Lithostratigraphy, sedimentology and diagenesis of the Ordovician carbonates, southern Thailand
Creators: name: Wongwanich, T
Publisher: name: University Of Tasmania
Year: 2023

description: The Ordovician Thung Song Group, in Satun Province, southern Thailand, is a 1,400 m thick sequence of tropical limestones, dolomites and calcareous shale ranging from the Upper Tremadoc to Upper Ashgill. The sequence conformably overlies the siliciclastic, Tarutao Group and is conformably overlain by black graptolitic shales and cherts with radiolarians of the Wang Tong Formation. The Ordovician-Silurian boundary is placed in the lower part of the Wang Tong Formation ~25 m above the Pa Kae Formation. At least seven distinctive lithostratigraphic units have been recognized and are formalised here as the Malaka Formation, Talo Dang Formation, La Nga Formation, Pa Nan Formation, Lae Tong Formation, Rung Nok Formation, and the Pa Kae Formation. The Pa Kae Formation is proposed for the uppermost formation of the Thung Song Group. The Thung Song carbonates were deposited on a homoclinar ramp. Environments during a major marine transgression with minor eustatic fluctuation included large tidal flats, stromatolite reefs, lagoons, local buildup-barrier reefs, and deeper water carbonates. The rocks are composed of twelve microfacies including stromatolites, micrite to fossiliferous micrite, biomicrite, intramicrite, poorly washed to unsorted biosparite, biopelsparite and pelbiosparite, pelsparite, intrasparite, flat pebble conglomerate, oosparite, biolithite and limestone breccia, micro- to megadolomite and dolomite breccia. These indicate sedimentation in a wide range of environments from supratidal, intertidal, subtidal to deeper-water zones. The lower portion of the sequence was deposited in peritidal, lagoonal and carbonate buildup environments and stromatolites played a major role in the sedimentation. Flat pebble conglomerates with algal laminite clasts, intrasparite and small-scale hummocky cross-stratification indicate high-energy storm events. The Pa Kae Formation is different from the underlying shallow water carbonates. It was deposited in a tropical setting, during a period of intense glaciation at high latitudes on Gondwanaland, as deeper water stromatolitic mounds. The rocks display rhythmic alternation of red stromatolitic limestones and dark red mudstones. The stromatolitic limestones are biomicrites with dense stromatolitic structures developed from flat laminites in the bottom to common columnar stromatolites, and some thrombolitic and oncolitic stromatolites on the top of each bed. They share several common features with the deep water stromatolites from the Canning Basin, Western Australia, such as fine lamination, rare or no fenestrae, and abundant pelagic faunas. They are different in the abundant ferromanganese micronodules, the occurrence of glauconitic pellets, bored calcite cements and very abundant rhombic spars. Based on a new proposed model developed to predict the possible palaeobathymetry of the carbonate rocks in tropical to temperate regions, the Pa Kae stromatolites accreted on the very distal part of the Thai Ordovician carbonate ramp at about 175-300 m depths and at about 15¬¨‚àûC. Their heavy isotopic signature (`- 3.1‚ÄövÑ‚àûPDB` `of ‚Äöv†vá^(18)˜ívº and +3.4‚ÄövÑ‚àûPDB` `of ‚Äöv†vá^(13)C`) and their cyclic nature suggest that the warm and cold climates repeated themselves many times throughout the Late Ordovician. The Thung Song carbonates have undergone a long and complex diagenetic history incJuding early marine, early to late meteoric, and burial cementation; dolomitization; physical and chemical compaction; and dedolomitization. Early cementation within the carbonate buildups of the Rung Nok Formation and the Pa Kae stromatolitic mounds are much more extensive than in the peritidal carbonates of the Malaka Formation and La Nga Formation but it is rare in the lagoonal facies of the Talo Dang Formation and Lae Tong Formation. Therefore, the degree of compaction of the lagoonal facies is extremely high in comparison with those of the peritidal buildups and deeper water stromatolitic mounds. Stylolites are very abundant in the Thung Song Group, especially in the peritidal carbonates and play a major role in the development of nodular structures in the limestone and shale of the lagoonal facies and of the deeper water stromatolitic facies. Most of the dolomites in the lower formation including the Malaka Formation, and La Nga Formation are early diagenetic microdolomites with minor dolomite cement. Later diagenetic meso- to megadolomites are common in the Rung Nok Formation. The dolomitization of the Pa Kae Formation occurred locally throughout its diagenetic history but normal sea water dolomite is dominant. The isotopic field of the Thung Song dolomite overlaps the marine calcite diagenetic field and extends up to the eteoric diagenetic field, possibly suggesting that these dolomites formed from marine to mixed-water dolomitizing fluids.