Th230-U234 Dating of Pedogenic Carbonates in Gravelly Desert Soils of Vidal Valley, Southeastern California

Th230-U234 dating of pedogenic carbonates in gravelly desert soils of Vidal Valley, southeastern California

TEH-LUNG KU Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 WILLIAM B. BULL Department of Geosciences, University of Arizona, Tucson, Arizona 85721 S. THOMAS FREEMAN Woodward-Clyde Consultants, 4000 W. Chapman Avenue, Orange, California 92668 KEVIN G. KNAUSS Lawrence Livermore Laboratory, University of California, Livermore, California 94550

ABSTRACT serve as a basis for further research into the a time span of about 350,000 yr, which is absolute dating of various types of impure considerably longer than the applicable Radioactive disequilibrium relationships carbonates of late Quaternary origin. range of C14. l1; among Th230, U234, and U238 can be used to date pedogenic carbonates formed in re- INTRODUCTION GEOMORPHIC SETTING gions of arid to semiarid climate. Samples suitable for dating consist of dense carbon- The time required to accumulate the The study area is located on the arid ate rinds around pebbles from the Cca soil pedogenic calcium carbonate in the soils of piedmont of Vidal Valley south of the horizon. Analytically, the method involves arid and semiarid regions has been sought Whipple Mountains in southeastern leaching the samples with dilute hydro- by several workers in order to estimate the California (Fig. 1). Mean annual precipita- chloric acid and measuring U238, U234, ages of geomorphic surfaces. Gile (1975) in tion of the area is 90 to 100 mm, two-thirds Th230, and Th232 in both the leachate and his work with the soils of the Rio Grande of which occurs during the winter. Gently residue fractions. As the soil carbonate River Valley determined the approximate sloping geomorphic surfaces have been dis- commonly incorporates silicate mineral- times necessary to develop morphologic sected by desert washes that drain into the bearing detritus, corrections are made to stages (Gile and others, 1966) of pedogenic Colorado River. An alluvial geomorphic account for possible introduction of detrital calcium carbonate development in southern surface is a mappable landform, formed Th230 and LP34 into the acid leachate. The New Mexico. He accomplished this with during a given time span and having dis- corrections are based on the assumptions some of the more common methods of dat- tinctive topographic, pedologic, and strat- that (1) the carbonate initially contains ing. Unfortunately, it is generally difficult to igraphic-sedimentologic characteristics. 232 230 negligible amounts of Th and Th , or find material such as charcoal, fossils, or Climatic changes during the Quaternary 230 232 has a Th /Th ratio similar to that in igneous rocks to date Quaternary deposits have resulted in two distinctly different 238 234 230 detrital minerals, (2) U , U , and Th in and soils in deserts. Radiometric dating of modes of operation of the fluvial systems of the detrital silicate phase are in secular pedogenic calcium carbonate using radio- the piedmont streams — the arid and equilibrium with each other, and (3) the carbon" methods has been attempted (for semiarid modes. The semiarid mode of the thorium isotopes in the detrital phase are example, Williams and Polach, 1971), but latest Pleistocene was followed by the much not fractionated by the acid leaching. Ap- with only marginal success. dryer and/or warmer mode of the Holocene. plication of the method to calcareous soils We present here the results of an investi- Dramatic contrasts between the two developed on upper Quaternary alluvial gation using the disequilibrium relationship climate-controlled modes are reflected in deposits of the eastern Mojave Desert in between Th230, U234, and U238 to assess ages the vegetation associations that grew on the southern California gives ages that are in- of the pedogenic carbonates deposited in hillslopes (Van Devender, 1977), the size ternally consistent and that agree with the gravelly desert soils of southeastern and stratigraphy of sediment derived from geomorphic and stratigraphic relative age California. We show that with judicious the hillslopes, and the thicknesses and dom- relationships. Fourteen samples from an selection of samples, this Th230-U234 method inant processes of soil-profile formation on upper Pleistocene geomorphic surface, gives highly consistent results. The method the alluvial geomorphic surfaces (Bull, Q2b, yielded an average age of 83,000 ± differs markedly from the C'4 technique in 1974). During the latest Pleistocene, soil 10,000 yr. This age is confirmed by a differ- principles and assumptions. Therefore, it carbonate accumulated beneath an argillic ent assessment of the data independent of can complement the C14 methodology and horizon and at depths of 0.3 to 0.5 m. Dur- the aforementioned assumptions. On one at the same time circumvent problems such ing the dryer mode of the Holocene, soil- specimen two different layers of a pebble as source(s) of carbon in C14 dating of profile development has been minimal and coating were dated, and a carbonate ac- pedogenic carbonate. Furthermore, owing carbonate accumulation has been restricted cumulation rate of about 1 mm/8,000 yr to the long half-life of Th230 (75,000 yr), to the upper 20 cm of most geomorphic sur- was obtained. It is hoped that this study will dating using this nuclide potentially covers faces.

Geological Society of America Bulletin, Part I, v. 90, p. 1063-1073, 6 figs., 7 tables, November 1979, Doc. no. 91108.

1063

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-N- COLORADO \ \ \ ® 1 •ARSTOW ®

10 15 20 25 30 SCALE IN KILOMETERS Figure 1. Location map of Vidal study area.

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The Colorado River also went through subsequent to its formation, the carbonate nosilicate minerals. Because these minerals repeated periods of downcutting and acts as a closed system with respect to the are always much older than the carbonate backfilling that changed the base levels of uranium and thorium isotopes. With these matrix, elimination or correction for this its tributary fluvial systems. The changes in assumptions, the age (t) of the carbonate "detrital contamination" is necessary. river levels, together with the climate- can be derived from the relationship. Complete separation of the carbonate ma-

controlled changes of the piedmont stream 230 238 trix from the silicate component through Th = U [1 - exp(-\„f)l, (1) systems, resulted in the formation of seven mechanical means has proven to be diffi- distinct geomorphic surfaces on the Vidal where k„ is the decay constant of Th230 cult. We subjected one sample to the follow- piedmont. The geomorphic surfaces of (9.22 X 10~6 yr"1), and notations of nu- ing mechanical separation steps: (a) pul- Table 1 can be recognized in many other clides refer to their activities. verized in an agate mortar, (b) passed areas along the Colorado River in Arizona Because in natural solutions U2 : often through a 125-/am sieve and then a magnet- and California (W. B. Bull, in prep.). found to be in excess of U238 (that is, activ- ic separator, (c) hand-picked free of visible Several of the geomorphic surfaces of the ity ratio U234/U238 > 1.00), the decay of this noncalcareous grains, (d) heated in a fur- 234 Vidal area have widespread (extending for excess U requires that equation 1 be nace to convert CaC03 to CaO, and (e) 3 to 5 km) varnished desert pavements un- modified to CaO concentrated by heavy-liquid density derlain by well-developed argillic and calcic separation. Although these treatments in- 230 238 soil horizons. These features indicate that Th = U [1 - exp(-Xof)] creased the CaC03 content of the sample + 234 _ U238 _ the undissected parts of these geomorphic (U )[Xo/(Xo À4)] from 45% to 88%, direct application of [1 - exp(\4i - V)], (2) surfaces are pre-Holocene in age and have equation 2 to the whole sample was still not 234 warranted. We have, therefore, adopted a undergone little erosion since their forma- where X4 is the U decay constant (2.79 x tion. The Th230-U234 dating technique was 10"6 yr-1). The modification necessitates combined chemical leaching and correction used mainly on the carbonate in soil profiles measurement of U234 in addition to U238 procedure to cope with the detrital con- of the Q2b geomorphic surface of late Pleis- and Th230 in the sample. The unknown t in tamination problem. The procedure is de- tocene age. Younger and older units also equation 2 can be computed iteratively or scribed later. were dated. through a graphic approach such as the one 2. Validity of the assumptions stated used by Kaufman and Broecker (1965). above. The first assumption is consistent Th230-U234 METHOD with current knowledge concerning the so- Application to Pedogenic Carbonate lution chemistry of uranium and thorium. The Th230-U234 method is a modification In natural solutions, uranium is rather sol- of the Th230-U238 geochronometer first In applying the Th230-U234 method to uble because of the formation of stable ura- 4 applied to coralline material by Potratz and pedogenic carbonate, two important con- nylanionic complexes such as U02(C03)3~ others (1955) and Barnes and others siderations must be made. (McKelvey and others, 1955; Starik and (1956). These workers assumed that (1) 1. Purity of the carbonate analyzed. Kolyadin, 1957), whereas the thorium carbonate when formed incorporates in it a Pedogenic carbonates are seldom pure; they isotopes are highly insoluble, owing to easy few parts per million of uranium, but neg- contain various amounts of detrital mate- hydrolysis of the Th+4 ions. Measurements ligible amounts of thorium isotopes, and (2) rial, most of which consists of alumi- have shown that in the majority of cases,

TABLE 1. CLASSIFICATION OF QUATERNARY GEOMORPHIC SURFACES OF VIDAL REGION

Geomorphic Remarks surface

Holocene Q4b Unvarnished bar-and-swale surfaces and stream channels. Q4b are active washes; Q4a are surfaces that are now flooded so Q4a infrequently that riparian trees, such as ironwood, are not present or are dead

Q3 Varnished bar-and-swale surfaces that may have A and Cca soil horizons but lack B horizons, and thus do not have evidence of having passed through long period of climate substantially wetter than present climate

Pleistocene Q2c Desert pavements underlain by argillic and calcic soil horizons. Q2b is product of both arid and semiarid climates, and Q2c is product Q2b of latest Pleistocene climate that was wetter and/or cooler than Holocene climate, which has only slightly modified soil formed under prior semiarid climate

Period of net erosion of alluvium

Q2a Highly dissected ridges or terraces with remnants of desert pavement that are underlain by argillic soil horizons

Period of net erosion of alluvium

Maximum age of Grand Canyon basalt flows (McKee and others, 1968) that are represented in gravel of younger terraces of Colorado River, but not Ql river terraces, is 1.2 m.y.

Qi Post—Bouse Formation (Metzger, 1968) alluvium that is so old and dissected that planar surfaces, pavements, and remnants of original soil horizons are no longer present on most piedmonts

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Th230/U234 activity ratios in continental waters are less than 0.1 (Cherdyntsev, 1971). The assumption will probably be strained for samples younger than about 10,000 yr. Whether pedogenic carbonates act as a closed system to U and Th is more difficult to evaluate. A direct confirmation would be to check the Th230/U234 ages with those obtained from other radiometric dating methods. Such methods were not available in the Vidal area; therefore, evaluations had to be made on the internal consistency of the data as well as their conformity with the relative ages of geomorphic surfaces. Coupled to these empirical arguments is the requirement of proper selection of samples based on their compactness, morphology of accumulation, locale of occurrence within the soil profile, and so forth, to be discussed below.

DEVELOPMENT OF PEDOGENIC CALCIUM CARBONATE — ITS APPROACH TO A CLOSED SYSTEM 10 15 20 The processes of leaching (eluviation) Figure 2. Type of pebble coating used for dating (from surface V). Densest and purest and precipitation (illuviation) determine the carbonate is closest to pebble. Scale is in millimetres. depths at which calcium carbonate is absent or present within the soil profile. The general situation in the Vidal area is for calcium carbonate to be dissolved from the A soil horizon and the upper part of the B horizon, and to be precipitated in the C d t "i kL > horizon and sometimes the lower part of y the B horizon. The parent material for the soils is a sandy gravel derived from schist •m mh and gneiss, which commonly has clasts of andesite and metavolcanic rocks. The sources of calcium carbonate are pedogenic carbonate fragments eroded from older alluvium and deposited as part of the parent material, and atmospheric carbonate dust. The combination of young and old sources of carbonate constitutes part of the difficulties in dating pedogenic carbonate by the C" method and necessitates careful sampling for uranium-series dating. The pre-Holocene soils of the Vidal area are polygenetic because they have been af- fected by both arid and semiarid climates. During periods of arid climate the carbonate is precipitated in the B horizon. In times of semiarid climate, the B-horizon carbonates may be partly dissolved and, along 10 15 20 with new atmospherically derived carbonate, will be precipitated in the underlying Figure 3. Two-stage pebble coating from surface U. In this case, youngest carbonate is Cca horizon. Thus, the B3ca horizon is sub- next to pebble where it precipitated because of carbonate salt splitting. Note lack of purity ject to alternating leaching and precipita- of outer part of coating. Scale is in millimetres. tion of carbonate, whereas the Cca horizon

is subjected only to periodic accumulation sampled for study, even though B-horizon Radiochemical analyses were made on of carbonate. pebble coatings commonly were thicker. the solution aliquots of both the leachate Two morphologies of calcium carbonate The type of pebble coating sample sought and the residue. Procedures for the isolation are precipitated in the B and C soil horizons for dating is shown in Figure 2. and purification of the uranium and of the Vidal gravelly soils: interstitial por- Age measurements by Th230-U234 analysis thorium isotopes followed those described ous fillings and coatings on the undersides of soil carbonate pebble coatings were at- by Ku (1965). Uranium was initially sepa- of pebbles. Samples of both kinds were tempted primarily on the Q2b geomorphic rated from an 8-N HC1 medium using dated, and it was found that the pebble surfaces. We mapped five extensive Q2b Dowex 1x8 anion-exchange resins, coatings with laminae of dense, massive surfaces in the Vidal area (Fig. 5). The whereas thorium was separated from a 4-N carbonate immediately next to the pebble criteria used in the field for evaluating the HC1 medium on a Dowex 50 X 12 cation- (see Fig. 2) best approached the desired surface for suitability of dating are sum- exchange column. Before purification, a so- closed system. Holocene pebble coatings marized in Table 2. The two surfaces lution containing known activities of U232 are thin and spotty and consist entirely of selected for sampling, U and V, both con- and Th228 was added to the sample aliquots porous carbonate. Pebble coatings from the tain pebble coatings that are relatively as yield tracer. Thin sources of the purified Q2b surface generally have porous car- thick, dense, and of good purity. The gravel uranium and thorium were counted in an bonate on the outsides and dense carbonate of surface V was derived primarily from alpha-spectrometer equipped with silicon next to the pebbles. This comparison gneiss and schist; the gravel of surface U surface-barrier detectors. The alpha energy suggests that as new carbonate is added, was derived primarily from andesite. Four- resolution (FWHM) is 60 to 80 keV at 5.5 part of it forms a new outermost increment teen samples were collected from seven pits MeV. of the pebble coating, and part of it moves dug in each of the two surfaces, spaced over through the porous parts of the coating to a distance of 3 km. EXPERIMENTAL RESULTS be precipitated next to an impermeable sur- Only the innermost 2 to 3 mm of the car- face. Initially this impermeable surface is bonate coating around a pebble was The analytical data for 14 carbonate the surface of the pebble itself, but eventu- analyzed. This material was carefully pebble coating samples from the Q2b sur- ally the lamina next to the pebble also be- scraped off with a surgical blade and faces V and U are listed in Tables 3 and 4, come impermeable. After the impermeable cleaned of any visible noncarbonate grains respectively. Results of the analyses done on lamina stage is reached, the innermost part by hand-picking with tweezers. The scrap- samples from younger and older geomor- of the pebble coating should approximate ing was facilitated by wetting the sample, phic surfaces are presented in Table 5. the closed system assumption. Thus, care which also served to remove any water- These latter samples were analyzed in the must be taken in selecting suitable pebble soluble salts. earlier phase of the study; some of them are coatings. About 5 to 8 g of the dry sample was then carbonate nodules, which commonly are leached with 1 N HC1. The HCl-insoluble less dense (therefore may be less approach- FIELD AND LABORATORY residue was separated from the leachate by ing a closed system) than the pebble coat- PROCEDURES centrifugation, and then dried and weighed. ings. In these tables, the errors indicated are It was then dissolved with HF-HCIO3. standard deviations (1 cr) derived from Each pebble coating was broken in the field to visually evaluate the density and purity of the carbonate. Several types of pebble coatings were avoided. The presence of truncated laminations indicated that coatings had undergone a period of erosion (probably during stream transport) and then renewed carbonate accretion. This type was rejected because of the possibility of its consisting of carbonates of distinctly different ages. Another type of pebble coating avoided was one in which salt splitting allowed young carbonate to be precipitated between the pebble and the oldest carbonate layer (Fig. 3). This occurs most fre- quently next to schist clasts and is recognized by the slivers of rock within a carbonate matrix of varying color and density. Figure 4 shows halite accumulation that has split a structurally weak rock, increased the local permeability, and allowed young carbonate to accrete next to the pebble. On the basis of the above observations, only the best suited pebble coatings from the Cca horizon of the soil profile were

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counting statistics. The activity unit, dpm/g, is an abbreviation for disintegration per minute per gram of leachate (L), or of residue (R).

AGE ESTIMATION FOR THE CARBONATE

The notations used below are as follows. The symbol of a nuclide — for example, Th230 — denotes specific activity (for example, dpm/g) of that nuclide. Subscripts C, D, L, and R refer to carbonate, detrital mineral, leachate, and 1 N HCl-insoluble residue, respectively. The letter f denotes

weight fraction (hence fL + fH = f(: + f0 =

1). It is held that fi: - fL and fD - fR. A first-order approximation can be made by assuming data on the leachate fraction to represent those for the carbonate coating — that is, Th230 = Th2?0, and so forth. Ages of the samples, calculated via equation 2 from the leachate data, are listed in Table 6, column A. These ages should be amenable to refinement, considering the following possibilities: (1) Differential solubilization of U234 and Th230 could occur during the acid treatment of samples; U234 may be preferentially leached from the detrital minerals relative to Th230, as suggested by the excess Th230 activities (for example, Th230 > U234) in residues. This effect tends to render the ages too young. (2) The presence of measurable amounts of Th232 in leachates BEDROCK 230 Areas of Q2b alluvium indicates that not all the leachable Th is (gneiss, schist and andesite) in situ produced in the carbonate; some of it 232 Y Index letter of Q2b alluvial surface must have accompanied the observed Th 230 ' referred to in the text as part of the "common" Th — that is, Th230 originally incorporated in the car- • - 94 Sample locality and Th"°/U2" age (1000 years) bonate as well as present in the detrital phase. Its presence would have the effect of Figure 5. Location map of Q2b geomorphic surfaces U through Y, Vidal, California. rendering the ages calculated on the basis of Numbers assigned to U and V surfaces denote ages of dated samples in thousands of years. leachates too old. TABLE 2. FIELD CRITERIA FOR SELECTION OF Q2b GEOMORPHIC SURFACES FOR TESTING OF Th^-U234 DATING METHOD

Surface* Ease of recognition of Degree of dissection Pebble-coating characteristics older and younger geomorphic of pavement area Typical Density Purity surfaces, and lack of by streams thickness multiple Q2b surfaces (mm)

U Excellent to good None to moderate 3 to 8 Excellent Excellent V Excellent Moderate to highly 2 to 6 Excellent Excellent dissected to good W Fair Minimal to moderate 2 to 6 Excellent Excellent to good X Good Minimal to highly 2 to 4 Excellent Poor to excellent dissected to good Y Excellent None to moderate 1 to 3 Good to Excellent to excellent good, except where salt splitting has caused impurities

* See Figure 5 for locations of surfaces.

Correction Scheme librium exists in the detrital phase — that and 8 is, U?? = Uff = Thf°. (3) Upon the acid UP8 = Uf38 - Thf32 (Thf/Thff2) To deal with the above possibilities, a leaching of detrital minerals, no fractiona- - (Mi.) (Thf" - Uf). (5) correction scheme is designed and applied tion of the thorium isotopes occurs — that to the leachate data to yield the radiochem- is, Th|30/ThiF = Thff/Th?,32. The correction term Thf2 (Th^'VTh'if) ical make-up for the carbonate phase. The These assumptions lead to the rela- refers to that amount of Th230 (or its equiv- correction scheme is based on the following tionships alent U234 and U238) leached from the detri- assumptions: (1) Initially, the carbonate 234 Thf0 = Th?30 - Th232(Th«""/Th|p), (3) tal phase. The correction terms (/¡¡//i) (U 232 230 3 contains negligible amounts of Th and - Th )and (fRlft.) (Ug» - Th?, "), re- 230 230 232 4 34 32 2 30 2 32 234 Th , or it has a Th /Th ratio similar to UP = U? - Thf (Th R /Th R ) spectively, refer to that amount of U and 23S that in the detrital phase. (2) Secular equi- - (Mi.) (Thf0 - US"), (4) U leached from the detritals in excess (or

TABLE 3. RADIOMETRIC DATA ON Q2b SAMPLES FROM GEOMORPHIC SURFACE V

Sample /¡+ U238 Uf^ Th^ Th232 no.* (dpm/g)

0.617 ± 0.021 0.691 ± 0.022 0.389 ± 0.016 0.090 ± 0.006 51-2b-l ^ 0.772 0.906 ± 0.037 0.865 + 0.036 1.020 ± 0.034 1.670 ± 0.048

0.788 ± 0.039 1.130 ± 0.050 0.580 ± 0.024 0.097 ± 0.008 52-2b-2 ^ 0.768 1.060 ± 0.038 1.053 ± 0.037 1.417 + 0.070 1.815 ± 0.084

1.369 ± 0.034 1.761 + 0.041 0.927 + 0.024 0.089 ± 0.006 53-2b-3 ^ 0.687 0.926 ± 0.030 0.884 ± 0.031 1.012 ± 0.034 1.072 ± 0.035

0.987 ± 0.028 1.198 ± 0.032 0.635 + 0.023 0.093 ± 0.007 54-2b-4 ^ 0.647 0.591 ± 0.022 0.649 ± 0.025 0.640 ± 0.033 1.077 ± 0.047

1.002 ± 0.034 1.200 ± 0.039 0.676 ± 0.027 0.101 ± 0.008 55-2b-5 ^ 0.678 0.605 ± 0.020 0.623 ± 0.021 0.711 + 0.018 1.043 ± 0.032

0.710 ± 0.022 0.897 + 0.025 0.408 ± 0.014 0.088 ± 0.006 56-2b-6 ^ 0.825 1.344 ± 0.041 1.355 + 0.041 2.106 + 0.100 2.347 ± 0.109

0.791 ± 0.024 0.988 ± 0.028 0.579 + 0.021 0.124 ± 0.008 57-2b-7 ^ 0.739 0.856 ± 0.029 0.812 ± 0.028 1.030 ± 0.048 1.495 ± 0.063

* L and R refer to leachate fraction and residue fraction, respectively. All samples are from innermost part of carbonate pebble coatings (about 2 to 3 mm thick). Fraction of sample weight teachable with 1 N HC1.

TABLE 4. RADIOMETRIC DATA ON Q2b SAMPLES FROM GEOMORPHIC SURFACE U

Sample /¡. U238 U^ Th230 TV no.* (dpm/g)

0.860 ± 0.030 58-2b-8 0.756 0.990 ± 0.033 0.567 ± 0.029 0.104 ± 0.010 1.011 ± 0.053 1.042 ± 0.054 1.254 ± 0.064 1.497 ± 0.073

0.559 ± 0.019 0.669 ± 0.021 0.245 ± 0.009 0.077 ± 0.004 59-2b-9a 0.840 1.615 ± 0.053 1.366 ± 0.047 1.918 ± 0.111 2.522 ± 0.137

0.640 ± 0.019 0.784 ± 0.022 0.433 ± 0.014 0.117 ± 0.006 59-2b-9 0.823 1.468 ± 0.063 1.360 ± 0.060 1.541 ± 0.094 1.710 ± 0.102

0.626 ± 0.026 0.755 ± 0.030 0.389 ± 0.015 0.097 ± 0.007 60-2b-10 0.806 1.229 ± 0.053 1.087 + 0.049 1.182 ± 0.050 1.616 ± 0.062

1.104 ± 0.051 1.366 ± 0.061 0.825 ± 0.030 0.084 ± 0.006 61-2b-ll 0.906 2.836 ± 0.111 2.548 ± 0.104 2.583 ± 0.112 3.375 ± 0.149

0.925 ± 0.028 1.083 ± 0.032 0.680 ± 0.022 0.140 ± 0.007 62-2b-12 0.688 0.995 ± 0.047 0.930 ± 0.045 0.950 ± 0.052 1.380 ± 0.070

0.687 ± 0.026 0.790 ± 0.029 0.451 ± 0.016 0.158 ± 0.008 63-2b-13 0.738 1.282 ± 0.047 1.193 + 0.045 1.505 ± 0.087 2.064 ± 0.113

0.688 ± 0.029 0.849 ± 0.034 0.167 ± 0.011 64-2b-14 0.722 0.429 ± 0.020 1.146 ± 0.044 1.040 ± 0.042 1.240 ± 0.059 1.844 ± 0.082

* L and R refer to leachate fraction and residue fraction, respectively. Sample 59-2b-9a is from outer part, about 3 to 6 mm from base of coating. All other samples are from innermost part of carbonate pebble coatings (about 2 to 3 m thick).

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deficit, as in a few cases) relative to Th230. 5, ages of the carbonates are then calculated the corrected values. It implies that the Both terms, therefore, are for correcting the from equation 2. They are listed in column two aforementioned complications — effect of differential acid-extraction of Th230 B of Table 6. Comparison between column preferential solution of U234 over Th230 and and the two uranium isotopes from the A and column B indicates that in general, contamination of Th230 from detritals — detrital phase. the uncorrected data for the leached part may have largely offset each other under After corrections via formulae 3 through give dates that are not very different from the acid-leach condition adopted. However,

TABLE 5. RADIOMETRIC DATA ON SAMPLES FROM OTHER GEOMORPHIC SURFACES

|J238 234 230 232 Sample Geomorphic k U Th Th no.1 surface (dpm/g)

Ku-1 L Qi 0.652 0.926 ± 0.044 1.130 ± 0.050 1.265 ± 0.045 0.278 ± 0.019 0.610 ± 0.015 0.665 ± 0.016 0.372 ± 0.013 0.089 ± 0.005 Ku-2 L 0.854 Qi 1.771 ± 0.066 1.683 ± 0.063 2.777 ± 0.200 2.134 ± 0.168 0.809 ± 0.022 0.890 ± 0.024 0.765 ± 0.033 0.067 ± 0.007 Ku-4 L Q2a 0.920 1.477 ± 0.110 1.479 ± 0.110 1.771 ± 0.097 1.071 ± 0.101 0.537 ± 0.015 0.665 ± 0.019 0.319 ± 0.010 0.067 ± 0.005 Ku-5 L Q2a 0.892 1.122 ± 0.073 1.096 ± 0.071 1.594 ± 0.063 1.555 ± 0.072 1.014 ± 0.034 1.207 ± 0.039 0.857 ± 0.034 0.127 ± 0.010 42-2a-3 Î: Q2a 0.862 K 1.999 ± 0.088 1.899 ± 0.086 2.431 ± 0.078 2.134 ± 0.072 0.814 ± 0.022 1.004 ± 0.030 0.713 ± 0.023 0.185 ± 0.009 42-2a-4 ^ Q2a 0.866 2.232 ± 0.081 1.982 ± 0.077 2.418 ± 0.122 2.311 ± 0.118 0.426 ± 0.015 0.503 ± 0.015 0.242 ± 0.008 0.070 ± 0.005 Ku-8 L Q2c 0.915 1.683 ± 0.110 1.633 ± 0.111 1.812 ± 0.112 2.496 ± 0.144 1.889 ± 0.071 2.323 ± 0.084 0.392 ± 0.090 0.182 ± 0.023 Ku-9 L 0.504 Q3 0.838 ± 0.058 0.821 ± 0.056 0.944 ± 0.080 0.638 ± 0.096

* L and R refer to leachate fraction and residue fraction, respectively. Samples Ku-4, Ku-5, and Ku-8 are carbonate nodules. Other samples are from innermost part of carbonate pebble coatings (about 2 to 3 mm thick).

300 TABLE 6. AGE ESTIMATES FOR PEDOGENIC CARBONATES FROM VIDAL AREA

Sample Geomorphic Age1 (X 103 yr) no. surface A B

51-2b-l Q2b 87 ± 7 90 ± 7 52-2b-2 Q2b 74 ± 7 78 ± 7 53-2b-3 Q2b 79 ± 5 77 ± 5 c CD 54-2b-4 Q2b 80 ± 6 75 ± 6 in a) 55-2b-5 Q2b 87 ± 7 85 ± 7 200 • 56-2b-6 Q2b 64 ± 5 72 ± 6 a> Figure 6. Results of 57-2b-7 Q2b 93 ± 7 94 ± 7 o Th230-U234 dating of al- 58-2b-8 Q2b 90 ± 9 91 ± 9 H— 59-2b-9a Q2b 49 ± 3 50 ± 3 a> luvial pedogenic car- XI 59-2b-9 Q2b 84 ± 7 75 + 6 bonate from Vidal, 60-2b-10 Q2b 78 ± 7 70 ± 6 o California. Bars repre- 61-2b-ll Q2b 98 ± 9 93 ± 9 £ sent minimum age of 62-2b-12 Q2b 104 ± 7 96 ± 7 90 ± 7 96 ± 7 each geomorphic sur- 63-2b-13 Q2b 64-2b-14 Q2b 75 ± 6 70 ± 6 face. Circled points are Ku-1 >350 "O Qi c carbonate nodule sam- Ku-2 88 ± 8 131 ± 12 a 100 • Qi X ples, which generally are Ku-4 Q2a 200 ± 28 172 ± 25 1 less dense and compact Ku-5 Q2a 69 ± 3 66 ± 3 X g 42-2a-3 Q2a 128 ± 15 134 ± 15 X than pebble coatings. XfX 42-2a-4 Q2a 127 ± 12 124 ± 12 1 t1 Ku-8 Q2c 70 ± 5 61 ± 5 3 > Ku-9 Q3 20 ± 3 6 ± 1 0) (1) o o CJ i1 * Column A values are based on uncorrected leachate data; column B values are derived from -> CO CO leachate data after correction. Uncertainties re- flect analytical (lcr counting) errors of leachate fractions only. Q3 Q2c 02b 02a Q 1

one must not overlook the large discrep- Q2b samples from surfaces U and V have a the pebbles during roughly 10,000 yr of ancy in the cases of Ku-2 and Ku-9. Given "common" age and Tho30/Th232 ratio. With arid climate. For one Q2b pebble coating the assumption that Th?30/U?34 = 1, any this assumption, one sets out to solve itera- (sample 59-2b-9) we dated the outer rind as preferential leaching of U234 over Th230 from tively the two nonlinear parameters t and well as the inner part next to the pebble. the detrital phase would have tended to A in equation 6 which will best fit the 14 These dates indicate an average rate of car- make samples of old age (Th230/Up4 = 1) too sets of experimental data on Th230/Th232, bonate deposition of about 1 mm/8,000 yr, 230 234 young and of young age (Th /U « 1) U238/Xh232 and U234/Th232 f()r Q2b during a time span that includes both arid too old, if no correction is made on the samples (Tables 3 and 4, excluding sample and semiarid climates. It appears that car- leachate data. This is believed to be the case 59-2b-9a). The iteration was done using a bonate accumulates at even slower rates in for Ku-2 and Ku-9; their uncorrected ages least-squares differential-correction tech- the drier eastern Mojave Desert than in are in error. One should also note that the nique for determining nonlinear parameters other areas of the southwest. Rates of car- corrected age of Ku-9 is in line with the (McCalla, 1967, p. 256). The results of the bonate accumulation probably were much Holocene origin for the Q3 geomorphic computation gave t = 88,400 yr and more rapid during Pleistocene times of surface deduced from field evidence (Table Tho30/Th232 = 0.656. semiarid climate than during the Holocene. 1). This "common" age of 84,400 yr is com- The dates are minimum ages for the time of pared to the average age of 83,000 ± initial carbonate illuviation, because ap- Independent Evaluation of 10,000 yr for the 14 Q2b samples (Table 6, proximately 16,000 to 24,000 yr are the Correction Scheme column B). The agreement suggests that the needed to accrete the 2 to 3 mm of pebble assumptions used in the correction scheme coating sampled for dating. In most cases, UR34 = U238 within error on leachate data are at least applicable to One should also consider the time needed limits (Tables 3 through 5). This provides samples from the study area. to deposit the thin alluvial fans that com- some justification for assumption 2 and by prise geomorphic surfaces U and V (Fig. 5). inference for assumption 3 as well, of the DISCUSSION A typical sequence of alluvial-fan formation correction scheme. However, one can argue is for initial deposition to occur in the fan- that detrital minerals — especially clays, Ages of the Q2b Samples and head area. Then as the stream channel be- owing to their adsorption-exchange capac- Carbonate Accumulation Rate comes incised into the surface, the loci of ities — may not act as closed systems before deposition shift downslope, and the sur- carbonate deposition. If so, assumption 2 As shown in Figure 5, the Q2b samples ficial deposits at the fan head become a becomes questionable. collected from the U surface, spaced over a stable surface where a soil profile forms. In the following, an evaluation of this distance of about 3 km, show ages ranging With increased incision the areas pro- problem is attempted. It entails the deter- from 70,000 to 96,000 yr (averaging gressively farther from the initial fan apex mination of a "common" age for the Q2b 84,400 yr). The ages obtained on the V sur- become stable and allow soil profiles to samples based on assumptions that are in- face range from 72,000 to 94,000 yr (av- form. Carbonate ages, therefore, should be dependent of those used in the correction eraging 81,600 yr). The mean age for all 14 progressively younger downslope on this scheme. samples is 83,000 yr, with a standard de- hypothetical fan. Although the pattern of If the pedogenic carbonate is treated as viation of ±10,000 yr, which is only ages for the surface U samples shows no an entity (that is, detrital as well as slightly larger than the laboratory mea- trend, that for surface V does seem to ex- carbonate), then the presence of sizable surement errors for the individual ages — hibit a downfan decrease in age (Fig. 5). It 3 4 amounts of common Th230 requires that a ±5,000 to 9,000 yr (Table 6). These ages seems likely that at least 10 to 10 yr were term, Th2,30 exp (—be added at the show a high degree of internal consistency, needed to deposit the two fans of Q2b age right-hand side of equation 2, Th230 being especially when one considers that the that were dated. the common Th230. With this additional samples are from widely separated parts of term, the age formula of equation 2 can be two alluvial fans and that accumulation of Minimum Ages for Geomorphic rewritten as pedogenic carbonate could be a very slow Surfaces in the Vidal Area process. v = A • exp (- \0t) The individual dates represent averages + x { [ 1 - exp ( - V)] The reliability of the Q2b ages presented - [1 - exp (\4t - k„t) ] [X„/ (\0 - 1 } of the ages of a 2 to 3 mm thickness of car- here may be further inferred from addi- + z [1 - exp(\4i - A0/)] bonate laminae included in each analysis. tional dates for samples collected from [X„/(A - \ ) ], (6) 0 4 Sigalove (1969) showed by C14 dating that other alluvial units (Q3, Q2c, Q2a, and Q1; where y = Th230/Th232, A = Th230/Th232, about 2,000 yr are needed to accrete each see Table 1) in the Vidal region (Table 6). x = U238/Th232, and z = U234/Th232. millimeter of carbonate on the bottom of a These samples were collected and dated basalt boulder northeast of Flagstaff, during that phase of the project when the The notations for the nuclides refer to Arizona. In the Rio Grande Valley, New sampling methodology was being evalu- their specific activities in the whole sample Mexico, about 2,000 to 5,000 yr are needed ated. Some of the samples are not the peb- 230 30 !0 (for example, Th = /¡.Th? + /RTh?; , for 1 mm of pedogenic carbonate to be ac- ble coating type carefully selected for the and so forth). There are two unknowns in cumulated (Gile and others, 1966; Gile, Q2b unit. Thus, their ages should be viewed this equation: t and A ( = Thf'/Th232), and 1975). Examination of the carbonate coat- as preliminary at this stage. However, the in principle, they can be solved simulta- ings from beneath the darkly varnished and maximum ages for the various surfaces neously from two or more coeval samples weathered geomorphic surfaces of the Vidal occur in the correct stratigraphic sequence with identical TWI;'"jTh'"2 ratios. area that are early Holocene in age indicates from Q1 to Q3, as shown by the bar graph In this study, it is assumed that the 14 that about 0.5 mm of carbonate accretes on of Figure 6. These ages represent the mini-

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mum age estimates for the formation of containing abundant hydroxy iron minerals ing the method's applicability to other im- their respective geomorphic surfaces. Table give spurious age results. pure carbonate systems containing various 7 provides a listing of the locations of all amounts of silicate-mineral—bearing de- the samples dated. CONCLUSIONS tritus. Such systems may include the non- AND SUGGESTIONS pedogenic calcic deposits described by Nature of Noncarbonate as a Criterion FOR FUTURE RESEARCH Lattman (1973) and various carbonate for Applicability of the Method cementations through the action of ground This study has demonstrated that with or surface waters. Another extension would It is apparent from the assumptions used prudent sample selection, carbonates from be to ascertain the feasibility of applying the in correcting the leachate data that knowl- arid-zone soils and paleosols can be dated technique to pedogenic carbonates formed edge about the radiochemical nature of the successfully with the Th230-U234 method. in environments more humid than desert.

noncarbonate detrital minerals is of impor- The method dates the time at which CaC03 As the investigation described here is still tance to the applicability of the dating precipitates from a solution, which is inde- preliminary in nature, experiments to method. In the absence of direct analyses of pendent of the past age history of the source further evaluate and refine the methodology

such isolated minerals, some of the knowl- material for CaC03. Its applicable age should be done. A few of these experiments edge can be gathered by inference from the range spans from a few thousand years to are suggested below. data on residues. As seen from Tables 3 about 350,000 yr. 1. The equation 6 approach, as con- through 5, the uranium and thorium con- The inner parts of the dense carbonate ducted here for the Q2b samples, should be centrations of the residues are in the range 1 pebble coatings from soil horizon Cca are tested on specimens from a single locality to 10 ppm, with an average Th/U weight considered to be the most suitable for dat- and on aliquots of a single specimen with ratio of about 4. This is generally expected ing. Analyses of 14 such specimens col- different carbonate percentages. It is possi- from an average shale (Adams and others, lected from an upper Pleistocene geomor- ble that this approach may itself be estab- 234 238 1959). Also, the U /U ratios in residue phic surface, Q2b, located in Vidal Valley lished as a valid methodology. fractions mostly are near the secular of California's eastern Mojave Desert, gave 2. The secular equilibrium relationship equilibrium value of unity. We suggest that ages ranging from 70,000 to 96,000 yr. The assumed for the detrital phase should be the above observations be taken as age spread is small, considering the ana- checked. The measurement is to be per- "norms" for the radiochemical characteris- lytical uncertainties and the time intervals formed on mechanically isolated detrital tics of the detritals or residues that are (of the order of 104 yr) generally involved in minerals (or if this proves infeasible, on visualized as common silicate minerals or the formation of individual pebble coatings sample aliquots of varying carbonate con- clays. Any large deviations from these and in the deposition of different parts of an tents). "norms" can then be used as criteria for alluvial fan. The maximum Th230/U234 ages 3. Stepwise leaching of insoluble residues identifying samples unsuitable for dating by obtained on samples from the other alluvial using different kinds and strengths of acid the present methodology. For instance, it units in the study area are consonant with should be carried out to secure conditions has been noted (Ku, unpub. report) that their stratigraphic sequence. under which no fractionation of the samples yielding anomalously high U con- The apparent success in dating arid-zone thorium isotopes occurs. centrations (>40 ppm) in their residues, or paleosols points to the possibility of extend- Because of the difficulty in assessing the initial Th230 activities, dating impure car- TABLE 7. SAMPLE LOCATIONS bonates with Th230/U234 probably will have inherent uncertainties of the order of Sample Geomorphic Depth of Location no. surface sample ±10% (or larger for Holocene samples). (m) Nevertheless, in view of the lack of absolute dating tools for these deposits, limitations Ku-1 Q1 0.40 to 0.81 . NE'ASEVisec. 10, T. 2 N., R. 22 E. in precision of such magnitude can certainly Ku-2 Ql Surface NE'ASEVisec. 10, T. 2 N., R. 22 E. Ku-4 Q2a 2.0 SE'ASEVisec. 8, T. 1 N., R. 24 E. be tolerated at this time. An ultimate eval- Ku-5 Q2a 0.75 to 1.0 SEViSEVisec. 8, T. 1 N., R. 24 E. uation would be calibration with an inde- 42-2a-3 Q2a Surface SW'ASWVisec. 23, T. 2 N., R. 23 E. pendent method, such as K-Ar dating of 42-2a-4 Q2a Surface SWASEVisec. 1, T. 1 N., R. 24 E. Quaternary volcanics associated with the 0.31 to 0.59 SW'ASE'Asec. 1, T. 1 N., R. 23 E. 51-2b-l Q2b pedogenic carbonate. 52-2b-2 Q2b 0.30 to 0.56 SE'ASE'Asec. 11, T. 1 N., R. 23 E. 53-2b-3 Q2b 0.35 to 0.63 SWA NE Vi sec. 12, T. 1 N., R. 23 E. 54-2b-4 Q2b 0.27 to 0.61 NW'ASWVisec. 18, T. 1 N., R. 24 E. ACKNOWLEDGMENTS 55-2b-5 Q2b 0.35 to 0.56 SE'ASE'Asec. 12, T. 1 N., R. 23 E. SE'A NE'A sec. 12, T. 1 N., R. 23 E. 56-2b-6 Q2b 0.42 to 0.85 This study was conducted in connection 57-2b-7 Q2b 0.39 to 0.52 NW'ASE'Asec. 1, T. 1 N., R. 23 E. 58-2b-8 Q2b 0.37 to 0.61 NE'A NW'A sec. 11, T. 1 N., R. 23 E. with a detailed geological investigation of 59-2b-9a Q2b 0.44 to 0.75 NE'ANWVisec. 11, T. 1 N., R. 23 E. the Vidal area by Woodward-Clyde Con- 59-2b-9 Q2b 0.44 to 0.75 NE'ANW'Asec. 11, T. 1 N.,R. 23 E. sultants for Southern California Edison 60-2b-10 Q2b 0.24 to 0.60 NE'ASWVisec. 11, T. 1 N., R. 23 E. Company. We acknowledge the generous 61-2b-ll Q2b 0.49 to 0.70 NW'A SW Vi sec. 11, T. 1 N., R. 23 E. 62-2b-12 Q2b 0.51 to 0.80 SW'ASW'Asec. 11, T. 1 N., R. 23 E. support of Woodward-Clyde Consultants 63-2b-13 Q2b 0.28 to 0.60 SW'ANWVisec. 14, T. 1 N., R. 23 E. and the Southern California Edison Com- 64-2 b-14 Q2b 0.38 to 0.55 NE'ASWVisec. 14, T. 1 N., R. 23 E. pany in preparation of this report. We are Ku-8 Q2c 2.0 SWV4NWV4sec. 16, T. 1 N., R. 24 E. grateful to Gail Hunt, Ray Sholes, and Pat Ku-9 Q3 0.07 NE'A NE'A sec. 16, T. 1 N., R. 24 E. Hamilton of Southern California Edison

Company for their assistance, encourage- Gile, L. H., 1975, Holocene soils and soil- Metzger, D. G., 1968, The Bouse Formation ment, and reviews during both the field geomorphic relations in an arid region of (Pliocene) of the Parker-Blythe-Cibola area, work and the reporting stages. We also southern New Mexico: Quaternary Re- Arizona and California, in Geological Sur- search, v. 5, p. 321-360. vey research 1968: U.S. Geological Survey thank George Brogan, Edward Heath, and Gile, L. H., Peterson, F. F., and Grossman, R. R., Professional Paper 600-D, p. 126-136. Ray Sugiura, who offered guidance and 1966, Morphological and genetic sequences Potratz, H. A., Barnes, J. W., and Lang, E. J., help. Special thanks are due to Quay Nary, of carbonate accumulation in desert soils: 1955, A radiochemical procedure for Mu-Ching Lin, Chow-Long Chu, and Pei- Soil Science, v. 101, p. 347-360. thorium and its application to the determi- Shiun Chen, who provided valuable discus- Kaufman, A., and Broecker, W. S., 1965, Com- nation of ionium in coral limestone: Los parison of Th230 and C14 ages for carbonate Alamos Science Laboratory Publication sions and technical assistance in various de- materials from Lakes Lahontan and Bon- LA-1845, 19 p. velopment stages of the methodology. T. L. neville: Journal of Geophysical Research, v. Sigalove, J. J., 1969, Carbon-14 content and Ku received partial support from National 70, p. 4039-4054.. origin of caliche [M.S. thesis]: Tucson, Uni- 234 238 Science Foundation Grant EAR77-13680. Ku, T. L., 1965, An evaluation of the U /U versity of Arizona, 72 p. method as a tool for dating pelagic sedi- Starik, I. E., and Kolyadin, L. B., 1957, The oc- ments: Journal of Geophysical Research, v. curence of uranium in ocean water: REFERENCES CITED 70, p. 3457 -3474. Geochemistry, v. 3, p. 245 —256. Lattman, L. H., 1973, Calcium carbonate cemen- Van Devender, T. R., 1977, Holocene woodlands Adams, J.A.S., Osmond, J. K., and Rogers, J. J., tation of alluvial, fans in southern Nevada: in the southwestern deserts: Science, v. 198, 1959, The geochemistry of uranium and Geological Society of America Bulletin, v. p. 189-192. thorium: Physics and Chemistry of the 84, p. 3013-3028. Williams, G. E., and Polach, H. A., 1971, Earth, v. 3, p. 298-343. McCalla, T. R., 1967, Introduction to numerical Radiocarbon dating of arid-zone calcareous Barnes, J. W., Lang, E. J., and Potratz, H. A., methods and FORTRAN programming: paleosols: Geological Society of America 1956, Ratio of ionium to uranium in coral New York, John Wiley &C Sons, 349, p. Bulletin, v. 82, p. 3069-3086. limestone: Science, v. 124, p. 175-176. McKee, E. D., Hamblin, W. K., and Damon, Bull, W. B., 1974, Effects of Holocene climate on P. E., 1968, K/Ar age of lava dam in Grand MANUSCRIPT RECEIVED BY THE SOCIETY DE- arid fluvial systems, Whipple Mountains, Canyon: Geological Society of America Bul- CEMBER 16, 1977 California: American Quaternary Associa- letin, v. 79, p. 233-236. REVISED MANUSCRIPT RECEIVED DECEMBER 20, tion, Biennial Meeting, 3rd, Abstracts, McKelvey, V. E., Everhart, D. L., and Garrels, 1978 p. 64. R. M., 1955, Origin of uranium deposits, in MANUSCRIPT ACCEPTED JANUARY 22, 1979 234 Cherdyntsev, V. V.,. 1971, Uranium : Bateman, A. M., ed., Economic geology, CONTRIBUTION NO. 370, DEPARTMENT OF Jerusalem, Israel Program for Scientific 15th anniversary volume, Part 1: Lancaster, GEOLOGICAL SCIENCES, UNIVERSITY OF Translation, 234 p. Pa., Lancaster Press, p. 464-533. SOUTHERN CALIFORNIA

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