Stop0 visit DMR’s rock samples collection Stop6 Paleo-tethys of Permian Ophiolite suite; see also in Excerpt paper of Stop1 Permian-Triassic Supaiwan Aggregate Chutakositkanon and Hisada (2008), Limestone Mine, Sukothai Foldbelt; see also Putthapiban (2002) and Watita and Metcalfe in Excerpt paper of Sone et al. (2012) (2005) Stop2 Triassic Pong Nam Ron Formation; Stop7 Permian limestone of Indochina fore argillite facies, trench area; see also in arc; see also in Excerpt paper of Excerpt paper of Sone et al. (2012) Charoentitirat (2002) Stop3 Jurassic-Cretaceous Laem Sink Stop8 Paleo-tethys of Permian ophiolite suite; Formation, the fault between Sukothai see also in Excerpt paper of Saesaengseerung foldbelt-trench area; see also in Excerpt paper et al., (2009) of Sone et al. (2012) Stop9 Typical Mélange Permian clastic Stop4 Triassic-Jurassic I-type Granite, in sedimentary rock suture area after plate collision; see also in Excerpt paper of Putthapiban (2002) Stop10 Inferred Pre-Cambrian Gneiss of Sibumasu Cratonic Area; see also in Excerpt Stop5 Permian Oceanic crust with Triassic paper of Kanjanapayont et al., (2013) chert, trench area; see also in Excerpt paper of Sone et al. (2012). STOP0 THE DMR’S REFERENCE ROCKS SAMPLES COLLECTION Location: Klaeng, District, Geographic Reference: 12.643323N 101.505470 E General Information: Department of Mineral Resources (DMR) by Division of Geological Survey has launched Geological Collection Project since 2010. The collections have been derived from exploration geologists of DMR in several geological works entire the country. Formerly, the selected samples are scattered recording and difficult to gather into the same format due to the difference purposes of collecting sample. After this project began, the Division of Geological Survey had made a typical format for gathering the necessary information. However, not only the rock samples are kept but rare and beautiful mineral or fossils are needed as well. In addition, the rare samples from foreign country had been derived by exchanging samples from foreign country. Nowadays, the collections are currently storage at Mineral and Rock Research Center, Rayong province. All collections will be moved to Geo-Collection building and ready to display for ordinary people and scientific researcher soon. The main objective is to provide efficient storage and database services in order to support scientific researches and developments. The project aims to collect and store geological samples systematically. Samples are basically informative references and support descriptions for further studies. All samples are classified and recorded systematically. The detailed of each sample contains basic information such as coordinate, rock descriptions, photography, texture and so on. The derived collections are divided into two main categories: geological materials and supplementary geosciences data. Geological materials are defined as rock samples derived from domestic and foreign countries. Supplementary geosciences data are composed of geologic maps, reports and photographs on geological view. Until now the collections had been collected rapidly up to 6,469 samples. The Division of Geological Survey planned to fully public the collection via website, http://bgs.go.th. In web page, researchers can access the basically detailed samples by only typing keyword such as rock name. On the other hand, researchers can click on symbol in webpage for finding rock type with selected location. Detailed sampling includes sampling locations, rock type, geochemical analyses e.g. XRD, XRF, thin sections and samples and so on. Additionally, all are welcome to site visit or even borrow samples for researches. However, the both latter services are now unavailable due to Geo-collection building and services system are remaining not complete. Highlight Visit DMR’s rocks sample collection. (a) RMRC building billboard and (b) a Geo-collection building. (Right) Displays samples storage at RMRC and photographs of sample code: area base (A), fossil sample (F), stratigraphic sample (S), foreign sample (FS) and extra large size (X)

Displays (a) http\\bgs.or.th homepage that showing rock sampling location in and (b) page of rock sample detail that showing coordinate and location, rock sample picture, basic rock description, and so on.

STOP10 INFERRED PRE-CAMBRIAN GNEISS OF SIBUMASU CRATONIC AREA Location: Abandon quarry, Nong Yai urabn, Nong Yai District, Chon Buri province Geographic Reference: 13.18083N 101.39983 E General Information: The high-grade metamorphic rock in eastern Thailand is considered to be Precambrian age (Salyapongse, 2002). They are known as “Nong yai Gneiss” or Khao Chao Gneiss” (Areesiri, 1983) and bounded by Klaeng fault zone (NW-SE direction). According to Areesiri (1983), this high-grade metamorphic rock sequence consists of mainly paragneiss, amphibolite and migmatite gneiss, and could be subdivided into six units of (1) biotite-feldspar-quartz gneiss, (2) biotite-hornblende gneiss, (3) biotite gneiss, (4) hornblende diopside gneiss, (5) biotite sillimanite gneiss and (6) biotite diopside gneiss. The marble and calcsilicate rocks occurred locally as massive lenses, generally intercalate in contact of the biotite-hornblende-diopside gneiss and the biotite hornblende. The average chemical compositions of these gneisses when plotted in variation diagrams are compatible to graywack, subgraywacke and arkosic parent rocks. Amphibolites are commonly observed in most unit as 2-10 cm thickness layers. These amphibolites include normal amphibolite, biotite amphibolite, diopside amphibolite garnet amphibolite and hornblende amphibolite. Their major and minor elements geochemistry pointed out that the parent rock of Khao Chao amphibolite would rather be mixture of pelite and Cr-rich mafic tuff in addition to some igneous origin. The metamorphic condition estimated have ranged about from amphibolite facies to upper- amphibolite and even granulite facies (Areesiri, 1983; Kawakami et al., 2008; Nantasin et al., 2011). The P-T condition is estimated to be about 730-840 C and 7.0-9.5 kbar for metamorphic peak, and 580-700 C and 5.0-7.1 kbar for the retrograde re-equilibrium stage base on the thermobarometry and microstructural information (Kawakami et al., 2008). According to Kanjanapayont et al. (2013), these gneisses were trough to have three distinct episodes of deformation (D1-D3). The first deformation phase (D1) formed large scale NW-SE asymmetric isoclinal fold (F1) that were subsequently overprinted by small scale tight to open folds (F2) in the second deformation phase (D2). Both deformation phases generated asymmetric folds (F1 and F2). D1 and D2 are interpreted as having resulted from NE-SW shortening during the Indosinian orogeny before crosscut by leucogranite intrusion. While the leucogranite intrusion took place at 78.6±0.7 Ma in the Late Cretaceous before the second crystallization event in this period (67±1 - 72±0.6 Ma) and shortly after the first crystallization. This magmatism can be related to West Burma and Shan Thai Collison or with a development of Andean-type margin. The third deformation phase (D3) was characterized by penetrative sinistal shearing that formed a promanent S3 shear planes and associated L3 mineral stretching lineations. D3 is interpreted to have occurred under a peak metamorphic condition in the Eocene during the phases of the India-Asia collision. This stop is an old abandoned quarry located on the middle of Khao Chao paragneiss sequence. It is equivalent to the of Kanjanapayont’s sampling location (UK18A). Highlight: Inferred Pre-cambrian gneiss of Sibumasu cratonic area.

The deformation phases; (a and b) photograph (left) and representation (right) of the (a) structural superimposition of F 2 over F 1 fold indicating refolded S 1 /S 2 type 0 3 , and (b) S 2 crenulation cleavage preserved in the hinge zone of F 2 folds, (c) F 1 tight isoclinal folds overprinted by ductile sinistral shearing of D 3 , (d) D 3 sinistral movement preserved in the leucogranite that cross cut the D 2 fabrics, (e) shear band boudins and (f) asymmetric folds of D 3 indicating sinistral movement.

Geological map of the Klaeng fault zone and adjacent area (modified from the Department of Mineral Resources (1982)), showing the location of the two samples dated in this study.

Regional Geology of Eastern Thailand Reference: Department of Mineral Resources, 2014: Geology of Thailand

The Eastern region was covered an area from southern rim of the Phetchabun Range and the Khorat plateau down to northern part of the Gulf of Thailand. Bantad mountain range divides Thailand from Cambodia in the east. The Eastern Region consists of , Rayong, , Trat, Chachoengsao, Prachin Buri, Nakhon Nayok, and Sa Kaeo provinces. Landscape of northern part of the region is flat plain and undulating terrain in between mountainous area in connection to the Khorat plateau and the central mountain range within the region. Elevation of undulating terrain is between 50-150 m above mean sea level. Trending of the central flat plains and undulating terrains is in north-south direction. The highest mountain range is Khao Soi Dao mountain range reaching 1,640 m height. Flat plains in this region lie along the major rivers, consisting of fluvial floodplain, lower terrace with elevation between 5-20 m, intermediate terrace with elevation of 20-30 m and higher terrace with elevation of 30-100 m above mean sea level. Topography of lower part of the region comprises the interchanging of low land and undulating hills. This kind of landscape forms a narrow belt that runs along the sea shore from Chonburi, Rayong, Chanthaburi provinces to . Elevation of the low relief ranges from 1 to 50 m above mean sea level. Flat area boundary along shoreline of the eastern region restricts between undulating terrain and hill slope and sea shore. Deposition of sediment occurred during transgression of the sea. Width of the area is around 5-10 km, consisting of old and present sand ridges, estuary, lagoon, delta and wave-built terrace. Average elevation of this flat area is 1-10 m above mean sea level. Estuaries and river deltas were built along the eastern coast of the Gulf of Thailand on the mouths of several rivers in the region such as Bang Pa Kong river, Rayong river, Pra Sae river, and Weru river etc. Elevation of this part of the area is 1-5 m height. Most of the area is flooded during high tide period leaving top of old and new sand ridges to be seen as narrow and long tracks exposing over mean sea level with 2-10 m height. Wave cut terrace with 20-25 m height is farther beyond sand ridges and lagoon. It is appeared in Chonburi and Rayong province.

1. The Precambrian Roks Precambrian rocks in Thailand are thought to consist of various medium to high grade metamorphic rocks underlying Cambrian rocks. The rocks at different locations contain unique characteristics which are easily recognized. Gneissic rocks show segregation banding with intercalations of schist, calc-silicates and marble indicating metamorphism of sedimentary rocks. Migmatites in Hot district, Chiang Mai province, and Mae Sariang district, Mae Hong Son province show melting of pre- existing rocks. Granite intruded this sequence which extends from the north to the south and is also found in the east. These rocks will be described by region within Thailand, i.e. the northern and the upper western region; the eastern region; the lower western region and the southern region. Regional metamorphic rocks in the eastern region, exposing in Chachoengsao, Chon Buri and Rayong provinces were called Chon Buri Massif by Campbell (1975) and Areesiri (1982). They lie in northwestern-southeastern direction. Brief descriptions of these rocks were given by Brown et al. (1951) and Nakinbodee et al. (1985). Areesiri (1982) has described in detail the rocks in the Khao Chao area, Nong Yai district, Chon Buri province as follows.

1.1 Khao Chao area, Phanat Nikhom-Nong Yai districts, Chon Buri province Sequence of high-grade metamorphic rocks in Chon Buri province, arranged in order from younger to older rock, is as follows.

1.1.1 Biotite-Microcline Gneiss This high-grade metamorphic unit is fine grained biotite-microcline gneiss in addition with inclusions of hornblende-diopside gneiss, amphibolite and biotite gneiss. Major mineral constituents are diopside, biotite, potash feldspar, plagioclase and quartz, with some apatite and zircon. 1.1.2 Biotite-Sillimanite Gneiss The biotite-sillimanite gneiss unit changes to sillimanite-biotite-potash feldspar augen gneiss in the upper part. It changes to garnet-biotite-sillimanite-quartz schist in the lower part. Some c ordierite was found in the lower part of the unit. 1.1.3 Hornblende-Diopside Gneiss The hornblende-diopside gneiss unit is a fine to medium crystalline rock, consisting mainly of hornblende, plagioclase and diopside with some potash feldspar, apatite and sphene. In some area, biotite-hornblende-diopside gneiss, amphibolite, and biotite-sillimanite schist are included. 1.1.4 Biotite Gneiss The biotite gneiss is fine to medium crystalline unit with a continuous contact to the hornblende-diopside gneiss unit. It mainly comprises quartz, potash feldspar, plagioclase and biotite with some zircon, magnetite and apatite. 1.1.5 Biotite-Hornblende Gneiss The biotite-hornblende gneiss is a medium to coarsely crystalline unit, with inclusions of amphibolite, calc-silicate and biotite-sillimanite schist. Major rock forming minerals are biotite, hornblende, plagioclase, quartz, potash feldspar and garnet. Minor minerals are apatite, zircon and magnetite. 1.1.6 Biotite-Feldspar-Quartz Gneiss The biotite-feldspar-quartz gneiss unit is comprised alternations between medium and coarsely crystalline gneiss with interbedded thin bands of amphibolite. Some parts are altered to migmatite with remaining fragments of amphibolite.

Areesiri (1982) separately described other two high grade metamorphic rocks, comprising marble, calc-silicate and amphibolite as follows. Marble-calc-silicate unit: Marble occurs as lenses interbedded in biotite-hornblende gneiss. It is consisted of calcite, dolomite and some phlogopites. Calc-silicate is also interbedded in the gneiss. This rock consists of diopside, calcite and sphene. Amphibolite unit: This unit occurs as subunits ranging from 2 cm to 10 m in thickness. It is interbedded in gneissic rocks and consists of hornblende and plagioclase with some biotite, diopside, quartz, garnet, apatite and sphene. The amphibolite unit can be divided into biotite amphibolite, diopside amphibolite, garnet amphibolite and hornblende amphibolite.

1.2 Northern Part of Khao Chamao, , Rayong province High grade metamorphic rocks in this area are presumably of Precambrian age. This location is considered the farthest east occurrence of the high gade metamorphics. It is in fault contact with younger rocks which gives rise to a long narrow mountain range of mica schist, hornblende schist, and calc- silicates. Biotite-muscovite granite, aplite and pegmatite veins lie parallel to foliation of the schist. Large garnet crystals are disseminated in both the granite and the schist (Figure 2.12). Gneiss and mica schist are exposed in rolling terrain between Khao Chamao and Klaeng district Office. 2. The Lower Paleozoic rocks

The Lower Paleozoic rocks of Thailand are ranging in age from Cambrian to Devonian and up to the early Carboniferous in some rock units. In Shan-Thai terrane, these rocks are widespread in the western mountain area extending from Kanchanaburi province in the west to Chiang Mai, Mae Hong Son in the north; and to Chon Buri in the east. The rocks in the Peninsula are widely distributed in north- south direction alongside of the Khao Luang mountain range, extending from Surat Thani province through Satun province to North Malaysia. In Indochina terrane, only upper part of the Lower Paleozoic rocks are exposed in northeastern part of Loei province extending across Thai-Laos border to Western Laos. In general, The Lower Paleozoic rocks of Thailand have been subjected to low grade regional metamorphism (greenschist facies) and contact metamorphism in the area nearby granite. The rocks are also strongly folded and faulted. These rocks are closely associated with and overlie unconformable on the Pre-Cambrian high-grade metamorphic rocks in the Shan-Thai terrane. The Lower Paleozoic rocks of Thailand can be divided into two conformable rock units in ascending order as: a lower siliciclastic, the Tarutao Group and an upper carbonate, the Thung Song Group. The Tarutao Group, type area at the Tarutao island, Satun province, is a thick sequence of red sandstone, siltstone, shale and conglomerate found the Late Cambrian trilobite on upper part of the sequence (Brown et al., 1951; Javanaphet, 1969). However, most of the rocks on the mainland are quartzite. This group is a shallow shelf sequence periodically subjected to storm. The overlying Ordovician Carbonates, the Thung Song Group, is a thick sequence of tropical limestones, dolomites and calcareous shales. They were deposited in peritidal environment on a homoclinal ramp during a long marine transgression in Early Ordovician, followed by shoaling, reefs, and a deeper subtidal in a Middle and a greater deepening during the Late Ordovician. Lithologically, the carbonate rock of the Thung Song Group is very similar throughout Thailand. However, the age of the similar rock unit in the east (Si Chang island) is still in controversy. The upper part of the Lower Paleozoic rocks of Thailand, Silurian-Devonian, in the Shan-Thai terrane which overlies conformably on the Cambrian-Ordovician rocks can be differentiated into three major north-south facial belts : the shelf-basin in the west and the Peninsula, the magmatic arc facies in the Sukhothai fold belt and Eastern part of the Peninsula and the eastern belt. The first belt comprises rocks in the western Thailand from Kanchanaburi to Mae Hong Son provinces and the Peninsula are black graptolitic and tentaculitic shale, chert, sandstone, siltstone and variegated stromatolitic nodular limestone of shelf to back-arc basin facies. These rocks now are defined as the Thong Pha Phum Group. In the Peninsula from Surat Thani to Satun provinces, this rock is represented by a continuous succession of those deeper water siliciclastic and carbonate rocks. Studies on the graptolite, trilobite, brachiopod, ammonite and conodont by many paleontologists revealed that the upper Lower Paleozoic rocks of this shelf-basin facial belt in the Peninsula are ranging in age from Late Ordovician to Early Carboniferous. They seem to be conformably underlied the Upper Paleozoic rocks. The second belt of the Silurian-Devonian rocks lies along the Sukhothai fold belt of Shan Thai terrane, in Central part of the Northern Highland, the Eastern Gulf and Eastern part of the south can be separated into three sub-facial belts including 1) back-arc-basin facies in the west, 2) volcanic arc facies in the central and the east and 3) fore-arc facies along western edge of the Khorat plateau. The back-arc basin facies are black 2.1 Cambrian Rock 2.1.1 Quartzite Ko Lan quartzite was named by Bunopas (1981) using its type section at Ko Lan, an island 10 km west of beach or south of Chon Buri province. The Ko Lan quartzite, Total thickness of 500 m, comprises variation of massive to well bedded folded, brown quartzite, grayish brown slaty shale and brown to black slate. Age of the rock unit is now unclear. The Cambrian age was based on stratigraphic correlation as the rock possibly underlies the possible Ordovician limestone at Ko Si Chang. However, many geologists believe that age of the Ko Lan quartzite can be as young as Carboniferous and Permian by comparison with similar quartzite rocks exposed on land from district to Satahip district. This onland quartzite at Bangphra reservoir underlies rocks that found fossil aged Permian (Polprasit and Prasomsap, 1984; Charusiri, 1984; Chaodumrong (a, 1994); Salyapongse, 1997) It may be controversial that there are two similar sets of quartzite yield different age or only one aged quartzite.

2.2 Ordovician Rock

2.2.1 Si Chang limestone, Thung Song Group The Si Chang limestone was named by Buravas (1957) for limestone crops out at the Si Chang island. The limestone is strongly deformed and recumbent isoclinal folds. This formation was later included in the Thung Song Group in the east by Nakinbodee et al. (1976). Type section of the rock unit located on the Si Chang island, 11 km west of . Another part of the formation is found along the coast from Si Racha to Bang Lamung, south of Chon Buri province. The Si Chang limestone, approximately 400 m thick, lying over quartzite and quartz schist, consists of dark gray, well-bedded to massive muddy limestone. Its lithological characteristics are similar to the Thung Song Group in Thailand. Nakinbodee et al. (1976) found very poorly preserved and deformed nautiloids on Si Chang island which were identified as Multicameroceras sp. dated Early to Middle Ordovician. The age of this limestone remains contentious and some geologists believe that the formation may be Permian and are correlate to the limestone at Khao Rewadee on the coast opposite Si Chang island.

2.3 Silurian-Devonian-Carboniferous Rocks 2.3.1 Sattahip Shale Satahip shale was named by Bunopas (1981) represent slaty shale, argillite, fine-grained sandstone and laminated limestone at Chon Buri and Rayong provinces. Type section of the formation located on Satahip naval base, Chon Buri province. General Characteristic of the formation is well- cleavage developed and tight folded. Color of rocks is generally greenish gray and brownish gray shale and sandstone became brownish gray and white when weathered. Fossils were not found in this formation 2.3.2 Klaeng schist and phyllite Klaeng schist and phyllite was named by Bunopas (1981). Distribution of the rock unit is situated on undulating terrane parallel to the inferred Precambrian rocks in eastern Klaeng district, Rayong province. Klaeng schist and phyllite consists of interbedded of quartz mica schist, tuffaceous quartzite and brown tuffaceous phyllite in the lower part. The upper part consists of interbedded of white to grayish brown quartzite, slaty shale, chert, phyllite and bands of limestone. Most of the rocks are well cleavage developed and medium-tight folded. They were tilted 55o to the northeast. Fossil of small nautiloids, Michelinoceras michelini of possibly Silurian age was only found in chert at Laem Thong Lang close to Klaeng district. Klaeng schist and phyllite are equivalent to the Bo Phloi formation and Thong Pha Phum Group in western Thailand (Bunopas, 1981).

3. The Upper Paleozoic rocks The Upper Paleozoic covers the Carboniferous Period (approximately 360 Ma to 299 Ma) to the end of Permian Period (approximately 251 Ma). Upper Paleozoic rocks are widespread in Thailand, consisting of limestones, volcanic sediments and siliciclastics deposited in a wide range of paleoenvironments. Their lithostratigraphy differs across the country and will be discussed region by region. 3.1 Carboniferous rocks Dating of the Carboniferous rocks in the eastern region was previously problematic since there are too few fossils, a lack of stratigraphic continuity and a complex structure. Recently, more detailed paleontological studies have been carried out and dating is therefore more accurate, especially using microfossils like foraminifera and radiolaria. Structural and tectonic studies and isotope dating have helped to form a more accurate understanding of geological evolution of the area. On the eastern coast from Muang district to , Chon Buri province, rocks were intensively metamorphosed. Although, nautiloid fossil was found in limestone interbedded with shale on Si Chang island and tentatively determined as Ellesmerocerida Multicameroceras (?) dated as Early to Middle Ordovician (Nakinbodee et al., 1976). The fossil was highly deformed thus exact dating was not possible. Salyapongse and Jungyusuk (1980) determined age of the rocks cropping out along the eastern coast of the Gulf of Thailand from Muang, , Sattahip districts, Chon Buri province to be Carboniferous to Permian. The lower part comprises bedded quartzite (Figure 5.1), quartz mica schist and quartz schist (Figure 5.2) siliceous shale and thin bedded limestone. The upper part is composed of thick bedded limestone such as at Khao Phu, Khao Choeng Thian, Khao Chee On and Khao Chee Chan. Limestone is green to brown colored and shows an alternation of thin argillaceous or arenaceous layers with calcareous layers and occasional interbeds of dolomitic limestone. At the base of this sequence, interbeds of chert are also present. The rocks have been subjected to at least two phases of intensive metamorphism; low grade, greenschist facies in the lower part of the sequence. In some areas they were later affected by contact metamorphism. In the middle and southern coast, the Carboniferous rocks are found in north of Khao Cha Mun and Khao Yai mountain range, Ban Thammarat Nai area. The rocks strike in northwestern-southeastern direction. They comprise slaty shale, siltstone and sandstone with alignment of black minerals. Some beds are metamorphosed to graphitic schist, especially in areas near faults and in contact with granite (Pholprasithi and Prasomsup, 1984). Nakinbodee et al. (1976) divided the Carboniferous into formations named Cp1 and Cp2. The rocks are distributed in areas of and Sanam Chai Khet district, ; Bo Thong district, Chon Buri province; , ; and Klaeng district, Rayong province and consist of micaceous siltstone, shale, limestone, quartzite, chert and conglomerate. Cp1, yields the bryozoa Penniretepora sp., Fenestella cf. F. triserialis, Fenestella sp., Polypora sp., and brachiopod Cleiothyridina sp. which indicate a Lower Visean to Upper Tournaisian age ( Lower Carboniferous). Kittisarn and Assavapachara (1987) divided the Carboniferous into two formations named C1 and C2. These formations are distributed in areas of Ban Tha Kloi, Ban Thammarat Nai, Ban Khlong Taklao, west of Ban Nong Khok, Khlong Taklao subdistrict and Khao Phai , Khao Ying of Ban Thammarat Nai in Sanam Chai Khet district, Chachoengsao province. This belt of occurrences extends southwesterly along rim of volcanic belt at Khao Yai-Khao Ang Rue Nai. The lower part of C1 formation often overlies Triassic granite and Permian-Triassic volcanic rocks. In the eastern part, the formation is unconformably overlain by Triassic sedimentary rocks. In the western area, the C1 formation conformably underlies low grade metamorphic rocks of the C2 formation in an area west of Ban Thammarat Nai. Thickness of the C1 formation is 1,500 m. The C1 formation comprises sedimentary rocks that were slightly metamorphosed. In general, it consists of alternations of sandstone, micaceous siltstone, shale, slaty shale, slate, chert, quartzite, limestone and conglomerate. The rocks near contact with igneous rocks are harder as result of silicification by hydrothermal fluids as at Lum Manao Temple, northeast of Khao Yai, and pale green to gray silicified siltstone in area west of Khao Phrik. Fossils of the bryozoan Fenestella sp. and foraminifera were found along the eastern and northern rim of Khao Yai. Light gray to dark gray limestone was found in two areas. The first area occurs as a belt extending from the eastern rim of Khao Yai to south of Ban Thammarat Nai-Khao Phai-Khao Ying, is 30-50 m thick and occurs as lenses in sandstone, micaceous siltstone and shale. The foraminiferans Endothyra sp., Pseudoendothyra sp., and Plectogyranopsis sp., an echinoid spine and fragments of bryozoa are present in limestone. The second area is around Tham Rat Temple, northeast of Ban Nong Khok, Sanam Chai Khet district, Chachoengsao province. Here, dense crystalline limestone occurs as lenses in light gray quartzitic sandstone with a thickness of 50 m and at Ban Thammarat Nai are the smaller foraminifera Plectogyranopsis cf. dendrei, Endohydra sp., Paraforaminifera sp., Tetrataxis sp., Mediocris sp. (?), Glomospira sp., echinoid spine and fragments of bryozoa which indicate a Carboniferous age. A 3 m thick dark gray limestone is also found west of Khao Yai Mo Noi showing recrystallization and cross-cutting calcite veins. Corals indicate a Carboniferous age (Fontaine and Salyapongse, 1997), 150 m south of the previous location, dark gray to brown chert crops out on flat terrain. Dark gray limestone with quartzitic sandstone also crops out northeast of Khao Ang Rue Nai, south of Ban Soi 16, in the area around the Office of Khao Ang Rue Nai forest reserve, Khlong Ta Klao subdistrict, Sanam Chai Khet district, Chachoengsao province. The C2 formation mostly comprises low grade metamorphic rocks and sub- metamorphosed sedimentary rocks. The formation consists of interbedded black slate, phyllite, slaty shale, graphitic schist, amphibolite and quartz schist. Quartz veins cut across the formation. Sequence of rocks in this formation is clearly seen to the east of Ban Khlong Yang, northwest of Khao Thammarat Nai. The Carboniferous rocks of the C2 formation can be divided into the C1-2 formation and the C3 formation. The C1-2 formation consists of sandstone, shale, mudstone and limestone and is dated as Lower to ‘Middle’ Carboniferous. The corals Kueichouphyllum?, Siphonodendon sp., Solenodendron sp. and Hexaphyllia sp. indicate a Late Visean-Serpukhovian age. The foraminifera Eostaffella sp., Millerella sp., Endothyra sp. indicate the Bashkirian. The C3 formation consists of metasediments comprising conglomeratic phyllite, graphitic phyllite, meta-pyroclastic sandstone and slate.

4. Permian Rocks

Permian sequences in Thailand are often thick limestones and intercalated clastics. The former usually display spectacular tower karst topography. These karsted terrains are regions of scenic beauty such as the group of small islands in Phangnga bay, the Angthong Archipelago in Surat Thani province, Tham Lod in Kanchanaburi province and Doi Chiang Dao in Chiang Mai province. Outcrops of the Permian rocks distributed in all regions of Thailand except the northeastern region, where they, however, occur in the subsurface.

Limestone, clastic rocks, pyroclastic rocks, low grade metamorphic rocks and mélange of Permian are distributed over the eastern region. Based on stratigraphy, rock types and fossil assemblages, the Permian rocks in this region can be divided into three areas, i.e. western coast, central part and an area near the Thai-Cambodia border. 4.1 Western Coast of Eastern Region Permian rocks at Khao Rewadee near Bang Phra water reservoir, Si Racha district, Chon Buri province, are dated as Late Carboniferous to Early Permian by Pseudofusulina sp. in limestone lens, interbedded in shale (Nakinbodee et al., 1976). Polprasit and Prasomsap (1984) described the rocks as containing arkosic sandstone, graywacke, interbedded with siltstone, shale, mudstone, thick bedded chert and dark gray to black limestone. Fossils of foraminifera, algae, and bryozoa at Khao Rewadee indicate Middle Permian. These Permian rocks unconformably overlie Carboniferous rocks. The rocks are fractured and secondary dolomitic limestone was found within fracture and fault zones (Chaodumrong, 1992).

4.2 Central Part of Eastern Region The Permian rocks consist mainly of metatuff. The lower part is metamorphosed conglomerate. The ascending sequence comprises volcanics, chert, shale and siltstone with some interbedded limestone. These rocks lie unconformably over Carboniferous rocks and have a fault contact with Triassic rocks (Polprasit and Prasomsap, 1984). The brachiopods Leptodus sp. productid sp., and a bivalve in pyroclastic shales, at Khao E-Prink, 18 km east of Klaeng district, Rayong province indicate Middle to Upper Permian (Bunopas et al., 1983).

4.3 Area near Thai-Cambodia Border Sivaborvorn et al. (1976) named the limestone interbedded with shale near the Thai-Cambodia border as the Ratburi Group. It contains the fusulinid Verbeekina verbeeki dated as Kazanian or Upper Permian. Bunopas (1981) named the Chanthaburi Groupconsisting of two formations: the Srakaew Formation and Khao Chakan Formation. The Mueangkaew Formation consists of radiolarian chert, limestone and a composite mélange assemblage of ultra-mafic rocks which may be regarded as a dismembered ophiolite. It contains Middle to Upper Permian fossils. The Khao Chakan Formation is mostly composed of limestone in Sakaeo province and along the Thai-Cambodian border. Chaodumrong (1992) divided the Permian rocks into two formations as Wang Nam Yen formation and Khao Ta Ngog formation. The Wang Nam Yen formation occurs in a narrow belt in , Chantaburi province, starting between km 83-94 on the road Chantaburi-Sa Kaeo, Khao Laem, Khlong Yai-In, Khao Poon, and Khao Wai. This formation is presumably a thrust sheet complex, comprising red and gray chert with Middle Permian radiolaria. The Khao Ta Ngog formation exposed along the Thai-Cambodian border from Khao Ta Ngog limestone mountain, Khao Chong Kaeb, Khao Pha Phung, Khao Prik to Khao Chakan. The formation mostly consists of massive bedded limestone interbedded with shale. The limestone contains numerous Middle-Late Permian fusulinids and crinoids originating in a barrier reef. Fragmentary fossils found at Khao Prik and Khao Chakan indicate breaking and transportation of the fragments before deposition. 5. Stratigraphic Correlation Sedimentary rocks of the Upper Paleozoic Era in Thailand deposited in a marine basin. Most of the rocks unconformably overlie older rocks, except in the northern region, for examples Mae Tha Group and Dan Lan Hoi Group lies unconformably over Sukhothai Group; Phrae Group unconformably lies on Pha Som group (Bunopas, 1981). The sedimentary rocks had been continuously deposited from the Carboniferous to Permian in almost the whole country except in the southern region which indicates a hiatus of deposition, possibly some mild tectonic activity, but no major orogenic event. There are both conformable and unconformable contacts between the Permian and Triassic rocks in Thailand. In the northern region, the Ngao Group conformably underlies the Lampang Group. In the eastern region, the Chantaburi group unconformably underlies the Pong Nam Ron Formation, whereas in the southern region the Ratburi Group underlies Sai Bon formation. 6. Mesozoic Rocks

Study of Mesozoic rocks in Thailand was originally based on the facies of the rocks. All of red beds of the Mesozoic Era were originally called Khorat Series, named after the Khorat plateau, where the rocks were widely distributed. Later the name was changed to Khorat Group. Continental rocks occur sporadically in the southern and the western regions where they are called the Thung Yai Group. Marine deposits are mainly found in the northern, the western and the southern region and are assigned to the Lampang Group and Umphang Group. The Khorat Group comprises nine formal and two informal formations: in ascending order of Huai Hin Lat, Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, Khok Kruat, Maha Sarakham, Phu Thok Formation and Khao Ya Puk and Phu Khat formations. The rock-types include sandstone, pebbly sandstone, siltstone, mudstone, shale, rock salt, gypsum and anhydrite. Based on occurrences of vertebrate fossils of dinosaurs, turtle, crocodile, fish and invertebrate fossils of bivalve and pollen, ages of Late Triassic to Early Tertiary are attributed to this group. The rocks unconformably overlie the Paleozoic rocks. In some places, the rocks overlie Permo-Triassic igneous rocks or marine Triassic rocks. Distribution of marine Mesozoic rocks is as wide as the continental Mesozoic rocks. The marine rocks consist of Triassic-Jurassic limestone, shale, sandstone and conglomerate. The Triassic rocks were found in the northern, western, eastern and southern regions. The Jurassic rocks distributed in more restricted area as they were found along Thai-Burmese border from Mae Hong Son to Kanchanaburi provinces and in some areas of the southern region. Biostratigraphy was constructed using bivalve and ammonoid fossils, found in Lampang province. It indicates the rocks of Early to Late Triassic or during Griesbachian, Anisian, Ladinian, Late Carnian and Early Norian. Fossils in Jurassic rocks were bivalve, coral and ammonoid, indicating deposition in shallow basin during Early Jurassic to Middle Jurassic or during Toarcian and Early Bajocian. Abundant fossils were locally found in marine Triassic and Jurassic rocks giving an almost complete succession from Early Triassic to early Middle Jurassic. The sea regressed out from most of Thailand in the Middle Jurassic. 7. Marine Rocks

7.1 Triassic Rocks In Thailand, marine Triassic rocks can be locally divided into two belts. The first belt starts from east of the northern region (Lampang-Chiang Rai-Nan provinces) to the eastern and southern regions. The second belt extends from Mae Sariang district, Mae Sot district and Kanchanaburi province southward to the South. Originally, marine Triassic rocks were defined by study of fossils in the northern region (HÖgbom, 1914). The fossils of algae, coral, brachiopod, bivalve and ammonite were dated as Late Triassic (Gregory 1930). They were found in limestone that exposed along Thai-Burmese border. Pitakpaivan (1955) undertook stratigraphic study of the Triassic rocks in Mae Moh area, Lampang province. He also found fossil of ammonoid in the limestone. In his study, he unofficially named new formations. The results of geological mapping by cooperative Thai-German team of geologists produced a considerable amount of data on fossils in the North. The systematic study of fossils from numerous locations was published by Chonglakmani (1981). Thus the origin of the Triassic rocks in the North is reasonably well understood. Other stratigraphic study of Triassic rocks were published by Pitakpaivan (1955), Piyasin (1972), Chonglakmani (1981), Bunopas (1981), Chonglakmani (1982), Chaodumrong (1992), Charoenprawat et al. (1992), Chaodumrong and Burrett (1997), Sardsud (1997 & 2002), Meesook et al. (2002), Ishida et al. (2006), and Weerahong (2007). Triassic rocks in the eastern region can be divided into four formations, two informal and two formal, (Chaodumrong, 1992a), from older to younger: the Sukpaiwan and Noen Po formations; Pong Nam Ron and Noen Phu Yai Yua Formations. Sukpaiwan formation has restricted distribution in area northeast of Klaeng district, Rayong province, especially at Khao Sukpaiwan, Khao Wong, Khao Chong Lom and Khao Cha-ang On. These small hills locate at eastern side of Khao Chamao. This formation is more than 100 m thick (Fontaine and Salyapongse, 1997a), comprising gray to dark gray, purplish brown limestone (Figure 5.13). The beds are fine to medium bedded (3-30 cm thick). Oncoids with diameters less than 0.5 cm, peloids and bioclasts are present in the limestone. The lithology varies from grainstone to lime-mudstone. Shale is interbedded with limestone in the lower part of the formation. Clasts of volcanic rocks are found. This formation has not experienced strong deformation. It unconformably overlies Late Permian rocks. Fossils of foraminifera Meandrospira pusilla Ho, Glomospira tenuifistula Ho, and algae Aeolissacus tintinniforis Misik indicate an Early-Middle Triassic (Scythian-Ladinian) age. Noen Po formation distributed from northern Chantaburi province southeastward to Laem Ngop district, Trat province. It is well exposed at Ban Noen Po, Khao Cham-Han and Ban Nong Chim. This formation consists of dark gray to black mudstone and shale with some coal. Locally, layers of gray chert occur in 3-5 cm thick beds. Fossils of radiolaria Triassocampe, Syringocapsa, Archaeospongoprunum indicate Middle to Late Triassic age (Hada et al., 1997). Color of the rock changes to be light color when weathered. Large amount of feldspar and Pong Nam Ron Formation was named after the by Sivaborvorn et al. (1976), has been widely used but in a different sense (Tansathien et al., 1975; Chaodumrong, 1992a; and Tansuwan, 1997). Outcrops of this formation are clearly visualised in area east of Chantaburi province, especially on eastern side of the Chanthaburi-Sa Kaeo road and in the Pong Nam Ron and Wang Nam Yen districts along border with Cambodia. Thickness of this formation is more than 200 m, consisting of thick to massive dark gray feldspathic sandstones which became greenish gray when weathered. This sandstone is poorly sorted with clasts and grains of volcanic rock fragments, feldspar, with minor quartz, sedimentary and metamorphic rock fragments. Graded bedding was also found. Interbeds of mudstone and conglomerate were found in sandstone locally. Clasts in the conglomerates are well rounded and consist of volcanic rocks and fossiliferous limestone containing fusulinids. Clasts in conglomerate are both well and poorly sorted. Noen Phu Yai Yua Formation is well exposed in the Noen Phu Yai Yua Buddhist sanctuary, Khao Ploy Waen, Muang district, Chanthaburi province, Pong Nam Ron Canal and Ban Nong Bon. This formation is more than 300 m thick, conformably overlying the Pong Nam Ron Formation. It consists of medium grained dark gray sandstone, becoming light gray when weathered, with interbedded mudstone. Thickness of sandstone and mudstone beds is similar, varying between 10 and 20 cm. The thickness is constant over a considerable area. Bouma sequence and graded bedding are found in the sandstone. Contacts between sandstone and the overlying mudstone are sharp and planar. Ratio between sandstone to mudstone varies from 2:1 to 1:1. Mudstone increases in upper part of the formation. These sediments indicate deposition in submarine fans in a deep marine basin (Chaodumrong, 1994). 8. Continental Rocks

8.1 Jurassic Rocks Terrestrial rocks in the eastern part of the country were distributed in quite a limited area, covering the south of Chanthaburi province, along Khao Banthat mountain range running along the border with Kampuchea, and on Ko Kut Island, Trat province. Formations of Jurassic age are as follows : Laem Sing Formation unconformably overlies marine Triassic rocks seen in Laem Sing, Khao Tha Le Sai, Khao Takrup, and Khlong Rabom dam (Sivaborvorn, et al., 1976). Its type section is at Laem Sing, Chanthaburi province. It is composed of red sandstone and siltstone, reddish purple, light gray, light brown, claystone and some conglomerate. Detrital grains in the sandstone are mostly composed of quartz. Cross bedding, well sorted, and graded bedding are found in these sediments. The beds are steeply dipping and locally overturned. By stratigraphic correlation with the lower part of the Khorat Group, this formation was dated as Jurassic. Phu Kradung Formation comprises reddish, light gray shale with interbedded sandstone and siltstone. It is found in Banthat mountain range along the border with Kampuchea. The beds have moderate dips. Volcanic ash occurs in the basal parts of shale beds. Plant remains are found in gray shale. The reddish purple to gray, fine to medium grained, well sorted, and well cemented arkosic sandstones were deposited in a meandering river system. By stratigraphic correlation with the lower part of the Khorat Group, this formation was dated as Jurassic. The Phu Kradung Formation in Ko Kut comprises red, brown sandstone, siltstone and conglomeratic sandstone. Plant remains and vertebrate bones were found in conglomeratic sandstone. This formation was formed in a meandering river system in a coastal environment. It was dated as Jurassic, on the evidence of sharks teeth, teeth of the fish Lepidotes, freshwater turtle plates and freshwater crocodile and plesiosaur teeth (Buffetaut and Ingavat, 1983). The plesiosaur is similar to one found in Tchepone, Lao PDR (Hoffet and Le Maitre, 1939). 8.2 Jurassic-Cretaceous Rocks The Phra Wihan Formation conformably overlies the Phu Kradung Formation at Khao Banthat Mountain, comprising sandstone with interbedded shale. Thick bedded, well cemented sandstone is composed mostly of light gray to white gray, brown, coarse grained quartz. Cross bedding is evident. The rocks were deposited in a braided river system. The formation was dated as Jurassic by stratigraphic correlation with the lower part of the Khorat Group.

9. Quaternary Rocks and Sediments Quaternary is the last period in geological time scale from 2.6 Ma to the present. This period is further divided into two epochs namely the Pleistocene from 2.6 Ma to 10,000 years B.P. and the Holocene from 10,000 years B.P. to the present. The fossils in this period are mostly comparable to modern taxa. Sedimentation has taken place from the lower layers to the upper layers in successions where many sedimentary horizons are hidden below the ground surface. Studying the Quaternary geology is thus mainly available from drilling, sedimentary profiles in cliffs, road cuts, quarries and along river banks. Quaternary rocks and sediments in Thailand are observable in various regions of the country as follows: 9.1 Central plain basin margin The area between the central plain and mountainous area possesses a distinctive geologic character from the nearby areas. It is characterized by undulating terrain and hills with elevation less than 100 m. Some areas have widely distributed by lower hills and monadnocks. The area along the basin margin therefore consists of hard rocks and unconsolidated sediments surrounded the wide central plain. Some rocks are the sources of sediment accumulated in the central plain basin. The area of the central plain margin is herein divided into 2 areas, the eastern and western sides.

9.2 Eastern central plain basin margin This area covers the area in the east of Sukhothai province continuous southward to the east of Chachoengsao province. The surface sediments or unconsolidated sediments in these areas are Quaternary in age occurring in different geomorphologic landforms. Naming the sedimentary groups is thus in accordance with the geomorphologic characteristics they occur such as colluviums are derived from the source rocks and deposited along the foothills comprising mainly granules and boulders and alluvial fan sediments are derived from stream flows from the mountains to the central plain. When the stream flows are abruptly changed the gradient, the sediments will be dispersed forming the fan-shape sedimentary deposits along the margin of the basin. The deposits are composed of various sizes of sediments ranging from granules to boulders accumulating on the clayey sand layers. Undulating terrain is elevated and ranges from 10-20 m above mean sea level. It is characterized by a series of rolling hills mainly composed of sandy clay with red mottled lateritic soil overlain by a laterite layer. In the area where the bedrock is shallow as in Nakhon Nayok and Prachinburi provinces, the lateritic soil and laterite are the successions directly over the bedrock (Sinsakul, 1996). Terrace deposit This sedimentary deposit is the result of changes in river flow directions and lowering the channel by river deepening retaining some levels of terraces along the river. The sediments in this group consist of gravel, sand and clay with lateritic soil on top in some places. The gravel sediments show characteristics indicating that deposition was under a high velocity of river flow and when the velocity was reduced, the sand and clay were deposited. The terrace deposit is mainly divided in accordance with the elevation of the terraces, the high terrace and low terrace. The terraces have similar sedimentary successions but different thicknesses. The terrace sediments are generally usable as construction materials. Fluvial deposit is defined as sediments transported and deposited by streams or rivers particularly during the overflow in raining season, weater current will carry the sediments to deposit in the channel and overflow on both sides of the river to deposit on the wide flood plain. The sediments are composed of fine sand, silt and clay. When the flood sediments accumulate and continuously get thicker, the previously wide channel will narrow with time to become a narrow channel. Finally, the channel will disappear retaining only a wide flat plain. Age of the sedimentary successions The central plain basin developed and evolved as a result of Cenozoic extension of the landmass until around Late Pliocene-Pleistocene (about 2.6 Ma) the fault block basin was developed. The eastern basin margin had moved up forming the mountain ranges and then weathered and eroded huge volumes of sediments to accumulate along the eastern basin margin. Age of the sediments is uncertain due to lack of fossils. However, the age Pleistocene is widely proposed on the basis of its underlying the marine fossil-rich sedimentary formation, the Holocene Bangkok Clay, which is dated at less than 10,000 years B.P. So far, the exact age of each sedimentary group in the eastern basin margin is still unknown. The Pleistocene Epoch is therefore assigned as the age of the whole sedimentary groups. For the topmost sediments in the area is regarded as recent flood plain sediments. 10. Igneous and Metamorphic Rocks

Igneous Rocks are solidified from magma and lava. These molten rocks originate from the depth beneath earth surface. Intrusive igneous rocks are products of magma that slowly ascended at some depth, cooled and solidified. In this case, minerals consisting in these rocks slowly cooled and precipitated from magma. Thus, they appear as coarse crystal grains with interlocking edge. When magma extrudes to the surface, it is distinctly called lava. Solidified lava is called extrusive igneous rock or volcanic rock. Cooling and solidification of lava on the earth surface occurs rapidly. Precipitation of lava occur quickly, therefore the crystals are normally fine grained until they are almost invincible by naked eyes. However, lava always brings up earlier precipitated crystals with it from the depth. Wall rock fragments were also brought up with lava. Obsidian is the product of rapid lava cooling. Gas and volatile always are included in lava. When this kind of lava solidified, gas and volatile will cause vesicles in the rock. Igneous rocks cover about 8% of surface in Thailand. Most of them are intrusive granitic igneous rocks. Some other igneous rocks are dark to black i.e. diorite, gabbro, and ultramafic rocks. Extrusive igneous rocks are less than the intrusive igneous rocks. They comprise of rocks solidified from both lava and pyroclastic debris i.e. basalt, rhyolite, andesite, perlite and other related rocks. Igneous rocks are important as they are the sources of metallic, non-metallic, and precious minerals such as tin, tungsten, feldspar, fluorite, sapphire etc. More, beauty and hardness of igneous itself is popular to be used as facing stone and valuable construction materials. These igneous rocks occurred in different of geologic times from Devonian to Quaternary. Metamorphic Rocks occurred by metamorphism of sedimentary rocks and igneous rocks beneath heat and/or pressure. Orientation of crystals is apparent. However, metamorphic rocks occurred mainly by heat may not show orientation of crystals. Metamorphic rocks have to be consisted of new formed crystals by metamorphic processes. Some minerals can determine temperature and pressure of metamorphic processes. Chemical composition of metamorphic rocks may be the same, similar or definitely different from the pre-existing rocks. In this chapter, description of rocks found in Thailand will be made in sequence of intrusive igneous rocks, extrusive igneous rocks and metamorphic rocks.

10.1 Intrusive Igneous Rock 10.1.1 Granites Granite is a type of intrusive igneous rocks widely found in Southeast Asian countries. In Thailand, it occurs both in mountainous and flat terrain. Granitic rocks normally relate to occur of economic minerals. Occurrences of metallic and non-metallic minerals were found in the vicinity of granite intrusions. Because of its beauty, granite is popular to be used as facing stone and construction rocks. Intrusion of granitic magma occurred in several episodes starting from Permian or about 250 m yrs ago. 10.1.2 Mafic and Ultramafic Intrusive Rocks Mafic and ultramafic intrusive rocks cooled under the earth surface. They formed bodies like veins, bands, batholith or xenolith in basalts. They comprise mafic or dark minerals more than 90%. This group of rocks consists of four types, i.e. mafic rocks, peridotites, pyroxenite and hornblendite. In Thailand, there are three belts of mafic and ultramafic intrusive rocks, i.e. Pha Som Ultramafic Belt, Sa Kaeo Ultramafic Belt and Narathiwat Ultramafic Belt (German Geological Mission to Thailand Reports, 1972, 1975).

10.1.3 Sra Kaeo Ultramafic Belt Sra Kaeo Ultramafic Belt or Sra Kaeo ultramafics comprises serpentinite and hornblendite. The belt extends from 30 km to the south. The rocks crop out at location 7 km west of the road between Sa Kaeo and Chantaburi provinces. They form small rolling hills in adjacent to mountains of Late Permian limestone and red chert. Relation between these rocks was unclear (Bunopas, 1981). Some similar ultramafic belt was found in area of Khao Phai, Khlong Phun Nip lying in northeast-southwest direction with 40 km length. This belt cuts across Sa Kaeo-Chanthaburi road at Ban Tha Kham. Ultramafic rock in this belt consists of serpentinite and hornblendite. Another ultramafic belt at Ban Ratam, Pong Nam Ron district, Chanthaburi province is serpentinite covering area of 45 sq km. It lies in the same trend as the belt at Ban Khao Phai, Khong Phun Nip. Ultramafic at Ban Ratam may be alteration product of peridotite. Geochemical data and magnetic survey data indicate that the rocks in this area lie in northeast-southwest direction (Bunopas, 1981). Serpentinite was found underlying laterite at Nong Wa-en-Nong Kra, Prachin Buri province and at , Rayong province. Ultramafic rock in this area lies in the dame trend of northeast-southwest (Suwanasing, 1973). The rocks in east of Ban Khai district, Rayong province are serpentinite and peridotite underlying gabbro (drilling data by Sahat Muenlek) Sa Kaeo ultramafic belt is a part of ophiolite belt in contact between Sukhothai fold belt and Loei fold belt (Bunopas, 1981). It is possibly formed in the same period as Pha Som ultramafic belt. 10.2 Extrusive Igneous Rocks 10.2.1 The East Coast Rhyolite and dacite in the east coast were distributed along Sa Kaeo-Chantaburi road in area south of Sa Kaeo District, north of Khao Soi Dao and Ko Chang Island. Extrusion occurred during Late Permian to Early Triassic. Most of older volcanic were metamorphosed as green schist to amphibolite facies (Salyapongse, and Jungyusuk, 1980). This volcanic underlay chert bed containing fossil radiolaria, limestone bed containing sponge spicule and foraminifera, dated Permian. Late Permian-Early Triassic volcanic was found in Ban Tha Kham, south of Wang Nam Yen district. They comprise spilitic pillow lava, diabase, andesite, agglomerate and volcanic breccias. These rocks were found in between thin beds of red chert containing numerous Permian micro fossils. Basalt formed a belt lying in Northwest-Southeast direction. Spilitic pillow lava and chert were deposited on the ocean floor. Most of volcanic rocks in Sa Kaeo province are greenish gray and grayish purple rhyolitic tuff. Some andesite dikes cross cut the rocks. Lithic crystal tuff formed as pyroxenite, basaltic andesite porphyry, hornblendite, andesite porphyry and crystal tuff. Crystals of pyroxene and plagioclase were found in these rocks. In Ko Chang Island, most of the volcanic rock is rhyolitic agglomerate, occurring in area Khao Chang Sam Look, north of the island. Size of the rock fragments varies from pebble to boulder. They are well rounded. Most of rhyolite was porphyry containing plagioclase and quartz. Intrusive rocks, such as quartz monzonite and granodiorite, were found in the middle of the island. 10.2.2 Basalt Basalt was found in Chanthaburi and Trat provinces. It is the source of gems in this region. Basalt flowed in low land area, lying in north-south direction. At Khao Phloi Waen, Tha Mai district, Chanthaburi province, the rock formed high hills as remains of old distinct volcanoes. Basalt can be classified in according with locations as follow.

10.2.2.1 Chanthaburi basalt can be found at Khao Phloi Waen, Khao Wuea, Tha Mai district, Chanthaburi province. It is fine grained, black colored with ultramafic rock fragments, and big crystals of spinel, pyroxene, and olivine. Olivine and clinopyroxene phenocrysts were embedded in matrix of clinopyroxene, nepheline and opaque minerals. Barr and Macdonald (1978) defined the basalt at Khao Phloi Waen as nephelinite. Basalt in east of Chanthaburi province covers small hills, lying in north-south direction. It was found at Ban Bo Weru, Ban Ang Ed, Ban Bo E-ram, Ban Si Siad, Ban Saphan Hin, and Khlong E-tak. Gems deposits were discovered at Ban Bo Weru, Ban Ang Ed, and Ban Bo E-ram. Gems bearing and gems barren basalts can be distinguished (Vichit et al., 1978). These 2 groups of basalt can be described as follows. Basalt at Ban Saphan Hin is porphyritic to glomeroporphyritic texture and holocrystalline, consisting of numerous olivine crystals with some clinopyroxene in matrix of clinopyroxene, feldspar, and opaque minerals. It is defined as alkaline-olivine basalt, and divided into 2 units with total thickness of 40 m. The lower unit is vitric pillow lava. The upper unit is vesicular hawaiite. Sirinawin (1981) described basalt at Khlong E-tak as composing of dunite rock fragments, and olivine crystals in matrix of brown vitric pyroxene, olivine, and some opaque minerals. Basalt in other areas is composed of olivine crystals, and numerous clinopyroxene crystals, embedded in matrix of feldspar, pyroxene and opaque minerals. 10.2.2.2 Pong Nam Ron basalt covers southeast of Pong Nam Ron district, Chanthaburi province, lying in north-south trending. The basalt is dense with dark gray to black color, comprising olivine and clinopyroxene embedding in matrix of tabular shape feldspar, pyroxene, opaque minerals and glass. Barr and Macdonald (1978) defied this basalt as basanite basalt. 10.2.2.3 Trat basalt was found in areas of Ban Nong Bon, Ban Bo Rai, , Trat province. The basalt flowed into valley lying in north-south direction, over Carboniferous- Permian sandstone, and shale. It is consisted of olivine, feldspar and clinopyroxene, which embedded in brown vitreous matrix of feldspar, clinopyroxene, olivine, and opaque minerals. It is typical for Trat basalt to comprise large garnet crystals, clinopyroxene, spinel, ilmenite and ultramafic rock fragments. This basalt is defined as olivine nephelinite.

10.2.2.4 Ko Kut basalt was exposed on west of Ko Kut island, in south of Trat province. Jurassic sandstone was overflowed this basalt. The rock is fine grained, dark gray, and vesiculous. Mineral assemblage in the rock is mostly olivine with some clinopyroxene. The phenocrysts embedded in matrix of feldspar, clinopyroxene, olivine, and opaque minerals. 10.2.2.5 Ban Sae-O basalt occurs in small areas of Ban Sae-O, , Sa Kaeo province, south of Khorat Plateau. Red soil, weathering product of basalt covers area of 8 sq. km. The rock is fine grained, dark gray, and vesiculous. Hyalite opal commonly crystallized in vesicles. The rock is consisted of olivine, embedded in matrix of feldspar, clinopyroxene, and magnetite.

10.3 Metamorphic Rocks in Thailand Metamorphic rocks in the eastern region are distributed in Wang Somboon district, Nam Yen, Mueang, and Watthana Nakhon districts of Sa Kaeo province, with extension to Kabin Buri of Prachin Buri province (Salyapongse, 1992). The lower part of this IP/IT metamorphic rock is classified as amphibolite facies. Green colored rock is intercalated with light colored rock. The green colored rock is hornblende - plagioclase - quartz schist and the light colored rock is quartz - plagioclase - muscovite schist. Plagioclase is 15% anorthite. Some meta-andesitic tuff was found in the bottom part of the IP/IT metamorphic rock. Oscillatory zoning was found in plagioclase crystals. The light colored rock was proved to be deformed from ignimbrite, featuring plagioclase embedded in foliated felsite. Grade of metamorphism lowered to greenschist facies on top of the sequence. It comprises of large lenses of marble, interbedded with metavolcaniclastics. The metavolcaniclastics are composed of metaandesitic tuff, metarhyolitic tuff and meta-ignimbrite with interbedded metachert in top of the unit