khúc et aỉ. 2000. - Hệ tầng Đơn Dương: Vũ Khúc, Bùi Phú Mỹ eí aỉ. 1990 - Hệ tầng Dầu Tiếng'. Phan Doân Thích {in Nguyền Ngọc Hoa et al. 1995). - Hệ tầng Phú Quốc: Nguyễn Ngọc Hoa el al. 1995.

In the holostratotype, the base of the section is not exposed, the upper boundary of the formation

was not observed, and the formation succession is as follows in ascending order. (1). Grey to brown-violet, massive (usually 220-250 cm) siltstone with some interbeds of claystone, sandstone and gritstone, 30-50 m thick. Under the microscope die grains of siltstone consist mainly of quartz and some sericitizcd feldspar; the cement of sericitized feldspar occupying 97-98% together with some ore grains. The grains of sandstone and gritstone consist of well-rounded quartz. (2). Intercalated greyish to pinkish-grey, massive (150-200 cm) sandstone, grey to chocolate-colorcd, massive siltstone and some interbeds of greenish-grey claystone, 40-45 m thick. Under the microscope sandstone and siltstone have a granular composition mainly of quartz. This member is penetrated by a granite vein of the Late Jurassic – Early Cretaceous Dinh Quan Complex Fm, that has been observed in the quarry situated on the southern slope of the range. (3). Light-grey, brownish-grey, fine- to coarse-graincd polymictic sandstone containing, locally, grit and gravel with some interbeds of quartz gritstone and siltstone; 40-45 m thick. (4). Light-grey or yellowish-brownish, thick-bedded, locally cross-bedded, medium- to coarse-grained sandstone grading upwards into fine-grained sandstone with interbeds of crossbedded quartz gritstone and some siltstone, 50-70 m thick. Under the microscope the grain composition of the sandstone and gritstone consists mainly of quartz. The sandstone yields the spore Selaginella aff. media. The total thickness of the formation in this section is 160-220 m.

On Minh Hoa Island (Gulf of Thailand), the beds attributed to the Ta Pa Formation occurring are exposed on its eastern side. There, the beds have an isoclinal structure, but are broken by some small faults, at the beginning they plunge southeastwards, then mainly southwestwards. From the north southwards the bed strike changes from 140, 210 to 245° and the dip - from 50 to 20°. The section includes 2 members. (1). At the beginning, there is a 9 m thick bed of greenish-grey, thick-bedded (200-250 cm) conglomerate with the granules composed of quartz, quartãte, argillaceous shale and limestone, well-rounded pebbles, and cement of tuffaceous sandstone and siltstone. It grades upwards into greenish-grey, medium- to thick-bedded and cross-bedded in the upper part, arkosic sandstone, some of its beds contain quartz gravel and grit parallel to the bedding surface. The sandstone contains some interbeds of chocolate-coloured silty sandstone; 145 m. Under the microscope quartz constitutes only 50-57% of the clastic components of the sandstone and siltstone, besides which there are plagioclase (albite), potash feldspar (orthoclase) and accessory minerals; some samples still contain carbonate (3-10%, sometimes up to 25%). (2). Yellowish-grey to greenish-grey, rarely violet-grey, fine- to medium-grained, thick-bedded sandstone interbedded with mainly violet-grey, massive siltstone, 250 m thick. Sandstone contains remains similar to crino ids, but indeterminate due to their poor preservation.

The Ta Pa Formation is very poor in fossils. At the present time only Mesozoic spores (Selaginella) occur in the Ta Pa Section and possible remains of crinoids - on Minh Hoa Island.