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Th chronology for landform evolution in the Sorbas basin, southeast Spain
Quaternary Science Reviews 19 (2000) 995}1010
A calcrete-based U/Th chronology for landform evolution in the Sorbas basin, southeast Spain M. Kelly *, S. Black, J.S. Rowan Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ, UK Postgraduate Research Institute for Sedimentology, University of Reading, PO Box 227, Reading RG6 6AB, UK Department of Geography, University of Dundee, Dundee DD1 4HN, UK
Abstract Nodular and massive calcretes, from alluvial terraces in the Sorbas area of southeast Spain, have been dated by the U/Th isochron method to between 8 } '350 ka. Alternative approaches have been used to generate the multiple samples for the isochron method for several of the calcretes. Dates agreed closely when obtained by total dissolution of a number of nodules or by the sequential dissolution in di!erent acids of a single nodule. However, fractional dissolution of a single nodule in the same acid revealed two stages of precipitation of di!erent age. It is considered that the calcretes may not be strictly pedogenic in origin and that groundwater processes linked to the alluvial environment may have been involved also. Nevertheless, their dates provide minimum ages for a chronostratigraphic framework for terrace evolution. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The Betic Cordillera in southeast Spain is an area of basin and range topography which has been tectonically active from the Neogene to the present (Weijermars et al., 1985). The drainage systems of the area were initiated by the late Miocene - early Pliocene uplift above sea level of Tertiary marine sedimentary basins partially separated by elevated ranges of Palaeozoic rocks. The evolution of a number of these drainage systems has been interpreted from sequences of alluvial deposits and terraces formed during the subsequent erosion of the basin in"lls (Harvey, 1987; Harvey and Wells, 1987; Mather and Harvey, 1995; Mather et al., 1995; Wenzens and Wenzens, 1995). Hitherto, an absolute chronological framework has not been available for the landform evolution of these basins and the discussion of evolution rates and correlation with the climate-stratigraphic units of the Quaternary has necessarily been limited. The present investigation examines the extent to which absolute dating can be provided by U/Th dating of calcretes precipitated in alluvial sediments and related soils of a terrace sequence. Its principle aim is methodological but the implications for landform chronology
* Corresponding author. E-mail address: m. [email protected] (M. Kelly).
are considered. The term calcrete is used here in a broad sense (Dixon, 1994), for secondary carbonate accumulations of a variety of morphologies (nodular to massive) formed in a near surface terrestrial environment. The study area is the Rio Aguas drainage system in the Sorbas basin, Almeria, for which a detailed relative chronology has been established (Harvey and Wells, 1987; Mather et al., 1991; Miller, 1991; Harvey et al., 1995; Mather and Harvey, 1995). According to their work, the initial drainage was a subsequent system, which locally developed antecedent components as continuing tectonic uplift and erosion exposed the basement bedrock of the Alhamilla-Cabrera range (Fig. 1). This period of incision is represented by a narrow valley cut by the protoAguas/Feos river across the Alhamilla-Cabrera range and by an upper, descending series of alluvial terrace surfaces (A, B, C) in the Aguas-Feos catchment. Subsequently, aggressive headward erosion by an easterly #owing river from the neighbouring Vera basin captured the Rio Aguas near Los Molinos (point marked `Cola, Fig. 1). A lower descending terrace sequence (D1, D2, D3, E, F) in the upper Rio Aguas records further incision of the channel in response to post-capture base level adjustment and/or continuing tectonic uplift. Terrace deposits indicate that the erosional history of the catchment was interrupted by periods of aggradation, determined by a number of possible factors including climatic change (Mather and Harvey, 1995). The contrasting competence
0277-3791/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 5 0 - 5
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M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Fig. 1. General topography map and bedrock geology of the Sorbas area, southeast Spain. The inset map shows the sample localities.
of the di!erent lithological units of the basin in"ll has also in#uenced the basin geomorphology and its evolution; with limestones and gypsum being the more competent units and marls, sands and gravels the less competent (Weijermars et al., 1985). The stratigraphy of soil pro"les developed on the alluvial terraces, and the nature of the calcretes, have been described by Miller (1991) and Harvey et al. (1995). They derived estimates of terrace ages, based on the relative development of calcrete and rubi"ed soil horizons and on a C date (2310#80!90 BP) for an E terrace. A detailed description of calcrete morphology and stratigraphy has been given by Nash and Smith (1998) for the adjacent Tabernas basin, west of Sorbas. U/Th dates have been previously reported for calcretes from the adjacent coastal area of southeast Spain (Radtke et al., 1988). However, little contextual information is provided and the correction procedure used for detrital Th is only approximate. U/Th ages have also been reported for marine molluscs from Quaternary sediments in the coastal part of this area (Stearns and Thurber, 1965; Hillaire-Marcel et al., 1986).
2. Methods Samples were collected from exposures located on terraces conveniently chosen to cover a range of relative
ages, as de"ned by Harvey and Wells (1987), Mather et al. (1991) and Miller (1991) (Fig. 1). The exposure stratigraphy was logged and calcrete samples representative of the main morphological types were taken from fresh surfaces for analysis. In the event, only the most developed calcrete was analysed from each site. Sampling was restricted laterally to a few tens of cm and vertically to generally (10 cm (Fig. 2). Where possible, calcretes with evidence of obvious post-formational carbonate precipitation (in joints etc.) were avoided. Munsell soil colours were determined on moist samples for some sections. The analytical basis for dating calcretes is the measurement of Th ingrown from the decay of authigenic U and, indirectly, U, the uranium having been co-precipitated from solution with the carbonate. The calcrete, however, is an impure mixture of calcium carbonate and incorporated detrital minerals, resulting in the same radionuclides being present in both the authigenic and detrital (allogenic) fractions. In addition, a minor amount of allogenic thorium could be included by subsequent scavenging from solution. Our basic approach has been to use an isochron method to determine the authigenic radionuclide components and, from that, the age (Bischo! and Fitzpatrick, 1991). This method requires the assumption that the coeval samples used for an isochron are mixtures, in di!erent proportions, of detrital and authigenic carbonate end members. In addition, it is assumed that the
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
respective end members producing the mixtures are compositionally homogeneous in terms of the radionuclide ratios. The detrital component can include silicate, carbonate or other mineral phases, and radioactive equilibrium or disequilibrium may or may not exist in these mixtures, so long as the homogoneity criterion is met. The analyses, carried out by SB, used three di!erent methods to obtain the 3}6 coeval, carbonate/detrital mixtures (subsamples) required for each isochron age determination of a sample. These methods are summarised below: (i) In the standard `total dissolutiona method (sample code /T), the coeval subsamples were either separate nodules or subsamples of a massive carbonate. These were dried and surface material discarded prior to hand picking and crushing. Between 2.9}17 g of subsample were dissolved in a HCl}HNO }HF mixture, following the methods outlined in Black et al. (1997) and based on those of Bischo! and Fitzpatrick (1991) and Luo and Ku, (1991). (ii) In the `sequential dissolutiona method (sample code /S), three di!erent coeval mixtures were obtained by chemically fractionating a single sample. This was achieved by obtaining three solutions for analysis by successive leaching of a crushed sample in 0.1 M HCl and then 0.1 M HNO and, "nally, dissolving the residue in HCl}HNO }HF. The following sample mass range was obtained for each fraction: 0.1 M HCl leachates 0.9}3.6 g; 0.1 M HNO leachates 5.4}12.7 g; HF soluble residues 1.0}6.9 g. Before leaching, the sample was "red by heating to 6003C. Each fraction was leached and then dried and weighed prior to continuing with the next step. Additional aliquots of yield monitor were added after each successive leachate step. The absorption of the yield monitor by samples that were later discarded was checked for. Each fraction absorbed (0.02% of the yield monitor under the exact conditions used to obtain the isochrons by maintaining acidic solutions. This method owes something to the earlier Leachate}Residue (e.g. Ku and Liang, 1984) and Leachate}Leachate (Schwarcz and Latham, 1989) methods but di!ers in the isochron approach. The sequential dissolution method is more stringent in its requirements than total dissolution, being susceptible to fractionation of radioisotopes (i.e. of a given element) during preparation and element redistribution (e.g. Th scavenging by the residual phase during leaching). The latter was minimised by "ring and the use of the acid molarities indicated by Bischo! and Fitzpatrick, (1991). (iii) In the fractionated total dissolution method (sample code /F), 3 apparently coeval nodules were sequentially fractionated by slow dissolution in 0.1 M HNO into outer, middle and inner fractions. Care was taken to keep reaction rates slow, by periodically lifting the nodule from the solution and washing in
997
Milli-Q water. For the purposes of de"ning the fractions during dissolution of the sectioned nodules, the outer fraction was delimited by visible banding whilst the middle and inner fractions were based only on equal dissolution times. Fraction masses ranged as follows: outer 1.2}3.8 g; middle 2.2}3.8 g; inner 3.1}7.0 g. These (plus any washings) were then treated as separate samples using the total dissolution method. After dissolution, U and Th were separated by ion exchange chromatography techniques and electrodeposited onto stainless steel planchettes. The activity concentrations of Th, Th, U and U were determined by alpha spectrometry using solid state silicon surface barrier detectors. A standardised U } Th tracer was used as a yield monitor, with a correction being made for the presence of detrital Th and the ingrowth of Ra. Th/Th equilibrium was checked on unspiked samples for at least one total dissolution subsample for each sample and various leachates which were run simultaneously. The (Th/U) and (U/U) activity ratios of the authigenic fraction were calculated from the slope of a 3-D isochron "tted to the XYZ data (U/Th, Th/Th and U/Th) using the method of Minimum Likelyhood Estimation of Ludwig and Titterington (1994), in which the analytical data are weighted for analytical errors and error correlations. The apparent age was determined from these authigenic fraction activity ratios (Broecker, 1963). A number of sources of uncertainty contribute to the quoted uncertainty in the age determination. The analytical errors are one such source and, usefully, the Ludwig and Titterington method quanti"es the probability that the uncertainty in the age is due to these ascertained analytical errors alone, through calculation of the mean square of weighted deviates (MSWD). Consequently, a low probability (or MSWD'1) indicates that the uncertainty in the age is due to other sources of error, probably of geological origin, e.g. subsamples are not coeval or mixture end members are not homogenous, due to spatial irregularity in carbonate precipitation history and in detrital composition. Another cause could be unquanti"ed analytical errors. In addition, open system radionuclide uptake or loss by the calcrete could result in a bias in the determined age. The 95% con"dence interval quoted for the isochron ages takes into account the MSWD value.
3. Results 3.1. Stratigraphy and lithology The stratigraphy of the sample sites is shown in Fig. 2 with the sample numbers of analysed material. This comprised a variety of calcrete types, each one being the
Fig. 2. Stratigraphic logs for calcrete samples (terrace designation from Harvey et al. (1995) and Harvey pers. comm. (1998).
998 M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
most developed form in its pro"le (sample number in brackets): (i) individual nodules formed of irregular but more or less elongate aggregates up to 4 cm (401) or 6 cm (402), composed of rounded micrite subnodules of mm to &1 cm scale, with a moderate content of #oating detrital grains (20}30% HF residues), (ii) massive, more or less planar calcrete formed of coalesced, #attened, nodular micrite aggregates with component micrite sub-nodules up to 1 cm, as in (i) (608), (iii) massive, intergranular micrite, in a more or less clast supported coarse gravel, although the sedimentary fabric is probably distorted, with "ne gravel and sand clasts #oating in the cement (403, 603, 613), (iv) massive laminated micrite with a low content of #oating detrital grains, laminae (1 mm to '1 cm, typically with sinuous-wavy boundaries on a mm scale (506 (also present at 508 and 613 but not analysed)), (v) as (iv) but unlaminated (503), also brecciated, with secondary sparite and secondary porosity (in 508 the lamination post-dates the brecciation). These calcrete morphologies correspond to stages II (type i), III (types ii and iii) and IV (types v and vi) of the developmental schemes for pedogenic secondary carbonates of Gile et al. (1966) and Machette (1985). However, these schemes may not be rigorously applicable because of the modes of formation of the calcretes, as discussed below. 3.2. Analytical results The analytical data are given in Table 1 and the authigenic or detrital radionuclide activity ratios derived from the isochron and the isochron date with its 95% con"dence interval are given in Table 2, together with locality details. The isochron diagrams are shown in Figs. 3}5 as 2-D versions (X> & XZ plots) for illustration purposes only. The regression lines are derived by the MLE method and the calculated data uncertainties are shown by 1p error ellipses where space permits. As explained, the age was calculated from the authigenic fraction Th/U and U/U ratios obtained by a 3-D (X>Z) "t to the data. Only one sample did not produce an isochron (506), because of low variability in Th/Th ratios in this laminar calcrete. The U and Th mass concentrations in the calcretes (total dissolutions only), essentially the U and Th concentrations, are low but very variable with a mean of 1.1$1.0 ppm and 0.8$0.7 ppm (1p, n"42), respectively. The mean U/Th mass ratio of 2.3$2.6 indicates the signi"cant but also variable detrital silicate contribution to the calcrete, re#ecting the changes in sediment sources during catchment evolution as progressive incision into the basin bedrock sequences occurs. Unfortunately, the bedrock U and Th signatures are not known.
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The initial U ratios (U/U) show a trend in time (Fig. 6), with generally higher ratios from older calcretes (conversely their present ratios are relatively uniform). This could be explained by a change in the relative importance of the mechanisms releasing U to solution from the source rocks, i.e. selective U leaching due to the recoil mechanism and non-selective U release by oxidative chemical weathering (Osmond and Ivanovich, 1992). Such a change could be climate related, from more arid to more humid, or simply re#ect the increasing extent of chemical weathering. In addition, it might be explained by a changing groundwater regime, with a lesser contribution from deeper reducing waters, perhaps as a consequence of the progressive terrain dissection. A similar trend of increasing (U/U) values with age has been seen in fossil marine shells from SE Spain and Mallorca, where it is explained either by U uptake on fossilisation from groundwaters which have U ratios evolving with time due to progressive weathering (Hillaire-Marcel et al., 1986) or by gradual open system replacement of U from groundwater with a high U/U ratio (Hillaire-Marcel et al., 1996). Isochron ages derived from di!erent analytical techniques on the same sample show very good agreement. That is, for each of the three youngest samples (401, 402 and 403), dates agree closely when produced by either analysis of 4 separate nodules (total dissolution) or of 3 chemical fractions of a single nodule or massive calcrete sample (sequential dissolution). For each sample, the most precise age estimate is given by combining the data from the two methods (Table 2 and Fig. 3) (for 401/S#T, the HCl acid leach data are excluded because of a relatively large analytical error). This shows the validity of the sequential dissolution approach, at least when isotope fractionation during analysis can be avoided (see Method section). Conversely, it indicates the uniformity in the apparent age of the material dated, over a spatial scale of (10 cm in the sediments. The probability and MSWD values, given by the error treatment of Ludwig and Titterington, (1994), indicate that only for dates based on 2 samples (401/S and 402/T) are the assigned analytical and correlation errors capable of accounting for the age uncertainty. For the remainder, the geological factors identi"ed earlier, i.e. subsamples di!er in age and/or in detrital radionuclide composition, are likely to be major contributors to the age uncertainty, with the potential for this increasing with apparent age.
4. Discussion 4.1. Interpretation of calcrete dates Calcretes are known to have a variety of origins, both pedogenic and non-pedogenic. Essentially, pedogenic
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M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Table 1 Analytical data for calcretes, Sorbas, Spain Sample no.
SubSample
Th-230 (Bg/kg)
1p
Th-232 (Bg/kg)
1p
U-234 (Bg/kg)
1p
U-238 (Bg/kg)
1p
401/T
1 2 3 4
5.302 6.622 8.595 3.773
0.101 0.086 0.112 0.049
0.745 0.946 1.264 0.546
0.011 0.018 0.024 0.010
8.282 8.618 7.205 4.592
0.166 0.147 0.123 0.078
7.598 8.054 6.671 4.100
0.156 0.139 0.113 0.070
401/S
a b c
4.905 0.549 54.601
0.093 0.110 1.037
0.694 0.079 8.464
0.010 0.002 0.129
7.472 0.569 18.799
0.149 0.011 0.376
6.732 0.470 19.027
0.135 0.009 0.381
402/T
1 2 3 4
1.912 3.675 0.884 3.125
0.022 0.043 0.010 0.036
0.597 0.799 0.400 0.812
0.008 0.010 0.005 0.101
13.547 32.013 4.003 25.290
0.348 0.848 0.106 0.670
11.851 29.370 3.543 23.858
0.318 0.778 0.044 0.622
402/S
a b c
5.985 0.322 11.117
0.040 0.002 0.056
3.420 0.046 6.948
0.029 0.001 0.035
10.875 3.205 13.200
0.084 0.018 0.109
10.771 3.057 13.368
0.083 0.017 0.111
403/T
1 2 3 4
8.598 9.707 12.013 9.005
0.142 0.224 0.233 0.190
0.818 0.772 1.261 0.597
0.016 0.015 0.024 0.012
12.273 17.374 12.299 18.854
0.221 0.313 0.322 0.439
9.091 13.263 9.684 15.328
0.136 0.199 0.145 0.229
403/S
a b c
6.352 1.776 105.313
0.133 0.037 2.211
0.371 0.117 12.976
0.007 0.003 0.247
15.262 3.641 53.422
0.382 0.091 1.336
11.390 3.034 48.566
0.251 0.067 1.068
501/T
1 2 3 4
5.640 5.237 1.324 1.084
0.041 0.141 0.020 0.014
2.350 2.618 1.891 1.041
0.013 0.111 0.051 0.044
10.035 10.141 3.662 2.678
0.110 0.099 0.111 0.112
9.840 9.840 3.325 2.277
0.090 0.101 0.141 0.131
503/T
1 2 3 4
3.515 6.103 4.163 3.700
0.045 0.039 0.091 0.030
3.028 5.382 4.935 4.593
0.033 0.036 0.100 0.034
4.973 7.212 6.255 5.526
0.046 0.060 0.091 0.045
4.281 6.947 5.636 4.746
0.042 0.059 0.036 0.042
506/T
1 2 3 4
5.047 4.993 7.070 4.202
0.043 0.043 0.115 0.045
2.561 2.970 4.228 2.431
0.029 0.032 0.084 0.033
7.693 8.514 9.778 9.131
0.097 0.102 0.123 0.100
8.060 8.062 9.085 8.019
0.099 0.099 0.117 0.092
508/T
1 2 3 4
5.920 7.766 9.315 10.020
0.062 0.069 0.142 0.079
2.189 2.303 1.085 1.443
0.074 0.034 0.042 0.026
7.156 8.578 10.373 11.744
0.061 0.074 0.086 0.082
6.914 7.962 10.252 10.584
0.060 0.070 0.086 0.076
603/T
1 2 3 4 5 6
7.506 11.394 13.701 31.133 7.275 27.852
0.094 0.101 0.099 0.107 0.096 0.097
2.244 3.076 3.111 6.142 1.197 6.143
0.071 0.091 0.088 0.101 0.093 0.090
4.374 5.829 12.618 30.986 8.512 25.880
0.091 0.091 0.111 0.096 0.049 0.087
3.538 4.894 10.720 25.843 7.281 22.563
0.092 0.081 0.091 0.093 0.059 0.079
607/R (silt)
1 2
11.599 20.142
0.311 0.669
28.221 31.142
0.616 0.871
12.897 31.952
0.443 0.614
13.001 32.738
0.515 0.717
608/T
1 2 3 4
5.525 7.768 6.309 13.365
0.094 0.083 0.091 0.099
6.514 7.114 4.073 10.011
0.101 0.086 0.088 0.096
8.173 11.062 9.551 20.493
0.099 0.102 0.097 0.111
6.759 9.628 8.156 17.728
0.093 0.099 0.099 0.090
Mass leached (%)
64 19 17
71 24 37
51 20 29
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
1001
Table 1 Continued Sample no.
SubSample
Th-230 (Bg/kg)
1p
608/F (outer)
1 2 3
3.510 2.866 3.990
0.066 0.039 0.047
608/F (middle)
1 2 3
11.773 11.107 7.319
608/F (inner)
1 2 3 4
608/R
613/T
Th-232 (Bg/kg)
1p
U-234 (Bg/kg)
1p
U-238 (Bg/kg)
1p
Mass leached (%)
4.077 5.164 6.116
0.091 0.089 0.102
12.300 5.211 12.917
0.097 0.093 0.107
10.603 4.571 11.637
0.095 0.081 0.098
10 8 3
0.099 0.097 0.061
7.118 8.119 11.141
0.081 0.092 0.107
18.129 15.053 8.389
0.111 0.101 0.091
15.234 13.210 7.109
0.107 0.117 0.090
31 23 36
10.524 11.756 13.494 11.957
0.099 0.103 0.121 0.109
6.017 8.141 12.146 13.111
0.071 0.091 0.107 0.113
14.849 17.104 18.887 15.589
0.133 0.117 0.201 0.188
12.478 14.873 16.863 13.796
0.116 0.109 0.199 0.101
59 69 61 63
1 2
11.790 16.909
0.641 0.717
24.461 31.666
0.874 0.994
12.451 24.414
0.346 0.894
13.093 24.736
0.361 0.911
1 2 3 4
48.570 43.638 53.599 55.325
0.103 0.171 0.141 0.099
6.716 7.232 10.117 12.114
0.096 0.099 0.111 0.099
59.806 50.869 56.321 49.146
0.117 0.104 0.105 0.088
50.131 42.041 48.096 42.477
0.106 0.099 0.106 0.089
/T: total dissolution; /S: sequential dissolution; /F: fractionation and total dissolution; /R: residual fraction. HNO3; b: HCl; c: HF
carbonate precipitation occurs in soils of arid or semi-arid climates due to leaching of calcium carbonate from sur"cial soils by downward percolating rain and its precipitation in the vadose zone (e.g. Gile et al., 1966; Machette, 1985). The source carbonate may be alluvial clasts or, as is more usually considered to be the case, aeolian dust. A relevant purely non-pedogenic category is groundwater calcretes; formed by precipitation within the phreatic zone of calcium carbonate introduced by groundwater #ow, as a result of evaporation or CO degassing (e.g. Arakel and McConchie, 1982). However, in a semi-arid alluvial environment, a continuum of processes is likely to occur, which includes both the pedogenic and non-pedogenic categories referred to above, together with intermediaries (Khadkikar et al., 1998). Examples of the last would be where a river contributes calcium carbonate, for pedogenic calcrete formation, by #oodwater in"ltration or through recharge of a perched water table supplying soil moisture from a capillary fringe (e.g. Arakel and McConchie, 1982; Semeniuk and Searle, 1985; Quade, 1986). Of the calcretes analysed, 2 are from sites (402, 608) with populations of nodules which resemble rhizoliths precipitated around roots by a mechanism that is still uncertain (Klappa, 1980), i.e. consistent with a pedogenic origin. These two calcretes and 401 are also from pro"les with an upper rubi"ed zone typical of semi-arid soil pro"les. The origins of the other calcretes associated with gravels are less certain, despite most having a rubi"ed upper zone (603, 613, 608, 501, 508), since this could post-date calcrete formation
and even intervening erosion. However, both sets of calcretes can be "tted into the mixed origin model of Khadkikar et al. (1998) for alluvial calcretes, particularly taking into account the importance of carbonate bearing bedrocks in the Rio Aguas catchment as a source of dissolved and detrital calcium carbonate (Weijermars et al., 1985). Both groundwater and pedogenic calcretes have been identi"ed in the neighbouring Tabernas basin (Nash and Smith, 2000), in some cases in the same pro"le. One context for groundwater calcrete formation found there, above impermeable bedrock, applies also to the calcrete at site L9 (sample 503). It is inherent in models of pedogenic calcrete formation that the quantity and morphology of the secondary carbonate evolve with time, as indicated by the morphological / developmental stage schemes of Gile et al. (1966) and Machette (1985) for calcretes from the southwestern United States of America, where the evolutionary lifespan of a calcrete can be '1 Ma. However, because of climate change, or changes in local conditions (tectonism, erosion, sedimentation), the rate of calcrete precipitation is unlikely to have been constant or even continuous (Gile et al., 1966; Machette, 1985). Non-pedogenic calcretes are potentially more rapidly formed (Khadkikar et al., 1998). The association with an active river system in an erosional environment also raises the possibility that the main periods of calcrete formation might be narrowly restricted in time, being inhibited or halted by incision of the river below the point at which lateral groundwater recharge and #ooding occur, i.e.
L7 Gochar B L8 Sorbas A L9 Cerro Molatas A
0.952 1.076 0.872
1.101 1.223 1.136 1.073 1.050 1.060 1.266 1.295 1.331 1.162 1.226 1.237 1.116 1.217 1.166 1.144 1.165
Authigenic U-234/U-238
0.049 0.097 0.51
0.056 0.161 0.021 0.036 0.030 0.023 0.079 0.122 0.036 0.043 0.515 0.029 0.091 0.105 0.142 0.205 0.038
1p
0.694 0.956 1.230
0.065 0.085 0.080 0.085 0.084 0.084 0.316 0.326 0.327 0.545 !0.018 0.658 0.668 0.765 0.679 0.141 0.673 0.028 0.100 0.555
0.044 0.139 0.023 0.003 0.003 0.001 0.042 0.052 0.020 0.098 0.334 0.045 0.065 0.072 0.101 0.095 0.026
Authigenic Th-230/U-238 1p
Note: Terrace designation from Harvey et al. (1995) and Harvey pers. comm.(1998). T: total dissolution, /S: sequential dissolution, /F: fractionated (O-outer, M*middle, I*inner); excludes 401/Sc.
D2 D2 D2 D1 D1 D1 Early D1? Early D1? Early D1? C C C C C C C C
Massive Massive Massive
Urra Urra Urra Urra Urra Urra Urra Urra Urra Urra Urra Urra `Cola `Cola `Cola `Cola `Cola
L1 L1 L1 L2 L2 L2 L3 L3 L3 L5 L5 L4 L6 L6 L6 L6 L6
Nodule Nodule Nodule Nodule Nodule Nodule Massive Massive Massive Massive Laminar Massive Massive Mass-nod. Mass-nod. Mass-nod. Mass-nod.
Terrace/ deposit
401/T 401/S 401/S#T 402/T 402/S 402/S#T 403/T 403/S 403/S#T 603/T 506/T 613/T 608/T 608/F(I) 608/F(M) 608/F(O) 608/T#F (1#M) 501/T 508/T 503/T
Locality
Material
Sample no.
Table 2 Sample data and isochron dates
1.443 2.028 *
1.122 1.251 1.162 1.101 1.077 1.087 1.381 1.415 1.450 1.408 * 1.552 1.472 1.629 1.513 1.190 1.507
Intial U-234/U-238
0.074 0.183 *
0.057 0.165 0.021 0.037 0.031 0.024 0.086 0.133 0.039 0.052 * 0.036 0.120 0.141 0.184 0.213 0.049
1p
1.050 20.600 38
0.506 4.050 0.337 0.513 2.780 1.970 1.770 2.520 2.060 8.100 29.9 12.400 3.460 6.830 17.200 14.800 14.800
MSWD
0.38 0 0
0.73 0.017 0.95 0.73 0.062 0.032 0.13 0.08 0.02 0 0 0 0.006 0 0 0 0
Probability of "t
147 224 '350
6.6 8 7.9 9.02 9.01 8.97 30.9 31.2 30.3 68 * 80 98 103 92 14 91
Age (ka)
25 160 *
9.2 27 4.7 0.71 0.7 0.28 10 12 4.4 32 * 19 30 33 41 20 11
95% C.I. #/! [ka]
1002 M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
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Fig. 3. Calcrete isochron plots (2-D versions) based on both total and sequential dissolution of nodules for each calcrete, with 1 p error ellipses (dates are based on a 3-D isochron).
precipitation events could be short relative to the calcrete age. However, subsequent phases of carbonate precipitation might occur by purely pedogenic processes, governed by the climatic regime.
A polygenetic history for some nodules is indicated by the analytical data for 608, which give apparently uniform U/Th ages for the inner and middle nodule fractions (608/F(I): 103$33 ka and 608/F(M): 92$41 ka) and
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M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Fig. 4. Calcrete isochron plots (2-D versions) based on both total and fractional dissolution of individual nodules; for the outer (O) and combined inner and middle fraction (I and M) plus total (dates are based on a 3-D isochron).
a much younger age for the outer (608/F(O): 14$20 ka) (Fig. 4). Because of the complex geometry of the small nodules, the fractions were de"ned only by dissolution time. The methodologically de"ned outer fraction corresponded to a mean of 7% of nodule mass. However, it is likely that the young phase of calcrete precipitate does form only a small percentage of the mass since total dissolution of nodules gave an age similar to the inner and middle fractions (608/T: 98$30 ka). The most precise age for 608 was given by combining the total, inner and middle dissolution data (91$11 ka). This is compatible with the above evolutionary model, with the inclusion of a second phase of calcrete precipitation. An alternative explanation would be post-formational U uptake by the outer fraction. However, thin section examination of 608 shows the presence of secondary sparite cement which could be responsible for the younger age, although it probably forms a smaller proportion than 7% of the mass. The very precise age of nodules from another site (402/S#T: 8.97$0.28 ka), obtained by two contrasting analytical methods, is also
indicative of a very uniform composition and, hence, one main period of precipitation which was short relative to the age. In theory, therefore, it is usually necessary to regard the pure authigenic component of a calcrete (the isochron end member) as having a time averaged composition, which integrates the di!erent time periods and rates of carbonate accumulation. Consequently, the U/Th date gives an apparent age which is the minimum for commencement of signi"cant carbonate precipitation and, in the present context, for the end of alluviation at a site. It is also a maximum age estimate for the latest signi"cant carbonate precipitation. In addition, because of the possible smallscale morphological complexity of the precipitate, subsamples are unlikely to be homogeneous with respect to this average authigenic end member, i.e. not coeval. 4.2. Terrace chronology There is a general relationship between the calcrete dates and the relative geomorphic ages of the associated
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
terraces indicated by their height relative to the local channel (Harvey and Wells, 1987; Harvey et al., 1995). However, the U/Th chronology is generally younger than the ages inferred by Harvey et al. (1995) from soil development, including the stage of calcrete formation (Tables 2 and 3). The age of the A terrace is very uncertain, with a "nite age estimate of 224$160 ka (508/T) and another beyond the limit of the U/Th method ('350 ka) (503/T). The former is from an obviously polygenetic calcrete and the age of initial calcrete formation will be in the upper part of the range or non-"nite. The measured non-"nite age is for a thin deposit of calcrete in a gravel matrix above limestone bedrock. Terrace B, with a minimum age estimate of 147$25 ka (501/T), can be broadly correlated with Oxygen Isotope Stage 5e-6 (Martinson et al., 1987). A more precise minimum age estimate is possible for the C terrace, which has a particular signi"cance in the evolution of the Rio Aguas drainage since it is considered to closely predate capture by what is now the easterly #owing lower Rio Aguas. Six isochron dates from two areas with di!erent calcrete morphologies relate to this
1005
feature (Table 2). The best estimate for the minimum age for the event should be the date of 91$11 ka from the capture site (`Cola, Fig. 1) (608/T#F(I)#F(M)). This date is for a massive layer of coalesced nodules, although a laminar layer is also present locally above this, corresponding to stage III/IV. (This locality is comparable to RA4 of Harvey et al. (1995)). Younger dates, but within the 95% C I of the `Cola date, come from the Urra area (68$34 (603/T) and 80$17 ka (613/T)), from massive intergranular calcretes below well developed laminar layers (stage IV), giving an overall mean from both sites of 88$28 ka. (The younger age is from a locality comparable to RA3 of Harvey et al. (1995)). Whilst the `Cola calcrete is considered to be pedogenic those at Urra are less obviously so, with the intergranular calcrete there resembling vadose cement but with the laminar calcrete indicating retention of vertically draining surface water. However, in the broad context of an alluvial environment the implications of the dates are the same, i.e. they give a minimum age for calcrete formation and cessation of aggradation which suggests that these events occurred early in the last glaciation (OIS 5a}d).
Fig. 5. Calcrete isochron plots (2-D versions) based on total dissolutions alone (dates are based on a 3-D isochron).
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M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Fig. 5. Continued.
The post-C terrace history of the Rio Aguas was apparently predominantly erosional, with &25 m of downcutting (at Urra) before the D1 terrace aggradation. The date of 30.3$4.4 ka (403/S#T), from a calcrete cemented gravel on a low terrace remnant just above the main D1 surface (early D1 according to Harvey (pers.
comm., 1998)), represents a minimum age for a late stage in this incision (OIS 3). The precise early Holocene minimum age (8.97$0.28 ka) for the calcrete from the D1 terrace (402) is most at variance with previous expectation (Harvey et al., 1995). (This locality is essentially identical to their RA2). As
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
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Fig. 6. Variation of initial U/U ratios with time.
4.3. Landform evolution and controls
Table 3 Comparison of age estimates for terrace features Terrace
Soil and landform development/ka
U/Th date/ka
D3 D2 D1 Early D1? C B A
10}20
* 8 9 31 88 (68-104) 145 224}'380
early Wurm (80 ?) '100 700}(1600
From Harvey et al. (1995) and Harvey pers. comm. (1998)
discussed earlier, the calcrete is thought to have formed in an active alluvial environment, with the date indicating correlation of a late stage in the aggradation with early OIS 1 or late OIS 2. After the initial dissection of the D1 terrace to near present river level, up to 35 m of mainly silts and sands were deposited in ponded water (D2) (Mather et al., 1991) before further cycles of erosion and aggradation (D3, E, F). The poorly de"ned date for 401, is from an early stage in the erosion of the D2 sediments, again indicating a Holocene age not statistically di!erent from that of 402 for D1, although the nodule morphology suggests a younger age.
The controls on drainage evolution identi"ed by previous work in the area (e.g. Mather and Harvey, 1995; Mather et al, 1995) include the extrinsic factors (relative to the drainage catchment) of tectonics, sea level and climate, and intrinsic factors such as river capture, landslide dams and gypsum diapirism. The general tectonic evolution of the area is known, with the present height of fossiliferous sediments related to the last marine incursion giving a mean uplift rate since the early Pliocene of 0.1 m/ka (Mather and Harvey, 1995). Fig. 7 shows schematically the terrace evolution for the last 100 ka for the Urra area, in terms of the approximate rates of degradation and aggradation, based on the minimum age estimates for terrace surfaces, assuming that the calcretes post-date terrace aggradation, and using the stratigraphic and topographic information given by Mather et al. (1991). This indicates a valley incision rate from terrace C to D1 of &0.4 m/ka. A similar rate is given with lesser precision for the B to C interval. The similarity of these incremental rates to the mean tectonic uplift rate quoted above, allowing for the uncertainties involved in the calculations, suggests that regional tectonism could have been the driving mechanism, although post-capture channel adjustment is also likely to have been a factor. The evolution of the
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Fig. 7. Schematic diagram of terrace evolution rates at Urra (terrace identi"cation from Harvey et al. (1995), and Harvey pers. comm. (1998).
neighbouring Tabernas basin to the west is also considered to have been controlled by tectonism, with calcrete formation occurring during periods of stability, to be terminated by renewed uplift (Nash and Smith, 2000). In contrast, the U/Th dates imply very rapid erosion and sedimentation rates in the early Holocene, of the order of &10 m/ka, made up principally of &41 m of post-D1 erosion, '35 m of D2 sedimentation and a similar amount of subsequent erosion (Mather et al., 1991). They interpreted the limnic/paludal facies of D2 and its syn- and post-depositional disturbance as due to gypsum diapirism and its brief local ponding of the river, following rapid erosion of the overlying Sorbas Formation marls. Our data are compatible with this interpretation, but whether these events were initiated solely by the intrinsic process of the upstream migration of the capture knick-point or whether they were all driven by a period of enhanced tectonic activity remains an open question. In addition, there is the possible role of the landslide/rockfall dam of gypsum debris, which still partly blocks the present gorge through the gypsum outcrop. This may well date from this time interval and could have been seismically generated. Comparable events, although not necessarily synchronous, have been described by
Mather et al. (1995) from the nearby Mula basin, with 36 m of erosion in the Holocene of paludal sediments initially deposited behind a seismically induced landslide. It is di$cult to recognise the operation of any climatic control on the evolution of the Rio Aguas, superimposed on the tectonic and intrinsic controls suggested above, unless it is expressed by the aggradation/degradation cycles. In general, in southeast Spain and the western Mediterranean, aggradation is considered to occur during the colder more arid glacial periods, when there is an increased coarse sediment availability due to frost action and reduced vegetation cover, due to the replacement of Mediterranean woodland of the warmer intervals by steppe scrub in the colder more arid ones (Harvey, 1984; PeH rez-Obiol and Julia, 1994; Pons and Reille, 1988; Van Andel and Tzedakis, 1996). Prentice et al. (1992) have suggested for the eastern Mediterranean that high runo! events capable of transporting coarse sediment are not inconsistent with steppe vegetation when rainfall is restricted to winters. The best dated evidence from the wider area comes from Mallorca, where alluvial deposition events have been dated by OSL to OIS 5d, 5b, 4, 2 (Rose and Meng, 1999), with which our poorer resolution dates for C and D1 aggradation are not inconsistent.
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
5. Conclusion The U/Th isochron method gives internally consistent dates for the calcretes of the terrace sequences in the Rio Aguas drainage system, southeast Spain. The results show that a number of approaches can be used to generate the isochron data, i.e. multiple samples or chemically de"ned fractions of a single sample. Serial dating of one calcrete provides a limited amount of evidence that calcrete formation has been episodic. Lithological evidence of this also exists. The dates, which range from 7 to 350 ka, provide minimum ages for periods of calcrete formation and, hence, minimum ages for the deposition of the host alluvium. Overall, they describe the Middle and Late Quaternary evolution of the drainage basin, during which over 150 m of erosional relief developed. A signi"cant river capture event, identi"ed by previous investigators, is dated to &100 ka. The implied slow rates of valley incision over much of this period (0.4 m/ka) is compatible with the mean uplift rate for the Tertiary and Quaternary. However, in the Holocene, high degradation and aggradation rates (&10 m/ka) in the middle reaches of the Rio Aguas could be due to one or more of a number of processes, most of which could be related to increased tectonic activity, i.e. migration of post-capture nick point, tectonic uplift, gypsum diapirism, landslides. In general, it is not possible to isolate a climatic signal from the chronology of terrace evolution, other than to point out that two major periods of terrace aggradation (D2 and C) occurred at the beginning and end of the Last Glaciation.
Acknowledgements We are grateful to K. Ludwig, USGS, for the provision of computer software for the evaluation of errors associated with isochron dating. We thank also P. Rowe, A. Harvey, A. Mather and another, anonymous, referee for helpful comments on an earlier version of the manuscript.
References Arakel, A., McConchie, D., 1982. Classi"cation and genesis of calcrete and gypsite lithofacies in paleodrainage systems of inland Australia and their relationship to carnotite mineralisation. Journal of Sedimentary Petrology 52, 1149}1170. Bischo!, J.L., Fitzpatrick, J.A., 1991. U-series dating of impure carbonates: An isochron technique using total sample dissolution. Geochimica Cosmochimica Acta 55, 543}554. Black, S., Macdonald, R., Kelly, M., 1997. Crustal origin for peralkaline rhyolites from Kenya: Evidence from U-series disequilibria and Th-isotopes. Journal of Petrology 38, 277}297. Broecker, W.S., 1963. A preliminary evaluation of uranium series inequilibrium as a tool for absolute age measurement on marine carbonates. Journal of Geophysical Research 68, 2817}2834.
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Dixon, J.C., 1994. Duricrusts. In: Abrahams, A.D., Parsons, A.P. (Eds.), Geomorphology of Desert Environments. Chapman & Hall, London, pp. 82}105. Gile, L.H., Petersen, F.F., Grossman, R.B., 1966. Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science 101, 347}360. Harvey, A.M., 1984. Aggradation and dissection sequences on Spanish alluvial fans: in#uence on morphological development. Catena 11, 289}304. Harvey, A.M., 1987. Patterns of Quaternary aggradational and dissectional landform development in the Almeria region, southeast Spain: a dry-region, tectonically active landscape. Die Erde 118, 193}215. Harvey, A.M., Wells, S.G., 1987. Response of Quaternary #uvial systems to di!erent epeirogenic uplift: Aguas and Feos river systems, southeast Spain. Geology 15, 689}693. Harvey, A.M. Miller, S.Y., Wells, S.G., 1995. Quaternary soil and river terrace sequences in the Aguas/Feos river systems, Southeast Spain. In: Lewin, J., Macklin, M.G., Woodward, J.C. (Eds.), Mediterranean Quaternary River Environments. Balkema: Rotterdam, pp. 263}281. Hillaire-Marcel, C., Carro, O., Causse, C., Goy, J.-L., Zazo, C., 1986. Th/U dating of Strombus bubonius - bearing marine terraces in southeastern Spain. Geology, 14, 613}616. Hillaire-Marcel, C., GarieH py, C., Ghaleb, B., Goy, J.-L., Zazo, C., Barcelo, J.C., 1996. U-series measurements in Tyrrhenian deposits from Mallorca - further evidence for two Last-Interglacial high sea levels in the Balearic islands. Quaternary Science Reviews 15, 53}62. Klappa, C.F., 1980. Rhizoliths in terrestrial carbonates: classi"cation, recognition, genesis and signi"cance. Sedimentology 27, 613}629. Khadkikar, A.S., Merh, S.S., Malik, J.N., Chamyal, L.S., 1998. Calcretes in semi-arid alluvial systems: Formative pathways and sinks. Sedimentary Geology 116, 251}260. Ku, T.-L., Liang, Z.C., 1984. The dating of impure carbonates with decay-series isotopes. Nuclear Instruments and Methods of Physical Research 223, 563}571. Ludwig, K.R., Titterington, D.M., 1994. Calculation of Th/U isochrons, ages, and errors. Geochimica Cosmochimica Acta 58, 5031}5042. Luo, S., Ku, T.-L., 1991. U-series isochron dating: a generalised method employing total sample dissolution. Geochimica Cosmochimica Acta 55, 555}564. Machette, M.N., 1985. Calcitic soils and calcretes of the southwestern United States. In: Weide, D.L., Faber, M.L. (Eds.), Soils and Quaternary Geology of the Southwestern United States. Geol. Soc. Amer. Special Paper, Vol. 203, pp. 1}21. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research 27, 1}29. Mather, A.E., Harvey, A.M., Brenchley, P.J., 1991. Halokinetic deformation of Quaternary river terraces in the Sorbas Basin, SouthEast Spain. Zeitschrift fuK r Geomorphologie N.F., Supl. Bd 82, 87}97. Mather, A.E., Harvey, A.M., 1995. Controls on drainage evolution in the Sorbas basin southeast Spain. In: Lewin, J., Macklin, M.G., Woodward, J.C. (Eds.), Mediterranean Quaternary River Environments. Balkema, Rotterdam, pp. 65}76. Mather, A.E., Silva, P.G., Goy, J.L., Harvey, A.M., Zazo, C., 1995. Tectonics versus climate: an example from late Quaternary aggradational and dissectional sequences on the Mula basin, southeast Spain. In: Lewin, J., Macklin, M.G., Woodward, J.C. (Eds.), Mediterranean Quaternary River Environments. Balkema, Rotterdam, pp. 77}87. Miller, S.Y., 1991. Soil chronosequences and #uvial landform development: studies in S.E. Spain and N.W. England. University of Liverpool, Ph.D. Thesis.
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Nash, D.J., Smith, R.F., 2000. Multiple calcrete pro"les in the Tabernas basin, southeast Spain: their origins and geomorphic implications. Earth Surface Processes and Landforms 23, 1009}1029. Osmond, J.K., Ivanovich, M., 1992. Uranium-Series mobilisation and surface hydrology. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium-Series Disequilibrium, 2nd edition. Clarenden, Oxford, pp. 259}289. PeH rez-Obiol, R., Julia, R., 1994. Climatic change on the Iberian Peninsula recorded in a 30,000-year pollen record from lake Banyoles. Quaternary Research 41, 91}97. Pons, A., Reille, M., 1988. The Holocene and upper Pleistocene pollen record from Padul (Granada Spain): a new study. Palaeogeography, Paleoclimatology, Paleoecology 66, 243}263. Prentice, I.C., Guiot, J., Harrison, S.P., 1992. Mediterranean vegetation, lake levels and palaeoclimate at the Last Glacial maximum. Nature 360, 658}660. Quade, J., 1986. Late Quaternary environmental changes in the upper Las Vegas Valley Nevada. Quaternary Research 26, 340}357. Radtke, U., BruK ckner, H., Mangini, A., Hausmann, R., 1988. Problems encountered with absolute dating (U-series. ESR) of Spanish calcretes. Quaternary Science Review 7, 439}445. Rose, J., Meng, X., 1999. River activity in small catchments over the last 140 ka, northeast Mallorca, Spain. In: Brown, A.G., Quine, T.A. (Eds.), Fluvial Processes and Environmental Change. Wiley, Chichester, pp. 91}102.
Stearns, C.E., Thurber, D.L., 1965. Th-230}U-234 dates of late Pleistocene marine fossils from the Mediterranean and Moroccan littorals. Quaternaria, VII, 29}42. Schwarcz, H.P., Latham, A.G., 1989. Dirty calcites 1. Uranium-series dating of contaminated calcite using leachates alone. Chemical Geology 80, 35}43. Semeniuk, V., Searle, D.J., 1985. Distribution of calcrete in Holocene coastal sands in relationship to climate, southwestern Australia. Journal of Sedimentary Petrology 55, 86}95. Van Andel, T.H., Tzedakis, P.C., 1996. Palaeolithic landscapes of Europe and environs, 150,000}25,000 years ago: an overview. Quaternary Science Reviews 15, 481}500. Weijermars, R., Roep, Th.B., Van den Eeckout, B., Postma, G., Kleverlaan, K., 1985. Uplift history of a Betic fold nappe inferred from Neogene- Quaternary sedimentation and tectonics (in the Sierra Alhamilla and Almeria, Sorbas and Tabernas Basins of the Betic Cordilleras SE Spain). Geologie en Mijnbouw 64, 397}411. Wenzens, E., Wenzens, G., 1995. The in#uence of Quaternary tectonics on river capture and drainage basin patterns in the Huercal-Overa basin, southeastern Spain. In: Lewin, J., Macklin, M.G., Woodward, J.C. (Eds.), Mediterranean Quaternary River Environments. Balkema, Rotterdam, pp. 77}87.
A calcrete-based U/Th chronology for landform evolution in the Sorbas basin, southeast Spain M. Kelly *, S. Black, J.S. Rowan Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ, UK Postgraduate Research Institute for Sedimentology, University of Reading, PO Box 227, Reading RG6 6AB, UK Department of Geography, University of Dundee, Dundee DD1 4HN, UK
Abstract Nodular and massive calcretes, from alluvial terraces in the Sorbas area of southeast Spain, have been dated by the U/Th isochron method to between 8 } '350 ka. Alternative approaches have been used to generate the multiple samples for the isochron method for several of the calcretes. Dates agreed closely when obtained by total dissolution of a number of nodules or by the sequential dissolution in di!erent acids of a single nodule. However, fractional dissolution of a single nodule in the same acid revealed two stages of precipitation of di!erent age. It is considered that the calcretes may not be strictly pedogenic in origin and that groundwater processes linked to the alluvial environment may have been involved also. Nevertheless, their dates provide minimum ages for a chronostratigraphic framework for terrace evolution. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The Betic Cordillera in southeast Spain is an area of basin and range topography which has been tectonically active from the Neogene to the present (Weijermars et al., 1985). The drainage systems of the area were initiated by the late Miocene - early Pliocene uplift above sea level of Tertiary marine sedimentary basins partially separated by elevated ranges of Palaeozoic rocks. The evolution of a number of these drainage systems has been interpreted from sequences of alluvial deposits and terraces formed during the subsequent erosion of the basin in"lls (Harvey, 1987; Harvey and Wells, 1987; Mather and Harvey, 1995; Mather et al., 1995; Wenzens and Wenzens, 1995). Hitherto, an absolute chronological framework has not been available for the landform evolution of these basins and the discussion of evolution rates and correlation with the climate-stratigraphic units of the Quaternary has necessarily been limited. The present investigation examines the extent to which absolute dating can be provided by U/Th dating of calcretes precipitated in alluvial sediments and related soils of a terrace sequence. Its principle aim is methodological but the implications for landform chronology
* Corresponding author. E-mail address: m. [email protected] (M. Kelly).
are considered. The term calcrete is used here in a broad sense (Dixon, 1994), for secondary carbonate accumulations of a variety of morphologies (nodular to massive) formed in a near surface terrestrial environment. The study area is the Rio Aguas drainage system in the Sorbas basin, Almeria, for which a detailed relative chronology has been established (Harvey and Wells, 1987; Mather et al., 1991; Miller, 1991; Harvey et al., 1995; Mather and Harvey, 1995). According to their work, the initial drainage was a subsequent system, which locally developed antecedent components as continuing tectonic uplift and erosion exposed the basement bedrock of the Alhamilla-Cabrera range (Fig. 1). This period of incision is represented by a narrow valley cut by the protoAguas/Feos river across the Alhamilla-Cabrera range and by an upper, descending series of alluvial terrace surfaces (A, B, C) in the Aguas-Feos catchment. Subsequently, aggressive headward erosion by an easterly #owing river from the neighbouring Vera basin captured the Rio Aguas near Los Molinos (point marked `Cola, Fig. 1). A lower descending terrace sequence (D1, D2, D3, E, F) in the upper Rio Aguas records further incision of the channel in response to post-capture base level adjustment and/or continuing tectonic uplift. Terrace deposits indicate that the erosional history of the catchment was interrupted by periods of aggradation, determined by a number of possible factors including climatic change (Mather and Harvey, 1995). The contrasting competence
0277-3791/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 5 0 - 5
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M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Fig. 1. General topography map and bedrock geology of the Sorbas area, southeast Spain. The inset map shows the sample localities.
of the di!erent lithological units of the basin in"ll has also in#uenced the basin geomorphology and its evolution; with limestones and gypsum being the more competent units and marls, sands and gravels the less competent (Weijermars et al., 1985). The stratigraphy of soil pro"les developed on the alluvial terraces, and the nature of the calcretes, have been described by Miller (1991) and Harvey et al. (1995). They derived estimates of terrace ages, based on the relative development of calcrete and rubi"ed soil horizons and on a C date (2310#80!90 BP) for an E terrace. A detailed description of calcrete morphology and stratigraphy has been given by Nash and Smith (1998) for the adjacent Tabernas basin, west of Sorbas. U/Th dates have been previously reported for calcretes from the adjacent coastal area of southeast Spain (Radtke et al., 1988). However, little contextual information is provided and the correction procedure used for detrital Th is only approximate. U/Th ages have also been reported for marine molluscs from Quaternary sediments in the coastal part of this area (Stearns and Thurber, 1965; Hillaire-Marcel et al., 1986).
2. Methods Samples were collected from exposures located on terraces conveniently chosen to cover a range of relative
ages, as de"ned by Harvey and Wells (1987), Mather et al. (1991) and Miller (1991) (Fig. 1). The exposure stratigraphy was logged and calcrete samples representative of the main morphological types were taken from fresh surfaces for analysis. In the event, only the most developed calcrete was analysed from each site. Sampling was restricted laterally to a few tens of cm and vertically to generally (10 cm (Fig. 2). Where possible, calcretes with evidence of obvious post-formational carbonate precipitation (in joints etc.) were avoided. Munsell soil colours were determined on moist samples for some sections. The analytical basis for dating calcretes is the measurement of Th ingrown from the decay of authigenic U and, indirectly, U, the uranium having been co-precipitated from solution with the carbonate. The calcrete, however, is an impure mixture of calcium carbonate and incorporated detrital minerals, resulting in the same radionuclides being present in both the authigenic and detrital (allogenic) fractions. In addition, a minor amount of allogenic thorium could be included by subsequent scavenging from solution. Our basic approach has been to use an isochron method to determine the authigenic radionuclide components and, from that, the age (Bischo! and Fitzpatrick, 1991). This method requires the assumption that the coeval samples used for an isochron are mixtures, in di!erent proportions, of detrital and authigenic carbonate end members. In addition, it is assumed that the
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
respective end members producing the mixtures are compositionally homogeneous in terms of the radionuclide ratios. The detrital component can include silicate, carbonate or other mineral phases, and radioactive equilibrium or disequilibrium may or may not exist in these mixtures, so long as the homogoneity criterion is met. The analyses, carried out by SB, used three di!erent methods to obtain the 3}6 coeval, carbonate/detrital mixtures (subsamples) required for each isochron age determination of a sample. These methods are summarised below: (i) In the standard `total dissolutiona method (sample code /T), the coeval subsamples were either separate nodules or subsamples of a massive carbonate. These were dried and surface material discarded prior to hand picking and crushing. Between 2.9}17 g of subsample were dissolved in a HCl}HNO }HF mixture, following the methods outlined in Black et al. (1997) and based on those of Bischo! and Fitzpatrick (1991) and Luo and Ku, (1991). (ii) In the `sequential dissolutiona method (sample code /S), three di!erent coeval mixtures were obtained by chemically fractionating a single sample. This was achieved by obtaining three solutions for analysis by successive leaching of a crushed sample in 0.1 M HCl and then 0.1 M HNO and, "nally, dissolving the residue in HCl}HNO }HF. The following sample mass range was obtained for each fraction: 0.1 M HCl leachates 0.9}3.6 g; 0.1 M HNO leachates 5.4}12.7 g; HF soluble residues 1.0}6.9 g. Before leaching, the sample was "red by heating to 6003C. Each fraction was leached and then dried and weighed prior to continuing with the next step. Additional aliquots of yield monitor were added after each successive leachate step. The absorption of the yield monitor by samples that were later discarded was checked for. Each fraction absorbed (0.02% of the yield monitor under the exact conditions used to obtain the isochrons by maintaining acidic solutions. This method owes something to the earlier Leachate}Residue (e.g. Ku and Liang, 1984) and Leachate}Leachate (Schwarcz and Latham, 1989) methods but di!ers in the isochron approach. The sequential dissolution method is more stringent in its requirements than total dissolution, being susceptible to fractionation of radioisotopes (i.e. of a given element) during preparation and element redistribution (e.g. Th scavenging by the residual phase during leaching). The latter was minimised by "ring and the use of the acid molarities indicated by Bischo! and Fitzpatrick, (1991). (iii) In the fractionated total dissolution method (sample code /F), 3 apparently coeval nodules were sequentially fractionated by slow dissolution in 0.1 M HNO into outer, middle and inner fractions. Care was taken to keep reaction rates slow, by periodically lifting the nodule from the solution and washing in
997
Milli-Q water. For the purposes of de"ning the fractions during dissolution of the sectioned nodules, the outer fraction was delimited by visible banding whilst the middle and inner fractions were based only on equal dissolution times. Fraction masses ranged as follows: outer 1.2}3.8 g; middle 2.2}3.8 g; inner 3.1}7.0 g. These (plus any washings) were then treated as separate samples using the total dissolution method. After dissolution, U and Th were separated by ion exchange chromatography techniques and electrodeposited onto stainless steel planchettes. The activity concentrations of Th, Th, U and U were determined by alpha spectrometry using solid state silicon surface barrier detectors. A standardised U } Th tracer was used as a yield monitor, with a correction being made for the presence of detrital Th and the ingrowth of Ra. Th/Th equilibrium was checked on unspiked samples for at least one total dissolution subsample for each sample and various leachates which were run simultaneously. The (Th/U) and (U/U) activity ratios of the authigenic fraction were calculated from the slope of a 3-D isochron "tted to the XYZ data (U/Th, Th/Th and U/Th) using the method of Minimum Likelyhood Estimation of Ludwig and Titterington (1994), in which the analytical data are weighted for analytical errors and error correlations. The apparent age was determined from these authigenic fraction activity ratios (Broecker, 1963). A number of sources of uncertainty contribute to the quoted uncertainty in the age determination. The analytical errors are one such source and, usefully, the Ludwig and Titterington method quanti"es the probability that the uncertainty in the age is due to these ascertained analytical errors alone, through calculation of the mean square of weighted deviates (MSWD). Consequently, a low probability (or MSWD'1) indicates that the uncertainty in the age is due to other sources of error, probably of geological origin, e.g. subsamples are not coeval or mixture end members are not homogenous, due to spatial irregularity in carbonate precipitation history and in detrital composition. Another cause could be unquanti"ed analytical errors. In addition, open system radionuclide uptake or loss by the calcrete could result in a bias in the determined age. The 95% con"dence interval quoted for the isochron ages takes into account the MSWD value.
3. Results 3.1. Stratigraphy and lithology The stratigraphy of the sample sites is shown in Fig. 2 with the sample numbers of analysed material. This comprised a variety of calcrete types, each one being the
Fig. 2. Stratigraphic logs for calcrete samples (terrace designation from Harvey et al. (1995) and Harvey pers. comm. (1998).
998 M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
most developed form in its pro"le (sample number in brackets): (i) individual nodules formed of irregular but more or less elongate aggregates up to 4 cm (401) or 6 cm (402), composed of rounded micrite subnodules of mm to &1 cm scale, with a moderate content of #oating detrital grains (20}30% HF residues), (ii) massive, more or less planar calcrete formed of coalesced, #attened, nodular micrite aggregates with component micrite sub-nodules up to 1 cm, as in (i) (608), (iii) massive, intergranular micrite, in a more or less clast supported coarse gravel, although the sedimentary fabric is probably distorted, with "ne gravel and sand clasts #oating in the cement (403, 603, 613), (iv) massive laminated micrite with a low content of #oating detrital grains, laminae (1 mm to '1 cm, typically with sinuous-wavy boundaries on a mm scale (506 (also present at 508 and 613 but not analysed)), (v) as (iv) but unlaminated (503), also brecciated, with secondary sparite and secondary porosity (in 508 the lamination post-dates the brecciation). These calcrete morphologies correspond to stages II (type i), III (types ii and iii) and IV (types v and vi) of the developmental schemes for pedogenic secondary carbonates of Gile et al. (1966) and Machette (1985). However, these schemes may not be rigorously applicable because of the modes of formation of the calcretes, as discussed below. 3.2. Analytical results The analytical data are given in Table 1 and the authigenic or detrital radionuclide activity ratios derived from the isochron and the isochron date with its 95% con"dence interval are given in Table 2, together with locality details. The isochron diagrams are shown in Figs. 3}5 as 2-D versions (X> & XZ plots) for illustration purposes only. The regression lines are derived by the MLE method and the calculated data uncertainties are shown by 1p error ellipses where space permits. As explained, the age was calculated from the authigenic fraction Th/U and U/U ratios obtained by a 3-D (X>Z) "t to the data. Only one sample did not produce an isochron (506), because of low variability in Th/Th ratios in this laminar calcrete. The U and Th mass concentrations in the calcretes (total dissolutions only), essentially the U and Th concentrations, are low but very variable with a mean of 1.1$1.0 ppm and 0.8$0.7 ppm (1p, n"42), respectively. The mean U/Th mass ratio of 2.3$2.6 indicates the signi"cant but also variable detrital silicate contribution to the calcrete, re#ecting the changes in sediment sources during catchment evolution as progressive incision into the basin bedrock sequences occurs. Unfortunately, the bedrock U and Th signatures are not known.
999
The initial U ratios (U/U) show a trend in time (Fig. 6), with generally higher ratios from older calcretes (conversely their present ratios are relatively uniform). This could be explained by a change in the relative importance of the mechanisms releasing U to solution from the source rocks, i.e. selective U leaching due to the recoil mechanism and non-selective U release by oxidative chemical weathering (Osmond and Ivanovich, 1992). Such a change could be climate related, from more arid to more humid, or simply re#ect the increasing extent of chemical weathering. In addition, it might be explained by a changing groundwater regime, with a lesser contribution from deeper reducing waters, perhaps as a consequence of the progressive terrain dissection. A similar trend of increasing (U/U) values with age has been seen in fossil marine shells from SE Spain and Mallorca, where it is explained either by U uptake on fossilisation from groundwaters which have U ratios evolving with time due to progressive weathering (Hillaire-Marcel et al., 1986) or by gradual open system replacement of U from groundwater with a high U/U ratio (Hillaire-Marcel et al., 1996). Isochron ages derived from di!erent analytical techniques on the same sample show very good agreement. That is, for each of the three youngest samples (401, 402 and 403), dates agree closely when produced by either analysis of 4 separate nodules (total dissolution) or of 3 chemical fractions of a single nodule or massive calcrete sample (sequential dissolution). For each sample, the most precise age estimate is given by combining the data from the two methods (Table 2 and Fig. 3) (for 401/S#T, the HCl acid leach data are excluded because of a relatively large analytical error). This shows the validity of the sequential dissolution approach, at least when isotope fractionation during analysis can be avoided (see Method section). Conversely, it indicates the uniformity in the apparent age of the material dated, over a spatial scale of (10 cm in the sediments. The probability and MSWD values, given by the error treatment of Ludwig and Titterington, (1994), indicate that only for dates based on 2 samples (401/S and 402/T) are the assigned analytical and correlation errors capable of accounting for the age uncertainty. For the remainder, the geological factors identi"ed earlier, i.e. subsamples di!er in age and/or in detrital radionuclide composition, are likely to be major contributors to the age uncertainty, with the potential for this increasing with apparent age.
4. Discussion 4.1. Interpretation of calcrete dates Calcretes are known to have a variety of origins, both pedogenic and non-pedogenic. Essentially, pedogenic
1000
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Table 1 Analytical data for calcretes, Sorbas, Spain Sample no.
SubSample
Th-230 (Bg/kg)
1p
Th-232 (Bg/kg)
1p
U-234 (Bg/kg)
1p
U-238 (Bg/kg)
1p
401/T
1 2 3 4
5.302 6.622 8.595 3.773
0.101 0.086 0.112 0.049
0.745 0.946 1.264 0.546
0.011 0.018 0.024 0.010
8.282 8.618 7.205 4.592
0.166 0.147 0.123 0.078
7.598 8.054 6.671 4.100
0.156 0.139 0.113 0.070
401/S
a b c
4.905 0.549 54.601
0.093 0.110 1.037
0.694 0.079 8.464
0.010 0.002 0.129
7.472 0.569 18.799
0.149 0.011 0.376
6.732 0.470 19.027
0.135 0.009 0.381
402/T
1 2 3 4
1.912 3.675 0.884 3.125
0.022 0.043 0.010 0.036
0.597 0.799 0.400 0.812
0.008 0.010 0.005 0.101
13.547 32.013 4.003 25.290
0.348 0.848 0.106 0.670
11.851 29.370 3.543 23.858
0.318 0.778 0.044 0.622
402/S
a b c
5.985 0.322 11.117
0.040 0.002 0.056
3.420 0.046 6.948
0.029 0.001 0.035
10.875 3.205 13.200
0.084 0.018 0.109
10.771 3.057 13.368
0.083 0.017 0.111
403/T
1 2 3 4
8.598 9.707 12.013 9.005
0.142 0.224 0.233 0.190
0.818 0.772 1.261 0.597
0.016 0.015 0.024 0.012
12.273 17.374 12.299 18.854
0.221 0.313 0.322 0.439
9.091 13.263 9.684 15.328
0.136 0.199 0.145 0.229
403/S
a b c
6.352 1.776 105.313
0.133 0.037 2.211
0.371 0.117 12.976
0.007 0.003 0.247
15.262 3.641 53.422
0.382 0.091 1.336
11.390 3.034 48.566
0.251 0.067 1.068
501/T
1 2 3 4
5.640 5.237 1.324 1.084
0.041 0.141 0.020 0.014
2.350 2.618 1.891 1.041
0.013 0.111 0.051 0.044
10.035 10.141 3.662 2.678
0.110 0.099 0.111 0.112
9.840 9.840 3.325 2.277
0.090 0.101 0.141 0.131
503/T
1 2 3 4
3.515 6.103 4.163 3.700
0.045 0.039 0.091 0.030
3.028 5.382 4.935 4.593
0.033 0.036 0.100 0.034
4.973 7.212 6.255 5.526
0.046 0.060 0.091 0.045
4.281 6.947 5.636 4.746
0.042 0.059 0.036 0.042
506/T
1 2 3 4
5.047 4.993 7.070 4.202
0.043 0.043 0.115 0.045
2.561 2.970 4.228 2.431
0.029 0.032 0.084 0.033
7.693 8.514 9.778 9.131
0.097 0.102 0.123 0.100
8.060 8.062 9.085 8.019
0.099 0.099 0.117 0.092
508/T
1 2 3 4
5.920 7.766 9.315 10.020
0.062 0.069 0.142 0.079
2.189 2.303 1.085 1.443
0.074 0.034 0.042 0.026
7.156 8.578 10.373 11.744
0.061 0.074 0.086 0.082
6.914 7.962 10.252 10.584
0.060 0.070 0.086 0.076
603/T
1 2 3 4 5 6
7.506 11.394 13.701 31.133 7.275 27.852
0.094 0.101 0.099 0.107 0.096 0.097
2.244 3.076 3.111 6.142 1.197 6.143
0.071 0.091 0.088 0.101 0.093 0.090
4.374 5.829 12.618 30.986 8.512 25.880
0.091 0.091 0.111 0.096 0.049 0.087
3.538 4.894 10.720 25.843 7.281 22.563
0.092 0.081 0.091 0.093 0.059 0.079
607/R (silt)
1 2
11.599 20.142
0.311 0.669
28.221 31.142
0.616 0.871
12.897 31.952
0.443 0.614
13.001 32.738
0.515 0.717
608/T
1 2 3 4
5.525 7.768 6.309 13.365
0.094 0.083 0.091 0.099
6.514 7.114 4.073 10.011
0.101 0.086 0.088 0.096
8.173 11.062 9.551 20.493
0.099 0.102 0.097 0.111
6.759 9.628 8.156 17.728
0.093 0.099 0.099 0.090
Mass leached (%)
64 19 17
71 24 37
51 20 29
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
1001
Table 1 Continued Sample no.
SubSample
Th-230 (Bg/kg)
1p
608/F (outer)
1 2 3
3.510 2.866 3.990
0.066 0.039 0.047
608/F (middle)
1 2 3
11.773 11.107 7.319
608/F (inner)
1 2 3 4
608/R
613/T
Th-232 (Bg/kg)
1p
U-234 (Bg/kg)
1p
U-238 (Bg/kg)
1p
Mass leached (%)
4.077 5.164 6.116
0.091 0.089 0.102
12.300 5.211 12.917
0.097 0.093 0.107
10.603 4.571 11.637
0.095 0.081 0.098
10 8 3
0.099 0.097 0.061
7.118 8.119 11.141
0.081 0.092 0.107
18.129 15.053 8.389
0.111 0.101 0.091
15.234 13.210 7.109
0.107 0.117 0.090
31 23 36
10.524 11.756 13.494 11.957
0.099 0.103 0.121 0.109
6.017 8.141 12.146 13.111
0.071 0.091 0.107 0.113
14.849 17.104 18.887 15.589
0.133 0.117 0.201 0.188
12.478 14.873 16.863 13.796
0.116 0.109 0.199 0.101
59 69 61 63
1 2
11.790 16.909
0.641 0.717
24.461 31.666
0.874 0.994
12.451 24.414
0.346 0.894
13.093 24.736
0.361 0.911
1 2 3 4
48.570 43.638 53.599 55.325
0.103 0.171 0.141 0.099
6.716 7.232 10.117 12.114
0.096 0.099 0.111 0.099
59.806 50.869 56.321 49.146
0.117 0.104 0.105 0.088
50.131 42.041 48.096 42.477
0.106 0.099 0.106 0.089
/T: total dissolution; /S: sequential dissolution; /F: fractionation and total dissolution; /R: residual fraction. HNO3; b: HCl; c: HF
carbonate precipitation occurs in soils of arid or semi-arid climates due to leaching of calcium carbonate from sur"cial soils by downward percolating rain and its precipitation in the vadose zone (e.g. Gile et al., 1966; Machette, 1985). The source carbonate may be alluvial clasts or, as is more usually considered to be the case, aeolian dust. A relevant purely non-pedogenic category is groundwater calcretes; formed by precipitation within the phreatic zone of calcium carbonate introduced by groundwater #ow, as a result of evaporation or CO degassing (e.g. Arakel and McConchie, 1982). However, in a semi-arid alluvial environment, a continuum of processes is likely to occur, which includes both the pedogenic and non-pedogenic categories referred to above, together with intermediaries (Khadkikar et al., 1998). Examples of the last would be where a river contributes calcium carbonate, for pedogenic calcrete formation, by #oodwater in"ltration or through recharge of a perched water table supplying soil moisture from a capillary fringe (e.g. Arakel and McConchie, 1982; Semeniuk and Searle, 1985; Quade, 1986). Of the calcretes analysed, 2 are from sites (402, 608) with populations of nodules which resemble rhizoliths precipitated around roots by a mechanism that is still uncertain (Klappa, 1980), i.e. consistent with a pedogenic origin. These two calcretes and 401 are also from pro"les with an upper rubi"ed zone typical of semi-arid soil pro"les. The origins of the other calcretes associated with gravels are less certain, despite most having a rubi"ed upper zone (603, 613, 608, 501, 508), since this could post-date calcrete formation
and even intervening erosion. However, both sets of calcretes can be "tted into the mixed origin model of Khadkikar et al. (1998) for alluvial calcretes, particularly taking into account the importance of carbonate bearing bedrocks in the Rio Aguas catchment as a source of dissolved and detrital calcium carbonate (Weijermars et al., 1985). Both groundwater and pedogenic calcretes have been identi"ed in the neighbouring Tabernas basin (Nash and Smith, 2000), in some cases in the same pro"le. One context for groundwater calcrete formation found there, above impermeable bedrock, applies also to the calcrete at site L9 (sample 503). It is inherent in models of pedogenic calcrete formation that the quantity and morphology of the secondary carbonate evolve with time, as indicated by the morphological / developmental stage schemes of Gile et al. (1966) and Machette (1985) for calcretes from the southwestern United States of America, where the evolutionary lifespan of a calcrete can be '1 Ma. However, because of climate change, or changes in local conditions (tectonism, erosion, sedimentation), the rate of calcrete precipitation is unlikely to have been constant or even continuous (Gile et al., 1966; Machette, 1985). Non-pedogenic calcretes are potentially more rapidly formed (Khadkikar et al., 1998). The association with an active river system in an erosional environment also raises the possibility that the main periods of calcrete formation might be narrowly restricted in time, being inhibited or halted by incision of the river below the point at which lateral groundwater recharge and #ooding occur, i.e.
L7 Gochar B L8 Sorbas A L9 Cerro Molatas A
0.952 1.076 0.872
1.101 1.223 1.136 1.073 1.050 1.060 1.266 1.295 1.331 1.162 1.226 1.237 1.116 1.217 1.166 1.144 1.165
Authigenic U-234/U-238
0.049 0.097 0.51
0.056 0.161 0.021 0.036 0.030 0.023 0.079 0.122 0.036 0.043 0.515 0.029 0.091 0.105 0.142 0.205 0.038
1p
0.694 0.956 1.230
0.065 0.085 0.080 0.085 0.084 0.084 0.316 0.326 0.327 0.545 !0.018 0.658 0.668 0.765 0.679 0.141 0.673 0.028 0.100 0.555
0.044 0.139 0.023 0.003 0.003 0.001 0.042 0.052 0.020 0.098 0.334 0.045 0.065 0.072 0.101 0.095 0.026
Authigenic Th-230/U-238 1p
Note: Terrace designation from Harvey et al. (1995) and Harvey pers. comm.(1998). T: total dissolution, /S: sequential dissolution, /F: fractionated (O-outer, M*middle, I*inner); excludes 401/Sc.
D2 D2 D2 D1 D1 D1 Early D1? Early D1? Early D1? C C C C C C C C
Massive Massive Massive
Urra Urra Urra Urra Urra Urra Urra Urra Urra Urra Urra Urra `Cola `Cola `Cola `Cola `Cola
L1 L1 L1 L2 L2 L2 L3 L3 L3 L5 L5 L4 L6 L6 L6 L6 L6
Nodule Nodule Nodule Nodule Nodule Nodule Massive Massive Massive Massive Laminar Massive Massive Mass-nod. Mass-nod. Mass-nod. Mass-nod.
Terrace/ deposit
401/T 401/S 401/S#T 402/T 402/S 402/S#T 403/T 403/S 403/S#T 603/T 506/T 613/T 608/T 608/F(I) 608/F(M) 608/F(O) 608/T#F (1#M) 501/T 508/T 503/T
Locality
Material
Sample no.
Table 2 Sample data and isochron dates
1.443 2.028 *
1.122 1.251 1.162 1.101 1.077 1.087 1.381 1.415 1.450 1.408 * 1.552 1.472 1.629 1.513 1.190 1.507
Intial U-234/U-238
0.074 0.183 *
0.057 0.165 0.021 0.037 0.031 0.024 0.086 0.133 0.039 0.052 * 0.036 0.120 0.141 0.184 0.213 0.049
1p
1.050 20.600 38
0.506 4.050 0.337 0.513 2.780 1.970 1.770 2.520 2.060 8.100 29.9 12.400 3.460 6.830 17.200 14.800 14.800
MSWD
0.38 0 0
0.73 0.017 0.95 0.73 0.062 0.032 0.13 0.08 0.02 0 0 0 0.006 0 0 0 0
Probability of "t
147 224 '350
6.6 8 7.9 9.02 9.01 8.97 30.9 31.2 30.3 68 * 80 98 103 92 14 91
Age (ka)
25 160 *
9.2 27 4.7 0.71 0.7 0.28 10 12 4.4 32 * 19 30 33 41 20 11
95% C.I. #/! [ka]
1002 M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
1003
Fig. 3. Calcrete isochron plots (2-D versions) based on both total and sequential dissolution of nodules for each calcrete, with 1 p error ellipses (dates are based on a 3-D isochron).
precipitation events could be short relative to the calcrete age. However, subsequent phases of carbonate precipitation might occur by purely pedogenic processes, governed by the climatic regime.
A polygenetic history for some nodules is indicated by the analytical data for 608, which give apparently uniform U/Th ages for the inner and middle nodule fractions (608/F(I): 103$33 ka and 608/F(M): 92$41 ka) and
1004
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
Fig. 4. Calcrete isochron plots (2-D versions) based on both total and fractional dissolution of individual nodules; for the outer (O) and combined inner and middle fraction (I and M) plus total (dates are based on a 3-D isochron).
a much younger age for the outer (608/F(O): 14$20 ka) (Fig. 4). Because of the complex geometry of the small nodules, the fractions were de"ned only by dissolution time. The methodologically de"ned outer fraction corresponded to a mean of 7% of nodule mass. However, it is likely that the young phase of calcrete precipitate does form only a small percentage of the mass since total dissolution of nodules gave an age similar to the inner and middle fractions (608/T: 98$30 ka). The most precise age for 608 was given by combining the total, inner and middle dissolution data (91$11 ka). This is compatible with the above evolutionary model, with the inclusion of a second phase of calcrete precipitation. An alternative explanation would be post-formational U uptake by the outer fraction. However, thin section examination of 608 shows the presence of secondary sparite cement which could be responsible for the younger age, although it probably forms a smaller proportion than 7% of the mass. The very precise age of nodules from another site (402/S#T: 8.97$0.28 ka), obtained by two contrasting analytical methods, is also
indicative of a very uniform composition and, hence, one main period of precipitation which was short relative to the age. In theory, therefore, it is usually necessary to regard the pure authigenic component of a calcrete (the isochron end member) as having a time averaged composition, which integrates the di!erent time periods and rates of carbonate accumulation. Consequently, the U/Th date gives an apparent age which is the minimum for commencement of signi"cant carbonate precipitation and, in the present context, for the end of alluviation at a site. It is also a maximum age estimate for the latest signi"cant carbonate precipitation. In addition, because of the possible smallscale morphological complexity of the precipitate, subsamples are unlikely to be homogeneous with respect to this average authigenic end member, i.e. not coeval. 4.2. Terrace chronology There is a general relationship between the calcrete dates and the relative geomorphic ages of the associated
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
terraces indicated by their height relative to the local channel (Harvey and Wells, 1987; Harvey et al., 1995). However, the U/Th chronology is generally younger than the ages inferred by Harvey et al. (1995) from soil development, including the stage of calcrete formation (Tables 2 and 3). The age of the A terrace is very uncertain, with a "nite age estimate of 224$160 ka (508/T) and another beyond the limit of the U/Th method ('350 ka) (503/T). The former is from an obviously polygenetic calcrete and the age of initial calcrete formation will be in the upper part of the range or non-"nite. The measured non-"nite age is for a thin deposit of calcrete in a gravel matrix above limestone bedrock. Terrace B, with a minimum age estimate of 147$25 ka (501/T), can be broadly correlated with Oxygen Isotope Stage 5e-6 (Martinson et al., 1987). A more precise minimum age estimate is possible for the C terrace, which has a particular signi"cance in the evolution of the Rio Aguas drainage since it is considered to closely predate capture by what is now the easterly #owing lower Rio Aguas. Six isochron dates from two areas with di!erent calcrete morphologies relate to this
1005
feature (Table 2). The best estimate for the minimum age for the event should be the date of 91$11 ka from the capture site (`Cola, Fig. 1) (608/T#F(I)#F(M)). This date is for a massive layer of coalesced nodules, although a laminar layer is also present locally above this, corresponding to stage III/IV. (This locality is comparable to RA4 of Harvey et al. (1995)). Younger dates, but within the 95% C I of the `Cola date, come from the Urra area (68$34 (603/T) and 80$17 ka (613/T)), from massive intergranular calcretes below well developed laminar layers (stage IV), giving an overall mean from both sites of 88$28 ka. (The younger age is from a locality comparable to RA3 of Harvey et al. (1995)). Whilst the `Cola calcrete is considered to be pedogenic those at Urra are less obviously so, with the intergranular calcrete there resembling vadose cement but with the laminar calcrete indicating retention of vertically draining surface water. However, in the broad context of an alluvial environment the implications of the dates are the same, i.e. they give a minimum age for calcrete formation and cessation of aggradation which suggests that these events occurred early in the last glaciation (OIS 5a}d).
Fig. 5. Calcrete isochron plots (2-D versions) based on total dissolutions alone (dates are based on a 3-D isochron).
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Fig. 5. Continued.
The post-C terrace history of the Rio Aguas was apparently predominantly erosional, with &25 m of downcutting (at Urra) before the D1 terrace aggradation. The date of 30.3$4.4 ka (403/S#T), from a calcrete cemented gravel on a low terrace remnant just above the main D1 surface (early D1 according to Harvey (pers.
comm., 1998)), represents a minimum age for a late stage in this incision (OIS 3). The precise early Holocene minimum age (8.97$0.28 ka) for the calcrete from the D1 terrace (402) is most at variance with previous expectation (Harvey et al., 1995). (This locality is essentially identical to their RA2). As
M. Kelly et al. / Quaternary Science Reviews 19 (2000) 995}1010
1007
Fig. 6. Variation of initial U/U ratios with time.
4.3. Landform evolution and controls
Table 3 Comparison of age estimates for terrace features Terrace
Soil and landform development/ka
U/Th date/ka
D3 D2 D1 Early D1? C B A
10}20
* 8 9 31 88 (68-104) 145 224}'380
early Wurm (80 ?) '100 700}(1600
From Harvey et al. (1995) and Harvey pers. comm. (1998)
discussed earlier, the calcrete is thought to have formed in an active alluvial environment, with the date indicating correlation of a late stage in the aggradation with early OIS 1 or late OIS 2. After the initial dissection of the D1 terrace to near present river level, up to 35 m of mainly silts and sands were deposited in ponded water (D2) (Mather et al., 1991) before further cycles of erosion and aggradation (D3, E, F). The poorly de"ned date for 401, is from an early stage in the erosion of the D2 sediments, again indicating a Holocene age not statistically di!erent from that of 402 for D1, although the nodule morphology suggests a younger age.
The controls on drainage evolution identi"ed by previous work in the area (e.g. Mather and Harvey, 1995; Mather et al, 1995) include the extrinsic factors (relative to the drainage catchment) of tectonics, sea level and climate, and intrinsic factors such as river capture, landslide dams and gypsum diapirism. The general tectonic evolution of the area is known, with the present height of fossiliferous sediments related to the last marine incursion giving a mean uplift rate since the early Pliocene of 0.1 m/ka (Mather and Harvey, 1995). Fig. 7 shows schematically the terrace evolution for the last 100 ka for the Urra area, in terms of the approximate rates of degradation and aggradation, based on the minimum age estimates for terrace surfaces, assuming that the calcretes post-date terrace aggradation, and using the stratigraphic and topographic information given by Mather et al. (1991). This indicates a valley incision rate from terrace C to D1 of &0.4 m/ka. A similar rate is given with lesser precision for the B to C interval. The similarity of these incremental rates to the mean tectonic uplift rate quoted above, allowing for the uncertainties involved in the calculations, suggests that regional tectonism could have been the driving mechanism, although post-capture channel adjustment is also likely to have been a factor. The evolution of the
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Fig. 7. Schematic diagram of terrace evolution rates at Urra (terrace identi"cation from Harvey et al. (1995), and Harvey pers. comm. (1998).
neighbouring Tabernas basin to the west is also considered to have been controlled by tectonism, with calcrete formation occurring during periods of stability, to be terminated by renewed uplift (Nash and Smith, 2000). In contrast, the U/Th dates imply very rapid erosion and sedimentation rates in the early Holocene, of the order of &10 m/ka, made up principally of &41 m of post-D1 erosion, '35 m of D2 sedimentation and a similar amount of subsequent erosion (Mather et al., 1991). They interpreted the limnic/paludal facies of D2 and its syn- and post-depositional disturbance as due to gypsum diapirism and its brief local ponding of the river, following rapid erosion of the overlying Sorbas Formation marls. Our data are compatible with this interpretation, but whether these events were initiated solely by the intrinsic process of the upstream migration of the capture knick-point or whether they were all driven by a period of enhanced tectonic activity remains an open question. In addition, there is the possible role of the landslide/rockfall dam of gypsum debris, which still partly blocks the present gorge through the gypsum outcrop. This may well date from this time interval and could have been seismically generated. Comparable events, although not necessarily synchronous, have been described by
Mather et al. (1995) from the nearby Mula basin, with 36 m of erosion in the Holocene of paludal sediments initially deposited behind a seismically induced landslide. It is di$cult to recognise the operation of any climatic control on the evolution of the Rio Aguas, superimposed on the tectonic and intrinsic controls suggested above, unless it is expressed by the aggradation/degradation cycles. In general, in southeast Spain and the western Mediterranean, aggradation is considered to occur during the colder more arid glacial periods, when there is an increased coarse sediment availability due to frost action and reduced vegetation cover, due to the replacement of Mediterranean woodland of the warmer intervals by steppe scrub in the colder more arid ones (Harvey, 1984; PeH rez-Obiol and Julia, 1994; Pons and Reille, 1988; Van Andel and Tzedakis, 1996). Prentice et al. (1992) have suggested for the eastern Mediterranean that high runo! events capable of transporting coarse sediment are not inconsistent with steppe vegetation when rainfall is restricted to winters. The best dated evidence from the wider area comes from Mallorca, where alluvial deposition events have been dated by OSL to OIS 5d, 5b, 4, 2 (Rose and Meng, 1999), with which our poorer resolution dates for C and D1 aggradation are not inconsistent.
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5. Conclusion The U/Th isochron method gives internally consistent dates for the calcretes of the terrace sequences in the Rio Aguas drainage system, southeast Spain. The results show that a number of approaches can be used to generate the isochron data, i.e. multiple samples or chemically de"ned fractions of a single sample. Serial dating of one calcrete provides a limited amount of evidence that calcrete formation has been episodic. Lithological evidence of this also exists. The dates, which range from 7 to 350 ka, provide minimum ages for periods of calcrete formation and, hence, minimum ages for the deposition of the host alluvium. Overall, they describe the Middle and Late Quaternary evolution of the drainage basin, during which over 150 m of erosional relief developed. A signi"cant river capture event, identi"ed by previous investigators, is dated to &100 ka. The implied slow rates of valley incision over much of this period (0.4 m/ka) is compatible with the mean uplift rate for the Tertiary and Quaternary. However, in the Holocene, high degradation and aggradation rates (&10 m/ka) in the middle reaches of the Rio Aguas could be due to one or more of a number of processes, most of which could be related to increased tectonic activity, i.e. migration of post-capture nick point, tectonic uplift, gypsum diapirism, landslides. In general, it is not possible to isolate a climatic signal from the chronology of terrace evolution, other than to point out that two major periods of terrace aggradation (D2 and C) occurred at the beginning and end of the Last Glaciation.
Acknowledgements We are grateful to K. Ludwig, USGS, for the provision of computer software for the evaluation of errors associated with isochron dating. We thank also P. Rowe, A. Harvey, A. Mather and another, anonymous, referee for helpful comments on an earlier version of the manuscript.
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