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Keywords: Tibet; Da; Lo; Climate; GIS; Chemical index of alteration

Irrawaddy, and Red rivers. Accumulation records ofmaterial delivered by these rivers will enable one to tracethe evolution of the Tibetan Plateau without the

Sedimentary Geology 194 (20 Corresponding author. School of Earth and Environmental Sciences,1. Introduction

The collision of India and Asia and the resulting build-up of the Himalayas and the Tibetan Plateau (HTP) are themost significant tectonic events in the Cenozoic. Thesouthern periphery of the HTP is an especially importantsource of terrigenous sediments on Earth today asevidenced by the large suspended sediment load carried

by the GangesBrahmaputra (Milliman and Meade,1983). The Bengal and Indus fans preserve records ofthe evolution of this system, and one can make links touplift rates and strength of the Asian monsoon systemusing isotopic and sedimentologic data (France-Lanordet al., 1993; Clift et al., 2004). The eastern periphery of theHTP is also an important source of terrigenous sedimentssupplying the Yellow, Yangtze, Mekong, Salween,Abstract

The Red (Hong) River straddles southwestern China and northern Vietnam and drains the eastern Indo-Asian collision zone. Wecollected bed sediments from its tributaries and main channel and report the petrographic point counts of framework grains andmajor oxide compositions as well as organic and inorganic carbon contents. The Q:F:Rf ratios and Q:F:(LLc) ratios of the bed-load indicate quartz-poor, mineralogically immature sediments of recycled orogen provenance. The weathering indices based onmajor oxides the chemical index of alteration (CIA) and the weathering index of Parker are also consistent with the recycledsedimentary nature of the bed sediments. Using geographic information system (GIS) we calculated for each sample basin suchparameters as temperature, precipitation, potential evapotranspiration, runoff, basin length, area, relief, and areal exposure ofigneous, metamorphic and sedimentary rocks. Statistically meaningful correlations are obtained between the two weatheringindices, between CIA and sedimentary to metamorphic rock fragments ratio, S / (S+M), and between CIA and sedimentary rockcover, but otherwise correlations are poor. The bed sediments preserve signatures of their provenance, but the effect of weatheringis not clearly seen. Subtle differences in the bed sediments are observed between the Red and the Himalayan rivers (Indus, Ganges,and Brahmaputra) as well as between sub-basins within the Red River system and are attributed mainly to differences in lithology. 2006 Elsevier B.V. All rights reserved.b School of Earth and Environmental Sciences, Seoul National University, San 56-1, Sillim 9-dong, Gwanak-gu, Seoul 151-747, Korea

Received 18 January 2006; received in revised form 10 May 2006; accepted 22 May 2006Petrography and chemistry of theChina and Vietnam: Proven

Joniell Borges a, Ya Department of Geological Sciences, Northwestern UniSeoul National University, San 56-1, Sillim 9-dong, Gwanak-gu, Seoul151-747, Korea. Tel.: +82 2 880 9167; fax: +82 2 871 3269.

E-mail address: yhuh@snu.ac.kr (Y. Huh).

0037-0738/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2006.05.029d sediments of the Red River ine and chemical weathering

ngsook Huh a,b,

y, 1850 Campus Drive, Evanston, IL 60208-2150, USA

07) 155168www.elsevier.com/locate/sedgeointervening Himalayas (Clift et al., 2004). Moreover,river capture events that accompanied the uplift of the

ary GHTP could be dated (Clark et al., 2004). The Red Riverlies at the heart of this problem, as these large rivers oncefed the proto-Red River draining into the Gulf of Tonkin(Clark et al., 2004). The configuration has since changedsuch that at present the Yangtze flows into the East ChinaSea, the Mekong and Salween to the South China Sea,while the Red River has been truncated into a short river.In order to interpret records of past Red River outflow,it is necessary to understand the present day weatheringprovenance (combination of source rock composition,tectonics, climate and relief) (Pettijohn et al., 1987,p. 297) of its sediments. As an effort toward this goal, westudied the petrography and chemistry of bed sedimentsof the RedRiver and the relationship to climate, relief, andlithology. We specifically address the effects of sourcerocks and evaluate the relative merit of various indicatorsof chemical alteration. To our knowledge, this is one ofthe first studies that combine bed-load petrography andchemistry with GIS for the Red River system.

2. Geological and hydrographical setting

The Red River is relatively small in comparison tothe major river systems originating in the HTP, e.g. theYellow, Yangtze, Mekong, Salween, Brahmaputra,Ganges, and Indus. It drains a 0.12106 km2 zone thatextends from southwestern China to the Gulf of Tonkin(Meybeck and Ragu, 1997)(Fig. 1).With its 123 km3/y ofwater discharge, it transports 18106 t/y of dissolved and130106 t/y of suspended sediment load to the Gulf ofTonkin (Meybeck and Ragu, 1997). The bed-load flux isdifficult to determine but is often assumed to be 10% orless of the total sediment load (Meade, 1988).

The climate is tropical to sub-tropical with an averageprecipitation of 1500 mm/y (Meybeck and Ragu,1997). Most of the water and sediment delivery occursduring the wet season (May to September) when the waterdischarge above the delta reaches 14,000 m3/s and airtemperatures are above 20 C (Fig. 2). During the dryseason, water discharge is 1200 m3/s, and temperaturesare in the low teens.

The Red River system consists of the main channeland the two main tributariesthe Da on the right bankdraining the Indochina block and the Lo on the left bankdraining the Yangzi Paraplatform (Fig. 1). The threechannels merge approximately 50 km northwest ofHanoi. The Red River basin is centered around a majorCenozoic shear zone, the Ailao ShanRed River shearzone (RRSZ), where the Indochina block has extrudedfrom the Tibetan Plateau (Tapponnier et al., 2001). Thegeological history that emerges from past geomorpho-

156 J. Borges, Y. Huh / Sedimentlogic and structural studies is that the Ailao Shan shearzone was actively unroofing till as late as 10 Ma andunderwent a period of deep weathering and erosion, untilin the Pliocene uplift occurred as part of the larger easternmargin of the Tibetan Plateau and the river incised intothe elevated low-relief landscape in response (Schoen-bohm et al., 2004). The total river incision over the twophases is estimated to be 1400 m (Schoenbohm et al.,2004).

The Red main channel runs along the RRSZ, whichextends NWSE, from the Hengduan Mountains insouthwestern China to the Gulf of Tonkin in Vietnam(Tapponnier et al., 1990; Leloup et al., 1995). The RRSZis exposed as a belt of metamorphic massifs of high-grade gneisses to low-grade schists (Leloup et al., 2001):from the southeast, the Day Nui Con Voi, Ailao Shan,Diancang Shan, and the Xuelong Shan off the map to thenorthwest (Fig. 1a). The headwaters of the main channeldrains the Chuxiong Basin composed of thick UpperTriassic to Lower Cenozoic sequences of coal-bearingcontinental clastic rocks (Leloup et al., 1995; Burchfieland Wang, 2003)(Fig. 1). The Da drains the Yangbi andSimao basins with Paleozoic and Mesozoic cover ofcontinental red beds and, close to the RRSZ, Permo-Carboniferous limestones (Leloup et al., 1995). Felsicand ultramafic rocks are limited to a few scatteredexposures near the RRSZ. The Lo drains the South ChinaFold Belt, which is characterized by a thick marinesuccession of Triassic limestone and fine-grained clasticrocks (Burchfiel and Wang, 2003) (Fig. 1). Between theLo main channel and the Chay tributary, Proterozoic toLower Paleozoic low-grade metamorphic and sedimen-tary rocks are exposed along with small granitoidintrusions (Wysocka and Swierczewska, 2003) (Fig. 1).

In summary, the three main tributaries of the RedRiverdrainage system have considerable differences in theirlithologies. The Red main channel is dominated by meta-morphic rocks, except in the upper reaches withMesozoicsedimentary deposits. The Da drainage is composed ofsedimentary rocks of Mesozoic and Paleozoic age withminor felsic intrusions. The Lo drainage has low-grademetamorphic rocks and Proterozoic to Paleozoic sedi-mentary rocks with some granitoid intrusions.

3. Materials and methods

We retrieved bed sediment samples from river banksunder flowing water during two expeditions in wet(AugustSeptember of 2001) and dry (DecemberJanuary of 20022003) seasons. The samples were air-dried and sieved to remove the gravel and plant debris(>500 m). About 1.5 g of

Fig. 1. (a) Simplified geologic map of the Red River drainage basin modified from Wakita et al. (2004). RRSZ: Red River Ailao Shan Shear Zone.(b) Sample location map showing major tributaries of the Red River system.

157J. Borges, Y. Huh / Sedimentary Geology 194 (2007) 155168

Fig. 2. Average monthly precipitation, potential evapotranspiration, and temperature in the drainage basins of the Red River main channel (MC) andits two largest tributaries, the Da and Lo.

158 J. Borges, Y. Huh / Sedimentary Geology 194 (2007) 155168

Table 1Petrography of bed sediments (63500 m) of the Red River

River Name SampleID

Datemm/dd/yy

63500 m(wt.%)

263 m(wt.%)

GazziDickinson method Standard method

L Tot % in sample Rf Tot % in sample

Q F Lv Lc Lch Ls Lm Al Op Hm Q F Pl V Ca Ch S M Al Op Hm

DaBabian Jiang RD121 09/07/01 66.6 31.6 64 1 0 0 2 21 13 100 4 2 2 57 0 0 0 0 2 27 13 100 8 2 0Babian Jiang RD218 01/07/03 59.6 37.4 67 6 0 0 1 8 17 100 5 2 1 51 2 2 1 2 1 22 17 100 4 2 0Amo Jiang RD120 09/07/01 89.9 9.0 65 1 0 0 4 16 14 100 2 4 3 57 0 0 0 0 3 20 20 100 5 3 2Amo Jiang RD217 01/07/03 65.6 33.9 62 2 0 0 2 18 16 100 6 7 8 53 1 2 0 0 0 29 15 100 8 8 4Da @ Lai Chau RD203 12/29/02 97.6 0.9 69 1 0 0 2 9 19 100 2 1 5 57 2 1 1 0 4 23 12 100 6 2 2Namna @ Lai Chau RD204 12/29/02 96.3 1.1 64 4 0 1 0 6 25 100 8 3 10 40 5 6 1 3 1 8 35 100 16 6 8Da @ Muong La, ab. res. RD103 08/18/01 96.9 2.1 70 3 0 0 2 7 18 100 6 2 5 57 3 1 0 0 3 17 19 100 7 2 1Da @ Muong La, ab. res. RD202 12/28/02 94.7 1.7 67 1 0 0 1 15 15 100 5 5 4 51 2 1 0 1 1 19 25 100 8 2 3

Red Main ChannelUpper Lishe Jiang, LB RD229 01/12/03 94.4 2.6 78 1 0 2 4 12 4 100 1 1 1 58 1 1 2 4 7 20 7 100 6 2 2Red @ Yuan Jiang RD119 09/05/01 78.4 19.2 63 3 0 3 1 18 12 100 5 3 5 53 1 0 1 8 0 23 13 100 4 3 4Huanien He RD215 01/06/03 82.1 13.1 73 0 0 0 1 21 4 100 2 1 1 49 4 3 1 0 3 34 6 100 1 0 1Red @ Cua Khao, nr. Lao Cai RD205 12/30/02 81.2 16.5 62 11 0 1 0 9 16 100 2 8 13 53 13 2 0 1 1 16 15 100 11 11 12Namthe @ Lao Cai RD206 12/31/02 78.9 17.4 54 1 0 0 0 5 39 100 8 6 5 35 2 1 0 1 1 19 42 100 11 5 3Red @ Yen Bai RD208 12/31/02 99.5 0.3 68 13 0 2 1 5 11 100 1 3 7 52 14 2 2 4 2 9 14 100 5 3 4Red @ Phu Tho RD214 01/02/03 95.5 3.5 65 3 0 0 0 11 20 100 7 6 11 63 2 2 0 1 1 14 16 100 11 6 10

LoLo @ Ha Jiang RD111 08/24/01 96.7 2.2 70 11 0 0 1 5 14 100 7 2 18 65 12 4 0 0 1 1 16 100 10 3 15Gam @ Chiem Hoa RD210 01/01/02 95.2 4.6 64 2 0 0 4 8 22 100 4 3 8 59 3 2 0 1 5 9 22 100 8 4 6Chay @ Bao Yen RD207 12/31/02 80.8 14.9 57 10 0 0 0 2 30 100 3 5 11 55 11 4 0 1 1 5 23 100 8 12 9Chay @ Doan Hung, bl. res. RD209 01/01/03 97.9 0.04 77 16 0 0 0 2 4 100 1 1 4 70 17 7 0 0 0 1 6 100 2 1 4Lo @ Doan Hung, bl. Chay RD213 01/02/03 69.8 27.3 55 6 0 0 3 29 7 100 2 1 3 51 7 2 0 0 2 36 2 100 4 3 2

Bank SedimentChay @ Doan Hung, bl. res. RD209Bk 01/01/03 100 0 88 8 0 0 0 0 4 100 2 5 29 79 5 4 0 0 0 5 7 100 1 7 32

Res = reservoir; ab. = above; bl. = below; nr. = near; LB = left bank.The sample ID in bold indicates the furthest downstream sample for the three tributary systems.Q = total quartz; F = total feldspar; L = Lv+Lc+Lch+Ls+Lm; Lv = volcanic; Lc = total carbonate grains; Lch = chert; Ls = sedimentary; Lm = metamorphic.Al = alterites; Op = opaque minerals; Hm = heavy minerals; Rf = total rock fragments; Pl = plutonic; V = volcanic; Ca = carbonate; Ch = chert; S = sedimentary; M = metamorphic.

159J.

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Y.Huh

/Sedim

entaryGeology

194(2007)

155168

from the finer silt and clay size fractions. The 96wt.%of the total initial sample, and the clay fraction was neg-ligible (Table 1). Thin sections were prepared from thesand-size fraction and subjected to both standard andGazziDickinson petrographic analyses (Potter et al.,2001; Garzanti et al., 2005), counting 200 frameworkgrains per sample (Table 1). Aliquots of the

pholo

ngth

ary GTable 3GIS parameters for the Red River sub-basins

SampleID

Climatic Geomor

Air T PRC PET Runoff Basin le

C mm/y mm/y mm/y km

DaRD121 19.0 2184 1041 1390 233RD218 8.9 176 719 322 233RD120 19.8 2308 1086 1256 144RD217 9.8 198 730 266 144RD203 10.8 227 739 303 515RD204 12.1 210 723 247 54RD103 21.2 2402 1174 1551 664RD202 11.9 218 736 317 664

Red Main ChannelRD229 7.2 114 685 336 54RD119 18.3 2062 993 969 313RD215 9.2 178 697 77 52RD205 8.9 164 695 184 545RD206 10.7 185 655 239 25RD208 10.1 169 694 223 704

J. Borges, Y. Huh / SedimentArcGIS. Drainage basin area is modified from Hearnet al. (2001). Following Summerfield and Hulton (1994),relief is calculated as maximumminimum elevationwithin each 10-minute grid cell using the USGSGTOPO30 digital elevation model (DEM) with 30-second resolution. Percentage of area covered by igneous,metamorphic and sedimentary rocks is calculated fromWakita et al. (2004). Statistical analyses were made withSPSS v.12 and are for 5% significance level or 95%confidence interval unless otherwise mentioned.

4. Results

For ease of discussion we separated the samples intothree groupsthe Da, Red main channel (MC), and Lo.The data set includes percent sand and silt size fractions,point counts of framework grains (Table 1), major oxides,and inorganic and organic carbon (Table 2). At threelocations we were able to sample in both wet and dryseasons, but in general wet season sampling was sparsedue to high flood conditions.

RD214 10.6 171 696 231 789

LoRD111 20.2 1807 1172 1233 193RD210 11.5 195 597 337 186RD207 12.2 182 666 375 147RD209 13.3 183 676 376 234RD213 11.9 189 626 333 365

Climatic parameters are for the months sampled. T = air temperature; PRC =CCWIa = Cumulative Chemical Weathering Index = RunoffBasin length /The sample ID in bold indicates the furthest downstream sample for the thregic Lithologic CCWIa

Area Relief Ig Met Sed

103 km2 m % area m/y

5.60 1027 0 2 98 3155.60 1027 0 2 98 733.66 1328 5 4 91 1363.66 1328 5 4 91 2926.36 1068 5 1 94 1466.54 1180 18 5 78 1143.47 1154 8 3 89 89243.47 1154 8 3 89 182

0.89 677 11 0 89 2719.87 857 1 14 86 3542.40 760 9 39 53 533.20 962 5 23 72 1044.01 962 2 19 79 643.80 1015 11 20 70 155

161eology 194 (2007) 1551684.1. Petrography

The bed sediments are mostly sand-sized (median94 wt.%), and the silt size fraction is significant(25 wt.%) in only 4 samples (RD121, 218, 217, 213).The framework grains of all 21 samples are angular tosub-angular, suggesting that they have not been in thesedimentary mill for long periods of time. The frameworkgrains were point counted once according to the standardmethod to obtainQ:F:Rf ratioswhich can be interpreted interms of the mineralogical maturity (Fig. 3a). They werecounted a second time according to the GazziDickinsonmethod to obtain Q:F:L ratios which yield information ontectonic provenance (Fig. 4a). The main differencebetween the two methods is in the assignment of grainswithin larger fragments to the category of the largerfragment for the standard method and to the category ofthe individual grain for the GazziDickinson method(Ingersoll et al., 1984). Quartz and rock fragments arefound in subequal proportions, and feldspars are minor;average of the three lowermost samples of the Da, Red

47.53 982 13 19 69 185

7.13 653 7 2 91 36415.06 716 1 5 95 884.30 1103 7 33 60 506.30 962 14 39 46 9136.30 788 4 20 76 154

precipitation; PET = potential evapotranspiration.Relief (Grantham and Velbel, 1988).e tributary systems.

Fig. 3. (a) QFRf discrimination diagram with the compositional classification of Pettijohn et al. (1987) for the Red River system. (b) Ternary diagramshowing relative abundance of volcanic, sedimentary and plutonic+metamorphic rock fragments.

162 J. Borges, Y. Huh / Sedimentary Geology 194 (2007) 155168MC, and Lo is Q:F:Rf=46:3:51. On the QFRf discrim-ination diagram, the samples occupy the lithic arenitefield (Pettijohn et al., 1987, p.158) with one exception(Fig. 3a) and on the QFL diagram, the recycled orogenfield (Dickinson et al., 1983) (Fig. 4a). The ratio of stableto unstable framework grains, Q/ (Q+Rf ), is intermediateand ranges from0.29 to 0.52. An exceptional sample from

the Lo drainage (RD209) has a high Q/(Q+Rf ) ratio of

Fig. 4. (a) QFL diagram with tectonic discrimination according to Dickinsonindicate the lowermost samples of the three sub-basins. (b) QFL diagram wiBrahmaputra and Indus Rivers (Garzanti et al., 2004, 2005).0.77 and straddles the sublitharenite and subarkose fields(Fig. 3a). Since it was sampled below the Thac Bareservoir, it is likely affected by local input from sand-stone and conglomerate from the Lo River banks whichalso have similar lithic arenite to sublitharenite compo-sition (Wysocka and Swierczewska, 2003). It is in greatcontrast to the sample 90 km upstream on the Chay

above the reservoir (RD207) or the sample downstream

et al. (1983) for the Red River system. Filled or highlighted samplesth 90% confidence interval about the mean (Weltje, 2002) for the Red,

(Yellow) and Chang Jiang (Yangtze) (0.091.27 and0.731.65 wt.%, respectively) (Yang et al., 2004). Thereis no correlation between OC and P2O5 (Table 2), and weassume that all P2O5 is associated with apatite, Ca5(PO4)3, such that 215 mol% of Ca in our bed sedimentsamples is from apatite. Assuming that all IC in the bed-load is from carbonates (calcite and dolomite), and thatcalcite:dolomite mass ratio is 2.29 (average Phanerozoiccarbonate) (Mackenzie and Morse, 1992), we calculatethat the Ca in the Da and Red main channel samples isassociated mostly with carbonates (median 74 mol% Ca)and that the Lo drainage samples have significant Caassociated with silicates (median 58 mol% Ca).

4.4. Weathering indices

Various weathering indices have been developed andused for characterizing the extent of weathering in soilprofiles. Since our samples are not from weatheringprofiles developed on homogeneous parent rock but

Fig. 5. (a) Compositionalmaturity (Herron, 1988) and (b) tectonic settingdiscrimination (Roser and Korsch, 1988) diagrams. Average sandstoneand UCC values are also plotted for reference. Filled or highlightedsamples indicate the lowermost samples of the three sub-basins.

ary Gafter confluence with the Lo main channel (RD213)(Figs. 1b, 3a). The dry river bank sediment (RD209Bk) atthe same location as RD209, on the other hand, has QFRfcomposition similar to the other samples (Fig. 3a).

The rock fragments in the Red River bed sedimentsconsist predominantly of sedimentary and metamorphicgrains with rare volcanic or plutonic grains (Table 1,Fig. 3b), as is also the case of sandstones and conglo-merates of the Lo River banks (Wysocka and Swierc-zewska, 2003), and reflect well the source lithologyof the Red River basin. Sample RD209 is again anexception with very high (33%) plutonic rock frag-ments and relatively high (22%) heavy minerals, and itsbank sediment (RD209bk) has low (5%) plutonic grainsbut high (65%) heavy minerals (Table 1, Fig. 3b). Thelowermost Lo sample (RD213), unlike other Lo systemsamples, is almost exclusively sedimentary rock frag-ments (69%).

4.2. Major oxides

Most samples have SiO2, Al2O3, Fe2O3, MnO, Na2O,and K2O concentrations that fall within the rangecovered by the Upper Continental Crust (UCC)(Rudnick and Gao, 2003) and the Average PhanerozoicSandstone (Condie, 1993)(Table 2). However, they aredepleted in MgO, CaO, and P2O5 and enriched in TiO2.This is consistent with the relative mobility of theseelements during weathering. Some noticeable outliersamples are RD207 above the Thac Ba Reservoir whichis high in Fe2O3 (11%) and MnO (0.25%) and RD209below the reservoir which has high SiO2 (86%) and K2O(2.4%) and is depleted in other major oxides.

Among the many plotting schemes that have beenused to show sediment maturity and tectonic settingusing major oxides, we opted for two (Roser and Korsch,1986; Herron, 1988) that utilize Fe2O3/K2O, SiO2/Al2O3, and K2O/Na2O ratios (Fig. 5). As was the casewith petrographic data, the samples are classified aslithic arenites according to the major oxides data, exceptfor a few that straddle the boundary with wacke and twothat extend into subarkose (RD209) and sublitharenite(RD207) fields (Fig. 5a). In terms of tectonic setting, allthe Da samples plot in the passive margin field, andthe Red and Lo samples are spread over the activecontinental margin and passive margin fields (Fig. 5b).

4.3. Organic and inorganic carbon

The organic carbon (OC) and inorganic carbon (IC)contents are

undergone sorting during transportation in river chan-nels, the weathering indices could reflect variation inparent rock composition rather than the degree ofweathering. However, as one of the conditions of auseful weathering index is that it should permitcomparison of material developed from different parentrock types (Price and Velbel, 2003), we adopt two ofthem for river bed sediments. The most commonly usedis the chemical index of alteration (CIA) (Nesbitt andYoung, 1982).

CIA 100xAl2O3=Al2O3 K2O Na2O CaO*;

where the major oxide concentrations are in molar units.The CaO is from silicates and is calculated by sub-tracting from total CaO the fractions from apatite andcarbonate (See Section 4.3) (Table 2). The CIA rangesbetween 45 (RD111 of the Lo) and 77 (RD121, 217, and218 of the Da). The latter three high-CIA samples alsohave high silt-size fractions (>30%)(Table 1). The Da

more sensitive to variation than the CIA and especiallyappropriate for applications where parent rock is hete-rogeneous (Price and Velbel, 2003).

WIP 100xf2Na2O=0:35 MgO=0:9 2K2O=0:25 CaO*=0:7g;

where the concentrations are in molar units and lowerWIP values indicate more intensive weathering. WIPvaries from 13 (RD229) to 49 (RD111) and has a similarspread to CIA (Table 2, Fig. 6b). The negative corre-lation with CIA is highly significant ( p

length, area, and relief govern the duration of weathering(Grantham and Velbel, 1988). These two effects arecombined in the Cumulative Chemical Weathering Index(CCWI) of Grantham and Velbel (1988), where

CCWI runoff=relief=basin length:

The CCWI ranges from 5 (RD215, 206) for the dryseason of the Red MC to 890 (RD103) for the wetseason of lowermost Da (Table 3).

We chose three parameters, Q/ (Q+Rf ), S / (S+M),and CIA to summarize the petrographic and chemical dataand carried out stepwise multiple regression against eachof the GIS-based parameters (Table 3). Among the threeparameters, there is significant correlation between CIAand S/ (S+M) (p=0.01). No correlation was found at 5%significance interval for Q/ (Q+Rf ) and S/ (S+M). Thestepwise multiple regression indicated that the importantGIS parameters for CIA were the percentage ofsedimentary rock cover within the drainage basin andtemperature (Fig. 7). The correlation between CIA andother climatic or geomorphologic factors or CCWI

J. Borges, Y. Huh / Sedimentary GFig. 7. (a) Chemical Index of Alteration (CIA) has a positive

relationship with percentage of drainage area covered by sedimentaryrocks. (b) Relationship between CIA and air temperature.was insignificant. The primary association of CIA withS/ (S+M) and% sedimentary rock cover suggests that theCIA values of bed sediments mainly reflect the exposedarea of primary versus recycled source rocks. Temperatureis the second factor considered significant in the multipleregression and is negatively related to CIA. We think thisrelationship may be spuriously driven by a limitednumber of summer samples (Fig. 7b). Thus, the chemistryof the bed-load is affected by multiple factors with sourcerock lithology, specifically its sedimentary or recycledcharacter, acting as the primary control.

5. Discussion

5.1. Seasonal, downstream, and interbasin variations

Three locations in the Da drainage were sampledduring both wet and dry seasons. The Da at Muong La,above the Song Da Reservoir (RD103, 202) is thelowermost sample on the Da; the other two are head-water tributaries. The seasonal variations in Q/(Q +Rf ),S / (S+M), and WIP at these three locations are small(Fig. 8). The Q/ (Q+Rf ) is higher and thus miner-alogically more mature during the wet season, but the S /(S+M) or WIP vary inconsistently with season atdifferent locations.

We used three sites on the Da and five on the Redmain channel to evaluate the downstream change inbed-load composition, and where both seasons areavailable, we used the average (Fig. 9). We do not haveadequate number of Lo main channel samples. The Q/(Q+Rf ) ratios are unvarying downstream except forsample RD205 of the Red main channel at the borderbetween China and Vietnam (Fig. 9a). This sample hasextremely high rare earth element concentrations andwe suspect it has been affected by local input of highREE minerals. This is consistent with the low Q/ (Q+Rf ) ratio since quartz is poor in REE. The rockfragment composition changes from sedimentaryupstream to metamorphic downstream (Fig. 9b). Thecovariation of S / (S+M) and CIA and the unorthodoxdownstream trend in CIA towards fresher materialcorroborates that the CIA of bed sediments is drivennot by the progressive weathering of bed sediments inthe channels but by the variability in the sourcematerial (Fig. 9c). The sediments are diluted by lessweathered material as they are transported downstream.Thus, the CIA results serve as a provenance index ofrecycled sediments rather than an index of presentweathering regime. CIA as originally defined byNesbitt and Young (1982) was to be applied to soil

165eology 194 (2007) 155168profiles developed on a single type of bedrock. Our

CIA (55 and 56, respectively) are probably derived fromlocal igneous rocks (Fig. 1a).

Inter-basin variation between the three tributary systemsis statistically insignificant in terms of their frameworkgrain and rock fragment compositions (Figs. 3a, 4). Thoughthere are individual samples that have distinctive composi-tions, the three basins at the lowermost locations havevery similar petrographic compositions (Figs. 3a, 4).We donot observe any consistent inter-basin variation in majorelement composition, but the CIA indicates that the Da hasa higher recycled component than the Red main channelor the Lo (Fig. 6). On both mineralogical maturity andtectonic classifications, the two Da drainage samples(RD204 and RD217) are different from other Da samplesand plot closer to the Red main channel samples. They are

ary Geology 194 (2007) 155168166 J. Borges, Y. Huh / Sedimentresult casts doubt on its uncritical usage where thematerial originates from multiple sources.

There are two reservoirs in the Red River basin,the Song Da Reservoir on the Da (23,500 ha) and theThac Ba Reservoir on the Chay (72,800 ha) tributaries(Fig. 1b). Samples above (RD207) and below (RD209)the Thac Ba reservoir can be used to examine the effectof the reservoir on its bed sediments. Above the reser-voir the sample has 15% of silt size sediments whichdecrease to almost none below the reservoir. The SiO2content is also higher above the reservoir (86% versus64%), reflecting the higher quartz content. Thesefeatures suggest sediment trap effect, especially of thefine particles, of the reservoir. The relatively highfeldspar point counts in both samples (16% and 10%above and below reservoir, respectively) and the low

geographically juxtaposed to theRedmain channel, but it isunclear why the summer sample of RD217 does not also

Fig. 8. Seasonal variation for three Da river samples in (a) frameworkgrains, (b) rock fragments, and (c) major oxide-based weatheringindex. Error bars indicate propagated analytical uncertainty.Fig. 9. Downstream change in maturity of bed sediments as reflected in(a) framework grains, (b) rock fragments, and (c) major oxide-based

weathering index. See text for definition of CIA (Chemical Index ofAlteration).

ary Gfollow this trend (Fig. 5a,b). On average, inorganic carboncontent is significantly greater in the Red river mainchannel drainage area (0.52wt.%) than in theLo (0.2wt.%)and Da (0.18 wt.%), but the organic carbon content issimilar in the three sub-basins.

5.2. Comparison to the Himalayan rivers

We compared our data for the Red River to that of theIndus and the Brahmaputra which drain the two syntaxesof the Himalayas (Garzanti et al., 2004, 2005). Petro-graphically, all three rivers are quartz-poor, with the bulkof the samples falling in the lithicarenite field and in therecycled orogen field consistent with their tectonic setting(Fig. 4b). The high relief and physical erosion, as evi-denced by high suspended sediment flux (Meybeck andRagu, 1997), explains the immaturity of the bed-load inthe three rivers. However, there are subtle differences aswell. The immaturity ismanifested as high rock fragmentscontent in the Red and as high feldspar content in theIndus and Brahmaputra. The average Q:F:Rf for the Red,Indus, and Brahmaputra, are (46:4:50), (43:24:33), and(57:21:22), respectively. Stated differently, theHimalayansamples extend into the subarkose (Pettijohn et al., 1987)and continental block fields (Dickinson et al., 1983)(Garzanti et al., 2004, 2005). This we attribute to adifference in source rock lithology sedimentary andmetamorphic rocks of the Red versus the two syntaxes ofthe Himalayas the southeast Karakoram of the Indusand the Namche Barwa of the Brahmaputra that are themajor suppliers of felsic bed sediments to the Indus andBrahmaputra (Garzanti et al., 2004, 2005). The CIAvalues for the Ganges (68) and Brahmaputra (73) aresimilar to the Red (66) (Potter, 1978). The Indus has lowerCIA values of 51 in the headwaters of Ladakh Himalayaand 46 in the Salt Range and near the mouth (Maynardet al., 1991; Ahmad et al., 1998). Thus, recycled sedi-mentary material makes significant contributions to thesediment load of both the Red and the Himalayan riverswith the major distinction coming from the two Hima-layan syntaxes which are unique suppliers of felsic sedi-ments to the Indus and Brahmaputra.

6. Conclusions

We analyzed 21 bed sediment samples from the RedRiver for petrography and chemistry. The Q/(Q+Rf ) andmajor oxide ratios indicate that they are petrographicallyand compositionally immature, and the bulk of thesamples can be classified as lithic arenite. On tectonicdiscrimination diagrams some samples lie in recycled

J. Borges, Y. Huh / Sedimentorogen and others in passive or active margin fields. ThisAcknowledgments

We thank P.E. Potter for his guidance in sandpetrography, J. Qin of the Chengdu Institute of Geologyand Mineral Resources and Nguyen van Pho of theInstitute of Geological Sciences, Hanoi for the logisticalsupport during field sampling, S. Moon for the discus-sions and sharing data, and B. Sageman for access to thecoulometer at the N.U. Reviews by E. Garzanti and B.Maynard greatly improved the quality of this paper. Thiswork was supported by the NSF-OCE 9911416 and NSF-EAR 0134966 to Y. Huh.

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Petrography and chemistry of the bed sediments of the Red River in China and Vietnam: Provenanc.....IntroductionGeological and hydrographical settingMaterials and methodsResultsPetrographyMajor oxidesOrganic and inorganic carbonWeathering indicesRelationship to climate, geomorphology, and lithology

DiscussionSeasonal, downstream, and interbasin variationsComparison to the Himalayan rivers

ConclusionsAcknowledgmentsReferences