PETROGRAPHY, GEOCHEMISTRY, AND CLAY MINERALOGY OF

A PALEOSOL IN THE DOCKUM GROUP (TRIASSIC),

TEXAS PANHANDLE

by KHAMCHANH KANHALANGSY, B.A.

A THESIS

IN

GEOSCIENCE

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCE

Approved

December, 1997 C' ACKNOWLEDGEMENTS / W^

^ I would like to extend my appreciation to Dr. Tom Lehman, my advisor, for

suggesting this project. Dr. Lehman's guidance and advice of this study were invaluable.

I am also grateful for Dr. Hal Karlsson and Dr. Calvin Barnes for their enlightening

discussions and helpful criticism and advice.

I want to thank Mike Gower for his assistance in preparation of thin sections and

slides and James Browning for his assistance in the isotope lab. Special thanks goes to

Dr. Necip Guven and Anish Kumar for the interpretation of X-ray diffraction patterns and

Trina Burling for the analyses of major and trace elements.

I want to thank Nora Kanhalangsy for being a part of my life. If not for you, I

would not be able to complete the goals in my life.

11 TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

ABSTRACT vi

LIST OF TABLES viii

LIST OF FIGURES x

CHAPTER

I. INTRODUCTION 1

Purpose ...... 2

Stratigraphic Summary .... 2

Study Location ..... 5

n. PETROGRAPHY 14

Methods of Investigation 14

Grain Size Variation . 14

Grain Roundness Variation .... 22

Parent Rock Mineralogy .... 24

Detrital Mineralogy of the Profile ... 24

Quartz ..... 26

Rock fi-agments . . 28

Feldspars . . . 30

Heavy/accessory minerals . . 30

Summary of detrital grain variation . 30

iii Cements ...... 32

Summary of cement variation 33

Rock Classification ..... 36

Tectonic Setting . 41

Micromorphologic Features .... 45

lUuvial channels .... 45

Voids ...... 49

Cutans ..... 49

Calcrete ..... 49

Silcrete . 53

III. GEOCHEMISTRY 57

Methods of Investigation 57

Patterns of Element Concentration 57

Chemical variation .... 62

Intensity of weathering conditions 76

Oxygen and carbon isotopes ... 78

IV. CLAY MINERALOGY 89

Methods of Investigation .... 89

Bulk powder analysis . 89

Clay mineral analysis .... 89

Mineralogy fi-om Bulk Sample Analysis 102

Mineralogy from Clay-Separation Analysis . 102

IV V. DISCUSSION 106

Review of Processes and Factors of Soil Formation . 106

Alteration of Soil Profile After Burial 107

Development of the Soil Profile 108

Formation of the Clay Minerals 109

Paleoenvironment of Soil Formation . 112

REFERENCES 114

APPENDIX 120 ABSTRACT

A Triassic paleosol, informally known as the "Palo Duro geosol" in Texas and

New Mexico, is the lowermost unit of the Triassic Dockum Group. This buried soil developed on sands of the Permian Quartermaster Formation in Texas. An excellent section of the paleosol is exposed at Caprock Canyons State Park, in Briscoe County,

Texas. This paleosol marker bed is found around the Southem High Plains in Texas and

New Mexico. A correlative unit, the "purple mottled" horizon, is also found in Triassic strata of Arizona and Colorado. The Palo Duro geosol is an Ultisol with two presumed major horizons identified here as A and B horizons. The A horizon consists of white sandstone with local accumulations of nodular and lamellar carbonate and chert layers at the top of the horizon. The B horizon consists of red, purple, and yellow mottled sandstone, siltstone, and mudstone. Petrographic analysis indicate that quartz, and to a lesser amount, sedimentary rock fi*agmentsar e the major detrital grains in the entire profile. Iron oxide and chert cements predominate throughout the profile.

Micromorphologic features observed in the profile include illuviation channels, voids, cutans, calcrete, and silcrete. Elemental enrichment and depletion trends in the Palo Duro geosol are correlated with those of modem soils with AI2O3 and Ti02 retained in the profile. Most of the elemental constituents are depleted fi-omth e profile relative to AI2O3 and Ti02. The clay minerals present in the profile are kaolinite, smectite, and illite. The

Palo Duro geosol profile developed as a consequence of intense leaching under well drained environments in a climate with high precipitation rates. The occurrences of

VI calcrete and silcrete samples suggests a climatic change fi"om humid to dryer condition occurred later during soil formation. Isotopic data of several peodogenic calcrete and silcrete suggest that these micromorphologic features formed at low temperature («9 to

38°C) compatible with their origin as a product of soil formation.

vu LIST OF TABLES

2.1. Grain size data for the Palo Duro geosol (in percent) . . 15

2.2. Overall median and mean grain sizes, and skewness and sediment

sorting values for grain size distribution in the Palo Duro geosol 15

2.3. Grain roundness data for the Palo Duro geosol (in percent) 16

2.4. Detrital grain mineralogy for the Palo Duro geosol (in percent). 16

2.5. Detrital quartz types in the Palo Duro geosol (in percent) 17

2.6. Cement types in the Palo Duro geosol (in percent). 17 2.7. Proportion of detrital grains, cement, pore space, and restored original porosity for the Palo Duro geosol (in percent) . . 18

2.8. Petrographic data for the Palo Duro geosol. .... 44

2.9. Proportion of monocrystalline quartz types and polycrystalline quartz in the Palo Duro geosol (in percent) ..... 46

3.1. Major and minor element abundances in the Palo Duro geosol (in

oxide weight percent) . . 58

3.2. Trace element abundances in the Palo Duro geosol (in ppm) . 58

3.3. Chemical index ofalteration (CIA) values in the Palo Duro geosol. . 79 3.4. Stable carbon and oxygen isotope ratios of selected carbonate and chert samples fi-om the A horizon of the Palo Duro geosol. 83

3.5. Apparent temperatures of formation of selected carbonate and chert samples ...... 87

4.1. Mineraology of the Palo Duro geosol from X-ray diffraction of bulk powder samples...... 90

4.2. Mineralogy of the Palo Duro geosol from X-ray diffraction of clay-sized (<2 micron) slide samples...... 98

viii A.l Detrital Grains ...... 121

A.2 Grain Size ...... 123

A.3 Grain Roundness . . . . . 123

IX LIST OF FIGURES

1.1. Stratigraphic sections of Triassic strata exposed along the eastern escarpment of the southem High Plains in Texas ... 3

1.2. Stratigraphic sections of Triassic strata exposed along the westem escarpment of the Southem High Plains and Pecos River valley in New Mexico and Texas ...... 4

1.3. Stratigraphic sections of Triassic strata along the Canadian River valley in Texas and New Mexico ..... 6

1.4. Map showing the surface exposures of Triassic strata around the Southem High Plains of Texas and New Mexico ... 7

1.5. Topographic map of the westem portion of the Caprock Canyons State Park ...... 8

1.6. Palo Duro geosol outcrop at John Hayne's Ridge in Caprock Canyons State Park, Briscoe County, Texas. .... 9

1.7. A close up view of the Palo Duro geosol outcrop at John Hayne's Ridge in Caprock Canyons State Park, Briscoe County, Texas. 10

1.8. A detailed view of the Palo Duro geosol outcrop at John Hayne's Ridge in Caprock Canyons State Park, Briscoe County, Texas showingcontactofthe A and B horizons. . . 11

1.9. Sketch of the Palo Duro geosol cross-section exposed along John

Hayne's Ridge in Caprock Canyons State Park, Briscoe County, Texas 12

2.1. Variations in grain size in different parts of the Palo Duro geosol profile. 20

2.2. Cumulative grain size for samples of the Palo Duro geosol profile. . 21 2.3. Variations in grain roundness in different parts of the Palo Duro

geosol profile...... 23

2.4. Major detrital mineral components in the Palo Duro geosol profile. . 25

2.5. Distribution of "genetic" quartz types at various parts of the Palo Duro geosol profile...... • 27 2.6. Distribution of sedimentary rock fragment types in various parts ofthe Palo Duro geosol profile...... 29

2.7. Photomicrograph of zircon (Z) and magnetite (M) grains in the presumed parent rock (Tr-7) sample ofthe Palo Duro geosol profile . 31

2.8. Proportions of detrital components, cement, and pore space in the Palo Duro geosol profile...... 34

2.9. Proportions ofcement types in the Palo Duro geosol profile. . 35

2.10. Photomicrograph showing calcite cement surrounding detrital grains in the Palo Duro geosol profile . 36

2.11. Classification of detrital grain composition for the Palo Duro geosol using the main quartz-feldspar-rock fragment (QFR) triangle . 38

2.12. Classification of detrital components ofthe Palo Duro geosol. A triangle representing a breakdown of rock fragment types including sedimentary rock fragments (SRF), volcanic rock fragments VRF), and metamorphic rock fragments (MRF) 39

2.13. Classification of detrital components ofthe Palo Duro geosol. A triangle representing a breakdown of sedimentary rock fragment types including sandstone, siltstone, or shale fragments (SS,SI,SH), carbonate fragments (CF), and chert fragments (CHF)... 40

2.14. QFL plot of detrital components ofthe Palo Duro geosol . 42

2.15. QmFLt plot of detrital components of the Palo Duro geosol . 43

2.16. Source rock identification based on detrital quartz types from the

Palo Duro geosol ...... 47

2.17. Photomicrograph of illuvial channels in the Palo Duro geosol profile . 48

2.18. Photomicrograph of intergranular voids in the Palo Duro geosol profile 50

2.19. Photomicrograph of clay coatings (cutans) on detrital grains in the Palo Duro geosol profile . . 51 2.20. Photomicrograph of rhythmic concentration of iron oxides in the Palo Duro geosol profile 52

xi 2.21. Photomicrograph of a silcrete nodule in the Palo Duro geosol profile . 55

2.22. Photomicrograph of a chert mass having fractures filled by secondary calcite . . . 56

3.1. Variation of major and minor element abundance (in oxide weight %) in the Palo Duro geosol plotted as a function of depth (in weight percent) . . 59

3.2. Variation in trace element abundances (in ppm) in the Palo Duro geosol plotted as a function of position in the profile ... 60

3.3. Plots of Ti/Zr and Ti/Y as functions of positive in the profile . 64

3.4. Concentration ratio diagram for the Palo Duro geosol samples showing enrichment and depletion of constituents Si02, Ti02, Fe203, MgO, and CaO relative to AI2O3 .... 65

3.5. Concentration ratio diagram for the Palo Duro geosol samples showing enrichment and depletion of constituents Si02, AI2O3, Fe203, MgO, and CaO relative to Ti02. .... 66

3.6. Plots of weight-percent ratios of Si02 to R2O3 (Al203+Fe203+Ti02) in the Palo Duro geosol as a function of position in the profile showing depletion of Si02 relative to immobile constituents (Al203+Fe203+Ti02) compared to the presumed parent rock . . 68

3.7. Concentration ratio diagram showing depletion and enrichment of constituents MnO, K2O, Na20, and P2O5 relative to AI2O3. 70

3.8. Concentration ratio diagram showing depletion and enrichment of constituents MnO, K2O, NazO, and P2O5 relative to Ti02 71

3.9. Concentration ratio diagram showing depletion and enrichment of trace elements Zr, Ba, Sr, Y, Cr, Rb, and V relative to AI2O3 . 72

3.10. Concentration ratio diagram showing depletion and enrichment of trace elements Zr, Ba, Sr, Y, Cr, Rb, and V relative to Ti02 . 73

3.11. Concentration ratio diagram showing depletion and enrichment of trace elements Zn, Nb, Ni, Sc, Cu, and Be relative to AI2O3... 74

3.12. Concentration ratio diagram showing depletion and enrichment of trace elements Zn, Nb, Ni, Sc, Cu, and Be relative to Ti02 ... 75 xii 3.13. Selected molecular weathering ratios showing base loss (Al203/(CaO+MgO+Na20+K20)), clayeness (Al203/Si02), calcification ((Ca0+Mg0)/Al203), and salinization (NaiO/KiO) 77

3.14. Chemical index ofalteration (CIA) in the profile plotted as a function ofposition in the profile...... 80

3.15. Climatic settings based on carbon and oxygen isotopes signatures of soil carbonate...... 84

3.16. Calibration curves for the A'^0 (quartz-calcite) fractionation of Clayton and Sharp and Kirschner ..... 86

4.1. A diagram showing the concentration of day-sized particles of less than 2 microns (in weight percent) represented at different depths ofthe Palo Duro geosol profile . . 104

4.2. The relative abundances of clay minerals in the Palo Duro geosol profile as indicated by the X-ray diffraction ofthe oriented glass slide samples ...... 105

Xlll CHAPTER I

ESTTRODUCTION

Paleosols are soils that formed on landscapes ofthe past, and are found at hiatuses, diastems, disconformities and unconformities in the non-marine stratigraphic record (Retallack, 1986; Bronger and Catt, 1989). Soils have evolved through geologic time along with Earth's atmosphere and hfe. The oldest paleosol recognized is about 3.1 biUion years old, and is developed on granitic basement rocks of South Africa (Edelman et al., 1983). Some ancient soils may even represent the oldest sedimentary rock record, formed approximately 3.8 billion years before present (Retallack, 1986).

Over the last decade there has been a growing interest in the study of paleosols.

The main reason for studies of ancient soils is that they provide information on past continental environments, including the climate, topographic relief, and types of vegetation present at the time of their formation (Retallack, 1990). Paleosols also record former atmospheric conditions because the chemistry of soil minerals reflects interaction with atmospheric oxygen, water, and carbon dioxide (Maynard, 1992). Paleosols offer one ofthe best or only means to study former continental climates. Although there are numerous publications on ancient soils, the vast majority of work is associated with recent Quartemary geomorphology. Relatively few reports on older paleosols are available, but such studies are growing in number (Mack et al., 1993). Purpose

The purpose ofthe present study is to describe a well developed and laterally

extensive paleosol found at the base ofthe Triassic Dockum Group in West Texas and

eastern New Mexico. In this report, I will refer to this paleosol informally as the "Palo

Duro geosol." Although recognized for some time, no detailed studies have been done on

this Triassic paleosol (Dubiel, 1987; Lehman et al, 1992; Lucas and Hayden; 1991; May,

1988). No detailed analysis ofthe soil profile has been presented. The primary objective

of this study is to investigate the mineralogical composition, the major and trace element

distributions, micromorphologic features, and pedogenic history ofthe soil profile.

Stratigraphic Summary

The Palo Duro geosol is the lowermost unit ofthe Triassic Dockum Group

redbeds exposed around the Southem High Plains (Lehman et al., unpubl.). The paleosol

formed at the unconformity developed on top of Permian strata along the margins ofthe northwest trending Dockum Basin in westem Texas and eastern New Mexico during

Triassic time. Along the eastem escarpment ofthe Southem High Plains (Figure 1.1), the

Palo Duro geosol rests on underlying Permian Quartermaster Formation or unnamed

Triassic aeolian strata from Armstrong County southward into Floyd County in Texas

(May, 1988; May and Lehman, 1989). The unit also occurs north of Amarillo, but is tmncated at the westem end ofthe Canadian River valley (Figure 1.2). A correlative paleosol is recognized by Lucas and Anderson (1993) and Pipringos and O'SuUivan

(1978) from Triassic strata in other regions ofthe westem United States. In Arizona, X H

X c H a: o z -a X E u X! O 00 (U o s s

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X H o u 8 I NORTH SOUTH REDONOA FM N Guodolupo E Guodolupe lllroiion IN" (llll abort iia Itvtl) S Guadalupe

.COOPER CANYON FORMATION 5000-

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New ufKOnfofm'Ifi on MtMico Ptrmion tiraia

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Figure 1.2. Stratigraphic sections of Triassic strata exposed along the westem escarpment ofthe Southem High Plains and Pecos River valley in New Mexico and Texas showing the position ofthe Palo Duro geosol (from Lehman et al., unpubl.). southwestem Colorado, and westem New Mexico the correlative unit (Figure 1.3) is referred to informally as the "purple-mottled unit" or the "mottled strata" within the

Shinammp Member ofthe Chinle Formation or at the top ofthe Moenkopi Formation

(Lucas and Hayden, 1991; Dubiel, 1987). The maximum thickness of this paleosol interval is up to 11 meters in some areas (Lehman et al., 1992).

Studv Location

Although exposures ofthe Palo Duro geosol occur at many places around the

High Plains of Texas and New Mexico, a single exposure was selected for detailed description. This study area is located in Caprock Canyons State Park, Briscoe County,

Texas (Figure 1.4). Samples were collected from the paleosol profile at an excellent exposure along the south-facing cHff of John Hayne's Ridge in the park (Figures 1.5, 1.6,

1.7 and 1.8). The same set of samples were used for thin-section preparation, major and trace element analysis, and X-ray diffractometry. Additional samples were collected from exposures at a nearby profile in Caprock Canyons State Park, the Currie Ranch in

Little Sunday Canyon northwest of Palo Duro Canyon State Park in Randall County, a roadcut on Highway 256 just north of Caprock Canyons State Park, and from the west side ofthe Pecos River dovmstream from Sumner Lake in eastem New Mexico.

The exposure ofthe paleosol is approximately 3.5 meters thick at the detailed sampling locality along John Hayne's Ridge (Figure 1.9). The paleosol is readily subdivided into two major horizons. The upper part ofthe profile (presumed A horizon) consists of stmctureless white quartzose sandstone. In places, nodular and lamellar o o X s(U I- tJIIIUi ^ < zu •o § ^ X CJ H c • ^^ >^ (U CT3 . > •^^ 3 > 3 O. Qi c 3 C en •^ -o CO 03 , . ^ o U ^ (U J *^ 0> 00 J c o E cS o •a ^ (U ^*—^ (n ^••^ O t/o3 o ex p CO O

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r : ROOSEVELT'jBAJLEY I j V'»J\^ ' • '-*"8 HALE I rijOYO ' CHAVES f "-n I , [- fc^7^f

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"!J^L0viNG^7 r\ » jWlNKLER.ER.ANDREW" ECTOSR j MIDLANMIDLANDI . ND \t^OW&HO.Y\\\:^/^yr^HOCXti}^^^,' ^"V"^ ^^ VX4- L., J. CI

A_Aw JCRANE I UPTOM ii STERLINGS. COKE "''^'^ IREACAN' I I .___] *•"""*:•.>-. CROCKETT^

i J

Figure 1.4. Map showing the surface exposures of Triassic strata around the Southem High Plains of Texas and New Mexico (from Lehman et al., 1992). Arrow indicates the location where samples ofthe Palo Duro geosol profile were collected. -s

E (u o -c 03 ^ ^ o c3 '"5 •n^. CoO

00 o

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00 rrnjT^'WrV'^WW ^f^r^wif^'w-f ; ~^''^9^^l^f!f^--'y^''',' ^"^'ftrfm-''" -iv-7|p?i^^»»?^f^

Fieure 1 6 Palo Duro geosol outcrop at John Hayne's Ridge in Caprock Canyons State F.gure .6. Fa^o ^ ^^^^g ^ ^^^^^^ ^^^^^ ^^^ .^^.^^^^^ ^j,^ p,,„ Duro geosol. Figure 1.7. A close up view ofthe Palo Duro geosol outcrop at John Hayne's Ridge ii n Caprock Canyons State Park, Briscoe County, Texas.

10 •u

Figure 1.8. A detailed view ofthe Palo Duro geosol outcrop at John Hayne's Ridge in Caprock Canyons State Park, Briscoe Coimty, Texas showing contact ofthe A and B horizons.

11 meter

Tecovas Formation

Horizon 4 _

3 - 4-Tr-2

<-Tr-3 Palo Duro geosol 2 -

B

1 -

0 J Permian Quartermaster Formation

Figure 1.9. Sketch ofthe Palo Duro geosol cross-section exposed along John Hayne's Ridge in Caprock Canyons State Park, Briscoe County, Texas (modified from Lehman et al., impubl.). Also shown are locations of samples collected for analysis.

12 carbonate and chert layers occur at the top of this horizon and grade downuard into the white quartzose sandstone. The nodular carbonate and chert in the horizon resemble that found in soil-formed calcrete and silcrete. The white sandstone grades downv. ard into red, purple, and yellow mottled sandstone, siltstone, and mudstone (presumed B horizon).

The color-mottled lower horizon is rich in clay and iron oxides and has cylindrical burrows and root traces. The lowermost portion ofthe profile grades downward into the presumed parent material (C horizon) ofthe buried soil that consists of uniformly bedded reddish-orange siltstone and mudstone ofthe underlying Permian Quartermaster

Formation. Other pedogenic features observed in the outcrop include root traces and massive, blocky columnar stmcture.

13 CFiAPTER II

PETROGRAPHY

Methods of Investigation

Six thin-sections obtained from samples collected at different depths in the paleosol profile and one thin-section from a parent rock sample were analyzed opticall>' to determine the overall detrital and authigenic mineral composition in various parts of the Palo Duro geosol profile. Thin-sections were systematically point counted using a petrographic microscope. A minimum of 300 to 600 points were counted per slide in order to obtain at least 300 counts for essential detrital components. Each slide was counted in its entirety in order to obtain results in confidence interval of 4-5% relative according to the method of Van der Plas and Tobi (1965). A complete statistical breakdown ofthe point count data is given in Appendix A. Relative abundances of grain sizes and shapes, detrital mineral grains and authigenic cement types at various depths in the profile are given in Tables 2.1 through 2.7. An attempt was also made to discriminate the major varieties of quartz, lithic fragments, and feldspars. The different varieties of both essential and non-essential constituents were also categorized following the method of Folk (1974).

Grain Size Variation

Measurements of detrital grain sizes were taken during petrographic analysis. The long axis of each detrital grain counted in thin-section was measured and the results were

14 Table 2.1. Grain size data for the Palo Duro geosol (in percent).

Grain size (O) >5 5 4 3 2 1 0 -1 clay-medium coarse very fine fine medium coarse very coarse silt silt sand sand sand sand sand granule Sample

Tr-1 1 6 11 24 43 14 1 0 Tr-2 2 3 16 27 41 9 1 0 Tr-3 1 4 13 19 43 12 3 4 Tr-4 1 7 18 28 38 7 1 0 Tr-5 4 3 19 28 35 11 0 1 Tr-6 2 8 14 26 36 12 2 0 Tr-7 0 2 42 33 20 3 0 0

clay-medium silt = .03 mm or less, coarse silt = .03-.06 mm, very fine sand = .06-. 13 mm, fine sand = .13-.25 mm, medium sand = .25-.50 mm, coarse sand = .50-1.0 mm, very coarse sand = 1.0-2.0 mm, granule = 2.0-4.0 mm.

Table 2.2. Overall median and mean grain sizes, and skewness and sediment sorting values for grain size distribution in the Palo Duro geosol (method of Folk, 1974). Median and mean values are given in O values (uncertainties = ±.05) and approximate millimeter equivalents. Skewness and sorting values are explained in the text.

Sample Median (Mdo) Mean (Mz) Skewness values (Sko) Sorting values (ac)

Tr-1 1.80 (0.29 mm) 2.03 (0.25 mm) 0.297 1.08 Tr-2 1.95 (0.26 mm) 2.18 (0.22 mm) 0.299 1.04 Tr-3 1.70 (0.31 mm) 1.92 (0.25 mm) 0.166 1.30 Tr-4 2.20 (0.22 mm) 2.37 (0.20 mm) 0.205 1.09 Tr-5 2.15 (0.23 mm) 2.27 (0.21 mm) 0.153 1.12 Tr-6 2.00 (0.25 mm) 2.25 (0.21 mm) 0.268 1.24 Tr-7 2.80 (0.14 mm) 2.72 (0.15 mm) -0.168 0.89

15 Table 2.3. Grain roundness data for the Palo Duro geosol (in percent).

Well Very Sample rounded Rounded Subrounded Subangular Angular angular

Tr-1 1 28 58 13 0 0 Tr-2 1 11 64 24 1 0 Tr-3 0 11 70 19 0 0 Tr-4 0 12 48 29 10 1 Tr-5 1 16 66 14 3 0 Tr-6 2 37 52 9 1 0 Tr-7 3 34 51 10 2 0

Table 2.4. Detrital grain mineralogy for the Palo Duro geosol (in percent).

Total Rock fragment types Sedimentary rock fragment types Sample Qtz RF Fds SRF MRF VRF CH LFC LSC ss

Tr-1 93 7 0 100 0 0 88 0 8 4 Tr-2 92 8 0 100 0 0 100 0 0 0 Tr-3 81 19 0 100 0 0 75 13 6 6 Tr-4 94 6 0 100 0 0 100 0 0 0 Tr-5 83 17 0 100 0 0 100 0 0 0 Tr-6 80 20 0 100 0 0 91 4 2 2 Tr-7 92 4 3 44 56 0 100 0 0 0

Total = quartz (Qtz), rock fragments (RF), feldspar (Fds) Rock fragment types = volcanic (VRF), metamorphic (MRF), sedimentary (SRF) Sedimentary rock fragment types = chert (CH), length-fast chalcedony (LFC), length- slow chalcedony (LSC), and sandstone/siltstone/shale (SS).

16 Table 2.5. Detrital quartz types in the Palo Duro geosol (in percent).

(Quart z types 1 Quartz types 2 Sample MS MU PS PU PLU MM REW

Tr-1 91 5 3 1 93 2 5 Tr-2 76 7 14 3 84 8 8 Tr-3 77 4 18 1 77 6 17 Tr-4 74 9 14 3 86 8 6 Tr-5 80 7 10 3 77 6 17 Tr-6 80 14 5 1 79 3 18 Tr-7 83 6 8 2 91 7 2

Quartz types 1 (extinction properties) = monocrystalline/straight extinction (MS), monocrystalline/undulatory extinction (MU), polycrystalline/straight extinction (PS), polycrystalline/undulatory extinction (PU)

Quartz types 2 (empirical/genetic) = plutonic (PLU), metamorphic (MM), reworked sedimentary (REW)

Table 2.6. Cement types in the Palo Duro geosol (in percent).

Sample Chert Hematite Calcite Limonite

Tr-1 88 7 5 0 Tr-2 69 16 7 9 Tr-3 85 15 0 0 Tr-4 4 95 0 1 Tr-5 74 26 0 0 Tr-6 13 84 1 3 Tr-7 0 53 47 0

17 Table 2.7. Proportions of detrital grains, cement, pore space, and restored original porosity for the Palo Duro geosol (in percent).

Apparent Sample Detrital grain Cement Pore space original porosity

Tr-1 62 35 3 38 Tr-2 61 35 4 39 Tr-3 66 30 4 34 Tr-4 43 54 3 57 Tr-5 47 52 1 53 Tr-6 42 57 1 58 Tr-7 70 3 27 30

Apparent original porosity = Cement + Pore space

18 tabulated according to the method of Folk (1974). The Palo Duro geosol contains

mineral grains ranging from clay to granule sizes. The entire soil profile consists mostly

of medium to fine sand (Figure 2.1). The Palo Duro geosol presents a coarsening-

upward profile. The grain size ofthe presumed parent rock for the soil profile (sample

Tr-7) comprises about 56% material coarser than fine sand. The lower B horizon

comprises from 76% (sample Tr-6) to 74% (samples Tr-5 and Tr-4) ofthe coarser

fractions. In contrast, the upper A horizon contains at least 78% (sample Tr-2), 81%

(sample Tr-3), and up to 82% (sample Tr-1) ofthe coarser fractions. Figure 2.2 illustrates

the overall grain size variation within the profile.

The median grain sizes (Mdo), corresponding to the 50th percentile on the

cumulative curve, are approximately 1.80, 1.95, 1.70, 2.20, 2.15, 2.00, and 2.800,

respectively for samples Tr-1 through Tr-7 (Table 2.2). These values indicate that detrital

grain size increases upward through the profile, in samples Tr-1, Tr-2, Tr-3 averaging

medium sand, while samples Tr-4, Tr-5, Tr-6, and Tr-7 are fine sand.

The average grain size (mean) may also be measured using the Graphic Mean

formula M^ = (Ol6 + O50 + 084)/3 (Folk, 1974). The calculated values are 2.03, 2.18,

1.92, 2.37, 2.27, 2.25, and 2.720, respectively, for samples Tr-1 through Tr-7, also

showing an upward-coarsening trend (Table 2.2). These values suggest that the average

grain size in samples Tr-1, Tr-2, Tr-4, Tr-5, Tr-6 and Tr-7 are fine sand, while medium

sand is the average grain size in sample Tr-3.

The degree of symmetry (or skewness) ofthe grain size distribution is determined by the Graphic Skewness formula Skc = ((Ol6 + 084) - (2 x O50))/2(O84 - Ol6) + (05 +

19 Grain size (%)

20 40 60 80 100

—I— —I—

2 "f" Clay and silt

3 .. Gravel

(73 V) B 4 Sand

Figure 2.1. Variations in grain size in different parts ofthe Palo Duro geosol profile. Plotted according to data of Table 2.1.

20 100

80 ^ (U >

•«-*

•^•a4 a B 60 s U a o •^^ CJ O) v> 1 4U

• aVi4 JS H

20

-1 2 3 >5 Grain Size ((D)

Figure 2.2. Cumulative grain size for samples ofthe Palo Duro geosol profile. Numbers refer to samples Tr-1 through Tr-7. These curves are plotted from data of Table 2.1.

21 095) - ((2 X 2O50))/2(O95 - 05) (Folk, 1974). The calculated values are 0.297, 0.299,

0.166, 0.205, 0.153, 0.268, and -0.168, respectively for samples Tr-1 through Tr-7 (Table

2.2). The degree of symmetry of grain size distributions for samples of A and B horizons are fine-skewed, whereas the grain size distribution for the parent rock sample (Tr-7) is coarse-skewed.

The uniformity or sorting of sediments in the samples is determined by the

Graphic Standard Deviation formula QQ = (084-Ol6)/4 + (095-05)/6.6 (Folk, 1974). The calculated sorting values are 1.08, 1.04, 1.30, 1.09, 1.12, 1.24, and 0.89, respectively for samples Tr-1 through Tr-7 (Table 2.2). These values indicate that sediments in the samples of A and B horizon are poorly sorted, while sediments in the parent rock sample

(Tr-7) are moderately sorted.

Grain Roundness Variation

Figure 2.3 illustrates the variation in grain roundness in different parts ofthe paleosol profile. The profile consists of framework grains with various shapes ranging from very angular to well rounded. The majority ofthe grains are subrounded, rounded, and subangular. Minor occurrences of angular and well rounded grains were also counted. The abundance of well rounded and rounded detrital grains generally increases towards the bottom ofthe soil profile. Subrounded, subangular, angular, and very angular grains are mostly concentrated in the middle portions ofthe profile.

22 100

^ o^ c cr o u

Well rounded Rounded Subrounded Subangular Angular Very Angular

Figure 2.3. Variations in grain roundness in different parts ofthe Palo Duro geosol profile. Numbers refer to samples Tr-1 through Tr-7. Plotted according to data of Table 2.3.

23 Parent Rock Mineralogy

The presumed parent rock ofthe soil profile, represented in sample Tr-7, has quartz as the major detrital mineral component (Table 2.4). Figure 2.4 illustrates the variation of detrital components in the profile. Approximately 92% ofthe detrital grains counted are quartz, with minor amounts of feldspars (3%), including plagioclase, orthoclase, and microcline. All three feldspar t3T3es occur as subrounded to rounded very fine sand grains. Other detrital grains (4%) occurring in the parent rock are sedimentary and metamorphic rock fragments. Sedimentary rock fragment types include limestone and chert. Quartz grains with sutured intercrystalline boundaries and mica crystallites are common metamorphic rock fragments. Micas also occur in minute amounts (<1%) as single grains and are rare in the sample. Zircon and opaque minerals are the only two accessory minerals observed, and both are as rare as the micas. Zircon grains are subangular and subrounded and are very fine sand size. Opaque mineral grains are mostly subangular to subrounded coarse silt and very fine sand size.

Detrital Mineralogy ofthe Profile

The predominant detrital grains in the profile are quartz (80-94% of essential grains) and sedimentary rock fragments (6-20% of essential grains), as shown in Figure

2.4. Feldspars are extremely rare (<1%).

24 Essential detrital constituents (%)

20 40 60 80 100 Horizon

A

B

Figure 2.4. Major detrital mineral components in the Palo Duro geosol profile. Plotted according to data of Table 2.4.

25 Quartz

Various quartz types were differentiated as empirical "genetic" types of Folk

(1974) and using extinction properties based on the cut-off boundary of stage rotation at

5° (Basu et al.,1975). Monocrystalline quartz with straight extinction (MS) consists of single crystal grains showing straight to slightly undulose extinction. Monocrystalline quartz with undulose extinction (MU) comprise grains of single crystals displaying undulose extinction. Polycrystalline quartz with straight extinction (PS) consists of grains with multiple crystals showing straight to slightly undulose extinction.

Polycrystalline quartz with undulose extinction (PU) comprise grains with multiple crystals displaying undulose extinction. To differentiate between the microquartz varieties, a decision based on the size of individual crystallites was used. Chert crystallites are smaller (<60 jam) compared to polycrystalline quartz grains that showed larger crystallites.

Quartz grains make up the greatest percentage of all detrital grains in the Palo

Duro geosol profile and occur in a variety of grain sizes and shapes. Quartz grains are mostly fine and medium sand size. Quartz grains are typically subrounded. Overall, quartz represents between 80% and 94% of all detrital grain at various depths ofthe profile (Table 2.4).

Most ofthe quartz grains in the profile are monocrystalline with straight to slightly undulose extinction (Figure 2.5). These represent between 74% and 83% of all quartz grains at most depths ofthe profile. Monocrystalline straight-extinction quartz is concentrated in the upper portion ofthe A horizon where 91% is represented.

26 Quartz Types 1 (%) 20 40 60 80 ^00 Horizon

B

Figure 2.5. Distribution of "genetic" quartz types at various parts ofthe Palo Duro geosol profile. MS = monocrystalline grains with straight extiction, MU= monocrystalline grains with undulose extiction, PU = polycrystalline grains with undulose extinction. Plotted according to data of Table 2.5.

27 Polycrystalline quartz with straight to slightly undulose extinction is the next most

abundant quartz type occurring in the profile, comprising 18% in the lower portion,

decreasing to 3% in the upper portion ofthe A horizon. This quartz type represents about

7 to 14%) of all quartz grains in the B horizon. Between 4% and 7% of this quartz type is

found in samples ofthe A horizon. Overall, polycrystalline quartz with undulose

extinction is evenly distributed throughout the profile with little variations in amounts,

between 1 to 3%). Quartz with undulose extinction decreases toward the upper half of the

profile, according to the calibration curve of Clayton (unpubl.).

Rock fragments

Virtually all ofthe rock fragment grains in the paleosol profile are of sedimentary

types (Table 2.4). Rock fragments in the presumed parent rock consist of about 44%

sedimentary rock fragment grains and 56% metamorphic rock fragment grains. The

abundance of rock fragment grains ranges from between 7% in the uppermost portion of

the A horizon to as high as 20% in the lowermost portion ofthe B horizon (Figure 2.4).

Various sedimentary rock fragment types are found in the soil profile (Table 2.4).

By far, chert is the most common type in the profile where detrital chert grains make up between 75 to 100% ofthe rock fragments (Figure 2.6). Length-fast chalcedony grains are the second most common sedimentary rock fragment type occurring mainly in the lower portion ofthe A horizon (13%) and in the lower portion ofthe B horizon (4%).

Other sedimentary rock fragment types occurring in the profile but in smaller amounts are

28 Sedimentary rock fragment types (%) 20 40 60 80 100 Horizon

Figure 2.6. Distribution of sedimentary rock fragment types in various parts ofthe Palo Duro geosol profile. LFC = length-fast chalcedony, LSC = length-slow chalcedony, SS = sandstone/siltstone/shale. Plotted according to data of Table 2.4.

29 length-slow chalcedony and sandstone and siltstone fragments. Both of these rock

fragment types are abundant mainly in the A horizon and in the lower B horizon.

Feldspars

Feldspars are less abundant in the paleosol profile compared to the parent

material. No feldspar grains were counted in any ofthe point counts ofthe

paleosolsamples, although a few grains of microcline were observed. Feldspar abundance

is therefore substantially less than 1%.

Heavy/accessory minerals

Zircon is present in the presumed parent rock and throughout A and B horizons,

but is rare. Zircon grains are increasingly more spherical and smaller upward in the paleosol horizon (Figure 2.7). These grains tend to be mostly subrounded to rounded.

Opaque mineral grains were found in the parent material but not in the profile.

Summary of detrital grain variation

In general, the parent material (Tr-7) has (1) more feldspar, (2) a greater variety of rock fragments, (3) a greater variety of quartz types, and (4) a greater variety of heavy/accessory minerals than the paleosol profile (Tr-1 through Tr-6). The upper part of the profile consists almost entirely of monocrystalline quartz grains with straight extinction, and a few chert grains. These observations are in keeping with an increase in

30 Figure 2.7. Photomicrograph of zircon (Z) and opaque mineral (O) grains in the presumed parent rock (Tr-7) sample ofthe Palo Duro geosol profile. Width of field of view is .5 mm. Light is cross-polarized.

31 weathering upward in the profile resulting in a reduction of chemically and mechanically unstable components.

The mineralogy ofthe presumed parent rock and soil profile suggests that detrital grains in the paleosol may have originated from three source areas (Table 2.5). The abundance of mainly monocrystalline quartz grains in these samples, accounting for between 77% and 93% of all quartz grains is indicative of ultimate derivation from plutonic source rocks (Folk, 1974). The presence of some polycrystalline quartz grains composed of equidimensional interlocking crystals with straight to slightly undulose extinction (mainly in the presumed parent rock) may indicate a subordinate metamorphic rock or deformed plutonic rock source as well (Folk, 1974). In addition, the occurrence of chert grains indicates that the sediment source for the paleosol likely included sedimentary rocks. The predominance of monocrystalline quartz with straight to slight undulose extinction and the lack of feldspars and ferromagnesian silicates in the profile suggests that much ofthe quartz was recycled from older sedimentary rocks and was not

"first-generation" quartz released by initial weathering of plutonic rocks.

Cements

Microcrystalline quartz (chert), calcite, clay coatings, ferric-oxide (hematite), and ferric hydroxide (limonite) make up the cements observed in the Palo Duro geosol profile but only ferric-oxide and calcite cement were found in the parent rock sample (Table 2.6).

These cements account for between 30% and 57% ofthe cross-sectional area of thin- section samples from A and B horizons (Table 2.7). Cements concentrated in the B

32 horizon comprise over 50% ofthe rock (Figure 2.8). When percentages of cements and pore spaces are combined, the apparent original porosity at various depths increases toward the lower portion ofthe profile. There is very little cement (3%) in the parent rock.

Chert cement is, by far, the major component ofthe matrix in the paleosol profile

(Figure 2.9). Chert cement is present in all samples of A and B horizons. Chert cement is predominant in the A horizon where up to 88% of all cement is chert in the upper portion of A horizon. This cement is less common in the B horizon were as little as 4% occurs.

Hematite cement is also observed throughout the profile but is more common in the B horizon (Figure 2.9). Up to 95% of all cement is hematite in the upper B horizon.

In thin section, hematite cement reveals no crystalline form and fills inter-granular pore spaces. Calcite cement is only common in the parent rock. Where present in the paleosol profile, calcite usually exhibits a poikilotopic cementing pattern where single crystals of calcite surround several detrital grains (Figure 2.10). Limonite is also rare in the profile.

Where present, limonite cement shows no crystalline form.

Summary ofcement variation

In general, cementation is much more thorough in the paleosol profile than in the parent rock, and mostly so in the B horizon. Iron oxides and chert predominate in the B horizon, whereas only chert is common in the A horizon. The small amount ofcement in the parent rock includes calcite as well as iron oxides. Most, if not, all of this

33 Proportions (%) 0 20 40 60 80 1 ^ ^°° Horizon

Pore space

Detrital grains

C/3 5 4

B

Figure 2.8. Proportions of detrital components, cement, and pore space in the Palo Duro geosol profile. Plotted according to data of Table 2.7.

34 Cement Types (%) 20 40 60 80 100 Horizon

B

Figure 2.9. Proportions ofcement types in the Palo Duro geosol profile. Plotted according to data of Table 2.6.

35 Figure 2.10. Photomicrograph showing calcite cement surrounding detrital grains in the Palo Duro geosol profile. Width of field of view is 1 mm. Light is cross- polarized.

36 cementation occurred during soil formation (not as a result of later diagenesis). This is

indicated by the extremely high pre-cementation porosity values (exceeding 50% in the B

horizon) that could only result from early, pre-compaction, precipitation. The cement

morphologies also reflect a pedogenic origin (see "micromorphologic features" below).

Rock Classification

Ternary diagrams illustrating various groupings of detrital components in the Palo

Duro geosol samples are used in classifying the rock types in the profile and parent rock.

Figure 2.11 is a standard quartz-feldspar-rock fragment (QFR) plot based on the method

of Folk (1974). Also, the samples were plotted on a daughter diagram showing a

breakdown of rock fragment types that include sedimentary rock fragments, volcanic rock

fragments, and metamorphic rock fragments (Figure 2.12). The samples were also

plotted on a triangular daughter diagram representing a breakdown of sedimentary rock

fragment types that consist of sandstone, siltstone, or shale fragments, carbonate

fragments, and chert fragments (Figure 2.13).

The plots indicate that samples collected from the A and B horizons and the

parent rock ofthe paleosol are sublitharenites (Figure 2.11). More specifically, all

samples ofthe paleosol and parent material are sedarenites (Figure 2.12) and in detail chertarenites (Figure 2.13). These rock classification schemes are designed for sandstone

samples, and these schemes are appHed here to sample ofthe presumed parent rock for the paleosol, as well as paleosol samples. However, the paleosol samples plot within the

same compositional fields as the presumed parent rock sample. This suggests that

37 a = Quartzarenite b = Subarkose c = Sublitharenite d = Arkose e = Lithic Arkose f = Feldspathic Litharenite g = Litharenite

R

Figure 2.11. Classification of detrital grain composition for the Palo Duro geosol using the main quartz-feldspar-rock fragment (QFR) triangle (after Folk, 1974). An explanation of symbols is given. Plotted according to data of Table 2.3. Numbers refer to samples Tr-1 through Tr-7.

38 SRF

1.2.3.4.5.6

VRF MRF

Figure 2.12. Classification of detrital components ofthe Palo Duro geosol (after Folk, 1974). A triangle representing a breakdown of rock fragment types including sedimentary rock fragments(SRF) , volcanic rock fragments VRF), and metamorphic rock fragments(MRF) . Plotted according to data of Table 2.3. Numbers refer to samples Tr-1 through Tr-7.

39 SS,SI,SH

.H,^,7

CF CHF

Figure 2.13. Classification of detrital components ofthe Palo Duro geosol (after Folk, 1974). A triangle representing a breakdown of sedimentary rock fragment types including sandstone, siltstone, or shale fragments(SS,SI,SH) , carbonate fragments (CF), and chert fragments(CHF) . Plotted according to data of Table 2.4. Ntimbers refer to samples Tr-1 through Tr-7.

40 weathering has not dramatically changed the bulk mineralogical composition ofthe parent rock. Therefore, the mineralogical composition ofthe paleosol samples is used in describing their rock types and to determine the tectonic provenance ofthe sediment as well.

Tectonic Setting

Dickinson and Suczek (1979) and Dickinson et al. (1983) used various ternary diagrams to show that petrographic data from terrigeneous sandstones can be used to determine the tectonic provenance ofthe sediment. Their ternary diagrams consist of separated compositional fields that are characteristic of sandstones derived from different types of provenance. Their QFL (Figure 2.14) and QmFLt (Figure 2.15) plots were used in this study. Table 2.8 shows the arrangement of data required for these plots. Samples from the Palo Duro geosol, as shown in Figure 2.14, plot in the "recycled orogenic" provenance. More specifically, the QmFLt plot suggests that the paleosol sediments plot in the "quartzose recycled" field (Figure 2.15). According to Dickinson et al. (1983), the

"recycled orogenic" provenance is typical for sediment sources ofthe cratonic regions.

Sediments derived from eroding uplifted terranes, folded and faulted rocks of subduction areas including deformed oceanic sediment, continental-continental collision areas formed along crustal sutures, and foreland uplift areas are included (Dickinson and

Suczek, 1979).

Basu et al. (1975) demonstrated that information on natures of detrital quartz grains can also be usefiil for provenance determination. They stated that quartz from a

41 Craton Interior

Transitional Continental

Basement Uplift

15 50

Figure 2.14. QFL plot of detrital components ofthe Palo Duro geosol (after Dickinson et al., 1983). Q = total quartzose grains, F = total feldspar grains, L = total rock fragment grains. Plotted according to data of Table 2.8. Numbers refer to samples Tr-1 through Tr-7.

42 Qm Craton Interior

Quartzose Recycled Transitional Continental

Transitional Recycled Basement Uplift

Lithic Recycled

Lt

Figure 2.15. QmFLt plot of detrital components ofthe Palo Duro geosol (after Dickinson et al., 1983). Qm = monocrystalline quartz, F = feldspar Lt = total lithics including polycrystalline quartz. Plotted according to data of Table 2.8. Numbers refer to samples Tr-1 through Tr-7.

43 Table 2.8. Petrographic data for the Palo Duro geosol (arranged using the methods of Folk, 1974; Dickinson and Suczek, 1979; and Dickinson et al., 1983).

Sample Q F L Qm F Lt Qp Lv Ls Qm P K

Tr-1 93 0 7 89 0 11 36 0 64 100 0 0 Tr-2 92 0 8 77 0 23 67 0 33 100 0 0 Tr-3 81 0 19 65 0 35 45 0 55 100 0 0 Tr-4 94 0 6 78 0 22 71 0 29 100 0 0 Tr-5 83 0 17 72 0 28 38 0 62 100 0 0 Tr-6 80 0 20 76 0 24 18 0 82 100 0 0 Tr-7 92 3 4 84 3 12 81 0 19 98 1 1

Q = total quartzose grains, including monocrystalline quartz (Qm) and polycrystalline quartz (Qp) varieties, F = total feldspar grains, L = total rock fragment grains, Lt = total lithics including Qp, Lv = volcanic lithics, Ls = sedimentary lithics, P = plagioclase, K = orthoclase

44 variety of source rocks can be differentiated on the basis of optical extinction ofthe quartz grains. According to Basu et al. (1975) recent and ancient sands of plutonic, low- rank, and high-rank metamorphic source rocks can be discriminated from each other by noticing the undulosity of monocrystalline quartz, coupled with observation ofthe amount of polycrystalline quartz and number of crystal units per grain of polycrystalline quartz. Table 2.9 shows the relative percentages of total quartz which are monocrystalline undulatory, monocrystalline non-undulatory, and polycrystalline at various parts ofthe profile. The diamond diagram contrasts four variables in the nature ofthe quartz grain population, and suggests that the ultimate source rock for the quartz in the parent rock sample for the paleosol is plutonic rock (Figure 2.16). It is likely, however that most or all ofthe quartz was recycled through erosion of older sedimentary rocks, themselves ultimately derived from plutonic rock.

Micromorphologic Features

Illuvial channels

One ofthe most striking features occurring in the Palo Duro geosol are illuviation structures. Illuvial channels are observed in all parts ofthe profile (Figure 2.17). Illuvial channels are better developed towards the middle portion ofthe B horizon. The illuviation structures in samples ofthe B horizon are much larger in diameter than those ofthe A horizon. Most ofthe illuvial channels are partly or completely filled by feme- oxide, ferric-hydroxide, and some calcite. These channels are evidence for translocation of clay by soil water (Brewer, 1976).

45 Table 2.9. Proportions of monocrystalline quartz types and polycrystalline quartz in the Palo Duro geosol (in percent).

Monocrystalline Quartz Polycrystalline Quartz Sample Straight Undulose 2-3 crystals/grain >3 crystals/grain

Tr-1 95 5 63 37 Tr-2 91 9 56 44 Tr-3 95 5 57 43 Tr-4 89 11 58 42 Tr-5 92 8 56 44 Tr-6 85 15 82 18 Tr-7 93 7 44 56

46 Polycrystalline quart/, (2-3 crystal units per grain;^ 75% of lota! polycrystalline quartz)

PLUTONIC

Non- undulatory Undulatory quartz quartz

Polycrystalline quartz (> 3 crystal units per grain; > 25% of total polycrystalline quartz)

Figure 2.16. Source rock identification based on detrital quartz types from the Palo Duro geosol as indicated using plot of Basu et al. (1975). Plotted according to data of Table 2.9, Numbers refer to samples Tr-1 through Tr-7.

47 Figure 2.17. Photomicrograph of illuvial channels in the Palo Duro geosol profile as indicated by an arrow. Width of field of view is 1 mm. Photomicrograph is under plane light.

48 Void?

Intergranular voids are also common structures in the profile (Figure 2.18). Voids are developed mainly in the B horizon. Intergranular voids are produced by contraction during dessication ofthe cement matrix (Summerfield, 1983). These structures are mainly filled by secondary chert, iron oxides, carbonates, or clays.

Cutans

Clay coatings or cutans occur at all depths ofthe profile (Figure 2.19). These clay coatings are developed by the progressive deposition of clay particles on the surfaces of mineral grains or along the walls of larger voids (Fitzgerald, 1993) or result from the accumulation of clay through dissolution of siHcate matrix. The features are indicators of soil water movement. Cutans are typically brovm or red in color, indicating that amorphous or cryptocrystalline iron oxides are associated with the clays, and formed by the reduction, movement, and oxidation of Fe and Mn within the soil matrix (Stoops and

Eswaran, 1985). Some of these clay coatings formed as concentric whorls produced by rhythmic concentrations of iron oxide within the cement matrix (Figure 2.20).

Calcrete

In many areas, the upper part ofthe profile contains layers and nodules of microcrystalline calcite. These carbonate accumulations are partially or completely replaced by microcrystalline quartz in places. The carbonate and silica layers resemble pedogenic "calcrete" and "silcrete" described in modem soils. Such deposits are not well

49 Figure 2.18. Photomicrograph of intergranular voids in the Palo Duro geosol profile as indicated by an arrow. Width of field of view is 1 mm. Light is cross- polarized.

50 Figure 2.19. Photomicrograph of clay coatings (cutans) on detrital grains in the Palo Duro geosol profile. Width of field of view is 1 mm. Photomicrograph is under plane light.

51 •^^i

^SsBib-

Figure 2.20. Photomicrograph of rhythmic concentration of iron oxides between detrital grains in the Palo Duro geosol profile Width of field of view is 1 mm. Photomicrograph is under plane light.

52 developed in the profile studied at Caprock Canyons, but are found nearby typically within or replacing the A horizon described above. Calcretes are recognized in the field as white and light gray layers or equant to disc-shaped nodules lying perpendicular to bedding planes. Portions of many calcrete nodules are replaced by secondary silica.

Silcrete

The varied types of authigenic silica recognized in the Palo Duro geosol are described using the classification of Folk (1965) and McBride and Thomson (1970) for chert. Included are megaquartz, microcrystalline quartz, length-fast chalcedony, length- slow chalcedony, and lutecite. Megaquartz forms as equant to elongated crystals in quartz overgrov^hs and in cavity and vein fillings. Microcrystalline quartz forms as equant crystal aggregates. Length-fast chalcedony grains consist of sheaf-like bundles of thin radiating fibers with extinction parallel with fibers. Length-slow chalcedony forms as fibers having parallel extinction. Lutecite usually has a length-slow pseudofibrous structure with oblique extinction.

Silcrete mainly occurs as chert cement in sand and gravel of ancient soil profiles

(Wopfiier, 1978). The A horizon ofthe Palo Duro geosol profile contains silcrete cement and replacement nodules. Silcrete is often considered to be rare in paleosols (Goudie,

1973). In hand sample, these silcretes are smoky-gray, yellowish-green, or brownish-red.

They are recognized in the field as massive layers and discrete, equant to disc-shaped nodules lying parallel to bedding planes. These probably resulted from replacement of carbonate nodules. Most silcrete nodules are composed of chalcedony and some consist

53 of alternating chert with megaquartz in the center, surrounded by length-slow chalcedony and lutecite (Figure 2.21). Some of these replacement structures are rimmed by iron oxide stain with remnant calcite surrounding the outermost part ofthe nodule. The successive layers of these chert types suggest that changes occurred in the chemical environment during various stages of replacement. These nodules mostly occur in the middle and upper portions ofthe A horizon. There appears to be no predominant size or shape of the nodules.

Irregular masses and layers of chert are also found in the A horizon. In hand samples, the characteristic shapes of these chert masses include tabular sheets, spherical nodules, elongated pods, and lenticular stringers. Some ofthe chert masses show iron oxide staining. In thin-sections, the samples are represented by microcrystalline quartz matrix within the chert masses, suggesting rapid silica precipitation from a silica solution

(Leckie and Cheel, 1990). Within these chert masses are small pores and fractures that are filled with length-fast chalcedony, length-slow chalcedony, and calcite. More common are fillings of these cavities by mainly calcite (Figure 2.22). Some cavities are filled by iron oxides, representing different stages of filling (Bowers and Reaser, 1996).

54 *

•' ••*•• ^ '' ^ T l^% L^ /•; ^-^^^^'i A **i-j^. m

^^-^^ •T iBiKt ^ 'i^f^^Ji'r''' "^ '^Tlii'iv-. - ^L*^Br-:i.-^"laii^-'/^ •»i«5av '^ • '••: '>'^''1;*^> W ifciBf :i.^p^> >>:•' 1 l^J^.I" *

• i '. p^s*v-.: \

Figure 2.21. Photomicrograph of a silcrete nodule in the Palo Duro geosol profile Width of field of view is 1 mm. Light is cross-polarized. Figure 2.22. Photomicrograph of a chert mass having fractures filled by secondary calcite. Width of field of view is 1mm. Light is cross-polarized.

56 CHAPTER III

GEOCHEMISTRY

Methods of Investigation

Parts ofthe seven whole-rock samples collected from different depths ofthe paleosol profile and parent rock were pulverized in a jaw crusher. Crushed samples were then powdered into fine size in an alumina shatterbox. Powdered samples were fused with a lithium metaborate flux, and then quenched in a weak hydrochloric acid solution.

The solutions were diluted to the appropriate concentration for major element analyses.

Major, minor and trace element concentrations were determined by inductively coupled plasma (ICP) spectrometry with precision determined with U.S.G.S. and in-house standards. Precision is within 2% for major and minor elements, and trace elements (Ba,

Sr, Y, and Zr). Precision is within 5% for other trace elements, with the exception of Cr,

Ni, and Cu (20%)) due to graphite crucible contamination (up to 20 ppm) and small concentrations in samples.

Patterns of Element Concentration

Table 3.1 lists the major and minor elements and Table 3.2 shows the trace element abundances in the Palo Duro geosol profile. Major and minor element concentrations are given in weight percent as oxides, trace elements in parts per million.

Variations of these elements within the profile as a function of depth are shown in Figure

3.1 for major and minor elements and Figure 3.2 for trace elements. A discussion

57 Table 3.1. Major and minor element abimdances in the Palo Duro geosol (in oxide weight percent). Precision is ±2Vo.

Sample SiO? AhO^ Fe203 Ti02 MgO CaO K2O Na20 P2O5 MnO LOI Total

Tr-1 92.39 4.05 0.21 0.58 0.07 0.16 0.04 BD 0.01 0 2.05 99.63 Tr-2 85.80 6.64 2.24 0.49 0.12 0.54 0.05 BD 0.02 0.01 3.59 99.62 Tr-3 88.58 7.04 0.45 0.50 0.16 0.08 0.09 BD 0.03 0 3.35 100.44 Tr-4 80.61 7.65 7.19 0.50 0.26 0.09 0.21 BD 0.01 0.01 3.88 100.67 Tr-5 78.76 8.59 1.93 0.59 1.48 0.46 0.37 0.16 0 0.01 7.69 101.52 Tr-6 76.71 10.51 4.40 0.55 0.75 0.22 0.46 0.07 0.02 0.03 6.59 101.06 Tr-7 85.57 5.69 1.87 0.61 0.74 1.11 1.45 1.27 0.09 0.05 2.03 101.22

BD = below detection limits LOI = loss on ignition

Table 3.2. Trace element abundances in the Palo Duro geosol (in ppm). Precisions are within ±2% for Zr, Ba, Sr, and Y, ±20% for Cr, Ni, and Cu, and ±5% for V, Rb, Zn, Nb, Sc, and Be.

Sample Zr Ba Sr Y Y Cr Rb Zn Nb Ni Sc Cu Be Tr-1 279 80 25 12.9 129 16 3 3 14 10 3.4 5 0.3 Tr-2 241 47 31 11.2 62 25 3 1 10 10 6.5 6 0.3 Tr-3 297 47 39 11.9 46 14 4 2 10 8 4.6 2 0.3 Tr-4 255 84 68 12.3 132 42 9 6 8 12 6.6 5 0.7 Tr-5 219 83 261 21 42 31 17 17 12 13 22.3 7 0.4 Tr-6 219 101 143 21.4 31 35 22 15 10 12 12 3 0.5 Tr-7 919 537 99 18.5 35 17 39 16 11 7 3.3 6 0.7

58 S102 AliQj FeiOj TIG: MgO CaO

K2O NaiO MnO

0 1 0 1 2 0 0.05 0.1

Figure 3.1. Variation of major and minor element abimdances (in oxide weight %) in the Palo Duro geosol plotted as a ftmction ofposition in the profile (in weight percent). 59 o

CO aO (41 o c § o . ^H 4—• o S

o

o O 00

Q

PL4 o

S

o CO o O

C/3 C

c E

C9 o

%m >I 2a.

ti.

60 o 4

s ••-c> u o C/3

u

o C4

o CM

s

a

61 regarding the individual behavior of each element in the soil profile is made by analysis of relative chemical associations.

The geochemical trends in mature paleosol profiles reflect the weathering of minerals, gravitational processes of leaching and translocation, and the capillary processes that produce calcification or salinization (Gill and Yemane, 1996). Postburial alteration processes can change primary chemical signatures, to some degree, within the paleosol profile (Retallack, 1990). This is apparently not the case in the Palo Duro geosol profile. The mineralogical and chemical signatures within the profile probably reflect its pedogenic origin because it formed on a relatively homogeneous alluvial sand deposit within which mineral variations are minimal, most mineralogical and chemical trends are

strongly directional, and the outcrop has not been exposed to substantial pedogenic

alteration after soil profile formation.

Chemical variation

AI2O3 is commonly assumed to be retained in the soil during weathering because

Al compounds are relatively insoluble under normal pH conditions and much ofthe Al is

incorporated in clay minerals (Birkeland, 1974). Therefore the concentration of Al

generally increases in the profile relative to the parent rock during weathering since it is

retained in the soil, while other components are removed by leaching. This is generally

the case in the Palo Duro geosol profile. Here AI2O3 content increases from the parent

rock (5.69%)) to the paleosol profile (7.41%, the average from the profile). Al is most

abundant in the B horizon where clays are concentrated (Chapter IV). Although there is a

62 slight decrease in Ti02 content from the parent rock (0.61%) to the average concentration

in the profile (0.54%)), Ti02 is believed to be retained in a soil profile because Ti

compounds are also relatively immobile during weathering (Sposito, 1989). The ratios of

the immobile trace elements Zr and Y relative to Ti should remain constant throughout

the profile and parent rock. This appears to be the case for the Ti:Zr ratio but not strictly

so for the Ti:Y ratio (Figure 3.3).

Because Al and Ti are conserved in the soil profile, the nature of chemical

weathering can be determined by making concentration ratio diagrams based on the

assumption that Al and Ti are retained within the soil profile. Naturally, some Al and Ti

are moved during weathering, although their solubility and mobility is relatively low

(Gay and Grandstaff, 1980). The relative changes in abundances ofthe major elements

(Si02, Fe203, MgO, and CaO) in the profile relative to AI2O3 and Ti02 are shown in

Figures 3.4 and 3.5.

Si02 shows slight enrichment at the top ofthe profile relative to AI2O3 (Figure

3.4). Relative to Ti02, Si02 is very slightly enriched evenly throughout the profile, with

the exception ofthe lower portion ofthe B horizon (Figure 3.5). There is a gradual

upward increase in Si02 content from the parent rock to the paleosol (Figure 3.1). In modem soils the behavior of Si02 is important because it is often used to verify the intensity of leaching during soil formation. Under low to moderate leaching conditions,

Si02 is conserved in the soil profile (Sposito, 1989). Si02 is usually retained in clay minerals under such conditions (Gay and Grandstaff, 1980). However, under intense

leaching conditions, Si02 is removed (Sposito, 1989). Figure 3.4 illustrates that silicon

63 TIA' and Ti/Zr 100 200 '°° Horizon

A

B

C

Figure 3.3. Plots of Ti/Zr and Ti/Y as a ftmction ofposition in the profile (methods of Muhs et al., 1987). The abundance of Ti (in oxide weight %) is converted to ppm in order to obtain these ratios.

64 Concentration ratio 0.01 6.1 1 ' ^° Horizon

B

Figure 3.4. Concentration ratio diagram for the Palo Duro geosol samples showing enrichment and depletion of constituents Si02, Ti02, Fe203, MgO, and CaO relative to AI2O3. The equation used to obtain concentration ratio (CR) is CR = (Mweathered/Mparcnt)/(Al203weathered/Al203parent) where Mweathered is the concentration of oxide or element in a sample and Mparent is the concentration in the parent rock. Components plotting on the left side ofthe diagram are depleted relative to parent rock. Components plotting on the right side are enriched with respect to AI2O3 (method of Gay and Grandstaff, 1980).

65 Concentration ratio 0.01 0.1 1 10 Horizon

A

ENRICHMENT

CaO to 3 4

B DEPLETION

Figure 3.5. Concentration ratio diagram for the Palo Duro geosol samples showing enrichment and depletion of constituents Si02, AI2O3, Fe203, MgO, and CaO relative to Ti02. The equation used to obtain concentration ratio (CR) is CR = (Mweathered/Mparent)/(Ti02weathered/Ti02parent) where Mweathered is the concentration of oxide or element in a sample and Mparent is the concentration in the parent rock. Components plotting on the left side ofthe diagram are depleted relative to parent rock. Components plotting on the right side are enriched with respect to Ti02 (method of Gay and Grandstaff, 1980).

66 was conserved or only slightly depleted through most ofthe profile, but enriched in the uppermost part ofthe profile. Overall, relative Si02 content runs opposite to that of

AI2O3, indicating that weathering resulted in conservation of Al (and depletion of Si) in the lower part ofthe profile and the opposite higher in the profile. This is illustrated in

Figure 3.6. In addition, the presence of chert cement especially in the upper half of the profile suggests that Si02 has been enriched pedogenically.

Fe203 concentration increases from the parent rock to the paleosol B horizon but decreases in the A horizon (Figure 3.1). Fe203 shows enrichment in the middle ofthe profile, but Fe203 is depleted from the surface ofthe profile (Figures 3.4 and 3.5).

During weathering, the oxidation state of iron changes (Gay and Grandstaff, 1980). In modem soils oxidation occurs toward the surface and reduction may occur at lower depths in the soil (Soil Survey Staff, 1975). However iron oxidation states change during diagenesis as well. Diagenesis usually changes the iron from ferric (Fe^^) to ferrous

(Fe^O state, with little changes in the total Fe content (Veizer, 1973).

Bulk changes in iron content can however be used to show whether oxidation or reduction has occurred during weathering. If conditions are oxidizing during soil formation, iron mobilized by dissolution of primary minerals is oxidized to insoluble Fe^" and precipitates in the soil profile as ferric iron oxide or hydroxide minerals (Soil Survey

Staff, 1975). Under reducing conditions iron is reduced to the more soluble Fe^*, which may be removed by groundwater (Soil Survey Staff, 1975). Figures 3.4 and 3.5 illustrate that iron was retained or enriched in the B horizon, whereas most was lost from the A horizon. Therefore surface loss and subsurface gain of iron in the profile suggests that

67 Si02/R203

15 20 10 Horizon

B

C

Figure 3.6. Plots of weight-percent ratios of Si02 to R2O3 (Al203+Fe203+Ti02) in the Palo Duro geosol as a ftmction ofposition in the profile showing depletion of Si02 relative to immobile constituents (Al203+Fe203+Ti02) compared to the presumed parent rock (method of Muhs et al., 1987).

68 during weathering, soil waters were imder oxidizing conditions. Color mottling occurring in the profile suggest that reduction (gleying) may have occurred. Gleyed soils occur when local variations in the water table resulted in iron reduction and loss (Soil Survey

Staff, 1975). Color mottling formed during pedogenesis, resulting in iron oxidation state variations would not be expected to survive diagenesis, which also modifies oxidation states (Yaalon, 1971). Diagenesis may have had an impact in causing color changes in the paleosol profile.

The concentrations of MgO, CaO, K2O, Na20, P2O5, MnO, Zr, Ba, Sr, Y, Rb, Zn,

and Be decrease from the parent rock to the paleosol (Figures 3.1 and 3.2). In modem

soils, most of these constituents are depleted from the profile (Sposito, 1989). This is the

case for the Palo Duro geosol. MgO is depleted in the profile relative to AI2O3 and Ti02

(Figures 3.4 and 3.5), but shows shght enrichment in the middle part ofthe B horizon

along with Fe. Ca, K, Na, P, Zr, Ba, Sr, Y, Rb, Zn, and Be are all depleted in the profile

(Figures 3.4, 3.5, 3.7, 3.8, 3.9, 3.10, 3.11, and 3.12). Thebehavior of these elements

suggests that leaching was moderate to intense during soil formation.

The concentrations of V and Cr increase from the parent rock to the paleosol

(Figure 3.2). In modem soils, these trace elements are usually depleted in soil profiles

(Sposito, 1989). However, this is not the case for the Palo Duro geosol. V and Cr are relatively enriched especially in the uppermost and middle portions ofthe profile (Figures

3.9 and 3.10). Overall, the enrichment of these elements generally increases toward the

upper half of the profile, suggesting that V and Cr were originally associated with organic

matter in the surface horizon ofthe soil.

69 Concentration ratio

0.01 0.1 ""^ Horizon

2 . A

B 4

DEPLETION B

ENRICHMENT

C

Figure 3.7. Concentration ratio diagram showing depletion and enrichment of constituents MnO, K2O, Na20, and P2O5 relative to AI2O3 (method of Gay and Grandstaff, 1980).

70 Concentration ratio 0.01 0.1 1 10 Horizon

2 .

C>5 to

DEPLETION B

ENRICHMENT

Figure 3.8. Concentration ratio diagram showing depletion and enrichment of constituents MnO, K2O, Na20, and P2O5 relative to Ti02 (method of Gay and Grandstaff, 1980).

71 Concetration ratio 0.0 0.r—1 ^ 1.0 10.0 Horizon

C/5 CO 3 4

DEPLETION B ENRICHMENT

C

Figure 3.9. Concentration ratio diagram showing depletion and enrichment of trace elements Zr, Ba, Sr, Y, Cr, Rb, and V relative to AI2O3 (method of Gray and Grandstaff, 1980).

72 Concentration ratio 0.01 0.1 1 Horizon

Figure 3.10. Concentration ratio diagram showing depletion and enrichment of trace elements Zr, Ba, Sr, Y, Cr, Rb, and V relative to Ti02 (method of Gay and Grandstaff, 1980).

73 Concentration ratio 0.01 0.1 1 10 Horizon

2 .

3

to 3 4

B

DEPLETION ENRICHMENT

Figure 3.11. Concentration ratio diagram showing depletion and enrichment of trace elements Zn, Nb, Ni, Sc, Cu, and Be relative to AI2O3 (method of Gay and Grandstaff, 1980).

74 Concentration ratio 0.01 0.1 1 ^° Horizon

2

C/3 to 3 4

CO

B

DEPLETION ENRICHMENT

Figure 3.12. Concentration ratio diagram showing depletion and enrichment of trace elements Zn, Nb, Ni, Sc, Cu, and Be relative to Ti02 (method of Gay and Grandstaff, 1980).

75 The concentrations of Nb and Ni are slightly enriched from the parent rock to the paleosol (Figure 3.2). Nb and Ni are usually depleted from profiles of modem soils

(Sposito, 1989). However, in this paleosol Nb and Ni are sUghtly enriched (Figures 3.11 and 3.12). Overall, the enrichment of these elements is generally greatest in the lower half of the profile along with clay and iron oxides.

The concentrations of Sc and Cu are similar those of AI2O3, Ti02, and Si02. Sc

and Cu are enriched relative to AI2O3 in the uppermost part ofthe profile, but are

depleted throughout the rest ofthe profile (Figure 3.11). Relative to Ti02, Cu is enriched

in the profile with the exception ofthe middle part (Figure 3.12).

Intensity of weathering conditions

One method of measuring the degree of chemical weathering (or leaching) in the

soil profile is obtained by observing molecular weathering ratios as given by Gill and

Yemane (1996) for example. In the Palo Duro geosol, relative base loss (Al203/CaO +

MgO + Na20 + K2O) is very strong (Figure 3.13). According to Gill and Yemane (1996)

these values indicate removal of mobile cations by extreme leaching. The Al203:Si02

ratio or relative "clayeyness" suggests significant clay accumulation especially in the

bottom portion ofthe B horizon and consequent Al enrichment (Chapter W). Such

strong loss of mobile cations and clay enrichment is typical of soils that form in warm

tropical and subtropical environments (Brady, 1990). Neither calcification (reflected in

the CaO + MgO/Al203 ratio) or salinization (shown in the Na20/K20 ratio) are evident in

the paleosol profile.

76 ^ 53 c o ^2 in o d

+ »-

?^ om d a o o a ^ o u

If) «^9 T— e Si o 'y—t ^^

r< V) o T— CO "^ (/) o a> (/} .2 f B 2 O >> o in A •M o 'C ^ « c S o

c3 *^ 3 !« . "T? o ^

V) - ^ s C3 o o s 00 S>^ O) ro rn

Sample bO

77 Another method used in measuring the degree of chemical weathering ofthe Palo

Duro geosol can be obtained by calculating the chemical index ofalteration (CIA) ofthe profile in terms given by Nesbitt and Young (1982) using molecular proportions:

CIA = Al203/(Al203 + CaO + Na20 + K2O) x 100.

CL\ values in the profile are given in Table 3.3 and changes in these CIA values are

illustrated in Figure 3.14. CIA values for the profile are more than 85 (profile average =

91.2), suggesting the presumed parent material was weathered under intense leaching

conditions. Changes in CIA are reflected mainly by changes in the proportion of

feldspars and clay minerals or caused by grain size sorting during original deposition of

the parent material for the profile (Nesbitt and Young, 1982).

Oxygen and carbon isotopes

Stable carbon and oxygen isotope studies of calcrete (pedogenic carbonate) are

mostly confined to modem soils or to geologically young paleosols (Cerling, 1984).

However, there are some reports on studies of carbon and oxygen isotopes from calcrete

of older paleosols (e.g., Mora et al., 1991). The main reason for the fewer reports is that

most calcretes in paleosols of Precambrian through Mesozoic age may have undergone

extensive post-pedogenic (i.e., diagenetic) modification of carbonate chemistry (Mora et

al., 1991). Diagenesis typically involves the dissolution, replacement and/or

recrystallization of calcretes. As a result, diagenesis could alter the isotopic signatures of

these calcretes, resetting the isotopic values at each stage of replacement or crystallization

and so removing the original isotopic information (Cerling, 1984). Because of

78 Table 3.3. Chemical index ofalteration (CIA) values in the Palo Duro geosol.

Samplg CIA

Tr-1 92.4 Tr-2 86.5 Tr-3 96.7 Tr-4 95.1 Tr-5 85.1 Tr-6 91.2 Tr-7 50.1

79 CIA 50 60 70 80 90 100 Horizon

Figure 3.14. Chemical index ofalteration (CL\) in the profile plotted as a function of position in the profile (method of Nesbitt and Young, 1982).

80 diagenesis, the carbon and oxygen isotopic signatures in samples ofthe Palo Duro geosol calcrete may not reflect the original isotopic signatures. Here isotope data of calcretes in the Palo Duro geosol are compared with those from younger paleosols in the Quatemary

Blackwater Draw and Blanco Formations (Soliz, 1996) and Caprock caliche ofthe

Ogallala Formation (O'Reilly, 1996) in the Panhandle of Texas. Isotopic data for silcretes in the Palo Duro geosol are compared with those from cherts in the silicified

Permian Alibates Dolomite and silcrete from the Caprock caHche in the Ogallala

Formation (Bowers and Reaser, 1996). This may provide insight into the diagenetic history of calcrete and silcrete in the Palo Duro geosol.

Five representative nodule samples containing both chert and carbonate from the

Palo Duro geosol were analyzed for stable carbon and oxygen isotope in the Geoscience

Stable Isotope Laboratory. Samples labeled C, B, and A were collected from the Currie

Ranch in Little Sunday Canyon northwest of Caprock Canyon State Park in Randall

County. Sample SRC-IB was collected from a roadcut on highway 256 exposed east of

Silverton just north of Caprock Canyons State Park. Sample CC-IP was collected from a nearby profile in Caprock Canyons State Park.

Isotopic data are expressed in terms ofthe 6 notation, 6'^OPDB, and 6'^OSMOW,

6'^CPDB, where :

6''0 (permil) = ((''0/^'0)sample - (^^O/^'O)standard)/ (•'0/''0)standard) x 1000

(for carbon the ratio used is '^C/'^C). The standards used are from Pee Dee Belemnite

(PDB) and Standard Mean Ocean Water (SMOW).

81 Results ofthe analyses are given in Table 3.4. Samples C, B, and A are taken from different depths in the upper A horizon ofthe profile, and represent, successively. the top, middle, and bottom. The Palo Duro geosol has 6'^0(PDB) values of-2.98, -3.69, and -3.94 permil relative to PDB, respectively for the carbonate portions of samples C, B. and A. The decrease in 6'^0 values relative to PDB and SMOW with increasing depth is correlated with 6'^C(PDB) values for the carbonate of-4.96, -5.73, and -6.42 permil, respectively in samples C, B, and A. This gradual upward enrichment trend is common in soil carbonates ofthe Blackwater Draw and Blanco Formations (Soliz, 1996) and

Caprock caliche (O'Reilly, 1996) and also in soil carbonates of modem soils (Cerling,

1984). 6^^0(SMow) values in soil carbonates ofthe Blackwater Draw and Blanco

Formations and Caprock caHche (range = +26 to +25 permil) are lower than those in the

Palo Duro geosol (range = +27.8 to +26.8 permil).

The relationship between the 6'^0(PDB) and 6'^C(PDB) values for calcrete in modem soils is used by Cerling (1984) to illustrate the climate settings at the time of soil formation (Figure 3.15). The average 6*^0(PDB) and 6'^C(PDB) values in calcretes ofthe

Blackwater Draw Formation plot in the typical continental setting whereas the Caprock caliche is plotted between the typical continental and monsoonal settings. The average

6''0(pDB) and 6''C(PDB) values from the Palo Duro geosol calcretes plot just below the typical continental values in the coastal environment setting. This reflects slightly enriched 6''O(PDB) values and/or depleted 6*'C(PDB) values relative to typical continental calcretes. The relationship between 6'^0(PDB) in calcrete and mean annual temperature

82 Table 3.4. Stable carbon and oxygen isotope ratios of selected carbonate and chert samples from the A horizon ofthe Palo Duro geosol.

CARBONATE CHERT Sample Yield (%) 6'^C(PDB) 5'«0(PDB) 5'«0(SMOW) Yield (%) 5'«0(SMOW)

c 86.8 -4.96 -2.98 +27.66 102 +32.8 B 89.4 -5.73 -3.69 +27.05 101 +33.4 A 83.4 -6.42 -3.94 +26.79 105 +33.8 SRC-IB — — — — 99 +29.5 CC-IP — — — — 101 +31.8

Chert samples were treated with hydrochloric acid to remove carbonate from the chert samples. The overall precision ofthe isotope data is ±0.1 permil for the carbon and oxygen isotopes. Yield is the amount of carbon and oxygen released relative to the theoretical amount.

83 5 - r -100

0 UJ

O -5 03 Q: (D < O -30- o O -10 fO -10- » • CO —0- A. Continental C. Monsoonal -15 B. Coastal D. Periglacial i 1 1 1 -20 -15 -10 -5 0 5sl8nCARBONATE 0 UpQB

Figure 3.15. Climatic settings based on carbon and oxygen isotopes signatures of soil carbonate (from Cerling, 1984). Plotted are the average isotope signatures from the Palo Duro geosol (X), Ogallala Formation Caprock caliche (•), and Blackwater Draw and Blanco Formations (A). Isotopic data for the Caprock caliche are from O'Reilly (1996). Isotopic data for the Blackwater Draw Formation are from Soliz (1996).

84 suggests a temperature of 20'^C for precipitation ofthe Palo Duro geosol calcrete (Cerling,

1984).

The overall 6'^0(SMOW) compositional values ofthe chert portion ofthe Palo Duro

geosol vary with the localities at which the nodules were collected (Table 3.4). Samples

C, B, A collected from the Currie Ranch have 5'^0(SMOW) values of+32.8, +33.4, and

+33.8 permil, respectively. A sample from the roadcut east of Silverton has a 5'^0(SMOW)

value of+29.5 permil and the sample from a nearby profile has a value of+31.8 permil.

These high 5'^O(SMOW) values are also found in cherts ofthe Alibates Dolomite (range =

+28.2 to +32.2 permil, average = +30.0 permil) and Ogallala Formation (range = +29.7 to

+31.1, average = +30.4 permil). Because of these high 5^^0(SMOW) values in the Alibates

and Ogallala samples. Bowers and Reaser (1996) suggested that these cherts crystallized

in a relatively cold, oxygen-enriched environment. Their interpretation may also be

applied to the chert portion ofthe Palo Duro geosol calcrete because of its similar values.

Assuming that pedogenic calcrete and silcrete in the Palo Duro geosol are in

isotopic equilibrium, calibration curves for A'^O(SMOW) (quartz-calcite) fractionation are

used to estimate the apparent temperature of their formation. These calibration curves are

shown in Figure 3.16 and estimated apparent temperature of formation of selected calcrete and silcrete samples are listed in Table 3.5. According to the calibration curve of

Clayton (unpubl.) the precipitation ofthe calcrete and silcrete occurred under low temperature («9 to 38°C) compatible with its origin as a product of soil formation at or near the land surface where mean soil temperatures rarely exceed 30^C. This also agrees with the mean annual temperature estimate of about 20°C based on the A'^OSMOW values

85 Quartz-Calcite Oxygen Isotope Fractionation

X - 106/T2

Sharp and Kirschner (1S94) linear A - 0.87'X

o o Sharp and Kirschner (1994) poly A - 1.02'X-0.004-x2 O oo

Temperature CC)

Figure 3.16. Calibration curves for the A'^0 (quartz-calcite) fractionation of Clayton (unplubl.) from Karlsson (personal commimication) and Sharp and Kirschner (1994). The estimated apparent temperature of forrmation of sample A was undetermined according to polynomial curves of Clayton and Sharp and Kirschner, suggesting non-equilibrium of isotopes.

86 Table 3.5. Apparent temperatures of formation of selected carbonate and chert samples, estimated according to calibration curves for the A'^0 (quartz-calcite) fractionation of Clayton (unpubl.) and Sharp and Kirschner (1994). A'^0 (quartz-calcite) values are obtained from Table 3.4. Their calibration curves are shown in Figure 3.16.

Clayton Sharp and Kirschner Sample A'^OrSMOW) (permil^ polvnomial polvnomial linear

C 5.14 38°C 115°C 140°C B 6.35 9°C 36°C 98°C A 7.01 79°C

The overall precision ofthe estimated apparent temperatures is ±3°C.

87 ofthe carbonate. In contrast, the apparent temperature of formation estimated from

calibration curves of Sharp and Kirschner (1994) are much higher («36 to 115°C from a polynomial curve and «79 to 140°C from a linear curve). These ranges of temperature

suggest that the calcrete and silcrete may have formed at deeper depths as a result of

diagenesis, not in a near surface environment.

The 5^^C(PDB) values for calcrete suggest the presence of at least 50% C4 flora at

the time ofthe paleosol formed (Cerling, 1984). However, it is not likely that C4 flora

existed during Triassic time. Therefore it is more probable that a significant atmospheric

CO2 component was involved in calcrete precipitation.

88 CHAPTER TV

CLAY MINERALOGY

Methods of Investigation

Bulk powder analysis

Bulk whole rock samples from the Palo Duro geosol were ground lightly by hand using a mortar and pestle. The same samples were used for thin-section petrography and major/trace element analysis. An attempt was made to reduce the mostly fine to medium sand sized samples into fine grained powders. Prolonged grinding was required to obtain completely uniform particle sizes for each ofthe samples.

Small amounts ofthe powdered sample were packed into an aluminum holder and exposed to Cu K a radiation generated by a Phillips X-ray diffractometer. The samples were scanned from 2 to 70 degrees at a speed of 2 degrees 2 6 per minute at 40 Kv and 20 mA with a 0.2 mm receiving slit. The scanning angle of up to 70 degrees was chosen because it is the angle at which most ofthe common minerals show their diffraction peaks. The diffraction pattems from a chart recording were used to identify the minerals present in the samples. The bulk mineralogy from the seven samples is listed in Table

4.1.

Clay mineral analvsis

One gram of each powdered sample was placed into a glass vial. Distilled water was added to the samples in order to make a suspension. The glass vial was shaken for

89 Table 4.1. Mineralogy ofthe Palo Duro geosol from X-ray diffraction of bulk powder samples.

Sample Tr-1 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 6.14 7.21 9.2 2000 184 1.56 kaolinite 6.75 6.56 6.1 2000 122 1.03 microcline (?) 10.02 4.43 6.6 2000 132 1.12 zircon (?) 10.47 4.24 98.8 2000 1976 16.75 quartz 12.46 3.57 11.5 2000 230 1.95 kaolinite 13.33 3.34 59 20000 11800 100.00 quartz 14.74 3.03 4.6 2000 92 0.78 calcite 17.5 2.56 6.1 2000 122 1.03 kaolinite 18.26 2.46 54.4 2000 1088 9.22 quartz 18.86 2.38 6 2000 120 1.02 kaolinite 19.25 2.34 7.8 2000 156 1.32 kaolinite 19.72 2.28 49 2000 980 8.31 quartz 20.16 2.24 37.6 2000 752 6.37 quartz 21.23 2.13 51.8 2000 1036 8.78 quartz 22.28 2.03 5.6 2000 112 0.95 dolomite (?) 22.91 1.98 14.4 5000 720 6.10 quartz 25.08 1.82 43.1 5000 2155 18.26 quartz 27.46 1.67 39.4 2000 788 6.68 quartz 27.68 1.66 19.8 2000 396 3.36 quartz 28.61 1.61 6.3 2000 126 1.07 quartz 30 1.54 92.4 2000 1848 15.66 quartz 31.17 1.49 7.5 2000 150 1.27 calcite (?) 32.03 1.45 7 5000 350 2.97 quartz 32.93 1.42 3.6 5000 180 1.53 quartz 33.89 1.38 26 5000 1300 11.02 quartz 34.13 1.37 42.5 5000 2125 18.01 quartz

90 Table 4.1 Contmued

Sample Tr-2 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 6.17 7.17 14.3 2000 286 2.77 kaolmite 10.02 4.43 9.5 2000 190 1.84 zircon (?) 10.45 4.43 67.4 2000 1348 13.06 quartz 12.43 3.58 15.2 2000 304 2.95 kaolinite 13.33 3.34 51.6 20000 10320 100.00 quartz 14.73 3.03 8.4 2000 168 1.63 calcite 16.65 2.69 6.7 2000 134 1.30 hematite 17.5 2.56 7.9 2000 158 1.53 kaolinite 17.86 2.51 9.3 2000 186 1.80 hematite 18.28 2.46 44.4 2000 888 8.60 quartz 19.3 2.33 10 2000 200 1.94 kaolmite (?) 19.74 2.28 40.9 2000 818 7.93 quartz 20.18 2.24 26.7 2000 534 5.17 quartz 21.25 2.13 31 2000 620 6.01 quartz 22.93 1.98 10.8 5000 540 5.23 quartz 25.1 1.82 36.3 5000 1815 17.59 quartz 27.48 1.67 34.9 2000 698 6.76 quartz 27.7 1.66 17.5 2000 350 3.39 quartz 28.66 1.61 6.5 2000 130 1.26 quartz 30 1.54 66.8 2000 1336 12.95 quartz 31.13 1.49 9.8 2000 196 1.90 calcite (?) 32.08 1.45 16.4 2000 328 3.18 quartz

91 Table 4.1 Continued

Sample Tr-3 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 6.12 7.23 13 2000 260 2.50 kaolinite 6.69 6.62 6.4 2000 128 1.23 microclme (?) 9.98 4.45 9.4 2000 188 1.81 zircon (?) 10.5 4.24 97 2000 1940 18.65 quartz 12.42 3.58 16.8 2000 336 3.23 kaolinite 13.33 3.34 52 20000 10400 100.00 quartz 17.5 2.56 8.2 2000 164 1.58 kaolinite 17.91 2.51 8.7 2000 174 1.67 hematite 18.28 2.46 49.9 2000 998 9.60 quartz 18.88 2.38 7.6 2000 152 1.46 kaolinite 19.26 2.34 11.5 2000 230 2.21 kaolinite (?) 19.75 2.28 50.2 2000 1004 9.65 quartz 20.19 2.24 24 2000 480 4.62 quartz 21.26 2.13 35.1 2000 702 6.75 quartz 22.27 2.03 8.3 2000 166 1.60 dolomite (?) 22.94 1.98 28.1 2000 562 5.40 quartz 25.1 1.82 98.5 2000 1970 18.94 quartz 27.48 1.67 38.1 2000 762 7.33 quartz 27.7 1.66 19.3 2000 386 3.71 quartz 28.56 1.61 7 2000 140 1.35 quartz 30.01 1.54 73 2000 1460 14.04 quartz 31.11 1.49 10.6 2000 212 2.04 calcite (?) 32 1.45 23.2 2000 464 4.46 quartz 32.83 1.42 7.4 2000 148 1.42 quartz

92 Table 4.1 Continued

Sample Tr-4 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 6.19 7.15 12.7 2000 254 3.32 kaolinite 10.02 4.43 8.8 2000 176 2.30 zircon (?) 10.48 4.24 79.2 2000 1584 20.68 quartz 11.17 3.98 12.9 2000 258 3.37 dolomite (?) 12.44 3.58 14.3 2000 286 3.73 kaolinite 13.33 3.34 38.3 20000 7660 100.00 quartz 14.86 3.01 6.1 2000 122 1.59 calcite 16.6 2.7 14.8 2000 296 3.86 hematite 17.51 2.56 8.1 2000 162 2.11 kaolinite 17.82 2.52 17.4 2000 348 4.54 hematite 18.27 2.46 35.9 2000 718 9.37 quartz 18.86 2.38 6.5 2000 130 1.70 kaolinite 19.29 2.33 9 2000 180 2.35 kaolinite (?) 19.73 2.28 34 2000 680 8.88 quartz 20.17 2.24 32.6 2000 652 8.51 quartz 20.42 2.21 8 2000 160 2.09 dolomite (?) 21.24 2.13 26.8 2000 536 7.00 quartz 22.3 2.03 6.8 2000 136 1.78 dolomite (?) 22.92 1.98 18.6 2000 372 4.86 quartz 24.77 1.84 9.8 2000 196 2.56 kaolinite (?) 25.09 1.82 67.5 2000 1350 17.62 quartz 27.09 1.69 10.9 2000 218 2.85 rutile 27.47 1.67 29 2000 580 7.57 quartz 27.65 1.66 12.9 2000 258 3.37 quartz 28.66 1.61 6.6 2000 132 1.72 quartz 30.01 1.54 61.3 2000 1226 16.01 quartz 31.19 1.49 13.5 2000 270 3.52 calcite (?) 32.19 1.45 19 2000 380 4.96 quartz 32.89 1.42 6.9 2000 138 1.80 quartz

93 Table 4.1 Continued

Sample Tr-5 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 3.03 14.58 21.6 2000 432 6.77 smectite 6.14 7.21 6.7 2000 134 2.10 kaolinite 9.9 4.48 17.2 2000 344 5.39 smectite 10.47 4.24 58.3 2000 1166 18.28 quartz 12.64 3.52 10.3 2000 206 3.23 kaolinite 13.33 3.34 31.9 20000 6380 100.00 quartz 17.52 2.56 14.1 2000 282 4.42 kaolinite 18.26 2.46 31.2 2000 624 9.78 quartz 19.72 2.46 27 2000 540 8.46 quartz 20.16 2.24 20.2 2000 404 6.33 quartz 21.23 2.13 34 2000 680 10.66 quartz 22.29 2.03 8.9 2000 178 2.79 dolomite (?) 22.91 1.98 18.6 2000 372 5.83 quartz 24 1.9 6.3 2000 126 1.97 anatase 25.08 1.82 82.1 2000 1642 25.74 quartz 27.45 1.67 22.2 2000 444 6.96 quartz 27.63 1.66 13.8 2000 276 4.33 quartz 28.55 1.61 8.2 2000 164 2.57 quartz 30 1.54 51.3 2000 1026 16.08 quartz 30.95 1.5 15.8 2000 316 4.95 calcite (?) 31.98 1.46 13.1 2000 262 4.11 quartz 32.93 1.42 8.7 2000 174 2.73 quartz

94 Table 4.1 Continued

Sample Tr-6 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 3.02 14.63 12.1 2000 242 3.78 smectite 6.14 7.21 13 2000 260 4.06 kaolinite 9.93 4.47 14.2 2000 284 4.44 smectite 10.46 4.24 44.4 2000 888 13.88 quartz 12.42 3.58 17.6 2000 352 5.50 kaolinite 13.33 3.34 32 20000 6400 100.00 quartz 16.54 2.71 10 2000 200 3.13 hematite 17.48 2.57 8.9 2000 178 2.78 kaolinite 17.8 2.52 17.1 2000 342 5.34 hematite 18.25 2.46 42.2 2000 844 13.19 quartz 18.82 2.39 10 2000 200 3.13 kaolinite 19.3 2.33 13.4 2000 268 4.19 kaolinite (?) 19.71 2.28 29 2000 580 9.06 quartz 20.15 2.24 20.4 2000 408 6.38 quartz 21.22 2.13 31.3 2000 626 9.78 quartz 22.9 1.98 24.8 2000 496 7.75 quartz 25.07 1.82 57 2000 1140 17.81 quartz 27.45 1.67 31.2 2000 624 9.75 quartz 27.64 1.66 18.3 2000 366 5.72 quartz 28.6 1.61 9 2000 180 2.81 quartz 29.99 1.54 60.6 2000 1212 18.94 quartz 31.1 1.49 14.8 2000 296 4.63 calcite (?) 31.97 1.46 18.4 2000 368 5.75 quartz

95 Table 4.1 Continued

Sample Tr-7 Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 6.82 6.49 8.3 2000 166 1.75 microcline 10.48 4.24 78.7 2000 1574 16.64 quartz 10.97 4.05 9.5 2000 190 2.01 dolomite (?) 11.79 3.77 10.1 2000 202 2.14 plagioclase 12.1 3.68 14.3 2000 286 3.02 plagioclase 12.79 3.48 10.4 2000 208 2.20 microcline 13.33 3.34 47.3 20000 9460 100.00 quartz 13.74 3.25 31.1 2000 622 6.58 microcline 13.98 3.19 33.3 2000 666 7.04 plagioclase 15.2 2.94 10.3 2000 206 2.18 magnetite 15.43 2.9 16.2 2000 324 3.42 dolomite 17.54 2.56 8.9 2000 178 1.88 magnetite (?) 18.25 2.46 47.1 2000 942 9.96 quartz 18.73 2.4 7.5 2000 150 1.59 anatase 19.72 2.28 40.3 2000 806 8.52 quartz 20.17 2.24 27 2000 540 5.71 quartz 20.51 2.2 8.9 2000 178 1.88 dolomite (?) 20.84 2.17 9 2000 180 1.90 dolomite (?) 21.22 2.13 41 2000 820 8.67 quartz 22.24 2.04 9 2000 180 1.90 dolomite (?) 22.9 1.98 27.3 2000 546 5.77 quartz 25.12 1.82 91.8 2000 1836 19.41 quartz 25.23 1.81 18.3 2000 366 3.87 quartz 26.56 1.72 13.8 2000 276 2.92 anatase (?) 27.42 1.67 29.6 2000 592 6.26 quartz 27.59 1.66 17 2000 340 3.59 quartz 28.62 1.61 8.8 2000 176 1.86 quartz 29.27 1.58 8.7 2000 174 1.84 quartz 30.01 1.54 67 2000 1340 14.16 quartz 30.91 1.5 8.7 2000 174 1.84 calcite (?) 31.97 1.46 22.5 2000 450 4.76 quartz

96 one minute until the sediments were saturated by the solution. The sample was placed in an automatic shaking machine for 15 more minutes in order to fully disperse the clay.

The sample was then placed in a ultrasonic cleaner for 5 minutes to increase the dispersion ofthe fine particles.

Separation ofthe clay sized fraction (2 microns or less) from the coarser fraction was accomplished by using settling rates according to Stoke's Law (Jackson, 1965).

Approximately two hours of settling time were needed to accomplish the separation per attempt. After two hours of settling, a clay suspension was made by pipetting the uppermost portion ofthe vial onto a 120 mL plastic cup. Distilled water was added into the vial after each pipette. The procedure was repeated until the water remained clear or until all ofthe clay fraction was taken out ofthe vial. The clay suspensions from the plastic cup were pipetted onto glass slides and allowed to dry at room temperature. The air-dried slides were scanned from 2 to 45 degrees at a speed of 2 degrees 2 0 per minute.

These angles covered the basal reflections of all the clay minerals.

The same slide was then sprayed with ethylene glycol and placed in a jar sealed with glycol saturated atmosphere ovemight to allow time for expansion ofthe clay minerals. A shift in the position ofthe clay mineral peaks indicated the existence of expandable clay minerals in the clay fractions ofthe samples. The glycolated slide was scanned from 2 to 40 degrees at a speed of 2 degrees 2 6 per minute on the same pattem as before so that the reflections are superimposed. The mineralogy ofthe clay sized fraction from the seven samples is listed in Table 4.2.

97 Table 4.2. Mineralogy ofthe Palo Duro geosol from X-ray diffraction ofthe clay-sized (<2 micron) slide samples.

Tr-1 SUde Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 6.18 7.16 17 2000 340 13.93 kaolinite 10.43 4.26 36.1 2000 722 29.59 quartz 12.44 3.58 23.3 2000 466 19.10 kaolinite 12.61 3.53 17.2 2000 344 14.10 kaolinite 13.33 3.34 48.8 5000 2440 100.00 quartz 18.24 2.46 18.2 2000 364 14.92 quartz 18.93 2.38 10.4 2000 208 8.52 kaolinite 19.73 2.28 13.7 2000 274 11.23 quartz 20.14 2.24 11.8 2000 236 9.67 quartz 21.21 2.13 15.3 2000 306 12.54 quartz

Tr-2 SUde Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 5.6 7.9 9.1 2000 182 11.43 (?) 6.21 7.13 27.2 2000 544 34.17 kaolinite 10.46 4.25 28.6 2000 572 35.93 quartz 12.46 3.57 30.9 2000 618 38.82 kaolinite 13.33 3.34 79.6 2000 1592 100.00 quartz 14.73 3.03 15.3 2000 306 19.22 calcite 16.63 2.69 13.2 2000 264 16.58 hematite 18.28 2.46 14.3 2000 286 17.96 quartz 18.76 2.4 11.3 2000 226 14.20 kaolinite 19.76 2.28 12.5 2000 250 15.70 quartz 20.18 2.23 10.6 2000 212 13.32 quartz 21.27 2.13 11.6 2000 232 14.57 quartz

98 Table 4.2 Continued

Tr-3 Slide Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 2.49 17.74 8.4 2000 168 11.20 smectite 3.58 12.35 7.4 2000 148 9.87 smectite 6.17 7.17 28.2 2000 564 37.60 kaolinite 10.43 4.26 23.5 2000 470 31.33 quartz 12.45 3.58 32 2000 640 42.67 kaolinite 13.33 3.34 75 2000 1500 100.00 quartz 17.09 2.62 13 2000 260 17.33 hematite (?) 18.26 2.46 13 2000 260 17.33 quartz 18.85 2.39 11.2 2000 224 14.93 kaolinite 19.29 2.33 10.4 2000 208 13.87 quartz 19.74 2.28 11.7 2000 234 15.60 quartz 20.22 2.23 10.3 2000 206 13.73 quartz 21.23 2.13 12.1 2000 242 16.13 quartz

Tr-4 SUde Theta (degree) d-spacing (A) Peak height Scale factofac r Peak Intensity I/I'(highest) mineral 2.46 17.96 11.1 2000 222 13.06 smectite 6.14 7.21 37.2 2000 744 43.76 kaolinite 10.37 4.28 23.5 2000 470 27.65 quartz 12.41 3.59 40.5 2000 810 47.65 kaolinite 13.33 3.34 85 2000 1700 100.00 quartz 16.49 2.72 14.9 2000 298 17.53 hematite 17.77 2.53 12.6 2000 252 14.82 hematite 18.21 2.47 12 2000 240 14.12 quartz 18.77 2.4 12.1 2000 242 14.24 kaolinite 19.71 2.29 10.8 2000 216 12.71 quartz 21.17 2.13 12.9 2000 258 15.18 quartz

99 Table 4.2 Continued

Tr-5 Slide Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 3.58 12.35 64.9 5000 3245 100.00 smectite 4.38 10.09 4.5 5000 225 6.93 illite 5.12 8.64 5.9 5000 295 9.09 smectite 6.15 7.2 6.5 5000 325 10.02 kaolinite 7.13 6.21 6.2 5000 310 9.55 smectite 10.41 4.27 9 5000 450 13.87 quartz 12.43 3.58 22.5 2000 450 13.87 kaolinite 13.33 3.34 67.5 2000 1350 41.60 quartz 14.33 3.11 36.2 2000 724 22.31 smectite 18.27 2.46 12.4 2000 248 7.64 quartz 18.83 2.39 11 2000 220 6.78 kaolinite 19.22 2.34 10.6 2000 212 6.53 kaolmite 20.11 2.24 10.7 2000 214 6.59 quartz 21.21 2.13 12.9 2000 258 7.95 quartz

Tr-6 SHde Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 3.54 12.49 38 2000 760 45.73 smectite 4.32 10.23 13 2000 260 15.64 illite 6.16 7.18 78.2 2000 1564 94.10 kaolinite 8.83 5.02 9.8 2000 196 11.79 smectite (?) 10.38 4.28 22.7 2000 454 27.32 quartz 12.44 3.58 77.6 2000 1552 93.38 kaolinite 13.33 3.34 83.1 2000 1662 100.00 quartz 14.33 3.11 18 2000 360 21.66 smectite 18.79 2.39 17 2000 340 20.46 kaolinite 21.17 2.13 12.8 2000 256 15.40 quartz

100 Table 4.2 Continued

Tr-7 Slide Theta (degree) d-spacing (A) Peak height Scale factor Peak Intensity I/I'(highest) mineral 2.41 18.33 8.2 2000 164 13.55 smectite 4.39 10.07 6.8 2000 136 11.24 illite 5.53 8 11.3 2000 226 18.68 smectite (?) 6.16 7.18 6.4 2000 128 10.58 kaolmite 6.76 6.55 10.5 2000 210 17.36 smectite (?) 10.4 4.27 18.2 2000 364 30.08 quartz 11.8 3.77 15.4 2000 308 25.45 plagioclase 12.11 3.67 15.5 2000 310 25.62 plagioclase 12.61 3.53 17.4 2000 348 28.76 anatase 13.33 3.34 60.5 2000 1210 100.00 quartz 13.71 3.25 21.4 2000 428 35.37 feldspar 13.95 3.2 23.3 2000 466 38.51 plagioclase 15.46 2.89 20.5 2000 410 33.88 dolomite 17.05 2.63 17.1 2000 342 28.26 magnetite (?) 18.23 2.46 12.5 2000 250 20.66 quartz 19.7 2.29 11 2000 220 18.18 quartz 20.11 2.24 10.1 2000 202 16.69 quartz 21.2 2.13 11.3 2000 226 18.68 quartz

101 Mineralogv from Bulk Sample Analysis

The parent rock (Tr-7) and profile samples (Tr-1, Tr-2, Tr-3, Tr-4, Tr-5, and Tr-6) consist mainly of quartz (Table 4.1). Besides quartz, the parent rock also has minerals microcline, plagioclase, magnetite, anatase, calcite (?), and dolomite (?). These minerals were also identified petrographically (Chapter II) with the exception of anatase.

The A horizon ofthe paleosol profile (Tr-1, Tr-2, and Tr-3) contains quartz, kaolinite, calcite, dolomite (?), zircon (?), and microcline (?). The middle and lower portion ofthe A horizon also contains hematite. The B horizon (Tr-4, Tr-5, and Tr-7) contains quartz, hematite, dolomite (?), and zircon (?). The middle and lower portion of the B horizon (Tr-5 and Tr-6) also has smectite and anatase. The bulk mineralogy ofthe profile is in keeping with observations based on thin-section petrography. The

occurrence of zircon and mtile (anatase) is, along with the predominance of quartz, an indication of prolonged chemical weathering ofthe profile.

Mineralogy from Clay-Separation Analysis

The minerals collected in the clay fraction (less than 2 microns) in the parent rock

and profile samples are listed in Table 4.2. Quartz remains a major component in the fine

fraction ofthe paleosol. The clay fraction ofthe parent rock sample (Tr-7) also contains

smectite, illite, kaolinite, plagioclase, anatase, feldspar, dolomite, and magnetite (?). The

clay fraction ofthe A horizon consists mostly of kaolinite. The middle portion ofthe A horizon also has hematite, and the lower portion ofthe horizon includes smectite. The B

horizon contains smectite, kaolinite, and hematite. The middle and lower portion ofthe B

102 horizon also contains illite. Figure 4.1 shows clay concentration at different zones of the profile. The A horizon contains about 35% clay while the B horizon contains up to 50% clay. The profile as a whole has a higher clay content than the parent rock (about 20% clay). These are relative percentages based on weighing the air-dried suspensions. Clay content determined by actual point counting of thin sections does not exceed 5% (Table

2.1). Nevertheless, point count data and weights ofthe air-dried suspensions both

indicate that mud content (clay and silt) is higher in the paleosol profile than in the parent material, and higher in the B horizon compared to the A horizon. This is consistent with preserved indications of clay illuviation observed in the profile (see micromorphologic

features. Chapter II).

Figure 4.2 illustrates the relative abundances of clay minerals in the paleosol. The

relative abundances of clay minerals are determined semi-quantitatively by comparison of

intensity peak readings of each clay mineral type in the paleosol with those of pure

standard clay minerals according to the method in Moore and Reynolds (1989). The parent rock (Tr-7) clays consist mostly of illite with small amounts of smectite and kaolinite. Upward through the paleosol profile, the amount of illite decreases, and

smectite increases, peaking in abundance with the upper B horizon. In the upper part of the A horizon, only kaolinite is present. A similar clay mineral zonation has been found in modem soils and other paleosols (Tremocoldi et al., 1994; Birkeland, 1969).

103 Relative clay content (%) 0 10 20 30 40 50 HorizoH

Figure 4.1. A diagram showing the concentration of clay-sized particles of less than 2 microns (in weight percent) represented at different depths ofthe Palo Duro geosol profile (according to Stoke's Law method after Jackson, 1965). This is determined by weighing the air-dried clay suspensions.

104 Clay minerals

20 40 60 flO 100 LLoiizuii

Kaolinite

C/) Smectite B 4

B

C

Figure 4.2. The relative abundance of clay minerals in the Palo Duro geosol profile as indicated by the X-ray diffraction ofthe oriented glass slide samples.

105 CHAPTER V

DISCUSSION

Review of Processes and Factors of Soil Formation

To investigate any soil (either recent or ancient), it is important to understand the processes and factors that resuh in the formation of soil. Soils form at the land surfaces as open systems (Sposito, 1989). The open system represents a boundary between earth and air through which energy and materials are exchanged with the surrounding atmosphere, biosphere, and hydrosphere. The basic types of movements of materials within and around soil include additions, subtractions, transfers, and transformations

(Buol et al., 1989). Additions include mineral grains or organic matter brought into the soil, for example as airbome dust. Subtractions include removal by surface erosion of minerals and organic matter, or for example materials lost in solution with water.

Transfers involve the movements of material within a soil profile typically to a lower position, and transformations are changes in the composition ofthe soil components.

The major factor in controlling soil composition and formation is the parent material ofthe soil (Buol et al., 1989). Soils derived from weathering of igneous, sedimentary, and metamorphic rocks are different from one another. Other factors, generally with lesser controlling effects are climate, topographic setting, organisms and time (Retallack, 1990).

Parent rock materials at the surface ofthe earth are typically not in equilibrium with the temperature, pressure, and moisture conditions ofthe atmosphere (Birkeland,

106 1984). Because ofthe disequilibrium, weathering occurs. Weathering results in the alteration of rocks and minerals and decomposition and/or modification of rocks and minerals to more stable forms at or near the surface environment (Buol et al., 1989). The two main processes of weathering are physical and chemical. Physical weathenng involves the breakup of rocks and minerals into smaller fragments without any chemical or mineralogical changes. Chemical weathering is a change in the chemical and/or mineralogical composition of rocks and minerals. The major chemical weathering processes of soil formation include hydrolysis, oxidation-reduction, dissolution, and hydration (Buol et al., 1989). The products of weathering result in accumulation of resistant minerals, and the formation of new clay minerals and iron oxides in soils.

Alteration of Soil Profile After Burial

Once a soil has formed, it may undergo later alteration at the surface or following burial by younger sediments. The term "diagenesis" is used to describe alteration after burial. Diagenesis ofthe soil begins at as little as 1 bar pressure and temperature from

84°C (Kimmins, 1987). With time, continued diagenesis will eventually turn loose soil material into rock or paleosol, and may obscure or destroy the original soil features.

There are several types ofalteration that paleosols are subjected to (Buol et al.,

1989). Compaction involves deformation ofthe soil profile by the lithostatic load from overlying sediment or rock. Cementation is the induration of loose grains or minerals in soils by precipitation of new minerals in pore spaces. Neomorphism involves the recrystallization of minerals in soils. Authigenesis is the formation of new minerals in

107 place within enclosing sediment. Replacement involves the changing of one mineral to another mineral. Dissolution is the removal of soil components by solution and transport in pore water. Hydration-dehydration reactions, common in soil environments, involve the addition or subtraction of water to mineral stmcture. Reduction-oxidation reactions involve the transfer of electrons. Cation exchange is the displacement of a cation bound to a site on the surface of a solid by a cation in solution.

Development ofthe Soil Profile

The Palo Duro geosol represents an "alluvial" soil, in that it was developed on older sediments, not on crystalline igneous or metamorphic bedrock ("residual" soil). The quartz and rock fragment types in the parent material indicate that these sediments were recycled from older sedimentary rocks in an orogenic provenance. Sediments ofthe paleosol and presumed parent rock were ultimately derived from plutonic rock. During weathering, unstable minerals (plagioclase, orthoclase, microcline, magnetite, and micas) present in the presumed parent rock were almost completely removed or transformed in the soil profile, while the more stable minerals (quartz, chert, and zircon) were retained.

Fine detrital grains in the parent rock were removed by erosion, solution, or translocation to lower positions in the soil, resulting in a coarsening-upward profile.

The elemental constituents AI2O3 and Ti02 were retained in the paleosol profile relative to the parent rock. This is observed and expected in modem soils and these constituents are used as references to determine the enrichment or depletion of other constituents in the profile relative to the parent rock. Overall, Si02 shows a distribution

108 similar to AI2O3 and Ti02 and is only shghtly enriched in the profile. Total Fe as Fe203, is depleted in the upper portions ofthe profile, but enriched in the B horizon. This is consistent with oxidation of iron in the profile and its translocation to a lower position

(podzolization in modem soils). Most ofthe minor elemental constituents MgO, CaO,

Na20, K2O, P2O5 and trace elements Zr, Ba, Sr, Y, Rb, Zn, and Be are mildly or strongly depleted, whereas V, Cr, Nb, and Ni show slight enrichment in the profile. The distribution of these constituents indicate that extreme leaching occurred during the time of soil formation, resulting in most ofthe mobile elements being removed in solution.

The Palo Duro geosol profile developed as a consequence of intense leaching by downward percolation of soil water. This resulted in the formation of a highly quartzose eluvial A horizon. Leaching dissolved virtually all ofthe unstable detrital components of the matrix from A horizon. At depth, dissolution and transformation products released by breakdov^m of unstable components were deposited with the formation of clays and iron oxides in the B horizon.

Formation ofthe Clay Minerals

According to Eberl (1984), clay minerals in soils originate by three mechanisms, including inheritance, neoformation, and transformation. Origin of clay minerals by inheritance results from reactions that occurred in another region during a previous stage in the rock cycle, and the clay is stable enough to remain unchanged in its present environment. Origin by neoformation involves the precipitation of clay minerals from solution as a resuh of in situ conditions. Origin by transformation requires that the cla\

109 has kept some of its inherited stmcture while undergoing chemical reactions through ion exchange or layer transformation. Ion exchange occurs when loosely bound ions are exchanged with those ofthe surrounding environment. Layer transformation involves the modification of arrangements of tightly bound octahedral, tetrahedral, or fixed interlayer cations.

Clay minerals in the Palo Duro geosol profile probably formed as a result of alteration of primary silicate minerals and subsequent clay mineral transformation.

During formation ofthe Palo Duro geosol, the feldspathic minerals ofthe presumed parent rock (orthoclase, plagioclase, and microcline) were almost completely destroyed or altered. The alteration ofthe feldspars probably led to the formation ofthe clay minerals in the profile (Abdel-Wahab and Tumer, 1991). Figure 4.2 illustrates the clay mineral distribution in the profile with depth. These changes may represent the "time sequence" of clay formation (Singer, 1980). The lower portions ofthe profile may contain the

"inherited" detrital clay minerals and are the clays in a relatively less advanced weathering stage. The upper portions ofthe profile usually contain the clay minerals most advanced in the weathering sequence. These observations suggest that kaolinite, concentrated mainly in the upper portions ofthe profile, is the most advanced clay mineral to have formed as a result of feldspar decomposition. Kaolinite typically can be precipitated from water solutions over a wide range of pH conditions (Dixon, 1977) and is represented by the following equation (Hurst and Irwin, 1982):

2KAISi308 + H^ + H2O -> Al2Si205(OH)4 + 4Si02 + 2K\ orthoclase kaolinite

110 Dissolution involves the K+ cation being removed or leached from the silicate stmcture by acidic water. Blanche and Whitaker (1978) beheved that flushing with water is an important process in kaolinization of feldspars. According to the above equation, the amount of kaolinite formed in the profile is dependent on the acidity and amount of water flushed through the profile during soil formation. The existence of kaolinite in the paleosol suggests that the profile was developed in a very well drained soil environment.

In modem soils, kaolinite is most abundant in warm climates (Dixon, 1977). Apart from the direct dissolution ofthe feldspars, kaolinite could also have been produced by the decomposition of other detrital silicates, including the micas or transformation of an earlier formed clay. Micas are observed in low amounts in the parent rock sample.

Smectite is concentrated mainly in the B horizon and in the lower portion of A horizon.

Figure 4.2 illustrates that the relative abundance of smectite is about 50% in the lower A horizon and upper B horizon. Like kaolinite, the smectite abimdance also decreases towards the base ofthe profile. During formation ofthe profile, smectite may have been produced by neoformation as a result of leaching of materials from the surficial A horizon. Smectites are often precipitated from soil solutions (Borchartdt, 1977). This usually involves the alteration of biotite and muscovite micas (Fanning and Keramidas,

1977). Illite is probably the original "inherited" detrital clay mineral in the profile, or could have been produced by partial degradation of detrital muscovite in the parent rock.

Illite is concentrated mainly in the lowermost portion ofthe soil profile and its relative abundance is highest in the parent rock sample (Figure 4.2). The illite in the parent rock probably transformed to smectite and/or ultimately to kaolinite during weathering.

Ill Paleoenvironment of Soil Formation

The Palo Duro geosol may be classified using the classification system of modem

soils in terms ofthe Soil Survey Staff (1975) as modified by Retallack (1988).

Retallack's modification ofthe USD A system was based on features that could be

observed in paleosols. The thick well differentiated profile ofthe Palo Duro geosol

consists of a surficial eluvial A horizon exhibiting total depletion or low concentrations of

Ca, Na, K, Mg, Mn, and P and with a subsurface illuvial B horizon (argillic or spodic

horizon) enriched in clay and iron oxides and elements Al, Fe, and Mg. The paleosol

contains pedogenic features (i.e., illuvial channels and clay coatings) that are still preserved in the profile. These features and soil horizonations are similar to those of

strongly developed modem soils (Alfisols, Spodosols, and Ultisols). These observations

and the alteration indices and molecular weathering ratios (Figure 3.13) indicate that the

Palo Duro geosol is probably an Ultisol (Retallack, 1990). An Ultisol is a soil formed in

well drained environments in a climate with high precipitation rates. The most likely

climate at the time of soil formation is humid, as indicated by the persistent occurrence of

kaolinite (Lander et al., 1991) and smectite (Borchardt, 1977) throughout the soil profile.

The illuvial stmctures observed in the paleosol indicate that the profile required at least

several thousand years to form (Birkeland, 1974).

In contrast, the presence of calcrete and silcrete observed in the Palo Duro geosol

contradict the inferred well drained humid climate under which the soil profile had

formed. Calcretes usually occur in soils of arid to semi-arid environments (Cerling,

112 1984). This suggests that the Palo Duro geosol may have evolved through different climatic conditions during a prolonged episode of landscape stability during Triassic time. The changes may have included the initial weathering ofthe presumed parent rock in a humid climate that resulted in the mineralogical and chemical variations observed in the profile. Later, the climatic conditions became dryer and resulted in the formation of calcrete and ultimately silcrete. These climatic changes would not have changed the leached kaolinitic character in the profile as seen in modem soils (Eberl, 1984).

Therefore perhaps two climatic signals are preserved in the Palo Duro geosol.

113 REFERENCES

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119 APPENDIX A.

POINT COUNT DATA FOR THE PALO DURO GEOSOL SAMPLES

120 Table A.l Detrital Grains

Ouartz

Sample Mono/Straighl : Mono/Undulose Polv/Straieht Polv/Undulose total Ouartz

Tr-1 347 19 14 2 382 Tr-2 235 23 44 8 310 Tr-3 190 9 45 2 246 Tr-4 162 20 30 6 218 Tr-5 99 9 13 3 124 Tr-6 166 30 10 1 207 Tr-7 303 23 30 8 364

Feldspar

Sample Plagioclase Orthoclase Microcline total Feldspar

Tr-1 0 0 0 0 Tr-2 0 0 0 0 Tr-3 0 0 0 0 Tr-4 0 0 0 0 Tr-5 0 0 0 0 Tr-6 0 0 0 0 Tr-7 3 5 5 13

Rock Fragments

Sample Sedimentarv Metamoroh ic Volcanic total Rock Fragments

Tr-1 25 0 0 25 Tr-2 25 0 0 25 Tr-3 53 0 0 53 Tr-4 13 0 0 13 Tr-5 26 0 0 26 Tr-6 47 0 0 47 Tr-7 4 5 0 9

121 Table A.l Continued

Sedimentary Rock Fragments

Siltstone/Shale/ total Sedimentary Sample Chert Sandstone Length-fast chalcedony Length-slow chalcedony Rock Fragments

Tr-1 22 1 0 2 25 Tr-2 25 0 0 0 25 Tr-3 40 3 7 3 53 Tr-4 13 0 0 0 13 Tr-5 26 0 0 0 26 Tr-6 43 1 2 1 47 Tr-7 4 0 0 0 4

Heaw/Accessorv Minerals

Sample Zircon Magnetite total Heavy/Accessory Minerals

Tr-1 0 0 0 Tr-2 1 0 1 Tr-3 1 0 1 Tr-4 0 0 0 Tr-5 0 0 0 Tr-6 0 0 0 Tr-7 2 3 5

CEMENTS and PORE SPACE total

Tr-1 206 12 16 0 234 21 255 Tr-2 137 13 31 17 198 23 221 Tr-3 113 0 20 0 133 17 150 Tr-4 13 1 278 2 294 13 307 Tr-5 125 0 43 0 168 1 169 Tr-6 45 4 298 11 353 2 355 Tr-7 0 8 9 0 17 152 169

122 Table A.2 Grain Size

coarse very fine fine medium coarse very coarse Sample mud silt sand sand sand sand sand granule

Tr-1 4 25 46 97 179 57 4 0 Tr-2 7 11 54 90 138 •30 4 0 Tr-3 3 13 40 59 131 37 10 12 Tr-4 1 17 42 65 90 17 2 0 Tr-5 6 5 30 44 56 18 0 1 Tr-6 6 20 35 68 93 30 5 0 Tr-7 0 8 176 139 84 10 0 0

Table A.3 Grain Roundness

well very Sample rounded rounded subrounded subangular angular angular

Tr-1 6 115 235 52 0 0 Tr-2 2 38 211 80 1 0 Tr-3 0 33 212 59 1 0 Tr-4 0 27 112 67 24 2 Tr-5 1 26 106 22 5 0 Tr-6 4 96 133 22 2 0 Tr-7 12 142 215 42 6 1

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