Tho Lam Fm

Western Quang Nam Province. Initially, the Tho Lam Group was established (Bourret, 1923) for describing fine-grained marine sediments containing Early Jurassic ammonites, grading upwards into continental sediments containing Middle Jurassic silicified wood formed in the Nong Son Depression of Central Trung Bo. Named after Tho Lam Reservoir. During the process of geologic mapping 1:200,000 scale, the unit was subdivided into 3 formations: Ban Co Fm of Hettangian, Khe Ren Fm of Sinemurian-Toarcian, and Hou Chanh Fm of Middle Jurassic (Vu Khuc, 1983). Considered as the Tho Lam Group with these 3 separate formations by Tong-Dzuy Thanh and Vu Khuc (2011), but retained as Tho Lam Fm by Tran Van Tri and Vu Khuc (2011; used here).

Synonym: Tho Lam Gr; Série de Tho-!am: Bourret R. 1925. - Loạt Thọ Lâm-. Vũ Khúc 1983; Vũ Khúc et a i 1984; Nguyễn Văn Trang et ai. 1995. - Diệp Thọ Lăm: Vũ Khúc, Bùi Phú Mỹ et al. 1990; - Terrains liasiques: Counillon H. 1908. - Poudingue siliceux à galets de quartz: Bourret R; 1925.

Characteristic section exposed in the foot of the An Bang mountain range has 1400 m. Lower unit ("Ban Co Fm"; 740 m thick) is thick-bedded massive quartz-chert gritstone and puddingstone, sometimes with interbeds of coarse quartz sandstone, and in the upper part are some interbeds of black-grey siltstone. Middle unit ("Khe Ren Fm"; 150 m thick) is dark grey to black-grey, thin-bedded siltstonc, grading upwards to grey thick-bedded marl and clayey limestone; and in the upper part gray medium-graied sandstone. Upper unit ("Huu Chanh Fm"; 500 m thick) is continental red beds of reddish-brown siltstone, sandstone and claystone.

Lower part of Middle unit ( Khe Ren Fm ) contains the Ammonite Tongdzuyites nongsonensis (= Psiloceras longipontinum, sensu Counillon H. 1908; Laqueoceras sp., sensu Repin in Vu Khuc et al. 1991; Jamesonites sp., sensu Vu Khuc et al. 1998), the Bclemnite Atractites sp., and the Bivalves Cardinia indosinensis, Cultriopsis counilloni, Goniomya fontainei, Protocardium philippianum, Modiolus saurini, Astarte (Nicaniella) sp., and Thracia sp. Based on ammonites, this fauna dates the host sediments as Pliensbachian. Upper part of Upper unit (Huu Chanh Fm ) at the Huu Chanh village area contains silicified wood trunks, that were described by Vozenin-Serra C. (1967, 1968) as Phyỉlocỉadoxylon vieínamense, Protophyllocỉađoxyỉon thylloides, Brachìoxylon sp., and Araucarioxyỉon sp., dated as Middle Jurassic.

[Figure: Thọ Lâm Fm: Middle Jurassic silicified wood (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Marine in lower and middle unit; Continental reds beds in upper unit.