QUATERNARY RESEARCH 37, 29-41 (1992)

Thermoluminescence and Excess **%a Decay Dating of Late Quaternary Fluvial Sands, East Alligator River, Australia

ANDREW MURRAY, **’ ELLEN WoHL,t” AND JON EASTMAN

*Alligator Rivers Region Reseurch Institute, Jab&u Easr, NT 0886, Australiu; and tDepartment of

Geosciences, University of Arizona, Tucson. Arizona 85721

Received August 7, 1990

Thermoluminescence (TL) dating was applied to seven samples of siliceous fluvial sands from the East Alligator River of Northern Australia, giving ages ranging from modem to 6000 yr B.P. Two methods of estimating the equivalent dose (ED), total bleach and regenerative, were applied to the 90- to 125~pm quartz fraction of the samples in order to determine the reliability and internal consistency of the technique. High-resolution y and (Y spectroscopy were used to measure radio- nuclide contents; these measurements revealed an excess 2’6Ra activity compared with Z3qh. This excess decreased with depth, and was used directly to derive mean sedimentation rates, and thus sediment agl:s. Both this method and one 14C date confirmed the validity of the TL values, which increased systematically with depth and were consistent with site stratigraphy. TL was of limited use in the dating of these late Holocene deposits because of age uncertainties of 500 to 1600 yr, resulting from a significant residual ED. This residual probably resulted from incomplete bleaching during reworking upstream of the sampling site. For Pleistocene deposits, the residual ED will be less significant because of higher total EDs, and TL dates will be correspondingly more accurate.

INITRODUCTION

As part of the development of design cri- teria for rehabilitated structures associated with uranium mines in the Alligator Rivers Region of the Northern Territory of Austra- lia, information is required on the fre- quency and magnitude of extreme rainfall and floods. This paper examines the suit- ability of fluvial deposits, some of which have been identified as flood levees, for dating by thermoluminescence (TL). It also compares the resulting TL dates with those based on the d.ecay of excess 226Ra and on 14C. The samples were collected in the East Alligator River gorge, and dating was un- dertaken as part of a broader study of the suitability of North Australian paleofloods as paleoclimatic indicators; future papers

’ Present address: CSIRO, Division of Water Re- sources, P.O. Box 1666. ACT 2601, Australia.

’ Present address: Department of Earth Resources, Colorado State University, Ft. Collins. CO 80523.

3 Present addre’ss: Bureau of Rural Resources, P.O. Box 858, ACT 26101, Australia.

will emphasize the hydrological and cli- matic aspects of this study.

The technique of TL dating has been ap- plied to Quaternary sediments with increas- ing success over the past 20 years. Austra- lia has large areas of Quaternary fluvial and eolian deposits and lesser areas of coastal deposits (Wopfner and Twidale, 1971; Thorn, 1978). A lack of material suitable for 14C dating in these sediments has presented problems of chronological reconstruction in prehistory, stratigraphy, and geomorphol- ogy. TL holds promise in all of these areas. Applications to dating Australian surficial deposits include the dating of eolian dune sands (Prescott, 1983; Gardner et al., 1987), beach dune sands (Huntley et al., 1985), sand aprons (Roberts et al., 1990), and fluvial sediments (Nanson and Young, 1987).

Again in this study, suitable 14C datable material was almost completely absent. However, the existence of an excess of 226Ra over its parent ‘3?h did provide an alternative dating method for comparison

29 0033-5894192 $3.00

30 MUKKA’, . WOHI . ,\Nl) t,ASI

with TL. This technique has already been used with success in the adjacent Magela Creek catchment (Johnston et ~1.. 1990).

The field area is located approximately 100 km upstream from the mouth of the East Alligator (Fig. I). The river drains an area of 4500 km’, two-thirds of which lies within the Arnhem Land plateau. This con- sists primarily of the Kombolgie Forma- tion, a resistant quartzose sandstone which exerts strong structural control on the drainage pattern. The region has marked seasonal precipitation, about 90% falling between November and March. Because of this seasonality and high rainfall intensity. large floods are produced. In reaches of confined cross-section, as in the gorge ex- amined in this study. these floods result in significant stage changes. The paleohydro- logical reconstruction employed in this area uses the resulting flood levees composed of slackwater deposits. In previous studies. slackwater deposits have proven to be use- ful indicators of paleoflood magnitude and frequency (Baker et al., 1979, 1983). but the technique requires an accurate strati- graphic chronology, and this need gave rise to the present study.

SAMPLE COLLECTiON AND PREPARATION

A possible slackwater depobtttonal site was identified on a narrow flood plain up- stream of the confluence of the East Alliga- tor River and a major tributary (Fig. I). Both trunk stream and tributary flow in gorges incised into Kombolgie Formation sandstone. Pits were excavated in each of three low terraces which form the flood plain (Fig. 6). A second site was located just downstream of a constriction in the main gorge. Only one sample was taken from this site (KTL28). Sedimentary strata were identified in the field on the basis of the existence of bedding planes and change in sediment texture. Individual strata were sampled by forcing a length of 5- cm-diameter PVC tubing into a freshly ex- posed stratum. The tube was then wrapped in opaque black plastic. and all further sam- ple preparation was conducted under low- level yellow light. After setting aside for ra- dionuclide analysis any portions which could have been exposed to sunlight during sampling, the remainder was sieved to sep- arate the 90- to 125~Frn fraction. This was

FIG I. Map showing location of the field si te.

TL DATING OF AUSTRALIAN SANDS 31

etched with hydrofluoric acid to remove the outer 10-20 pm of the grains. Because these samples were composed of predomi- nantly clean, well-sorted quartz sands, it proved unnecessary to explicitly remove any carbonates or organics; any feldspars would have been destroyed by the hy- drofluoric acid treatment. Heavy min- erals were removed by separation in a sodi- um polytungstate solution of specific grav- ity 2.7.

ESTIMATION OF EQUIVALENT DOSE

Treated quartz was loaded on stainless- steel discs smeared with silicon grease, each disc holding between 7.8 and 8.2 mg of sample. The TL was measured at 2S”C set - ’ in a nitrogen atmosphere, and typical glow curves for various treatments are shown in Figure 2. At least four replicates of each sample treatment (i.e., dose or light exposure) were prepared. and the four glow curves were temperature shifted to lower temperatures as necessary to produce the most consistent vertical alignment among the curves.

As a cross-check, two methods for esti- mating the equivalent dose (ED) were used: the total bleach method (Singhvi et al..

1982) and the regenerative method (Pros- zynska, 1983). The methodology employed was conventional, and no details will be given here. To illustrate the behavior of the material, Figure 3 shows the dependence of residual TL on light exposure, and Figures 4 and 5 show typical total bleach and regen- erative growth curves, and the resulting ED plateaux. The regenerative method as- sumes that the TL sensitivity of the sample does not change during laboratory expo- sure to light. The total bleach method pro- vides a check on any changes in sensitivity; in the absence of significant changes the two EDs should be in agreement. The EDs for all samples are given as EDob in Table 1. Each ED was evaluated qualitatively on the basis of data scatter and degree of plateau definition, and either accepted or rejected. Two samples, KTL31 and 32, require dis- cussion at this stage.

In KTL3 1, the total bleach method gave a large scatter in the plateau data, and the regenerative plateau was poorly defined. These EDs were also about 10 times those for other samples, in particular for the di- rectly overlying unit KTL30, although the dose rates proved similar. The likely expla- nation for this discontinuity is contamina-

__ Natural TL

I-‘\ Natural + 20 h sun

I’ \( -------Natural + 6 Gy : \

/’ - __ Background

,’ \ /’ ‘\

,-._’ ‘\

/’

..’ ,,. ,., ,. . ...” ” _L’ -I” I I I I I

200 250 300 350 400 450

Temperature (“C)

FIG. 2. TL glow curves of quartz fraction for sample KTL 34; the legend shows the treatment that each has received.

25

1 0

fl 01, > / c , 1 , , , ,m---

0 20 40 60 80 100 120 140 160

Sunlight Exposure (h)

FIN;. 3. Bleaching curves for quartr fraction for sample KTL 34 following exposure to sunlight

tion by the weathering of colluvium; heav- ily weathered colluvial fragments up to 100 mm across were recorded in this strati- graphic unit at the time of collection (Fig. 6). If in situ weathering and comminution of these fragments was releasing quartz grains, then the apparent ED would have a contribution from the partially bleached geological TL of the sandstone. Thus, both

methods of estimating the ED would tend to give the same result. although the calcu- lated age would then bear little relationship to the age of fluvial deposition.

Sample KTL32 had a poor total bleach plateau. although the ED obtained agreed with the better defined regenerative value. Both values were used to calculate a mean. although the regenerative ED dominated

0 Total Bleach Method

4x104

F’ 1x104

0 Regenerative Method

E /

ED = 6.99 Gy /’

ED = -6.05 Gy Natural light level

Bleached light level

0 I I I I

-10 0 10 20 30

Dose (Gy)

FIG. 4. Growth curves for the total bleach and regenerative methods at 330°C for sample E;TL. 34

TL DATING OF AUSTRALIAN SANDS 33

15 - * o

A Total Bleach. Plateau 320 to 395°C 0

0 _ 0 Regenerative. Plateau 310 to 390°C A%

‘;: 0

9 10 -

$

::

2 !2 >"

‘5 5- E

OA

O- A

I I I I I I 200 250 300 350 400 450 500

Temperature (“C)

FIG. 5. Equivalent dose plateaux for sample KTL 34: total bleach and regenerative method.

the average because of the much larger found, outside the normal range of varia- uncertainty associated with the total bility. bleach ED.

Although quartz grains are believed to be ESTIMATION OF RESIDUAL TL

unaffected by ;anomalous fading (Wintle, AT DEPOSITION

1978), two samples were tested for this phe- As a check on the efficacy of natural nomenon. No evidence of fading was bleaching of the sediments, a surface mono-

ED,, total, Cy ED,, regen. Gy ED,. Gy 238U, Bq kg’ ‘3qh. Bq kg ’ 226Ra, Bq kg ’ “‘Pb. Bq kg ’ “‘Ra, Bq kg ’ *-Th, Bq kgg’ 4”K+ Bq kg ’ Saturation W.C. % Fraction of

saturation Date, yr B.P. Random

uncertainty Total

uncertainty

TABLE 1. EDs, RADIONUCLIDECONTENTSAND TL DATES

KTL28 KTL30 KTW I KTL32 KTL33

3.42 3.83,, 74, 3.l,, 4.96,,

2.9, 6.22 8210 2.92 5.0, 0.2, 24, 73, -0.1, 2.0,

21.1,, 182 202 11, 35, 18.2, 17.3, 17.1,? 9.4, 21.6,,

36.5, 25.5, 19.4, 14.1, 58.9,

23.6," 25, 11.4, 17, 38.1,,

20.4, 30.7,, 37.1, 11.03 30.4,

22.2, 32.0, 38.6, 11.32 30.6,

62, 47, 67, 25.6,, 7% 33 25 30 31 25

0.21,, 0.28,, 0.23,z 0.32,, 0.76,, 0.713, 0.79,,

200 1600 59,000 -200 1300 2900 5600

500 1400 6.000 900 500 500 1100

500 1500 10,000 900 600 700

KTL34 KTL35

6.11,, 12.5,,

6.95,, 9.610 3.85 7&l

31, 31, 23.1, 26.5,:

33.6, 23.8,

26.8,? 32, 27.5, 26.4,

29.0s 21.3,

855 70,

26 19

Note. Analytical uncertainties given as subscripts in the least significant figures. Systematic uncertainties in radionuclide analyses are estimated at 3%. Uncertainties on “Fraction of saturation” are estimates only.

:ree

slope

C-14

39

0 f

100

a

C-14

98

i

1.2

-

(X

mod

em)

- 60

cm

Kom

bolgi

e

-0.2

+

0.9

ka

c-14

1.

2 +

0.15

ka

KT

L 32

Site

EA23

fins-

med

ium

sa

nds

- 77

cm

loo

sely

indu

mte

d 2.9

+

0.7

k.

m

34

5.6

2 1.

6 ko

Kn

35

/

0 20

m

l J1

90

cm

I--

loam

y aa

nch3

we

ll ln

dum

ted

TL DATING OF AUSTRALIAN SANDS 35

layer of grains from the study site was pro- cessed. The resulting ED value of 0.21 +- 0.13 Gy corresponds to an age of only 200 yr, using representative dosimetry, and in- dicates that sunlight exposure under ideal natural conditions effectively bleaches the natural TL. However, this test does not check on the bleaching process during ini- tial erosion or subsequent fluvial transport. R. G. Roberts (personal communication, 1989) has examined the process by which individual quartz grains are freed from the Arnhem Land plateau sandstone matrix, and transporteld to the fluvial system. He suggests that grains may be exposed to sun- light for many months before finally being detached from the matrix. He has con- firmed complete bleaching by measuring a quartz ED of a.bout 0.2 Gy, 0.5 cm deep in a l-m-diameter shallow sand-filled depres- sion in the sandstone surface. Similar EDs are also reported in Roberts et al. (1990) for recently eroded material. These values are similar to the laboratory ED reported above, and confirm effective bleaching of the geological TL. However, a sample taken at 20 cm depth in an adjacent 2- m-deep chanrrel connected to the drainage network gave an ED of about 9 Gy. This is evidence that transport through headwater channels may take thousands of years, al- lowing quartz to accumulate a significant ED before transport and bleaching in the main channel.

To investigate whether fluvial transport does bleach this more recently acquired ED, three samples were collected from the main channel of the East Alligator, one ad- jacent to the study site, and two further downstream. Partial burial of vegetation by the sampled sand units was evidence that these samples were deposited recently, probably within the last wet season. A sim- ilar sample was taken from a tributary, the Magela Creelk, also draining the plateau. Three samples were measured using the re- generative method, and one the total bleach. One ‘of the East Alligator samples did not give an acceptable plateau, although

the ED was within the range of the other values, of 3.5, 2.3, and 3.1 Gy, all with un- certainties of about kO.3 Gy. It appears that although the ED may be reset on each reworking of sediment as it moves down- stream, this bleaching is not complete. For this area an ED at deposition of 3.0 ? 0.5 Gy should be subtracted from the observed ED, to give the dose accumulated after de- position. This subtraction gives the final EDs (ED, in Table 1) used in the age calcu- lations.

ESTIMATION OF DOSE RATE

The TL age is obtained by dividing the ED by the annual dose rate. This dose rate is mainly derived from the radionuclides 13*U and ‘32Th (and their decay products), 40K, and cosmic rays. High-resolution y spectrometry (Murray et al., 1987; Murray and Aitken, 1988) and (Y spectrometry (Martin and Hancock, 1987) were used to determine these nuclide activities, summa- rized in Table 1.

In such sedimentary materials 234U should be close to secular equilibrium with its parent 238U (Scott, 1982). This has been experimentally confirmed for KTL28 (234U/ 238U = 1.06 -C 0.08), and is assumed for the remaining samples. From the similarity of the “8Ra and “*Th activities, it appears that nuclides below “*Ra in the 232Th chain are in equilibrium. It is assumed that ‘3ZTh and ‘28Ra are in equilibrium: this has been confirmed by measurement of 228Th/23’Th activity ratios by cx spectrometry for KTL28, 30, 32, and 35. In the ‘38U series, there is disequilibrium between 238U and 230Th in KTL28, 33, and 34, and between “26Ra and 230Th in all samples except KTL31 and perhaps KTL35. Based on 2’0Pb/22”Ra activity ratios, it is likely that there has been between 30 and 40% loss of “‘Rn from all samples, although the analyt- ical uncertainties in “‘Pb activities prevent useful comment for KTL30, 32, and 35.

Disequilibria between 22hRa and 230Th are of particular significance in these samples, because of the 226Ra 1600 yr half-life. If ex-

36 MUKKhY. WOHI.. \NI) I..\\1

cess “6Ra activity is unsupported by phys- ical ingress of ““Ra, e.g., by groundwatci movement, then this excess will have been decaying toward equilibrium with ““Th since the setting up of the excess. Such dis- equilibria exist in modern sediments in the adjacent Magela Creek flood plain (Johnston rt al., 1988, 1990): it is presumed that the 226Ra excess here was also present at deposition. It is demonstrated below that this excess decreases with depth in at least one profile (KTL33, 34. and 351, despite pronounced similarity in the activities of other nuclides, and it is concluded that the excess “‘Ra is not supported by physical ingress. This was the conclusion drawn in the Magela Creek catchment. In terms of dosimetry, the effect of assuming the ex- cess 2”6Ra is unsupported is to reduce the ages by up to 17%~ (for KTL33).

The modern fractional “‘Rn escape, as reflected by ““Pb/76Ra ratios. is assumed to have applied throughout the lifetime of the site. In calculating the dose rate, the radionuclide activities of Table 1 were then multiplied by the appropriate p and y dose rate conversion factors derived from Mur- ray (1981) to give the dose rate. The dose rate derived from the present day excess 226Ra (and daughters) was calculated sepa- rately from that ‘26Ra derived dose rate supported by ““Th. Although the effective OL dose component was included explicitly in the calculation of the age, the typical contribution was only I to %, and so the details are not given here. However, it is noted that the ratio of the observed thick source (Y count rate of grains after etching to that of grains before etching was usually between 0.05 and 0.1.

To enable correction of the dose rate for water content, field specimens preserving the natural sedimentary/pedological struc- ture of the samples were used to measure the water content at saturation. Because all samples lie close to the river. but above the dry season water table, gauge records for the East Alligator were used to estimate the fraction of time the samples would be sat-

uratcd. l‘hesc data gave an esttmate of the average fraction of saturation over the life- time of the site. These estimates and the saturated values (Table I) were rt<ed to cor- rect the p and y dose rates for uater content tc.g., Rendell. 1985). A cosmic ray dose rate of 0.15 2 0.03 Gy kyr- ’ was used for a latitude of IO” south (Prescott and Hutton. 1988). A B attenuation factor of 0.95 was included as a correction for etching (Mej- dahl, 1979).

CALCULATION OF TL DATES

The age calculation mentioned at the be- ginning of the previous section assumes a time independent dose rate. In this case the equation must be revised to

ED t=

D, ((7” - I j (1)

DI + it ..

where t is the TL age, D, the sum of the supported (Y, p, y, and cosmic dose rates. and D, the sum of the p and y dose rates derived from the excess “‘Ra. which has decayed with a decay constant I to the present level.

The TL ages are given in Table 1, with random and total uncertainties. In addition to the random component, the total in- cludes those uncertainties likely to be cor- related between samples when comparing dates from any one laboratory, e.g.. uncer- tainties in calibration of p source or of nu- elide analytical techniques. Uncertainties arising from the site water contents are of- ten included with this component. How- ever, in these cases it is unlikely that all the water content uncertainties necessarily be- have systematically and so these have been included in the random component. It is useful to consider the contribution to the overall uncertainty of the water content corrections. because of the sub.jective as- sumptions involved. Typically, these con- tribute up to 30% of the overall uncertainty. In the worst case, allowing the water con- tent to be either 0 or 100% of the observed

TL DATING OF AUSTRALIAN SANDS 37

saturated water content produces a change in the age of t- 22% (KTL3 1, 100% water content). The average change, independent of sign, in the worst case analysis is 16%. Even these extremes are usually smaller than the reported overall uncertainties.

Only in unit EA22, at the same level as KTL32, was sufficient material recovered for a comparable 14C date; difficulties in finding such carbonaceous material high- light the need for alternative dating proce- dures in these deposits. The 14C date was 1200 + 150 yr EI.P. (ANU-5613), to be com- pared with the TL date of -200 ? 900 yr. These two dates agree within the limits of the uncertainties.

The three TL dates obtained for site EA23 (KTL33, 34, 35) were from depths of 150, 1050, and 1620 mm, respectively, giv- ing mean accumulation rates between the first two depths of 0.56 ? 0.25 mm yr-’ and between the second two of 0.21 ? 0.09 mm yr-‘. The initial rate between 0 and 150 mm, of 0..12 5: 0.05 mm yr-‘. may be an underestimate because of surface stripping, although the nlext section suggests that this is negligible.

ESTIMATION OF SEDIMENT DATES USING RADIONUCLIDE DISEQUILIBRIA

The dosimetry data suggest excess 226Ra at site EA23 lmay decrease with depth. A similar observation was used by Johnston et al. (1990) in the flood plain of the Magela Creek to derive a sedimentation rate over the last 4000 yr. If sediment is deposited with constant initial 2’6Ra excess over 230Th of R,, then because the 226Ra half-life is short compared with its parent, that excess will decay exponentially to a value R after time t, i.e.,

R = R, e-l*, (2)

where I is the decay constant for 2’6Ra. If deposition is at a steady rate, s, with time then the accumulated depth D = s t. Sub- stituting into Eq. (2),

= R, epKD; (3)

i.e., it is expected that the depth profile of excess 226Ra will be exponential in form. A full discussion of the assumptions involved in this approach is given in Johnston et al., (1990). In summary these are

(a) constant sedimentation rate, or at least distinct periods of constancy.

(b) constant deposited “‘Ra and 230Th concentrations in time.

(c) chemical immobility of 226Ra and ‘-“Th since deposition.

(d) no significant bioturbation or other physical disturbance of the profile.

The constancy of 23(‘Th concentrations can be checked by direct observation, but con- stancy of 226Ra will always remain an as- sumption. 230Th is very immobile in a fresh- water environment, but little has been pub- lished about the behavior of 226Ra in such waters (Scott, 1982). Work in the Magela Creek catchment (Murray et al., unpub- lished data) found high values (circa 2.5 x

10’) for the distribution coefficient, K,, for both “(jRa and 230Th, and it appears that here these nuclides behave as essentially immobile species (Kd is the ratio of activity concentration on solids to that in water). Thus, decay of excess ‘26Ra may be used to provide an independent check on the valid- ity of the TL dates.

Site EA23 was resampled with depth 1 year after the initial sampling. Figure 7 summarizes the depth profiles of 238U, 230Th, and 226Ra, and the parent of the thorium series, 232Th. A surface excess of 226Ra over both the series parent 238U and the immediate parent ‘30Th exists, but Fig- ure 8a shows this excess is not a smooth function of depth, nor can it be well repre- sented by the unweighted best fit single ex- ponential function shown as a dashed line. However, the data are much better repre- sented by two exponential decreases with depth, whose unweighted best fits are shown as solid lines, with a change in the

38

0,) I I I , I1 I I , I I I , I I I I I

0 500 1000 1500 2000

Depth, mm

120

Depth, mm

FIG. 7. Concentration profiles for various radionuclides at site EA 23. (a) ‘jZTh and ‘jOTh. (b) ‘“hRa

and 238U

deposition rate occurring between 200 and 400 mm. This discontinuity in deposition rate coincides with a stratigraphic bound- ary at 300 mm, noted on the basis of lield- recorded morphology, including sediment particle size and color.

Despite the improvement in fit, the point at 600 mm is still well removed from the best fit line. This deviation could be due to further fluctuations in sedimentation rate, or to changes with time in excess 226Ra at deposition. This can be considered by reference to Figure 7a, which shows a strong correlation in the activities of 232Th and 230Th. Although these two isotopes

have different origins, their chemical fate once mobilized will be almost identical. Both are readily sorbed (see various au- thors in Ivanovich and Harmon, 1982) and such fluctuations may reflect changes in the very fine particulate (e.g., clays) concentra- tions relative to the bulk of the sediment, and perhaps also in the mineralogical de- tails. If so, the excess 226Ra/‘32Th profile should show reduced scatter compared with that of excess 226Ra alone. Such a pro- nounced correlation was used by Johnston et al. (1990); Murray et al. (1990) used such correlations to correct for variable mixing of coarse and fine particles in sedimentary

TL DATING OF AUSTRALIAN SANDS 39

FIG 8. (a) Depth profile of excess “‘Ra, site EA23. Cb) Depth profile of excess Z26Ra/‘72Th activity

11 I 81 I I I 81 I,, I I I ,I 1 !,I 0 500 1000 1500 20bo

Depth, mm

. Ro/Th = 1.92 e-o’oo33 ’ -

1 5

1500 2000

Ra/Th = 0.95 ebo.Oo” ’ -

I 0 1 1 1 I 8

500 1000 Depth, mm

ratio, site EA. 23.

profiles and 1.ransported sediment else- where in Australia. This nuclide ratio is shown in Figure 8b. and the two best fits give an improved representation of the data, with both now entirely consistent with the analytical uncertainties. The rates of decrease with depth are insensitive to the location of the break in sedimentation rate. Substituting thle fitted values for K into Eq. (3) gives a sedimentation rate of 0.12 2 0.02 mm yr -I to about 300 mm, and 0.39 ? 0.05 mm yr-’ below that depth. The uncertain- ties are based only on those derived from the fitting procedure, and are thus mini- mum values.

For comparison with TL. these sedimen-

tation rates can be converted to absolute dates if surface stripping is assumed negli- gible. The dates derived from 226Ra decay for the 150-, 1050-. and 1620-mm layers are 1250 +- 200,440O 2 500, and 5900 2 600 yr B.P. These compare favorably with the TL dates of 1300 5 600,290O -+ 700, and 5600 f 1600 yr, and provide direct support for the validity of the TL dates at this site, and thus support for the validity of the residual cor- rection of 3.0 + 0.5 Gy. It should be noted that surface stripping would make the dates based on 226Ra decay younger, and an un- derestimate of the residual value would have increased the TL dates. Only the co- incidence of an overestimate of the residual

value and surface stripping could have pro- duced spurious agreement. Except for the TL based value for the top 150 mm. the sedimentation rates based on either tech- nique are unaffected by surface stripping, or by uncertainties in the residual TL.

GEOMORPHOLOGICAL INTERPRETATION

Because a finite residual ED has signifi- cantly compromised the TL uncertainties, only a limited interpretation is possible. The middle to late Holocene saw pro- nounced fluvial deposition in northern Aus- tralia (Woodroffe er al., 1986). Eustatic sea level rises after the last glaciation reached a maximum early in the Holocene. Marine transgressions followed rising sea levels with associated stream base level changes and accelerated fluvial deposition. Recent studies of Holocene stratigraphy in channel and flood plain deposits in the adjacent Magela Creek catchment indicate this phase of aggradation commenced 5000 to 7000 yr ago, and remained approximately constant to the present (Nanson et trl., 1990; Wasson, 1990). The dates for the on- set of this phase of deposition agree with the TL date of 5600 f 1600 yr and the “hRa decay based date of 5900 ? 600 yr for the basal unit at site EA23.

CONCLUSIONS

TL dating of East Alligator siliceous flu- vial deposits is of limited value for Ho- locene material because of a substantial re- sidual ED of about 3 Gy, which results in age uncertainties of between 500 and 1600 yr. For Pleistocene deposits this residual would be less significant relative to the ED acquired at the final deposition site: the rel- ative uncertainties would be correspond- ingly less. The residual ED is readily bleached under laboratory conditions, and results from the incomplete bleaching, dur- ing reworking, of the dose acquired in long- term storage sites. such as levee banks. All dates were consistent with the stratigraphic record, and an excess of ‘2hRa over ‘jOTh at

the time of deposition allowed the, tndepen- dent estimation of sedimentation rates at one site. EA33. Dates based on these sedi- mentation rates were consistent with the TL dates. In addition. a single “c’ date at site EA22 was also consistent with the cor- responding TL date. These two indepen- dent checks confirm the validity of the re- sidual TL correction, and the reliability of the TL dates overall.

A basal date of 5600 -+ 1600 yr ITL) and 5900 ? 600 yr fzzhRa decay) for the deepest deposit is consistent with a major deposi- tional phase associated with a postglacial maximum sea level rise recorded in re- gional flood plain sediments. Because it is not known how widespread 12hRa excess is in sedimentary systems. it is not possible to speculate on the applicability of this decay based technique at other locations. How- ever, it is concluded that for deposits which are either barren or deficient in organics suitable for “C dating, TL is able to pro- vide at least an approximate chrclnology for Holocene deposits. and should providr a more precise time scale for Pleistocene ma- terials.

ACKNOWLEDGMENTS

We thank Cohn Mackintosh and Seng Ho Diep for

program development. and Rainer Marten. Paul Mar-

tin, and Gary Hancock for radionuclide analyses.

Ellen Wohl’s work was supported by grants from the

Fulhright Foundation. The Geological Society of

.4merica. the Geosciences Department elf the Univer-

sity of Arizona. and Sigma Xi. The Scientific Research

Society. Radiocarbon analyses were performed at the

Australian National University. Canberra

REFERENCES Baker-. V. K.. Kochel. K. C.. and Patton. I-‘. C‘. (1979).

Long-term flood frequency analysis using geological

data. IAHS-AISH Publicntion 128, 3-Y.

Baker. V. R.. Pickup, G.. and Polach. H. A. (1983).

Desert paleofloods in central Australia. .Ymlltre 301.

50?-504.

Gardner. G. J.. Mortlock. A. J.. Price. I>. M.. Read-

head. M. L.. and Wasson. R. J. (19871. Thermolu-

minescence and radiocarbon dating of /UIStrdkin

desert dunes. Artstrulicrn Jortrr~ul of‘ Etrrth Scienc~c~s 34. 343-357.

Huntley. D. J.. Hutton. J. T.. and Pre\<‘ott. J. K.

TL DATlNG OF AUSTRALIAN SANDS 41

(1985). South Australian sand dunes: A TL sediment test sequence. Preliminary results. Nuclear Tracks lO(4-6). 7.57-758.

Ivanovich, M.. and Harmon. R. S. (1982). “Uranium Series Disequilibrium: Applications to Environmen- tal Problems.” Oxford Univ. Press, London.

Johnston, A., Murray, A. S., Marten, R., and Martin, P. (1988). “The Transport and Deposition of Radi- onuclides Discharged into Creek Waters from the Ranger Uranium Mine.” Radiation Protection Prac- tice. 11, Seventh Annual Congress of the Interna- tional Radiation Protection Association, pp. 652- 655. Pergamon Press, New York.

Johnston, A., Murray, A. S.. and Wasson, R. J. (1990). Determination of sediment sources and sinks using U and Th series radionuclides. In “Modern Sedimentation and Late Quaternary Evolution of the Magela Creek Plain,” (R. J. Wasson, Ed.). In press.

Martin, P., and Hancock, G. (1987). Radionuclide an- alytical technique development. In “Alligator Riv- ers Region Research Institute Annual Research Summary 198&1987,” pp. 78-83. AGPS, Canberra.

Mejdahl, V. (1979). Thermoluminescence dating: Beta-dose attenuation in quartz grains. Archueome-

tiy 21, 61-72.

Murray, A. S. (1981). “Environmental Radioactivity Studies Relevant to Thermoluminescence Dating.” Unpublished Ph.D. thesis. Oxford University.

Murray, A. S.. Johnston, A., Marten, R., and Martin P. (1987). Analysis for naturally occurring radionu- elides at environmental levels by gamma spectrom- etry. Journal of Radioanalytical and Nucleur Chem-

isfry 115, 263-288. Murray, A. S., and Aitken, M. J. (1988). Analysis of

low-level natural radioactivity in small mineral sam- ples for use in thermoluminescence dating, using high-resolution gamma spectrometry. Infernational Journal of Applied Radiation and Isotopes 39, 145-

158. Murray. A. S.. Caitcheon, G., Olley, J. M.. and

Crockford, H. 111990). Methods for determining the sources of sediments reaching reservoirs: Targeting soil conservation. Proc. of “1989 Conference on Large Dams,” Ballarat, Australian National Com- mittee for Large Dams, ANCOLD Bulletin. Vol. 85, pp. 61-70.

Nanson, G. C., and Young, R. W. (1987). Comparison of thermoluminescence and radiocarbon age-deter- mination from Late-Pleistocene alluvial deposits near Sydney. ,4ustralia. Quurernary Research 21, 263-269.

Nanson, G. C., East, T. J., Roberts, R. G., Clark, R. L., and Murray. A. S. (1990). Quaternary evolu- tion and landform stability of the Magela Creek catchment near the Ranger Uranium Mine, North- ern Australia. The Supervising Scientist for the Al- ligator Rivers Region, OFR 63, Sydney.

Prescott, J. R. (1983). Thermoluminescence dating of sand dunes at Roonka, South Australia. PACT 9,

505-512. Prescott. J. R., and Hutton, J. T. (1988). Cosmic-ray

and gamma-ray dosimetry for TL and electron- spin-resonance. Nuclear Tracks and Radiurion

Measuremeni 14, 223-227. Proszynska, H. (1983). TL dating of some sub-aerial

sediments from Poland. PACT 9, 539-546. Rendell, H. M. (1985). The precision of water content

estimates in the thermoluminescence dating of loess from northern Pakistan. N&ear Trucks 10, 763- 768.

Roberts. R. G., Jones, R., and Smith, M. (1990). Ther- moluminescence dating of a 50,000-year-old human occupation site in northern Australia. Nafure 345, 153-156.

Scott. M. R. (1982). The chemistry of U and Th series nuclides in rivers. In “Uranium Series Disequilib- rium: Applications to Environmental Problems” (M. lvanovich and R. S. Harmon. Eds.). Oxford Univ. Press. London.

Singhvi, A. K.. Sharma, Y. P.. and Agrawal, D. P. (1982). Thermoluminescence dating of sand dunes in Rajasthan, India. Nature 295, 313-315.

Thorn. B. G. (1978). Coastal sand deposition in South- east Australia during the Holocene. In “Landform Evolution in Australasia” (J. L. Davies and M. A. J. Williams, Eds.). pp. 197-214. ANU Press, Canberra.

Wasson, R. J. (Ed.) (1990). Modern sedimentation and late Quaternary evolution of the Magela Creek plain. The Supervising Scientist for the Alligator Rivers Region, Sydney. in press.

Wintle, A. G. (1978). Anomalous fading. PACT 2(l). 240-244.

Woodroffe, C. D., Chappell, J. M. A., Thorn, B. G., and Wallensky, E. (1986). Geomorphological Dy- namics and Evolution of the South Alligator Tidal River and Plains, Northern Territory. North Austra- lia Research Unit, Mangrove Monograph No. 3.

Wopfner, H., and Twidale, C. R. (1971). Geomorpho- logical history of the Lake Eyre Basin. In “Ldnd- form Studies from Australia and New Guinea” (J. N. Jennings and J. A. Mabbutt, Eds.). pp. II9- 143. ANU Press, Canberra.