Rates and Processes of Soil Development on Quaternary Terraces in Cajon Pass, California

Rates and processes of soil development on Quaternary terraces in Cajon Pass, California

LESLIE D. McFADDEN Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 RAY J. WELDON II U.S. Geological Survey Branch of Engineering Seismology and Geology, M.S. 977, 345 Middlefield Road, Menlo Park, California 94025

ABSTRACT creased. This conversion of the soil from a soil development to date young deposits because noncolloidal system to a much more colloidal radiometrically datable materials or index fossils Field and laboratory analyses of soils on 11 system takes place over a relatively short pe- are absent or scarce in most terrigenous Quater- well-dated fluvisil terraces spanning the past riod of time (<4,000 yr) and is herein defined nary deposits and soils occur commonly on 0.5 m.y. demonstrate that a threshold governs as a type of pedologic threshold. In the Cajon Quaternary deposits. New techniques such as changes in several morphological and chemi- Pass area, the attainment of the threshold and uranium-trend and thermoluminescence dating cal characteristics of increasingly older soils. subsequent development of the argillic B ho- are experimental, hence uncertain. The geo- Correlations with respect to time among iron rizon of soils on latest Pleistocene terraces chronological information provided by soil species, soil morphology, and soil silt and clay occurred during the Holocene; thus, the ab- development is especially appropriate in cases in demonstrate that the chronosequence at sence of argillic horizons in soils on Holocene which the distinction between the ages of a Cajon Pass reflects primarily an evolutionary, terraces is attributable to simply their geomorphic surface and its substrate is a critical largely time-dependent trend and does not re- younger age rather than to the Pleistocene- consideration. flect differences in external factors such as to-Holocene climatic change. The threshold is Soils are highly complex natural systems and climate. Most of the soil development on a function of several variables, including in- are affected by variables that include topog- Holocene terraces of the Cajon Pass area is flux rate of eolian dust and initial soil per- raphy, parent materials, vegetation, climate, due to physical incorporation of eolian dust meability; therefore, the time required to and time. The state factor approach of Jenny and organic material into initially very per- attain the threshold will vary in chronose- (1941) provided a sound basis for using pedo- meable gravels. This process decreases soil quences characterized by geomorphic or logic data to infer age by holding the influence permeability and is conducive for an increase geographic settings that are different from of the other factors constant. Many studies have in the magnitude of chemical weathering. conditions found in Cajon Pass. shown that certain soil properties are related to Latest Pleistocene and older Pleistocene soils soil age; among these properties are morphology have developed clay and authigenic iron ox- INTRODUCTION of calcic horizons (Gile and others, 1966), mass ide-rich B horizons at the expense of organic- of secondary carbonate (Machette, 1978,1985), matter-rich A horizons and color B horizons The disciplines of tectonic geomorphology, thickness of stone weathering rinds (Colman as the extent of chemical weathering has in- neotectonics, and paleoclimatology often rely on and Pierce, 1981), soil morphological properties such its horizon thickness and clay content (Bockheim, 1980), total soil morphology (Harden, 1982; Birkeland, 1984), and iron oxide content (McFadden and Hendricks, 1985). Ab- solute rates of soil development, however, have 35 not been determined in most soil chronosequence studies because few absolute ages of the soils are known. Some variables, such as climate or influx of eolian dust, have changed with time and may have significantly affected the rate of ^

Figure 1. Map showing location and general geologic setting of the Cajon Pass study area (cp) and selected geographic features re- ferred to in this paper. Los Angeles (LA), San Bernardino (SB), Salton Sea (SS), San Andreas faulli (SAF), and Cleghorn fault (cf) are included for reference.

Additional material for this article (tables) may be obtained free of charge by requesting Supplementary Data 87-07 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 98, p. 280-293,9 figs., 3 tables, March 1987.

280 SOIL DEVELOPMENT ON QUATERNARY TERRACES, CALIFORNIA 281 soil development (Bockheim, 1980). For exam- 11 well-dated fluvial terraces in this area that developed during the past 0.5 m.y. and to evalu- ple, rates of clay or carbonate accumulation may range in age from 47 to -500,000 yr (Fig. 2). ate how soil-forming variables such as climatic in part reflect changes in the rate and magnitude The climate of this region is classically Mediter- change and eolian influx have influenced rates of of dust influx (Gile and others, 1966,1981; Yaa- ranean, characterized by hot, dry summers and soil development during the late Quaternary. lon and Ganor, 1973; Colman, 1982; McFad- cool, moist winters; annual precipitation gener- den and Tinsley, 1982, 1985; Machette, 1985) ally ranges from 630 to 730 mm (Alhborn, QUATERNARY HISTORY OF THE as well as the intensity of leaching and asso- 1982). Climatic variation across the study area is CAJON CREEK AREA ciated chemical weathering, all of which strong- minimal. With one exception, elevations of ter- ly depend on climate (Rogers, 1980; Jenny, races in the study area range from 710 to 950 m Cajon Creek is in the central Transverse 1980; Birkeland, 1984). For these reasons, soil above sea level. The oldest and best described Ranges and has formed a flight of terraces across chronosequence data do provide broad age es- soil in the study area is best preserved at an the San Andreas fault (Figs. 1 and 2). To charac- timates but are most useful for determining the elevation of 1,220 m, where the annual precipi- terize the tectonic deformation associated with relative ages of geomorphic surfaces. tation is only 430 mm. Terrace sediments are the San Andreas fault, the Quaternary deposits Recent studies of the late Cenozoic history of composed primarily of albite-epidote-mica- in the Cajon Creek drainage were mapped in the Cajon Creek area in the Transverse Ranges, chlorite schist and melanocratic to leucocratic detail and dated using 14C, magnetic stratig- California (Fig. 1), provide absolute ages of granitic rocks. Parent materials, vegetation, and raphy, and fossils (Weldon and Sieh, 1985; Holocene and Pleistocene deposits and geomor- topographic relief are similar on most of these Weldon, 1986). The ages of the surfaces that phic surfaces (Weldon, 1986; Weldon and Sieh, surfaces, which gives us an opportunity to de- formed on most late Quaternary deposits, given 1985). We have described and analyzed soils on termine the rates at which soil properties have in Table 1, can be closely constrained using 18

2000-1

Figure 2. Schematic section through Cajon Creek area, showing ages of deposits, surfaces, and the height of surfaces above the active channel of Cajon Creek. Details of terrace deposits (Qoa) and surfaces (Qt) are tabulated in Table 1 and discussed in the text. TABLE 1. AGE, TEXTURAL, AND CHEMICAL CHARACTERISTICS OF KEY QUATERNARY SOILS IN THE CAJON PASS AREA, SOUTHERN CALIFORNIA

Particle size, <2 mm (%)

Deposit, Surface age Number Profile Depth Color* Sand Silt Clay pH terrace (yr B.P.) horizon (cm) (dry; moist)

Qal-0 47 RW-9 A 0-27 2.5Y 5/2; 4/2 90.4 9.6 tr 6.3 Cu 27+ 2.5Y 6/2; 4/2 93.5 6.1 0.4 7.0

Qoa-a, 275 RW-18 O 1.0-0 Qt-6 +385 Al 0-3 2.5Y 4/2; 5Y 2.5/1 72.3 23.7 4.0 4.6 -75 2A2 3-18 2.5Y 5/2; 5Y 3/1 82.0 15.2 2.8 4.9 2AC 18-34.0 2.5Y 5/2; 5Y 3/2 83.7 13.7 2.6 5.4 2Cox 34-55 2.5Y 6/4; 3/2 89.4 7.7 2.9 5.6 2Cu 55+ 5Y 6/2; 5Y 3/2 91.2 6.9 1.9 5.7

Qoa-a, 275 RW-10 Al 0-5 10YR 4/3; 3/2 95.1 4.9 0.5 4.9 Qt-6 +385 A2 5-21 10YR 5/3; 2/2 84.4 12.8 2.8 5.4 -75 AC 21-26 10YR 5/3; 4/3 88.1 9.6 2.3 5.5 Cox 26-33 10YR 6/4; 4/4 96.0 3.4 0.6 5.7 Cu 33+ 10YR 6/3; 5/4 91.5 6.9 1.6 6.4

Qoa-c, 5900 RW-12 Al 0-18 10YR-2.5Y 5/2; 2/2 84.5 13.3 2.2 5.1 Qt-5 ± 900 A2 18-35 10YR-2.5Y 5/2; 3/2 87.1 10.9 2.0 5.0 Bw 35-63 2.5Y 6/3; 5/4 91.5 7.6 0.9 5.2 Coxl 63-73 2.5Y 6/2; 5/4 88.5 10.0 1.5 5.4 Cox2 73-90 2.5Y 6/2; 4/2 89.8 8.8 1.4 5.7 Cu/Cox 90+ 5Y 5/2; 4/2 94.8 3.9 1.3 5.8

Qoa-c, 7150 RW-15 Al 0-6 2.5Y 4/2; 3/2 69.0 30.0 1.0 5.2 Qt-4 ± 1200 A2 6-25 10YR 4/3; 68.8 31.2 tr 5.6 2.5Y/10YR 3/2 BA 25-32 10YR 5/3; 79.7 20.3 tr 5.6 2.5Y/10YR 3/3 Bw 32-45 10YR 5/4; 86.1 13.9 tr 5.8 2.5Y/10YR 3/2 Cox 45-89 2.5Y 5/4; 89.3 10.7 tr 5.9 2.5Y/5Y 4/2 Cu 89+ 5Y 6/2; 4/2 90.4 9.6 tr 6.2

Qoa-c, 8350 RW-13 Al 0-2.5 10YR 5/2; 2/2 70.8 28.8 0.5 4.6 Qt-3 +900 A2 2.5-7.5 10YR 5/3; 2/2 68.2 30.8 1.0 4.9 -500 AB 7.5-15 10YR 5/3; 4/3 71.6 27.4 1.0 5.0 2Bwl 15-26 10YR 5/4; 3/4 71.3 26.8 1.8 5.1 2Bw2 26-37 10YR-2.5Y 6/4; 82.5 16.5 1.0 5.2 10YR 4/3 2BC 37-53 10YR-2.5Y 4/4; 87.2 12.8 tr 5.2 2.5 Y 4/4 2Cox 53-100+ 2.5Y 6/3 94.9 5.1 tr 5.5 2Cu 3,000+ 5Y 5/2; 3/2 91.0 9.0 tr 5.6 2.5Y 4/2

Qoa-c 11.500 RW-6 Al 0-6 10YR 5/3; 3/2 70.0 27.2 2.8 5.4 Qt-2 +2000 A2 6-24 10YR 5/3; 3/3 69.6 26.8 3.6 5.7 -3000 2Btl 24-48 8.75YR 5/4; 3/4 69.0 25.6 5.4 5.4 2Bt2 48-59 10YR 5/6; 4/3 74.0 20.1 5.9 5.3 2Coxl 59-85 2.5Y 5/2; 3/2 94.8 4.2 1.0 5.4 2Cox2 85+ 5Y 5/2; 3/2 94.8 4.7 0.5 5.3

Qoa-c, 12,400 RW-17 O IM Qt-1 ± 1000 Al 0-3.5 10YR 4/3; 3/2 65.3 29.6 5.1 5.1 A2 3.5-12 10YR 5/3; 3/3 69.6 25.5 4.9 5.1 Btl 12-21 10YR 5/4; 3/3 71.4 22.4 6.2 5.3 Bt2 21-37 10YR 5/3; 4/3 71.0 19.6 9.4 5.3 Bt3 37-50 8.75YR 5/4; 74.6 15.1 10.3 5.4 8.75YR 4/4 Bt4 50-65 10YR 5/4; 10YR 4/4 75.5 14.8 9.7 5.3 BC 65-79 10YR-2Y 5/4; 10YR 4/3 80.7 11.7 7.6 5.4 Coxl 79-99 2.5Y 5/4; 2.5Y 4/2 84.4 9.3 6.3 5.2 Cox2 99-110+ 2.5Y 5/4-5Y 5/3; 85.4 9.2 5.4 5.3 2.5Y 4/2

Qoa-d 55,000 RW-11 Ol 7-0 ± 12,000 BA 0-9 7.5YR 5/4; 3/4 60.0 22.7 17.3 5.5 2Btl 9-42 5YR 4/6; 4/4 29.0 46.8 24.2 5.9 2Bt2 42-77 5YR 5/6; 4/4 52.5 25.7 21.8 5.8 2Bt3 77-99 6.25YR 5/4; 4/4 60.0 21.6 18.4 5.7 2BC 99-140 7.5YR 6/6; 4/6 81.3 16.9 1.8 5.5 2Coxl 140-190 8.75YR 6/4; 4/4 88.0 12.0 tr 5.3 2Cox2 190+ 10YR 7/4; 4/4 93.5 6.4 0.1 5.3

Qoa-e 500,000 RW-14 2BAt 0-13 2.5YR 4/6; 4/6 56.2 15.5 28.3 4.4 ±200,000 2Btl 13-36 2.5YR 4/6; 4/4 47.2 18.3 34.5 4.3 2Bt2 36-54 3.75YR 4/4; 5YR 5/6 62.7 15.0 22.3 5.5 2Bt3 54-142 5YR 5/6; 4/6 67.4 17.0 15.6 5.8 2Bt4 142-183 5YR 5/6; 4/6 75.4 10.8 13.8 5.3 2Bt5 183-335 5YR 5/6; 4/6 72.1 12.6 15.3 5.4 2Bt6 335-701 5YR 5/6; 4/6 77.7 10.3 12.0 5.2 2Bt7 701-1,460 6.25YR 5/6; 4/6 5.2 2Cox 1,460+ 10YR 5/6; 4/6 93.8 4.7 1.6 5.7

•From the Munsell Soil Color Chart. TABLE 1. (Continued)

Iron oxide contents and composition (%)

Organic Fe203d Fe203o Fe203p FeOT Ke2Oj Fe203T Location and comments carbon (%)

Mouth of Pitman Canyon at Cajon Creek 0.2 (granitic debris). 1938 flood deposit 0.1 burying the pre-1938 road. Lat. 34°14'32"N; Long. 117°26'23"W.

Lone Pine Canyon (Pelona Schist debris). 2.9 0.85 0.22 2.86 2.12 5.22 Two l4C dates provide minimum age 0.7 0.69 0.20 2.44 2.26 4.90 constraint; unit predates only last earth- 0.4 0.60 0.24 2.20 1.77 4.15 quake, providing maximum age con- 0.2 0.47 0.23 2.03 1.93 4.13 straint. Lat. 34°16'15"N; Long. 0.2 0.46 0.29 2.09 1.62 3.88 117°27'56*W.

0.7 0.29 0.11 0.003 1.69 0.52 2.40 Mouth of Rat Creek at Cajon Creek 1.1 0.60 0.20 0.01 0.96 3.33 4.40 (granitic debris). Age constraints same 0.4 0.58 0.15 0.01 0.99 3.74 4.84 as for RW=18. Ut. 34°17'25"N; 0.2 0.47 0.14 0.01 0.91 4.10 5.11 Long. 117°26'56"W. 0.46 0.13 0.01 0.79 3.59 4.47

0.8 0.55 0.15 0.01 3.17 0.78 4.30 Lone Pine Canyon (Pelona Schist debris). 0.5 0.58 0.15 0.01 3.41 1.35 5.13 Incision below Qt-5 isolated Lost Swamp 0.2 0.55 0.16 0.01 2.41 1.95 4.62 area from surface flow. Three 14C values 0.3 0.59 0.16 0.01 2.69 1.65 4.64 from Lost Swamp sediments date this 0.59 0.18 0.01 2.73 1.71 4.74 event. Ut. 34°16'17"N; 0.51 0.15 0.01 2.88 1.88 5.08 Long. 117°27'56"W.

4.3 0.90 0.17 0.02 4.20 0.86 5.52 Lone Pine Canyon (Pelona Schist debris). 1.6 0.79 0.21 0.02 3.97 0.96 5.37 Offset by the San Andreas fault; age inferred from offset and slip rate of 0.7 0.67 0.21 0.02 2.95 1.65 4.92 24.5 ± 3.5 mm/yr. Absolutely bracketed by Qt-3 and Qt-5. Ut. 34016'37"N; 0.5 0.63 0.19 0.01 2.61 1.76 4.66 Long. 117°28'22"W.

0.62 0.15 0.01 2.72 1.67 4.69

0.51 0.12 0.01 2.11 2.28 4.62

2.1 0.69 0.18 0.01 3.00 1.92 5.25 Lone Pine Canyon (Pelona Schist debris). 1.2 0.70 0.17 0.01 2.74 2.36 5.40 Lost Swamp sediments were deposited on 1.0 0.86 0.27 0.01 2.79 1.98 5.08 Qt-3 as soon as Lone Pine Creek 0.7 0.81 0.21 0.01 2.86 2.36 5.54 abandoned the surface; five l4C dates in 0.5 0.72 0.21 0.01 2.54 2.26 5.08 basal clays of Lost Swamp are used to infer the age of the surface. Ut. 0.72 0.22 0.01 2.46 2.20 4.93 34°16'43'N; Long. 117°28'25"W.

0.63 0.17 0.004 2.41 2.16 4.84 0.56 0.13 0.003 2.42 1.85 4.54

1.8 0.81 0.23 0.01 3.94 1.07 5.45 Lone Pine Canyon (Pelona Schist debris). 0.9 0.79 0.26 0.01 3.04 2.06 5.44 Minor cut into Qoa-c that appears to be 0.5 0.87 0.26 0.01 2.85 2.83 6.00 offset by the San Andreas fault almost as 0.85 0.26 0.01 2.88 2.15 5.35 much as Qt-1. Absolute age limits are 0.45 0.20 0.01 2.59 2.00 4.88 based on the ages of the higher and lower 0.44 0.18 0.004 2.77 1.52 4.60 Qt-1 and Qt-3. Ut. 34°16'6"N; Long. U7°27'58"W.

Lone Pine Canyon (Pelona Schist debris). 1.6 1.00 0.27 2.07 2.43 4.67 Six C dates in the Qoa-c deposit permit 0.6 0.89 0.31 1.78 2.27 4.20 estimate of surface age based on rate of 0.4 0.95 0.34 1.76 2.80 4.71 fill. C dates in younger units are con- 0.2 1.08 0.41 1.72 2.82 4.68 sistent with extrapolated age; age is 0.2 1.26 0.47 1.55 2.67 4.35 consistent with offset on San Andreas fault UL 34°16'48"N; 0.3 1.32 0.43 1.52 3.05 4.70 Long. 117028'19"W. 0.4 1.20 0.43 1.76 2.61 4.52 0.2 1.09 0.43 1.60 2.41 4.14 0.2 1.12 039 1.61 2.73 4.48

Freeway cut at San Andreas fault (granitic 1.5 1.50 0.36 2.00 6.25 and gneissic debris). Age based on 1.3- to 0.4 2.19 0.53 1.03 7.10 1.4-km offset by San Andreas fault, using 2.18 0.58 1.13 slip rate of 24.5 ± 3.5 mm/yr determined 1.50 0.46 1.52 from younger deposits. Similar 0.73-Ma 0.75 0.22 1.74 slip rate justifies extrapolation of the rate 0.55 0.18 1.65 to older deposits. Lat. 34°15'42"N; 0.37 0.16 1.63 Long. 117°26'51*W.

0.5 1.42 0.25 0.344 Summit Pass (mixed Pelona + granitic 0.4 0.63 0.10 0.133 debris). Unit is incised into Qoa-N (Fig. 1.27 0.12 0.140 2) that contains the Brunhes-Matuyama 0.48 0.07 0.167 polarity reversal (time scale of Harlind 0.88 0.12 and others, 1982). Age and offset of 0.70 0.12 Qoa-N yield consistent slip rates on 0.64 0.13 several faults. Lat. 34°19'I8"N; 0.49 0.16 Long. I17°25'54"W. 0.32 0.10 284 McFADDEN AND WELDON

14C dates obtained from the deposits into which however, soils developed on the surfaces of sodium pyrophosphate, wet-sieve separation of the terraces were cut and from overlying paludal Qoa-d and Qoa-e are included for comparison the sand and silt + clay fractions, and pipette and colluvial sediments. The amount of offset of with the younger soils. extraction for clay content. Organic material in the terraces across the San Andreas fault also The depositional-terrace deposits in the inner A, AC, and the upper part of the B horizons was can be used to estimate when particular surfaces gorge have yielded 18 radiocarbon ages from removed prior to particle size analysis by H2O were abandoned by the creek. Establishing the both within and, locally, at the top of them digestion. Organic-carbon content in these ho- ages of surfaces is crucial in determining rates of (Weldon, 1986). Rates of sedimentation esti- rizons was measured colorimetrically using a soil development, Available age control is usu- mated from radiocarbon ages in the fill can be Bausch and Lombe Spectronic 20 spectropho- ally obtained from deposits underlying the sur- extrapolated to estimate the age of the top of the tometer,2 after the method described by Metson face, and the time when stable surfaces became fill. Minimum ages of the terrace surfaces can and others (1979). Soil pH was measured in

established on deposits and the soils began to also be determined on the basis of the age of 1:10 soiil-to-water ratio in 0.01 M CaCl2. form can be only roughly estimated. sediments subsequently deposited on the cut ter- In well-drained, oxidizing soil environments, The modern Cajon Creek is the result of cap- races. For example, Qt-3 is dated on the basis of ferrous iron is progressively converted to essen- ture of an older drainage system in the central 14C tially insoluble ferric-iron oxides that accumu- Transverse Ranges followed by rapid incision ages obtained from sediments of Lost late in increasingly older and typically redder into Cenozoic sediments. Streams of the older Swamp (Fig. 2), as described in more detail in soils (Schwertmann and Taylor, 1977). Changes system flowed north, toward the western Mo- Weldon and Sieh (1985) and Weldon (1986). in soil iron oxide content and composition are jave Desert. The beginning of capture, deter- After a terrace surface was abandoned by Cajon closely related to degree and nature of soil de- mined on the basis of fossils, paleomagnetic Creek, offset across the San Andreas fault began velopment. Several methods, thus, were used to data, and the offset of key units by faults with to accumulate. Given the slip rate of the fault evaluate soil iron. Total soil iron (represented as relatively well constrained slip rates, was just (Weldon and Sieh, 1985), the offset of a terrace Fe203T) was extracted by using the hydrofluor- after 0.73 Ma (Weldon and others, 1981; Wel- by the fault provides an accurate way of estimat- ic, nitric, and perchloric acid digestion method don, 1986). Approximately 500 m of incision ing when the terrace was actually abandoned (Husler, 1969). Hydrofluoric and sulfuric acid has subsequently occurred in the central part of and when soil formation was initiated. After digestion, followed by potassium dichromate ti- the drainage (Fig. 2). abandonment of a terrace, rapid incision tended tration, was used to determine ferrous iron to isolate the broad, low-gradient terrace and The oldest deposit discussed herein, Qoa-e, is (FeOT) (Kolthoff and Sandell, 1961). The dif- prevented significant degradation of, or deposi- of middle Pleistccene age (0.5 Ma) and was ference between FeOT and Fe203T determines tion on, the terrace. formed as the result of a major period of aggra- the ferric component (Fe203) of total soil iron. Locally, soil-stratigraphic data show that dation during the early stages of incision of the Comparisons of Fe203, FeOT, and Fe2C>3T some of the terraces had been subjected to pre- creek. The deposit is dated by its position, early data among soil horizons allow us to estimate viously unrecognized erosion or colluviation, in the downcutting that began just after a 0.73- the degree of chemical alteration and relative chiefly at sites near hillslopes. Only soils from Ma magnetic polarity reversal (time scale of losses or gains in iron content due to soil forma- the geomorphically most stable sites were se- Harland and others, 1982) and by its offset of tion. Extraction of total ferric iron present in lected for detailed textural and chemical analysis ~ 1 km by the Cleghorn fault, which has a slip oxyhydroxide phases such as hematite and goe- (Table 1). Morphological data from these soils rate of 2 mm/yr (Weldon and others, 1981). thite (Fe203d) was accomplished by using the and less detailed data for the other soils are in- The 0.73-Ma magnetic polarity reversal occurs dithionite-citrate-bicarbonate procedure of cluded in Tables A and B,1 which are on file in a deposit (Qoa-N) that has been correlated Mehra and Jackson (1960). Ferric iron present with the Geological Society of America Data with the older alluvium of Noble (1954) and in poorly crystalline oxyhydroxide phases Repository. that is the youngest deposit predating downcut- (chiefly ferrihydrite) and organic complexes (Fe20 o) was extracted by using the oxalate ting of Cajon Creek. The next youngest major FIELD AND LABORATORY 3 extraction method of McKeague and Day datable deposit, Qoa-d, is of late Pleistocene age METHODS (-55,000 yr old). It is dated by its 1.3-km offset (1966). Because magnetite, which varies in these across the San Andreas fault, which has a slip Soil profiles from hand-dug pits were de- soils between 0.19 and 0.38 wt%, is slightly sol- rate of 25 mm/yr (Weldon and Sieh, 1985). scribed wherever possible. Because the depth of uble under the conditions of oxalate extraction Formation of Qoa-d was the result of as much weathering that is associated with many soils (Rhoton and others, 1981; Walker, 1983), it as 85 m of aggradation. Reincision into this exceeds 2 to 3 m, the deepest soil horizons de- was removed prior to Fe203o extraction by massive deposit created the "inner gorge" of scribed were in stream cuts or in road cuts, and using a strong magnet. Iron present in organic Cajon Creek (Fij;. 2). Within the inner gorge, the upper horizons were matched with horizons complexes (Fe203p) was extracted from se- there are a latest Pleistocene depositional terrace in pits. Soil profiles were described and sampled lected soils by using the method of McKeague (Qoa-c) and a late Holocene depositional ter- primarily according to the procedure and termi- (1967). Determination of Fe203d, Fe2030, and race (Qoa-a). A total of seven terraces have been nology of the Soil Survey Staff (1951, 1975). Fe203p provides additional data for evaluating recognized in the inner gorge (Fig. 2, Qt-l-Qt- Particle size distribution was determined by the magnitude of chemical alteration of iron- 7), two on the surfaces of the depositional ter- clay dispersal of the <2-mm fraction in 10% bearing minerals and of gains in iron oxides due races Qoa-c and Qoa-a and five erosional terraces formed during latest Pleistocene and Holocene time. This study focuses on the soils 'Tables A and B may be obtained free of charge by 2 Use of trade names in this paper is for descriptive formed on the dated terraces of the inner gorge; requesting Supplementary Data 87-07 from the GSA purpose:! only and does not constitute endorsement by Documents Secretary. the U.S. Geological Survey. SOIL DEVELOPMENT ON QUATERNARY TERRACES, CALIFORNIA 285

A SILT (%)

Figure 3. Estimated increase in pedo- 10 0 ,_ 10 10 o 10 20 10 20 ml I I L_ genic silt content (A silt %) in soils formed on terraces of Cajon Creek, showing initially E 20- u ? rapid accumulation of silt in Holocene soils. A T 40 silt % determination is based on maximum silt 0- content of the unaltered or least altered C Lil 60 Q subhorizon. Minimum increase in silt % considered pedogenic in origin = +3%. 40 yrs. B.P. 275 yrs. B.P 5900 yrs. B.P. 7100 yrs. B.P. 8300 yrs. B.P.

A SILT (%)

0 10 , 20 , 3,0 o with a soil profile that formed on an -2,000-yr- 20 old fan deposit near the study area is 80 cm thick (profile RW-16, Table A). The uppermost I 40 soil horizons contain large amounts of silt and X (- 60- organic carbon (Table 1, Fig. 3). The silt occurs a. HI mainly in the soil matrix, but some silt and or- Q 80 ganic matter are also present as thin coatings on stones. The color B horizon is present below the

11,500 yrs. B.P. 12,400 yrs. B.P. transitional AB or BA horizon. Significant silt has accumulated in the B horizon (Table 1, Fig. 3), some of which is present as coatings of silt on to additions by other processes. Extracted iron SOIL DEVELOPMENT ON skeletal grains. If conventional techniques of particle size analysis are used, very little clay is (Fe2C>3T, Fe2C>3d, Fe203o, Fe203p) was mea- HOLOCENE TERRACES sured by using a Perkin and Elmer 303 atomic detectable in the B horizon; however, micro- absorption spectrometer. Morphology, Chemistry, and Mineralogy morphologic evidence shows that a very small In order to determine the absolute amount of amount of clay is probably present, occurring as increase of an iron component in a given soil, The initially developed and most prominent colloidal stains on skeletal grains. In marked the amount of the iron component in the unal- soil horizon that occurs on late Holocene geo- contrast to stones in late Holocene soils, some of tered parent materials must be determined. On morphic surfaces is the darkened A horizon the schistose and many of the coarse-grained middle and late Holocene terraces, unaltered (Table 1). This horizon reflects the rapid estab- plutonic stones in middle and early Holocene parent materials are present, although mottling lishment of the dense chaparral vegetation soils have been weathered to a grus or near-grus of the matrix and partly grussified stones in community on abandoned flood plains. In late state. some cases are present and demonstrate that Holocene soils on Qt-6, a thin (7 to 32 cm), Holocene soils acquire slightly to moderately minor chemical alteration has taken place lo- slightly reddened horizon (Cox) is always pres- acidic pH values. The pH is lowest in the A cally. Unaltered parent materials of early Holo- ent below the transitional AC horizon. Redden- horizon and increases significantly with depth cene terraces are not present in the upper several ing is due partly to ferric oxide stains but is also (Table 1). metres, and completely fresh parent materials of due partly to reddish particles of silt that coat The maximum content of organic carbon oc- latest Pleistocene terraces are not present in the larger skeletal grain surfaces. The presence of curs in the A horizon. In the middle Holocene upper several metres. Completely fresh parent such silt coatings (siltans) indicates downward soils, significant amounts of organic carbon are materials of late to middle Pleistocene terraces translocation of silt. Stones in the late Holocene also present in transitional horizons of the upper probably no longer exist. As there are few sedi- soils are not visibly weathered, as indicated by part of the Bw or Bt horizon. The maximum mentological differences among different fluvial the sharp ring from a hammer blow, the absence total organic carbon (profile carbon) occurs in deposits, parent material characteristics for early of weathering rinds, and the presence of smooth, middle Holocene soils (Fig. 4). Holocene and Pleistocene soils in the study area stream-worn surfaces. Significant quantities of iron oxides have ac- can be estimated on the basis of parent material Soils on middle and early Holocene surfaces cumulated in Holocene soils (Table 1). Gains in data for younger Holocene soils. The gain in an (Qt-5, Qt-4, Qt-3) possess even thicker, darker Fe203d and Fe2C>3T content are most pro- iron component for the entire soil (profile con- epipedons (mollic A horizons) and reddened, nounced initially in the A horizon, but increas- tent) is calculated by summing the net increases silt-enriched, color B horizons (one type of Bw ing amounts of these constituents occur in the of the component (weight percent of iron com- horizon) (Tables 1 and A3). The thickness of the Bw horizons of early Holocene soils (Table 1). ponent in a measured horizon minus that in the A horizon generally ranges from 26 to 35 cm, In the soil on the Qt-5 surface (RW-12), the parent material x horizon thickness) of all hori- although a very thick mollic horizon associated FeOT and Fe2C>30 content and the Fe2C>3d and zons above the shallowest unoxidized or least Fe203T content of the Cu/Cox horizon are uni- oxidized horizon. 3See footnote 1. formly higher than those of the unaltered parent 286 McFADDEN AND WELDON

that eolian processes can contribute silt, clay, and calcium carbonate to soils. Other studies have demonstrated that the source of dust can be either local (Lattman, 1973; Peterson, 1980; — Organic Carbon McFadden and others, 1986) or thousands of — Epipedon Thickness kilometres distant (Prospero and others, 1981; P6we and others, 1981; Muhs, 1983). The Mo- jave Desert, located seasonally upwind of the study area, probably serves as a primary source of eolian dust in the Cajon Creek area. Evidence for eolian activity in the area includes the presence of ridge dunes along the northern edge of Cajon Pass and the extensive eolian deposits in the San Bernardino Valley to the south. Changes in the iron oxide content and composition of soils on Holocene terraces as depth and soil age increase also indicate that many of Time (years the accumulated iron oxides have been derived primarily by incorporation of eolian materials Figure 4. Changes in profile organic carbon content and epipedon thickness as a function rather than by chemical alteration. For example, of soil age in the Cajon Pass area. despite the well-drained, oxidizing, and acidic environment of Holocene soils, ferrous-iron content and the Fe0T/Fe203 ratio decrease with materials of other Holocene terraces or in the tents indicate that very little of the ferric iron in depth (Fig. 5). Furthermore, the very minor in- least altered materials at the base of Qoa-c ter- Holocene soils is organically complexed and creases in ferrihydrite relative to significant ac- race fill. This increase indicates that iron en- that content of organically complexed iron does cumulations of more crystalline ferric-iron richment is associated with stratigraphic varia- not increase with soil age (Table 1). Increases in oxides in Holocene soils also suggest that little tion or possibly with ground-water alteration Fe2030 content are thus attributable largely to chemical alteration of ferrous-iron to secondary and is not attributable to pedogenesis. The in- the accumulation of ferrihydrite. The presence ferric-iron oxides has occurred. This pattern of crease in Fe2C>3d and Fe2C>3T due to soil devel- of at least some organically bound iron, in asso- iron accumulation in Holocene soils is therefore opment thus is probably much greater than is ciation with the gains in organic-carbon content, attributed to incorporation of iron-bearing eo- apparent. The trends of ferric-iron (Fe2C>3) ac- however, is consistent with the edaphic envi- lian dusl: at a rate that exceeds the rate of chemi- cumulation in Holocene soils are similar to the ronment (extensive adsorption of organic com- cal alteration of ferrous-iron to secondary trends observed for Fe203d and Fe203T, except plexes) proposed by Schwertmann and Taylor ferric-iron oxides. Ferrous iron, present in ferro- for a usually low Fe2C>3 content in the A ho- (1977) that favors formation and prolonged sta- magnesian minerals, magnetite, and various oth- rizon (Table 1). The data also show that part of bility of ferrihydrite. er iron oxides, is a moderately abundant con- the accumulated Fe2C>3 is due to causes other stituent of eolian dust (Pewe and others, 1981). The relative depletion of ferrihydrite and the than simple increases in ferric iron present as Influence of Eolian Dust Influx ferric oxyhydroxides. For example, the increase low Fe203o/Fe203d ratio of the uppermost A horizons of Holocene soils also suggest that the in Fe2C>3 percentage in the 2Bwl horizon of the Two of the most important aspects in soil eolian dust transported through Cajon Pass has soil on the Qt-3 surface (RW-13) relative to the development during the late and middle Holo- less ferrihydrite than do the soil parent materials. Fe2C>3 percentage in the soil parent material is cene are the initially rapid accumulations of silt ~ 1.0%, compared i:o an Fe2C>3d increase of only and of organic matter (Figs. 3 and 4, Table 1). Statistical analysis of iron content and silt 0.3% to 0.4%. Gains in Fe203 thus might be Field observations, micromorphic studies, and content of the soil supports the hypothesis of partly due to addition of minerals that initially laboratory analyses indicate that almost no eolian iron addition. Use of the nonparametric have ferric iron, such as biotite or some clay chemical and physical weathering of the parent Spearman rank correlation test shows that the minerals. Changes in FeOT content are gener- materials of soils has occurred on late and mid- secondary iron oxide content and the silt content ally similar to changes in Fe203d and Fe203T dle Holocene terraces. Field observations also of Holocene soil horizons in the study area (n = content in Holocene soils, although the magni- show that grussification of boulders occurs in 30) are positively correlated; correlation is sig- tude of the increase of FeOT below the A ho- place and produces little matrix that is finer than nificant at the 99.5% level of confidence (a rizon relative to FeOT in soil parent materials is sand. These observations indicate that much of = 0.05). Ordinary least-squares regression anal- usually much smaller than that for associated the silt accumulating in Holocene soils is con- ysis strongly indicates a linear relation between increases in Fe20;,d, Fe203, and Fe203T con- tributed as eolian dust. Eolian processes are con- these two components. tent (Table 1). Increases in Fe2O30 content are sistent with the abundant coatings of silt present small compared to those observed in other iron on skeletal grains and stones, an association that Fe203d % = 0.01 silt % + 0.43, r = 0.83 components (Table 1). As in the case of Fe203, indicates downward translocation of silt from such gains typically occur in the B horizon the surface horizons. Similarly, analysis of ferrous-iron and silt con- rather than in the A horizon. Low Fe203P con- Many previous studies have demonstrated tent, using the Spearman test, shows that these SOIL DEVELOPMENT ON QUATERNARY TERRACES, CALIFORNIA 287

FeOT/ Fe203 mottled and bleached appearance suggests that 1.0 2.0 3.0 4.0 5.0 for unknown reasons, the soil was subjected to reducing conditions after formation of much of = J the argillic horizon had occurred. Chemical and .J mineralogical evidence do indicate some net loss 25 I of Fe203d from this horizon (Table 1). The argillic B horizon is at least 3.35 m thick, and E LEGEND exposures in road cuts suggest that the horizon - 275 yr. B. P may actually be considerably thicker. Weakly ftl 275 yr. B.F? altered parent materials that exhibit colors Q_ 5,900 yr. BP corresponding to Cox horizon colors of younger 0>75 O 7,100 yr B P soils were observed only at a depth of 15 m. 8,300 yr. B. P The maximum Fe203d, Fe2030, and Fe203 contents of latest Pleistocene soils, in contrast to 100 11,500 yr. B P 12,400 yr. BP most Holocene soils, occur in the argillic B horizon and not in the A horizon (Table 1). Profile Fe203d and Fe 0 o contents are also much 125 2 3 greater than in Holocene soils. Maximum FeOT Figure 5. Changes with depth in the ratio of total ferrous iron to total ferric iron (FeOT/ content, however, as in the case of Holocene Fe203) for Holocene and latest Pleistocene soils in the Cajon Pass area. The ratio increases in soils, occurs in the A horizon and decreases rap- the A, AC, or B horizon relative to the unaltered or least altered C horizon in all soils. idly with depth, although the minimum FeOT content of the soil on the Qt-1 surface occurs in the B horizon and not in the C horizon. In most two components are positively correlated (a = SOIL DEVELOPMENT ON respects, changes in Fe203p content and soil pH 0.025) in most Holocene soils. Simple linear PLEISTOCENE TERRACES of latest Pleistocene soils resemble those recog- regression analysis of these two components also nized in middle and early Holocene soils. Morphology, Chemistry, and Mineralogy indicates correlation of the two components. The maximum Fe203d and Fe203 contents Soils on the Pleistocene terraces are morpho- of late and middle Pleistocene soils, as in the A FeO % = 0.04 A silt % + 0.04, r = 0.57 logically much better developed than are Holo- case of latest Pleistocene soils, occur in the argil- cene soils, possessing argillic B horizons that lic horizon (Table 1). In the late Pleistocene soil Logarithmic transformation of silt content im- become progressively thicker, redder, and more on Qoa-d, maximum Fe 0 o content also oc- proves the degree of correlation. 2 3 clay rich with increasing age (Table 1). A signif- curs in the argillic horizon, the minimum FeOT A FeO % = 0.16 In A silt % -3.66, r = 0.79 icant amount of the clay in the argillic horizon is occurs in the argillic horizon, and Fe203p con- illuvial, occurring as coatings which thicken in- tent is uniformly low (Table 1). In the middle Statistical analysis indicates functional depend- creasingly with time on the grains or peds, as Pleistocene soil on Qoa-e, the minimum FeOT ence of the ferrous-iron and ferric-iron oxide pore-filling material, or as bridges. In late and content also occurs in the argillic horizon, al- content on the silt content, a relation consistent middle Pleistocene soils (RW-11, RW-14), though in contrast to younger Pleistocene soils, with addition of iron-bearing silt to Holocene stones are rare or have been completely weath- the Fe2030 content is also very low in the argil- soils. ered to a grus-like state. The epipedon thick- lic horizon. Slight reddening of subhorizons in Holocene ness and organic matter content of the late soils demonstrates that at least some authigenic Pleistocene soil are less than the epipedon thick- Formation of Authigenic Ferric-Iron Oxides ferric-iron oxides have accumulated because ness and profile organic matter content of most reddened soils require formation of minerals Holocene soils (Fig. 4). Exposures of the depos- Several aspects of soil development on in- such as hematite and ferrihydrite (Schwertmann its underlying Pleistocene surfaces show that the creasingly older Pleistocene terraces contrast and Taylor, 1977; Childs and others, 1979; Cox horizon extends to depths of at least 5 m. significantly with those of Holocene soil devel- Schwertmann and others, 1982). The relatively Alteration to depths of 20 to 30 m is indicated opment: an argillic B horizon appears, the A low Fe203 content of the uppermost A horizon by slight reddening of the matrix or the presence horizon declines, and the ferric oxides and clay is consistent with chemical alteration and of locally reddish mottles. accumulate, whereas the ferrous iron in the argil- downward translocation of ferric iron. As these The oldest surface (Qoa-e) is generally heav- lic horizon becomes depleted. The pattern of and others studies (Walker, 1967) have shown, ily dissected. We described the soil on the most iron oxide formation in Pleistocene soils strongly however, very little authigenic iron oxide ac- stable remnant of the surface at a site where the indicates that the increasingly larger quantities of cumulation is required to make soils or sedi- soil is buried by ~ 1 m of eolian sand that has a ferric-iron oxides in the argillic horizon are due ments considerably redder than the initial color weakly developed soil. Although we cannot be to chemical alteration and formation of authi- of the unaltered materials. For example, in- absolutely confident that no stripping of the ar- genic ferric-iron oxide minerals rather than to creases of 0.02% to 0.07% ferrihydrite appear to gillic horizon has occurred, the argillic B horizon incorporation of iron-bearing minerals present be sufficient to change the initial olive-gray color is characterized by the maximal reddish color, in eolian dust. of the parent material to a light brownish-gray clay content, and structure observed in the study Evidence of in situ chemical alteration is pro- or yellowish-brown color (Table 1). area. In part of the argillic horizon (2Bt3), vided by systematic changes in the Fe203d/ 288 McFADDEN AND WELDON

Fe 0 d / Fe 0 T 2 3 2 3 Changes in the Fe203o/Fe203d ratio also Fe0T/Fe203 0.1 0.2 0 3 0.4 0.5 provide evidence for formation of authigenic 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 IT j ferric-iron oxides in soils on Pleistocene terraces. r~ hp Í LEGEND Increases in profile Fe203o content in soils on 25 — 55,000 yrs. B P 25 increasingly older Pleistocene terraces (Fig. 8) 500,000 yrs. B P 50 II indicate that part of the depleted ferrous iron can .Composition of I least altered be attributed to alteration of ferrous-iron-bear- 75 50 J ! ing minerals to ferrihydrite, causing an increase horizon 100 in the profile Fe203o/Fe203d ratio. Ferrihy- I 75 drite, however, is a metastable mineral and §"125 eventually transforms to more stable minerals,

Q. primarily hematite (Schwertmann and Taylor, 150 I a> 100 O ¡ LEGEND 1977). The value of the profile Fe203o/Fe203d "1 275 yr. B P ratio thus cannot possibly continue to increase I500 5,900 yr. B P 125 with soil age; after some duration of soil devel- 8,350 yr.BP opment, the ratio should achieve a maximum 1525 II,500 yr.B.P value and decline thereafter in soil-forming en- 12,400 yr.B.P 150 vironments that ultimately cause transformation Figure 7. Changes with depth in the ratio 55,000 yr. B P of ferrihydrite to more crystalline hematite 500,000 yr. B.P of total ferrous iron to total ferric iron (McFadden and Hendricks, 1985). In the Cajon 175 (Fe0T/Fe203) for late and middle Pleisto- Creek chronosequence, the ratio is greatest dur- cene soils in the Cajon Pass area. Major de- Figure 6. Changes with depth in ratio of ing the latest Pleistocene and decreases after that crease in the ratio occurs in the maximal part total iron oxyhydroxides to total iron despite continuing formation of ferrihydrite of the argillic horizon. Arrows show this ratio (Fig. 8). (Fe203d/Fe203T) in soils of the Cajon Pass calculated on basis of data for least altered

area. Significant increase in this ratio occurs The net depletion of Fe203o in the argillic horizon of each soil; deposits of the terraces in the argillic horizon of latest Pleistocene horizon of the middle Pleistocene soil (Table 1, on which the soils occur have been altered to and older soils. Fig. 8) results in a negative profile ratio of a depth exceeding 20 m.

Fe203o/Fe203d and indicates that nearly all accumulated ferrihydrite has been converted to a more crystalline iron oxide. The increasingly

Fe203T, Fe0T/Fe203, and Fe203o/Fe203d bright color of these soils indicates that this min- supports the chemical evidence for the forma- ratios. Increases in the Fe203d/Fe203T ratio eral is very likely hematite. This conversion tion of a greater relative abundance or propor- (especially in B horizons) are closely related to probably accounts for the strong relation be- tion of ferric-iron oxides in Pleistocene soils an increasing degree of primary-mineral altera- tween soil color and Fe203d content of soils in discussed above. tion, soil development, and soil age (Rebertus the Cajon Creek area (Table 1). The increasing The growth of the argillic horizon that is in- and Buol, 1985), In Holocene soils of Cajon abundance of pedogenic hematite with increas- creasingly rich in authigenic iron oxides is ac- Creek, only slighl. increases in the ratio are ob- ing soil age has been documented in many stud- companied by thinning of the A horizon. This served (Fig. 6). In increasingly older Pleistocene ies (Childs and others, 1979; Torrent and others, process cannot be attributed to erosion or strip- soils, however, this ratio increases significantly 1980; Schwertmann and others, 1982) and ap- ping of a once thicker A horizon. Latest Pleisto- in the B horizon and attains maximum values in parently accounts for the increasingly bright red cene terraces that are completely isolated from the middle Pleistocene soil. The progressive in- colors of such soils. Using Hurst's (1977) redness fluvial flow exhibit little evidence of erosion. crease in the Fe203d/Fe203T ratio shows that index, R = (Hue * Value)/chroma, McFadden The remnants of the original bar-and-swale to- an increasingly larger proportion of ferrous iron and Hendricks (1985) demonstrated that the pography exposed in cliffs and in our pits and in primary minerals of parent materials or in linear relation between R and Fe203d percent- the lack of a mechanism to move large boulders, incorporated eolian dust has been converted to age is statistically significant for early Holocene in some cases exceeding 2 m in intermediate authigenic ferric-iron oxides. This conversion is and Pleistocene soils throughout the Transverse diameter, off the wide, undissected terraces consistent with i:he progressively marked de- Ranges. Statistical analysis of R (modified to clearly indicate that the surface has not been creases in the Fe0T/Fe203 ratio in subhorizons account for 5Y and 2.5Y hues), as well as of stripped. Surficial erosion of the finer material of the argillic horizon compared to that ratio in another index of reddening and brightening would progressively expose boulders; instead, the parent material of Pleistocene soils (Figs. 5 (rubification) defined by Harden (1982), and these boulders are gradually being buried. Field and 7). Increases in the Fe0T/Fe203 ratio in Fe203d content using both nonparametric observations show that the bar-and-swale topog- the A or uppermost B horizons of Pleistocene (Spearman rank correlation) and ordinary least raphy of a recently abandoned terrace is soils indicate continuing incorporation of ferrous squares regression tests shows that soil reddening gradually eliminated, primarily by infilling of iron in eolian dust; however, the rate of chemi- correlates very strongly with Fe203d in early swales with eolian material and with material cal alteration of ferrous to ferric iron in the max- Holocene and older soils (minimum a = 0.025; off the bars by sheetwash and bioturbation. imal argillic horizon exceeds the rate at which range in value of r = 0.76 to 0.92). The strong These processes produce smooth surfaces on ter- dust-derived ferrous iron is added. correlation of soil color and Fe203d content races as young as the early Holocene. Locally, SOIL DEVELOPMENT ON QUATERNARY TERRACES, CALIFORNIA 289

0.50r

o "O ro ro ovs <_> O CM CM £ £ 0 < < W - 0.25 Profile ratio

-0.50 • 500

400 Figure 8. Changes in Harden (1982) soil index, profile ferric-iron 400 - oxyhydroxides (Fe203d), profile poorly crystalline ferric-iron oxyhydroxides (Fe203o), and profile Fe2C>30/Fe2O3d ratio as a function of E soil age in the Cajon Pass area. 0 LEGEND 300 1 300 3 Q. — Ee203o , Profile CD ro X O •Fe203d , Profile 200

CL 100 - 100

100 1,000 10,000 100,000 Time ( years) TABLE 2. SOIL PROFILE INDICES AND THEIR WEIGHTED MEANS FOR HOLOCENE AND PLEISTOCENE SOILS IN THE CAJON PASS AREA, SOUTHERN CALIFORNIA

Soil Age Profile Weighted the relatively thick, silt-rich and nongravelly A logic development of a given soil can be quanti- profile (yrB.P.) index* mean horizons of early Holocene and latest Pleisto- fied by using an index derived by Harden (1982) cene soils are probably the result of cumulative that combines horizon thickness and other field RW-9 1947 2.97 0.06 RW-10 275 5.49 0.11 soil development in former swales. properties. The value of this index has been •385 The declining thickness of the A horizon must shown to increase systematically with soil age in -75 be attributed to upward thickening of the B ho- many chronosequences in diverse climates and RW-18 275 6.93 0.10 +385 rizon and to a concomitant increase in the mag- parent materials (Harden and Taylor, 1983; -75 nitude of oxidation of organic matter at the McFadden and others, 1986; Ponti, 1985). The RW-12 5900 13.65 0.14 expense of the A horizon (McFadden and Hen- soil profile indices of selected soils in the study ±900 RW-15 7150 14.60 0.15 dricks, 1985). Increases in water-holding capac- area (Table 2) correlate strongly with soil age (a ±1200 ity and decreases in infiltration rates favor an = 0.005, Spearman rank correlation). Ordinary RW-13 8350 13.04 0.13 increase in the magnitude of oxidation of soil least-squares regression analysis of the indices +900 -500 organic matter, particularly during hot, dry and soil age yields the following equations. RW-6 11,500 19.12 0.19 summer months. Decreases in permeability of (1) Profile index = 0.001 + 12.11 (terrace age), r +2000 the soil also may inhibit mechanical transloca- = 0.998, and (2) log (weighted mean) = 0.093 - -3000 RW-17 12,400 20.11 0.17 tion of large fragments of undecayed to partly 0.15 (terrace age), r = 0.957 (weighted mean is ±1000 decayed organic matter that readily accumulate the profile index/profile thickness). Similarly, RW-ll 55,000 50.97 0.24 in loose, gravelly Holocene soils. profile Fe203d (pF) is also strongly correlated ±12,000 with soil age (a = 0.005, Spearman rank correla- RW-14 500,000 277.99 0.40 ±200,000 RATES OF SOIL DEVELOPMENT tion). The rate of pF increase with age, estimated by using ordinary least-squares regression Note: parent-material properties for soils that are rich in Pelona Schist: dry color, 5Y 6/2; moist color, 5Y 4/2; texture = gravelly sand; structure = single Soil Morphology and Iron Oxide Content analysis, is (1) pF = 121.2 log (terrace age) - grain; dry consistence = loose; wet consistence - nonsticky and nonplastic; clay 378.9, r = 0.80, and (2) log (pF) = 0.62 log films = none; pH = 7.0 Parent-material properties for soils that have little Pelona Schist are the same except for the dry color, 10YR 6/3, and moist color, 10YR (terrace age) - 0.92, r = 0.94. In order to test 4/3. Maximum values for parameters used to calculate the profile index for Two soil characteristics that have systemati- soils in this study were calculated on the basis of morphologic data for soils cally increased during the past 0.5 m.y. in the further the use of these soil parameters to esti- reported by Harden (1982) except for soils formed in parent materials of Pelona mate terrace age, several soils must be analyzed Schist, in which maximum value of rubification equals 220 points. Cajon Creek area are soil morphologic devel- •Soil profile index (Harden, 1982) determined for 100-cm depth except for opment and ferric-iron oxide content (Figs. 6,7, on each terrace, thereby permitting determina- RW-I7 (110 cm), RW-I1 (200 cm), and RW-14 (701 cm), calculated on the basis of properties indicated in note. and 8; Table 1). The over-all degree of morpho- tion of the degree of variability of a given pa- 290 McFADDEN AND WELDON

A CLAY (%)

5 10 5 10 0 5 10 15 20 25 1 1 I 1 L

Figure 9. Changes with depth in pedogenic clay content (A clay %) in soils formed on Holocene and late Pleistocene terraces of Cajon Creek. A clay % determinations on the basis olf maximum clay content of least altered C subhorizon.

filled with silt and organic matter. For example, an increase in the silt content from 1% to 18%, with no change in clay content, theoretically in- 7100 yrs. B.P. 8300 yrs. B.P. 11,500 yrs. B.P. 12.400 yrs. B.P. 55.000 yrs. B.P. creases available water-holding capacity (AWC) (Birkeland, 1984) by as much as 35% and con- comitantly lowers infiltration rates. Increasing rameter for a given terrace. On the basis of data triggered by this climatic change (Weldon, the silt content to 28% further increases AWC for the Cajon Creek chronosequence, we con- 1983,1986); thus, soil development on the latest by another 37%. The concomitant accumulation clude that soil morphology and iron oxide con- Pleistocene surfaces has occurred almost entirely of organic matter probably also increases AWC; tent are potentially excellent indicators of during the Holocene. thus, AWC is potentially doubled by the ac- absolute age of Quaternary deposits in much of Even in the unlikely case that the climate did cumulations of silt and organic matter measured the Transverse Ranges over a span of 0.5 m.y. not change until the Holocene, soils on the latest in the Cajon Pass area. Pleistocene erraces could not have developed Latest Pleistocene to early Holocene eolian Clay Content an argillic horizon in coarse porous gTavels dur- influx rates may have been greater than subse- ing the 1,500 to 2,400 yr of Pleistocene climate quent influx rates, or eolian dust during the latest Pedogenic clay content also generally in- that the soils experienced. The climatic changes Pleistocene to early Holocene might have con- creases with soil age in the Cajon Pass area, a during the Holocene certainly have not been tained more clay than did the subsequent eolian feature noted elsewhere in the Transverse nearly as significant as the Pleistocene-to- dust. Either of these factors would accelerate the Ranges (Keller and others, 1982) and in many Holocene change; therefore, the rapid develop- rate of argillic horizon development, although other areas (for example, Bockheim, 1980; Gile ment of the argillic horizon that is observed in an initial period of accumulation of material that and others, 1981; Marchand and Allwardt, the study area cannot be attributed to climatic reduces soil permeability still would probably be 1981; Guccione, 1985; McFadden and Bull, change from relatively moist conditions, favor- required. Therefore, the formation of the argillic 1987). In contrast to these studies, however, clay ing rapid rates of chemical alteration, to drier horizon on the latest Pleistocene terraces cer- content increased very little during the first conditions, presumably favoring much lower tainly took place during the late Holocene, after 8,300 yr of soil development (Fig. 9). A rela- rates. a system that could hold the clay and produce tively sudden increase in the rate of clay accu- We attribute the relatively sudden appearance higher AWC had evolved. Older Pleistocene mulation during the subsequent 3,000 yr of soil of the argillic horizon to the significant increase soils presumably have also passed through this development is required to create the argillic ho- in silt content and its impact on soil permeability threshold. rizon of latest Pleistocene soils. and water balance. Although eolian dust con- It is difficult to distinguish clay produced by Such an apparent change in the rates or proc- tains clay, apparently little eolian clay is en- weathering from clay produced by addition of esses of soil development has often been trapped in gravelly sediments that have very eolian material in the Pleistocene argillic hori- attributed to the significant changes in climate high permeability. Most of the clay is translo- zons. Alteration of ferrous iron in fine-grained that have occurred during the Quaternary (Hunt cated out of the zone of A and B horizon devel- minerals, such as biotite in the parent materials, and Sokoloff, 1950; Morrison and Frye, 1965; opment, as shown by clay-bearing coatings on or in eolian dust certainly may result in in situ Yaalon, 1971; Gile and others, 1981; Chartres, stones at depths that exceed 5 m in Holocene formation of authigenic clay minerals. McFad- 1980). It is tempting to ascribe the presence of deposits and by the very low clay contents in den and Hendricks (1982), for example, re- increasing amount!! of ferric-iron oxides in argil- young soils. High soil permeability also de- ported that systematic increases in vermiculite lic B horizons of latest Pleistocene soils and the creases the time during which soil moisture is content occur in early Holocene to late Pleisto- lack of such horizons in early and middle Holo- retained and thereby limits the magnitude of cene soils on fluvial deposits throughout the cene soils to changes in climate and, by infer- chemical weathering. While silt and organic Transverse Ranges. Although vermiculite may ence, to changes in rates and processes of soil matter continued to accumulate in the upper be present in dust, the increasing abundance of development that occurred at the end of the profile, however, conditions favoring clay en- this mineral relative to other clay minerals is Pleistocene. Deposition of unit Qoa-c and its trapment and chemical alteration develop as the more likely caused by alteration of appropriate subsequent incision, however, was probably initially noncolloidal soil pores are gradually mafic minerals to vermiculite and iron oxides. SOIL DEVELOPMENT ON QUATERNARY TERRACES, CALIFORNIA 291

TABLE 3. PROFILE DATA FOR CHRONOSEQUENCE OF SOILS IN THE MERCED AREA, CALIFORNIA tion of clay, silt, and organic matter creates positive feedback that accelerates changes in soil Formation Surface age Profile Generalized Parent Profile Profile Weighted or (yr) horizon materia] depth index mean permeability and infiltration rates. Subsequent unit sequence (cm) soil development is progressively characterized by increasing accumulations of authigenic ferric- Post-Modesto 3,000 PM8 A/C fSL/SiL 76 13.10 0.17 iron oxide and clay due to chemical weathering Post-Modesto 3,000 PM16 A/Cox/C fSL/SL 236 16.28 0.07 under conditions of acidic pH and strong leach- Modesto, upper member 10,000 M31 A/AC/C fSL 254 19.89 0.08 ing. The transition from a permeable, noncolloi- Modesto, dal soil environment favoring dust incorporation upper member 10,000 M46 A/AC/Cox fSL 250 33.73 0.13 to a more strongly colloidal, less permeable sys- Modesto, 20,000 to lower member 70,000 M12 A/BI/B3/C SL 413 67.05 0.16 tem favoring chemical weathering occurs over a

Riverbank, relatively short period of time and constitutes an upper member 130,000 R9 A/B/B3/Cox SL/LS 400 115.85 0.29 extrinsic pedologic threshold. Soil thresholds, Riverbank, extrinsic or otherwise, have been recognized or upper member 130,000 R33 A/Bl/B3/Cox SL/LS 300 87.85 0.29 suggested in previous studies (for example, Turlock Lake 600,000 T6 A/Bt/BC SL 190 148.10 0.30 McFadden, 1981; Muhs, 1984; Birkeland, Turlock Lake 600,000 Til A/Bt/BC/Cox SL 500 148.78 0.78 1984) and are analogous to the geomorphic

Note: data from Harden and Marchand, 1977; Marchand and Allwardt, 1981; Harden, 1982; Harden and Taylor, 1983; and Harden, 1986. Profile index values thresholds described by Schumm (1973, 1977, determined on the basis of eight properties. *f, fine; LS, loamy sand; SL, sandy loam; SiL, silty loam. 1979) and Coates and Vitek (1980); in each situation, relatively constant processes produce sharp changes in the rate of formation of soil properties or analogous geomorphic parameters. Clay and Fe2C>3d content are correlated late and middle Pleistocene soils of the Trans- Comparison of morphologic data from the (Spearman rank correlation, linear regression), verse Ranges (McFadden and Hendricks, 1982). chronosequence of soils in the Merced area in especially in latest Pleistocene and the late Pleis- Because little kaolinite is present in soils the San Joaquin Valley of California (Table 3) tocene soils (minimum a = 0.01, range in value of throughout the Mojave Desert (McFadden, (Harden and Marchand, 1977; Marchand and r = 0.78 to 0.98). This relation has been ob- 1982; McFadden and others, 1986; McFadden Allwardt, 1981; Harden, 1982; Harden and Tay- served in soils formed on fluvial deposits else- and Bull, 1987), eolian dust derived from this lor, 1983; Harden, 1986) with data for the where in the Transverse Ranges (McFadden and region can probably supply little kaolinite to ter- Cajon Creek chronosequence (Tables 1 and 2) Hendricks, 1985) and suggests that processes re- races and fans of the Transverse Ranges, imply- permits an evaluation of the variables that may sulting in the accumulation of these two compo- ing a primarily authigenic origin for kaolinite in influence the timing or relative significance of a nents in soils are genetically related. Statistical the Cajon Pass area. pedologic threshold. The present climate of the analysis shows that silt and Fe2C>3d contents are Merced area (Mediterranean; mean annual pre- not correlated or are weakly correlated in latest RECOGNITION OF A PEDOLOGIC cipitation = 410 mm, mean annual temperature and late Pleistocene soils, in contrast to Holo- THRESHOLD AND IMPACT ON = 16 °C) is quite similar to the climate in the cene soils. Furthermore, silt and FeOT contents RATES OF SOIL DEVELOPMENT Cajon Creek area. The parent materials in the are more poorly correlated in latest Pleistocene Merced area, however, are typically finer soils than in Holocene soils and are actually The observed contrasts in rates of soil devel- grained, consisting typically of sandy loam, and weakly negatively correlated in the soil on the opment with respect to aspects of A- and B- are chiefly granitic. Deposit or surface ages are 14 late Pleistocene terrace (Qoa-d) (r = -0.66). horizon development are the result of the based on a variety of data, including C, ura- These results indicate that iron oxide and clay interdependent nature of factors that influence nium trend, K-Ar dating methods, and correla- accumulation are both increasingly related to soil development through time (Yaalon, 1971; tions with the marine oxygen-isotope stages. chemical weathering rather than to incorpora- Jenny, 1980; Birkeland, 1984). The initially Values of the Merced profile index also were tion of eolian dust. As shown previously, altera- rapid rate of A-horizon development in the calculated for soil depths that significantly ex- tion of ferrous iron in Pleistocene soils creates study area, for example, reflects the initially ceed depths for which the index was calculated authigenic ferric-iron oxides. Oxidation of fer- permeable nature of a parent material that fa- for most Cajon Creek soils. Differences in the rous iron in the eolian silt must account for vors incorporation of eolian dust, a sufficiently index value due to thickness, however, can weak or even ultimately negative correlation of moist climate, and a vegetation cover that pro- be significantly reduced by determining the FeOT and silt contents because these compo- vides abundant soil organic matter. Despite low weighted mean value of the index (Harden and nents are so strongly correlated in young un- pH and intense winter leaching, chemical altera- Taylor, 1983). weathered soils. Authigenic clay results from tion in these soils is limited to very slight altera- Comparison of the two study areas indicates hydrolytic weathering of most minerals; hence, tion of ferrous iron, weak soil reddening, and many similarities with respect to the morpholog- the strong correlation of clay and Fe203d con- partial grussification of large stones. Accumu- ical trends in soil development. The soils on the tent in Pleistocene soils is at least partly due to lated soil materials apparently consist almost en- upper member of the Modesto Formation co-formation of authigenic clay and ferric-iron tirely of material derived from eolian dust. As (-10,000 yr old, Harden, 1986), however, ap- oxides. These conclusions agree with regional the A horizon develops, soil permeability, water- parently are more weakly developed than are studies of the clay mineralogy that identify kao- holding capacity, and infiltration change, even- the soils on latest Pleistocene terraces of the linite as the ultimately predominant mineral in tually favoring accumulation of clay. Accumula- Cajon Creek area and are considerably more 292 McFADDEN AND WELDON

similar to those on the middle and early Holo- dust. Reheis observed logarithmic rates of soil ment, whereas in climates more humid than that cene terraces. The youngest deposit on which an development in more humid regions of Wyo- of the study area, intense leaching and chemical argillic-horizon-bearing soil is present is the ming, inferred to reflect the increased signifi- alteration may always dominate soil develop- 20,000- to 70,000-yr-old lower member of the cance of chemical weathering. As noted pre- ment. Regions that have significant dust influx Modesto Formation (Marchand and Allwardt, viously, long-term rates of soil development rates a nd moderately intense leaching, such as 1981; Harden and Taylor, 1983; Harden, 1986). were logarithmic in the Cajon Pass region. If the Cajon Pass area, may be unique in that peri- The timing of argillic horizon development in only soils of middle Holocene age or younger ods of time can be identified during which one the Merced area is therefore slightly to much are considered, however, some rates of soil de- process dominates over the other. Implicit in the older than the timing determined for the Cajon velopment in the Cajon Pass area can be de- concept of such a threshold is that episodes of Creek area. Note that values of the weighted scribed as linear: profile Fe2C>3d = 0.002 (terrace rapid rates of soil development do not necessar- means of soils on the 130,000-yr-old upper unit age) + 4.84. An initially linear rate of develop- ily require climatic regimes that were uniquely of the Riverbank Formation are almost identical ment may reflect the predominance of dust in- favorable for chemical weathering. The thresh- to those of the weighted means of the soil on the corporation. The over-all logarithmic rate re- old could be influenced significantly, however, 55,000-yr-old Qoa-d deposit. The 130,000-yr- flects the processes of dust incorporation and an by climatic changes that cause changes in rate, old age assignment, however, has been deter- increasing magnitude of chemical weathering, magnitude, and composition of aerosolic dust mined only on the basis of uraniun-trend dating and it masks the early linear-rate phase as well influx. Changes in soil properties in a given se- and correlation to the oxygen-isotope record as the thresholds that occur and are discernable quence of soils may thus be quite systematic, but and therefore is subject to some uncertainty. If over only relatively short periods of time. significant differences in the degree of soil devel- that age is correct, an over-all slower rate of opment on late Pleistocene and Holocene depos- morphological development in the Merced area CONCLUSIONS its that were observed in different regions may compared to that in the Cajon Creek area is occur, despite similar soil ages and climates, due indicated, at least during the initial 130,000 yr of to contrasting geomorphic settings that influence Studies of the well-dated sequence of soils in soil development. The 14C dates within deposits the local timing and relative importance of a the Cajon Pass area demonstrate that many soil of the upper member of the Modesto Formation pedologic threshold. An important implication characteristics change systematically with time. demonstrate a latest Pleistocene age, which is of this hypothesis is that caution should be exer- The rates and magnitude of soil development consistent with the outwash origin attributed to cised in the assignment of ages or correlations and the particular processes dominating soil de- this deposit by Marchand and Allwardt (1981). among late Quaternary deposits on the basis of velopment, however, have varied significantly Because the lower member of the Modesto is comparison of soils that exhibit relatively similar through time. The most important single varia- older than latest Pleistocene, the threshold of soil pedologic characteristics. ble affecting the initial phase of soil development development that was recognized in our study at Cajon Pass is the incorporation of eolian dust, probably was crossed in the Merced area after a which is the primary source of silt, most second- ACKNOWLEDGMENTS much longer period of soil development than is ary iron oxides, and some clay. The continuing required in the Cajon Creek area. Differences in accumulation of these components subsequently The authors thank C. Prentice, G. Martinez, soil parent materials, eolian influx rates, or other changes the initially permeable, noncolloidal soil L. Smith, P. Karas, M. Jackson, and T. Bullard as yet unknown factors account for the different environment to an increasingly less permeable who provided valuable assistance during the times required to attain the threshold in the two and more colloidal environment, which in con- field and laboratory phases of this study. Re- areas. The great difference in initial permeability junction with strong seasonal leaching and views of a preliminary manuscript of this paper of the fine-grainei Merced area soils compared acidic pH, promotes an increasing degree of by J. C. Tinsley, J. W. Harden, and V. T. Holli- to that in Cajon Pass probably in particular re- chemical weathering of the soil parent materials day resulted in improvements in this paper. duces the significance of the threshold. and the incorporated aerosolic materials. More- Laboratory analysis and field work conducted In an environment more arid than that of over, steady-state conditions of soil develop- during this study were partly supported through Cajon Pass, the threshold identified in this study ment are not attained over a time span of at least grants (U.S. Geological Survey Hazards Reduc- may be difficult to recognize. Shallow leaching half a million years, a conclusion also drawn by tion Programs, Contract Numbers 14-08-0001- and a relatively high rate of dust influx min- Muhs (1982) and by Harden and Marchand 16774, 18285, 19756, and 21275) to Kerry E. imizes the rate and magnitude of chemical (1977) in studies of soils elsewhere in California. Sieh, who served as thesis adviser to R. 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