Sulfur Isotopic Implication of Middle Miocene Marine Incursion in Northern Thailand

ScienceAsia 30 (2004): 43-58 Sulfur Isotopic Implication of Middle Miocene Marine Incursion in Northern Thailand Thanuchai Silaratana,a,* Benjavun Ratanasthien,a Katsumi Takayasu,b William S. Fyfe,c Pongpor Asnachinda,a Wittaya Kandharosaa and Minoru Kusakabed a Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. b Research Center for Coastal Lagoon Environments, Shimane University, Matsue, Shimane 690-8504, Japan. c Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada, N6A 5B8. d Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori-ken 682-0193, Japan. * Corresponding author, E-mail: [email protected], [email protected] Received 17 Jan 2003 Accepted 12 Nov 2003 ABSTRACT: A sulfur isotopic study was undertaken to identify environmental changes of the middle Miocene Mae Moh and Chiang Muan coal fields in northern Thailand. Samples of coal, gypsum, and pyrite were collected from the mine fronts and processed for sulfur isotopic analysis. In the Mae Moh coal field, the δ 34S values of pyrite from the Q and K coal zones are between +4.2 and +10.9 per mil and indicate that the source of sulfur came from organic sulfur. The δ 34S values of +11.2 to +12.1 per mil of gypsum from the K zone coal indicate non-marine sulfate. The δ 34S values of J zone coal are highly variable from -18.8 to +17.2 per mil. The δ 34S value of +17.2 per mil in the lower part of the J zone indicates a freshwater source. In the middle part of the J zone, the δ 34S values of -5.8 to +8.8 per mil may indicate a volcanic source, and are confirmed by volcanic debris. In the upper part of the J zone, the very low δ 34S values of –18.8 to –15.5 per mil may indicate a marine incursion during coal deposition in middle Miocene. This is confirmed by the δ 34S values of gypsum, which are +16.1 to +18.6 per mil, from red beds lying above the J zone. In a minor I coal zone, where the pyrite is replaced in gastropods, the δ 34S values of pyrite are between +30.6 to +34.2 per mil, suggesting that the sulfur was reworked from red beds by bacterial reduction. In the Chiang Muan coal field, the high positive δ 34sulfur values of total sulfur from LS and LM zones (+12.1 and +10.3 per mil, respectively) indicate that the LS and LM were deposited in fresh water environment. The δ 34sulfur values of pyrite from U2 coal zone are between –15.6 and –11.2 per mil, which show the associated with marine sulfate. The similar δ 34S values of black gypsum from the U1 coal zone show that the source of sulfur could have come from the oxidation of sulfide in coal. The different δ 34S values between the organic sulfur, total sulfur, and pyritic sulfur in the U2 zone suggest that marine incursion occurred during the deposition of the U2 coal. This is indicated by the similar δ 34S values of pyritic and organic sulfur. Keywords: sulfur isotope, Mae Moh coal field, Chiang Muan coal field, marine incursion, Middle Miocene, Northern Thailand. INTRODUCTION northern Thailand, the basins are rich in coal and oil shale deposits. Large deposits of oil shale occur in the Most of Cenozoic basins in Thailand originated Mae Sod basin, whereas large lignite and coal deposits during the Early Tertiary as a result of extensional occur in the Mae Moh and Li basins. rifting related to the collision between the Indian plate Most of coal in northern Thailand is mined to be and the Asian plate.1 These basins are characterized by used in power plants and in the cement industry. north-south trending half-grabens and grabens that Combustion of coal usually produces large quantities are filled with sedimentary rocks. The strata in these of polluting gases and waste, inducing environmental basins can be divided into syn-rift and post-rift problems that often lead to health hazards. The sequences. Rifting of the onshore basins continued environmental problems come mainly from the quality until Pliocene, whereas rifting of the offshore basins of coal used, especially from a high sulfur content continued only to the Early Miocene. This rifting coal.4 A large amount of sulfur in the form of pyrite and promoted extensive lacustrine deposition.2,3 In gypsum is usually found in coal and its associated rocks. 44 ScienceAsia 30 (2004) The study of depositional environments and sources of Mae Moh Group Straigraphic Succession sulfur in coal in northern Thailand is necessary to The three formations of the Mae Moh Group are, understand the high sulfur formation. A sulfur isotope in ascending order, the Huai King Formation, Na Khaem study is useful to identify the sources of sulfur in coal Formation, and Huai Luang Formation (Fig 2). Each and sediments. formation consists of clastic rocks that differ strongly The middle Miocene Mae Moh coal field and the in lithology, sedimentary structure, degree of Chiang Muan coal field are being mined and are the consolidation, and fossil types. Stratigraphic focus of this study due to their significantly high sulfur descriptions from the bottom to the top are as follows: content in some coal sequences that are being mined. Huai King Formation The Mae Moh basin, Thailand’s largest proven coal The Huai King Formation was the initial deposition deposit, is located in Lampang Province, in northern during rifting of the basin. It consists of semi- Thailand. The mine is operated by the Electricity consolidated, fine-grained and coarse-grained clastic Generating Authority of Thailand. The total coal rocks that have small scale fining upward sequences. reserves of the mine are approximately 1,047 million It underlies the coal of the S zone of the Na Khaem tons. Current annual production from the mine is Formation. The thickness of the Huai King Formation approximately 13 million tons, which supports 20 varies from less than 15 meters on the edge of the basin percent of Thailand’s electrical power requirements. to 150 meters in the central part of the basin. The Chiang Muan is a small mine with approximately 10 formation has two distinct parts. The lower part consists million tons of coal reserves. It is located in Payao mainly of coarse-grained strata that grade upward from Province, also in northern Thailand. The mined coal is conglomeratic sandstone to clayey siltstone. Breccias mainly sent to Mae Moh power plants for electricity of local origin occur at the base of this unit. The generation. sandstone is green, gray, and purple and is mottled red and yellow. The siltstone is gray to red and is also STUDY AREA mottled. Some thin carbonate layers that contain fossil shell debris occur within the siltstone sequence. The Mae Moh Coal Field upper part of the formation is a silty claystone sequence. The Mae Moh coal field is located at 18o 18' 21" This fining upward sequence grades from sandstone or north latitude and 99o 44' 02" east longitude, 26 conglomeratic sandstone to interbedded red and gray kilometers east of the city of Lampang and claystone and silty claystone. Calcareous and mottled approximately 630 kilometers north of Bangkok. The horizons also occur. The rocks in this part of the oval shaped basin covers an area of 104 square formation represent rifting and rapid filling with strong kilometers. Coal occurs in six zones, the S, R, Q, K, J, and to moderate fluvial energy. The lower part of the I zones. Present-day mining is restricted to the upper formation was deposited in a fluvial environment of part, comprises of Q, K, and J zones. braided rivers. The upper part of the formation is fluvial The Mae Moh basin developed as a Tertiary north- meandering river and overbank deposits.13 south faulted graben that was flanked by marine Triassic Na Khaem Formation rocks to the north, east, and west. These marine Triassic The Na Khaem Formation varies in thickness from rocks, which constitute the Lampang Group (Fig 1), 250 to 400 meters and consists of mudstone that has are mainly limestone, shale, and sandstone. The Tertiary five zones of coal. The mudstone is gray to greenish rocks of the Mae Moh basin constitute the Mae Moh gray, very calcareous, and contains abundant fossils of Group.5 This sequence has been divided into three gastropods, fish, ostracods, and plants. It also has formations on the basis of seismic data6 and deep burrows and borings. Near the coal zones, the borehole data. The paleomagnetic study of Mae Moh mudstone is either conglomeratic, calcareous, stratigraphy shows that the Mae Moh group was fossiliferous, or siliceous. The formation has three major deposited in Middle Miocene.7 It was supported by economic coal seams and divides into three members mammalian fossil, proboscidean, Stegolophodon sp., such as Member III, II. And I, in ascending order.14 rhinocerotid, Gaindatherium sp., and mustelid Member III (underburden) carnivore, Siamogale thailandica sp.8,9 Watanasak10,11 Member III is the lowest member, which varies in concluded on the basis of pollen and spore content of thickness from 150 to 230 meters, consists of claystone core samples from a drill hole that the Mae Moh referred interbedded with the coal of the R and S zones. These to Middle Miocene. Pleistocene basalt occurs at the beds are variously colored, very calcareous, and southern part of the Mae Moh basin.12 A thin veneer of conglomeratic. The gastropods Viviparus sp. and Physa Quaternary fluvial sediments, ranging up to 15 meters sp.

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