Moonpa K and Motanated K (2021), Carbonate microfacies and depositional model of...

ECB-ART-48896

Heliyon 2021 Oct 05;710:e08130. doi: 10.1016/j.heliyon.2021.e08130.

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Carbonate microfacies and depositional model of Triassic Pha Kan and Doi Long Formations, Lampang Group, Sukhothai Zone, Northern Thailand.

Moonpa K , Motanated K .

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Triassic carbonate rocks of Pha Kan and Doi Long Formations of Lampang Group consist mostly of carbonate successions deposited in the Sukhothai Zone, northern Thailand. These formations are mixed carbonate and siliciclastic sediments and widely exposed in the Lampang area. These deposits are important for the reconstruction of depositional environments and regional correlations of the Triassic carbonate rocks with respect to the volcanic arc of the Sukhothai Zone during the Triassic. The goal of this study is to analyze the lithology and microfacies of the carbonate rocks of the Pha Kan and Doi Long Formations and reevaluate the existing depositional models and carbonate settings. Both formations have diverse skeletal fossils, including foraminifers, sponges, calcimicrobes, calcareous algae, bivalves, echinoderm spines and fragments, ostracods, gastropods, and other fossils. Based on field observations and microfacies analysis, twelve major facies, corresponding to specific depositional environments within a carbonate setting, have been established. The inferred depositional environment is a carbonate platform with a well-developed lagoon, small-scale reefs, and bioclastic and oolitic shoals, as parts of a carbonate ramp. The microfacies and sedimentological study show that this carbonate system was influenced by the nearby volcanic arc. Consequently, our study shows that the Lampang Group in northern Thailand can be correlated with the Lincang Massif of the western Yunnan Province, PRC and the northwestern Lao PDR.

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	Figure 1. Stratigraphic chart of the Lampang and Song Groups. Ages and lithostratigraphic division are modified from Feng et al. (2005).
	Figure 2. Geological map of the Lampang and Phrae areas, northern Thailand and locations of the studied localities (black points) (modified from Hara et al., 2017). Dashed line is the boundary between the sub-basin modified from Feng et al. (2005).
	Figure 3. Geological setting and location of the studied area. a) Geotectonic subdivisions of Thailand that represent the orientation of the Sukhothai Arc Terrane in the northern Thailand. b) Locations of the studied area modified after Chonglakmani (2011) and Ueno and Charoentitirat (2011).
	Figure 4. Outcrop photographs of the Pha Kan Formation. a) Doi Pha Kan in Ban Tha Si (TS) area. b) Oncoidal limestone at the TS locality. c) The Pha Kan limestone at the Phratu Pha Shrine (PP) locality. d) The contact of limestone with the siliciclastic rocks of the Hong Hoi Formation at the PP locality. e) Volcanic grains in limestone samples are either scattered in limestone bed of the Pha Kan Formation at the TS locality. f) Limestone outcrop at the Ban Than (BT) locality. g) Oncoidal limestone at the Ban Mae Ang (MA) locality. h) and i) Limestone outcrops at the Phra That Muang Kham (MK) locality.
	Figure 5. Outcrop photographs of the Doi Long Formation. a) and b) Doi Wiang Ho (DWH) locality. c) The interbedding of limestone, sandy limestone, shale, and sandstone at the 434th km of the National Highway 11. d) Limestone outcrop of the Doi Long Formation at the Ban Sop Chang (SC) locality and a sponge (d1) from the middle part of the Doi Long Formation. e) Limestone outcrop at the Doi Huai Long (DHL) locality. f) Interbedding of limestone and shale in the upper part of the Doi Long Formation from the DHL locality.
	Figure 6. Biostratigraphic summary of the studied sections in the Pha Kan and Doi Long Formations, defined on the basis of foraminiferal associations compared with the previous studies of Kobayashi et al. (2006) and Miyahigashi et al. (2012).
	Figure 7. Lithostratigraphic columns and correlation of the studied sections of the Pha Kan Formation.
	Figure 8. Lithostratigraphic columns and correlation of the studied sections of the Doi Long Formation.
	Figure 9. Photomicrographs of fossils of the Pha Kan Formation, Lampang area. a) Pilammina densa, Phra That Muang Kham (MK) locality, sample MK-6-1, scale bar = 200 μm. b) Glomospirella lampangensis and Diplotremina sp., Ban Pha Lad (PL) locality, sample PL-2-1, scale bar = 200 μm. c) Bivalve fragment, Ban Than (BT) locality, sample BT-7-1, scale bar = 250 μm. d) Echinoderm spine, Phratu Pha Shrine (PP) locality, sample LP-NG-7-3, scale bar = 250 μm. e) Coral fragment, Ban Mae Ang (MA) locality, sample MA-2-1, scale bar = 300 μm. f) Dasycladacean algae, Doi Ton (DT) locality, sample DT-6-3, scale bar = 300 μm.
	Figure 10. Triassic foraminifers of the Lampang Group, a)-m) from the Pha Kan Formation and n)-v) from the Doi Long Formation. a)-f) Pilammina densa, a)-b) sample MK-5-3, c) sample MK-5-2, d) sample MK-5-1, e)-f) sample PL-3. g)-j) Meandrospira dinarica, g) sample MA-2-2, h)-i) sample LP-NG-3-3, j) sample LP-NG-6. k)-m) Glomospirella sigmoidalis from sample MK-6-1. n)-o) Aulotortus tumidus, n) sample DWH-2-1-2, o) sample DWH-2-2-1. p) Trochammina sp. from sample DWH-1. q) Endotriadella wirzi from sample DHL-5-1. r) Aulotortus sinuosus from sample DHL-4-1. s) Lamelliconus multispirus from sample DHW-2-1-1. t)-u) Agathammina austroalpina, t) sample DWH-2-2-2, u) sample DHL-4-2-1. v) Endothyroid foraminifers from sample DWH-2-1-2. Scale bars in a)-j) and n)-v) are 100 μm; scale bars in k)-m) are 50 μm.
	Figure 11. Photomicrographs of fossils of the Doi Long Formation, Lampang area. a) Aulotortus sinuosus, Ban Khun Mae Huat (KMH) locality, sample KMH-5-2, scale bar = 250 μm. b) Nodosaria sp., sample DHL-3, scale bar = 100 μm. c) Trochammina sp., sample DWH-2-1-1, scale bar = 200 μm. d) Photograph of longitudinal section of a gastropod (yellow arrow), Doi Wiang Ho (DWH) locality, sample DWH-3-1-1. e) Microbialite encrustations and the right points at encrusting foraminifera associated with microbialite crust at the Ban Sop Chang (SC) locality, sample SC-4-1. f) Tubiphytes (yellow arrows), Ban Sop Chang (SC) locality, sample SC-4-2-1. Scale bars in d)-f) are 200 μm.
	Figure 12. Photomicrographs of microfacies of the Pha Kan Formation in the central studied area. a) Bioclastic grainstone (Bg). Bioclasts are mostly fragments of Pilammina densa and Meandrospira dinarica, with minor skeletal bioclasts such as echinoderms, Glomospirella sigmoidalis, brachiopods, and bivalves, sample MK-6-1. b) Foraminifer and ostracods in bioclastic grainstone (Bg). Bioclasts are mainly of round-shaped and well-sorted, sample MK-7. c) Ooids in oolitic grainstone (Og). Note that the concentric structure is preserved, sample MK-6-2. d) Oncoid (yellow arrow) in oncoidal peloidal packstone (Opp), sample MK-5-1-2. e) Oncoidal wackestone (Ow) (yellow arrow points at an oncoid), sample MK-5-1-1. f) Peloidal grainstone (Pg), sample DT-10-1. Peloids are supported by sparry cement. g) Dasycladacean algae grainstone (Dag), sample DT-6-2. Dasycladacean algae and aggregate grain are indicated by the yellow and white arrows, respectively. h) Bioclastic wackestone (Bw) with fragment of a gastropod, sample NE-MK-1. i) Bioclastic wackestone (Bw), sample PL-2-1. Bioclasts are foraminifers (yellow arrows). j) Siliciclastic wackestone (Sw) with the fragments of foraminiferal Trochammina sp. (yellow arrow), sample PL-2-1. k) Photograph of the basal conglomerate of the Hong Hoi Formation (left), Clast of limestone containing a foraminifer (right), sample PL-4. Scale bars: a)-b) 200 μm, c)-e) 100 μm, f)-i) 250 μm, j) 150 μm, k) 3.0 cm (left) and 250 μm.
	Figure 13. Photomicrographs of microfacies of the Pha Kan Formation in the northeastern studied area. a) and b) Oolitic grainstone (Og), sample BT-5-2, characterized by concentric ooids with isopachous and blocky cements. The nucleus of ooids is gastropod (yellow arrows), sample BT-5-1. c) Oncoidal peloidal packstone (Opp) characterized by fine-grained peloidal packstone, largely composed of sorted peloids together with coated grains, and calcimicrobe oncoids, sample BT-4-1. d) The oncoids (yellow arrows) in oncoidal peloidal packstone (Opp), sample BT-6. The coated grains with the gastropod nucleus. e) Oncoid (yellow arrow) in oncoidal wackestone (Ow), sample BT-6. f) Peloidal grainstone (Pg), sample BT-4-2. g) Peloidal grainstone (Pg) composed of peloids and microbiallite (arrow), sample BT-4-1. The peloids are cemented by calcite and micritic matrix. Scale bars in a)-d) are 100 μm; scale bars in e)-g) are 250 μm.
	Figure 14. Photomicrographs of microfacies of the Pha Kan Formation in the northern studied area. a) Oncoidal wackestone (Ow) contains oncoids (yellow arrow) with bioclasts including fragments of crinoids and echinoids, brachiopod shells (white arrow), sample LP-NG-3-1. b) Oncoidal peloidal packstone (Opp) is dominated by large oncoid (arrow), sample MA-2-1-2. c) Oncoidal peloidal packstone (Opp), sample LP-NG-6-2. Yellow arrow indicates large oncoids. d) Fragment of echinoid spine in bioclastic wackestone (Bw), sample LP-NG-6-1. e) Bioclastic wackestone (Bw) composed of dark micrite and bivalve shells, sample MA-2-1-2. f) Peloidal grainstone (Pg) with micritized grains (yellow arrow), sample LP-NG-7-3. The peloids are largely micritized and the internal structure is not preserved. g) Dolomitized mudstone (Lm), sample MA-2-2. h) Siliciclastic wackestone (Sw) composed of quartz, lithic fragment, and carbonate grains (yellows arrow), sample TS-5-3. Scale bars: a)-b) 100 μm; c) and f) 250 μm; d), e), g), and h) 200 μm.
	Figure 15. Photomicrographs of microfacies of the Doi Long Formation in the central studied area. a) and b) Well-sorted oolitic grainstone (Og) composed of concentric ooids with radial structures in place, sample DWH-4-1. Subordinates are fragments of algae and intraclasts supported by coarse sparry calcite. Note bioclasts in the nuclei. c1) and c2) Cross-lamination in oolitic grainstone (Og) (yellow arrows) at the 434th km of the National Highway 11 (southeast of DWH), sample DWH-4-2. d) and e) Bioclastic grainstone (Bg) composed of micritized clasts, gastropods, corals, echinoderms, and shell fragments (yellow arrows), all cemented by microsparite, sample DWH-2-2-(1–2). f) Oncoidal packstone (Op) largely composed of sorted oncoids together with ooids, coarser bioclasts, aggregate grains, coated grains, and calcimicrobes, sample DWH-3-1. Yellow and red arrows indicate large oncoids and foraminifer, respectively. g) Bioclastic wackestone (Bw) composed of gastropods and foraminifers in a dark micritic matrix, sample DWH-1. h) Bioclastic wackestone (Bw) with a large mollusk fragment, sample DWH-5-1. i) Siliciclastic wackestone (Sw) with thin bivalve shell fragments (yellow arrow), sample DWH-4-2. j) Sandy limestone with abundant quartz grains and some distorted ooids, sample DWH-5-2. Scale bars: a)-b) 200 μm; c) 100 μm; d), e), i), and j) 250 μm; f) 50 μm; g)-h) 300 μm.
	Figure 16. Photomicrographs of microfacies of the Doi Long Formation in the northern studied area. a) Oncoids (yellow arrow) and microproblematica (white arrow) in oncoidal wackestone (Ow), sample SC-3. The interstitial sediment is filled with cement. b) Oncoidal peloidal packstone (Opp), sample SC-4-1, characterized by large oncoid (yellow arrow) associated with small peloids. c) Calcimicrobes (yellow arrow) in sponge-microbes boundstone (Smb) composed of fragments of algae and calcimicrobe, sample SC-4-2-2. d) Peloidal grainstone (Pg) composed of sorted peloids, sample KMH-3. e) Sponge-microbes boundstone (Smb), sample DHL-4-1. Partially encrusted foraminifera (yellow arrow). f) Siliciclastic wackestone (Sw) composed of quartz (yellow arrow), foraminifer (white arrow) and micritized grains, sample DHL-5-2. g) Calcareous siltstone (Cs), sample DHL-3. Clastic minerals (quartz) or lithics have been observed. Scale bars: a)-b) 100 μm; c) 200 μm; d)-g) 250 μm.
	Figure 17. Schematic depositional model of the Pha Kan Formation (see text for description). The abbreviations of each locality are MK for Phra That Muang Kham, NE of MK for Mae Tha Railway Station, DT for Doi Ton, PL for Ban Pha Lad, BT for Ban Than, TS for Ban Tha Si, MA for Ban Mae Ang, and PP for Phratu Pha Shrine.
	Figure 18. Schematic depositional model of the Doi Long Formation (see text for description). The abbreviations of each locality are DWH for Doi Wiang Ho, SC for Ban Sop Chang, DHL for Doi Huai Long, and KMH for Ban Khun Mae Huat.