Upper Nam Phong Fm

Type area: Nam Phong, Phu Kradung district, Loei province. Parent unit: Khorat Gr.

[Figure: The three sedimentary basins in which the marine Jurassic occurs, and their relationships to the main faults which might subsequently have displaced them (MYF, Mae Yuan Fault; MPF, Mae Ping Fault; TPF, Three Pagodas Fault; KMF, Khlong Marui Fault). The main outcrop areas of non-marine Jurassic rocks are also shown. In some areas, particularly on Peninsular Thailand, the Jurassic succession contains both marine and non-marine facies (From Meesook A. & Saengsrichan W., 2011. Redrawn from Meesook 1994; Charusiri et al. 2002).]

Synonym: หมวดหินน้ำพอง

Fining-upwards sequence. "In the central and western parts of the basin, the lower part of the formation is characterized by thick-bedded to massive reddish-brown fine- to medium-grained sandstones, with conglomerates common towards its base. … thus forming prominent ridges at outcrop along the Khorat Plateau Scarp. … In contrast, over the eastern and north-eastern parts of the basin, the lower part of the formation consists of grey to reddish-brown claystones and siltstones with minor thin-bedded fine-grained sandstones. Across the entire basin, the upper part of this formation consists mainly of reddish brown claystones and siltstones, with subordinate relatively thin (generally 1 – 2 m thick) light-grey to greyish-red fine- to very fine-grained sandstones." (Booth, J. & Sattayarak N., 2011, Chapter 9 Subsurface Carboniferous-Cretaceous geology of NE Thailand). Maximum of ca. 1000 m in the center of the basin..

Previously, the combined Nam Phong Fm was considered to span the Rhaetian- Hettangian (Geology of Thailand, 2011, page 139, Fig 6.2 Correlation chart of the principal Triassic sedimentary lithostratigraphic units in Thailand.). However, recognition of a major Unconformity III Event in middle, implies the Upper Nam Phong Fm spans the Late Jurassic (Oxf-Kimm here), and only the Lower Nam Phong Fm spans the Rhaetian-Hettangian following Booth, J. & Sattayarak N., 2011, Chapter 9 Subsurface Carboniferous-Cretaceous geology of NE Thailand.

Fluvial. "In the western and southern portions of the basin the lower part of the formation was deposited in predominantly fluvial environments, although it is unclear if these were braided or meandering-river systems. The lower part of the formation elsewhere and the upper part everywhere were deposited in mixed alluvial-fluvial floodplain environments, with meandering-river systems." [Booth, J. & Sattayarak N., 2011, Chapter 9 Subsurface Carboniferous-Cretaceous geology of NE Thailand]