Con Voi Fm

Upper formation of the Thuy Nguyen Gr (inland sections). The section is situated on the southern slope of the Nui Con Voi limestone Mount, extending to the small limestone block at the Le Chan Temple (N = 20°50', E = 106°33’).

Synonym: Điệp Con Vot Nguyễn Quang Hạp 1967(C₁). Calcaires noirs de ỉa Montagne de I'Elephant: Dcprat J. 1913, 1914 (Dinantien). Thong Carbon hạ: Dovjikov A.E. et al. 1965; Lê Hùng (ỉ« Trần Văn Trị et al. 1977). Diệp Sơn Liêu: Phạm Văn Quang et al. 1969 (C)). Hệ tầng ìlh Long: Nguyễn Văn Liêm 1974, 1978 (D₃fm-C₁t); Nguyễn Văn Licm, Lê Hùng (in Dương Xuân Hào et al. 1980 Đj?‘ C₁t); Đoàn Nhật Trường 1988(part.) (Djfr-C₁t); Lê Hùng, Trần Thị Ninh 1990 (D₃-C₁t); Đoàn Nhật Trường, Nguyễn Đức Khoa 1994 (D?fm-C₁t); Lê Hùng (part.) 1995. Hệ tầng Cát Bà: Nguyễn Công Lượng et al. 1980 (C₁); Vũ Khúc, Bùi Phú Mỳ et al 1990 (C₁). Trầm tích Devon thượng (part.): Tống Duy Thanh et al. 1986. Hệ tầng Phố Hàn (part.): Ngô Quang Toàn el al. 1994 (D₁-C₁). Hệ (ầng Núi Voi: Đoàn Nhật Trường (in Tống Duy Thanh et al. 1995; Đoàn Nhật Trường, Tạ Hòa Phương 1998 (D₃-C)

Limestone: (1). Grey to dark grey, medium- to thick-bedded, fine- to medium-grained dolomitic limestone, 10 m thick. (2). Grey to dark grey, thick-bedded to massive fine-grained limestone, 30 m thick; in the lower part containing the foraminifer Septabrunsiina grozdilovae, in the middle part - Septatournayella rauserae, S. potensa and the corals Cystophrentỉs koỉaohoensis, C. grandis, C. cf. roniewiczae, Syringopora reticulata; the above fossils date the host beds as Latest Famennian. The upper part contains the foraminifera Bisphaera malevkensis, Tournayellina beata, and T. septata, of Early Toumaisian age. (3). Grey, thick-bedded to massive limestone, locally with interbeds of thin-bedded limestone, 120 m thick; this member is exposed on the upper part of the massif bearing the temple of Le Chan, and in the foot of the southern slope of Nui Con Voi Mt. On the abraded surface of the limestone there are intercalated bands between light grey and dark grey limestones. On the bedding surface of the limestone there is a very thin bed of red-brown clay or marl. This member yields the foraminifera Bisphaera malevkensis, Septabrunsiina karakubensis, Septatournayella cf. segmentata and the corals Pseudouralinia cf. tangpakouensis, Syringopora reticulata, S. haiphongensis, S. distans, and S. geniculata haiphongensis; which date the host rocks as Early-Middle Toumaisian. (4). Dark grey, medium-bedded fine-grained limestone, organic limestone containing crinoid stems interbedded with cherty shale, and with chocolate colored marl in the upper part, 15 m thick; containing the coral Thysanophylluml sp. and the brachiopods Camarotoechia cf. baitalensis, and Cyrtospirifer sp. (Ngo Quang Toan el al. 1993). (5). Grey to brown-grey, thin-bedded chert and cherty shale with some interbeds of silty claystone and siltstone, 15 m thick. (6). Dark grey to black-grey, thin-bedded (10-20 cm) organic limestone with thin interbeds or nests of chert grading upwards into thick-bedded limestone, 20 m thick The total thickness of the formation in this section readies about 200 m.

[Figure: Con Voi Fm: Lithology and Tabulate fossils (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Containing Toumaisian faunas, among which corals are very abundant with: Rotiphyllum omalius, Septabrunsiina sp., Lophophyllum cf. koninckii, Keyserlingophyllum sp., Syringopora geniculata, and S. geniculata haiphongensis.

The Con Voi Fm characterizes the carbonate shelf facies. The presence of the coral Cystophrentis proves that this shelf surrounds an uplift. It is distinguished from other carbonate shelves which are limited by internal deep troughs on the shelf.