Thailand Was a Desert' During the Midcretaceous: Equatorward Shift

Home , Geology of Thailand, Khok Kruat Formation, Khorat Group

Island Arc (2010) 19, 605–621

Thematic Article ‘Thailand was a desert’ during the mid-Cretaceous: Equatorward shift of the subtropical high-pressure belt indicated by eolian deposits

(Phu Thok Formation) in the Khorat Basin, northeastern Thailandiar_728 605..621

HITOSHI HASEGAWA,1,*† SUVAPAK IMSAMUT,2 PUNYA CHARUSIRI,3 RYUJI TADA,1 YU HORIUCHI5 AND KEN-ICHIRO HISADA4 1Department of Earth and Planetary Science, the University of Tokyo, Tokyo 113-0033, Japan (email: [email protected]), 2Department of Mineral Resources, Bureau of Geological Survey, Bangkok 10400, Thailand, 3Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand, 4Graduate School of Life and Environment Sciences, University of Tsukuba, Tsukuba 305-8572, Japan, and 5Institute for Geo-Resources and Environment, AIST, Tsukuba, Ibaraki 305-8567 Japan

Abstract The Tibetan Plateau is a key factor in controlling the present-day climate and atmospheric circulation pattern in Asia. The pattern of atmospheric circulation after the uplift of the plateau is well known, whereas direct evidence is lacking regarding the nature of the circulation pattern prior to the uplift. The distribution of desert directly reﬂects the position of the subtropical high-pressure belt, and the prevailing surface-wind pattern recorded in desert deposits reveals the position of its divergence axis. Cretaceous eolian sandstone of the Phu Thok Formation is extensively exposed in the northern Khorat Basin, northeastern Thailand. We conducted a sedimentological study on this formation to reconstruct temporal changes in the latitude of the subtropical high-pressure belt in low-latitude Asia during the Cretaceous. Spatio-temporal changes in the paleo-wind directions recorded in the Phu Thok Formation reveal that the Khorat Basin mainly belonged to the northeast trade wind belt and subtropical high-pressure belt was situated to the north of the Khorat Basin during the initial stages of deposition, shifted southward to immediately above the basin during the main phase of deposition, and then shifted northward again to the north of the basin during the ﬁnal stages of deposition. The paleomagnetic polarity sequence obtained for the Phu Thok Formation comprises three zones of normal polarity and two of reversed polarity, correlating to chrons M1n to C34n of the geomagnetic polarity time scale. This result suggests that the Phu Thok Formation is mid-Cretaceous in age (from c. 126 Ma to c. 99–93 Ma), similar to the age of eolian sandstone in the Sichuan Basin, southern China (the Jiaguan Formation). These results, in combination with paleo-wind direction data, suggest the development of low-latitude desert and an equatorward shift of the subtropical high-pressure belt (relative to the present-day) in Asia during the mid- Cretaceous.

Key words: Cretaceous, desert, magnetostratigraphy, paleo-wind, subtropical high- pressure belt, Thailand.

INTRODUCTION

*Correspondence. The present-day climate of Southeast Asia is char- †Present address: Department of Natural History Science, Graduate school acterized by seasonal change between an arid of Science, Hokkaido University, Sapporo 060-0810, Japan. winter and a humid summer, resulting from Received 14 December 2009; accepted for publication 18 May 2010. the occurrence of distinct winter and summer © 2010 Blackwell Publishing Asia Pty Ltd doi:10.1111/j.1440-1738.2010.00728.x 606 H. Hasegawa et al. monsoons. Although the timing and cause of Asian (Fig. 1; Sattayarak 1983; Meesook 2000; Depart- monsoon development remain topics of debate, the ment of Mineral Resources (DMR) 2001; Charusiri uplift of the Tibetan Plateau and its orographic et al. 2006). The eolian sandstone of the Phu Thok influence is thought to be a key factor for the pres- Formation is important because it provides direct ence of monsoonal circulation in Southeast Asia evidence of desert development in low-latitude (An et al. 2001; Abe et al. 2003). In contrast, the Asia during the Cretaceous; however, no previous climate in other regions at the same latitude, study has examined the detailed sedimentology where no such high and large plateau exists, is and chronology of the formation. zonal as a result of meridional atmospheric circu- The purpose of this study is to reconstruct the lation. Thus, a subtropical arid climate generally lithofacies of the Phu Thok Formation and to prevails beneath the subtropical high-pressure identify spatio-temporal changes in paleo-wind belt at 20°–30° latitude in both hemispheres (Biga- directions recorded in the formation. Based on rella 1972; Livingstone & Warren 1996). Given that these data, we evaluate the depositional environ- the uplift of the Tibetan Plateau commenced after ment of the formation and the paleo-position of 40 Ma (Tapponnier et al. 2001), a subtropical arid the subtropical high-pressure belt in this area climate would have prevailed in Southeast Asia during the Cretaceous. We also constrain the age during the Cretaceous (Fluteau et al. 2007). of desert development in northeastern Thailand Deserts are the direct products of meridional based on a correlation of the paleomagnetic polar- atmospheric circulation. Modern deserts are gen- ity sequence in the Phu Thok Formation, as erally developed under the subtropical high- obtained by Imsamut (1996), with that in the Phu pressure belt as a result of downwelling of the Wua section from the northernmost part of the Hadley circulation. Hence, the equatorward and Khorat Basin. poleward parts of desert areas are dominated by trade winds and westerlies, respectively (Bigarella MATERIALS AND METHODS 1972; Livingstone & Warren 1996). Eolian dunes in desert areas migrate leeward of the wind, thereby GEOLOGIC AND STRATIGRAPHIC SETTING recording the direction of the prevailing surface- wind pattern (dominantly the wintertime wind Non-marine Late Jurassic–Cretaceous deposits flow) in the form of large-scale cross-sets. There- are widespread in the Khorat Basin, northeastern fore, the distribution of desert deposits and pre- Thailand (Fig. 1). The basin belongs to the vailing surface-wind patterns recorded in such Indochina block, which is thought to have moved deposits provide direct information on the paleo- and rotated during the Paleogene in response to position of the subtropical high-pressure belt and India–Eurasia collision (Chen et al. 1993; Ritcher its divergence axis. et al. 1993; Charusiri et al. 2006). The paleo- Cretaceous eolian sandstones are widely distrib- position of the Indochina block during the Creta- uted in low- to mid-latitude sedimentary basins in ceous remains a point of controversy. Chen et al. the Asian interior (Jiang & Li 1996; Jiang et al. (1993) argued that the Indochina and South China 1999, 2001; 2008; Hasegawa et al. 2009). The occur- blocks were located between 20°N and 30°N during rence of such eolian deposits in terrestrial sedi- the Cretaceous; however, a recent paleomagnetic mentary basins is thought to provide direct study (Charusiri et al. 2006) demonstrated that the evidence of desert development in areas beneath paleo-position of the Khorat Basin in the south- the subtropical high-pressure belt during the eastern Indochina block during the Cretaceous Cretaceous. was between 16.3 Ϯ 2.3°N and 21.6 Ϯ 4.0°N, with Sattayarak (1983) was the first to report Creta- 20–25° of clockwise rotation relative to the present, ceous eolian sandstone in the Khorat Basin, north- indicating that during the Cretaceous the Khorat eastern Thailand. The Khorat Basin is composed Basin was located at a much lower latitude than mainly of a Late Jurassic–Cretaceous non-marine that of the South China block (N25.5°–29.6°; Enkin sedimentary sequence (the Khorat Group) that et al. 1991). unconformably overlies deformed Paleozoic rocks Late Jurassic–Cretaceous non-marine deposits (Sattayarak 1983; Racey et al. 1996; Meesook of the Khorat Group consist of the Phu Kradung, 2000). The eolian sandstone deposit (the Phu Thok Phra Wihan, Sao Khua, Phu Phan, Khok Kruat, Formation), exposed in the northern part of the Maha Sarakham and Phu Thok Formations, in Khorat Basin, is thought to represent the youngest ascending stratigraphic order (; Sattayarak 1983; part of the sedimentary succession in the basin Racey et al. 1996; Meesook 2000; Charusiri et al. © 2010 Blackwell Publishing Asia Pty Ltd Thailand was a desert in the mid-Cretaceous 607

EE101°101° E102° E103° E104° E105° 050100(km) N 4 1 XX’’ 5 2 3 N18°

6 SSakhonakhon NNakhonakhon bbasinasin X 1100 7 9 8 N17° NNakhonakhon TThaihai basinbasin ((b)b)

South China WSW ENE NNakhonakhon ThaiThai basinbasin SSakhonakhon NakhonNakhon basinbasin MMyanmeryanmer LLaosaos

KKrr ddetailetail inin (B)(B) TThailandhailand ((c)c) CCambodiaambodia TThaihai VVietnamietnam LEGEND landland Phu Khat Fm (late Cretaceous) Phu Thok Fm (Apt.-Tur.)

Maha Sarakham Fm (Alb?-Cen.) Khok Kruat Fm (Apt?) MMalaysiaalaysia Phu Phan, Sao Khua, and Pre-Khorat Rocks SSumatraumatra ((a)a) Phra Wihan Fms (Ber.-Bar.)

Fig. 1 A schematic geological map of the Khorat Basin, northeastern Thailand. (a) A regional map of mainland Southeast Asia with the Khorat Basin (shaded area). (b) A geological map and location of the study sites of the Phu Thok Formation in northern Khorat Basin (modiﬁed after DMR 2001). Numbers indicate several studied sites; 1) Phu Wua; 2) Phu Langka; 3) Phu Thok; 4) Chet Si; 5) Phu Sing; 6) Wat Ahong; 7) Pak Man; 8) Na Haew; 9) Nakon Chum; 10) Chat Trakan. (c) A cross-section along the line X–X’ of (b).

2006 Fig. 2). The Phu Kradung, Phra Wihan, Sao Sarakham Formation consists of alternating beds Khua, Phu Phan, and Khok Kruat Formations are of halite, anhydrite, and siliciclastic redbeds (mud- composed mainly of alternating beds of mudstone, stone and siltstone, including calcretes containing siltstone, sandstone, and conglomerate, indicating desiccation cracks), indicating deposition within deposition in meandering and braided fluvial envi- an evaporitic inland playa lake (Denchok 2006) ronments; these deposits unconformably overlie and/or a shallow saline pan environment subjected deformed Paleozoic rocks (Fig. 1). The Phu to repeated inflows of marine water (El Tabakh Kradung Formation is assigned a Late Jurassic et al. 1999). The age of the Maha Sarakham age (Meesook 2000). Based on palynological evi- Formation is considered to be mid-Cretaceous dence, the age of the coal-bearing Phra Wihan For- (Albian–Cenomanian) based on the K/Ar (93.7 Ϯ mation is assigned to the Berriasian–Barremian; 2.7 Ma) and Rb/Sr (95.8 Ϯ 15 Ma) ages of potash consequently, the fluvial successions of the Phra and rock salt deposits (Hansen et al. 2002). Wihan, Sao Khua, Phu Phan, and Khok Kruat For- The Phu Thok Formation is exposed in the mations are generally regarded to be Early Cre- northeastern part (Sakhon Nakhon sub-basin) taceous in age (Racey et al. 1996; Meesook 2000; and northwestern part (Nakhon Thai sub-basin) Charusiri et al. 2006; Fig. 2). of the Khorat Basin (Fig. 1). In the Nakhon Thai The Khok Kruat Formation is unconformably sub-basin, the Phu Thok Formation overlies the overlain by the Maha Sarakham and Phu Thok fluvial-dominated Khok Kruat Formation (Satta- Formations (Sattayarak 1983; Racey et al. 1996; yarak 1983; Imsamut 1996; DMR 2001). In con- Meesook 2000). The Maha Sarakham Formation is trast, a drill-core investigation by DMR (2001) exposed widely in the northeastern part of the in the Sakhon Nakhon sub-basin revealed that Khorat Basin (Fig. 1). A recent drill-core investi- the Phu Thok Formation overlies the evaporite- gation by DMR (2001) revealed that the Maha dominated Maha Sarakham Formation (Charusiri © 2010 Blackwell Publishing Asia Pty Ltd 608 H. Hasegawa et al.

Sattayarak Racey et Time Meesook (2000) This study (Ma) (1983) al. (1996) Period Stage Maa. 70 ?

Cam. ? 80 PPhuhu KhatKhat San. ? PPhuhu ThokThok Con. ? 90 Tur. PPhuhu TThokhok ? ? Cen. ? 100 MMahaaha SSarakhamarakham MMahaaha SSarakhamarakham PhuPhu Alb. MMahaaha SSarakhamarakham ThokThok MMahaaha 110 SarakhamSarakham ?

Apt. 120 KKhokhok KKruatruat KKhokhok KruatKruat KKhokhok KKruatruat ? ? Bar. PPhuhu PPhanhan PPhuhu PPhanhan KKhokhok KruatKruat 130 Hau. SSaoao KKhuahua PPhuhu PhanPhan Early Cretaceous Late Cretaceous SSaoao KKhuahua ? SSaoao KhuaKhua Fig. 2 Stratigraphic divisions and PPhuhu PPhanhan Vlg. PPhrahra WWihanihan age assignments for the Thailand Creta- PPhrahra WWihanihan PPhrahra WihanWihan ceous Formations in previous studies 140 Ber. PPhuhu KradungKradung and comparison with that of this study.

et al. 2006). Because the Phu Thok Formation mainly of large-scale cross-bedded, yellowish-red, overlies the Khok Kruat Formation in the fine- to medium-grained sandstone interbedded Nakhon Thai sub-basin, whereas it changes later- with reddish-yellow, very fine-grained sandstone ally into the main part of the Maha Sarakham and siltstone. Strata are sub-horizontal, although Formation in the northern part of the Sakhon field and remote-sensing data reveal a broad Nakhon sub-basin (DMR 2001; Fig. 1), the Phu NW–SE-trending regional syncline with limbs Thok and Maha Sarakham Formations have an that dip at 5–10°. interfingering relationship, at least in part (e.g. Eolian dunes in desert areas migrate leeward of Sattayarak 1983). Thus, the age of the Phu Thok the wind, thereby recording the direction of the Formation is considered as Mid- to Late- paleo-wind flows (prevailing surface-wind pattern) Cretaceous (Charusiri et al. 2006). In the Nakhon in the form of large-scale cross-stratification Thai sub-basin, the Phu Thok Formation is con- (Hunter 1977; Brookfield 1992). To reconstruct the formably overlain by the Late Cretaceous Phu paleo-wind flow direction changes, we measured Khat Formation (Figs 1,2). the foreset azimuths of large-scale cross-strata, recorded in the Phu Thok Formation at several sites in northeastern Thailand. Paleo-wind direc- SEDIMENTOLOGICAL ANALYSIS tion data were corrected for post-Cretaceous rota- We conducted sedimentological analysis at several tion of the crust based on paleomagnetic data localities of the eolian sandstone deposits (Phu (Charusiri et al. 2006). Thok Formation) in the Khorat Basin during the field survey in August 2007 and February 2008 MAGNETOSTRATIGRAPHIC ANALYSIS (Fig. 1). We investigated the well exposed outcrops in the (1) Phu Wua, (2) Phu Langka, (3) Phu Thok, Paleomagnetic sampling in the Phu Wua section in (4) Chet Si, (5) Phu Sing, and (6) Wat Ahong, (7) the northernmost part of the Sakhon Nakhon sub- Pak Man, (8) Na Haew, (9) Nakon Chum, (10) Chat basin, which is the best-exposed section in this Trakan sections in the Nakhon Thai sub-basin, area (Imsamut 1996; DMR 2001; Figs 1,3). In total, northwestern part of the basin (Figs 1,3). The 185 paleomagnetic samples were collected from an best-exposed section in this area is the Phu Wua approximately 220-m-long section within the Phu section in the northernmost part of the Sakhon Thok Formation in the Phu Wua area (Imsamut Nakhon sub-basin (Fig. 3). The Phu Thok Forma- 1996). The samples were collected mainly from tion is totally 400–500 m thick and composed very fine-grained inter-dune sandstone layers, © 2010 Blackwell Publishing Asia Pty Ltd ae-iddrcin r xmndi eal oedarm hwtesrtgahccagsi h ae-iddrcin nec oaiis Paleo- localities. each data. in paleomagnetic directions on paleo-wind based the crust in the changes of stratigraphic rotation the post-Cretaceous show for diagrams corrected were Rose data detail. direction in examined are directions paleo-wind 3 Fig.

tairpi oun tteslce oaiiso h h hkFraini otenSko ahnsbbsn otesenTaln,where Thailand, northeastern sub-basin, Nakhon Sakhon northern in Formation Thok Phu the of localities selected the at columns Stratigraphic (1) Phu Wua

210

205 5 Zone 3 200 LEGEND 195 7 50 (m) (2) Phu Thok 190 165 Eolian dune sandstone 185 3 160 40 180 n = 14 2 Inter-dune siltstone 155 175 6 150 170 5 30 Mudstone (key bed) 145 165 4 140 23 160 20 Paleo-wind directions 135 155 1 130 20 150 125 10 145 1 120 10 (4) Phu Sing 140 115 90 3 0 135 10 Zone 2 110 85 2 130 Zone 2 105 80 125 13 15 100 75 15 120 95 70 115 5 7 90 (3) Phu Langka 65 3 (5) Chet Si 110 40 85 75 60 6 n = 46 Zone 2 105 7 35 80 n = 115 70 55 Zone 2 Zone 2 100 22 65 30 75 6 50 95 1

6 25 mid-Cretaceous the in desert a was Thailand 70 60 45 90 20 15 55 40 2 65 85 2 50 15 60 35 80 10 9 45 n = 23 30 n = 28 n = 84 55 3 75 2 5 50 40 25 70 0 38 45 35 20 3 65 2 30 40 15 60 25 4 35 10 mudstonesandstone 55 5 9 conglomerate 30 20 5 50 3 Zone 1 2 25 15 0 45 1 10 3 20 Zone 1 00BakelPbihn saPyLtd Pty Asia Publishing Blackwell 2010 © 40 4 Zone 1 15 5 mudstonesandstone 35 4 10 0 conglomerate 30 2 5 n = 7 25 0 3 mudstone 6 sandstone 20 n = 3 conglomerate 15 n = 15 mudstone 10 1 sandstone conglomerate 5

0 2

mudstonesandstone conglomerate wind 609 610 H. Hasegawa et al.

a b

4400 m

c d

Fig. 4 Outcrop photographs of the Phu Thok Formation in the northern Sakhon Nakhon sub-basin, northeastern Thailand. (a) Overview of the exposure of the Phu Thok Formation at the Phu Thok section.; (b) Large-scale cross-stratified sandstone (eolian dune) at the Phu Sing e section.; (c) A contact of medium to fine-grained, large-scale cross-stratified sandstone (upper: d) and horizontally- stratified, very fine sandstone and siltstone (lower: e) at the Phu Thok section.; (d) A close-up view of the medium to fine-grained, large-scale cross-stratified sandstone (eolian dune).; (e) A close-up view of the horizontally-stratified, very fine sandstone and siltstone, consisting of the deformation structure (inter-dune fluvio-eolian). free of deformation and weathering, using a hand- RESULTS drill corer and oriented with a magnetic compass. In the laboratory, samples were cut into standard LITHOFACIES OF THE PHU THOK FORMATION cylindrical specimens of 25 mm in diameter and 2.2 cm in length. The Phu Thok Formation is 400–500 m thick To identify the carrier of remanent magnetiza- and composed mainly of large-scale cross-bedded, tion and paleomagnetic directions, rock and paleo- yellowish-red, fine- to medium-grained sandstone magnetic experiments were performed at the interbedded with reddish-yellow, very fine-grained Centre of Paleomagnetic Laboratory, Chengdu sandstone and siltstone (Figs 3 and 4). The forma- Institute of Geology and Mineral Resources, China tion contains two lithofacies: (i) thick bedded, (Imsamut 1996; Charusiri et al. 2006). Natural fine- to medium-grained, large-scale cross-bedded remanent magnetization (NRM) was measured sandstone (Fig. 4b–d); and (ii) horizontally- using a Schonstedt DSM-2 spinner magnetometer bedded, alternating very fine-grained sandstone (Schonstedt, Kearneysville, WV, USA). The and siltstone that include deformation structures samples were then demagnetized progressively in (Fig. 4c,e). The large-scale cross-bedded sand- 9–14 steps for thermal demagnetization (ThD) stone consists of reverse-graded, millimeter- to from 100 to 730°C at intervals of 30–50°C. Charac- centimeter-scale sets of alternating beds of mod- teristic remanent magnetization (ChRM) direc- erately well-sorted, moderately well-rounded, tions were calculated by principal component very fine- to fine-grained sandstone and well- analysis (Kirschvink 1980). sorted, well-rounded, fine- to medium-grained © 2010 Blackwell Publishing Asia Pty Ltd Thailand was a desert in the mid-Cretaceous 611

EE101°101° E102° 1 (n=75) E105° ddetailetail inin N 050100(km) Fig.Fig. 9 4 (n=28) 1 4 5 (n=84) H 5 2 N18° 2 (n=118) 3 6 (n=62) 3 (n=30) NNakhonakhon TThaihai 6 10 (n=80) bbasinasin 7 (n=25) SSakhonakhon NNakhonakhon bbasinasin

1100 7 9 8 9 (n=38) 8 (n=32) N17°

Fig. 5 A geological map with the studied sites (upper) and reconstructed paleo-wind directions. Numbers indicate several studied sites; 1) Phu Wua; 2) Phu Langka; 3) Phu Thok; 4) Chet Si; 5) Phu Sing; 6) Wat Ahong; 7) Pak Man; 8) Na Haew; 9) Nakon Chum; 10) Chat Trakan. Location of the subtropical high-pressure belt is reconstructed based on the spatial distribution of the paleo-wind direction data.

quartz-rich sandstone (Fig. 4d). Each set of cross- tion of the the Phu Wua and Phu Langka sections, bedded strata is thickly bedded (>4 m) and extends where bi-modal paleo-wind directions (southeast- laterally for hundreds of meters. The average dip ward and southwestward flows) are dominant of cross-bedding is 26–30°, rarely exceeding 34°. (Fig. 5). The cross-bedded strata that we measured are Stratigraphic variations in paleo-wind direc- mostly tabular planar with dip spread less than tions in the Phu Thok Formation within the 60° and also have a unimodal dip direction. northernmost part of the Sakhon Nakorn sub- Thickly-bedded, large-scale cross-bedded sand- basin (Fig. 3; the Phu Wua, Phu Thok, and Phu stone is intercalated with 1–2 m thick horizontally- Langka sections) show marked changes from con- stratified, alternating sets of reddish-yellow, very sistent southwesterly flow (lower part of the fine-grained sandstone and siltstone (Fig. 4c,e). section; Zone 1) to southeasterly flow (main part Internally, very fine-grained sandstone and silt- of the section; Zone 2) and then a return to south- stone are horizontally laminated and locally westerly flow (upper part of the section; Zone 3). contain deformation structures (Fig. 4e). Stratigraphic correlations among the studied localities are based on a distinct mudstone horizon (i.e. a key bed) located in the middle part PALEO-WIND DIRECTIONS of Zone 2 (Fig. 3). Figures 3 and 5 show the paleo-wind directions, as calculated from the foreset azimuths of large- ROCK AND PALEOMAGNETIC EXPERIMENTS scale cross-strata, recorded in the Phu Thok For- mation at several sites in northeastern Thailand. The general patterns of stepwise thermal demag- Paleo-wind direction data were corrected for post- netization (ThD) in the analyzed samples are Cretaceous rotation of the crust based on paleo- shown in a Zijderveld diagram in Figure 6 magnetic data (Charusiri et al. 2006; Figs 3,5). (Zijderveld 1967). The average NRM is moderate The paleo-wind directions in the Nakhon Thai (~1.1–55.0 mA/m). The stepwise ThD reveals sub-basin are dominantly southward. The paleo- that some samples have a low-temperature compo- wind directions in the Sakhon Nakhon sub-basin nent, although it was completely removed at tem- are southward to southeastward, with the excep- peratures below 250°C. In the temperature range © 2010 Blackwell Publishing Asia Pty Ltd 612 H. Hasegawa et al.

(a) 37003A (b) 31082D (c) 31068B N N N 700°C 0°C 10 mA/m E, Up E, Up 1 mA/m 680°C 600°C 400°C 600°C 550°C 550°C 550°C 400°C 600°C E, Up 400°C 680°C 1 mA/m 250°C N 250°C N N 1.0 1.0 1.0 0°°CC 550°C 700°C 600°C W E W E W E Intensity Intensity Intensity J0 = 16.6 mA/m 680°C J0 = 3.94 mA/m J0 = 55 mA/m 0.0 0.0 680°C 0.0 680°C 250°C 0 700°C 600°C 0 700°C 0 700°C 555050°°CC 250°C Temperature S Temperature S Temperature S

Fig. 6 Examples of progressive thermal demagnetization for selected specimens of ﬁne-grained sandstone (a and b) and very ﬁne-grained sandstone (c). Upper: an orthogonal vector plot with solid (open) symbols for data projected onto the horizontal (vertical) plane (Zijderveld 1967). Lower right: a normalized plot of the intensity of magnetization versus demagnetization temperature. Lower left: equal-area projection.

between 250°C and 680°C, most samples show a characteristic component decaying toward the N= 102 origin, which is rapidly removed at 500 to 550°C Inc= 33.1 (25% NRM) and is almost entirely removed at 650 Dec= 24.6 to 680°C (less than 10% NRM) (Fig. 6). This block- A95= 4.7 ing temperature indicates that the main carrier of k= 10.0 magnetization in these samples is magnetite (tita- nomagnetite). Some samples show relatively coherent demagnetization behaviors with unblock- ing temperatures extending to 680°C, although they do not show concordant (stable) magnetization directions. After correction for tilted bedding, the ChRM N= 7 directions oriented north down and south up are Inc= -31.1 interpreted as normal and reversed ChRM, Dec=199.9 respectively. The mean directions of both ChRM A95= 28.8 groups were calculated using Fisher statistics k= 5.4 (Fisher 1953) and are plotted on an equal-area stereographic projection in Figure 7. The mean Fig. 7 An equal-area projection of paleomagnetic directions after directions of the normal and reversed ChRM bedding correction. Open and solid symbols indicate negative and posi- groups are I = 33.1°, D = 24.6° and I =-31.1°, tive inclinations, respectively. D = 199.9°, respectively. To evaluate the reliability of this result, we performed a reversal test based MAGNETOSTRATIGRAPHY on that proposed by McFadden and McElhinny (1990). The angle between the mean directions of The declination and inclination of each ChRM the normal and reversed polarities is 18.5°, less direction, along with their corresponding strati- than the critical angle of 20°. Therefore, the mean graphic position, yields the magnetic polarity paleomagnetic directions of the normal and sequence of the studied section (Fig. 8). The reversed polarities pass the reversal test (quality normal and reversed polarities of the magneto- classiﬁcation C; Fig. 7). This positive result indi- stratigraphic column were determined from cates that the Phu Thok Formation preserves reli- inclination latitudes approaching +90° and -90°, able primary magnetization. respectively. In total, we identiﬁed three zones of © 2010 Blackwell Publishing Asia Pty Ltd Thailand was a desert in the mid-Cretaceous 613

Phu Wua Paleo-wind Age Polarity Ds/(°) Is/(°) Stage (Ma) Chron -9090 270 -900 90 210 Zone 3 6655 205 5 C30 MMaast.aast. 200 C31 7700 195 7 C32 190

185 3 n = 14 7755 180 CCamp.amp. 175 6 C33 170 8800 165 4 160 ?

155 1 8855 SSan.an. 150 PTn3 CCon.on. 145 1 9900 140 TTur.ur. 135 3 130 Zone 2 9955 125 13 CCen.en. 120

115 5 110000 LEGEND 110 6 C34 Eolian dune 105 n = 46 110505 AAlb.lb. sandstone 100 95 1 PTr2 Inter-dune 90 siltstone 111010 85 2

Mudstone 80

Paleo-wind 75 111515 ? M-1r direction 70 PTn2 65 AApt.pt. 60 112020 55 50 (m) 50 3 M0r 45 112525 M1 BBar.ar. 40 4 40 Zone 1 M3 35 M5 2 113030 30 PTr1 M6-7 30 25 HHau.au. M8-9 M10 20 6 113535 20 15 M11 10 1 n = 15 Vlg. PPTn1Tn1 M12 10 5 114040 M14 0 2 M16 Ber. M17 0 M18 -9090 270 -900 90 mudstonesandstone conglomerate

Fig. 8 A lithostratigraphic column, paleowind direction data, and magnetic polarity sequence of the Phu Thok Formation at the Phu Wua section of the northern Sakhon Nakhon sub-basin, northeastern Thailand, and their correlation to the geomagnetic polarity time scale (GPTS) of the geological time scale 2004 (Gradstein et al. 2004). Magnetic polarity zones are shown by black (white) bars for normal (reversed) polarity.

normal polarity and two zones of reversed polarity are recognized in the lower part of section. in the studied section, which we numbered PTn1 to Because the short reverse interval (PTr2) is PTn3 (in this naming system, the ﬁrst two letters deﬁned in only one sample (Fig. 6c; sample indicate the locality; the third letter indicates 31068B), its reliability is considered to be rela- reversed (r) or normal (n) polarity; and the tively low. The normal polarity zones PTn2 to number indicates the stratigraphic level of the PTn3 occupy approximately 180 m of the upper polarity zone). Polarity zones PTn1 to PTn2 part of the stratigraphic section (Fig. 8). © 2010 Blackwell Publishing Asia Pty Ltd 614 H. Hasegawa et al.

H 1 (n=46) H N N N 1 (n=15) 4 (n=28) 1 4 1 1 (n=14) 1 5 (n=84) H 5

2 (n=3) 2 3 2 (n=115) 2 3 3 (n=7) 3 (n=23)

0 550(km)0(km) 0 550(km)0(km) 0 550(km)0(km) Zone 1 Zone 2 Zone 3

Fig. 9 Spatial and temporal distributions of the paleo-wind directions within the studied area (stratigraphic variations of the paleo-wind directions are shown in Figure 3), and reconstructed latitudinal changes in the paleo-position of the subtropical high pressure belt.

DISCUSSION wind directions are caused by the northeast trade winds, while north to northeastward directions DEPOSITIONAL ENVIRONMENTS OF THE PHU are caused by westerlies in desert areas of the THOK FORMATION Northern Hemisphere (Bigarella 1972; Living- stone & Warren 1996). In addition, southeasterly Sets of large-scale cross-bedding structures, winds occur in areas close to the divergent axis of which are predominantly developed in the fine- to the subtropical high-pressure belt. The spatial medium-grained sandstone of the Phu Thok For- distribution of the reconstructed paleo-wind mation, are interpreted as the foresets of eolian directions recorded in the Phu Thok Formation dunes. The interpretation of an eolian setting is shows south- to southeastward directions in the based on the following features: (i) the grain Sakhon Nakhon sub-basin and southward direc- shape, sorting, and compositional maturity of tions in the Nakhon Thail sub-basin (Fig. 5). the sandstones; (ii) the large three-dimensional Therefore, the Khorat Basin mainly belonged to size of cross-stratification; (iii) the high angle of the northeast trade wind belt, and paleo-position the cross-strata; and (iv) the millimeter- to of the subtropical high-pressure belt during the centimeter-scale coarsening-upward sets of cross- deposition of the Phu Thok Formation is esti- bedding (Hunter 1977; Brookfield 1992). The dis- mated to have been north of the Nakhon Thai tinctive large-scale, tabular cross-stratification sub-basin and/or immediately above the Sakhon with laterally continuous occurrence and unimo- Nakhon sub-basin (Fig. 5). dal paleo-wind direction suggest that those dunes In addition to this spatial pattern in paleo-wind were possibly of matured transverse type (sensu, directions, stratigraphical variations in paleo- Kocurek 1991; Brookfield 1992). Intercalated very wind directions within the Phu Thok Formation fine sandstone and siltstone beds with deforma- in the Sakhon Nakhon sub-basin show marked tion structures are interpreted to have resulted changes from southwesterly (Zone 1) to south- from sand-slides at the base of the eolian dunes in easterly (Zone 2) and then back to southwesterly the inter-dune fields, due to flash-flood events or (Zone 3) winds (Fig. 3). These winds are inter- high-energy wind-storms (Talbot 1985). Thus, the preted to represent the zone of northeast trade depositional environment of the Phu Thok Forma- winds, divergent axis of the trade winds and tion is interpreted as a broad eolian sand-dune westerlies, and the zone of northeast trade winds, desert with minor interfingering of a fluvio-eolian respectively (Figs 3,9). Therefore, the paleo-wind inter-dune system. directions recorded in the Phu Thok Formation are thought to record latitudinal changes in the divergent axis of the subtropical high-pressure PALEO-POSITION OF THE SUBTROPICAL HIGH-PRESSURE belt. Specifically, the subtropical high-pressure BELT IN NORTHEASTERN THAILAND belt was situated to the north of the Nakhon Thai The distribution of desert and paleo-wind pat- and the Sakhon Nakorn sub-basins during the terns provides important information on the posi- initial stages of deposition of the Phu Thok For- tion of the subtropical high-pressure belt in the mation (Zone 1), shifted southwards to immedi- past. In general, south- to southwestward paleo- ately above the Sakhon Nakorn sub-basin during © 2010 Blackwell Publishing Asia Pty Ltd Thailand was a desert in the mid-Cretaceous 615

Table 1 Paleontological age constraints of the Cretaceous deposits in Khorat Basin, northeastern Thailand (modiﬁed after, Racey et al. 1996; Meesook 2000)

Formation Lithology Fossil assemblage Age Phu Kradong Fm Reddish brown, sandstone, Pollen & Spores: Cyathidites minor, E. Cretaceous siltstone, & mudstone Baculati-sporites commaumensis, Corollina simplex Phra Wihan Fm Whitish-grey, conglomerate, Pollen & Spores: Cicatricosisporites Ber.–Brm. sandstone, mudstone, augustus, Dicheiropollis etruscus, & lignites Corollina spp., Araucariacites australis, Ischyosporites cf. variegatus, Gleichenidites senonicus, Laevigatosporites sp., Perinopollenites elatoides, Callialasporites dampieri, Anaplanisporites dawsonensis, Apiculatisporites spp., Osmundacidites wellmanii, Todisporites minor, Kraeuselisporites sp., Concavissmisporites sp. Sao Khua Fm Reddish brown, sandstone, Pollen & Spores: Vitreisporites cf. pallidus, E. Cretaceous siltstone, & mudstone Cicatricosisporites spp., Cyathidites minor spp., Ephedripites spp., ?Araucariacites australis Phu Phan Fm Whitish-grey, conglomerate, Pollen & Spores: Corollina spp., Cyathidites E. Cretaceous sandstone, mudstone minor, ?Todisporites sp. Khok Kruat Fm Reddish brown, sandstone, Pollen & Spores: no data presented* Apt.? siltstone, & mudstone Maha Sarakhan Fm Reddish, sandstone, siltstone, Pollen & Spores: no data presented* Alb.?–Cen.? salts, gypsums, & anhydrites Phu Thok Fm Reddish, eolian sandstone no fossils Apt.–Tur.? & siltstone Phu Khat Fm Reddish to whitish grey, no fossils L. Cretaceous sand-stone & mudstone

* Palynological age estimatioins of the Khok Kruat and Maha Sarakham Formations are quoted by Sattayarak et al. (1991a,b). However, no palyno-fossil assemblage data were presented.

the main phase of deposition (Zone 2), and then 1996; Meesook 2000; Charusiri et al. 2006; see shifted northward again to the north of both sub- Table 1). basins during the final stages of deposition (Zone Our field observations and analyses of the drill- 3) (Fig. 9). core further suggest that the Phu Thok Forma- tion has an interfingering relationship with the Maha Sarakham Formation, at least in part, as MAGNETOSTRATIGRAPHIC AGE CONSTRAINTS the Phu Thok Formation overlies the Khok Kruat The age of the Phu Thok Formation has been a point Formation in the Nakhon Thai sub-basin, of controversy for a long time, due to a lack of whereas it is laterally continuous with the main age-diagnostic fossils (Sattayarak 1983; Racey part of the Maha Sarakham Formation in the et al. 1996; Meesook 2000; Charusiri et al. 2006). Sakhon Nakhon sub-basin (Fig. 1). The age of Age constraints are provided by palynological evaporite in the Maha Sarakham Formation is evidence from underlying strata of the Khorat estimated to be mid-Cretaceous based on the Group, in which the lignite-bearing Phra Wihan results of K/Ar (93.7 Ϯ 2.7 Ma) and Rb/Sr Formation yields age-diagnostic palyno-fossils (95.8 Ϯ 15 Ma) ages of potash and rock salt such as Cicatricosisporites augustus (Berriasian deposits (Hansen et al. 2002). Thus, the age of the or younger) and Dicheiropollis etruscus (Barre- Maha Sarakham Formation is considered to be mian), as described by Racey et al. (1996). Thus, the mid-Cretaceous, most likely Albian–Cenomanian age of the Phra Wihan Formation is assigned to the (Hansen et al. 2002). Consequently, the age of the Early Cretaceous (Berriasian–Barremian), and Phu Thok Formation is considered younger than the age of the overlying Phu Thok Formation is Barremian and in part equal to the Albian– considered younger than Barremian (Racey et al. Cenomanian. © 2010 Blackwell Publishing Asia Pty Ltd 616 H. Hasegawa et al. Based on these age constraints, we correlated DEPOSITIONAL AGE OF EOLIAN SANDSTONE IN the magnetic polarity sequence obtained for THE SICHUAN BASIN the Phu Thok Formation with the geomagnetic Pan et al. (2004) demonstrated a magnetostrati- polarity time scale (GPTS) (Gradstein et al. 2004). graphic column for Cretaceous eolian sandstone The thick normal-polarity zones from PTn2 to of the Jiaguan Formation, Sichuan Basin, south- PTn3 can be correlated with Superchron C34n. ern China (Fig. 10). The authors reported short- Accordingly, the polarity sequence from PTn1 to duration zones of reversed–normal–reversed PTr2 corresponds to chrons M1n to M0r. Based polarity in the lowermost part of the formation and on this correlation, deposition of the Phu Thok a thick zone of normal polarity in the main part Formation began at approximately c. 126 Ma and (Fig. 10). The age of the Jiaguan Formation, as ended by c. 84 Ma or earlier (from the late well as the underlying Tianmashan Formation and Barremian–early Aptian to the late Santonian the overlying Guankou Formation, is well con- or earlier). Based on these ages, the sedimenta- strained by ostracod biostratigraphy (Li 1982; Hao tion rates of the Phu Thok Formation are esti- et al. 2000; Chen et al. 2006; see Table 2). The mated to be 0.9–2.5 cm/ky, with a mean rate of Jiaguan Formation yields age-diagnostic ostracod 1.5 cm/ky assemblages that contain Ziziphocypris orbita, Alternatively, the polarity sequence from PTr1 Cypridea (Bisulcocypridea) sp. (Cenomanian– to PTn3 can be correlated with chrons C33r to Turonian), and Latonia (Monosulcocypris) spp. C33n. Except for a short interval of normal polar- (Aptian), as described by Li (1982); consequently, ity (PTr2), it also appears straightforward to the age of this formation is mid-Cretaceous match the obtained magnetostratigraphic column (Aptian–Turonian) (see Table 2). with the GPTS in this way. However, the estimated Based on these paleontological age constraints, sedimentation rate based on this correlation (i.e. the long zone of normal polarity (300 m thick) in PTr1 correlated with chron C33r) is 0.4 cm/ky, the main part of the Jiaguan Formation correlates which is significantly less than that deduced from best with Superchron C34n of the GPTS (Grad- the above correlation and less than typical sedi- stein et al. 2004). According to this correlation, the mentation rates of fluvio-eolian deposits (e.g. sequence of reversed–normal–reversed polarity in Einsele 2000). Therefore, the former magneto- the lowermost part of the formation corresponds stratigraphic correlation is preferred over the to chrons M1r to M0r (Pan et al. 2004; Fig. 10). latter, although further chronological study is Specifically, deposition of the Jiaguan Formation required, particularly from the Nakhon Thai sub- began at approximately c. 127 Ma and ended no basin, where the complete succession of the Phu later than c. 84 Ma (from the late Barremian–early Thok Formation and overlying Phu Khat Forma- Aptian to no later than the late Santonian). Based tion is exposed (Fig. 1). on this correlation, sedimentation rates of the The main part of the Phu Thok Formation cor- Jiaguan Formation (400 m thick) vary from 0.9 to relates with the interval from M1n–M0r to Super- 1.5 cm/ky, with a mean rate of 1.2 cm/ky, similar to chron C34n of the GPTS (Gradstein et al. 2004), the rates calculated for the Phu Thok Formation in which corresponds to c. 126 Ma to no younger than the Khorat Basin. Based on these magnetostrati- c. 84 Ma (from the late Barremian–early Aptian to graphic correlations (Pan et al. 2004; Fig. 10) and no younger than the late Santonian; Fig. 8), paleontological age constraints (Li 1982; Hao et al. although the exact age of the upper limit remains 2000; Chen et al. 2006; Table 2), the age of the unclear. Because the total thickness of the Phu eolian sandstone in the Sichuan Basin (the Jiaguan Thok Formation is 400–500 m (as observed in the Formation) is estimated to be mid-Cretaceous Nakhon Thai sub-basin), the duration of deposition (between late Barremian–early Aptian and Turo- is calculated to be 26.7–33.3 million years, based on nian), being similar in age to eolian sandstone in an extrapolation of the mean sedimentation rate the Khorat Basin (Fig. 8). (1.5 cm/k.y). In this case, the upper limit of the Phu Thok Formation is estimated to be the latest Albian to middle Turonian. Consequently, the age LOW-LATITUDE DESERT DEVELOPMENT IN THAILAND AND SOUTHERN CHINA of the Phu Thok Formation is estimated to be mid-Cretaceous (from c. 126 Ma to c. 99–93 Ma; Cretaceous eolian sandstone deposits are also younger than late Barremian–early Aptian to recorded in several sedimentary basins in low- to older than early Cenomanian–middle Turonian; mid-latittude Asia (Jiang & Li 1996; Jiang et al. Fig. 8). 1999, 2001; 2008; Hasegawa et al. 2009). The above © 2010 Blackwell Publishing Asia Pty Ltd Thailand was a desert in the mid-Cretaceous 617

Ds/(°) Is/(°) Age Polarity Paleo-wind Stage (Ma) Chron 009090 180 -90 6655 C30 MMaast.aast. 2 C31 7700 C32 1

7755 2 CCamp.amp. C33 2 8800

2 8855 SSan.an.

2 CCon.on. n=16 9900 1 TTur.ur.

2 9955 CCen.en.

2 110000

2 C34 110505 AAlb.lb. 3

2 111010

Jiaguan Fm 1 111515 M-1r

3 AApt.pt. 112020 100 1 M0r 112525 (m) 3 M1 BBar.ar. lower partlower Upper part M3 3 n=22 113030 M5 M6-7 HHau.au. M8-9 50 1 M10 113535 M11 1 Vlg. M12 114040 M14 2 M16 0 Ber. M17 009090 180 -90 M18

Fig. 10 Correlation of the magnetostratigraphy of the Jiaguan Formation in Sichuan Basin, southern China (modified after, Pan et al. 2004) to the geomagnetic polarity time scale (GPTS) of the geological time scale 2004 (GTS2004) (Gradstein et al. 2004). Magnetic polarity zones are shown by black and white bars for normal and reversed polarities. Rose diagrams show stratigraphic changes in the paleo-wind directions in this area (modified after, Jiang et al. 1999). magnetostratigraphic correlation provides firm According to Jiang et al. (2008), eolian sand- evidence for the development of low-latitude stones in low-latitude Asia (e.g. Jiaguan Formation desert in Asia during the mid-Cretaceous. Eolian in Sichuan basin; Pashahe Formation in Simao sandstone of the Phu Thok Formation in the Basin) are characterized by southward- to Khorat Basin was deposited between c. 126 Ma southwestward-flow, suggesting that these basins (between the late Barremian and early Aptian) (Southern China) mainly belonged to the north- and approximately 99–93 Ma (between the early east trade wind belt. It is consistent with our find- Cenomanian and middle Turonian) (Fig. 8). Eolian ings that the southeastward to southwestward sandstone of the Jiaguan Formation in the Sichuan paleo-wind directions are dominant in the Phu Basin is of similar age (Aptian–Turonian; Hao Thok Formation and that the Khorat Basin mainly et al. 2000; Pan et al. 2004; Chen et al. 2006; belonged to the northeast trade wind belt (Fig. 5). Fig. 10). As with the paleo-wind directions recorded in © 2010 Blackwell Publishing Asia Pty Ltd 618 H. Hasegawa et al.

Table 2 Paleontological age constraints of the Cretaceous deposits in Sichuan Basin, southern China (modiﬁed after, Li 1982; Hao et al. 2000; Chen et al. 2006)

Formation Lithology Fossil assemblage Age Tianmashan Fm Reddish-purple conglomerate, Ostracodes: Deyangia lushanensis, D. postacuta, Ber.–Brm. sandstone, & mudstone Cypridea sp., Jingguella obtusura, Lycopterocypris sp., Minheella sp., Ziziphocypris sp., Mongolianella sp. Jiaguan Fm Red to purple, fine- to Ostracodes: Cypridea angusticaudata, C. Apt.–Tur. medium-grained, eolian sichuanensis, C. yunnanensis, C. gunzulingensis, sandstone C. enodata, C. cf. ampullaceousa, C. concise, C. tera, C. cf. gibbosa, C. setina frorida, C. sentina acerata, C. setina bellatula, C. (Bisulcocypridea) sp., C. (B.) chuxiongensis, C. (Morinina) monosulcata, Harbinia jingshanensin, Latonia (Monosulcocypris) spp., Sinocypris (Quadracypris) cf. favosa, Talicypridea (Cristocypridea) sp., Ziziphocypris orbita, Z. acuta, Jinggunella sp., Kaitunia cuneata,Darwinnela sp., Timiriasevia sp., Lycopterocypris sp., Pinnocypridea sp. Guankou Fm Red to purple, fine-grained Ostracodes: Cypridea gigantea, C. infidelis, Con.–Maas. sandstone, mudstone, Cristocypridea latiovata, C. chinensis, Sinocypris gypsum subfuningensis, Candona huangdianensis, C. qionglaiensis, Nonion sichuanensis,

the Phu Thok Formation in the Khorat Basin Cretaceous. (Figs 9,11). Given that deposition of (Figs 3,8,9), the stratigraphic change in the paleo- evaporite of the Maha Sarakham Formation in wind directions in the Jiaguan Formation in the the Khorat Basin occurred simultaneously with Sichuan Basin also show marked changes from the eolian sandstone of the Phu Thok Formation southward and eastward directions in the lower in the mid-Cretaceous (Albian–Cenomanian; El part to southeastward and northeastward direc- Tabakh et al. 1999; Hansen et al. 2002), the depo- tions in the upper part of the succession (Jiang sition of evaporite in northeastern Thailand et al. 1999; Fig. 10). The paleo-wind directions of appears to have been related to an equatorward the Jiaguan Formation are thought to record the shift in the subtropical high-pressure belt and the following latitudinal changes in the location of the development of a low-latitude desert (hot and dry divergent axis of the subtropical high-pressure climate) during the mid-Cretaceous (Fig. 11). belt: the belt was situated to the north or immedi- Widespread evaporite formation in the South ately above the Sichuan Basin during the early Atlantic and Thailand during the mid-Cretaceous part of the mid-Cretaceous, but shifted southward also indicates a possible link to the changes in the to south of the basin during the later part of the pattern of atmospheric circulation (equatorward mid-Cretaceous (Fig. 10). shift of the subtropical high-pressure belt) at that The paleolatitude of the Sichuan basin (South time. China block) during the Cretaceous is estimated to have been N25.5° to N29.6°, based on paleomagnetic data (Enkin et al. 1991). The paleolati- CONCLUSIONS tude of the Phu Thok Formation in the Khorat Basin at this time is calculated to have been We conducted the sedimentological and magneto- N16.8° to N18.0°, also based on paleomagnetic stratigraphic analysis of eolian sandstone deposits data (Charusiri et al. 2006). Therefore, based on (Phu Thok Formation) in the northern Khorat the latitudinal distribution of eolian sandstones, Basin, northeastern Thailand, to reconstruct in combination with paleo-wind direction data spatio-temporal changes in the latitude of the sub- from the two basins, we propose that the mid- tropical high-pressure belt in northeastern Thai- Cretaceous in Asia saw the development of low- land and to establish a chronology for desert latitude deserts and an equatorward shift in development in low-latitude Asia during the Cre- the subtropical high-pressure belt relative to the taceous. Spatio-temporal changes in paleo-wind present-day position occured in Asia during the directions recorded in the Phu Thok Formation © 2010 Blackwell Publishing Asia Pty Ltd Thailand was a desert in the mid-Cretaceous 619

N60° (a) mid-Cretaceous (b) Polar Cell

GGbb N45° Ferrel Cell OOrr TTrr JJhh SSbb N30° SScc SSmm KKrr Hadley N15° Desert area Cell

0 1000 2000(km) 0°

Paleo-wind Eolian dune Fluvial Coal Westerlies Northeast trades direction Desert zone

Fig. 11 (a) A paleogeographic map and distribution of climate-sensitive sediments (e.g. eolian dune, fluvial, and coal deposits) in sedimentary basins of Asia during the mid-Cretaceous: the Gobi (Gb) Basin in south Mongolia; the Ordos (Or), Tarim (Tr), Subei (Sb), Jianghan (Jh), Sichuan (Sc), and Simao (Sm) basins in China; and the Khorat (Kr) Basin in northeastern Thailand (modified after, Jerzykiewicz & Russell 1991; Jiang & Li 1996; Hao et al. 2000; Jiang et al. 2001). The distribution of the desert zone and position of the westerlies and trade winds suggest that the subtropical high-pressure belt was situated in the low latitude of the paleo-Asian continent (between N15° and N30°) during the mid-Cretaceous. (b) A schematic figure showing the general atmospheric circulation pattern and the distribution of desert area in the Northern Hemisphere.

reveal that the Khorat Basin mainly belonged to ACKNOWLEDGEMENTS the northeast trade wind belt and that the subtropical high-pressure belt was situated to the We thank the members of IGCP 507 ‘Paleoclimates north of the Khorat Basin during the initial stages in Asia during the Cretaceous’ for fruitful discus- of deposition, shifted southwards to immediately sions. This study was supported ﬁnancially by a above the basin during the main phase of deposi- Grant-in-Aid from a JSPS Research Fellowship tion, and then shifted northward again to north (No. 18-05250) and the 21st Century COE of the basin during the ﬁnal stages of deposition. Program at the Department of Earth and Plan- The paleomagnetic polarity sequence obtained etary Science, the University of Tokyo. Thanks are for the Phu Thok Formation comprises three zones also due to Prof. Jens Herrle and an anonymous of normal polarity and two zones of reversed polar- reviewer and the Associate Editor Prof. Helmut ity, which can be correlated with chron M1n to Weissert, who improved the manuscript. C34n of the GPTS. This result suggests that the Phu Thok Formation is mid-Cretaceous in age (from c. 126 Ma to c. 99-93 Ma; from younger than REFERENCES late Barremian–early Aptian to older than early Cenomanian–middle Turonian), and similar in ABE M., KITOH A.&YASUNARI T. 2003. An evolution of age to eolian sandstone in the Sichuan Basin, the Asian Summer Monsoon Associated with Moun- southern China (the Jiaguan Formation: Aptian to tain Uplift – Simulation with the MRI Atmosphere- Turonian). These results, in combination with Ocean Coupled GCM. Journal of the Meteorological Society of Japan 81, 909–33. paleo-wind direction data, suggest the mid- AN Z. S., KUTZBACH J. E., PRELL W.L.&PORTER S. C. Cretaceous development of low-latitude desert 2001. Evolution of Asian monsoons and phased uplift (hot and dry climate) and an equatorward shift in of the Himalaya-Tibetan plateau since late Miocene the subtropical high-pressure belt relative to the times. Nature 411, 62–6. present-day position occured in Asia during the BIGARELLA J. J. 1972. Eolian environments: Their char- mid-Cretaceous. acteristics, recognition, and importance. In Rigby J. © 2010 Blackwell Publishing Asia Pty Ltd 620 H. Hasegawa et al.

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