Lithostratigraphy of the Phu Khat Formation in Nakhon Thai Region, Thailand : Preliminary Result

Pradit Nulay1,2, Chongpan Chonglakmani1 and Weerachai Paengkaew2

1School of Geotechnology, Institute of Engineering, Suranaree University of Technology, Thailand 2Department of Mineral Resources, Ministry of Natural Resources and Environment, Thailand

E-mail: Pradit_2004@Yahoo.com

Keywords: The Phu Khat Formation, Nakhon Thai, Lithostratigraphy.

ABSTRACT

This report presents the detailed lithostatigraphy of the Phu Khat Formation based on the fieldwork during the program of post-graduated study of the first author. The Phu Khat Formation is the uppermost red bed in Nakhon Thai region. It is composed of a sequence of conglomerate, purple sandstone, reddish brown sandstone and fine-grained clastic sedimentary rocks. These strata are largely interpreted to have been deposited by alluvial fan and fluvial (braided stream) which overlie unconformably on the large-scale cross-bedding aeolian sandstone of the Khao Ya Puk Formation. The well exposed five reference site sections are selected and describe in detail. The lower contact of the Phu Khat Formation is in general placed at the presence of successions of polymictic conglomerate with inverse graded bedding but in some other places it is place below the reddish brown sandstone. As the Phu Khat Formation is the upper most red-bed, therefore the upper contact has not been recognized. However, in general the upper part of the formation is defined by the present of thick bedded, reddish brown, medium- to coarse- grained sandstone of fluvial braided stream system. The formation can be subdivided into two large units, the upper and lower sequence. The lower sequences are mainly characterized by the succession of alluvial fan facies deposits. They are composed mainly of the conglomerate and purple, coarse- grained sandstone of stream flow deposits in proximal fan (the lower Phu Khat Formation) and continuous even parallel beds, reddish brown, fine- to medium- grained sandstone grading up into siltstone and claystone in more distal fan (the middle Phu Khat Formation). The upper sequence comprises a succession of thick bedded coarse-grained sandstone of the braided stream facies (the upper Phu Khat Formation) which overlies conformably on the lower sequence. The thickness, throughout the formation as followed the detailed section measurement in the Nakhon Thai region is approximately 490 m. The age of the Phu Khat Formation is considered as Late Cretaceous to Early Tertiary with reservation that the maximum age of deposition may not older than the Campanian age and not younger than the Ypresian age based on tectonostratigraphy (Nulay et al. in prepare). The formation represents a post reactivation of interior tectonic movements which was formed by erosion of an uplifted orogene and deposited in restricted subsidence basin. It can be lithostratigraphically correlated with the Phu Soay Dao formation in Laos (Assavapatchara and Raksasakulwong, 2010).

1. INTRODUCTION The Phu Khat Formation is composed of a sequence of polymictic conglomerate and purple to reddish brown sandstone. They are well exposed in Phitsanulok and Loei Provinces formed a broad NE-SW trending synclinorium named the Nakhon Thai basin centered on the town of Nakhon Thai in Phitsanulok Province (Fig.1). The succession of the Phu Khat Formation from various localities are illustrated diagrammatically in (Fig. 2 and 3)

1.1 Objective and area of study

The stratigraphy of red beds in Thailand has long been studied by many geoscientists especially the red bed of the non- marine Khorat Group in the Khorat Plateau. The sequence of Khorat Group extends beyond the rim of Khorat Plateau to the Nakhon Thai region (Booth & Sattayarak, 2011; Sha et al., 2012) which the two regions are separated by the succession of pre-Cretaceous rock in the Loei- Phetchabun Fold Belt (Fig. 1a). The stratigraphy of the upper most part of the red bed is mentioned only in Nakhon Thai region as the presence of the Phu Khat Formation. Nonetheless, the detailed lithostratigraphy of the Phu Khat Formation is still ambiguous. So the main objective of this study is focusing on the facies characteristic of the Phu Khat Formation based on the basis of lithology, sedimentary structure, outcrop geometry, palaeocurrent pattern, and fossil in order to grains a better understanding of lithostratigraphy of the Phu Khat Formation. The study area is in the Nakhon Thai region where the Phu Khat Formation is well exposed. Five reference site sections (Fig. 1b) are selected and study in detail.

1.2 Previous work Kosuwan (1990) conducted the geological mapping of Nakhon Thai region on scale 1: 50000 and proposed two new informal name formations: (in ascending order) the Khao Ya Puk Formation and the Phu Khat Formation. The Phu Khat Formation was followed the name of the Phu Khat mountain located in the north of Nakhon Thai district. In the study of Kosuwan (1990) the Phu Khat Formations was interpreted overlying conformably on the Khao Ya Puk Formation and was roughly interpreted as having been deposited by fluvial process. The age of the Phu Khat Formation was inferred as the Latest Cretaceous. Heggemann (1994) proposed the name of the Upper conglomerate formation instead of Phu Khat name based on sedimentary evolution of the Khorat Group (the Upper Triassic to Paleogene) in the Khorat Plateau and the northern Thailand. He concluded that the upper most red bed formation in the Khorat Group is characterized by the Upper Cretaceous conglomerate succession of alluvial fan facies with 500 m thick and well exposed in Nakhon Thai region. He reported that the Upper conglomerate formation was underlain conformably by the aeolian sandstone Khao Ya Puk Formation which is lithostratigraphically correlated with the Phu Tok Formation in the Khorat Plateau. On the other hand, Meesook et al. (2002) and Assavapatchara & Raksasakulwong (2010) reported the unconformably contact between the Khao Ya Puk Formation and the Phu Khat Formation based on the presence of abrupt facies change. They interpreted that the Khao Ya Puk Formation had been deposited by occasional meandering river and by winds in semi-arid grading upward to arid desert in Latest Cretaceous to early Cenozoic. In contrast, the Phu Khat Formation was interpreted to have been deposited by alluvial fan and meandering fluvial in the semi-arid condition. They proposed the depositional age of the Phu Khat Formation as more or less in the Latest Cretaceous to Early Cenozoic. In summary, the previous researchers have tended to concern on the geological mapping without detailed study of lithostratigraphy and facies characteristic of the Phu Khat Formation. Additionally, most of them gave the different ideas about the age and depositional environment of the formation. Therefore, the detail lithostratigraphy and facies characteristic is more favoured in the present study in order to fill up these gaps.

2. GEOLOGIC AND TECTONIC SETTING It is widely accepted that Thailand is divided into four tectonic zones, i.e., the Sibumasu Block, the Inthanon Zone, the Sukhothai Zone and the Indochina Block respectively from west to east (Fig. 1A). The Indochina Block and the Sibumasu Block are two main continental blocks that were originally derived from the Gondwana in the Devonian and the Permian respectively (Ueno and Hisada, 2001; Li et al., 2004; Sone and Metcalfe, 2008; Metcalfe, 2011; 2013). After a long time of northward drifting, the Sibumasu Block collided and amalgamated with the Indochina Block in the southern Eurasia margin by closing of the Palaeo-Tethys in western Thailand during the Triassic time (Ueno and Hisada, 2001; Sone and Metcalfe, 2008; Chonglakmani, 2002; Feng et al, 2005). The Inthanon and the Sukhothai Zones were interpreted as an accretionary complex resulted from closure of the Palaeo-Tethys and a remnant of Permo-Triassic island arc induced by subduction of the Palaeo-Tethys respectively (Sone and Metcalfe, 2008, Hara et al., 2012). The Nan-Uttaradit Suture Zone has been interpreted as a remnant of a closed back arc basin (Nan back arc basin) (Ueno and Hisada, 2001; Sone and Metcalfe, 2008) or small ocean (Yang et al., 2008) divided between the Sukhothai Zone and the Indochina Block. It is a narrow ophiolite zone which comprises Carboniferous to Permian Pha Som Metamorphic Complex and the Permo-Triassic? Pak Pat volcanic (Barr and Macdonald, 1987; Singharajwarapan and Berry, 2000). The Nan back arc basin may close and amalgamated with the Indochina Block in the Permian-Triassic (Sone and Metcalfe; 2008; Chonglakmani, 2002). The Loei-Phetchabun Fold Belt is formed as a high mountain range in western part of the Indochina Block. It comprises the succession of Paleozoic to Early Mesozoic sedimentary rocks and the igneous rocks of a long age range from the Silurian to Tertiary (Ueno and Charoentitirat, 2011; Khin zaw et al., 2014; Kamvong et al, 2014; Salam et al, 2014; Khositanont, 2008; Intasopa, 1993).

Fig. 1.- a) Geotectonic subdivision map of Thailand (After Barr and Macdonald, 1991; Ueno and Hisada, 2001). b) Simplified geological map of Nakhon Thai region based on the Geological Map of Thailand (1:2,500,000)published by Department of Mineral Resources (1999). Apatite fission track age from Racey et al. (1997) and Upton (1999 cited in Morley, 2004).The K-Ar mica dating age from Ahrendt et al (1993). c) Schematic stratigraphic column of Nakhon Thai region since the Cretaceous (after Racey et al., 1996; Racey, 2009; Assavapatchara and Raksasakulwong, 2010)

Fig. 2. Succession of the Phu Khat Formation showing classification and facies relation of the various parts.

The Nakhon Thai region is located between the two tectonic terranes, the composite Nan-Uttaradit Suture Zone & Sukhothai Zone to the west and the Loei-Phetchabun Fold Belt and Indochina Block to the east (Fig. 1A). Geographically the Nakhon Thai region is dominated by a series of parallel mountain chain which comprises the sequences of Late Jurassic to Cretaceous non-marine red bed of mainly the Khorat Group and the younger units (Racey, 2009; Racey et al., 1996). These strata form a broad NE-SW trending synclinorium situated on western part of the Indochina Block. The Khorat Group and younger units (the Maha Sarakham and the Khao Ya Puk or the Phu Tok Formations) are believed to extend westward and northward to the Nakhon Thai region of north-central Thailand and the Sayabouri basin of Laos respectively (Booth and Sattayarak, 2011; Morley, 2012; Chonglakmani et al., 2010; Heggermann et al., 1994). The Nakhon Thai basin was formed subsequently by uplifted Nan-Uttaradit Suture Zone to the west and the Loei- Phetchabun Fold Belt to the east during the Tertiary Himalayan Orogeny (Booth and Sattayarak, 2011; Racey et al., 1997). As shown in the geologic map (Fig. 1B) of the Department of Mineral Resource (1999) the red bed of the Khorat Group is well represented in both the Nakhon Thai region and the Khorat Plateau. The Khorat Group in the Khorat Plateau is overlain unconformably by the succession of Mid-Cretaceous rock salt of the Maha Sarakham Formation and the aeolian sandstone of the Late Cretaceous Phu Tok Formation (Fig. 1C) (Sattayarak and Polachan, 1990; Lovatt Smith et al., 1996; Racey, 2009). However in the Nakhon Thai region, the Khao Ya Puk Formation is proposed for the aeolian sandstone unit, although it is lithostratigraphically identical with the Phu Tok Formation (Meesook, 2011; Sha et al., 2012). So, the uppermost formations in Nakhon Thai region are composed of the Khao Ya Puk and the overlying Phu Khat Formation while on the Khorat Plateau only the Phu Tok Formation has been reported (Meesook et al., 2002).

3. STRATIGRAPHY 3.1 Definition and type section The Phu Khat Formation was named by Kosuwan (1990). He mentioned the name of Phu Khat Formation in geological map 1: 50,000 on Thai Royal Survey Department topographic map series L7018 sheet 5143 I&II of Ban Nam Khum and Amphoe Nakhon Thai. He stated that the formation consists of reddish brown to purplish brown fine- to medium- grained

sandstone interbedded with reddish brown siltstone. Conglomerate beds were also occasionally reported as intercalated beds which the pebble are composed of quartz, chert, sandstone, siltstone, volcanic and few pebble limestone. As mentioned by Kosuwan (1990) the sequence of the formation was conformably underlain by the thick bedded sandstone of the Khao Ya Puk Formation. The type section designated by Kosuwan (1990) is broadly located on a local road from Ban Nam Ton to Ban Nam Chaeng Phatthana (Phu Khat). It is represent on the Thai Royal Survey Department topographic map series L7018 sheet 5143 I&II in scale 1: 50,000. Unfortunately, Kosuwan defined this location as the type section but without detailed lithostratigraphic explanation. In this study, as a single complete exposure of the formation is nowhere exposed, five reference localities are thus designated and measured.

Reference locality 1. Km 40+300 Road No 1237 (Amphoe Chat Trakan - Ban Bo Phak) This section is along the road-cut between Amphoe Chat Trakan and Ban Bo Phak located at milestone KM 40+300 to KM 40+600 about two kilometer north to Ban Bo Phak. It is represented on Thai Royal Survey Department topographic map series L7018 sheet 5144 II in scale 1: 50,000 grid reference 693869 E, 1938258 N. The section is 300 m long which the total measured thickness of the formation is 85 m representing the lower Phu Khat (Fig. 3).

Reference locality 2. Km 24 to Km23+600 Road No 1286 (Ban Lao Ko Hok - Amphoe Na Haeo) This section is along the road-cut between Ban Lao Ko Hok and Amphoe Na Haeo located at milestone KM 24 to KM 23+600 about three kilometer east of Ban Lao Ko Hok up on to the peak of the mountain. The road was cut through the escarpment of mountain and perpendicular to the strike of the bed rock which shows eastward dipping. It is represented on Thai Royal Survey Department topographic map series L7018 sheet 5143 I in scale 1: 50,000 from grid reference 702081 E, 1934500 N to 702872 E, 1934027 N. The section is about 800 m long which the total measured thickness is 80 m representing the upper part of lower Phu Khat (Fig. 3).

Reference locality 3. Km 0 to Km 3+600 Road No 2195 (Amphoe Na Haeo - Ban Pak Man) The Khao Ya Puk and the Phu Khat Fms are observed in this section which is along the road cut between Amphoe Na Haeo and Ban Pak Man located at milestone KM 0 to KM 3+600. The road was cut perpendicular through the mountain which lies in N-S trending in the western flank of major syncline structure. It is represented on Thai Royal Survey Department topographic map series L7018 sheet 5243 IV in scale 1: 50,000 from grid reference 720854 E, 1935824 N to 723495 E, 1934991 N. The section is about 3,600 m long and the total measured thickness of the two formations is 375 m including complete section of the Khao Ya Puk Formation about 265 m and the middle Phu Khat 110 m (Fig.3).

Reference locality 4. Ban Nam Ton to Ban Nam Chaeng Phatthana (Phu Khat)

This section is along the road climbing up to the top of the Phu Khat Mountain from Ban Nam Ton to Ban Nam Chaeng Phatthana (Phau Khat). The detailed section was measured where the out crop are well exposed at the contact between the upper Khao Ya Puk Formation and the Phu Khat Formation. It is represented on Thai Royal Survey Department topographic map series L7018 sheet 5143 II in scale 1: 50,000 grid reference 70246 E, 1904884 N. The section is 3,000 m long which the total measured 495 m thick including the upper Khao Ya Puk Formation 165 m and the middle and /or upper Phu Khat Formation 330 m (Fig. 3).

Reference locality 5. Dat Juang Waterfall

This section is located at six kilometer north of Ban Khok Phak Wan. The detail section was measured from

the waterfall namely Dat Juang. The out crop of thick bedded, cross-bedding sandstone are well exposed and

showing horizontally dipping. It is represented on Thai Royal Survey Department topographic map series

L7018 sheet 5143 IV in scale 1: 50,000 grid reference 676387 E, 1920590 N. The total measured thickness is 50

m representing the upper Phu Khat (Fig.3).

3.2 Contact 3.2.1 Lower contact Even though the contacts of the lower part of Phu Khat Formation in the field have not been found in this study, the unconformity contact either disconformity or paraunconformity are favoured based on the following features;

(I) The presence of polymictic conglomerates at the lower Phu Khat. The lower part are characterized by the sequence of polymictic, conglomerates which in some conglomerate beds show coarsening upward associated with cross stratified in thin bedded sandstone. These features are likely to indicate the sub-aerial fan environment (Stanistreet & McCarthy, 1993; Miall, 1996). The cobble mudstone and siltstone clasts formed in the conglomerates are derived from the older formation (probably from the Khao Ya Puk Formation) presumably as the resulted of tectonic uplifting.

(II) The presence of abrupt facies change between the Phu Khat Formation and those underlying the Khao Ya Puk Formation. Only one section at reference locality 4 has found the contact between the Phu Khat Formation and the older formation (Khao Ya Puk). It seems to be somewhat conformed bed between the upper Khao Ya Puk Formation and the Phu Khat Formation. However, it is only the middle Phu Khat Formation presented while the lower part is absent. Sedimentary structure of the Phu Khat Formation including cross-bedding, rip-up clast, channel structure, are apparent. These sedimentary structures indicate characteristic of fluvial process environment in more humid environment which clearly contrast with the underlying the Khao Ya Puk Formation. Sedimentary structure of the Khao Ya Puk Formation, i.e., giant cross bedding, high angle foresets cross bedding (30-35) with tangential base contact, very well sorted, high textural maturity and coarsening upward sequence of thin bedded sandstone, are typical of the aeolian sand dune deposit in more arid environment (Tucker, 2003; Swezey, 1998; Pye & Tsoar, 2009; Pye, 1995). All of these features give the evidence that the Khao Ya Puk Formation may formed by aeolian dune in desert palaeoclimate (Meesook, 2011; Hasekawa et al, 2010) and that is obviously contrasting with the Phu Khat Formation.

(III) Lack of structural dip in the Phu Khat Formation. The Phu Khat Formation shows beds dipping at a few degrees particularly in the sandstone beds. They show dipping angle about 5 to 10 degrees which differs apparently from those underlying sandstone beds of the Khao Ya Puk Formation that show high angle dipping 25-30 degree in average within continuous section. This feature is interpreted by us as supporting evidence for an unconformity contact.

(IV) The contrasting detrital zircon age cluster between the Phu Khat and the Khao Ya Puk Formations. One notable difference between the Phu Khat and the upper Khao Ya Puk Formations is detrital zircon age distribution (Nulay et al., in prepare). Such a difference of provenance ages across the boundary of the two formations may imply the presence of a tectonic event or a major erosional event subsequent to the deposition of the Khao Ya Puk Formation. Based on all of these features above we prefer the unconformity contact between the Phu Khat Formation and the underlying formation in out interpretation.

3.2.2 Upper contact From all reference locality sites, the upper contact of the Phu Khat Formation has not been found in the Nakhon Thai region. The reason for an absence of the contact in the upper part of the Phu Khat Formation may due to the Phu Khat Formation is the upper most part of the red bed sequence in this region which is consistent with the conclusion of Heggermann et al. (1994).

3.3 Lithology The Phu Khat Formation is composed mainly of conglomerate, conglomeratic sandstone and litharenite in the lower part. The middle part is dominated by alternate bed of sandstone, siltstone and mudstone with occasionally trace fossil in calcareous mudstone. The upper part is chiefly characterized by thick bedded litharenite with tubular cross bedding.

3.3.1 The lower Phu Khat Conglomerate; The conglomerates beds are well exposed and dominated in the reference locality 1 with rarely in reference locality 2. Their composition and texture is also varies. In the reference locality 1 the pebble and cobble are generally rounded to subangular which is composed of quartz, chert, sandstone, volcanic clastic, mudstone, siltstone and few pebble limestone. Resistant pebbles consisting of quartz, chert and sandstone comprise 70 % of the total clasts. Red chert clasts are common. The volcanic clast comprises 22% and the remainding 8% is mudstone and siltstone. The conglomerate bed can be classified as polymictic conglomerate. These polymictic conglomerates are matrix to clast supported. The clasts are ranging from 2- 14 cm in size. The matrix is sandy to siltly and normally is purple to reddish purple in colour. The conglomerate beds are 50 cm to 8 m thick. They often show coarsening up ward sequence and structureless frabric which are generally intercalated with thin-bedded, purple and cross stratified litharenite. The conglomerate in referent locality 2 is slightly different from the referent locality 1. The range of clast sizes are smaller than locality 1 with ranging from 0.5 cm to 4 cm in average. The clasts are consist mainly of quartz, chert, sandstone, volcanic, and mudstone and siltstone which shown more rounded than those locality 1. The matrix is siltly to sandy and normally is purple. Subsequently, in this locality conglomerate beds is thinner than locality 1 with 1-2 m thick The clasts composition from two sites locality are identical. Both of them contain mainly of quartz, chert, sandstone, volcanic, mudstone and siltstone. The textural maturity increase from locality 1 to locality 2 as shown in thickness and clast size decrease from locality 1 to locality 2 as same as the roundness index increase from the locality 1 to locality 2.

Sandstone; the sandstone beds are well exposed in both localities (1 and 2). They tend to be a main unit in locality 2 and it is minor unit in locality 1.

In referent locality 1 the sandstone are chiefly belong to litharenite with quartz contain 58%, feldspar 6% and predominant volcanic rock fragment 36 % (Fig. 4a). The rock is principally coarse- grained and poorly sorted texture (Fig. 4b). It is reddish brown to reddish purple in color, angular- to sub rounded- grains. The bed is ranging from medium to very thick bedded and capped commonly by fine grain sediment which in turn these beds are cut through by the conglomerate beds. Some beds show lenticular laminated beds within the conglomerate beds. Cross stratified are generally found in sandstone bed e.g., parallel laminated and some small dune form. Moreover, complete polygons mud-cracks with V-shape also have been found in some sandstone beds. In referent locality 2 the sandstones are also belong to litharenite following the classification of Folk (1974) (Fig. 4a). Their component is similar to the referent locality 1 with quartz, feldspar and rock fragment contain 58%, 4% and 38 % respectively. Their grained size is ranging from fine- to coarse- grained, angular- to rounded- grains with poorly to moderately sorted texture (Fig. 4c). Some sandstone beds are conglomeratic containing rounded pebble of volcanic and sandstone. The beds range in thickness from thin to very thick bedded. These beds are generally capped by red siltstone and/or mudstone. The sandstone to siltstone and/or mudstone ratio is likely to be more than 3. Red siltstone and mudstone; Theses lithologic type are found as either interbedded or intercalated beds within the conglomerate beds in referent locality 1 and sandstone beds in locality 2. Some parts are shown as the capped beds on top of the sandstone in referent locality 2. Siltstone and mudstone are generally 1 cm to 2m thick. They are composed predominantly of clay and silt which are mostly siliceous and calcareous. Subsequently, borrow trace fossil are normally observed within the thick beds calcareous shale and siltstone in locality 2.

3.3.2 The middle Phu Khat The middle Phu Khat Formation is mainly characterized by reddish brown sandstone and reddish brown siltstone and mudstone. In this study they are observed in the referent locality 3 and 4 which the fining upward sequence is commonly observed throughout the section. Generally, in both referent localities the middle Phu Khat Formation have been found somewhat overlie unconformably on top of the older formation (the Khao Ya Puk Fm.). The detail of their lithology is as following.

Sandstone; the sandstone are identical to the lower Phu Khat which is predominantly belong to litharenite with quartz contain 65% , feldspar 3% and rock fragment 32 % in average (Fig.4). Nevertheless, they grained size seem to be smaller than the lower Phu Khat and display more textural maturity (Fig. 4d). They are generally reddish brown, very fine- to medium- grained, sub angular- to rounded- grains, moderately sorted texture. The beds are ranging from medium- to very thick- bedded but in general the thickness decrease up into the upper sequence and capped commonly by fine-grained sediment showing continuous, even, parallel beds with fining and thinning upward sequence. The contacts between beds are sharp planar contact. Cross stratified are also found. It is chiefly by plane bedding and cross laminated with some tubular cross-bedding as well as ripple mark with linguoid crest in some beds. Complete polygonal mud-cracks are also observed locally. The palaeo-current obtained from cross stratified have shown SEE ward flow direction. Siltstone and mudstone; They are characterized by reddish brown to reddish purple siltstone and/or mudstone interbedded with sandstone beds. The sandstone to silt and shale ratio ranges from 3:1 to 1:1 from the lower sequence to upper sequence. Siltstone and mudstone are thinly- to thick- bedded which the thickness gradually decreases from the lower to the upper of unit.

3.3.3 The upper Phu Khat The upper Phu Khat Formation is mainly characterized by thick-bedded sandstone with cross-stratification. It is usually well exposed on the high mountain with elevation more than 1000 m above mean sea level and shows escarpment of the mountain. The well exposure out crop is in the reference locality 5 formed as waterfall namely Dat Jung. Lithologically, sandstone is mainly litharenite with quartz contain 56% , feldspar 6% and rock fragment 32 % in average (Fig.4a). Generally, it displays less textural maturity than the middle Phu Khat but is quite similar with the lower Phu Khat. It is reddish brown to light red in color, fine- to coarse- grained, angular- to sub round- grains, poorly- to moderately- sorted texture. The beds are ranging from thick- to very thick- bedded with 1m-4m thick in average. The sharp erosional contact and pebble lags are also evident. It is calcareous in some beds but the most are siliceous. The carbonaceous bed of siltstone and mudstone are observed as intercalated bed. Cross-stratifications are generally found in sandstone beds e.g., pane bedding and tabular cross-bedding with overturned foresets. Subsequently, asymmetry ripple marks are also observed. All cross stratified indicate SSW ward palaeocurrent direction.

3.3.4 Summary The lithology of the Phu Khat Formation is composed of three main parts i.e., the lower, the middle and the upper parts (Fig.5). The lower part consists mainly of the polymictic conglomerates, conglomeratic sandstone and poorly sorted litharenite sandstone. The polymictic conglomerates are matrix to clast-support within the matrix of sand. They are lacking of structure fabric but the cross-stratifications are recognized which indicate SEE ward palaeoflow direction in some part i.e., sandstone and clast-support conglomerates. The middle part is characterized by the tabular geometry litharenite and siltstone & mudstone showing fining upward sequence. The compositions are quite similar to sandstone in the lower part which palaeocurrent are also identical with SEE ward palaeo flow direction. The sandstone in the middle part displays more textural maturity than the lower part. The upper part is also predominated by poorly sorted litharenite. Most of them are chiefly shown thick- bedded, cross-stratified sandstone that clearly indicates fluvial process system with palaeo-current perpendicular to both former parts.

Fig. 4. Sandstone classification and Photomicrographs of clastic sandstone of the Phu Khat Formation. a) sandstone classification of the Phu Khat Formation after Folk (1974). b) coarse- grained, poorly sorted litharenite of the locality 1. c) medium- grained, poorly sorted litharenite of the locality 2. d) moderately sorted litharenite of the locality 3. e) poorly sorted litharenite of the locality 5. All photographs were under cross polarized. Q: Quartz, F: Feldspar, Lv: Volcanic lithic fragment.

3.4 Thickness and Extend The Phu Khat Formation is present throughout the area of this report and extends somewhat northeast to north into Uttaradit province and beyond the Thailand border into the Ken Thao area in Laos. It is bounded by the Nan-Uttaradit Suture zone to the west and Loei- Phetchabun Fold Belt to the east. In the report area the formation exposes in Chat Trakan and Nakhon Thai district of Phitsanulok province and Na Haeo and Dan Sai district of Loei province. The rock sequence are generally gently folded into broad synclines and anticlines which NE-SW trending parallel to the trending of Nan-Uttaradit Suture Zone.

Firstly, Kosuwan (1990) given the name of the Phu Khat Formation but did not mention thickness of the formation at type section. Heggermann (1994) suggested that the thickness of the Phu Khat Formation was about 500 m which coincides with Meesook et al. (2002) who stated that the Phu Khat Formation was 150 - 500 m thick. In this study the thickness of the Phu Khat Formation was measured throughout the three parts and indicates following Walther'law that total vertical thickness of the formation is 490 m thick.

3.5 Age and Correlation 3.5.1 The Age Due to a paucity of the fauna and flora, the age of the Phu Khat Formation has been generally inferred from the stratigraphic relation by several researchers. Heggermann (1994) explained the occurrence of the Phu Khat Formation (the Upper conglomerate Formation) as a result of the intense deformations in the Upper Cretaceous to Early Paleogene during the collision of Indian Plate with the Eurasian Plate. Consequently, the thrust fault block along the Nan- Uttaradit Suture Zone were reactivated and eastward thrust over the younger red bed of the Mesozoic Khorat Group. As the same time Heggermann et al. (1994) stated that the Khorat Basin had been subsided which is restricted to Nakhon Thai area and formed as a restricted basin for the alluvial fan sediment at the front of thrust fault zone. The sediments were fed by the thrust fault block and offered the thick succession of alluvial fan deposited in the Nakhon Thai area. So, based on the assumption above, Heggermann (1994); Heggermann et al. (1994) point out that the age of the Phu Khat Formation is the Late Cretaceous (Campanian - Maastrichtian). This given age is similar with Meesook (2002, 2011) who suggest that the age of the Phu Khat Formation is more or less at the Late Cretaceous - Tertiary based on its lithostratigraphic position that overlie the Late Cretaceous of the Khao Ya Puk Formation (Assavapatchara & Raksasakulwong, 2010).

In this study the age of Phu Khat Formation cannot indicate directly from absolute age due to a lacking of fossils. However, the relative inferred age which is considered from the previous studies together with tectonostratighaphic and detrital zircon grain dating indicate that the maximum deposition age of the Phu Khat Formation should not older than Campanian and not younger than Ypresian age (Nulay et al. in prepare).

3.5.2 Correlation Based on the lithostratigraphy and stratigraphic position, the Phu Khat Formation can be correlated with the Phu Soay Dao formation in Laos (Assavapatchara & Raksasakulwong, 2010). The rocks in both regions contain similar lithology. In Laos's side, the Phu Soay Dao formation is composed mainly of brown to maroon sandstone, siltstone and claystone with conglomerate in place. Moreover, its stratigraphic position is identical to the Phu Khat Formation where it is underlain unconformably by the Say Som Boun Formation which its lithostratigraphy equivalent to the Khao Ya Puk Formation (Assavapatchara & Raksasakulwong, 2010).

3.6 Origin and Depositional Environment The inferred depositional environment of the Phu Khat Formation are mainly based on facies study including lithology, sedimentary structure, palaeocurrents and geometry contained in the rock type that make up the unit. The formation is interpreted to have been deposited in alluvial fan and fluvial condition. It is considered as a result of post reactivate movement of Nan-Uttaradit Suture Zone as a thrust fault of the rock sequence in the suture zone over the younger red bed Khorat Group. Owing to the uplifting of the thrust fault block, the erosion takes place and feed the sediments to an accommodation area. The conglomerates beds in the lower part consist mainly of polymictic conglomerates with matrix to clast-support within the matrix of sand. Their sedimentary structures are composed of the channel structures and cross stratified in the sandstone and clast-support conglomerate. While the matrix support conglomerate is structureless. Some coarsening upward and fining upward are generally observed. Based on all feature above, it might imply the depositional condition under the alluvial fan deposit (Mial, 1996, Stanistreet & McCarthy, 1993; Blair, 1987; Heaward, 1977). The occurrence of coarsening upward in matrix supported conglomerates in the lower Phu Khat may reflect the

growth of fan during continuous faulting i.e., uplifting of source area or subsidence of fan region. The fan tends to prograde toward low-land area. In turn fining upward may imply the short phase of faulting and, subsequent by the retreat of the scarp front and lowering of the relief in the high land (Einsele, 1992). The presence of channel structure and cross stratified may reflect the confined stream flow which palaeo-current obtained from these structures indicate SEE ward flow direction. Non arid climate alluvial fan (possibly semi-arid) may also evident by the occurrence of fining upward sequence in sandstone and clast support conglomerate together with channel structure and the cross stratified which is rare in arid alluvial fan (Blair & McPherson, 1992). In addition the presence of complete polygonal mud-crack may elucidate sub aerial fan system (Stanistreet & McCarthy, 1993; Nichols, 2009). The middle Phu Khat is mostly located to the eastern area. The rocks are composed of predominantly alternate tabular geometry of sandstone and caped by thin bedded of siltstone & mudstone. The composition and palaeo-current of sandstone is identical to the sandstone in the lower Phu Khat which possibly indicated the same source area. They show more textural maturity and their gained size is generally smaller than the lower Phu Khat. The presence of these features may be the effect of gained sorting during transportation far away from the source area. The plane bed and cross-laminated with some tubular cross bedding and the ripple mark with linguoid crest are main cross-stratified in the middle Phu Khat. These sedimentary structures are likely to indicate fluvial environment within the shallow water in fluctuation of discharge (Selley, 2000; Collinson, 1996; Heward, 1977). Additionally, the complete polygonal mud-crack is evident of the sub-aerial or ephemeral flow (Tucker, 2003; Nichols, 2009). The paleo-current obtained from cross stratified also indicate flow direction the same as lower Phu Khat. Based on all of these facies association together with the lacking of the occurrence of bar microform and the low angle epsilon cross bedding which is diagnostic feature of lateral migration meander channel (Tunbridge, 1984; Mial, 1985; Hampton & Horton, 2007), the middle Phu Khat Formation, therefore, is interpreted to have been deposited by ephemeral stream with poorly confined to unconfined, shallow high energy drown fan environment. The source area of the lower and the middle Phu Khat may be derived from the western area as indicated by palaeo-current and an increasing of textural maturity as well as an increase of fine- grained sediment from west to east. The upper Phu Khat Formation is mainly characterized by thick- bedded sandstone with tabular cross bedding and some plane- bedding of sandstone. Channel scoured and pebble lags can be found at the base of sandstone beds. Litology of sandstone is also similar with the lower and the middle Phu Khat which is litharenite dominated by volcanic rock fragment. However, they are less texture maturity than the middle part. The palaeo-current indicated the SWW ward flow direction which is perpendicular to both former parts. The occurrence of thick bedded tabular cross bedding with channel scoured structure and rare flood plain sequence are likely to indicate braided stream environment (Collinson, 1996; Selley, 2000). The intercalated of tabular cross bedding and horizontal laminated sandstone may suggest oscillation between high flow and low flow condition of the stream (Hampton & Horton, 2007). The presence of fluvial braided stream in the upper Phu Khat Formation may imply a gradual change from semi-arid in the lower and middle to semi-humid environments in the upper Phu Khat and/or represent the last stage of basin filled.

Fig. 5. Schematic stratigraphic column of the Khao Ya Puk and The Phu Khat Formation. For the Khao Ya Puk Formation adapted from Monjai (2006) (Not to scale)

4. SUMMARY

The Phu Khat Formation can be subdivided into three parts i.e., the lower, the middle and the upper parts with total thickness about 490 m which lie unconformably on the Khao Ya Puk Formation. It is well exposed in Chat Trakan and Nakhon Thai district of Phitsanulok province and Na Haeo and Dan Sai district of Loei province and probably extends somewhat northeast to Uttaradit province and beyond Thailand border into the Ken Thao area in Laos. The lower Phu Khat is consists mainly of the sequence of proximal alluvial fan which its lithology are composed predominantly of conglomerate, sandstone and some siltstone. The coarsening upward and fining upward sequence can be found through the sequence. The middle Phu Khat is characterized by tubular geometry sandstone beds with chiefly horizontal stratified and gradually grade upward into fine grain siltstone and mudstone at the upper unit. Based on its facies characteristic, the middle Phu Khat is interpreted to have been deposited by ephemeral stream with poorly confined to unconfined, shallow high energy distal fan environment under non arid palaeoclimate (possibly semi-arid). Palaeocurrent direction flow indicated that both parts (the lower and middle) show SEE ward flow direction. The pretography study also reveals the same rock type in both parts as litharenite which volcanic rock fragment is dominant. Nonetheless, the sandstone in the middle Phu Khat displays higher textural maturity than the lower Phu Khat. So based on the palaeocurrent measurement and an increasing of textural maturity together with an increase of fine- grained sediment from NWW to SEE give the evident that source area of alluvial fan were likely from NWW. In contrast, the upper Phu Khat Formation is chiefly characterized by the sequence of braided fluvial sandstone. It is composed predominantly of thick bedded sandstone with tubular cross bedding intercalated with plane-bedding. The bases of sandstone are commonly sharp erosive with some pebble lags. The palaeo-currents obtained from cross-stratification indicate SWW ward flow direction which is perpendicular to the both former parts. The presence of the fluvial braided stream in the upper Phu Khat Formation may imply that the environment gradually change from semi-arid to semi-humid. The age of the Phu Khat Formation is still debatable because of a paucity of fauna and flora. Most of the proposed age is relative age which is inferred from its stratigraphic relation. In this study the proposed age is Late Cretaceous - Early Paeogene (not older than Campanian and not younger than Ypresian) based on the tectonostratigraphic together with stratigraphic relation. REFERENCES Ahrendt, H., Chonglakmani, C., Hansen, B. T., & Helmcke, D. (1993). Geochronological cross section through

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