Nam Mahoran Fm
Type Locality and Naming
Pha Nok Khao Platform (Loei region): Its type section is designated at Phu Tham Nam Mahoran about 25 km SE of Wang Saphung (DMR 1992), but has not been measured properly. Proposed by Charoenpravat et al. (1984) for Middle Permian limestone in the Loei area. A middle formation in the Loei Gr.
Synonym: Nam Maholan Fm
[Figure: Late Paleozoic major facies subdivisions on the western margin of the Indochina Block by Ueno and Charoentitirat (2011). (Slightly modified from Wielchowsky & Young 1985).]
Lithology and Thickness
"The formation is essentially massive to thick-bedded pure grey limestone with locally dolomitized levels, and gives rise to cliff-forming karst hills and mountains in the Loei area which are easily distinguished on topographic maps. … In the carbonates of the Nam Mahoran Formation, bioclastic packstone and grainstone are the major microfacies. Chert nodules and bands are common, and conglomeratic limestone and shale are sometimes observed. The thickness of the formation has not been measured and attempts have been hampered by the cliff-forming topography and the often obscure bedding. However, it is probably at least several hundred meters thick, judging from the size of the karst mountains." [Ueno and Charoentitirat (2011)]
Relationships and Distribution
Lower contact
It conformably overlies the lower part of the Wang Saphung Fm, but exhibits heteropic facies with the upper part.
Upper contact
"There are no strata directly overlying the Nam Mahoran Formation but it may have been over- lain by the Pha Dua Fm, although these two formations now have slightly different distributions. Moreover, in the west of Wang Saphung the Nam Mahoran Fm and E-Lert Fm probably show heteropic facies (see Figure)." [Ueno and Charoentitirat (2011)]
Regional extent
The outcrops are mainly distributed along the synclinal axis described earlier, east and SE of Loei. Other small and isolated outcrops are also known west and south of Wang Saphung. … Karst mountain-forming Early Permian carbonates are scattered in the Nam Som area of northwestern Udon Thani Province which Chonglakmani et al. (1985a) attributed to the Pha Nok Khao Formation, but Ueno and Charoentitirat (2011) include them in the Nam Mahoran Formation based mainly on their areal distribution.
[Figure: Distribution of Carboniferous and Permian in NE Thailand. Data mainly based on 1:1 million scale geological map of Thailand (DMR 1999) with minor modifications (Ueno and Charoentitirat 2011).]
GeoJSON
Fossils
"Brachiopods, corals, trilobites, calcareous algae, ammonoids and conodonts have been reported from the formation, suggesting an Early–Middle Permian age. Fusulines are also known from many localities. … Originally, the Nam Mahoran Formation was referred to the Early – Middle Permian (Bunopas 1988) but there is increasing evidence of Late Pennsylvanian ages also." [Ueno and Charoentitirat (2011)].
Age
Depositional setting
"The Nam Mahoran Formation was deposited on a wide carbonate platform. Essentially, shallow-marine conditions prevailed on this platform judging from the predominant microfacies of the formation."
Additional Information