journal of asian earth sciences 2012_2013/121.p… · triassic to early cretaceous age, but were...

Journal of Asian Earth Sciences 61 (2012) 62–77

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Journal of Asian Earth Sciences

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Identification of an anastomosing river system in the Early Cretaceous KhoratBasin, northeastern Thailand, using stratigraphy and paleosols

Yu Horiuchi a,⇑, Punya Charusiri b, Ken-ichiro Hisada c

a Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japanb Earthquake and Tectonic Geology Research Unit (EATGRU), c/o Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailandc Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan

a r t i c l e i n f o

Article history:Available online 10 September 2012

Keywords:Facies analysisFluvial depositAnastomosing riverPaleosolCalcreteKhorat Group

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⇑ Corresponding author. Tel.: +81 29 861 3678; faxE-mail address: [email protected] (Y. Horiuch

a b s t r a c t

The Phu Kradung and Phra Wihan formations of the Early Cretaceous Khorat Group were investigated tocharacterize the depositional system and paleoclimate, based on stratigraphy, paleosol features, paleosolprofiles, and reworked deposits. The sediments of both formations are classified into 12 facies on thebasis of their primary depositional attributes, such as bedding, grain size, texture, and sedimentary struc-tures. Furthermore, these facies are grouped into eight architectural elements based on stratigraphic rela-tionships and outcrop-scale macroforms. The proportions of each architectural element indicate that thedepositional system was that of an anastomosing river. The anastomosing river system in the studied sec-tion is characterized by a high rate of vertical floodplain aggradation and the development of a relativelystable crevasse channel. Paleosols are developed mainly in floodplain deposits, and are characterized bythe development of calcretes. A semi-arid to sub-humid paleoclimate is deduced from the occurrence ofthe calcretes. In addition, their microstructures suggest that these calcretes were formed under severaldifferent conditions, with micritic carbonate precipitating under relatively arid climates, and alveolarseptal structures under semi-arid to sub-humid conditions. Using the thicknesses of the calcrete hori-zons, we conclude that the difference in the amount of precipitation between the wet and dry periodswas highly variable, without any order, during deposition of the studied section. The thicknesses of inter-vals between successive calcrete horizons indicate that the inflow of fine materials occurred randomlyacross the floodplain. Partly superimposed, multiple calcrete horizons were developed during periodsof low sediment supply. A random deposition of floodplain sediments appears to characterize the anas-tomosing river system of the Khorat Basin.

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1. Introduction

Cretaceous continental basins, which frequently contain non-marine sequences with paleosols, are exposed sporadically in Eastand Southeast Asia (Horiuchi et al., 2009; Lee and Hisada, 1999;Lee et al., 2003; Meesook, 2001, 2011; Miki, 1992; Okada, 1999;Paik and Lee, 1995, 1998; Vincent and Allen, 1999). A paleosol isa soil that formed on a landscape of the past (Retallack, 2001; Ruhe,1965). Paleosols differ from modern soils as a result of compactionand alteration during burial, producing diagnostic changes inchemical properties, such as pH, Eh, and base saturation (Retallack,1990). Paleosols undergo alteration caused by groundwater, hydro-thermal activity, or diagenesis, but many diagnostic features, suchas traces of life, soil horizons, and soil structures, can survive suchalteration and be used for paleosol recognition (Retallack, 1988,1990, 1992, 1997).

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: +81 29 861 3717.i).

Calcretes are one of such survivors and a common product inpaleosols. Calcretes are commonly developed in alluvial sequencesdeposited under arid to semi-arid climates (Wright and Tucker,1991). It is generally accepted that a calcrete is composed mainlyof calcium carbonate, but there exists no uniform definition ofthe term in the literature (Quast et al., 2006). The term is com-monly used for carbonates from the vadose zone, but also forgroundwater carbonates precipitated in the phreatic soil zone(e.g., Quast et al., 2006; Stokes et al., 2007; Wright and Tucker,1991). In the present paper, the term ‘‘calcrete’’ is used to referto carbonate not only from the vadose zone but also from the phre-atic soil zone. The precipitation of calcium carbonate can occur viaseveral mechanisms, including evaporation, evapotranspiration,degassing of CO2, the common ion effect, and biological activity(Wright and Tucker, 1991). Calcrete formation is also influencedby climate, especially the amount of rainfall (Khadkikar et al.,2000; Retallack, 2005). Calcretes are considered a good indicatorof sedimentation rates, vegetation type, tectonic regime, climate,and sedimentary discontinuities (Alonso-Zarza, 2003; Gómez-Gras

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Fig. 1. Stratigraphy and age of the Khorat Group (after Racey, 2009). Stars representthe occurrences of calcretes reported by Mouret et al. (1993) and Racey et al.(1996).

Y. Horiuchi et al. / Journal of Asian Earth Sciences 61 (2012) 62–77 63

and Alonso-Zarza, 2003). Though a detailed depositional model canbe inferred using facies analysis, calcrete profiles in the sequenceprovide additional information on depositional cycles, such as peri-ods of sedimentation and stagnation (Kraus, 1999). Conversely, cal-cretes are easily liberated and reworked by superficial runoff,particularly where weakly developed within soft host rocks, suchas floodplain mudstones (Gómez-Gras and Alonso-Zarza, 2003).Such reworked deposits are important in interpreting the climaticand subsidence conditions under which the soils were formed andreworked (Gómez-Gras and Alonso-Zarza, 2003).

The Mesozoic Khorat Group is a non-marine sequence for themost part, distributed widely across northeastern Thailand, thoughthere are some minor marine incursions in places (Mouret, 1994).Calcretes in the Khorat Group were previously interpreted as lime-nodule conglomerates or nodules, and were only recently reinter-preted as paleosols and reworked calcretes (Meesook, 2001;Meesook and Wongprayoon, 2001; Mouret et al., 1993). The se-quence containing calcretes and reworked calcretes providesimportant information for characterizing the depositional systemsand paleoclimate during deposition of the Khorat Group.

The aim of the present study is to characterize the depositionalsystem and paleoclimate of the Khorat Basin during the Early Cre-taceous. The depositional environment of the studied section isinterpreted based on facies analysis, calcrete profiles, and the nat-ure of reworked deposits.

2. Geological setting and stratigraphy

The Mesozoic Khorat Group, which is distributed widely acrossnortheastern Thailand, consists primarily of clays, siltstones, sand-stones, and conglomerates. The base and top of the Khorat Groupare controversial, because of uncertain depositional age and sup-posed unconformities within the strata (Piyasin, 1985; Sattayarak,1983; Racey, 2009; Ridd et al., 2011). According to Racey (2009),stratigraphically, the Khorat Group is divided into six formations,which are, in ascending order, the Upper Nam Phong, Phu Kradung,Phra Wihan, Sao Khua, Phu Phan and Khok Kruat formations(Fig. 1). These formations were generally regarded as being of LateTriassic to Early Cretaceous age, but were reinterpreted recently asof Late Jurassic to Early Cretaceous age, based on palynological data(Racey and Goodall, 2009). Depositional environments in the Khor-at Group are thought to have been meandering river systems forthe Upper Nam Phong, Phu Kradung, Sao Khua, and Khok Kruat for-mations, and braided river systems for the Phra Wihan and PhuPhan formations (Mouret et al., 1993).

Paleosols with calcretes have been reported from the PhuKradung, Sao Khua, and Khok Kruat formations (Meesook, 2011;Mouret et al., 1993; Racey et al., 1996).

The depositional environment of the Phu Kradung Formationwas probably an anastomosed meandering river and containspaleosols in floodplain deposits, and reworked deposits in channelconglomerates (Mouret et al., 1993). The Phra Wihan Formationcontains reworked deposits near its base, and is interpreted ashaving been deposited by a semi-distal, braided stream system(Mouret et al., 1993). By definition, an anastomosing river consistsof two or more interconnected channels that enclose a flood basin,while braided rivers are regarded as rivers with a single channelbelt, but multiple thalwegs (Makaske, 2001). The Phu KradungFormation was deposited under environmental conditions oflong-term stability, and is therefore the best succession to use inevaluating the Early Cretaceous paleoenvironment.

The studied section is located at Nong Bua Lamphu (Fig. 2). Thesection is continuously well exposed for�2 km along highway 210.The sequence is �350 m thick in this section and corresponds tothe uppermost part of the Phu Kradung Formation and the lower-most part of the Phra Wihan Formation. The boundary between the

Phu Kradung and Phra Wihan formations in the northern part ofthe section is decided based on geological map of Chonglakmaniet al. (1979). The basal part of the Phra Wihan Formation is com-posed of conglomeratic sandstone and conformably overlies thePhu Kradung Formation. The strata in the section strike NW–SEto NNW–SSE and dip 10�–20�NE. The section is composed mainlyof sandstones, mudstones, and subordinate conglomerates. Thestratigraphy is characterized by several meter-to-decameter-thickcycles of fining-upward sequences, typically with an erosive sur-face at the base. The lower part of the fining-upward sequencesis composed mainly of sandstones and conglomerates; the middlepart is formed of massive, laminated, and rippled medium- tovery-fine-grained sandstones and siltstones, and the upper partcomprises red to pale green massive, argillaceous, fine- tovery-fine-grained sandstones and mudstones.

3. Facies analysis

3.1. Classification of lithofacies

The observation and classification of lithofacies are standardcomponents of the facies-analysis methodology for studying sedi-mentary rocks (Miall, 1996). The scale of an individual lithofaciesunit generally depends on the level of detail, varying from mappa-ble stratigraphic units to centimeter-by-centimeter logging (Miall,1996). For the purpose of architectural-element analysis, a rela-tively fine degree of description and subdivision is required (Miall,1996).

In the studied section, the sediments of the Phu Kradung andPhra Wihan formations are classified into 12 facies (I–XII) on thebasis of their primary depositional attributes, such as bedding,grain size, texture, and sedimentary structures (Table 1). The sed-imentary logs for the studied section are shown in Fig. 3.

(a) (b)

Fig. 2. Location map of Nong Bua Lamphu (a) and geological map of the studied section (b). Compiled from Chonglakmani et al. (1979) and Department of Mineral Resources(1999).

Table 1Facies classification of Phu Kradung and Phra Wihan formations in the study section at Nong Bua Lamphu.

Facies Lithofacies Sedimentary structures Faciescodesa

Interpretation

I Matrix-supportedconglomerate

Massive, or horizontal beds Gmm Debris flow

II Matrix-supportedconglomerate

Cross beds Gp, Gt Minor channel fills, transverse or deltaic growth bedforms

III Coarse sandstone togranule conglomerate

Thin beds or lenticular bodies, massive or weak normalgrading, small scour fill

Gcm, Sm Pseudoplastic debris flow, sediment gravity flow

IV Fine to very coarsesandstone

Massive, some gravel, rip-up clast, scours Sm, Ss Sediment gravity flow, turbated sand, scour fill

V Very fine to coarsesandstone

Low-angle cross beds, some gravel Sl Scour fills, humpback or washed-out dunes, antidunes

VI Fine to coarse sandstone Planar cross beds Sp Transverse and linguoid bedformsVII Fine to coarse sandstone Trough cross beds, some gravel St Sinuous-crested and linguoid dunesVIII Very fine to very coarse

sandstoneHorizontal beds, some gravel, parallel lamination, somebioturbation, convolution

Sh Plane-bed flow

IX Fine to medium sandstone Faint lamination, bioturbation Sm, P Turbated sand, weakly developed soilX Very fine to medium

sandstoneFine cross lamination, ripple, some loading structure,bioturbation, some calcretes

Sr, Fl, P Ripples, overbank, abandoned channel, or waning flooddeposits, soil with chemical precipitation

XI Mudstone to very finesandstone

Frequently intercareted thin sandstone layer, faintlamination, mottle, bioturbation, calcretes

P, Fl Soil with chemical precipitation, overbank, abandonedchannel, or waning flood deposits

XII Mudstone to very finesandstone

Massive, mottle, calcretes, slickenside Fm, Fsm, P Overbank, backswamp, abandoned channel or drapedeposits, soil with chemical precipitation

a Corresponding to facies codes by Miall (1996).

64 Y. Horiuchi et al. / Journal of Asian Earth Sciences 61 (2012) 62–77

Fig. 3. Detailed log of studied section, with facies. The facies are illustrated in the left sides of the columnar sections. See Tables 1 and 2 for symbols relevant to the facies andarchitectural elements.


Fig. 3. (continued)


Facies I is a matrix-supported conglomerate. This facies is poorlysorted and is massive or with horizontal bedding (Fig. 4A). Clastsare granule to pebble in size and sub-rounded to angular. The ma-

trix consists of sandstone and siltstone. The thickness of facies Iranges from 15 cm to 1.3 m. Facies I corresponds to Miall’s (1996)lithofacies Gmm, which is formed by high-strength debris flows.

I

1 m

E

5 cm

B

5 cm

G

5 cm

A

D

5 cm

H

10 cm

C

1 cm

M2 cm

K

L

4 cm

4 cm

J

F

Fig. 4. Photographs of the various facies. (A) Appearance of matrix-supported conglomerate corresponding to facies I; (B) rip-up clasts in facies II (dark color); (C) plantfragments in facies IV; (D) Facies V; (E) planar cross-bedding of facies VI; (F) boundary between facies VII (lower) and VIII (upper); (G) pseudo-climbing ripple in facies X; (H)patchy pale-green color in purplish-colored part of facies X; (I) facies XI, with sandstone layer showing an irregular boundary with underlying strata; (J) mottles withabundant bioturbation in facies XI; (K) tubular trace fossils with meniscate structure, facies XI; (L) slickensides in facies XII; (M) rootlet in facies XII. Photograph locations areshown in Fig. 3. Images B, J, and K show the bedding surface.


Facies II is a matrix-supported conglomerate with cross beddingand locally intercalations of sandy conglomerate. Clasts are verycoarse-grained sand to granule in size and are sub-rounded. Thematrix consists of sandstone with a calcareous cement, and is gray-ish-white, reddish-brown1 or white in color. The thickness of faciesII ranges from 30 cm to 90 cm. Facies II commonly has an erosivelower boundary, and locally contains rip-up clasts (Fig. 4B). This fa-cies corresponds to Miall’s (1996) lithofacies Gt and Gp, which areinterpreted as minor channel fills and transverse or deltaic growthbedforms, respectively.

Facies III is a massive, coarse-grained sandstone to granularconglomerate. This facies generally occurs as assemblages of sev-eral lenticular bodies of small scour fills and thin beds. Individualbeds are 10–20 cm thick and occasionally show weak, normalgrading of similar-sized grains. Facies III is frequently accompanied

1 For interpretation of color in Figs. 4, 6 and 8, the reader is referred to the webversion of this article.

by facies XII (see below) and corresponds to Miall’s (1996) lithofa-cies Gcm and Sm, which are interpreted as pseudoplastic debrisflow and sediment gravity flow deposits, respectively.

Facies IV is a fine- to very-coarse-grained sandstone with someconglomeratic intercalations, and has a whitish-gray to pale green-ish-white color. The facies is typically massive, with locally veryfaint and patchy laminations. The thicknesses of individual bedsranges from 5 cm to 4 m. Facies IV occasionally has an irregularlower boundary and often contains plant fragments (Fig. 4C) orrip-up clasts. The facies is sometimes found in association withbioturbated facies IX and X (see below) and corresponds to Miall’s(1996) lithofacies Sm and Ss, which are massive sandstones.Though lithofacies Sm is typically formed by sediment gravityflows, its massive texture may also be produced by post-depositional modification, such as dewatering and bioturbation(Miall, 1996). Based on the presence of accompanying bioturbatedfacies, some parts of facies IV are likely to have been modified bypost-depositional disturbance. Lithofacies Ss is characterized by

Table 2Architectural elements recognized in the Phu Kradung and Phra Wihan formations of the studied section at Nong Bua Lamphu.

Element Symbol Facies assemblage Interpretation

Channel-fill elementChannel CH IV, V Channel-fillSandy bedforms SB IV, V, VI, VIII, minor VII, I, X, XII, IX, II Channel-fill, minor bars, vertical aggradationLateral accretion LA IV, V, VI, minor II, III, X, I, XI, XII Channel-fill, point barDownstream accretion DA VI, X Channel-fill

Overbank elementLevee LV IX, X, XII Overbank floodingCrevasse channel CR X, VIII, VII, minor IV, I, II, V, VI Channel-fillCrevasse splay CS XI, X, IV, XII, minor VIII, IX, III, VII, VI, II Sheet splayFloodplain deposit FF XII, XI, X, minor VIII, IX, IV Deposition from suspension, swamp deposit, some sheet flow in proximal setting


poorly sorted sand that contains abundant intraclasts and lagmaterial such as plant debris (Miall, 1996). Lithofacies Ss is formedby rapid deposition over the irregular shape of a basal boundingsurface and is interpreted as scour–fill (Miall, 1996).

Facies V is a very-fine- to coarse-grained sandstone that showslow-angle cross-bedding (Fig. 4D) and that is locally intercalatedwith conglomeratic bodies and carbonaceous material. The faciesoccasionally has an erosive lower boundary and contains rip-upclasts, and varies from 30 cm to 3.5 m in thickness. The facies isfound in conjunction with facies IV, VI, and VII, and correspondsto Miall’s (1996) lithofacies Sl. Cross-bedding within lithofacies Slis formed by deposition on initially dipping surfaces such as scourhollows, and by washed-out or humpback dunes and antiduneswhich occur at the transition between subcritical and supercriticalflow (Miall, 1996).

Facies VI is a fine- to pale green to gray coarse-grained sand-stone with conglomeratic units. This facies is between 10 cm and2.8 m thick and contains planar cross-bedding (Fig. 4E). It co-occurs mainly with facies IV, V, and VIII, and occasionally showsan erosive lower boundary. Facies VI corresponds to Miall’s(1996) lithofacies Sp, in which cross-bedding is formed by dunemigration. A wide variety of internal structures within the cross-bedding may be generated, depending on flow velocity and the rateof sediment supply. Lithofacies Sp is interpreted to be deposited bytransverse and linguoid bedforms (Miall, 1996).

Facies VII is a gray to white, fine- to coarse-grained sandstonewith conglomeratic beds. The facies is 20 cm to 1 m thick andshows trough cross-bedding (Fig. 4F). It is found in juxtapositionwith various other facies, and corresponds to Miall’s (1996)litho-facies St, which is interpreted as minor channel fill bysinuous-crested and linguoid dunes.

Facies VIII consists of very-fine- to very-coarse-grained sand-stones, which are gray to purplish-white in color. This facies is15 cm to 2 m thick and is horizontally bedded (Fig. 4F). It corre-sponds to Miall’s (1996) litho-facies Sh, which represents upperplane-bed conditions at the transition from subcritical to supercrit-ical flow. Lithofacies Sh, up to several meters thick, may be depos-ited during a single dynamic event such as a flash flood (Miall,1996). Facies VIII may therefore have been deposited under a highflow velocity, such as during a flash flood event.

Facies IX consists of purplish, yellowish, and pale greenish gray,fine- and medium-grained sandstone. The facies varies from 15 cmto 3.5 m in thickness and frequently shows faint laminationsand bioturbation, locally containing calcretes. Facies IX is oftenaccompanied by facies IV and XI, and corresponds to Miall’s(1996) litho-facies Sm and P, which are lithologies modified bypost-depositional disturbance.

Facies X consists of very-fine- to medium-grained sandstone,showing ripple cross-laminations, and is 10 cm to 6.1 m thick. Itcontains loading structures, bioturbation, pseudo-climbing ripples(Fig. 4G), root traces, and calcretes. Facies X occasionally showspatches of pale green color in laminated sediments that are other-

wise purplish (Fig. 4H). It co-occurs with facies XI and XII, and cor-responds most closely to lithofacies Sr of Miall (1996), which isformed by ripple migration. The finely laminated part of facies X,however, corresponds to Miall’s (1996) lithofacies Fl, which repre-sents deposition from suspension and from weak traction currentsin overbank areas. Some parts of facies X with calcretes probablyalso correspond to Miall’s (1996) lithofacies P and are interpretedas soils containing chemical precipitates.

Facies XI comprises mudstone and very-fine-grained sandstonewith intercalations of coarser sandstone (Fig. 4I), which is red topale green in color. The facies is 10 cm to 5.9 m in thickness andthe beds show faint laminations. Mottles, bioturbation, loadingstructures, root traces, and calcretes are often observed in this fa-cies. The mottles are interpreted as representing glaebules of veryirregular shape, with diffuse boundaries (Retallack, 1997). Glae-bules are segregations of materials distinct from other parts ofthe soil, and include such features as nodules, concretions, and sep-taria (Retallack, 1997). Mottles in this facies occur as thin pale-green layers with abundant bioturbation (Fig. 4J). The sandstonelayers often show an irregular boundary with underlying strata. Fa-cies XI co-occurs with facies XII, and corresponds to Miall’s (1996)lithofacies Fl, like facies X, but can be distinguished from facies X inbeing finer-grained and being modified by abundant bioturbation,such as tubular trace fossils. The tubular traces occur in a randomorientation and occasionally include a meniscate structure(Fig. 4K). Parts of facies XI containing calcretes probably corre-spond to Miall’s (1996) lithofacies P, and are interpreted as soilsin which chemical precipitates formed.

Facies XII is composed of mudstone and very fine-grained sand-stone, and is red to pale green in color. This facies is massive andcommonly contains calcretes and mottles, and locally slickensidesand rootlets. Slickensides are striated and smeared surfaces thatare typically formed in clayey soils by repetitive swelling andshrinking during wetting and drying episodes, although they canalso be formed in paleosols by the crushing of peds during burialcompaction (Retallack, 1990). In facies XII, slickensides are ob-served in a grayish or greenish-brown siltstone. The planar struc-tures and the striations of the slickensides are developed invarious orientations (Fig. 4L). Small rootlets are usually gray toblack in color and are distributed randomly (Fig. 4M). The thick-ness of this facies varies from 15 cm to 10 m, and it is often foundin conjunction with facies XI. Facies XII corresponds to Miall’s(1996) lithofacies Fm, Fsm, or P, and is interpreted as overbank,backswamp, abandoned channel or drape deposits, and soils withchemical precipitates.

3.2. Architectural elements

An architectural element is defined as a three-dimensionalcomponent of a depositional system, characterized by a distinctivefacies assemblage, internal geometry, external form, and verticalprofile (Miall, 1996). Miall (1985, 1996) suggested that there are

N N

Paleocurrent flow direction from SB, LA and DA, n=18

Paleocurrent flow direction from LV, CR and CS, n=13

Fig. 5. Paleocurrent flow directions for the studied section. Data shown in blackrepresent paleocurrents from channel-fill elements (SB, LA, and DA), and datashown in white represent paleocurrents from overbank elements (LV, CR, and CS).


eight basic architectural elements in fluvial deposits: channels(CH), lateral-accretion deposits (LA), sediment-gravity-flow depos-its (SG), downstream-accretion macroforms (DA), gravel bars andbedforms (GB), sandy bedforms (SB), laminated sand-sheets (LS),and overbank fines (FF). These architectural elements were origi-nally applied to sandy fluvial systems (Miall, 1996).

Facies I–XII are classified into facies assemblages based on theirstratigraphic relationships and outcrop-scale macroforms. As a re-sult, eight architectural elements are recognized in the Phu Kra-dung and Phra Wihan formations (Table 2).

In the studied section, the channel architectural element (CH)comprises facies IV and V. This element is defined by its channeli-zed geometry and is interpreted as a channel-fill deposit. The CHelement has a concave-up erosional base, and is about 2.5 m deepand more than 25 m wide. CH appears in only one horizon in thestudied section.

Sandy bedforms (SB) are composed mainly of sandy facies IV, V,VI, and VIII, and subordinately of facies VII, I, X, XII, IX, and II. SBelements show massive or parallel/cross-bedded structures. Thiselement is interpreted as sandy bedforms that accumulated in achannel, predominantly by vertical aggradation. SB elements occa-sionally show an erosive boundary at their base and often co-occurwith lateral-accretion deposits (LA) and crevasse splay deposits (CS).

Lateral-accretion deposits (LA) consist mainly of sandy facies IV,V, and VI, and subordinately of facies II, III, X, I, XI, and XII. LAelements are defined by the presence of lateral accretion sets. Thiselement appears typically in the sandstone-dominated part of thesequence and is frequently accompanied by SB elements, whichwere deposited by vertical aggradation. LA elements are inter-preted as channel-fill and point-bar deposits that formed by thelateral migration of channels. Though LA is classified as a chan-nel-fill element, some LA deposits in the studied section may cor-respond to crevasse channel-fill deposits. These LA elements showribbon-like bodies and occasionally contain abundant calcareousclasts, very coarse-grained sand to granular in size, which areinterpreted as reworked calcretes.

Downstream-accretion macroforms (DA) are composed of faciesVI and X. DA elements are characterized by several co-sets ofdownstream-oriented, flow-regime bedforms dynamically relatedto each other by a hierarchy of internal, downstream-dipping,bounding surfaces. Though most sandstone elements accumulatedby both vertical aggradation and horizontal accretion, LA and DAelements correspond to those sediments displaying obvious evi-dence of horizontal accretion. Where vertical aggradation is dom-inant, the sediments are classified as SB elements. According tothe definition of Miall (1996), the orientation of an accretionarysurface of DA elements is parallel or oblique (within 60�) to localflow. In the case of orientations at an angle of more than 60�, the

sediments are classified as LA elements. DA elements are observedin only one horizon in the studied section.

Levee elements (LV) comprise facies IX, X, and XII, and showrhythmically bedded units of silty ripple-laminated sandstone.Laminations are frequently obscured by bioturbation. Accordingto Miall (1996), each rhythmic bed within LV elements representsa flood event. In the studied section, LV reaches about 5 m in thick-ness. In general, a levee constitutes a tapering, wedge-shaped de-posit, thinning and fining away from the channel margin (Miall,1996).

Crevasse channel elements (CR) are composed mainly of faciesX, VIII, and VII, and subordinately of facies IV, I, II, V, and VI. Theyare characterized by ribbon-like bodies with an erosional base andinternal scours. Lag deposits occasionally contain fossil remainssuch as bone fragments, and calcareous clasts that are interpretedas reworked calcretes. According to Miall (1996), the scale of cre-vasse channels depends on the scale of the river.

Crevasse splay deposits (CS) are seen primarily in facies XI, X,IV, and XII, and subordinately in facies VIII, IX, III, VII, VI, and II.CS elements show thin bedding and gently dipping accretion sur-faces with abundant hydrodynamic sedimentary structures, plantroots, and bioturbation. Ripple cross-laminations are common. CSdeposits are characterized by abundant surfaces of non-depositionand small-scale erosion. These surfaces reflect the origin of thesplays as periodic or irregular sheet floods (Miall, 1996). CS ele-ments can be discriminated from LV elements by their coarsergrain size and the presence of internal, small-scale erosive sur-faces. Though facies III is classified as representing a CS element,some units within that facies occur as lenticular bodies, which fillsmall scour surfaces and are independently intercalated in flood-plain deposits. Facies III is interpreted as deposits formed bypseudoplastic debris flows and sediment gravity flows. Therefore,as discussed below, facies III may represent ephemeral channelsdeveloping within the floodplain. CS elements appear throughoutthe studied section, and amalgamated beds occasionally reach8 m in thickness. Crevasse splays are delta-like deposits that formadjacent to the margins of main channels, interfingering at theirmargins with fine-grained floodplain deposits (Miall, 1996).

Floodplain deposits (FF) are represented mainly by the fine-grained clastic facies XII, XI, and X, and subordinately by facies VIII,IX, and IV. These facies are interpreted as floodplain deposits be-cause they show paleosol features in fine-grained flooding-relatedsediments. It appears that original sedimentary structures were re-moved by pedogenesis and bioturbation. FF deposits occurthroughout the studied section and can be more than 15 m thick.The uppermost part of FF elements is usually overlain by chan-nel-fill elements across an erosive boundary. CS and CR elementsare often intercalated within FF deposits and contain abundantbioturbation.

4. Paleocurrent data

Paleocurrent analysis is generally used as a tool to obtain infor-mation on: (1) vertical or lateral changes in fluvial style, (2) tribu-tary or distributary patterns, (3) local and regional paleoflowpatterns, (4) vertical changes in flow direction through a strati-graphic section as indicators of interacting fluvial systems, and(5) vertical changes in the orientation of a system (Miall, 1996).

Thirty-one paleocurrent readings were collected from the stud-ied section (Fig. 3). Of these, 18 were collected from cross-beddingin channel-fill elements, such as SB, LA and DA, and are repre-sented in Fig. 3 by arrows with solid circles. The other readingswere collected from overbank deposits, such as LV, CR and CS ele-ments, and are shown in Fig. 3 by arrows with dotted circles. Tilt-ing of the strata was corrected using the Wulff Net method of


stereographic projection to reconstruct the original paleocurrentdirections. Summary paleocurrent rose diagrams are shown inFig. 5. Data from the channel-fill deposits indicate a predominantlysouthwestward flow in the studied section, whilst the data fromthe overbank elements yield a north-to-northeastward flow(Fig. 5).

5. Calcrete horizons

In the Nong Bua Lamphu section, the Phu Kradung and Phra Wi-han formations are characterized by the presence of abundant cal-crete horizons in fine-grained sandstones or mudstones (FF, LV,and CS elements; Fig. 6A and B). Traditionally soil horizons havebeen designated by shorthand of letters and numbers (Retallack,1997). Upper case letters are used for the main horizons, A–Cdownward from the surface. The soil profile consists of a near-surface A horizon, a subsurface B horizon, and a less weatheredparent material C horizon (Retallack, 1997). Lower case symbolsare used with the horizon designation to denote special features:k is for the horizon with accumulation of carbonates. The calcretesformed by pedogenesis and generally occur within a specific hori-

A B

C D

E F

G H

Fig. 6. Calcrete structures and microstructures. (A and B) appearance in outcrop of calpatchiness due to iron staining (dark region); (D) euhedral, bladed crystals of calcite surro(arrows) and cracks filled with sparry calcite; (F) micrite and cracks/voids filled with spaC–G taken under cross-polarized light; H under plane-polarized light.

zon in the soil profile, which can usually be recognized as a Bk hori-zon. The Bk horizons in the studied section are mostly parallel tobedding and are occasionally truncated by overlying sediments.The interval between successive Bk horizons is highly variable,and the horizons range in thickness from 1 cm to 4 m.

5.1. Occurrence and microstructure of calcretes

Calcretes occur within 85 horizons in the Phu Kradung and PhraWihan formations in the studied section (Fig. 3). Of the 85 hori-zons, 79 are in the Phu Kradung Formation and 6 in the Phra WihanFormation. The calcretes are irregular-, disc- and rod-shaped, andsome of their shapes are strongly related to biogenic structures,such as root traces and burrows. They show various colors, includ-ing white, pale green, and brown. Though the diameter of the cal-cretes varies from less than 1 cm to more than 10 cm, in a singlehorizon they are of similar sizes and are arranged nearly parallelor oblique to the bedding planes.

Various microstructures are observed in the calcretes, such asdense microfabrics, floating detrital grains, cracks and voids, andsparry calcite. Dense microfabrics are commonly observed in cal-

cretes (whitish color) in reddish-colored mudstone; (C) micrite showing a diffuseunded by micrite (eu); (E) micrite showing bladed extinction, floating detrital grainsrry calcite (arrows); G: alveolar septal fabric; H: aggregations of micrite (p). Images

Fig. 7. Stratigraphy of the studied section, showing the distribution of reworked deposits (RW01–11).


cretes in the studied section. These comprise micrite, which showsa diffuse patchiness (Fig. 6C) due to variations in crystal size andthe presence of fine-grained clays and iron staining. Relativelylarge crystals occur at boundaries with the sedimentary matrixand voids filled with clay, iron stain, or sparry calcite. These large

crystals frequently have a euhedral, bladed shape (Fig. 6D). Asshown in Fig. 6E, some regions of the micrite show bladedextinction.

Floating detrital grains are frequently observed in the micriticgroundmass. In some cases, these grains are surrounded by a spar-


ry zone, showing a sharp contact with the outer part of themic-rite. Such sparry zones have been reported as grain coatingsor infills of circumgranular cracks (Tandon and Friend, 1989). Thesparry zones usually occur uniformly around a grain within theplane of section, and are therefore considered to be grain coatings.

Cracks and voids are occasionally observed within the micriteand are frequently filled with sparite (Fig. 6F) and clay, or, morerarely, occur as open, unfilled spaces. Where the cracks and voidsare filled with sparite, the sparry calcite crystals tend to be largertowards the inner part of the crack or void. Some clay-filled andopen voids within the micrite are surrounded by large crystalsand are probably relicts of parent matrix sediments, with the voidsrepresenting dissolved parts of the relicts. Tandon and Friend(1989) reported two types of coarse-grained carbonate: one withdiffuse margins (patches) and one with sharply defined margins(fenestrae or crack fills). In this study, sparry calcite with diffusemargins appears to occur as voids of irregular shape within the mi-crite (Fig. 6F). Conversely, sparry calcite with sharply defined mar-gins occurs as various shapes within voids, cracks, and veins(Fig. 6E).

A complex of ribbon-like calcite crystals is shown in Fig. 6G.These crystals form a tubular shape within the micrite, and appearto be tangled around each other. This fabric is similar to the alve-olar-septal structure described previously by Wright (1986). Analveolar-septal structure is composed of arcuate septa that consistof parallel-oriented, needle-fiber calcite within intergranularspaces or in voids such as root molds (Wright and Tucker, 1991).

Aggregations of micrite are shown in Fig. 6H. Such aggregationshave been documented by Solomon and Walkden (1985) as a pel-leted fabric. The observed pellets occur as an assemblage of vari-ously sized, spherical, micritic grains with irregular, slightlydiffuse outlines, which are partly amalgamated. Iron oxide anddetrital grains occur in the matrix.

5.2. Interpretation of calcrete microstructure

The micromorphology of calcretes can be classified into two endmembers (alpha and beta) controlled by climate (Wright, 1990;Wright and Tucker, 1991). Alpha-type calcretes have a densemicrofabric, typically with such features as floating detrital grainsand large euhedral crystals (Wright and Tucker, 1991). Theyform by chemical precipitation associated with evaporation,evapotran-spiration, and degassing (Wright and Tucker, 1991). Incontrast, beta-type calcretes exhibit microfabrics dominated bybiogenic features such as rhizocretions, needle-fiber calcites, alve-olar septal fabrics, and calcified pellets (Wright and Tucker, 1991).The calcretes of the studied section show characteristics of both al-pha- and beta-type calcretes, suggesting a mixture of biogenic andnon-biogenic origins.

Dense microfabrics and associated floating detrital grains aretypical components of alpha-type calcretes and have been reportedwidely from pedogenic calcretes (Esteban and Klappa, 1983;Goudie, 1983; Khadkikar et al., 2000; Tandon and Gibling, 1997;Wright and Tucker, 1991). Micrite forms via the simultaneousgrowth of closely spaced nuclei (Tandon and Friend, 1989), possi-bly in response to the rapid degassing of carbon dioxide duringevaporative processes (Wieder and Yaalon, 1974). Floating detritalgrains develop by textural inversion, caused by the growth of sec-ondary micrite in the spaces between grains (Wright and Tucker,1991). These microstructures are commonly observed in calcretesin the studied section and indicate precipitation by pedogenic pro-cess. Cracks and voids are probably formed physically or biogeni-cally by processes such as the swelling of the surroundingmicrite or the development of roots or burrows, respectively. It isknown that root structures often become infilled with clay or spa-rite after the roots have decayed (Wright and Tucker, 1991). In the

case of calcretes around roots, rootlets play a more significant rolein the formation of microsparite veins (Khadkikar et al., 2000).Tandon and Friend (1989) interpreted sparry calcite with sharplydefined margins as the result of the passive fill of cracks that wereempty of solids at the time of spar growth. In this study, therefore,most of the observed sparry calcite is thought to have formed byprecipitation after the formation of the surrounding micrite, asthe result of passive infill of cracks or voids, or the intrusion ofveins. Some of the sparry calcite with diffuse margins is likely tohave developed at a relatively early stage of precipitation, due toprocesses such as a difference in the local growth patterns of themicrite and the replacement of clay by calcite. Alveolar-septalstructures are considered to be of biogenic origin, such as fungalsheaths on the mycorrhiza (Wright and Tucker, 1991). Observedneedle-fiber calcite has a diffuse boundary with the surroundingmicrite and seems to be fragmented and recrystallized. Therefore,it is difficult to identify the type of mycorrhiza.

As mentioned above, Wright (1990) classified microstructuresof calcrete into alpha- and beta-types. Alpha- and beta-type cal-cretes are developed in relatively arid climates with little bioactiv-ity, and in semi-arid to sub-humid areas covered by vegetation,respectively (Wright and Tucker, 1991). The microstructures ofthe present study correspond to those of both alpha- and beta-typecalcretes, and were probably formed in several stages, includingmicritic carbonate precipitation, modification by biogenic activity,recrystallization, and the precipitation of sparry calcite. Therefore,the calcretes of the studied section are interpreted to have formedunder varying conditions, including a relatively arid climate for theprecipitation of micritic carbonate, and semi-arid to sub-humidconditions for the formation of alveolar-septal structures.

6. Reworked deposits

A compilation of the stratigraphy in the studied section isshown in Fig. 7. Sandy conglomerates and conglomeratic sand-stones, which contain calcareous clasts, are observed at 11 hori-zons (RW01–11) in the studied section (Figs 3 and 7). Variousmicritic microfabrics, similar to those seen in calcretes, are ob-served in the calcareous clasts. The calcareous conglomeraticdeposits in the studied section are therefore interpreted to be re-worked calcrete clasts.

The style and composition of reworked deposits are importantfor deducing the processes of soil formation on floodplains andfor determining the drainage network of the basin (Gómez-Grasand Alonso-Zarza, 2003). Reworked calcrete clasts of various sizeshave been found in different types of fluvial deposits, such asdunes, the bases of major channels, and sheet-flood deposits(Gómez-Gras and Alonso-Zarza, 2003; Khadkikar et al., 1998).Gómez-Gras and Alonso-Zarza (2003) classified reworked calcretedeposits in meandering fluvial depositional systems into three dif-ferent types, and discussed the aggradation rates of the floodplain,and climatic conditions. According to Gómez-Gras and Alonso-Zarza (2003), the three types that occur are: floodplain depositsof ephemeral channels draining interfluvial areas (type 1), crevassedeposits that break the natural levee of the major channels (type2), and channel-lag deposits of major channels that drain the allu-vial plain (type 3).

Reworked deposits in the studied section (RW01–11) occur assandstones and conglomerates in the SB, LA, CS, CR, and FF ele-ments. Reworked deposits are classified into three types basedon their occurrence, following Gómez-Gras and Alonso-Zarza(2003).

Type 1 reworked deposits are represented by units RW04,RW06, RW09, RW10, and RW11. This type occurs as sandstonesin CS and FF elements in the studied section. These deposits occur

CH (channel)

SB (sandy bedforms)

LA (lateral accretion)

DA (downstream accretion)

LV (levee)

CR (crevasse channel)

CS (crevasse splay)

FF (floodplain deposit)

CH (channel)

SB (sandy bedforms)

LA (lateral accretion)

DA (downstream accretion)

LV (levee)

CR (crevasse channel)

CS (crevasse splay)

FF (floodplain deposit)

Fig. 9. Frequency of occurrence of various elements in the studied section. CH, SB,LA, and DA are channel-fill elements that make up about 30% of the total thickness.LV, CR, CS and FF are overbank elements and occupy about 70% of total thickness.The sediment supply system of the studied section was characterized by thedominance of FF, CS, and SB.


as lenticular bodies overlying small scour surfaces, which are about10–20 cm thick. These lenticular bodies frequently occur as assem-blages of several layers. The deposits are relatively well sorted, andare composed mainly of reworked calcrete clasts. The clasts arevery coarse-grained sand and are rounded to subrounded(Fig. 8A). The clasts locally contain cracks filled with sparry calcite,branching and tapering towards the margin of a micritic clast(Fig. 8B). Micrite in some of the clasts is stained and shows bladedextinction radiating from the center of the clasts (Fig. 8B). Suchcracks and bladed extinction are strongly associated with theshape of clasts. Therefore, these features probably reflect an origi-nal texture, such as a root mold, or acquired their spherical shapeby abrasion during reworking. The sparry calcite in the cracks isinterpreted as a later stage of precipitation.

Type 2 reworked deposits occur in units RW03 and RW05,which occur as sandstones or conglomerates in CS deposits. Thesedeposits are characterized by the appearance of lenticular bodieswith small, scoured bases and convex tops, which are about30–40 cm in maximum thickness and that fine upwards. Cross-lamination and pseudo-climbing ripples are observed in the over-lying sediments (Fig. 8C). These deposits are composed mainly ofreworked calcrete clasts that are medium-grained sand to granulesand that are rounded to subrounded (Fig. 8D). Locally, the depositscontain biogenic fragments.

Type 3 reworked deposits are seen in units RW01 and RW02,which occur as sandy conglomerates in SB and LA deposits in thestudied section. They are poorly sorted and are matrix-supported.These deposits are composed of abundant reworked calcrete clastsand detrital grains. The reworked calcrete clasts are very coarse-grained sand to pebbles that are rounded to subangular. They arepale green to gray in color (Fig. 8E), and most of the clasts have amicritic texture. Concentric structures are observed in larger clasts(Fig. 8E). Such clasts contain many floating detrital grains withgrain coatings in the outer part of the concentric structure(Fig. 8F). The central part of the concentric structure occasionallycontains small voids that are arranged along the concentric bandand that are filled with sparry calcite (Fig. 8G).

A D

B E H

C F

Fig. 8. Occurrence of reworked deposits. (A) Occurrence of RW04; (B) fractured micriteclasts in RW05; (E) cut slab surface of RW02; clast towards the top shows concentric(RW02); (G) small holes filled with sparry calcite (RW02); (H) low-angle cross-bedded stsparite cement); (J) micrite clasts containing floating sediment grains, RW07. Images B, FStratigraphic locations of all horizons are shown in Figs 3 and 7.

In terms of their occurrence within channel-fill deposits, RW07and RW08 are similar to type 3 reworked deposits. RW07 andRW08 occur as sandstones or conglomerates in LA and CR elementsin the studied section and are characterized by large-scale, low-angled cross-bedding (Fig. 8H). They are relatively well sorted,show a clast-supported texture, and are sporadically cementedby sparite (Fig. 8I and J). These deposits are composed mainly of re-worked calcrete clasts of coarse-grained sand to granules that aresubrounded to angular (Fig. 8J). The deposits contain biogenic frag-ments. The reworked calcrete clasts are reddish in color and mosthave a micritic texture, occasionally with floating detrital grains(Fig. 8J).

7. Discussion

7.1. Depositional environment

Eight architectural elements of a depositional system (CH, SB,LA and DA for channel-fill; LV, CR, CS, and FF for overbank ele-

G

I

J

clast, showing bladed extinction. (RW06); (C) lenticular body of RW03; (D) micritestructure; (F) floating sediment grains and grain coatings observed within a clastructure in RW07 (between dotted lines); (I) cut slab surface of RW08 (white part isand J taken under cross-polarized light; images D and G under plane-polarized light.


ments) are recognized in the studied section. To characterize thedepositional system, the proportion of each element was calcu-lated, based on the ratio of the thickness of each element to the to-tal thickness of the section (Fig. 9). Channel-fill and overbankelements make up about 30% and 70% of the total thickness,respectively. Vertical aggradation, represented by SB elements, ispredominant among the channel-fill elements, though lateralaccretion represented by LA deposits is also a significant compo-nent. Vertical aggradation seems to be dominant over lateral accre-tion, thereby being characteristic of the sediment supply system ofthe studied section. In general, typical LA deposits show offlappedupper terminations followed by fine-grained facies of the FF ele-ment (Miall, 1996). However, in the case of the studied section,internal erosive surfaces are frequently observed in channel-fill.Furthermore, LA deposits are occasionally cut by overlying SB ele-ments or a different unit of LA deposits. Therefore, it is probablethat lateral channel migration was minimal, and point-bar depositsare rarely preserved in the studied section. Among the overbankelements, FF is the dominant component of the studied section.It is hypothesized that the depositional environment of the studiedsection was characterized by a well-developed floodplain. Further-more, CS deposits make up a considerable proportion of the flood-plain successions, representing 23% of the total thickness. It islikely that crevassing was the main aggradational process on thefloodplain.

Miall (1996) introduced 16 types of fluvial facies models. Ofthese, an anastomosing fluvial system is most consistent with thesedimentary characteristics of the studied section. Fig. 10 shows ageneral depositional environmental model of the studied section.Anastomosing river systems are characterized by a network of rel-atively stable, interconnected channels of various sinuosities(Smith and Smith, 1980). Most examples of anastomosing rivershave low-gradient floodplains and low stream power (Makaske,2001; Miall, 1996). Channel banks in anastomosing river systemsare typically cohesive and steep-sided, reflecting the low stream

LA

CS

SB

LV

Fig. 10. Depositional environments within an anastomosing river system, showing the delements discussed in the text: SB (sandy bedforms), LA (lateral accretion), DA (dow(floodplain deposit). Modified from Miall (1996).

power and the fine-grained sediments (Makaske, 2001; Miall,1996). Crevasse channels and splays are particularly importantcomponents of the anastomosing fluvial environment becausechannel evolution is driven by crevassing and the developmentof stable crevasse channels (Miall, 1996; Smith et al., 1989). Flowfrom crevasse channels may eventually rejoin the main channeldownstream, or be diverted into the course of a paleochannel, pro-ducing a significant diversion of flow (Richards et al., 1993; Smith,1983; Smith et al., 1989). Anastomosing fluvial systems are foundin both humid and arid climates, and the primary cause of anasto-mosis is generally thought to be the low gradient of the river(Miall, 1996). The damming of a river, caused by the appearanceof an impediment and a rise in base level, may reduce its slopeand result in an anastomosing river (Miall, 1996). Such river sys-tems have been interpreted as occurring where a river flows acrossa basin undergoing rapid subsidence, but with a downstream dam-ming effect caused by local bedrock control (Miall, 1996).

Maranate and Vella (1986) estimated continuous subsidence ofNortheast Thailand during deposition of the Khorat Group asdecreasing in rate exponentially, based on the thickness and ageof formations. This estimation was carried on the assumption ofthe depositional age in Jurassic and Cretaceous for the KhoratGroup, whereas more rapid rate of subsidence can be inferred inconsideration of possible age mostly in Cretaceous, as suggestedby Racey and Goodall (2009). Mouret et al. (1993) proposed thatthe depositional environment of the Upper Phu Kradung Formationwas an anastomosed meandering channel, related to a likely baselevel fall. However, in the Holocene deposits of the Rhine–Meusedelta, Törnqvist (1993, 1994) and Törnqvist et al. (1993) demon-strated that a rapid rise in base level led to the development ofan anastomosing fluvial style with high avulsion frequency,whereas a slower rise of base level resulted in the formation of ameandering channel system. Charoentitirat et al. (2009) andMouret (1994) reported a marine incursion near the top of thePhu Kradung Formation around Uttaradit and Phitsanulok, about

reworked deposit

FF

coarser sediments

finer sediments

plants

CR

DA

istribution of reworked deposits and the relationships of some of the architecturalnstream accretion), LV (levee), CR (crevasse channel), CS (crevasse splay) and FF


300 km west of Nong Bua Lamphu, Northern Thailand. Therefore,the deposition of the studied section may have coincided with abase-level rise.

7.2. Interpretation of reworked deposits

The presence of reworked calcrete deposits indicates that in situcalcretes were simultaneously formed and eroded in a depositionalsystem. The occurrences and characteristics of reworked depositsare important in interpreting the infill of terrestrial basins andthe construction of floodplains (Gómez-Gras and Alonso-Zarza,2003). Here, we summarize the occurrence of reworked depositsin the studied section and discuss the reworking system of thefloodplain in the Khorat Basin.

Type 1 reworked deposits of Gómez-Gras and Alonso-Zarza(2003) (i.e., RW04, RW06, RW09, RW10, and RW11 in the studiedsection) are interpreted as ephemeral channel and sheet flooddeposits in interfluvial drainage systems. Ephemeral channelsand sheet floods are formed after sporadically heavy rain, indrained local areas of the floodplain (Gómez-Gras and Alonso-Zarza, 2003; Marriott and Wright, 1993). The frequent occurrenceof such reworked deposits in the studied section indicates that thePhu Kradung and Phra Wihan formations were deposited under apaleoclimate with sporadic and heavy rainfall events.

Type 2 reworked deposits of Gómez-Gras and Alonso-Zarza(2003) (RW03 and RW05 in the present study) are interpreted ascrevasse splay deposits and were deposited by floodwaters flowingdown levees of the major streams and eroding poorly developedsoils near channels (Gómez-Gras and Alonso-Zarza, 2003). The ob-served reworked deposits are composed of relatively small claststhat may have resulted from the erosion of poorly developed soils.

Type 3 reworked deposits of Gómez-Gras and Alonso-Zarza(2003) (RW01 and RW02 in the studied section) occur in the basalpart of a channel deposit (Fig. 4A) and are interpreted as lag depos-its of a main channel or channel-fill deposits formed by debrisflows. The main processes of formation of reworked calcretedeposits as channel lags are the lateral migration of channels ortheir avulsion (Gómez-Gras and Alonso-Zarza, 2003). The concen-tric structure, which is observed in relatively large clasts inRW01 and RW02, was probably formed by the calcification ofroots. Root calcification structures are commonly observed in cal-cretes (Wright et al., 1995), and it is known that they occur whenroot molds are filled with sediments after the root has decayed(Wright and Tucker, 1991). The observed voids, filled with sparrycalcite, probably represent the former location of the root. Thoughthe time required for the formation of nodular to massive calcretesis highly variable, root calcification occurs relatively rapidly(Wright et al., 1995). Therefore, it is possible that the calcretes withroot calcification structures occurred in weakly developed soil onthe floodplain, which shows relatively rapid aggradation. Rapidcalcification of roots probably resulted in larger clast sizes in theobserved reworked deposits compared with other micritic clasts.Furthermore, such clasts contain many detrital grains in the outerpart of the concentric structure, indicating that compared withother micritic clasts, they resisted dissolution or abrasion by watercurrents in the main channel. It is known that reworked calcretesdeposited as channel lags (e.g., RW01 and RW02) are typicallyformed due to active erosion caused by channel migration duringlow rates of aggradation of the floodplain (Gómez-Gras andAlonso-Zarza, 2003). Though the reworked channel lag deposits oc-cur at only two horizons in the lower part of the studied section(Fig. 7), such rare occurrences are consistent with a depositionalenvironment of an anastomosing river system, examples of whichtypically show high rates of vertical aggradation of the floodplain.

RW07 and RW08 are interpreted as crevasse channel deposits.However, reworked deposits from crevasse channel-fills are rarely

reported from meandering fluvial depositional systems. Large-scale, low-angle cross-bedding probably corresponds to a lateralaccretion structure formed by lateral migration of the channel.Such well-developed lateral accretion structures indicate that thecrevasse channel in the studied section was relatively stable. Thedevelopment of stable crevasse channels is consistent with a depo-sitional environment of an anastomosing river system. It is inferredthat the calcretes that developed on the floodplain were eroded bythe lateral migration of a crevasse channel. The occurrences ofthese deposits in the studied section probably represent the depos-its of an anastomosing river system.

7.3. Distribution of calcrete horizons and pedogenesis in the PhuKradung and Phra Wihan formations

Here, we discuss the calcrete profiles in the floodplain of thestudied section, based mainly on the thicknesses of the calcretehorizons and the thicknesses of the intervals between successivecalcrete horizons.

The calcrete horizons in the studied section vary from 1 cm to4 m in thickness. According to Retallack (2005), there is a positivecorrelation between the mean annual range of precipitation (M,mm), which is the difference between maximum and minimumprecipitation within a year, and the thickness of carbonate nod-ule-bearing soils (T, cm), as shown by the formula:M = 0.79T + 13.71. As a result of calculations based on this formula,the mean annual range of precipitation is estimated to be between15 and 330 mm. On this basis, it is inferred that the differences inthe amount of precipitation between the wet and dry periods werehighly variable and without any order during the deposition of thePhu Kradung and Phra Wihan formations. However, in several cal-crete horizons, it is possible that the Bk horizon is partly superim-posed and that, as a result, multiple Bk horizons have amalgamatedinto a single, thick calcrete profile.

The thickness of the interval between successive calcrete hori-zons is highly variable through the studied section. Thus, the in-flow of fine-grained material probably occurred randomly acrossthe floodplain, because one calcrete horizon represents one periodof soil formation. It is inferred that partly superimposed, multiplecalcrete horizons were developed during periods with a low sedi-ment supply. As proposed by Bown and Kraus (1987), a pedofaciesrelationship between detritus supply and soil maturity is generallyapplied within an alluvial plain system: immature soils occur onthe alluvial ridge, whereas the most mature soils occur on the dis-tal floodplain. In an alluvial plain system, the maturity of paleosolsin a single floodplain profile broadly reflects the migration or avul-sion of the channel (Wright, 1992). Based on this pedofacies rela-tionship, it appears that the amount of sediment (i.e. theintervals between calcrete horizons) also reflects the migrationor avulsion of the channel. As mentioned previously, crevassechannels and splays are particularly important components ofthe anastomosing fluvial environment (Miall, 1996). Therefore,the random distribution of calcrete horizons in the studied sectionis probably caused by random avulsion and crevassing eventswithin an anastomosing river system, rather than constant lateralmigration of the channel. Furthermore, it is inferred that a highrate of vertical aggradation of the floodplain in an anastomosingriver system inhibited the development of soil and resulted inthe weak development of paleosol profiles across the studied sec-tion. Such weakly developed calcretes within floodplain mud-stones would be easily liberated and reworked by superficialrunoff (Gómez-Gras and Alonso-Zarza, 2003). As shown by thepresence of reworked calcrete horizons in the studied section, itis possible that erosion of the floodplain occurred frequently, lead-ing to the random distribution of calcrete horizons.


7.4. Paleoclimate of the Phu Kradung and Phra Wihan formations

Based on the occurrence of evaporitic cements in the UpperNam Phong Formation, and a significant decrease in the abun-dance of evaporitic minerals in the Lower Phu Kradung Forma-tion, Mouret et al. (1993) proposed that the aridity of theclimate lessened during the deposition of the Lower Phu KradungFormation. Subsequently a climate developed in which therewere two seasons per year, sub-arid and humid, based on theoccurrence of calcretes and abundant plant fragments with minorevaporitic minerals in the Upper Phu Kradung Formation (Mouretet al., 1993). Racey and Goodall (2009) suggested a warm, season-ally dry and subtropical climate for the Phu Kradung Formation,based on the occurrence of the palynomorph Corollina. Meesook(2001) and Meesook et al. (1999) reported bedded-nodular sil-cretes from paleosols in the Phu Kradung, Sao Khua, and KhokKruat formations, and reconstructed a semi-arid paleoclimatefor the Sao Khua Formation, based on low concentrations of tita-nium dioxide in the silcretes. Paleosols in the studied section con-tain numerous calcrete horizons, though there is a certain amountof variation in the occurrence of calcretes, such as their shape,size, and abundance. Calcretes observed in the studied sectionwere developed in red-colored and argillaceous soils, and occa-sionally include slickensides. Such calcrete associations are likelyto have formed under a semi-arid to sub-humid climate with100–700 mm in mean annual precipitation (Khadkikar et al.,2000). As mentioned above, the microstructures of the calcretesin the studied section show the characteristics of both alpha-and beta-type calcretes, and are interpreted to have formed undersemi-arid to sub-humid conditions. Furthermore, wet–dry cyclesare essential for carbonate precipitation (Breecker et al., 2009).Seasonal wet–dry cycles are typically shown in monsoon climatesfor present day, while in regions where precipitation is moreevenly distributed throughout the year, pedogenic carbonatemay form primarily during droughts and may accumulate onlyif droughts occur with sufficient frequency (Breecker et al.,2009). Therefore, the calcretes in the studied section were possi-bly formed by precipitation–drought cycles. However, in consid-ering that the pedogenic carbonate accumulated over severaldecades to hundreds years, wet–dry cycles undoubtedly contin-ued during deposition of the studied section. Thus, it is inferredthat paleosols in the Phu Kradung and Phra Wihan formationswere formed under semi-arid to sub-humid climate with wet–dry cycles. This conclusion is consistent with previous studies(e.g. Mouret et al., 1993; Racey and Goodall, 2009). To reconstructthe formative environment more precisely, chemical analyses ofcalcretes and silcretes in the Phu Kradung and Phra Wihan forma-tions are required.

8. Conclusion

Using facies analysis, it is interpreted that an anastomosing riv-er system was developed in the Phu Kradung and Phra Wihan for-mations of the Early Cretaceous Khorat Group. The paleoclimate ofthe Khorat Basin during the Early Cretaceous was semi-arid to sub-humid, with wet–dry cycles. The findings of this study are summa-rized as follows.

The channel-fill and overbank elements make up about 30% and70% of the total thickness, respectively. The depositional environ-ment of the studied section is characterized by a well-developedfloodplain. Crevassing was the main aggradational process on thefloodplain. Furthermore, vertical aggradation seems to have beenthe dominant process of channel-fill, rather than lateral accretion.The occurrences of reworked calcrete deposits indicate a high rateof vertical aggradation of the floodplain, and the development ofrelatively stable crevasse channels. As a result, it is concluded that

the depositional system of the studied section is an anastomosingriver.

Calcretes in the studied section are developed in red-colored,argillaceous paleosols, and occasionally contain slickensides. Theoccurrence of calcretes indicates that the paleosols were formedunder a semi-arid to sub-humid climate. The microstructures ofthe calcretes are interpreted to have formed through several stagesof precipitation, with relatively arid climates permitting the pre-cipitation of micritic carbonate, and semi-arid to sub-humid condi-tions enabling the formation of alveolar-septal structures.

The calcrete horizons in the studied section vary from 1 cm to4 m in thickness. The differences in the amount of precipitationbetween the wet and dry periods were highly variable, withoutany order during deposition, as deduced from the thicknesses ofthe calcrete horizons. However, in several calcrete horizons, it ispossible that the Bk horizon is partly superimposed and, as a re-sult, multiple Bk horizons are coalesced into a single, thick cal-crete profile. The thickness of the intervals between successivecalcrete horizons is highly variable through the studied section.Thus, the inflow of fine materials probably occurred randomlyacross the floodplain. Partly superimposed, multiple calcrete hori-zons were probably developed during periods of low sedimentsupply. It is concluded that the random distribution of calcretehorizons was caused by depositional processes characteristic ofan anastomosing river system, such as random avulsion and cre-vassing events.

Acknowledgements

We would like to thank Prof. Emeritus Kenshiro Ogasawara(University of Tsukuba), Dr. Veerote Daorerk (Chulalongkorn Uni-versity), and Dr. Hitoshi Hasegawa (Hokkaido University) for theirvaluable suggestions and discussions; Dr. Hidetoshi Hara (Geolog-ical Survey of Japan) for revising a draft version of the manuscript;and Mr. Suvapak Imsamut (Department of Mineral Resources), Dr.Thasinee Charoentitirat, Dr. Montri Choowong, Dr. Vichai Chutako-sitkanon, Ms. Teerarat Napradit, Ms. Rattana Theeratititum, Mr.Sarun Kaewmuangmun, Mr. Attiwat Wattanapitaksakul, Ms. Jensa-rin Vivatpinyo, Mr. Kiatkrahohn Nuchprasert (Chulalongkorn Uni-versity), and Dr. Hiroshi Kamikubo (Japan Oil, Gas and MetalsNational Corporation) for supporting our fieldwork in Thailand.

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