Submitted to the Department of Geology of The

Home , Atlantic (period), Early Pleistocene, Turlock Lake Formation

GEOLOGY AND GROUND WATER OF A PORTION OF EASTERN STANISLAUS COUNTY, SAN JOAQUIN VALLEY, CALIFORNIA

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOLOGY AND THE COMMITTEE ON GRADUATE STUDY OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Francis Ramey Hall June i960 Icertify that Ihave read this thesis and that in my opinion it Is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy,

Icertify that Ihave read this thesis and that in ray opinion it is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy.

Approved for the University Committee on Graduate Study:

II TABLE OF CONTENTS

AouxixiiAsa...... J. SUMMARY AND CONCLUSIONS ...... 4

INTRODUCTION *.♥...... 9 Location, Climate, Agriculture, Industry .... 9 Purpose and Scope 15 Previous Investigations ... 16 Location Numbers 19 Acknowledgments ...... 21

{x&OIJJUiX ...... a...**....*..*.. (~3

stratigraphy «.*.«"""«""".»."*. «-o Pre-Pllocene ...... 27 Pre-Cretaceous ...... 27 Cretaceous ..... 29 lone Formation . 31 Valley Springs Formation ...... 34 Lower and Middle Pliocene 37 Mehrten Formation , 37 Upper Pliocene and Pleistocene ...... 55 Differentiation of post-Mehrten Formations 55 Soils and Topographic Expression in Geologic Mapping , 57 Turlock Lake Formation 63 Riverbank Formation 75 Modesto Formation . 90

Recent """""""""*""""""".. 99 Sand Dunes " 99 Alluvium 104 iii TABLE OF CONTENTS-Continued

Structure 106 Geologic History...... 108 GROUND WATER 114 Occurrence of Ground Water . 114 !Tne Water Table 114 Slope and Shape 116 Depth to Water 119 Fluctuations 119 Recharge ...... 120 Precipitation 120 Irrigation ...... 124 Streams 124 Subsurface Inflow ... 125 Discharge 126 Evapotranspiration 126 Pumpage from Wells 127 Streams 127 Subsurface Outflow 130 Water-bearing Properties of the Sediments . 130 Data from Wells 131 Chemistry of Water 135 Chemical Character of Precipitation, Imported Irrigation Water, Saline Water and Streams 138 Precipitation 146 Irrigation Water 152 Streams 156 Saline Water 166 of Ground Chemical Character Water ♥ . 177 Normal Ground Water 179 Exceptions to Normal Ground Water . 182 Summary of Chemical Processes and Constituents and Physical Properties of Water 189

IV TABLE OF CQNTENTS-£ontlnued

REFERENCES CITED 210

APPENDICES A. Measured Sections ..... 218 B. Well Logs ... 251 C. Sieve Analyses ...... 27b D. Selected Chemical Analyses 280

LIST OF ILLUSTRATIONS

Plates

1. Cultural features of eastern Stanislaus County, California . 11 2. Geology of a portion of eastern Stanislaus County, California in pocket 3. Generalized geologic section . . 24 4. Chloride content of water from wells 50» - 150 1 deep 200

Figures

1. Index map showing location of area studied * 10 2. Average monthly precipitation at Modesto and Oakdale, average monthly potential evapotranspiration at Modesto, and average monthly pan evaporation at Woodward Reservoir .... 12 3. Mean monthly maximum, minimum, and average temperatures at Modesto and Oakdale ... 14 4. Summary of formations 25 5. Correlation chart of the Cenozoic (from Durham, 1954) 26

V LIST OF ILLUSTRATIONS-Sontinued

6. Upper surface of the Mehrten Formation. Dotted line gives approximate western limit of area of continuous outcrop of the formation 39 7. Mehrten Formation and post-Mehrten channel fill in road cut on Rodden Road near Orange Blossom Bridge east of Oakdale ... 40 8. Contact between Mehrten and Turlock Lake Formations in road cut on Rodden Road east of Oakdale 41 9. Lower part of Mehrten Formation in road cut just north of Knights Ferry. Exposure shows pronounced cross bedding 44 10. Cumulative curves for samples from the Mehrten Formation 46 11. Cumulative curves for samples from the Turlock Lake Formation ..... 648 12. Soil developed on Turlock Lake Formation on Steams Road east of Oakdale 66 13. Exposure of sand in the Turlock Lake Formation near the type section at Turlock Lake State Park 68 14. Gravel in Turlock Lake Formation at the type section at Turlock Lake State Park . . 70 15. Cumulative curves for samples from the Riverbank Formation ..... 77 16. Soil developed on the Modesto Formation overlying a buried soil developed on the Riverbank Formation near Waterford 80 17. Contact between Riverbank and Modesto (?) Formations on Highway 120 just north of uaicciaae ...... oa. 18. Upper surface of blue clay as determined from drillers1 logs . 85 19. Upper surface of Riverbank Formation* as determined from drillers logs and surface exposures. Wells marked with a cross indicate anomalous elevation .... 86

VI LIST OF ILLUSTRATIONS-Continued

20. Cumulative curves for samples from the Modesto Formation , 92 21. Sand and silt in Modesto (?) Formation on Highway 120 just north of Oakdale ... 95 22. Sand dune on Blue Gum Avenue west of Modesto 100 23. Cumulative curves for samples from dune sands ♥ . 105 24. Water-level map, late spring and early summer, 1957 117 25. Hydrograph of well 3-9-32 Al in Modesto . .. 121 26. Hydrograph of well 2-10-11 Nl in Oakdale ... 122 27. Location map for chemical analyses ...... 137 28. Trilinear graph and expanded fields 140 29. Regular scale trilinear graph, Stanford and U. S. Geological Survey analyses ,.. 142 30. Expanded scale trilinear graph, Stanford and U. S. Geological Survey analyses . .. 143 31. Regular scale trilinear graph, Twining and other analyses 144 32. Expanded scale trilinear graph, Twining and other analyses 145 33. Regular scale trilinear graph, selected analyses 149 34. Chloride content of Tuolumne River 16l 35. Chloride content and discharge of Tuolumne River at Tuolumne City 162 36. Depth to non-potable water as determined from chemical analyses of water and electric logs 167 37. Generalized hardness and sulfate map of the 50-150-foot zone 197

VII LIST OF ILLUSTRATIOHS-Continued

38. Chloride content of ground water from depths of oto 50 feet. Circles indicate wells sampled and figures indicate ppm chloride 201 39* Chloride content of ground water from depths of 50-130 feet in Modesto area . . . 202 40. Chloride content of ground water from depths of 150 feet to top of Mehrten Formation ...... 203 41. Chloride content of water from wells in the Mehrten Formation. Figures indicate ppm chloride 204

Tables

1. Mehrten Formation grain count 47 2. Turlock Lake Formation grain count 64A 3. Pebble counts 64C 4. Riverbank Formation grain count 78 5. Modesto Formation grain count . . 93 6. Sand dunes grain count ,, 101 7. Water usage, 1957 128 8. Summary of well data . . , . , 132 9. Bata for saline water 170 10. Summary of ratios for various waters 174

VIII ABSTRACT

The oldest rocks that crop out in the portion of eastern Stanislaus County covered in this study are the andesitic sands, silts, and gravels of the Mehrten Forma- tion of Early and Middle Pliocene age found along the eastern edge of the area. The Mehrten is underlaid In descending order by l) rhyolitic sands, silts, and gravels of the Valley Springs Formation of Miocene age, 2) sands, clays, and gravels of the lone Formation of Eocene age, 3) an unnamed marine unit of Eocene age, 4) marine shale and sandstone of Cretaceous age, and 5) basement rocks

composed of metamorphic rocks of Jurassic age with associated intrusives. All of the units older than the Mehrten crop out east of the study area except the unnamed

marine Eocene and the Cretaceous rocks which are found only in the subsurface. The Mehrten is overlaid In ascending order by alluvial fan deposits of granitic origin divided into three units: l) Turlock Lake Formation of Late Pliocene and Early Pleistocene age, 2) Riverbank

Formation of Middle Pleistocene age, and 3) Modesto Formation of Late Pleistocene age. All units are overlaid in places by Recent alluvium and sand dunes. The structure of the area Is simple with all formations gently dipping westward. Chemical quality of water data suggest the 1 2 existence of a westward trending structure beneath the Tuolumne River. Ground water of generally good chemical quality is found at shallow depths In the area. The slope of water levels Is gently westward and toward streams. The streams receive ground water except the Tuolumne River and Dry Creek at Modesto where surface water enters the Modesto pumping cone. The shallow ground water Is underlaid at varying depths in most of the area by saline and brackish water. Most ground-water recharge is from imported Irrigation water and some is from precipitation. Large amounts of ground water are pumped for agricultural, industrial, and public supply uses. Imported surface water and precipitation are calcium bicarbonate waters with low dissolved solids. Streams have generally calcium bicarbonate waters with rather low dissolved solids except the Tuolumne River which receives saline water from gas wells. The saline waters which underlie most of the area are sodium calcium chloride to calcium chloride waters with high dissolved solids. The hypothesis is made that the saline waters were derived from connate waters in marine Cretaceous and

Eocene rocks which moved into younger sediments and under- went natural hardening. The normal ground water of the area Is a calcium bicarbonate type with rather low dissolved solids which are lower in the east and higher in the west. Normal ground water is derived from 3 precipitation and irrigation water by concentration by evapotranspiration and increase in C02 in the 3oiland *>y base exchange and solution of minerals in the soil and sediments. Sodium bicarbonate waters in ponds and shallow ground waters are derived from normal ground waters by evaporation and precipitation of calcium and magnesium carbonates. Practically all ground waters in the area that are not normal ground waters, saline waters, or sodium bicarbonate waters can be shown to be a mixture of the other types. SUMMARY AND CONCLUSIONS

The contributions of this study are essentially of two types: l) factual findings and 2) development and application of new principles. The tangible results of the study are listed below. Some consideration is given also to the new principles. The concept of the use of soils in geologic mapping is not new; so, strictly speaking, no original contribution Is made in the use of soils. Nevertheless, the concept has not been applied widely, and the present study demonstrates clearly the great value of soils in geologic mapping. The work shows that the degree of development of soils on alluvial deposits can be used to delineata geologic units, obtain relative age sequence, and obtain an idea of absolute age. A more original contribution has been made in the application of ordinary chemical analyses of water to the study of ground water. The history of the ground water in an area has been developed by the use of chemical analyses in conjunction with a knowledge of the geology . and hydrology. Studies of this kind are rare, and little guidance was obtained from the works of others. Some noteworthy conclusions are: 1) samples for chemical analyses 4 5

should be carefully selected based on all available knowledge in order to obtain maximum information from a minimum number of samples, and 2) regular chemical analyses made by inexpensive methods are quite useful. Another contribution is the consideration of the amounts of dissolved solids brought to an area by rainfall and imported water. The results show that with only slight modification, considerable normal ground water could be derived nearly directly from rainfall and imported water through simple concentration by evapotranspiration. The emphasis on the Importance of the contribution of dissolved solids by rainfall may seem extreme; however, this aspect has been almost completely Ignored in a large number of

The more general conclusions of this study are summarized as follows: 1. The post-Mehrten sediments are derived from a granitic source, and they can be subdivided into at least three formations on the basis of development of soils and topographic expression of the alluvial fan deposits. The resulting units are from oldest to youngest the Turlock Lake, Riverbank, and Modesto Formations. The Turlock Lake probably represents more than one sequence of alluvial fan deposits, but field evidence is not sufficient to allow further subdivision. 2. Two units found in the subsurface are of value in studying the geology of the area. The younger Is a red 6

clay which represents the soil developed on top of the Riverbank Formation. The older is a blue clay present within the Riverbank Formation. The distribution of the blue clay Indicates that the study area was on the edge of a wide-spread lake of probable Middle Pleistocene age. 3. Chemical quality of water data from relatively shallow water wells suggest the presence of a westward trending structural feature at depth beneath the Tuolumne River. 4. The ground water is mainly recharged by surface water imported for irrigation and obtains only a relatively small amount from precipitation. 5. Water levels show that the ground-water gradient trends gently westward and toward the streams. 6. The streams all receive ground water during times of normal flow except at the Modesto pumping cone where water from Dry Creek and Tuolumne River enters the ground-water body. 7. The shallow or upper ground water of the area Is a calcium bicarbonate water with relatively low dissolved solids content. This type of ground water is called normal ground water in this study. Normal ground water is derived from precipitation and imported surface water by concentration by evapotranspiration and increase in C02 with some modifications by exchange and solution in the soil and sediments. 7

8. Brines present in gas wells along Tuolumne River show that the area between Dry Creek and Tuolumne River is underlaid by saline water with rather high dissolved solids content. Limited data from oil and/or gas test wells and deeper water wells indicate that the saline water is also present west of Modesto and Salida. The saline water is absent at least to a depth of 700 feet in the vicinity of Oakdale and Riverbank. Little is known about the area just north of Dry Creek but south of Oakdale and Riverbank. 9. Chemical data suggest that the saline water is derived from connate water trapped in Cretaceous and probably Eocene marine sediments. The connate water was modified by natural hardening and base exchange as it moved into overlying continental sediments. Sulfate reduction has occurred also. 10. Sodium bicarbonate waters are found in places In the western part of area. Evidence from alkali ponds shows that such waters can be derived from normal ground water by concentration by evaporation and precipitation of calcium and magnesium carbonate. 11. All ground waters in the area that are not normal ground water, saline water, or sodium bicarbonate water probably represent a mixture of the other types. 12. Saline water may undergo further natural hardening as it moves upward into sediments occupied by normal ground water. 8

13. The withdrawal of large amounts of shallow ground water may allow saline water to rise and mix with the shallower waters. This probably has occurred In the Modesto pumping cone. INTRODUCTION

LOCATION, CLIMATE, AGRICULTURE, INDUSTRY Eastern Stanislaus County is in the northeastern part of the San Joaquin Valley (Figure l) which makes up the southern two-thirds of the Central Valley physiographic province of California. The portion of eastern Stanislaus County to be discussed is bounded on the north by the Stanislaus River, on the south by the Tuolumne River, on the west by the San Joaquin River, and on the east by a line just east of Oakdale and Waterford (Plate l). The area enclosed within these boundaries is approximately 220 square miles. The major features of the climate are hot, dry summers and cool, wet winters. Temperature distribution is rather uniform throughout the area, but average annual rainfall increases from about 10 inches at the San Joaquin River to about 14 inches at the eastern boundary. Climatologic data are presented for weather stations at Modesto and at Woodward Reservoir north of Oakdale (Weather Bureau, 1958aand 1958b; Arkley, 1959, p. 2). The precipitation data (Figure 2) show that most of the rainfall occurs during December, January, February, and March and that practically no rainfall occurs during June, July, and August; whereas the pattern for potential

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FIG. 2 13 evapotranspiration and evaporation from a water surface (Figure 2) are just the reverse. The monthly and annual ranges of temperature are indicated in Figure 3. Summer temperatures commonly are above 85°F, and may exceed 100°F., but they are rarely In excess of 105°F. Winter temperatures commonly fall below 32°F., but they are rarely lower than 25°F. The hot, dry summers require the use of irrigation for the most important types of agriculture. A major portion of the water used for irrigation is supplied by local districts which are owned and controlled by the people within the districts. The three districts and the acreage served within the study area are; 1) the Oakdale Irrigation District which supplies water from the Stanislaus River to about 25,000 acres, 2) the Modesto Irrigation District which supplies water from the Tuolumne River to about 68,000 acres, and 3) the Waterford Irrigation District which supplies Tuolumne River water to about 2,000 acres. The Oakdale and Modesto Irrigation Districts also pump some ground water which is discharged into the irrigation canals where it mixes with the imported river water.

The types of agriculture are in large part a function of soil fertility which is controlled to consid erable extent by geologic factors. The oldest alluvial soils in the low hills to the east are used for grazing, unlrrigated grain, and, where capable of Irrigation, for Fig. 3.--Mean monthly maximum, minimum, and average temperatures at Modesto and Oakdale. OAKDALE-WOODWARD RA- 746541- 19

FIG. 3 15 clover and alfalfa. The older alluvial soils on the somewhat dissected, rolling plain west of the low hills are used for clover, pasture, and almonds. The soils with maximum fertility are on younger alluvium in the flat plain between Modesto and Salida. Crops raised include peaches, walnuts, apricots, grapes, berries, and various row crops. Toward the western edge of the area the soils are poorly drained and are used for row crops, clover, and alfalfa. In places where the poorly drained soils are high in alkali, they can be used only for unirrigated pasture. Eastern Stanislaus County has primarily an agrleul tural economy although considerable industrial expansion has taken place since World War 11. Most of the industry is related to agriculture and is heavily concentrated in the Modesto area. Many of the industrial plants require large quantities of water which they obtain from wells.

PURPOSE AND SCOPE

This report is based on work supported by a grant from the Stanford Research Institute. The grant was made for fundamental research in ground water, and eastern Stanislaus County was chosen as the study area. The entire project has been under the direction of Professor Stanley N. Davis of the School of Mineral Sciences, Stanford University, and the field work extended from 16 the summer of 1955 to the spring of 1958. Davis has done reconnaissance work over the entire area and detailed work south of the Tuolumne River. The present author made a detailed study of the geology and ground water in the area north of the Tuolumne River from the winter of 1956 to the spring of 1958. John Foster mapped the geology of the Cooperstown, Turlock Lake, Paulsell, and Montpeller Quadrangles which lie east and southeast of the present study area. Ronald Eddlngton collected water samples, located wells, and assembled subsurface data for parts of the area. Keith Kvenvolden logged and assembled other data from a test hole drilled in the western part of the area. The major emphasis of this study has been on the geology and chemical quality of water with a lesser emphasis on the hydrology of the area. The primary purpose of this report is to give a description of the geology and to interpret the chemical quality of water as considered within the geological and hydrologlcal framework of the area.

PREVIOUS INVESTIGATIONS Prior to the Stanford project no detailed studies of the geology or ground water of eastern Stanislaus County have been published. Various aspects of the geology and ground water, however, have been discussed in earlier reports. The first published reports concerning 17

the occurrence of ground water and its chemical quality in Stanislaus and Merced counties can be found in the Tenth Annual Report of the State Mineralogist (Watts, 1890a, 1890b). These reports show an awareness of the area of mineralized ground water which lies along both sides of the San Joaquin River. Also, mention is made of a well 1070 feet deep at the Stanislaus County Hospital in Modesto which yielded inflammable gas and mineral water. The United States Geological Survey made the first important study of hydrology, geology, and chemical quality of water In the San Joaquin Valley. The results of this work are included in preliminary reports by Mendenhall (1908) and Van Winkle and Eaton (1910) and in a final report by Mendenhall, and others (1916). The report by Mendenhall, and others (1916) contains about 450 chemical analyses, of which 15 are from the present study area. Well records are given to go with the analyses, and records are Included for some wells that were not sampled. A water-level map for the San Joaquin Valley is given, and it shows an artesian area parallel to the San Joaquin River. In the report by Mendenhall, and others (1916, pp. 38-141) Dole came to the following important conclusions concerning regional trends in the San Joaquin Valley: l) on the east side, water from 20-1000 feet is generally of the calcium carbonate type, moderate in total 18 solids, and low in sulfate, commonly less than 5 ppm; 2) on the east side, water from less than 50 feet deep is usually poorer In quality than water from wells slightly deeper; 3) axial and west side waters are high in sulfate, commonly 100 ppm or greater; 4) axial waters tend to be relatively high In sodium and potassium; 5) the San Joaquin River forms the approximate boundary between high sulfate on the west and low sulfate on the east; and 6) ground water from below 1100 feet has a dissolved solids content more than 10 times larger in the northern end than in the southern end of the San Joaquin Valley. The difference In sulfate content on the east and west sides was considered to be due to the different sources of sediments. The east side sediments came from sulfate-free granitic and metamorphic rocks; whereas the west side sediments came from sedimentary rocks rich in gypsum. Published reports of the Ground Water Branch of the U. S. Geological Survey and the California Department of Water Resources and unpublished reports of the U. S. Bureau of Reclamation contain the results of many geologic, hydrologic, and quality of water studies in the San Joaquin Valley. None of these reports, however, covers the study area in eastern Stanislaus County in any detail. A recent report by the U. S. Geological Survey (Davis, and others, 1957) is the first major report on the San Joaquin Valley since the work by Mendenhall, and others, (1916), 19

and it verifies many of the conclusions of the earlier work. The report by Davis, and others (1957) presents several geologic and geochemical sections across the study area and gives some description of the geology and specific yield of the sediments. The soils of eastern Stanislaus and eastern Merced counties have been mapped in detail (Arkley, 1954, 1959). The soil reports also contain some geologic and hydrologic information. A recent report by the Stanislaus County Planning Commission (Anonymous, 1957a) contains a compilation of data concerning water usage in Stanislaus County.

The basic chemical data obtained during the

Stanford project along with some preliminary Interpretation and a sketch of the geology and hydrology of eastern Stanislaus and northern Merced counties have been presented in a report by Davis and Hall (1959).

LOCATION NUMBERS

Thelocations of wells, river-sampling stations, measured sections, and geologic samples are given according to a code number based on the public land survey system. The code number is identical to the numbering system used for the designation of wells by the U. S. Geological Survey and the California Department of Water Resources. The first number is the township south of the Mount Diablo base, the second number is the 20 range east of the Mount Diablo meridian, and the third number is the section. The quarter-quarter section is indicated by a letter according to the following arrange- ment within the section:

DC B A EF G H ML X J NP Q R The following examples illustrate the use of the code number.

Location Number Standard Land Office Designation 2-10-30 B NW£ of the NEi of Sec. 30, T. 2 S., R, 10 E., MDB&M 4-12-5 C NEi of the NWi of Sec. 5, T. 4 S., R, 12 E,, MDB&M For wells the letter is followed by a final number indicating the order in which the wells have been inventoried. For example, 3-10-27 Jl indicates that this is the first well to be inventoried in the NE£ of the SEj- of Sec. 27, T. 3 S., R. 10 E., MDB&M. In a few grain counts and cumulative curves more than one sample was taken at a field locality. If the field locality is described in Appendix A the samples are differentiated by unit number, for example 4-9-2J, unit 8. If the field locality Is not described in Appendix A the differentiation is made by a lower case letter in parentheses, for example, 2-8-28 H(c). 21

ACKNOWLfiDGMENTS

The entire ground water research project In eastern Stanislaus County has been supported by the Stanford Research Institute with the interest and encouragement of Dr. Donald L. Benedict and Dr. Richard M. Foose of the Institute. The author expresses his personal thanks to the Institute and to these men for the aid received during his part of the project. The work has been done under the direction of and in close association with Professor Stanley N. Davis of the School of Mineral Sciences, Stanford University, and the author owes deepest gratitude to him for his constant interest and inspiration. James L, Hatchett of Stanford Research Institute reviewed the chemistry of water section Most of the office and laboratory work was done In the Geology and Mineral Engineering Departments of the School of Mineral Sciences, Stanford University. The Modesto

Irrigation District furnished office space during the summer of 1957. The success of this study is due in large part to the wholehearted cooperation and interest of many individ- uals and organizations in eastern Stanislaus County. Particular thanks are due to Charles Crawford, Carl Washburn,, Edward Ames, Marvin Ray, Allen Holbrook, and Howard Hennings. 22

The Standard Oil Company of California and the Tidewater Associated Oil Company supplied electric logs from test wells drilled In the study area. ' ■»» in mil miiiiiiiii

STRATIGRAPHY The Mehrten Formation, Early and Middle Pliocene in age, which is the oldest formation that crops out

in the study area is restricted to a thin belt along the eastern boundary (Plate 2). Rocks older than the Mehrten crop out further east and dip westward beneath the area. The geologic cross section from Cooperstown to the San Joaquin River (Plate 3), which crosses the study area, shows the general subsurface relations of the sedimentary rocks and the trend of the basement rocks. A summary of the formations that crop out in the area and those that occur in the subsurface is given in Figure 4. The relative dating and correlation of Cenozoic rocks in California are based on provincial stages. At present three sets of stages are in use (Figure 5): mammalian (nonmarine), megafaunal (marine), and microfaunal (marine). Work is also being done with fossil floras, and

the results seem in general to agree with the mammalian

stages.

The epochs of the Cenozoic depend on the definition of stages and their type sections in Europe. Gaps occur between some stages, and disagreements in correlation exist within the European type sections. As a result 23 2U

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MAMMALIAN PROVINCIAL AGES, MEGAFAUNAL'STAGES* AND MICROFAUNAL STAGES ON THE PACIFIC COAST-

Figure 5

Correlation Chart of the Cenozoic

(from Durham, 1554) 27

boundary problems arise between the Miocene and Pliocene and Pliocene and Pleistocene in Europe. This creates a major problem in California because epoch boundaries are determined by correlation of provincial stages with European stages (Axelrod, 1957* P. 23). The differences are primarily between megafaunal correlations on one side and mammalian, floral, and microfaunal correlations on the other.

The only boundary problem that is of major concern in the present study is the Mio-Pliocene boundary which involves the Valley Springs and Mehrten Formations. This boundary problem and related problems in California have been discussed in a number of recent papers (Axelrod, 1956; Savage, 1955; Stirton, 1951), but no real solution has been offered. Dating and correlation in the present study follow generally the usage of vertebrate paleontologists and paleobotanists because most of the geologic units of interest contain fossil floras and vertebrate faunas. Wherever the formations can be correlated &ith marine sections, then the current usage of marine invertebrate paleontologists will also be Indicated.

Pre-Pliocene Pre-Cretaceous The study area is underlain at varying depths by what is commonly referred to as basement or bedrock. The 28

exact character of this bedrock is not known because of limited subsurface information. However, an oil and gas test well drilled several miles west of Modesto (3-8-34 Ql) encountered basement at about 13*600 feet below sea level, and bottom cores Indicated that the material was granite. An oil and gas test well (3-11-23 ) drilled several miles east of Waterford encountered basement at about 3100 feet below sea level, but the published record (Jennings and Hart, 1956, p. 92-93) does not give the rock type. Bedrock is exposed at the surface about eleven miles east of well 3-11-23 at an elevation of approximately 300 feet above sea level. Wells 3-11-23 and 3-8-3^ 01 have been projected Into the geologic cross section (Plate 3). The bedrock slope is 270 feet/ mile from Cooperstown to just east of Waterford, 560 feet/mile from just east of Waterford to west of Modesto, and an average of 450 feet/mile from Cooperstown to west of Modesto. Between Knights Ferry and LaGrange the most abundant rock at the surface is metavolcanic schist with some chert, slate, and conglomerate. The rocks have been intruded by narrow, light-colored dikes, larger bodies of quartz porphyry, and a small body of granodiorlte which is exposed in the bed of the Stanislaus River at Knights Ferry. The metavolcanic rocks have been mapped as part of the Logtown Ridge Formation of Jurassic age (Eric, and others, 19551 Taliaferro and Solarl, 19^9). The Logtown 29

Ridge Formation is named for exposures of metamorphosed pyroclastic andesite along the Cosumnes River in Eldorado and Amador counties, California (Taliaferro, 1943). The most abundant rock between LaGrange and Merced Falls is a gray slate. These rocks also have been intruded by narrow light-colored dikes. The slate is probably a part of the Mariposa Formation of Jurassic age, which is named from exposures in Mariposa County, California (Becker, 1885, p. lB| Eric, and others, 1955* p. 12). The igneous intrusives in the Logtown Ridge and Mariposa Formations are probably either Late Jurassic or Early Cretaceous in age. Similar intrusives elsewhere in the foothills of the Sierra Nevada have been dated as Late Jurassic (Curtis, and others, 1958, p. 10).

Cretaceous Rocks of Cretaceous age do not crop out in eastern Stanislaus County or elsewhere along this part of the Sierra Nevada foothills. The nearest exposures of Cretaceous on the east side of the Central Valley are at Folsom about 70 miles north of Oakdale (Piper, and others, 1939, P. 87). Marine sandstone and shale of Cretaceous age have been encountered In oil and gas test wells in eastern Stanislaus County (Anonymous, 19^3* p. 661; Oakeshott and others, 1952, p. 45, 74$ Jennings and Hart, 1956, p. 92-93). well data and the slope of the pre-Cretaceous bedrock suggest that the Cretaceous 30 rocks wedge out In the subsurface a few miles east of Oakdale, Waterford, and Montpeller. A possible exception is indicated by a well (3-13-20 ) drilled some ten mile3 east of Waterford which is reported to have encountered Cretaceous (?) rocks (Anonymous, 1943* P. 661). The thickness of the Cretaceous varies from a feather edge in the east to more than 9500 feet in the west (Plate 3). A recent north-south correlation section through the central San Joaquin Valley (Church, and others, 1958) includes two wells that were used in constructing the cross section of the present report (Plate 3). Tidewater Associated Overton 3^-16, (well 3-17-16 Fl) is not shown on Plate 3 but was consulted, and Standard Young Community 1 (well 3-8-34 Ql) is shown on Plate 3. Both sections use the same data for the top of the Cretaceous. The correlation section (Church, and others, 1958) shows that most of the Cretaceous is Late Cretaceous in age. A major point of disagreement between the sections is that the correlation section shows the Eocene to be absent beneath the study area; whereas the cross section of the present report (Plate 3) has up to 800 feet of Eocene in the subsurface. The correlation section contains little further information about the post-Cretaceous sediments. The only published description of the Cretaceous in the vicinity is a brief summary of a well log from near Oakdale (Piper, and others, 1939* Plate 6). The well penetrated about 2100 feet of Cretaceous with 31 approximately 1280 feet of shale above and 850 feet of sandstone below. Electric logs from three wells passing through the entire thickness of the Cretaceous, two in eastern Stanislaus County and one in northern Merced County (Oakeshott, and others, 1952, p. 74), show that about 60 percent of the thickness is shale and 40 percent is sandstone. Limestone makes up less than 1 percent. On the west side of the San Joaquin Valley opposite eastern Stanislaus County a thick section is found consisting of more than 10,000 feet of Upper Cretaceous marine shale and sandstone and a thin section of 0-500 feet of Lower Cretaceous marine shale (Huey, 1948, p. 16). These Cretaceous rocks dip eastward beneath the San Joaquin Valley and are probably continuous in the subsurface with the Cretaceous rocks that underlie the study area. lone Formation The lone Formation is the oldest unmetamorphosed sedimentary unit exposed at the surface on the east side of this part of the San Joaquin Valley. The lone lies unconforraably on pre-Cretaceous bedrock at the surface. Stratigraphlc relationships to older marine Eocene and Cretaceous rocks in the subsurface are not known with any certainty, although an unconformity probably occurs between the lone and Cretaceous. The lone Is overlapped or eroded In places by the younger Valley Springs and 32

Mehrten Formations. The type section consists of exposures near lone In Amador County, California (Lindgren, 1894; Allen, 1929* P. 353-35^). In the type section the lone is characterized in the lower part by white clay and

sand beds which contain coal seams and by sandstone and quartz conglomerate in the upper part (Turner, 1894, p. 462-465). The uppermost clay rock or tuff unit described by Turner is now called the Valley Springs Formation. The sandstone may be white, pink, yellow, red, or gray In color. The lone Formation of eastern Stanislaus

County is quite similar to the type section except that

no coal seams are exposed, although coal is present In the subsurface in what is probably lone in well 3-8-34 Ql west of Modesto.

The thickness of the lone Formation in the outcrop area in eastern Stanislaus County is not known definitely, but based on outcrop width and regional dip it is estimated to be about 200 feet. The formation thickens to the west in the subsurface and, together with earlier unnamed marine Eocene rocks, reaches a thickness of about 800 feet. The unnamed marine Eocene is probably present near Oakdale (Piper and others, 1939* Plate 6, p. 85) and just west of Modesto (well 3-8-3^ 01). Marine Eocene is definitely present in a well in northern Merced County

(Oakeshott, and others, 1952, p. 74). Nothing further is known concerning the unnamed marine Eocene In the study area or immediate vicinity. 33

A correlation section down the central part of the San Joaquin Valley has the Eocene cut out to the north and south and absent beneath the study area with a Mehrten-Valley Springs unit resting directly on the Cretaceous (Church, and others, 1958). This is in disagreement with data given in the present report which indicate the presence of Eocene rocks in the subsurface. The presence of Eocene rocks in some wells and the fact that the lone Formation crops out on the east side of the Valley and an equivalent unit is found on the west side make it difficult to explain the absence of the Eocene in the subsurface as shown on the correlation section of Church and others (1958). The lone Formation of eastern Stanislaus County has been correlated with the type section by Allen (1929) who mapped the formation from the type section southward to the north bank of the San Joaquin River near Fresno.

Allen (1941j, p. 272) has also correlated the lone Formation with the Tesla Formation of Corral Hollow (Huey, 1948, p. 33-38) and similar rocks in the Coast Range on the west side of the San Joaquin Valley. The formation is mainly fluvial in origin but contains some lacustrine and lagoonal deposits. Part of the lone is marine because fossil marine pelecypods have been found in the upper part of the formation near Cooperstown (2-13-21 L) and Merced Falls (5-15-10 M). The Merced Falls locality is probably the same as the one described by Turner (1896, p. 659). 34

The fossils collected near Cooperstown are tentatively identified as Venerlcardla sp., a genus which has been reported from near the type section (Piper, and others, 1939* P. 84). On the basis of marine fossils the lone Formation has been correlated with the Capay (megafossil) stage (Merrlam and Turner, 1937; Allen, 1941, p. 272) which is considered to be Early Eocene in age (Durham, 195*0. However, if the underlying Meganos stage is considered Eocene rather than Paleocene then the Capay would be Middle Eocene, The lone Formation is not known to be penetrated by any water wells in the study area as It occurs at considerable depth beneath the surface. In general, the lone Is not a very good aquifer in its outcrop area because of a large amount of clay and the compactness of the sandstone and conglomerate. The presence of marine fossils In part of the lone Indicates that It is a potential source of saline water. The unnamed marine Eocene rocks where present beneath the lone Formation are also a possible source of saline water. Saline water is indicated in the electric logs of wells that penetrate the lone and marine Eocene,

Valley Springs Formation ■_ii-j--*>w-M-wii*--t^»tJL«-«-B---«^^ MOMWi" —I ■""" *>*iim»mwm<*mmmmt' The Valley Springs Formation crops out east of the study area (Plate 3) arid seems conformable with the under lying lone Formation and the overlying Mehrten Formation. 35

The Valley Springs, however, is either overlapped or missing in places, and the Mehrten rests on lone or pre-Cretaceous bedrock. The Valley Springs Formation is derived from erosion of rhyolitic material in the foothills and main Sierra Nevada, and consists of rhyolitic ash, sandy clay to clayey sand, and siliceous gravel all apparently deposited by streams. The clays in the lower part of the formation are similar to clays In the lone

Formation, and the contact is usually arbitrarily placed at the first occurrence of rhyolitic material, although this may be a number of feet above the actual contact. White pumice layers In the lower part of the Mehrten resemble the Valley Springs, although the pumice layers are set in a matrix of fine-grained andesitic sand. The top of the Valley Springs is considered to be the first occurrence of andesitic material. The rhyolitic sediments were first thought to be the upper part of the lone Formation (Turner, 1894, pp. 462 465), but they were separated from the lone by Allen (1929, p. 353). The name Valley Springs Formation was first applied by Piper, and others (1939* P. 72), and the type section is near Valley Springs in western Calaveras County, twenty-six miles north of Oakdale. The maximum thickness at the type section is about 450 feet; however, in eastern Stanislaus County the thickness seems to be about 270 feet (Davis and Hall, 1959* P. 8). At the type 36

section the Valley Springs consists of rhyolitic tuff or tuff breccia, conglomerate, pumice bearing sandstone and siltstone, and sandstone. The rhyolitic sediments of eastern Stanislaus County are considered equivalent to the type Valley „ __. aPrt s..m or u-u.«, MM» position between the lone and Mehrten Formations. Taliaferro and Solar! (19^9) also mapped Valley Springs in northern Stanislaus County, and, where their map overlaps that of the Stanford project, the outcrop patterns agree closely (Davis and Hall, 1959* p. 9). The age of the Valley Springs Formation near the type section has been determined as Late Miocene or Mio-Pliocene on the basis of three fossil floras found in a channel extending from Mokelumne Hill to Valley Springs (Axelrod, 1944a). Axelrod states that the Valley Springs floras are closely related to the Table Mountain flora of the Mehrten Formation, and that there was no hiatus between rhyolitic and following andesitic eruptions which furnished the material for the two formations (see section on Mehrten Formation). Axelrod (1956, p. 264) also correlates the Valley Springs with the Cierbo Formation at Corral Hollow In the Coast Range opposite eastern Stanislaus County. The Valley Springs has been correlated in the subsurface of Madera County with the Zilch (nonmarlne) zone (Hoots, and others, 195^* Plate 6) which underlies the Santa Margarita Formation of Late 37

Miocene age and overlies the Kreyenhagen Shale of Late Eocene age. The Zilch is considered to be Middle and Early (?) Miocene in age, and marine invertebrate paleontologists would probably consider the Valley Springs Formation to be Middle Miocene in age. In so far as is known no water wells In the study area penetrate the Valley Springs formation. The main reasons for this are that the Valley Springs is at considerable depth and is overlaid by the Mehrten a much better aquifer (Plate 3). The Valley Springs may also contain saline water. As the Valley Springs Is nonmarine in origin any saline water present probably migrated from other rocks. At the surface the Valley Springs is not a good aquifer, except locally, because of the presence of fine-grained material and clay (Davis and

Hall, 1959* P. 9).

Lower and Middle Pliocene Mehrten Formation - General Description, Type Section. Thickness The Mehrten Formation is a distinctive geologic unit in eastern Stanislaus County. It consists of rather dark-colored sandstone, conglomerate, siltstone, and claystone. The outstanding feature of these rocks is that they are mainly derived from volcanic rocks and consist of from 50 to nearly 90 percent andesitic material. The upper part of the Mehrten Formation crops out in a narrow belt along the 38

eastern boundary of the study area (Plate 2). The remainder of the formation is exposed to the east of the study area and dips beneath It (Plate 3). The general nature of the upper surface of the Mehrten Formation is shown in Figure 6. As described in the section on the Valley Springs, white pumice occurs In the lower part of the Mehrten that resembles Valley Springs, and the contact Is set arbitrarily at the first occurrence of andesitic material. Neverthe- less the Mehrten overlaps the Valley Springs and rests on lone or bedrock in the eastern part of the area. Most likely this overlap Is due to erosion and subsequent deposition in channels rather than to an angular discordance Contact relations with the overlying Turlock Lake and younger formations indicate local relief on top of the Mehrten in places. On the left-hand side of the road cut shown In Figure 7 at channel fill of probable Riverbank age was deposited in a channel cut into the Mehrten Formation. At some places the contact with the Turlock Lake Is sharp as shown in the road cut in Figure 8; whereas at other localities the contact is transitional. The material of the transitional unit ranges in size from fine-grained sand or silt up to gravel and is characterized by a decreasing content of andesitic material and an increasing content of quartz, feldspar, and biotite. Examples of the transitional unit are shown in measured sections 2-11-4G, unit 2 and 3-13-31K, L, P, and Q, units O i 39 3

* " " " " " » . " \1 T.3S. MODESTO o \ o .~> / ' V^, FIGURE 6 \ r UPPER SURFACE

CONTROL POINTS O a MEHRTEN- POST MEHRTEN TURLOCK \ CONTACT AND TOPOGRAPHY s o WELL SCALE V- 40

— Fig. 7. Mehrten Formation and post-Mehrten channel fill in road cut on Rodden Road near Orange Blossom Bridge east of Oakdale. 41

— Fig. 8. Contact between Mehrten and Turlock Lake Formations in road cut on Rodden Road east of Oakdale. 42

10 and 11 (Appendix A). The transitional unit can be seen in the upper right hand corner of the road cut shown in Figure 7. The maximum thickness of the transitional unit is not known but reaches at least 10 feet. The type section of the Mehrten Formation is near the Mokelumne River in eastern San Joaquin County about thirty miles north of Oakdale (Piper, and others, 1939* p. 6l). The Mehrten of the type section consists of siltstone resembling the underlying Valley Springs Formation, andesitic sandstone, siltstone, clay, and andesitic breccia. The maximum thickness near the type section is about 400 feet. A total exposed thickness of about 800 feet is found along the Stanislaus River between Knights Ferry and Oakdale. The thickness increases to about 1200 feet in the subsurface west of Modesto (Plate 3). - Lithology A characteristic bluish color is an interesting feature of the Mehrten Formation and other formations containing andesitic sand. These are commonly referred to as blue sands. In the field the color actually ranges from gray to blue gray to grayish blue. Lerbekmo (1957) studied the blue coating and cement of andesitic sands from the Mehrten Formation of the central Sierra Nevada, the Neroly of San Francisco Bay, the Purisima near Santa Cruz, and the Etchegoin near Coalinga. His general conclusions concerning the nature and origin of the cement coating ares 1) It Is an authigenlc cement of montmorillonlod 43

with a probable composition intermediate between beidellite and nontronlte; 2) the blue color Is a superficial optical effect when the coating is less than 0.01 mm thick; 3) the color is blue only when dry and takes on rock color when wet; 4) grains have a tan color unless stained by iron oxide when the thickness is greater than 0.01 mm; 5) the coating is found only in andesitic sands and is not related to distance from the volcanic source; and 6) the coating is equally common In marine and nonmarlne sandstones. The Mehrten Formation can be described in terms of three major subunits which have not been mapped because of poor exposures and great variability (Davis and Hall, 1959* p. 10). The lowest subunit consists of scoriaceous and/or pumiceous sand and conglomerate which locally displays tabular cross-bedding (Figure 9). The subunit occurs as deposits filling channels on the surface of older rocks, and it has a variable thickness with a maximum of about 100 feet. The lower unit is in the subsurface beneath the study area, but good exposures are found near Knights Ferry.

A sample of scoriaceous andesitic sand was collected from the lowest subunit near Knights Ferry in the road cut shown in Figure 9 (field locality 1-12-29G). The color of the sample is medium dark gray (Munsell color N 4; see Goddard, and others, 1948). A sieve analysis indicates that the sample is a rather well-sorted (So- 1.48), 44

— Fig. 9. Lower part of Mehrten Formation in road cut just north of Knights Ferry. (Note pronounced cross bedding. ) 45 fine- to medium-grained sand (Figure 10; Appendix C).

The sample, and In fact all of the material at the outcrop, was unconsolidated. Everything coarser than a few millimeters consists of rounded scoriaceous pebbles and small cobbles. A qualitative examination of three size fractions of the sample was made with a binocular microscope, and the results of a grain count are given in Table 1. The coarser fraction has 95 percent volcanic material including a large amount of the bluish colored andesite grains. The medium-grained fraction of the sample (1-12-290) contains 83 percent andesitic material. The andesitic grains and mineral grains tend to be angular to subangular. The finer-grained fraction has 60 percent volcanic material, and most grains are angular to subangular. The overall color of the sample Is medium dark gray (N 4), but the coarser fractions is more bluish. The smaller fractions became increasingly brown with decrease in size until the silt and clay fraction is light brown. This same feature was observed in the other Mehrten samples described below. The middle subunit of the Mehrten Formation is a sequence of alternating layers of andesitic gravel, sand, and silt. Some of the layers display cross-bedding. The individual beds and layers are from a few feet to twenty feet or more In thickness, and they may persist for a lateral distance of a mile or so. The outcrop width and regional dip indicate a thickness of about 400 feet. This 46

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subunit is exposed best to the west and southwest of Knights Ferry; however, in the study area it occurs only in the subsurface. A sample of the middle subunit was collected at a spot known locally as Lovers Leap on the south bluff of the Stanislaus River a few miles west of Knights Ferry (field locality 1-12-31C). A sieve analysis (Figure 10; Appendix C) and a grain count (Table 1) show the material to be a well-sorted (So « 1.28), fine- to medium-grained, andesitic sand. The sample color is grayish blue (7.5 PB 5/1). The medium-grained fraction is 9^ percent volcanic including a large portion of blue andesitic grains, and the grains are subangular to subrounded. The finer-grained fraction is 6l percent volcanic, and the grains are subangular and rather broken in appearance. The upper subunit is a sequence of rather soft clay, silt, sand, and minor amounts of gravel. Cross- bedding is shown in some of the beds. A distinctive feature of the upper part of the subunit is the presence of pink clays (Munsell color; grayish orange pink, 5 YR 6/2) which are from 2 to 4 feet thick. One of these clays is shown just below the Mehrten-Turlock Lake contact in Figure 8. These pink clays are also characteristic of the top of the Mehrten in many drillers' logs. The upper subunit crops out along the eastern boundary of the study area (Plate 2). It has a thickness of about 300 feet along the Stanislaus River east of Oakdale. The general 49 character of the upper subunit is shown in two measured sections from road cuts along the north 3ide of the Stanislaus River a few miles east of Oakdale (Appendix A, measured section 2-11-4G and measured section 2-10-1H and 2-11-6F and C). Two samples were collected from the upper subunit along the south side of Stanislaus River east of Oakdale. Sieve analyses and a brief microscopic examination were made of the samples (Figure 10 and Appendix C). Sample 2-11-5P(a) is a well-sorted (So = 1.4l), somewhat Indurated, fine- to medium-grained sand that is pale grayish blue (5 PB 6/1). Sample 2-11-s(b) is a well-sorted (So = 1.37) somewhat indurated, medium-grained sand that is grayish blue (2.5 PB 5/1). The results of the grain counts for the two samples are given in Table 1. The two are similar except that 2-11-5P(a) has a greater percentage of volcanic material. In both samples, the coarser fractions are rounded, and the two smaller fraction are mainly angular to subangular. The Mehrten Formation is not exposed well south of the Stanislaus River until the Tuolumne River is encountered where there are good exposures upstream from Waterford. The general nature of the Mehrten in the subsurface is shown in some of the well logs In Appendix B. - Source and Origin The presence in the foothills and main Sierra Nevada of wide-spread andesitic material, Including 50

breccias, tuffs, mud flows, and lava flows, underlaid in places by rhyolitic material has been known for a long time (Turner, 1894, p. 487-495; Turner, 1896, p. 657-716; Lindgren, 1911; Ransome, 1898). In fact, the west slope of the Sierra Nevada from at least as far south as Yosemite National Park northward to Mount Lassen was covered by large amounts of volcanic material, mainly andesitic, during the approximate period Late Miocene to Middle Pliocene. The source of the material was from vents and other openings now 3hown by old plugs, domes, and dikes (Curtis, 1954; Durrell, 1944).

The erosional products of the earlier rhyolitic (see section on Valley Springs Formation) and later andesitic eruptions were deposited along the east side of the Central Valley. Early workers mentioned this, and Louderback (1924, p. 17-18) clearly expressed It, particularly for the andesitic material. In the Mokelumne report (Piper, and others, 1939. p. 61-67), the Mehrten Formation is considered to be derived from andesitic material deposited in a fluviatile environment. In the Sierran foothills, however, some of the Mehrten probably was deposited by mud flows. The paper by Curtis (1954) gives a good description of the character of the eruptive andesitic material in the Sierra Nevada. His work, however, involves a dualism in nomenclature because he calls the eruptive andesitic material the Mehrten Formation, whereas the name Mehrten 51

was first applied primarily to the eroslonal products of the eruptive material (Piper, and others, 1939, p. 61-67). The important point is that a clear distinction should be

made particularly in time relations between the volcanics and their erosional products. The same distinction should also be made between the eruptive rhyolitic material and its eroslonal product, the Valley Springs Formation. The good sorting and shapes of the cumulative curves for the four Mehrten samples (Figure 10) Indicate deposition in water. The deposits which fill channels and depressions and the tabular cross-bedding are all Indicative of running water. A number of mammalian faunas and continental floras have been described from various localities in the Mehrten Formation, and a few specific examples are discussed in the section on age and correlation. Plant remains can be found in many clay beds throughout the Mehrten Formation In eastern Stanislaus County. These features are evidence also for deposition by streams. Nevertheless, on the basis of lithology and texture at least some of the beds In the Mehrten could be called tuffs (see Appendix A, measured section 2-11-4G, units 12 and 13).

Age and Correlation - The Mehrten Formation of eastern

Stanislaus County has been correlated with the type section on the basis of lithology and stratigraphic position. Although the Mehrten has not been mapped across the 52

intervening area, reconnaissance field work Indicates continuity along the strike. No fossils were found in the Mehrten Formation of the type area, but Its age was thought to be Late Miocene and possibly Early Pliocene in part (Piper, and others,

1939, P. 70-71). More satisfactory evidence has been found for the age of the Mehrten in eastern Stanislaus County. The lower part of the Mehrten just east of Knights Ferry has been dated as Early Pliocene on the basis of two vertebrate faunas (Stirton and Goeriz, 1942). The upper part of the Mehrten just east of Oakdale has been dated as Middle Pliocene on the basis of the Oakdale flora (Axelrod, 1944b, p. 147-167) and the Oakdale fauna (Stirton and Goeriz, 1942, who also reinterpreted earlier work by Vanderhoof, 1933). The Mehrten Formation further east in the Sierran foothills has been determined to be Early Pliocene or Mio-Pliocene in age on the basis of several floras (Axelrod, 1944a). All of these age determinations are based on correlation with mammalian provincial California stages supported by fossil floras. The ages do not quite agree with those given by correlation with marine invertebrate provincial stages (Figure 5). As far as is known, the Mehrten Formation has been dated approximately by the use of marine fossils at only one locality on the east side of the San Joaquin Valley.

A set of samples was described by the Bureau of Reclamation 53

from a well just northeast of Chowchilla, Madera County, about 56 miles southeast of Modesto (Appendix B, well 9-16-228). Marine Pliocene rocks are found beneath the Mehrten Formation in this well. These rocks are called Pliocene on the basis of a marine diatom flora. The Mehrten Formation seems to be entirely of continental origin along the east side of the Central Valley. The Mehrten Is continuous, however, in the subsurface with the Neroly Formation of the Coast Ranges which is of marine origin In part. The Neroly consists of andesitic siltstone, sandstone, and conglomerate. It contains mammalian vertebrate fossils and fossil floras in the Tesla area opposite eastern Stanislaus County and marine invertebrate fossils further north near Mount Diablo (Huey, 1948, p. 44-47). The age of the Neroly Is considered to be transitional Mio-Pliocene on the basis of the floras and vertebrate faunas (Axelrod, 1944a; Stirton, 1939). Its age would probably be considered Late Miocene by invertebrate paleontologists on the basis of the marine fossils, and therefore, the Mehrten would also be considered Late Miocene. Blue andesitic sands occur in rocks of the Coalinga and Kettleman Hills region, and they are dated by marine fossils as Early and Middle Pliocene (Woodring, and others, 1940). The formations and their ages are: Jacalitos Formation, Early Pliocene; and Etchegoin Formation, 54

Middle Pliocene. Also, some blue sand occurs in the base of the San Joaquin Formation, Late Pliocene. The andesitic eruptions of the Sierra Nevada are the most probable source of the andesitic material; so the Mehrten Formation probably correlates at least in part with the Jacalitos and Etchegoin Formations. The Purisima Forma- tion near Santa Cruz contains blue andesitic sand, and it probably correlates in time with part of the Mehrten

Water-bearing Properties - The Mehrten Formation is an Important aquifer in the area bounded by Oakdale, Waterford, Empire, and Riverbank. In the eastern part of this area, wells may penetrate 400 feet or more into the Mehrten, whereas west of Empire and Riverbank few wells reach the Mehrten (Plate 3). The chemical quality of water in the Mehrten is usually good in the northern part of the area including Riverbank and Oakdale. Water quality in the Mehrten may be poor, depending on depth, along the Tuolumne River east of Empire and is generally very poor west of Empire and Riverbank. Saline water present in the Mehrten probably migrated in because the Mehrten is a continental formation. The Mehrten Formation has undergone some consolidation and cementation in the subsurface; as a result, permeabilities of the sands and gravels may have been reduced. Wells In the Mehrten are usually drilled deeper 55 than wells in the younger formations in order to produce the same amount of water. Numerous domestic and stock wells obtain adequate amounts of water from the Mehrten. Also, a number of large irrigation, public supply, and industrial wells obtain small to large amounts of water from the Mehrten. Many of these wells penetrate younger* formations and also obtain water from above the Mehrten. Records from twenty larger wells that obtain more than 50 percent of their water from the Mehrten show a range in yield of 314-2780 gallons per minute (gpm) with an average of 1278 gpm. Most of these wells are more than 400 feet deep.

Upper Pliocene and Pleistocene Differentiation of post-Mehrten Formations Only a limited amount of work has been done with post-Mehrten sediments on the east central side of the Central Valley. In fact, these sediments are usually mapped simply as Quaternary alluvium. Somewhat more attention has been paid to equivalent sediments found in oil fields on the west side of the San Joaquin Valley. The Mokelumne report (Piper, and others, 1939) includes the only published detailed descriptions of post-Mehrten formations in the northeastern part of the San Joaquin Valley. The formations defined in this report have served as a standard for much of the east side of the San Joaquin and Sacramento valleys. The Mokelumne area lies 56 about thirty miles north of the Stanislaus River, and no detailed geologic mapping has been done in the intervening area. The post-Mehrten formations as mapped in eastern Stanislaus County are not quite the same as those of the Mokelumne area, and it has seemed best to give them new names (Davis and Hall, 1959* p. 10). The major criteria used for mapping in the Mokelumne area were topographic expression and the projection of old land surfaces into the subsurface. In eastern Stanislaus County, the major criteria have been topographic expression and soil development. The formations of eastern Stanislaus County have been traced northward into the Mokelumne area by reconnaissance field work, and a suggested correlation chart is given below. The Turlock Lake Formation is probably represented in the Mokelumne area by the Laguna Formation. However, the Arroyo Seco Gravel has not been recognized in eastern Stanislaus County. The Modesto and Riverbank Formations together represent the Victor Formation, and they can be traced northward into the Mokelumne area. The presence of sand dunes was not mentioned in the Mokelumne report (Piper, and others, 1939)* but eolian deposits were described in an earlier report on the same area (Steams, and others, 1930, p. 18-23). 57

Eastern Stanislaus Count Mokelumne Area

Recent alluvium ------Recent alluvium Sand dunes ------Sand dune3(?) Modesto Formation Victor Formation Riverbank Formation (Not present) Arroyo Seco Gravel Turlock Lake Formation ----- Laguna Formation Mehrten Formation ------Mehrten Formation The Arroyo Seco Gravel is considered to be a deposit left on the Arroyo Seco pediment which was formed during an erosional interval between deposition of the Laguna and Victor Formations (Piper, and others, 1939* p. 20-22; Howard, 1951* p. 102-103). Such a gravel has not been found in the study area or as far south as the Merced River. Scattered gravel occurs on the present surface of the Turlock Lake Formation, but most of it can be accounted for by the presence of gravel beds in the Turlock Lake. South of the Merced River a pronounced erosional surface can be seen which may be a pediment. The well-developed soil of the Turlock Lake Formation, as contrasted with the less developed soils of the Riverbank Formation, and the presence of a pronounced topographic unconformity between the two formations are Indicative of a long interval of erosion. Because a well-defined gravel unit Is not present in the study area the terms Arroyo Seco pediment and Arroyo Seco Gravel are not used in this report.

Soils and Topographic Expression in Geologic Mapping|- An important problem in attempting to differentiate geologic 58

units in alluvial areas is that the sediments commonly are derived from a single source and are deposited in a similar environment. All or nearly all of the alluvium of the east side of the San Joaquin Valley is derived from granitic and associated rocks of the Sierra Nevada which lies to the east. Thus, the formations offer no textural or lithologlc bases for subdivision. Neverthe- less, the use of the topographic expression of the units in conjunction with the development of their soils makes It possible to define formations. In the study area, the older formations have been exposed the longest, and they have been the most eroded and dissected. As a result topographic unconformities occur between the outcrop areas of the formations. The topographic unconformity is more distinct if tilting or downwarplng has taken place between periods of deposition. The sharpness of the topographic unconformity also depends upon the interval of time between deposition of the formations. The topographic unconformities can be observed both in the field and on topographic maps. Modern 1/24,000 topographic maps with 5- or 10-foot contour intervals are available for the study area (United States Geological Survey 7^-minute quadrangles: Avena, Brush Lake, Ceres, Dcnair, Escalon, Oakdale, Ripon, Riverbank, Salida,

Waterford, and Westley), and the topographic unconformities 59 are shown well on them. The geologic cros3 section (Plate 3) also shows the topographic expression of the different formations. Starting from the west and going toward the east there is a flat, gently sloping surface of the Modesto Formation which ends against a more dissected and gently rolling surface of the Riverbank Formation. The Riverbank surface in turn ends against the Turlock Lake Formation which has a much more dissected surface with steeper hills. The development of a soil depends on five major variables: parent material, climate, organic activity, slope, and time (Jenny, 194l). If the use of soils in geologic mapping is confined to an alluvial area such as the east side of the San Joaquin Valley where parent material, climate, and organic activity are approximately the same, then the differences in the development of the soils depend only on the variables, slope and time. In the study area, the use of soils has been restricted to gently sloping, topographically high localities that have not been eroded or covered by younger deposits. Under these conditions, time is the main variable causing differences of soil development.

As a soil develops, three master horizons are formed. These soil horizons are given letter designations and are described as follows (Soil Survey Staff, 1951* p. 178-180): 60

A horizon: characterized by leaching and downward movement of iron, aluminum, and clay minerals, and the concentration of more resistant minerals. The A horizon is lighter in color than the underlying

B horizon: characterized by accumulation of clay, Iron, or aluminum and It shows a more or less blocky or prismatic structure. The B horizon has stronger colors that are different than those of the A or underlying horizons. C horizon: a layer of unconsolidated material which has been little affected by processes undergone by the overlying horizons. It is presumed to be similar to the material from which the overlying horizons have been developed. For the purposes of geologic mapping, the main r„,u„ * „ «-»«-, v *.***-.» or « b horizon. The thickness, amount of clay, degree of compactness, amount of blocky or columnar structure, and deepness of color resulting from oxidation are all functions of time. The soils must be drained well in order to use these factors in mapping. Soil scientists utilize these same criteria in determining the maturity of soil profiles. Shaw (1928) has given a description of the increasing maturity of the soil series in a soil family developed on granitic alluvial fan and terrace 61

deposits in the San Joaquin Valley, The soil family is

the San Joaquin which includes In order of increasing maturity the Tujunga, Hanford, Greenfield, Placentia, and San Joaquin series. The Tujunga, Hanford, and San Joaquin soils are important in geologic mapping in the study area. Shaw also describes the Fresno family which develops on the same parent material but under conditions of restricted drainage. Some of the Fresno soils are found in the western part of the study area. Storie and Harradine (1957) describe the soil regions of California and discuss soil type as related to parent material, annual rainfall, geographic location, native vegetation, and topographic position. The part on the San Joaquin Valley gives a description of the San Joaquin and Fresno soils and also describes soils like the Delhi which are developed on sand dunes from the same parent material as the other two. Soils developed on other types of parent material are also discussed. The soils of eastern Merced and eastern Stanislaus Counties have been mapped in detail (Arkley, 1954* 1959). The development of the B horizon is an index to the maturity of a soil; therefore, soils developed on similar parent material can be placed in a relative age sequence. The geologic formations on which the soils are developed can also be placed in a relative age sequence. Finally, some idea as to the geologic age of a soil, and thus the age of the geologic formation, can be 62 obtained. This concept has been investigated in the central United States (Thorp, and others, 1951) with particular emphasis on the color of the B horizon. Accurate determinations of soil colors by the Munsell Color Chart were made on soils of different ages, and a definite relation was found between color (hue on the Munsell Color Chart) and age. The study showed also that time is che major factor, although a warm climate during formation may contribute to deepening of color. The use of the B horizon in dating and the relation of soil color to time has been considered in some detail by Carter (1956; 1957) who has been concerned primarily with the dating of archaeological sites in San Diego region of Southern California. He has given a summary of the use of the B horizon as an indicator of age, and his general conclusions concerning the color of the B horizon and degree of weathering are (Carter, 1957* P. 119- 120): 1. Soil color determinations based on the Munsell Color Chart show that the hues of modern soils developed in loess of Wisconsin age are rarely redder than 7.5 YR and usually no redder than 10 YR. 2. For glacial moraines of the Sierra Nevada the youngest moraines are light gray (2.5 V 7/2) and pale yellow (2.5 V 8/4). The slightly older moraines are very pale brown (10 YR 7/6) and yellow (10 YR 7/4)* 63

and the older moraines are yellowish red (5 YR 5/6). 3. Fresh granitic material is late glacial and younger; stained and moderately decayed granite may be early Wisconsin; and thoroughly rotted granite is older than Wisconsin time. 4. Soil weathering that is 5 to 6 feet deep is pre-Recent in its inception.

Turlock Lake Formation General Description c Section, Thickness - The Turlock Lake Formation is a sequence of fluvial sands, silts, and gravels primarily of granitic origin. The change from the Mehrten containing more than 50 percent andesitic material to the overlying Turlock Lake containing more than 50 percent quartz, feldspar, and biotite Is striking. The Turlock Lake Formation crops out along the eastern side of the study area (Plate 2) and dips gently to the west (Plate 3), Tongues of the Turlock Lake extend west of the main outcrop area in several places. The Turlock Lake Is found also as outliers capping hills of Mehrten east of the study area. The Turlock Lake Formation seems generally conformable with the Mehrten Formation, although exposures in the Oakdale area indicate that in places relief due to erosion is found on the top of the Mehrten. In places the contact Is sharp (Figure 8), but in others a transitional zone occurs which is characterized by decreasing andesitic 64 material and increasing amounts of quartz, feldspars, and biotite (Figure 7). This transitional zone was described in the section on the Mehrten Formation. At the surface the bedding of the Turlock Lake seems generally parallel with the overlying Riverbank Formation, although local dlsconformities may occur between the two. In the subsurface, the dip of the Riverbank is gentler than that of the Turlock Lake (Plate 3).

The type section of the Turlock Lake Formation consists of a series of exposures in road cuts on a hill in Turlock Lake State Park In eastern Stanislaus County

(Davis and Hall, 1959* P. 13). A detailed description of the type section is included in the present report (Appendix A, 3-13-31 X, L, P, k Q). Also, grain counts (Table 2) and sieve analyses (Figure 11; Appendix C) for two samples and a pebble count (Table 3) for one sample from the type section are given. The thickness of the Turlock Lake Formation is not known definitely because of difficulties in picking the upper contact In the subsurface. The probable thickness as shown on the geologic cross section (Plate 3) ranges from 300 to 850 feet.

Soils, Topographic Expression, Lithology - The Turlock

Lake Formation is expressed topographically as dissected, rolling hills with up to sixty feet of relief. The topographic unconformity between the Turlock Lake and the 64A

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— Fig. ia Soil developed on Turlock Lake Forma- tion on Steams Road east of Oakdale. 67

The sediments of the Turlock Lake Formation in the sand and silt sizes consist of quartz, feldspars, biotite, and other minerals typical of a granitic source (Table 2). The coarser-grained material consists of rock types such as granodiorlte* andesite, quartzite, and schist (Table 3). This lithology is characteristic also of the Riverbank and Modesto Formations. The sediments of the type section (Appendix A, 3-13-31K, L, P, & Q) tend to be rather coarse- grained, but the formation commonly contains considerable amounts of silt, clay, and fine sand (Appendix A, 2-11-218, G, & X). Epical colors of the sediments are gray to grayish orange to grayish pink silts and clays and gray, yellowish brown to brown, and reddish brown sands. The lighter colors are typical of the Riverbank and Modesto Formations, but the brown and reddish brown colors of the oxidized zones are mainly restricted to the Turlock Lake. The sample from unit 2 of the type section- (Appendix A, 3-13-34 X, L, P, and Q) is a well-sorted (So 1.55)* sandy silt (Figure 11) that is grayish orange in color (10 YR 7/2). The unit Is cross bedded. The mineral grains are angular to subangular. The sample from unit sof the type section is a well -sorted (So = 1.45), medium- to coarse-grained sand (Figure 11) that is yellowish brown (7.5 YR 5/5). The coarser grains are subrounded, but the finer material is angular to subangular. Granite pebbles in the samples are somewhat weathered. Figure 13 gives the details of coarse sand in the Turlock Lake near the type 68

Fig. 13.--Exposure of sand in the Turlock Lake Formation near the type section at Turlock Lake State Park. 69 section.

A pebble count was made of gravel from the type section (Table 3). The general character of the gravel Is shown in Figure 14. The pebbles and cobbles tend to be subrounded, but the shapes are commonly tabular or dlscoidal rather than spherical. The maximum length of a tabular pebble is 70 ram, and the greatest spherical diameter is about 45 mm. The sizes range from 5-70 mm but most are greater then 15 mm. Some Interesting aspects of this gravel are: l) many of the pebbles and cobbles have a silica coating and some of them seem to be etched, 2) granite is rotted and disintegrates easily, 3) schist and coarser volcanic rocks are weathered and break easily, and 4) only quartz!te and fine-grained siliceous rocks are relatively unweathered except along cracks. The gravel has a coarse-grained matrix with much rotted granite that is brown in color (5 YR 6/4), A pebble count was made of a gravel sample from near Oakdale (Table 3). The gravel Is mainly less than 50 mm in diameter but ranges up to 130 mm. The material is subrounded to rounded, and the shapes are discoidal to spherical. Rock types Include granodiorite, andesite* rhyollte, quartzite, and schist. The coarser-grained types are weathered. A deeply weathered component occurs that appears to be composed of granitic and coarse-grained volcanic material. At the sample locality and in the surrounding area many rhyollte pebbles and cobbles are 70

Fig. 14.— Gravel In Turlock Lake Formation at the type section at Turlock Lake State Park. 71 found lying on the ground and in place in and below the soil. The Turlock Lake Formation in the subsurface is similar to the exposures at the surface except that it has more finer-grained material. The general character of the Turlock Lake in the subsurface is shown in some of the well logs in Appendix B. An excellent example of Turlock Lake or its equivalent Is given in the description of well 9-16-228 (Appendix B) for the interval 126-403 feet. Many drillers 1 logs show one or more red clays in the subsurface. No success has been attained in attempting to correlate these clays, but they most likely represent buried soils. In the section on the Riverbank Formation a similar red clay is described that is definitely a buried soil. In so far as can be determined, these red clays are quite similar to well-developed B horizons.

Source and Origin - The large amounts of quartz, feldspars, and blotite in the sand and silt size grades as well as the type of heavy minerals show origin from a granitic source. This is also indicated by the presence of granitic pebbles and cobbles in the coarser size grades. Volcanic and metamorphic materials are present also which indicate other, but less Important sources, of sediments. The distribution of the Turlock Lake Formation on the east side of the San Joaquin Valley indicates an eastern source for the materials. The minor amounts of volcanic and 72

metamorphic material show that most of the Sierran

volcanic cover had been stripped off, and that the

streams were eroding granitic rocks of the Sierra and some

volcanic and metamorphic rocks from the Sierra and its foothills. Nothing in the sediments of the exposed Turlock Lake indicate any contribution from a source to the west although such material may be present in the

The sedimentary features such as cross bedding, degree of sorting (Figure 11), lenticularity, angularity of sand grains, heterogeneity of material, channeling, and channel filling are all indicative of deposition in a fluviatile environment. The broad sheet-like form of the Turlock Lake which thickens down dip is indicative of deposition in broad, gently sloping alluvial fans. If the red clays represent buried soils, then the Turlock Lake Formation Is probably a composite unit of a number of coalescing alluvial fans. The age and amount of erosion and dissection of the Turlock Lake make it difficult or impossible to subdivide it further.

Age and Correlation - The Turlock Lake Formation seems equivalent to the Laguna Formation of the Mokelumne area (Piper* and others, 1939* P. 57-6l). It probably correlates at least in part with the Tulare Formation as mapped on the west side of the San Joaquin Valley (Huey* 1948, p. 48-49; Reiche, 1950, p. 6-9). The Turlock Lake 73

probably correlates with part of a thick sequence of marine, brackish, lacustrine, and fluviatile sediments in the southern San Joaquin Valley mapped as the San Joaquin Clay and Tulare Formation. (Barbat and Galloway, 1934). No evidence from fossils is available to assist in determining the age of the Turlock Lake Formation; so the age must be arrived at Indirectly, The upper part of the underlying Mehrten Formation in eastern Stanislaus County 13 Middle Pliocene In age. The Laguna Formation of the Mokelumne area is considered to be Pliocene and possibly Early Pleistocene in age (Piper, and others, 1939* p. 60- 6l), The Tulare Formation on the west side of the San Joaquin Valley is considered to be Pliocene to Late Pleistocene by Huey (1948, p. 48-49) and Plio-Pleistocene by Reiche (1950, p, 6-9). The San Joaquin Clay and Tulare Formation are considered to be Late Pliocene and Early Pleistocene and Pleistocene respectively by Barbat

and Galloway (1934). The color of a typical B horizon of the Turlock Lake Formation is 5 YR 5/4, and this suggests an age of Middle Pleistocene or older (Carter, 1957* P. 119-120). If the deposits south of La Grange are oldest Turlock Lake, then their color (2,5 YR 4/6) suggests an age of earliest Pleistocene or older. Early workers considered these deposits to be Pleistocene (Turner and Ransome, 1897). The indirect evidence as to age, together with the 74

generally conformable relations with the Mehrten suggest that the Turlock Lake ranges from Late Pliocene into Early Pleistocene.

Water-bearing—— Properties— — - The HIWin— IMM II III "W^l-nl— Turlock Lake Formation is !■ II■«■■■■—■■■■111I» _■_-! >WWiWM 1IMIIWIKtII ■— H— Hill an important aquifer in the study area particularly in the vicinity of Oakdale, Because of older, impermeable soils and rather rugged topography, however, the outcrop area of the Turlock Lake has not been extensively developed for irrigated agriculture. Wells may penetrate 300 feet or more Into the Turlock Lake. Many wells obtain water from both the Turlock Lake and Mehrten Formations in the eastern part of the study area. Water quality is good in the outcrop area and in the subsurface about as far west as Empire and Riverbank. Some poor quality water is found depending on depth in the strip along Tuolumne

River between Waterford and Empire. West of Empire and Riverbank water in the Turlock Lake becomes Increasingly saline. This saline water probably migrated into the nonmarine rocks. The Turlock Lake Formation has undergone some consolidation and cementation in the zone of weathering and oxidation, and this reduces the permeability of the sands and gravels. Beneath the zone of weathering and oxidation, however, unconsolidated sands and gravels are found, although local variations in permeability occur because of the lenticular nature of the deposits. The 75 range of yields of larger wells given for the Mehrten, 314-2780 gpm with an average of about 1278 gpm, seems valid for the Turlock Lake. In fact, problems arise in differentiating between yields from the two formations because most large wells obtain water from both. Wells In the Turlock Lake tend to be shallower; so the Turlock Lake is probably the better aquifer.

Riverbank Formation

General Description c Section, Thickness - The Riverbank Formation consists of a series of sands, silts, and gravels deposited by streams. It also contains two persistent clay layers that can be traced over a wide area. There is no distinctive change in lithology from the underlying Turlock Lake Formation. The Riverbank Formation crops out in a northwest-southeast trending belt that lies west of Oakdale and Waterford but east of

Modesto (Plate 2). A tongue of the Riverbank thinly covered by Modesto Formation extends eastward from the main belt along the Tuolumne River. At the surface, the bedding in the Riverbank Formation seems generally parallel with the underlying Turlock Lake Formation, although local disconformities may occur between the two. In the subsurface, the dip of the Riverbank is gentler than the Turlock Lake (Plate 3). The bedding of the Riverbank at the surface seems parallel with the overlying Modesto Formation, but the Riverbank 76 appears to have a steeper dip in the subsurface (Plate 3). In the vicinity of Oakdale and farther east channels have been cut Into the Mehrten Formation and filled with post-Mehrten material (Figure 7). The degree of weathering and the elevation of some of these suggest that they are part of the Riverbank Formation. The Riverbank Formation was named for exposures in the south bluff of the Stanislaus River in the town of Riverbank, (Davis and Hall, 1959* p. 16-17). A detailed description of the type section is included in this report (Appendix A, 2-9-26K). Also sieve analyses (Figure 15; Appendix C) and grain counts (Table 4) are given for two samples from the type section. The thickness of the Riverbank can not be given with certainty because of difficulties in picking the lower contact In the subsurface. The probable thickness as shown on the geologic cross section (Plate 3) is 150-200 feet.

Topographic ression. Soils. Litholo The topography of the Riverbank outcrop area Is that of low, slightly dissected hills with 10-20 feet of relief to nearly flat land. The topographic unconformity with the more dissected Turlock Lake is usually distinct. The topographic unconformity with the Modesto Formation is not as sharp because it represents the intersection of a very flat surface with a nearly flat surface. The topographic 77

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unconformity can be seen in the field, however, between Riverbank and Empire where it is fairly distinct when viewed from the west. The degree of weathering and oxidation of the B horizon of the soil represents a condition intermediate between the weathering of the B horizons of the Modesto and Turlock Lake Formations. The Munsell color of a typical Riverbank B horizon is 7.5 YR 5/4. A well- developed B horizon may be up to 6 feet in thickness. The B horizon is fairly compact with considerable clay and grains of quartz and feldspar. Coarser-grained material has been weathered and stained, but granite pebbles and

cobbles are commonly intact. A distinctive feature of the Riverbank Formation is the presence of a hard resistant silica hardpan in the lower part of the soil profile. The general nature and color of a soil developed on the Riverbank are shown In Figure 16. At this location the Riverbank soil is buried beneath a few feet of Modesto Formation (Appendix A, section 3-11-33 Iand F), An example of the oxidized zone at the contact of Riverbank and probable Modesto Formations is shown in Figure 17 (Appendix A, section 2-10-10 B, G, and H). The lithology of the Riverbank Formation is similar

to the Turlock Lake Formation except that the Riverbank has more fine-grained material and less gravel. The minerals present are typical of a granitic source and consist of quartz, feldspars, and biotite with minor 80

Fig. 16, --Soil developed on the Modesto Forma- tion overlying a buried soil developed on the River- bank Formation near Waterford, 81

— Fig. 17. Contact between Riverbank and Modesto (?) Formations on Highway 120 just north of Oakdale. 82

amounts of other minerals. The coarser-grained material consists of such rock types as granodiorite, andesite, rhyollte, quartzite, and schist. In Appendix A a number of measured sections are given for the Riverbank, but sections 2-9-26K and 3-11-33E and F are the best in the area. The colors of the sediments are gray to grayish orange silts and clays and gray, yellowish brown, and

light brown sands. The samples from the type section and other locations (Figure 15; Table 4) are so similar that they can be described together. They are well-sorted (So - range 1.33 1.62 with an average of 1.45) and vary from very fine- to very coarse-grained sands (Appendix C), The coarser grains are subrounded* but most finer material is angular to subangular. Some of the coarse size grades contain rock fragments, but the dominant minerals are quartz, feldspars, biotlte, and minor amounts of magnetite, hornblende, and other dark minerals. Colors are gray, yellowish brown, and light brown. Many of these sands are from cross-bedded units. Granitic pebbles associated with some of these sands are still nearly intact and relatively unweathered.

A pebble count was made for gravel exposed in the bank of Dry Creek (Table 3). The pebbles are 10-80 mm in diameter, and the shapes are pyramidal, tabular, and discoidal. Most of the material is subrounded. Rock types represented include andesitic, rhyolitic, granitic, 83 and metamorphic with the volcanics being most abundant. The granitic and other coarse-grained rocks are weathered and some deeply so. The Riverbank Formation in the subsurface Is similar to the surface exposures. The general nature of ' the Riverbank is shown in some of the drillers logs in Appendix B. The log from well 3-7-21H1 is based on a detailed sample description of a test hole drilled in the western part of the area. The sample description was done as part of a Master's problem by Keith Kvenvolden. The top of the Riverbank lies about 100 feet below land surface, A feature of interest is the gravel from 225- 255 feet which consists of chert, schist, metagraywacke, and dark quartzite. Practically no igneous intrusive rocks occur in this gravel or in other samples from the hole although volcanic rocks were found. The gravel is quite different from any other gravel seen in the post- Mehrten rocks. This suggests a possible source in the Coast Ranges on the opposite side of the Valley. Two distinctive clay units are found in the Riverbank Formation in the subsurface. The oldest clay is logged as blue clay by local drillers. The unit is a silty, light bluish-gray to greenish-gray clay which ranges in thickness from 3 to 75 feet. The clay probably is equivalent to what has been called the Corcoran Clay Member of the upper part of the Tulare Formation (Frink 84 and Kues, 1954); however, the local drillers 1 term will be retained as an informal name until the correlation is more firmly established. Figure 18 shows contours on top of the blue clay. The blue clay is limited to the western part of the study area, and, although well logs were carefully examined, it could not be traced much further east than Modesto and Salida. In fact, the blue clay does not always seem to be present In the western part of the area. It was not definitely identified in a carefully studied test hole (Appendix B, well 3-7-21 Hi), and it seems to be missing beneath parts of Modesto. The study area may be close to the edge of the old Pleistocene lake in which the clay was deposited.

The second distinctive unit in the Riverbank Forma- tion is a red clay layer some 5 to 12 feet thick that occurs at the top of the formation. This red clay represents the soil developed on the Riverbank surface and can be traced in the subsurface eastward to where the Riverbank crops out. The nature of the clay can be observed in the north bluffs of the Tuolumne River at Waterford where the soil developed on the Riverbank is buried by the younger

Modesto Formation (Figure 16; Appendix kt 3-H-.33 E & F. unit 2). Figure 19 shows contours on top of the red clay in the subsurface. The red clay is given on some drillers 1 logs in Appendix A, and both the blue clay and red clay are shown on the log for well 3-8-24 81. oi? 86

R. 7 E. R. 8 E. R. 9 E. R. 10 E. R. I I E.

' R T 2 S. OA DALE S ...."■"■"*- \ s \ \ \ \v. \ A S L_S \ ®o \ \ T. 3 S. \ \ \a N ■■ ®^ o \ \ MODESTO ...... -"■ v-- O o \ \ \ \ TOO* / \ 9 OCy0 Cy^ \ A^E \ o\ o T. 4 S. J> FIGURE 19 0 v^ czp RECONSTRUCTED SURFACE \ OF OLD ALLUVIUM li o \°o° °\ REFERENCE POINTS^ O WELLS; ATOPOGRAPHY I — o \turloSk° Sa'""""■" A ----- UPPER SURFACE FROM WELL LOGS \ \ SURFACE RECONSTRUCTED FROM TOPOGRAPHY -P, PRESENT CONTACT BETWEEN YOUNGER ALLUVIUM AND OLDER ALLUVIUM CONTOUR INTERVAL: 50 FEET 87 - Source and Origin The lithology and sedimentary features of the Riverbank Formation are quite similar to those of the Turlock Lake Formation, and indicate the same source and mode of origin. That is, the Riverbank was derived primarily from granitic rocks of the Sierra Nevada and

wa3 deposited in broad, gently sloping, coalescing alluvial fans. Minor sources of metamorphic and volcanic rocks existed In the foothills and main Sierra Nevada as shown by pebbles and cobbles of the coarser size grades. Some gravel In the subsurface in the west (Appendix B, well 3-7-21 Hl, 225-255 feet) may have been derived from the Coast Ranges. Little evidence has been found for buried soils within the Riverbank; so the formation probably represents a single episode of fan building. The blue clay is a lacustrine deposit from a very extensive fresh water lake In the San Joaquin Valley (Frlnk and Kues, 1954; Davis, and others, 1957* p. 157- 163; Barbat and Galloway, 1934, p. 492). The stratigraphic position of the blue clay indicates that the lake was present during deposition of the middle part of the Riverbank Formation. Its general absence east of Modesto and Sallda suggests proximity to the lake margin. - Age and Correlation The Riverbank Formation is equivalent to the lower part of the Victor Formation of the Mokelumne area (Piper, and others, 1939* p. 38-49) and the Victor is considered to be Late Pleistocene in age. The Riverbank 88

probably correlates with at least part of the Tulare Formation of the west side of the San Joaquin Valley (Reiche, 1950; Barbat and Galloway, 1934). Reiche thought that clay present in an exposure of the Tulare Formation west of Tracy correlated with the blue clay, although the outcrop is a number of miles west of the nearest occurrence of blue clay In the subsurface. Frink and Kues (1954* p. 2364) state that the Corcoran Clay lies in or over the upper part of the Tulare Formation. Frink and Kues (1954* p. 2368) also state that the Corcoran Clay occurs stratigraphically above the surface formed by San Joaquin (hardpan) and Cometa soils. In eastern Stanislaus County, however, the blue clay occurs beneath the San Joaquin (hardpan) soil which is found at the top of the Riverbank Formation. A firm dating for the blue clay would be Invaluable In assigning an age to the Riverbank Formation, but at present no agreement has been reached as to its age. The Corcoran Clay Is considered to be Pleistocene by Frink and Kues (1954, p. 2364-2368) and Reiche (1950* p. 9) and Late Pliocene by Davis, and others (1957* p. 162), If the Riverbank Formation is Middle Pleistocene as suggested below, then the blue clay is also Middle Pleistocene in age. The Munsell color of the B horizon (7.5 YR 5/4) suggests a pre-Wisconsin age for the Riverbank Formation (Carter, 1957* p. 119-120). This is indicated also by the 89

degree of weathering of granitic material and the amount of development of the soils. As the underlying Turlock Lake Formation is inferred to be Late Pliocene and Early Pleistocene, the suggestion is made that the Riverbank is Middle Pleistocene in age.

Water-bearing Properties - The Riverbank Formation is the most important aquifer in the study area. In the outcrop area, the Riverbank furnishes large amounts of water to many Irrigation wells, some municipal and industrial wells, and numerous small domestic wells. The Riverbank is penetrated in the subsurface by many wells drilled through the Modesto Formation. Water quality is good in the shallower zones, but water quality problems exist west of Modesto and Sallda. Saline water present in the Riverbank in the subsurface must have migrated into the continental beds. The Riverbank Formation is a better aquifer than the Mehrten or Turlock Lake Formations because shallower wells will yield as much water as deeper wells in the two older units. Wells in the Riverbank also yield more water than wells of equal depth in the Modesto. Twenty-one irrigation wells obtaining large amounts of their water from the Riverbank have pumping rates ranging from 1008-2965 gpm with an average of 2200 gpm. Several public supply wells In Modesto have been tested at 3500 gpm or more, and these wells draw a large part of their water from the Riverbank Formation. 90

Modesto Formation General Description, Type Section, Thickness - The Modesto

Formation closely resembles the Turlock Lake and Riverbank Formations and contains an alternating sequence of sands, silts, and gravels. The formation crops out in a northwest- southeast trending belt extending from slightly east of Modesto and Sallda nearly to the San Joaquin River (Plate 2). Tongues of the Modesto extend eastward from the main belt along the Stanislaus and Tuolumne Rivers. The bedding of the Modesto Formation Is generally parallel with the Riverbank Formation but seems to have a gentler dip in the subsurface. The lower contact with the Riverbank is defined well by the red clay. The contact with younger Recent alluvium is easily observed where the westward flowing major streams are entrenched below the surface on the Modesto. The contact is probably gradational along the San Joaquin River where it can not be picked on well logs; so the contact has been projected into the subsurface on Plate 3. The Modesto Formation occurs also as deposits on terraces eroded on the Mehrten Formation east of Oakdale (Appendix A, section 2-11-SR) and probably as deposits filling some channels cut Into the Mehrten. The type section of the Modesto Formation is along a private road down the south bluff of the Tuolumne River in eastern Modesto (Davis and Hall, 1959* P. 20-21). A detailed description of the type section is included in this report (Appendix A, section 4-9-2J), and sieve analyses 91

(Figure 20; Appendix C) and grain counts (Table 5) are given for three samples from the type section. Along the Tuolumne and Stanislaus Rivers terraces are found below the surface of the Modesto Formation and above the flood plain deposits mapped as Recent alluvium. These terraces have been included in the Modesto Formation of this report. A satisfactory thickness for the Modesto Formation is difficult to obtain because of problems in picking the upper contact in the subsurface. As shown on the geologic cross section (Plate 3), however, the thickness Is probably 50 to 100 feet.

Topographic Expression, Soils, Lithology - The topography of the upper surface of the Modesto Formation is flat and sloping gently westward. A few shallow drainage courses occur on the surface. The surface is also covered In places with stabilized sand dunes and closed depressions from a more recent period of eollan deposition and erosion. The topographic unconformity with the Riverbank Formation is not sharp but can be seen in the field and on topographic maps. The topographic unconformity with the Recent

alluvium is easily seen along the Stanislaus and Tuolumne Rivers but is hard to distinguish along the San Joaquin River, This topographic unconformity has been defined mainly on the basis of the degree of development of the recent drainage pattern. 92

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The soils of the Modesto Formation are not developed well, but the major horizons can be differentiated, A typical B horizon has a Munsell color of 2.5 V 6/2. The A and B horizons on the Modesto Formation are less than 5 feet thick. The B horizon is somewhat compact with minor amounts of clay and abundant sand grains of quartz and feldspar. Coarser-grained material has been only slightly weathered, and granitic material is usually rather fresh. An example of Modesto soil overlying better developed Riverbank soil is shown In Figure 16. The Modesto Formation is similar in composition to the Turlock Lake and Riverbank Formations, although it has considerable fine-grained material. The sand, silt, and gravel beds are lenticular and commonly cross-bedded. An exposure of sand and silt in the Modesto (?) Formation Is shown in Figure 21. This example can be contrasted with coarser sand of the Turlock Lake shown in Figure 13. The coarser materials consist of such rocks as granodiorite, andesite, rhyollte, quartzite, schist, and other volcanic and metamorphic rocks. In the finer grade sizes, the dominant minerals are quartz, feldspars and blotite with minor amounts of magnetite, hornblende, and other dark minerals. The colors of the sediments are gray to grayish orange silts and clays and gray and yellowish brown to pale brown sands. 95

Fig. 21.— Sand and silt in Modesto (?) Forma- tion on Highway 120 just north of Oakdale. 96

Grain counts (Table 5) and sieve analyses (Figure 20; Appendix C) were made for six samples from the Modesto Formation, and one pebble count was made (Table 3), The results are rather similar, and the samples can be discussed together. The sieve analysis for 4-9-2 A indicates the possibility of a mixed sample; so the sample is not included on Figure 20, Sample 4-10-1DIs from a terrace that may be slightly younger than typical Modesto. The samples, except 4-9-2A, are well-sorted (So range - 1.31 1.67 with an average of 1.4l), medium- to coarse- grained sands. Most of the coarser material is subrounded, but the finer material is angular to subangular. Colors are gray, yellowish brown, and dark yellowish orange. Granite pebbles associated with some of the sands may be broken but are relatively unweathered. A pebble count , as made for one sample from the Modesto Formation (Table 3). The sample (4-9-2A) is the coarse fraction from the mixed sample mentioned above. The pebbles are 4~16 mm in diameter with most greater than 6 mm. They are subrounded to rounded, and are pyramidal, tabular, and discoldal In shape. Many of the pebbles have a calcareous coating. Rock types include igneous mainly granitic, volcanic mainly andesitic, and metamorphic. The exposure from which the sample was taken seems to be Modesto, and no known exposures of older rocks occur nearby. Nevertheless, the degree of weathering and 97

breaking of coarser material are more characteristic of the Riverbank. The Modesto Formation in the subsurface is similar to the surface exposures. Its general character Is shown in some of the drillers 1 logs in Appendix B, and a particularly good example is well 3-7-21H1.

Source and Origin - The composition and texture of the Modesto Formation indicate deposition as a rather thin series of coalescing fans. The formation displays much the same sedimentary features as the Riverbank and Turlock Lake Formations, that is, cross-bedding, lenticular!ty, and angularity of grains all suggestive of stream deposition. The slope of the surface of the Modesto indicates that the fans were rather flat and broad. The lack of a distinct contact with Recent alluvium along the San Joaquin River suggests that the Modesto grades into the

Recent alluvium. The Modesto was probably deposited during a single episode of fan building, although what seems to be a burled Immature soil in the type section (Appendix A, 4-9-2J, unit 10) presents the possibility of at least temporary periods of weathering and soil formation. The sediments of the Modesto Formation show derivation from the granitic and associated rocks of the Sierra Nevada and foothills. Some Coast Range material may be present in the subsurface. 98

Age and Correlation - The Modesto Formation is equivalent to the upper part of the Victor Formation of the Mokelumne area (Piper, and others, 1939* p. 38-49), and the Victor Formation is considered to be Late Pleistocene in age. The Modesto may correlate In part with the Tulare Formation of the west side of the San Joaquin Valley (Reiche, 1950, p. 6-9), but the Modesto is probably younger than the Tulare. The color of the B horizon of the Modesto Formation indicates a fairly recent age. This is supported also by the slight amount of weathering of the granitic material, the relatively slight development of the soil, and the lack of erosion along the entrenched streams. These suggest that the Modesto Formation is probably Late Pleistocene in age.

Water-bearing Properties - The Modesto Formation is an important aquifer and supplies large amounts of water to many drainage wells, some irrigation wells, some municipal and industrial wells, and numerous domestic and stock wells. The chemical quality of water is generally good, although areas with poor quality water are found near the San Joaquin River. Well yields in the Modesto Formation are difficult to separate from those in the Riverbank Formation because many wells penetrate both. Data from 74 drainage wells give a range of 246-2543 gpm with an average of 1005 gpm. 99

Nearly all of these wells are less than 200 feet deep, and many of them obtain a large portion of their water

Recent Sand Dunes The surface of the Modesto Formation and to some extent that of the Riverbank Formation have been modified by wind action in fairly recent times. Numerous isolated sand dunes are found in the study area, and the larger ones are shown on Plate 2. Extensive tracts of sand dunes occur north and south of the study area. Most of the dunes are stabilized by vegetation, although some may have been activated by modern agriculture. The dunes generally range from 1000-3000 feet In length, 100-500 feet in width, and 10-25 feet In height. The most common dune form is longitudinal, but parabolic and complex forms occur also The sand dunes are accompanied in many places by depressions. Some of the depressions are oriented approximately east-west and appear to lie along former distributary channels on the surface of the alluvial fans. However, many depressions are oriented nearly at right angles to the old drainage pattern, tend to be oval in outline, and have no outlets. Probably these oval depressions were produced by deflation because l) many of them are associated with sand dunes, 2) they are oriented parallel to the present dominant wind direction 100

Fig. 22.— Sand dune on Blue Gum Avenue west of Modesto. 101

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(northwest-southeast), and 3) they could not have formed hy solution subsidence because soluble material Is not present in the subsurface. The oval depressions are of many sizes, but in the study area a maximum size is about 1500 feet long, 500 feet wide, and 5-10 feet deep. A large number of these depressions have been altered by land-leveling operations. In the study area, most dunes are isolated, but a small dune field can be seen at the Del Rio Country Club (Plate 2, 2-9-19* 20 & 21). The dune field Is oriented east-west and is about 7000 feet long, 2000 feet wide, and 25 feet high. Another small dune field can be observed just west of the town at Riverbank, An example of an isolated dune may be found on Blue Gum Avenue six miles west of Modesto, and Figure 22 shows the dune where it has been cut by land-leveling operations. This dune is 1000 feet long, 400 feet wide, and 18 feet high, and It is oriented northwest-southeast. Many of the isolated dunes especially along the south bluff of the Stanislaus River are oriented east-west whereas the extensive longitudinal dunes of the Turlock-Delhi area to the south and the Avena quadrangle to the north are oriented northwest-southeast. Some of the dunes that show east-west orientation may be parabolic dunes that have lost their tails, and possibly some dunes received an orientation because they formed along the east -west trending south bluff of the Stanislaus River. 103

Sieve analyses (Figure 23; Appendix C) were made for 5 samples, and grain counts (Table 6) were made for 4 samples from sand dunes. One sample (2-9-26K, unit l) is from dune sand overlying the Riverbank Formation type section. A sieve analysis was made of active dune sand from south of the area (6-11-20C). The sieve analyses were made with the hope that they would provide definite criteria for eollan origin. The range in sorting coefficients 1.44-1.59 with an average of 1.52 is no better, however, than that for fluviatile sands, although they do show well-sorted material. The sample of active dune sand (6-11-20C) has an So i1.59. The sands are all fine- to medium-grained and contain few grains larger than 2 ram and none greater than 5 mm. Uniform appearance, uniform color, and general lack of coarser-grained material are all characteristic of the dune sands. Observations with a binocular microscope suggest that the dune sands may be slightly better rounded than the fluviatile sands in the same grade sizes. The grain counts (Table 6) show a mineral composition similar to the other post-Mehrten sediments. The soil profiles on the sand dunes are only slightly developed with some accumulation of organic material in the top 2-3 inches of the soil. This minor amount of soil development along with the fact that the dunes are on top of the Modesto Formation indicate that the dunes 104 are young, possibly completely post-Wisconsin in age. The dune sands lie above the water table and are not aquifers. They are uniform and permeable and should serve as good recharge areas particularly for artificial recharge. Furthermore, water that percolates through the dune sands would not pick up many dissolved solids. At present most of the dunes overlie areas of rather high water table; so their recharge potential can not be utilized.

Alluvium Recent alluvium occurs in narrow strips along the entrenched channels of the Stanislaus and Tuolumne Rivers and Dry Creek, It overlies and probably grades into the Modesto Formation along the San Joaquin River. The lower contact has not been distinguished in the subsurface, but a probable maximum thickness is about 50 feet. The sediments of the Recent alluvium are deposited by streams draining the Sierra Nevada and foothills and are quite similar to the other post-Mehrten formations. The size distribution ranges from coarse gravel near the foothills to silt along the San Joaquin River. Gravel pits and sand bars show that rather coarse gravel occurs at least as far west as near Salida on the Stanislaus and Empire on the Tuolumne. The streams are now under artificial control, and probably they are not transporting 105

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such coarse loads except during extreme floods. A characteristic constituent of these streams is biotite which is present even during periods of low flow. High water in the spring of 1958 deposited large amounts of fine silt and sand up to several feet thick along the flood plains of the major streams.

A few shallow wells obtain water from the alluvium along the Stanislaus, Tuolumne, and San Joaquin Rivers. Water quality along the Stanislaus and Tuolumne seems good although little information is available. The water along the San Joaquin may have a rather high iron content but be of good quality otherwise, or else It may contain high sodium, chloride, and bicarbonate and be of rather poor quality.

STRUCTURE

The general structural picture of eastern Stanislaus County (Plate32 and 3) consists simply of a prism of post-Jurassic sediments dipping gently to the west and underlaid by granitic and metamorphic basement rocks.

Structures in the basement rocks at the surface to the east all seem to be pre-lone; however, little work has been done on basement rocks during the Stanford project. In the younger rocks at the surface, practically no evidence is found for folding or faulting with one possible exception: near Knights Ferry contact features occur between the lone and Mehrten Formations and between the lone and post-Mehrten sediments that seem to be old 107

channels but which might be due to faulting. On the subsurface maps (Figures 6, 18, and 19) some features are suggestive of folding, but these should be viewed with caution because the details are based on scant data. The gentle westward dip of the pre-Riverbank formations is due at least in part to tilting of the Sierra Nevada in the Late Pliocene and Early Pleistocene. The presence of gas wells along the Tuolumne River from Modesto to east of Waterford Is indirect evidence for some sort of an east-west trending structure in the subsurface. Some oil and/or gas test wells have been drilled rather close to this trend. The existing gas wells produce small amounts of methane and larger amounts of saline water which the more easterly wells apparently obtain from the lone Formation and the westerly wells from the Mehrten Formation. Electric logs from the few major test wells drilled in and close to the study area have been collected and examined, but not enough data are available to show much about the structure. The study of the ground water in the area indicates that chemical quality of water is subject to a pronounced subsurface control. More specifically the chloride contours (isochlors) for ground water from several depth zones show a definite linear trend along the Tuolumne River (Davis and Hall, 1959). At present not enough data are available to precisely define the trend or character of the structural feature. One possible explanation is 108

that an east-west fault has brought beds containing saline water closer to the surface along the river. The correlation section of Church, and others (1958) Indicates that the study area may overlie a broad upwarp in pre-Miocene rocks. As discussed previously, the present report is in disagreement with several interpretations on the correlation section. Nevertheless, if such an upwarp is present in the subsurface it might bring saline water-bearing beds closer to the surface. An upwarp might also form a structural situation favorable for up dip migration of saline water.

GEOLOGIC HISTORY The oldest rocks that crop out in eastern Stanislaus County are the Mariposa and Logtown Ridge Formations and related intrusives of the Sierra Nevada foothills. These rocks show a history of sedimentation, volcanism, low grade metamorphlsm, and minor intrusions during the Jurassic and ending in the Late Jurassic or possibly Early Cretaceous. The Intrusions and metaraorphism are a manifestation of much larger scale activity farther east in the main Sierra Nevada. Insofar as the study area Is concerned these rocks furnish the basement on which the younger rocks have been deposited. The presence of marine rocks in the subsurface shows that the study area was covered by a sea during the Cretaceous. Limited evidence In the subsurface and 109 evidence from the west side of the San Joaquin Valley Indicate that the i_ate Cretaceous was probably the major time of marine inundation. The Cretaceous rocks are overlaid in places by unnamed marine Eocene rocks, but little is known concerning them. The marine Eocene is overlaid by the lone Formation, and where the marine Eocene is absent the lone lies directly on the Cretaceous. The lone and marine Eocene are of Early Eocene age; so during the period Late Cretaceous to Early Eocene there was an eroslonal interval possibly with tectonic activity. The lone Formation consists of fluviatile, lacustrine, and marine sediments of Early Eocene age. Presumably, most of the sediments were deposited in a deltaic environment on a low plain near the ocean (Allen, 1929), The amount of clay and clean sand and the presence of an intensely weathered zone between the lone and bedrock are Indicative of active chemical weathering, and they are considered to be the products of weathering In a humid, tropical climate (Allen, 1929). The coal beds which are present In part of the lone were undoubtedly deposited in swamps. The marine fossils found east of the study area show that marine tongues extended across the plain during part of the time, A considerable portion of the Central Valley had a similar history at this time because the lone Formation and its equivalents extend from near Fresno northward Into the Sacramento Valley on 110 the east side, and from near Tracy northward toward Mt Diablo on the west side. The lone Formation is overlaid by the Valley Springs Formation of Late Miocene age. The period from Early Eocene to Late Miocene was a time of nondeposition and erosion, but little tectonic activity took place because the formations are nearly conformable. The Valley Springs was derived from erosion of rhyollte resulting from fairly widespread rhyolitic eruptions in the Sierra Nevada. The rhyolitic materials were deposited mainly by streams although some of the ash may have been airborne. The rhyolitic eruptions were followed rather closely by very extensive eruptions of andesite. In fact, the western slope of the Sierra Nevada became covered with andesitic debris. Erosion proceeded rapidly and the Mehrten Formation was deposited In the San Joaquin Valley. The Valley Springs and Mehrten have conformable and somewhat gradational contact relations; so there was neither much of an erosional interval nor a time of tectonic activity. The Mehrten was deposited in a fluviatile environment during the period Early Pliocene (Late Miocene farther east) to Middle Pliocene. Andesitic sediments similar to the Mehrten Forma- tion are found in rocks as far south as Coalinga, as far north as Mt. Diablo, and as far west as Santa Cruz. The rocks at Coalinga, Mt. Diablo, and farther west are marine 111 in part, but those along the east side of the Central Valley are continental, and those between Coalinga and Mt, Diablo on the west side of the San Joaquin Valley are mainly continental. Apparently, much of the east side of the Central Valley, including the study area, was a broad plain on which andesitic debris was being deposited and across which material was being transported

By Middle Pliocene, stripping of the volcanics was nearly complete, and the streams began eroding older granitic and metamorphic rocks. The contact between the Mehrten and the overlying Turlock Lake Formation is characterized by a distinct change from andesitic material to sediments consisting mainly of quartz, feldspar, and biotite. In some places the contact is gradational with an intervening transitional zone. However, in other places, the contact is sharp and may be of an erosional nature. Possibly the contact is gradational down dip and erosional up dip. The lower part of the Turlock Lake Formation is conformable with the Mehrten Formation, but the upper part has an increasingly gentle dip. The Turlock Lake probably represents a composite series of alluvial fans with intervening periods of erosion. During the time of deposition of the alluvial fans, Late Pliocene to Early (?) Pleistocene, the Sierra Nevada was undergoing tilting to the west. The angular discordance might be explained on 112 the basis of climatic change. Evidence has been found, however, indicating that the Middle Pliocene climate was similar to the present (Axelrod, 1944, p. 147-167); so probably no major climatic changes occurred until the beginning of Pleistocene glaciation. The upper part of the Turlock Lake Formation, Early (?) Pleistocene, the Riverbank Formation, Middle (?) Pleistocene, and the Modesto Formation, Late (?) Pleistocene probably are related to Pleistocene glaciation. The alluvial fans which make up the formations were most likely deposited during glacial periods, and the soils between them probably represent Interglaclal periods. Evidence obtained in the study area is not sufficient to allow correlation with established glacial stages In the Sierra Nevada. The blue clay In the Riverbank Formation shows the presence of a widespread lake during the Middle (?) Pleistocene,

Recent alluvium is depositedr in trenches eroded into older formations by major westward flowing streams. The contact with the Modesto Formation is gradational along the San Joaquin River, The streams are downcutting now during a period of relatively high sea level; therefore, it might be inferred that the study area was not greatly affected by lowering of sea level during the Pleistocene, The sand dunes lying on the Modesto Formation are indicative of a drier climate in the recent past, probably 113 post-Wisconsin, The dunes have been stabilized by vegetation suggesting that the climate has become moister very recently. GROUND WATER

OCCURRENCE OF GROUND WATER

The Water Table The term "water table" is not too satisfactory because of ambiguities in its definition. For example, the water table may be defined as the level at which unconfined water stands in a well, or it may be defined as the top of the saturated zone which is above the level at which unconfined water stands in a well. Also, because of local conditions some wells may be under confined or partially confined conditions, and the level at which water stands may be either above or below the level at which unconfined water stands. Therefore, In this report water table is used only in a general sense, and Figure 24 is deliberately entitled "water-level map." In eastern Stanislaus County, most of the shallower ground water is unconfined or possibly partially confined in some areas. Along Dry, Creek and the Stanislaus River, however, some shallow flowing wells are found. The water level In these wells is generally only a few feet above the water table which is usually close to land surface. The ground water In the vicinity of these wells may be confined under local clay and silt lenses. These streams

114 115 particularly Dry Creek might receive a significant quantity of inflow from the confined ground water if leakage occurs upward through the confining layer. In the eastern part of the area north of Dry Creek, the water down to a depth of 800 feet or so seems to be unconfined, but little is known about deeper water. South of Dry Creek there is evidence that some deeper ground water is confined. From Waterford west to Modesto some wells 300 feet and deeper have water that rises above the water table and in some cases flows at the surface. Many of the wells have salty water, and they may also have gas. Gas wells up to 2400 feet deep along the Tuolumne River yield salty water that flows at the surface. The water in these wells obtains its lift either from confined conditions or gas pressure or else a combination of the two. In the western part of the area, little evidence has been found as to whether or not the deeper ground water is under pressure. Practically the only wells deeper than a few hundred feet have been gas or oil test wells In which water levels were not taken. The fact that saline water is close to the surface beneath Modesto (see section on chemistry of water) may be evidence that deeper water is under pressure and will rise closer to the surface. The blue clay in the Riverbank Formation may be a confining layer or at least have an effect on movement of 116

ground water, but little supporting evidence has been found. In other parts of the San Joaquin Valley, the blue clay has confined water beneath it (Davis and others, 1957* p. 169). As mentioned in the section on geology the blue clay seems discontinuous beneath parts of the study area. The red clay at the top of the Riverbank Formation may also have some influence on the occurrence of ground water, but again no real evidence has been found.

Slope and Shape The general form of the water table, or at least the level at which water stands in wells, Is shown for the late spring and early summer of 1957 in Figure 24, In the preparation of this map, the assumption was made that the ground water was unconfined and intersected the streams. Although this Is not quite true in some places, the magnitude of error at the map scale is small. The water table has a general slope from east to west with an average gradient of about five feet per mile. Locally the gradient varies from about 4 to 12 feet per mile. The steeper gradients are found near the Stanislaus

I An examination of existing records indicates that with the exceptions listed in the text the map is approximately valid for the last ten years or so.

118 and Tuolumne Rivers and at the Modesto pumping cone of depression. The lower gradients are found along the ground water divide running east-west down the central part of the area, in the irrigated area around Oakdale, and in the western part of the area toward the San Joaquin River. The more interesting and significant aspects of the shape of the water table are the exceptions to the general east to west slope. These aspects may be summarized as follows: l) the Stanislaus, Tuolumne, and San Joaquin Rivers are all gaining ground water except during flood stage and except for the Tuolumne at Modesto, 2) the pumping cone at Modesto receives water from the ground water body over a large area and surface water from Dry Creek and the Tuolumne River, 3) fc&e Modesto cone and the Tuolumne River together drain over one half of the ground water of the area, and the San Joaquin and Stanislaus each receive about one half of the remainder, 4) Dry Creek flows down the water table for most of its length in the area and gains little unconfined ground water but may gain some inflow from leakage upward of locally confined ground water, and 5) ground water pumpage at Oakdale, Riverbank, and Salida does not seem to have developed noticeable cones of depression, and the same is probably true at Empire and Waterford. 119

Depth to Water Depth from the land surface to water ranges from a foot or less in places near the San Joaquin River to about 100 feet southeast of Oakdale. The Modesto Irrigation District has an extensive drainage well program to lower the water table In the western and central parts of the area. The Oakdale Irrigation District has a drainage program also, but this is mainly to handle water perched on soils of the Riverbank and Turlock Lake Formations. Depth to water beneath Modesto is 40 feet or more. Pumping lifts in wells, which is the sum of the depth to ground water and the drawdown, range from less than 20 feet near the San Joaquin River to more than 130 feet near Oakdale. Beneath Modesto pumping lifts may be as much as 75 feet.

Fluctuations Fluctuations of water levels are of two main types: l) seasonal and 2) long term. These fluctuations are controlled In the study area by irrigation practices, drainage techniques, and intense local pumpage. Seasonal fluctuations in irrigated areas involve a rising water table during the irrigation season, roughly April to September, and a declining one during the rest of the year. In areas of intense summer pumpage such as Modesto, the water table declines during the summer and rises during 120

the winter (see Figure 25). Long-term fluctuations are shown by Oakdale Irrigation District records which indicate that the water table over much of the district has declined about one foot per year since 1944 (see Figure 26). This has been due to two factors: l) heavy pumpage from irrigation wells and 2) slow recharge through the well- developed soils of the Riverbank and Turlock Lake Formations. The Modesto Irrigation District on the other hand has trouble with a rising water table in much of its area due to large scale recharge of Irrigation water through the Modesto Formation. In the City of Modesto water levels in the deeper part of the pumping cone have declined about 22 feet since 1924 with a considerable portion of this taking place after World War 11.

Recharge

Precipitation

The average annual precipitation in the study area is about 12 inches and ranges from 10 inches at the San Joaquin River to 14 inches at the eastern boundary. Average annual precipitation at Modesto is 10.99 inches (Figure 2), but annual precipitation has varied from 4 to 19 inches during the period of record. Average annual precipitation near Oakdale is 13.96 inches (Figure 2), but annual precipitation has varied from 6 to 28 Inches CO

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during the period of record. The bar graphs in Figure 2 show that most of the rainfall occurs in December, January, February, and March whereas practically none occurs in June, July, and August. The study area comprises about 220 square miles or 140,000 acres; so the average annual precipitation is approximately 140,000 acre-feet and annual precipitation has varied from about 58,000 to 280,000 acre-feet during the period of record. An important problem to be considered is the amount of rainfall available for ground-water recharge. The average annual potential evapotranspiration at Modesto is 31.7 inches of water (Arkley, 1959* p. 2), and Its monthly distribution is shown in Figure 2. Precipitation exceeds potential evapotranspiration by a total of 4.1 inches during January, February, March, and December, but potential evapotranspiration exceeds precipitation by 24.7 inches during the period April to October. Therefore, only about 4 inches or 47,000 acre-feet of rainfall are available for soil moisture requirements and recharge during an average year. Once soil moisture requirements are fulfilled then possibly 12,000 acre-feet or less would become available for ground-water recharge. Naturally this would vary from year to year and could be nearly zero In a dry year and many thousands of acre-feet in wet years. Comparison of the approximate figure for recharge with the amount of ground-water withdrawal and 124 usage discussed below shows that rainfall is not the major source of ground water replenishment at present.

Irrigation

Large quantities of surface water are Imported by the irrigation districts during the growing season. In 1957 about 433,000 acre-feet were spread on some 95.000 acres of land. Some of this water is actually lost by leakage from canals, but part of the leakage returns to the ground water anyway. With an irrigation efficiency of about 60-70$ some 130,000 to 170,000 acre-feet of the imported water would be available for recharge. Actually, part of this water Is lost by evaporation and drainage facilities,but the amount left is still large. During the same period of time about 16,000 acre-feet of ground water was used for irrigation, and some 5,000-6,000 acre-feet of this might be available for recharge. Although the above figures are for 1957 they should be fairly representative of conditions in recent years. The magnitude of these figures shows that irrigation is the major source of ground-water replenishment.

Streams The water-level map for the spring and summer of 1957 (Figure 24) shows that streams can be a source of recharge to the ground water body only at Modesto except during periods of flood flow. The pattern of the water- level contours around the Modesto pumping cone indicates 125 that both Dry Creek and the Tuolumne River are losing water. The magnitude of this ground-water recharge is unknown but probably is only a fraction of the flow of the streams. Most likely, the main effect is on chemical quality of ground water rather than on amount of water (see section on

Subsurface Inflow The pattern of water-level contours in Figure 24 shows that the only place subsurface inflow of shallow ground water can occur Is In a relatively small area southeast of Oakdale, All other shallow subsurface inflow is directed toward the Stanislaus and Tuolumne Rivers, Average annual rainfall east of Oakdale ranges up to 18 Inches, but the flat water level gradient of 4 feet per mile and the general absence of irrigation to the east suggest that subsurface inflow is rather small. The older sedimentary rocks east of Oakdale may receive replenishment which serves to recharge the deeper aquifers to the west.

This might account for some of the pressure in deeper wells that penetrate the Mehrten Formation and older rocks. Deeper saline water is moving upward in parts of the area (see section on chemistry of water), and some of this may be inflow from adjacent areas. 126

Discharge Evapotranspiration Evapotranspiration is an Important type of discharge particularly in irrigated areas. Of course evapotranspiration can only take shallow ground water in and near the root zone. However, it serves to intercept water i^hich might otherwise become ground water. The average annual potential evapotranspiration at Modesto is 31.7 inches of water (Figure 2), and this figure is assumed to be valid for the rest of the area. The average annual precipitation over the area is about 12 inches; so, the monthly distribution of evapotranspiration and precipitation (Figure 2) Indicate an annual deficit of water of about 26 inches. According to Arkley (1959* P* 3) the average annual consumptive use of Irrigation water by plants in the Modesto Irrigation Dis- trict for 1952-54 was 30.2 inches. The figure of 30.2 inches compares quite favorably with the deficit of about 26 Inches derived independently and suggests that the calculated potential evapotranspiration is close to actual evapotranspiration. The data indicate that annual evapotranspiration from the study area is on the order of 350,000 acre-feet. This does not account for evapotranspiration by water-loving plants along the rivers nor does it account for water lost by evaporation from areas of a very shallow water table. 127

Pumpage from Wells Pumpage from wells for the year 1957 is given in Table 7. Irrigation and drainage pumpage varies from year to year according to the amount of surface water available, length of irrigation period, and amount of irrigated acreage; however, it has tended to remain fairly constant in recent years. Pumpage for industrial, public, and rural supplies has been increasing especially since

World War 11, but the 1957 figures give a good idea of water usage in recent years. Not all of the ground water pumpage Is lost to the area because part of the water used for irrigation, lawn watering, and sewage is returned to the ground water body. However, in Modesto, Riverbank, Oakdale, and Waterford most oomestlc and Industrial sewage is discharged into the rivers. Out of the 124,000 acre-feet pumped in 1957 possibly 100,000 to 110,000 acre-feet were lost from the

wtrearns The Stanislaus, Tuolumne, and San Joaquin Rivers are all gaining ground water with only one exception at Modesto (Figure 24). and as a result they remove large quantities of ground water from the area. Dry Creek receives discharge from drainage ditches and may receive some ground-water inflow particularly from partially confined ground water. A detailed analysis of stream 128

TABLE 7 WATER PUMPAGE - 1957 acre- eet

JAN FEB, APRIL MAY JUNE JULY AUG. SEPT OCT. NOV. DEC TOTAL Surface Water Oakdale Irrigation District 5,258 19,233 24,901 28,075 29,605 25,502 15,105 4,716 152,395 Modesto Irrigation District 327 7,382 35,314 30,545 !52,614 53,750 45,683 33,471 11,653 337 271,076 Waterford Irrigation District 9.50011 Total 327 12,640 54,547 55,446 80,689 83,355 71,185 48,576 16,369 337 432,97112 Ground Water Drainage Wells Oakdale Irrigation District 47 164 265 219 396 427 383 390 98 1 2,390 Modesto Irrigation District 1,173 8,965 8,782 9,064 9,417 9,759 9,166 4,240 1055 418 62,039 Subtotal 47 1,173 9,129 9,047 9,283 9,813 10,186 9,549 4,630 1153 419 64,429 Shallow sumps and drains3" Oakdale Irrigation District 1 2 2 869 1,614 2,092 2,723 2,964 2,824 2,201 587 8 15,887 Modesto Irrigation District 23 1 19 630 941 676 839 953 831 715 3 15 5,646

SubtctaJ i 24 21 1,499 2,555 2,768 3,562 3,917 3,655 2,916 590 23 21,533 Total Drainage 71 1,194 10,628 11,602 12,051 13,375 14,103 13,204 7,546 1743 442 85,962 Irrigation Wells 2 Oakdale Irrigation District 15 4 5 116 121 372 1,054 1,447 1,569 1,480 298 2 6,483 Modesto Irrigation District 37 3,878 1,210 253 533 3,138 215 9 9,273 Total Irrigation 15 42 3,994 1,331 625 1,587 4,585 1,784 1,489 298 15,756

Municipal and Industrial Modesto, west of Dry Creek City of Modesto 332 307 445 758 875 1,440 1,590 1,450 1,180 630 448 365 9,820 Industrial5 124 131 137 217 279 354 193 307 525 641 525 217 3,650 Del Este6 53 65 96 142 192 249 281 273 234 106 91 68 1,850 Snail Companies 18 22 33 48 53 83 94 88 77 35 30 22 603 Subtotal 527 525 711 1,165 1,399 2,126 2,158 2,118 2,016 1,412 1094 672 15,923

Modesto east of Dry Creek 2,071 Del Este6.' 60 73 110 158 218 280 314 305 259 118 101 75 Industrial7 95 87 62 74 99 72 51 141 161 124 122 76 1,164 3,236 Subtotal 155 160 172 232 317 352 365 446 420 242 223 151 Empire 7 S 12 17 24 31 35 34 29 13 11 8 229 72 1,417 Oakdale 61 52 48 121 124 151 208 202 168 103 107 22 22 21 35 39 59 76 74 56 38 26 21 489 Riverbank 13 11 8 6 8 12 17 23 30 33 33 28 222 Salida° 40 45 43 38 17 15 11 299 Waterford6 9 11 16 23 31 1,610 1,957 2,789 2,920 2,950 2,755 1,838 1487 943 21,814 Muni, and Ind. Total 787 786 992 496 688 769 753 654 362 242 235 5,172 Rural8 (Domestic) 159 183 264 364 18,651 2,391 18,397 ,235 128,704 976 2,492 16,596 5,386 16,153 i11 3773 1622 Ground Water Total 1032 127,604 12 5,132 71,143 0,832 96,842 102,006 '3,576 K6.973 3773 1959 56i,675 Total Water Pumpage 1359 976

9 Includes some pumpage from west of Dry creek 1. At Welch Drop " . X" 10- No account has...been made of some private Industrie within nmviu 2. Includes Improvement Di.tricts their Qwn wlter N(J estißates ar£ Bade 3. Mixed ground and surface water for stock water or private irrigation. The magnitude of these is probably not large. *" Includes drain and reclamationpumps 11. An approximation 5- Metered by City of Modesto __,_«, v ■ , acre-feet that are not shown in monthly,mv use...e fieurefigure perpc tapv 12. Includes 9500 the 6-- Based on number of taps times Bonthly columns. 7 Supplied b/ Del Este Water Company by times 8. Estimated rural population of 40,000 divided 5 in footnote number 6. Also conversion factor described «»d Modesto State include, Improvement Districts in OID 129 flow and other records would be necessary in order to obtain good information about the amount of ground water removed by the streams. However, rough calculations can be made to obtain an idea of the amount of water returned to the streams. Streamflow records for the water year October 1955 to September 1956 (Wells, and others, 1959) show that 2,755*000 acre-feet of water passed the stream gage on the Tuolumne River above the La Grange Dam. Just below this gage the Modesto and Turlock Irrigation Districts diverted 1,025*000 acre-feet leaving 1,730,000 acre-feet in the Tuolumne, and 1,999*000 acre-feet passed the gage at Modesto. Therefore, 269,000 acre-feet were gained between La Grange and Modesto a distance of about 40 river miles. The only stream that enters the Tuolumne River In this stretch Is Dry Creek at Modesto with a flow of about 20,000 acre-feet per year. The slight amount of irrigation upstream from Waterford indicates that more than one half of the Increase in streamflow probably occurs between Waterford and Modesto a distance of 17 river miles. Three assumptions are made to simplify calculations l) about 150,000 acre-feet per year, excluding flow from Dry Creek, enter the Tuolumne River between Waterford and Modesto, 2) the same amount enters the Tuolumne between Modesto and the mouth, a distance of sixteen river miles, and 3) equal amounts enter the Tuolumne from each side of 130

the river. Under these assumptions, 150,000 acre-feet per year enter the Tuolumne from the study area between Waterford and the mouth. Similar calculations were not made for the Stanislaus or San Joaquin Rivers, but the water-level map (Figure 24) indicates less ground-water flow into them than into the Tuolumne.

Subsurface Outflow No subsurface outflow of shallow ground water from the study area Is taking place now (Figure 24), and presumably none has occurred in the past. Some of the deeper ground water especially under confined conditions may move into adjacent areas, but little is known about this.

Water-bearing Properties of the Sediments A detailed investigation of the water-bearing properties of the sediments such as permeability and porosity was not made in this study. Nor was an investlga tion made of the related properties of transmissibillty and storage coefficient of the aquifers. Some general conclusions can be drawn, however, from data collected concerning the performance of irrigation and drainage wells. A pumping test made with shallow wells just north of Modesto indicated a transmissibillty of the order of 140,000 gallons per day per foot which suggests a 131 permeability of the magnitude of 1000 gallons per day per square foot. The storage coefficient was about 10~3 which is In the artesian range. Probably the well casings are landed on clay lenses, and the ground water Is confined beneath the lenses. Recent work by the U. S. Geological Survey indicates an average specific yield for the 50-200 feet depth zone of 10.3^ on the east side of the San Joaquin Valley In the vicinity of Modesto (Davis and others, 1957* p. 430). Specific yield is essentially the effective porosity of the sediments. It is equivalent also to the coefficient of storage of sediments under water table conditions.

Data from Wells The data for well performance come mainly from records of the Modesto and Oakdale Irrigation Districts, and they are summarized in Table 8. In order to handle the data approximations had to be made concerning elevation of land surface and static water levels. In the statistical analyses the medians and averages are close together suggesting a normal distribution of the data; so the results probably can be accepted with some confidence. The statistics given for the wells Include the average, median, and range for yield, depth, and specific capacity (Table 8). The specific capacity of a well is the yield per foot of pumping drawdown. Specific capacity 132

v vo rrt CO in in CC rt I— i CO if ■«* 1 H a Ifl 9« lA f O t- rH t~- O 1J CO IN CO Or-1O r-1 CM f~ vo in 1 vo O Ov I C- - I— ■5 O O 1 o l rH Q Ch "^ rH rH VO rH Q~ej o CM rrt I CM CO 00 I CM a 1 -rt CM CM "<* ■«tf ■# 10 a "Q rH I— l CM CO O iH H o 0 cc CO -a CC CO < H

>HH O C/2 v 4-> 9 O C O "H <+H

M * 41 ■11 CQ m CO Dh -H 44 i-rt -rt "H rH -rt 4! CO 0) O 4) 4J c is CO "* 3 o Oh a r-H «HH Mrt 4J >-H ■rt rH o O V 0 s CO to c3 CD V oj bo bo a v o bC C -h Mrt to C -H co co eg oj "H co co ai a) CO CO «U oj 11 (h -h bo -5 «H (4 -H bO -Q lU -H bo -Q T3 OJ t3 C S "H 4> T3 c c -C 1) T3 C E -H r t) S 3 o > eu CO 3 4-1 > oo a 3 CO CJ < s cc £ V < 3 DC Z Oh < S CC Z a "H Oh o >H C/2 —t rH rH (S i-H I— l ►H " 133

Is not a constant value for a well because It tends to decline with time, that is, it is a function of time as well as physical properties of the sediments. Specific capacity is affected also by such factors as rate of

pumpage, saturated thickness penetrated, location and number of perforations, and method of well construction. Therefore, specific capacity is more of an index of individual well performance than of aquifer performance. Data were taken from a number of wells constructed in a similar fashion and which take water from the same aquifer in a rather small area. The statistical results should at least give a relative comparison of the various formations, and probably they give some insight into actual water-bearing abilities of the formations. Specific information about the water-bearing ability of the various formations is hard to obtain, because the data summarized in Table 8 are for wells penetrating two or more formations. Nevertheless, some general statements and conclusions can be made: 1. The 20 irrigation, drainage, and municipal wells In the Oakdale area obtain 50$ or more of their water from the Turlock Lake and Mehrten Formations. A few wells obtain some water from the Modesto, and many obtain up to 25$ or more from the Riverbank formation. The average yield is 1278 gpm with a range of 314-2780 gpm. 2, The 21 irrigation wells of the Modesto Irrigation 134

District are located mainly east of Modesto and Salida, and they obtain 50$ or more of their water from the Riverbank Formation. Many wells obtain considerable water from the Turlock Lake Formation, some obtain water from the Modesto Formation, and a few may obtain some water from the Mehrten Formation. The average yield is 2200 gpm with a range of 1008-2965 gpm. 3. The 74 drainage wells of the Modesto Irrigation District are located mainly west of Modesto and Salida, and they draw 50$ or more of their water from the Riverbank Formation. Only a few wells obtain any water from the Turlock Lake Formation. The wells obtain considerable water from the Modesto Formation, and many obtain some water from Recent alluvium. The average yield is 1005 gpm with a range of 246-2543 gpm. 4. The Riverbank Formation Is the best water-bearing formation followed In approximate order by the Turlock Lake, Modesto, and Mehrten formations and Recent alluvium. The same order is valid also for quantities of water supplied to wells. In addition to the data summarized in Table 8, information is available for other wells in the area. Most of this information Is in reasonable agreement with the results given in Table 8. Tests on two large public supply wells In Modesto indicate yields on the order of 3500 gpm at least during a short period of pumping mainly 135

from the Riverbank Formation. This, also, is the largest well yield reported in the study area.

CHEMISTRY OF WATER The discussion on chemistry of water of the study area is based on data from chemical analyses made at Stanford during the ground water research project supplemented by data from governmental agencies and private laboratories. Stanley N, Davis of the Geology Department, Stanford University made a number of chemical analyses of surface and ground waters in eastern Stanislaus and northern Merced counties during 1955 &nd 1956. Davis covered the entire area, but his major effort was concentrated south of the Tuolumne River. In 1957 and 1958, the present author made a detailed study of the area north of the Tuolumne River. A chemical sampling program was planned to make use of all available analyses. The analyses and their locations were recorded and considerable effort was made to fill in the gaps in the distribution of analyses. Complete analyses were made where necessary, and partial analyses were used to fill In areas between complete analyses. The analyses made at Stanford along with some preliminary interpretations have been published (Davis and Hall, 1959). The California Department of Water Resources has maintained an extensive program of sampling of surface 136 waters since 1951* and the analyses through 1956 have been published (Plumb, and others, 1956 and 1957). The analyses since 1956 have been released in monthly reports by the Department of Water Resources (Anonymous, 1957b). A number of private and governmental analyses of surface and ground waters in the lower San Joaquin Valley have been published by the Department of Water Resources (McNealy, and others, 1956).

Many chemical analyses of ground water have been made by the Twining Laboratory of Fresno for the Modesto Irrigation District, and the Twining Laboratory has made some analyses for the City of Modesto. The rest of the analyses used in the study were made by the U. S. Geological Survey, the California Department of Health, the University of California Extension Service, and private laboratories other than Twining. The locations of 127 selected wells with complete chemical analyses are given in Figure 27. Of these analyses 67 were made at Stanford and most of the rest were made by the Twining Laboratory for the Modesto Irrigation District. The 127 analyses furnish the fundamental background for much of the discussion of chemistry of water that follows. These selected analyses are supplemented, however, by the data from several hundred other complete and partial analyses. In addition to the ground water data, a number of surface water analyses are available for certain stations on the Tuolumne and

Stanislaus Rivers.

138

Chemical Character of Precipitation, Imported Irrigation Water, Saline Water and Streams In order to understand the chemical character of ground water a knowledge Is required of the kinds of water that make up the ground water body. When the different kinds of water are differentiated then consideration can be given to possible mixtures, chemical reactions, and modifications from ion exchange. The major contributors to the ground water in the study area are imported irrigation water and to a lesser extent precipitation. Streams, except for the Tuolumne River and Dry Creek at Modesto, do not contribute to the ground water, but they do give information as to the chemical character of the ground water body which they drain. The deep saline waters which underly much of the area must be considered in connection with contamination of the shallower main fresh-water body. The results of chemical analyses are usually given in parts per million (ppm) which are a weight per weight unit. Parts per million can be converted to equivalents per million (epm) by dividing the ppm by the equivalent weight of the ion. Some of the more common methods of expressing analyses and calculations that can be made with them are given by Davis and Hall (1959* PP. 25-30). Equivalents per million are useful because one epm of an ion will exactly combine with, or replace, one epm of any other ion. Also, in water the sum of cations should 139

equal the sum of anions in epm (Wilcox, 1955, P. 3); therefore, differences between anion and cation totals help to Indicate the amount of lons not determined or the magnitude of analytical error. The epm can be converted to percentages of cations and anions to give percentage reacting values. In this report, the cations and anions are each calculated to 100$. Either epm or percentage reacting values lend themselves readily to graphical representations or to statistical studies. Preliminary evaluation of the data indicates that graphical methods are the most meaningful way of presenting the chemical analyses; however, statistical results will be given where necessary. Various methods of graphical representation are summarized well by Schoeller (1955* p. 42-51). After an investigation of the methods the trilinear graph was chosen as the most effective way to present the data. The trilinear graph was developed by Piper (1944) and is illustrated in Figure 28. The major features of this graph are a triangular cation field in the lower left, a triangular anion field in the lower right, and a diamond shaped combined field in the center. A chemical analysis is first plotted in the cation and anion fields and then projected into the combined field. In this fashion, waters of different characteristics and concentrations can be compared and classified. In the present study, the total lliO

CATIONS PERCENTAGE REACTING VALUES ANIONS

WATER- ANALYSIS DIAGRAM

FIG. 28 TRILINEAR GRAPH 141 dissolved solids for the analyses are not indicated on the trilinear diagram. The graphs can be used also to determine whether a certain water Is a mixture of two or more other waters. The graphs are not satisfactory for consideration of minor constituents such as bromide, fluoride, boron, and silica. Only a few nitrate determinations were made, and these are combined with chloride in the anion field causing anomalously high chloride values where the nitrate concentration is high. Potassium is combined with sodium on the cation field; so potassium variations can not be shown. Variations in potassium are small, however, In most of the study area. Carbonate is combined with bicarbonate in the anion field, and this causes difficulty in the few samples where carbonate content is high. These shortcomings do not cause much trouble In most analyses, and they can be treated in the text without modifying the use of the graphs.

A refinement of the trilinear graph is to plot expanded fields (Piper, 1944). This Is particularly useful when large numbers of analyses fall into a particular field. A large percentage of the analyses from the study area plot in the parts of the fields outlined with heavy lines on Figure 28. Therefore, these fields on the regular trilinear graphs of Figures 29 and 31 have been expanded in Figures 30 and 32 respectively. 142

Stanford

CATIONS PERCENTAGE REACTING VALUES ANIONS

WATER- ANALYSIS DIAGRAM

FIG. 29 GROUND WATER 143

♥ Stanford Analyses

CATIONS PERCENTAGE REACTING VALUES ANIONS

WATER -ANALYSIS DIAGRAM

FIG. 30 GROUND WATER 144

CATIONS PERCENTAGE REACTING VALUES ANIONS

WATER- ANALYSIS DIAGRAM

FIG. 31 GROUND WATER 145

" Twining and other analyses

10 "wv\

CI4N03 +F CATIONS PERCENTAGE REACTING VALUES5 ANIONS

WATER -ANALYSIS DIAGRAM

FIG. 32 GROUND WATER 146

Piper (1944) gives a binomial system for classifying waters which is used also in this report. The symbol is given in the form of a decimal as for example 55.70 which means that Ca plus Mg equals 55 percent of the cations and COg plus HCOo equals 70 percent of the anions. The first part of the number is the complement of the sodium percentage which is -widely used in considerations of irrigation waters, that is, Na plus X equals 45 percent in the example given. The first part of the decimal can be used as an index of hardness because hardness is defined on the basis of Ca plus Mg. Therefore, all hardness is carbonate hardness as long as the second part of the decimal is greater than or equal to the first part. When the second part is less than the first part then the difference is nonearbonate hardness. The binomial symbol has further use because It immediately locates an analysis in the combined field of the trilinear graph.

Precipitation

Rainfall Influences the ground water of the study area in two major ways: l) some part of the precipitation serves as recharge to the ground water and 2) the major part of the dissolved solids content of rainfall eventually reaches the ground water. The first point was discussed in the section on occurrence of ground water, and the second point will be considered here. At present not much information is available concerning the chemical composition 147

of precipitation in or near the study area. The analytical techniques used for the chemical analyses (Davis and Hall,

1959* P. 24-29) are not sufficiently accurate to detect the minor concentrations in rainfall. As a result a chemical analysis of rainfall has been synthesized from the literature. The study of atmospheric chemistry is a relatively new field, although interest has been displayed in it for a number of years in Scandinavia, Western Europe, and Australia. In recent years much research has been devoted to the field, and some of the data are now becoming available for the United States. A series of publications (Junge, 1958a, 1958b; and Junge and Gustafson, 1957) give the results of sampling of rainfall over the United States during 1955 and 1957 in the form of maps showing lines of equal concentration of such constituents as calcium, chloride, nitrate, and sulfate. Junge (1958a) and Eriksson (1952) discuss the principles of atmospheric chemistry and give ratios and ranges of ratios of anions for varying conditions such as distance from the ocean and proximity to industrial and heavily populated centers. Junge (1958a, p. 45-49, 76-78) also gives a detailed discussion of the relation of the partial pressure of C02 in the air to the amount of HCO3 in water in contact with air. The chemical analysis for rainfall was synthesized in the following manner; l) calcium, sulfate, chloride, and 148 nitrate were interpolated from the maps in Junge (1958a, 1958b) and Junge and Gustafson (1957) and rough averages were taken giving greater weight to winter values when most of the rainfall occurs, 2) potassium was taken as an average value from Eriksson (1952), 3) magnesium and sodium were obtained by the use of ratios for Cl/Na, Mg/Ca, and Mg/Cl given in Junge (1953a) and Eriksson (1952), and 4) the anions and cations were summed and bicarbonate was taken as the difference. The pH of rainfall is around

5.5-5.7 depending on the partial pressure of C02*2* and at this pH little or no carbonate is present. At the partial pressure of C02 in the air (3xlo~^ atmosphere) less than 1 ppm of HCO3 will be dissolved in pure water (Junge, 1953a, p. 76-78), The value obtained for HCO3 by difference is 4 ppm. This higher value probably can be explained by the presence of calcium carbonate dust in the air which would supply the extra HCO3. The synthetic analysis is given in Appendix D and Is shown in Figure 33. The rainfall has a dissolved solids content of about 7 ppm and a classification number of 65.50. Rainfall in the study area is about 12 Inches per year on the average, but only one Inch or less is actually available for ground water recharge (see section on occurrence of ground water). The remainder of the rainfall becomes soil-moisture replenishment and is lost by evapotranspiration. The dissolved solids content of the rainfall is not lost in this use of the water but it does — Fig. 33. Selected Analyses 1. Center of points for gas wells 2. Sea water 3. Tuolumne River at Tuolumne City 4. Tuolumne River near Hickman 5. Synthetic analysis for precipitation 6. Dry Creek near mouth 7. Stanislaus River at mouth 8. Tuolumne River below Don Pedro Dam 9. Stanislaus River below Tulloch Dam 10. Alkali Pond 149

CATIONS PERCENTAGE REACTING VALUES ANIONS

WATER -ANALYSIS DIAGRAM FIG. 33 SELECTED ANALYSES 150 become concentrated. If the assumption Is made that a steady state condition exists in the soil with as many dissolved solids leaving as entering the soil then 7 ppm dissolved solids in 12 inches of rainfall will become 84 ppm in one inch of water. Concentration is not the only important effect, as a change occurs also in the partial pressure of C02. The C02 content in soil is increased by normal plant processes and organic decay, and an increased partial pressure of C02 means an increase in dissolved and disassociated C02 which in turn leads to an increase in carbonic acid (H2CO3) and bicarbonate. If calcium carbonate is present in the soil, which is true for most of the soils in the study area (Arkley, 1959* Table 4), some CaCO3 will be dissolved. Another result of the increase in carbonic acid Is that chemical reactions will take place with minerals present In the soil, and constituents such as Ca, Mg, Na, F, B, and silica will go into solution. Assuming that CaCO3 is present in the soil, the following limits to some of the chemical changes can be sets 1) at 25°C, pH of 8.31* and a partial pressure of (this an average C02 of 0.00032 atmospheres is about figure for normal air) there will be 50 milligrams per liter (mg/l) solution of CaCO^ (Foster, 1950) or 6l ppm HCO3; 2) at average partial pressures in the soil of .02 151 to .05 atmospheres (Foster, 1950; Yaalon, 1958) there will be about 120-340 ppm HCO3; and 3) at a probable maximum partial pressure in soil of .05 to .2 atmospheres (Foster, 1950; and Yaalon, 1958) there will be about 340-430 ppm HCO3. The values in 2) and 3) are only approximations because account has not been taken of such factors as temperature, time, or pH. Also, the upper limit of .2 atmospheres seems rather extreme. Nevertheless, a reasonable conclusion is that ground water will not contain more than about 400 ppm HCO3 from action in the soil horizon; therefore, greater concentrations require other processes. One possible way in which abnormally high values of HCO3 can occur is by evaporation in so called alkali ponds in the western part of the area (Davis and Hall, 1959* P. 39-40). Chemical changes in the form of ion exchange occur in the soil. Exchange reactions take place between ions present in rain water and clays in the soil probably with the exchange of Ca in clay for Na In solution. Presumably, some Kis also lost by exchange. The relative concentration of SO^ in rainfall is about 20 percent whereas most surface and ground waters have 10 percent or less. This suggests that SO^ reduction takes place also in the soil. On Figure 33 a relative increase of about 30 percent HCO3 and 5 percent Ca plus Mg along with a loss in SO^ would modify the rainfall analysis so that It would 152

resemble the ground water analyses (classification number on the order of 70,80 at the center of the points) shown in Figures 30 and 32. This shift on the trilinear plot is in agreement with changes in the soil described above. Figure 33 shows also that a 40 percent increase in HCO3 and a 10 percent increase in Ca (with a few percent decrease in Mg) and a loss In SO4 would modify the rainfall analysis so that it resembles the surface water analyses (classification number of about 75.90) for the Tuolumne River at Don- Pedro and both stations on the Stanislaus River. A to 4-fold concentration of rain water along with chemical changes in the soil would match the dissolved solids content of the streams at Don Pedro and Tulloch Dams. A simple 12-fold concentration of dissolved solids of rain water would raise the epm to 1.68 which is close to that of many ordinary ground waters In the study area.

Irrigation Water

The surface water imported for irrigation is of excellent chemical quality and has a low dissolved solids content. The Oakdale Irrigation District Imports water from the Stanislaus River east of Knights Ferry, and the Modesto Irrigation District imports water from the Tuolumne River east of La Grange. Water is taken from both streams just before they leave the foothills of the Sierra Nevada. The Department of Water Resources maintains sampling stations near the points of diversion, and the analyses can 153 be found in State publications (Plumb, and others, 1956 and 1957; Anonymous, 1957b). Rather than make an extensive statistical study of the limited number of complete analyses in order to arrive at representative analyses for the irrigation season it seems justifiable to simply pick complete analyses for each stream from the records. The selection is based on limited sampling by Stanford (Davis and Hall, 1959* Figure 16, Table 2) augmented by knowledge of chemical content as related to stream flow. The analyses selected for the streams show zero chloride in one case and zero sulfate in the other. The rainfall analysis discussed In the prior section indicates that chloride and presumably sulfate should be present In all runoff. An Inspection of available analyses shows that both are usually present in the river waters with concentrations on the order of 1 ppm or less. The analytical results are reported to the nearest 0.1 ppm Indicating an accuracy of 0.05 ppm; therefore, less than 0.05 ppm of CI or SO4 are present when a zero value is given. The analyses selected for the irrigation water seem reasonably representative with the probable exception that minor amounts of chloride and sulfate are always present. The analyses are plotted on Figure 33 and given in Appendix D, The sample for the Stanislaus River was taken at Tulloch Dam, and has a dissolved solids content of about 41 ppm and a classification number of 81.97. The sample 154 for the Tuolumne River was taken below Don Pedro Dam and has a dissolved solids content of about 25 ppm and a classification number of 67.83. The selected analyses show a difference in chemical quality between the two rivers. An examination of th" other available analyses indicate that the Stanislaus generally has a slightly higher dissolved solids content. Both streams have about the same amount of runoff in terms of acre-foot/acre of drainage basin. The drainage basins of both streams have similar type Igneous and metamorphic rocks, but the Stanislaus has more Tertiary volcanic rocks. Weathering of the volcanic rocks may account for some of the difference in dissolved solids. In addition, the Stanislaus basin has rather extensive belts of limestone which might also account for some of the difference. The analyses are for the water as it comes from the rivers and do not include any changes that might occur while in transit to the study area. Where canals are unllned an Increase in dissolved solids might occur; however, seepage is usually outward as the canals are above the water table. The Modesto Irrigation District uses a Th# reservoir which undergoes considerable evaporation. The limited data (Davis and Hall, 1959* Figure 20, Table 2) indicate, however, that the change in chemical quality in transit probably is not large. Once inside the study area, the chemical quality of water in the irrigation canals shows 155

an increase in dissolved solids as well as other changes because of mixing with water from drainage systems and irrigation wells that empty into the canals. The effect of this becomes quite significant in the western part of the area (Davis and Hall, 1959* Figure 16). A considerable amount of imported irrigation water is not used by the crops as shown in the section on occurrence of ground water. About 1/3 of the water was left after evapotranspiration in 1957* a fairly typical year. Much of this excess water is available for ground-water recharge, although some is routed to nearby streams by drainage systems. If 4l ppm of dissolved solids were present in the imported water of the Oakdale Irrigation District, there would be about 123 ppm present in the 1/3 left for recharge. If about 25 ppm dissolved solids were present in the water imported by the Modesto Irrigation District there would be about 75 Ppm in the 1/3 left for recharge. The analyses are plotted on Figure 33 which can be compared with plots of typical ground waters on Figures 30 and 32 (average classification number of about 70.80). The Stanislaus River water needs about a 20 percent decrease in HCO3 and about a 20 percent increase in CI in the anion field, and about a 10 percent Increase in Ca and 10 percent increase in Mg in the cation field to plot in the same field as the ground waters. The Tuolumne River 156 water needs only a 5 percent decrease In HCO3 and a minor decrease in CI in the anion field, but it needs a 20 percent increase in Mg and a 20 percent decrease in Ca in the cation field to plot in the same field as the ground waters. The change of the imported Irrigation water to average ground water is harder to explain than was the change for precipitation. The decrease in HCO3 indicates the possibility of precipitation of CaCO3 in the soil. This would also cause a decrease in Ca and a relative increase in Mg, The Ca and Mg content may also be affected by ion exchange. Nevertheless, It is not clear why Ca and Mg should Increase in one water and Mg Increase at the expense of Ca in the

Streams Representative analyses for the streams where they cross the study area have been selected in essentially the same fashion as those for irrigation water. No analysis, however, is given for the San Joaquin River which does obtain ground water from the area (Figure 24). Both the Stanislaus and Tuolumne Rivers drain areas on their north and south banks in which the sediments are the same, and the ground waters are of similar chemical character. Therefore, the assumption is made that even if the north and south sides do not contribute equal volumes they at least contribute water of similar quality. The situation along the San Joaquin River is more complex because both 157 the type of sediments and the character of ground water are different on the two sides of the stream. For example, ground water on the west side of the San Joaquin Valley has a higher sulfate content than does ground water on the east side. The San Joaquin River is influenced also by upstream activities. An examination of analyses of samples from Maze Road Bridge at the western boundary of the study area shows that among other differences sulfate, fluo^rlde, and boron are all higher than in analyses from the Tuolumne and Stanislaus near their mouths. Because of these complexities it seems best not to attempt to draw conclusions concerning chemical quality of ground water of the study area from analyses of San Joaquin River water.

Stanislaus River - The Stanislaus River is sampled regularly at two stations that are of Interest In this report. The station near Tulloch Dam has been discussed- in the section on irrigation water. The analysis chosen for the mouth of the Stanislaus shows a dissolved solids content of 171 ppm and a classification number of 72.83, The dissolved solids content increases from 4l ppm at Tulloch Dam to 171 ppm at the mouth. The analyses show a little change in relative percentages of cations between Tulloch Dam and the mouth with only a slight decrease in Ca and increase in Mg and Na, The anions, however, show a relative decrease in HCO3 of about 14 percent and an 158

Increase in CI of 5 percent and SO4 of 9 percent. Part of the change may be due to the influence of sewage effluent and drainage waters emptying into the river as well as to gain from ground water.

Dry Creek - A representative analysis for Dry Creek Is difficult to pick because only a few analyses are available, and they date back to the early 1930!s (McNealy, and others, 1956). A number of partial analyses were made of water from Dry Creek during the course of the Stanford research project (Davis and Hall, 1959* Table 2). A comparison of the partial analyses indicates that the concentration and character of Dry Creek is similar from the eastern part of the area to Modesto where it empties into the Tuolumne River. Also, except during occasional periods of high runoff, the quality is fairly constant. The average hardness, alkalinity, and chloride determined at Stanford from stations near the mouth and further east closely match the same constituents from an analysis given by McNealy, and others (1956, Table C-4); so this analysis has been selected as representative of Dry Creek (Appendix D), The dissolved solids content is 172 ppm, and the classification number is 69.62. Dry Creek receives little inflow from shallow unconfined ground water (Figure 24) but may receive inflow from leakage upwards of shallow confined ground water. 159

The water-level contours indicate that the limited inflow usually comes from the north side. At Modesto, Dry Creek loses water to the Modesto pumping cone. Dry Creek receives considerable drainage water from the Oakdale Irrigation District to the north and some overflow from the Modesto Irrigation District's main canal where it crosses the creek. Therefore, Dry Creek water should resemble ground water from the area It drains particularly on the north side. A comparison of Figure 33 with Figure 30 which has many points from many ground-water analyses from wells in the Oakdale Irrigation District centered at about 75.85 shows that the cations need only a slight increase in percentage of Ca and Mg to be in agreement with the ground waters. The anions are, however, about 20 percent low in HCO3, 15 percent or more high In SO4, and a few percent high in CI, Dry Creek has a rather sluggish flow in many places and is overgrown with aquatic plants. The water probably has a high organic content as indicated by a yellowish or brownish tint that is always present. The sluggish, nearly stagnant water may have a pH on the order of 6.9 or lower as opposed to 7.6 or so In more actively flowing water. Under these circumstances the stream might lose enough C02 to lower the HCO3. However, this does not explain the sulfate content of 26 ppm which is much higher than in nearby ground waters both in percentage of anions 160 and in ppm. Another sulfate determination of 8 ppm for Dry Creek is given in McNealy, and others (1956, Table C-4),

Tuolumne River - The Department of Water Resources maintains sampling stations on the Tuolumne River near Don Pedro Dam, at Hickman Bridge near Waterford, and at Tuolumne City near the mouth. No sampling station has been established at Modesto; so the quality of the Tuolumne River at Modesto is obtained from indirect evidence. Spot samples for partial analyses were taken from the Tuolumne River and Dry Creek by S. N. Davis during 1955 and 1956. The present author maintained a periodic sampling program for analyses along the Tuolumne River and Dry Creek from June 1957 to May 1953. This program was designed to fit in where possible with the regular program of the Department of Water Resources. The analyses of the samples are given in Davis and Hall (1959* Table 2). The chloride determinations along Tuolumne River are shown graphically in Figure 34. Only enough samples are plotted to show the form of the curves. The analyses for 6/29/57 and 7/2/55 are characteristic of low flows in most years, and the one for 11/22/57 is characteristic of higher flows during the wet months. Hardness and alkalinity were also determined in the sampling program. The chloride content at Tuolumne City and its variations with discharge are given in Figure 35. 161

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The analysis at Don Pedro was discussed in the section on irrigation water. Representative analyses for Hickman and Tuolumne City were picked by comparing average Stanford values for chloride, hardness, and alkalinity during low flows with low-flow analyses made during the State program. The analysis at Hickman Bridge shows a dissolved solids content of 311 ppm and a classification number of 46.36 (Appendix D). The analysis at Tuolumne City shows a dissolved solids content of 443 ppm and a classification number of 45,30 (Appendix D). The Stanford analyses indicate that the hardness, chloride, and alkalinity at Modesto are about the same as or a little higher than those at Tuolumne City (Figure 34); so the Tuolumne City analysis is considered to be representative of the water at Modesto. The analyses for Hickman Bridge and Tuolumne City are plotted on Figure 33. It can be seen that, although the two analyses plot fairly close together, they do not plot close to the other stream analyses including the Tuolumne River at Don Pedro Dam. Furthermore, the analyses for Hickman Bridge and Tuolumne City fall outside of the expanded fields which include most of the ground water (Figures 30 and 32), streams, and rainfall. The major difference at these stations is that there has been a large increase in percentage reacting values and concentrations particularly in regard to sodium and chloride. The 164 problem of chloride In the Tuolumne River has been discussed previously (Davis and Hall, 1959* p. 48-50), and it was concluded that saline water from gas wells was the probable major source. The saline waters will be discussed below, but at this point the effect of the discharge of the gas wells on the quality of the Tuolumne River will be discussed in more detail. The relation of the gas wells to chloride in the river is shown well in Figure 34. Ten gas wells were located by the Stanford project and by the Department of Water Resources (Slater, 1957). Their estimated combined discharge is about 16 cubic feet per second (cfs) with a weighted dissolved solids content of about 8700 ppm. The summer flows in the Tuolumne are on the order of 300 to 400 cfs;sothe^s wells must have a pronounced influence on the river. Trilinear graphs are useful in the consideration of the mixing of two or more waters, and mixing can be demonstrated by rather precise analytical and graphical methods (Piper, 1944). The element of uncertainty in the choice of Tuolumne River analyses makes it inadvisable to use the precise methods; however, mixing can be demonstrated in a qualitative fashion. If a given water is a mixture of two other waters then it must lie on a straight line connecting the two waters in each of the three fields of a trilinear 165 diagram, and the percentage reacting values and concentrations of constituents of the mixture must be Intermediate between the two source waters (Piper, 1944). The stream analyses for the Tuolumne River are plotted on Figure 33 and the gas well analyses are plotted on Figure 29. Average values for the gas wells plot at about 38.05 on the combined field; at about 28 percent Ca, 10 percent Mg, and 62 percent Na plus X on the cation field; and at about 95 percent CI, 5 percent HCO3, and 0 percent SO4 on the anion field. If these values are transposed to Figure 33 then a straight line can be drawn between the average points for the saline water and the analysis of the Tuolumne River at Don Pedro, In the combined and anion fields, the analyses at Hickman Bridge and Tuolumne City plot near the line; however, in the cation field the agreement Is not quite so good. The disagreement In the cation field might be due to some chemical reactions in the water or else more likely to the method of choosing the stream analyses. Nevertheless, considering the nature of the data, the disagreement does not seem serious. The analyses on the diagrams are all in correct position to demonstrate mixing. That is, Hickman Bridge and Tuolumne City fall between the saline water and Don Pedro. This is in agreement with the reacting percentages and the dissolved solids which are also intermediate 166

between Don Pedro and the saline waters.

Saline Water

A considerable portion of the study area is underlaid by brackish to saline water. This is particularly true of the area south of Dry Creek and west of Salida and Modesto. The general depth to the saline water is shown on Figure 36. In Figure 36, nonpotable water is defined as having more than 2000 ppm total dissolved solids or 400 ppm chloride. The character of the saline water is shown well by chemical analyses from 9 gas wells along the Tuolumne River. The analyses have been plotted on Figure 29 where they have an average classifica tion number of 38.05 with about a 5 percent variation in cations and anions around the average. The dissolved solids content of the analyses ranges from 916 to 16,900 ppm and chloride ranges from 462 to 10,400 ppm. Seven of the analyses were made by the U, S, Geological Survey for the Department of Water Resources (Slater, 1957) and are given in Appendix D, The other two were made at Stanford (Davis and Hall, 1959* Table 1, wells 3-12-3582 and 3-13-32D1).

Although the total concentrations vary, the proportions of various ions in waters from the gas wells are all quite similar as shown by the close grouping of points on Figure 29. The saline waters are widespread 167

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because the wells are distributed for about 30 miles along the Tuolumne River (Figures 27 and 34), The chemical character of saline water north of the Tuolumne River (Figure 36) is not known well but Is assumed to be similar to the saline water beneath the river. The deep saline waters are of importance in the study area because they are a source of contamination both of shallower ground waters and of the Tuolumne River through the discharge of gas wells. The first point will be discussed in the section on chemical character of ground water and the second has already been discussed in the section on streams. The nature and origin of the saline water is of importance in investigating its effects on the shallower ground waters. The saline waters beneath the study area are similar to oil-field brines that are high in chloride. Oil-field brines have been discussed in detail by a number of workers (Piper, and others, 1953* Schoeller, 1955* Chebotarev, 1955). The data for oil-field brines dominated by chloride have been assembled and summarized by White (1957* p. 1662-1674). Oil-field brines may be composed of surficial, or meteoric, waters that have circulated to great depth and that have obtained their chemical character by solution at depth (Chebotarev, 1955). More likely, however, the oil-field brines are derived from sea water that was trapped during deposition of marine sediments (tohite, 1957* p. 1662-1674). Water 169

such as ancient sea water that is present in the interstices of rocks and that has been out of contact with the atmosphere for a considerable length of time can be

called connate (White, 1957* P. l66l). The term connate will be used in this sense here. The western part of the study area is underlaid by about 9500 feet of marine Cretaceous rocks and about 800 feet of marine Eocene and the lone Formation which Is probably marine in part. This thick section thins toward the east until at the easternmost gas well (Figures 27 and 34) only a relatively few hundred feet of lone and possibly a thin edge of Cretaceous are present. Connate water derived from ancient sea water trapped in the marine rocks is suggested as the most probable source of the saline water. The chemical analyses from 9 gas wells are plotted on Figure 29* and an average value Is plotted on Figure 33. Pertinent data and ratios from 10 gas wells are summarized in Table 9 along with data for sea water and oil-field brines. A comparison of the results for the saline waters with data for many oil-field brines high in chloride (Piper, and others, 1953* Figure 6, Tables 8 and 29; White, 1957* Tables 1 and 4) shows that, although the anions agree closely, the cations of the saline waters are rather high in percentage of Ca and Mg. Nevertheless, all of the ratios of Table 9 are within the 170

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ranges of values for oil-field brines. The plots on Figure 33 and the data of Table 9 make it possible to show how the saline water may have been derived from sea water. Revelle (1941) gives a good summary of the criteria for such changes. In the anion field there has been a nearly complete reduction in SO4, and an equivalent increase in HCO3 and CI. A comparison of the HCO3/CI and SOij/Cl ratios1 of Table 9 with those for sea water (White, 1957) shows an increase in HCO-, with respect to CI and a decrease in SO4 with respect to CI indicating that most of the gain has been in HCO3. Throughout this report chloride is assumed to be unaffected by exchange reactions or precipitation from solution (Foster, 1942; Piper, and others, 1953* p. 85-92; Revelle, 1941) although this may not be strictly true (Correns, 1956; Carroll, 1959* p. 765-767). Therefore, chloride is considered to furnish a good basis for computing ratios. Sulfate has most likely been reduced by SO4 reducing bacteria (Foster, 1942; Revelle, 1941). The gas wells are discharging methane gas and have an odor of hydrogen sulfide, and this is indicative of reduction. Nitrate might be expected to be reduced to ammonia in a

1■""All ratios in this section on saline water have been computed by absolute weight (ppm) to facilitate comparison with the data given by White (1957). 172

reducing environment; however, nitrate in the saline waters (about 1 ppm) is about the same as in sea water (0.7 ppm). Data for oil-field brines (White, 1957* Table 1; Piper, and others, 1953* Table 29) seem to indicate that nitrate does not follow the same trend as sulfate as it may be either reduced or concentrated when SOjj is reduced. Some nitrate reported in saline water might be from contamination by shallower ground water. Data for some of the minor constituents in the saline waters (Table 9) show that the ratios of Br/CI (2 values), B/Cl (7 values), and F/Cl (7 out of 8 values give zero F) all range around the values for sea water (white, 1957* Table l). This may Indicate that Br, B, and F have been relatively unaffected by changes in the water; however, the evidence for Br and F is based on limited data. The large number of zero values of Fis difficult to explain although it might be due either to precipitation, for example by Ca, or to anion exchange for OH on mica. In normal determinations of fluoride a zero value means less than 0.1 ppm. The general changes In the cation field can be traced on Figure 33. There has been a relative increase in Ca plus Mg with respect to Na plus X, This change Is not typical of many brines of the chloride type as the increase is usually in Na plus K. The cation triangle indicates that the major change has been a large increase 173 in Ca over Mg which is typical of most brines, A comparison of saline water with data for sea water and oil-field brines (Table 9) shows that the Mg/Ca ratio for saline water is within the range for brines. The K/Na ratios of the saline waters (Table 9) are of interest because they are similar to the ratio for sea water. Normally, the i^/Na ratio found in sea water decreases in oil-field brines particularly with increasing salinity (White, 1957* p. 1668). Presumably most of the Kis lost by cation exchange. The saline waters have either obtained additional X from some source such as potash feldspars or else sea water was trapped In sediments that were already in equilibrium with sea water with respect to Na and K. The first alternative is possible but requires somewhat of a coincidence for the K/Na ratio to be so close to sea water in all of the analyses. The second alternative Is favored because, as shown in a later section on Na and X, many different physical environments have a K/Na ratio close to that of sea water. The K/fcJa ratio might be expected to decrease as connate water moved into andesitic material of the Mehrten Formation where considerably more Na than X would be available. However, as shown In Table 10 typical ground waters including those from the Mehrten have a K/Na ratio as high or higher than saline water or sea water. 174

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The next problem to be considered is the implication of the Ca+Mg/Na+K ratios which are much higher in the saline waters of the study area than in oil-field brines of the chloride type. The increase in Ca plus Mg Indicates that the water has undergone natural hardening whereas most oil-field brines of the chloride type have undergone

natural softening (Piper, and others, 1953* P. 88), Under certain circumstances where ground water is contaminated by sea water or oil-field brines there may be natural hardening (Piper, and others, 1953* p. 87* 182), Natural hardening may have taken place when connate water from marine Cretaceous and Eocene rocks moved up into the Valley Springs and Mehrten Formations. Little is known about saline water in the Valley Springs, but it seems similar to that in the Mehrten, Most of the analyses of saline water are from the Mehrten. The post-lone rocks are of continental origin, and therefore, initially did not contain connate water derived from sea water* At present, typical ground water in the Mehrten is of the calcium bicarbonate type with rather low total dissolved solids, and probably the same was true in the past. ) The silica (Si02 content of the saline waters has a range of 57-102 ppm with an average of 74. These values are much higher than sea water but are -within the range for oil-field brines (Table 9). In fact, these values 176

are fairly close to the saturation point of silica in fresh water (Krauskopf, 1956) and are higher than those found in ordinary ground water (Davis and Hall, 1959,

P. 39). Temperature has a strong influence on the solubility of silica which ranges from 60-80 ppm at 0-s°c to 300-380 ppm at 85-95° C(Krauskopf, 1956), The saline waters have an average temperature of 27°Cwith a range of 22-36°Cindicating depths on the order of 350- 1300 feet (Davis and Hall, 1959* Figure 9). The rather high silica values are probably due to the higher temperatures and to sufficient time to dissolve considerable silica. The relative concentration of a number of anions have been unchanged in the transition from sea water to connate water to saline water, although the dissolved solids content has been decreased. The saline waters have dissolved solids content of 2.7-49 percent of that of sea water and a chloride content of 2.4-55 percent of that of sea water. This indicates that the concentration of chloride decreased but was otherwise unaffected by chemical reactions during the change. The ratios for Br/CI, B/Cl, and F/Cl for saline water are similar to those for sea water. Therefore, Br, B, and Fas well as CI retained their relative concentrations during the change. These facts make it difficult to explain a decrease in dissolved solids by precipitation of constituents 177

or by dilution with fresher waters. The problems of decrease and also concentration of dissolved solids of connate waters with respect to sea water are puzzling, but no suitable explanation has yet been given (White,

1957* P. 1667-1668),

Chemical Character of Ground Water The term "ground water" is used for the uppermost water in the subsurface, and the term "saline water" Is used for the underlying water of poor quality. In the Oakdale area the ground water extends to a depth of 800 feet or more, whereas in the rest of the area the ground water is underlaid at depths of less than 800 feet by saline water (Figure 36). A sharp boundary probably does not exist between ground water and saline water, but chemical analyses furnish sufficient information at least to define a transitional zone between them. In the preceding section the chemical character was described for water that enters the area (precipitation and imported irrigation water), that leaves the area (streams), and that underlies most of the area (saline water). The ground water of the area is basically a mixture of precipitation and imported irrigation water, and its chemical character is formed by reactions of the mixture with the physical, chemical, and biological environment in which it Is found and through which it has passed. The chemical character of the ground water 178 is modified where it comes in contact with saline water. Modification may also occur where ground water is close enough to land surface to be affected by evaporation. Dry Creek, Stanislaus River, and Tuolumne River at Don Pedro reflect the chemical character of the areas that they drain. The Tuolumne River below La Grange Is also influenced by the discharge of gas wells. The chemical analyses from 118 wells yielding ground water and 9 wells yielding saline water are plotted on Figures 29 and 32 where 29 and 31 are regular scale trilinear graphs and 30 and 32 are the expanded fields outlined on Figure 28. Sixty-seven analyses made during the Stanford project (Davis and Hall, 1959* Table l) and 7 analyses made by the U. S. Geological Survey (Slater, 1957) are plotted In Figures 29 and 30. Forty-nine analyses by the Twining Laboratory, two by the University of California Extension Service, one by the U. S. Geological Survey, and one by the California Department of Health are plotted in Figures 31 and 32. Some of the analyses on Figures 31 and 32 dating before 1955 have been published (McNealy, and others, 1956), and a selected group of the more recent analyses are given In Appendix D. The analyses are shown on 4 graphs rather than combined on 2 in order to keep the diagrams from being too crowded and to separate the Stanford analyses from the Twining analyses. The reasons for separating the Stanford and Twining analyses 179 are: l) Na and X were determined for most of the Stanford analyses (Davis and Hall, 1959* p. 27-28) whereas Na plus X was obtained by difference for the Twining analyses and 2) SO^ was determined by a rather accurate method for most of the Stanford analyses (Davis and Hall, 1959* p. 26). The most conspicuous feature of the trilinear graphs is the very large number of analyses that are In the expanded fields on Figures 30 and 32 as compared to the relatively few analyses that plot outside of the expanded fields as shown on Figures 29 and 31. An analysis is plotted on Figures 29 and 31 only If it does not plot in all three fields of the expanded graphs. A less conspicuous feature of the graphs is that the analyses not falling on the expanded fields tend to form several important groups. The patterns in which the analyses are found on the trilinear graphs suggest that the ground water of the area can be described in terms of a "normal" ground water which plots on the expanded fields and the exceptions which do not. Most of the discussion that follows is based on the Stanford analyses supplemented where necessary by data from analyses by Twining and others.

Normal Ground Water Normal ground water is a predominantly calcium bicarbonate water with relatively small amounts of chloride and sulfate. The classification numbers for the analyses tend to cluster around 70.80 (Figures 30 and 32). 180

The general character of normal ground water can be expressed in a series of ratios. The analyses are plotted by percentage reacting values which are derived from concentrations expressed as equivalents per million; therefore, the ratios in equivalents of various constituents can be obtained directly from the graphs. Sodium and potassium are combined on the graph, and K/_4a can not be obtained. Also, because F and NO3 are combined with CI, some ratios involving CI may not be precise. The important ratios that can be read from Figure 30 and their ranges along with computed values for K/Na are given in Table 10. Some of the ratios are interrelated, for example, Ca+Mg/Na+K and Na+K/Cl usually have an inverse relationship to each other. The same is generally true for HCO3/CI and SO4/CI. The Na+K/Cl ratio is called the alkali number (Loewengart, 1958). This ratio is useful in considering contamination by saline water. The analyses on the expanded fields of Figure 30 have another feature in common as they are all relatively low in dissolved solids. The range in dissolved solids expressed as equivalents per million is 1.34-6.79 (approximately 80-410 ppm) with a median of about 2.3 epm. The lower values are well within the range of concentrations in the soil of dissolved solids of precipitation and irrigation water described previously. Even the higher values can be obtained without the need of special processes 181 or extreme chemical activity. Of course some modifications occurred because of solution of minerals, ion exchange, and sulfate reduction. These will be discussed below. The pattern of the Twining and other analyses in the expanded fields of Figure 32 is very similar to the Stanford analyses. They group also around a classification number of 70.80, and the values of the various ratios are quite similar. The only large difference between the Stanford and Twining analyses is in total dissolved solids. The analyses on Figure 32 have a range of 1.99-9.36 epm (approximately 120-560 ppm) with a median of 3.8 epm. The difference in dissolved solids can be explained by observing the distribution of analyses on Figure 27. The Stanford analyses are concentrated in the eastern part of the area particularly north and east of Modesto and in the vicinity of Oakdale. In this area the water table Is deep enough to prevent evaporation, saline water is either absent or deep, and irrigation water is low in dissolved solids. The Twining analyses are concentrated in the western part of the area where the water table is shallow, saline water is closer to the surface, and irrigation water has a higher content of dissolved solids. The data show that ion ratios of normal ground water are the same throughout most of the area, but the water becomes more concentrated in the western part. 182

Exceptions to Normal Ground Water

A small number of analyses fall Into one or two of the expanded fields of the trilinear graphs but just miss falling into all three. Generally the total dissolved solids of these analyses are within the range for normal ground water. Most of these are normal ground water that has too high or too low a Mg/Ca ratio to fall into the expanded cation field or that is slightly contaminated by saline water or sodium bicarbonate water as described

Some analyses on Figures 29 and 31 are sodium bicarbonate waters having more than 50 percent Na plus X and more than 50 percent HCO3. A few of these have a dissolved solids content above the range for normal ground water. In addition, some analyses on all of the trilinear graphs have more than 50 percent HCO3 but slightly less than 50 percent Na plus K. A few of these have a rather high dissolved solids content. An extreme example is an analysis with a classification number of 55.78 (Figure 29) with about 20 epm dissolved solids. All of these analyses are from shallow wells in the western part of the area. Ground water of the sodium bicarbonate type can be shown commonly to be derived from ordinary calcium bicarbonate water by natural softening. The usual type of natural softening that is described in the literature (Foster, 1942 and 1950; Piper, and others, 1953) takes 183

place by cation exchange when water high in Ca and Mg invades an aquifer with considerable adsorbed Na (Hem, 1959* p. 221). This type of natural softening is not adequate to explain the sodium bicarbonate waters or the increase in dissolved solids in the study area. The water-bearing sediments in which the sodium bicarbonate waters are found are continental in origin and presumably have never been exceptionally high In Na. Furthermore, the ground-water recharge entering the sediments probably has always been a calcium bicarbonate water. In other words, there seems to be no source of extra Na to replace Ca and Mg in the water. If Ca and Mg are removed from solution, there will be an increase in the relative percentage of Na which is an alternative method of natural softening. A way in which this might take place is illustrated by alkali ponds in the western part of the area. The ponds occur where the shallow water table intersects depressions in the land surface. Evaporation rates are high, and the water in these ponds may become quite concentrated. During this concentration the solubilities of calcium carbonate and magnesium carbonate are exceeded probably, and calcium and magnesium are removed by precipitation (Hem, 1959* P. 71-72, 80-82). The analyses for several alkali ponds are given by Davis and Hall (1959* Table 2), and one from the western part of the study area (3-7-27G) Is 184

plotted on Figure 33. The classification number of the pond water is 01.72, and its total dissolved solids is about 34l epm. The HCO3 content of the pond water Is about 11,500 ppm, and the CO3 content is about 1750 ppm. The presence of CO3 indicates apH higher than 8.2. The high content of HCO3 and CO3 requires some process of concentration such as evaporation because water in

contact with the highest partial pressure of C02 in the soil can only have a maximum of about 400 ppm HCO3 (see section on precipitation). The position of the pond water analysis on Figure 33 shows that it could be derived from normal ground water by a relative decrease of Ca and

The sodium bicarbonate waters and those trending toward this type shown on Figures 29 to 32 lie between normal ground water and the alkali pond water on Figure 33. This indicates that these waters may be either undergoing processes similar to those that formed the pond water or else that normal ground water is mixing with alkali pond water. Where the water table is close to the surface evaporation will concentrate dissolved solids and salts may be precipitated. This can be observed in many places in the western part of the area which is characterized by a shallow water table and alkali soils (Arkley, 1959). Also, in the section on chloride evidence is given that in places shallow ground water is more concentrated than 185

someiirhat deeper water. If an alkali pond were buried, for example by wind deposits, wells drilled into or close to the old pond might yield sodium bicarbonate water (Davis and Hall, 1959* P. 39-40). An important type of exception to normal ground water occurs where ground water is contaminated by saline water. The chemical character of saline water has been discussed in a previous section, and some typical analyses centered about 38.05 are plotted on Figure 29. The simplest type of contamination happens when normal ground water mixes with saline water with no chemical reactions. This is likely to take place where saline water is close to the surface and where there is active ground water circulation due to well pumpage. A good example of simple mixing is shown by three analyses centered near 56.39 on Figure 29. These analyses are from wells in the eastern part of the Modesto pumping cone. The graphical criteria for mixing of normal ground water and saline water are pretty well fulfilled (Piper, 19^4). On Figure 31 several analyses plot rather closely to 56.39* and one of these is from the Modesto cone. In addition, on Figures 30, 31 and 32 some analyses close to 60.50 indicate contamination by saline water, and several of these are from Modesto. A similar but slightly aifferent type of contamination seems to be illustrated on Figure 31 by two analyses at 45.45 and 40,43. These may represent a 186

mixture of saline water and sodium bicarbonate water. The analyses are from shallow wells west of Modesto where sodium bicarbonate water is found and saline water is rather close to the surface. The analysis at 40.48 appears to be one of this group; however, its position on the anion field indicates that it is one of the few high sulfate waters in the area. Sulfate will be discussed below. The suggestion was made in the section on saline water that saline water was derived from connate water formed in the deeply burled marine rocks. The suggestion was made also that the high Ca+Mg/fca+K ratio of saline water can be explained by natural hardening as connate water moved up into the Mehrten and Valley Springs Forma- tions and possibly the lone Formation. Some analyses have a very high CatMg/Na+K ratio which could be accounted for by further natural hardening as the saline water moved up higher into the freshwater-bearing upper Mehrten and younger formations. An analysis plotted at 69.08 on Figure 29 seems best explained by this process. Seventeen analyses dating between 1949 and 1957 have been made for water from this well (Stanford No. 3-10-27J1* Modesto Irrigation District No. 200). The analysis plotted on Figure 29 is by Stanford, and it is from a sample taken in the spring of 1957 after the well had been idle for a year (Davis and Hall, 1959* Table l). A number of analyses 187

for the well are given in Appendix D. Well 3-10-27J1 was drilled to a depth of 565 feet in 1949 and bottomed In the Mehrten Formation, The first water from the well has a classification number of 66.07 and is quite similar in most respects to the analysis by Stanford, After the well was drilled it was plugged first at 466 feet then at 344 feet In an attempt to reduce the amount of poor-quality water. The analyses show that the hardened saline water was still present, although in less quantity. Mixing was taking place with normal ground water because as the dissolved solids decreased from a high of about 28 epm to a low of 8 or 9 epm the classification numbers trended toward 64.28. When the well was allowed to stand for extended periods of time the first water pumped was quite similar to the original water, but the quality gradually Improved with the length of pumpage. The alkali number, Na+K/Cl, follows the changes perfectly as It has a value of .37 when the water Is concentrated and Increases nearly to 1as the quality Improves. The alkali number for normal ground water ranges from 1-7 and is about .65 for saline water. Analyses at 67.21 and 58.22 on Figure 29 probably represent a mixture of hardened saline water and normal ground water. These analyses are from wells close to well 3-10-27J1 described above. 188

The types of exceptions to normal ground water shown on the trilinear graphs may be summarized as follows: l) A few analyses in which the Mg/Ca ratio is too high or too low and the analyses do not plot in the expanded cation field. 2) A few analyses In which normal ground water shows evidence of slight contamination by saline or sodium bicarbonate water. 3) An important type occurs in the western part of the area where normal ground water Is concentrated by evaporation, and calcium and magnesium are precipitated. The result is a tendency to form a sodium bicarbonate water. This process can be observed in present-day alkali ponds. Normal ground water may also be partly mixed with sodium bicarbonate water in some places. 4) At the Modesto pumping cone and elsewhere In the area, particularly toward the west, contamination occurs by a simple mixing of saline water and normal ground water. A slight modification of this is where sodium bicarbonate water mixes with saline water. 5) Saline water that has moved up into fresh water sediments may be modified by further natural hardening. Some wells east of Modesto produce this type of water. Some wells in the same vicinity produce water that is probably a mixture of normal ground water and hardened saline water. 189

Summary of Chemical Processes and Constituents and Physical Properties of Water pH - Determinations of pH were made in the field for 98 samples of ground water in the study area. The range of values is 7.0-8.0 with an average of 7.6 and a median of 7.7. The pH determinations for the Twining and other analyses are in agreement with these values but also Include a few above 8.0 and a few below 7.0. All of the ground water In the area contains appreciable HCO3. Most likely the pH values are due primarily to the buffering action of CO-J-HCO3 at partial pressures of C02 prevalent in the soil and water-bearing sediments and to hydrolysis of carbonate and bicarbonate salts (Hem, 1959* p. 44-46). The saline waters have a pH range of 7.1-7.7* based on 2 determinations by Stanford and 7 by the U. S. Geological Survey, with an average of 7.4 and a median of 7.3. The saline water probably has sufficient HCO3 to account for this pH range. The saline water in place in the subsurface may have a lower pH and adjustment may take place as it rises to the surface. Some evidence for this Is indicated by the facts that pH values taken in the field are the lowest (7.1 and 7.2) and that some change in pH can be observed in the field after saline water has been collected. The highest pH obtained in the study area is a value in excess of 8.4 for sodium bicarbonate water in an alkali pond. Probably some of the sodium bicarbonate ground 190 waters have pH- values in excess of 8.0. Temperature The temperature range of ground water is from about 18° C for shallow wells to about 24° C for deep irrigation wells. The saline waters from the gas wells have a temperature range of 22° C to 36° C. The major importance of temperature is in determining the depth or range of depths from which water might come (Davis and Hall, 1959* p. 31-34). It may also have significance in the solubility of some constituents.

Calcium and Magnesium - Calcium and magnesium have much in common because their chemistry is similar, and they usually occur together. Calcium and magnesium are widespread in the earth, and ground water may obtain them from sources such as weathering of igneous and metamorphic rock forming minerals, limestone, and dolomite (Hem, 1959* p. 70-71* 79- 80). Considerable Ca and Mg in normal ground water in the study area come from imported surface water and rainfall. Ordinarily, lesser amounts are obtained by weathering of minerals primarily in the soil. Hardness is an important property of water. In this report hardness is considered to be determined on the basis of the total content of Ca and Mg. The normal ground water of the study area is a calcium bicarbonate water with a range of hardness from about 50 to nearly 300 ppm. Saline water may have a hardness of as much as 5400 ppm. The distribution of hardness in a zone 50-150 191

feet below land surface which is mainly a zone of normal ground water is shown in Figure 37. The lower values tend to be found in the eastern part of the area and the higher in the western part. Local exceptions occur in the western part where sodium bicarbonate water may have low values of hardness. The points plotted on the trilinear graphs (Figures 29-32) show that hardness is to be expected in nearly all the waters because with the

exception of a few sodium bicarbonate waters all the analyses have more than 30 percent Ca plus Mg and most have more than 50 percent Ca plus Mg. The Mg/Ca ratio Is of interest in considering natural waters, and values for some waters are listed in Table 10. These ratios indicate that, with the exception of the loss of Mg in the change from sea water to connate

to saline water, Mg and Ca rather closely follow each other in the ground waters. The higher Mg/Ca ratios in normal ground water are found in the eastern part of the study area and the lower ones in the western part (Davis and Hall, 1959* P. 38). This variation in ratios Is somewhat difficult to explain but may be due to relative availability of Ca and Mg in the sediments (Hem, 1959* p. 82-84). For example, more andesitic material is found toward the east, and the andesitic material probably has more available Ca than do the granitic materials. i i 192 1 R.7E. , R.BE. R. 9 E. i R. 10 E. R. II E.

1 . , r— / O re \ y \

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PPM i A fIMII HARDNESS GREATER THAN 200 I %^ < " TURLOCK— * WELL SAMPLED cc I 15 WELLS IN MODESTO NOT SHOWN .

i X\J SCALE ( V 5 0 5 MILES 193

The Ca+Mg/Na+K ratio (see Table 10) is of value in tracing natural softening (decreasing ratio) or natural hardening (increasing ratio).

Sodium and Potassium - Sodium and potassium are chemically similar, but X is more readily absorbed and held by base exchange material (Hem, 1959* P. 89-91). Common sources of Na and X in natural waters are weathering of Igneous and metamorphic minerals particularly the feldspars, some clays, evaporates, and impurities In carbonates. In the study area a considerable amount of Na and X comes from rainfall and imported irrigation water. Most likely the weathering of feldspars in the soil is also an important source of Na and K. The K/Ma ratio is useful In studying natural waters, however, caution should be used as X concentrations are usually small and may not vary greatly. Some values for waters in the study area are given in Table 10. The higher values for streams occur upstream at Don Pedro and Tulloch Dams, and the lower values occur downstream where the streams are gaining ground water. The lowest values are for the Tuolumne River below the gas wells. A frequency analysis of K/Ha, for normal ground water shows two modes, one at about 0.50 and one at about 0.14. The higher values are found in the eastern part of the area where only rainfall and excellent quality irrigation water are contributing to the ground water. These waters 194

also have lower dissolved solids. According to White (1957* P. 1668) the K/Na ratio is usually rather high in streams and dilute ground waters. The lower values from the frequency analysis are found throughout the rest of the area. The lower values for normal ground water are similar to those for saline water and even sea water. Rock type does not seem to have influenced the K/Na ratio, although less X would be available in andesitic material of the Mehrten Formation than in granitic material of the post-Mehrten formations. However, K/Na ratios in water from the Mehrten are sometimes higher and never lower than those from water in post-Mehrten sediments. Apparently the K/Na ratio approaches a similar value in different physical environments. The Na+K/Cl ratio or alkali number is very useful in considering contamination of normal ground waters. Some values for this ratio are listed in Table 10. These results show that contamination of normal ground water will lead to values of less than 1. Normal ground waters have a Na+K/Cl range of 1-7* but on Figure 29 contaminated water from the Modesto pumping cone centered at 56.39 has a Na+K/Cl ratio of .89. The ratio can also be used for tracing natural hardening (see section on exceptions to normal ground water), but it does not seem useful for tracing natural softening. 195

Bicarbonate and Carbonate - Bicarbonate has been discussed in several previous sections particularly the ones on precipitation and pH. Carbonate is not present in significant amounts unless the pH is 8.2 or higher; so carbonate Is important only in the few sodium bicarbonate waters in the western part of the area. The major sources of bicarbonate are the partial pressure of C02 in the soil and soluble carbonates present in the soil. Presumably, sufficient calcium carbonate is present in the soil and sediments so that the amount of bicarbonate is really a function of the partial pressure of C02. A maximum of about 400 ppm bicarbonate may be present in ground water based on a partial pressure range of .02-.2 atmospheres in the soil. Partial pressures of this magnitude are attained in the soil by organic processes including SO4 reduction which release C02. Only three analyses of ground water have more than 400 ppm bicarbonate, and these are all sodium bicarbonate or sodium calcium bicarbonate waters from shallow wells in the western part of the area. The alkali pond water has a very high content of HCO3 and CO3 due to concentration by evaporation. The HCO3/CI ratio is helpful in tracing contamination by saline waters. It does not seem useful in investigating sodium bicarbonate waters. Some values of the ratio are given in Table 10; however, very high values from the 196

Tuolumne River at Don Pedro Dam and the Stanislaus at Tulloch Dam are not Included because of doubts about the CI concentrations. Lowest values for the streams are for the Tuolumne below the gas wells, otherwise all stream ratios are greater than 4.9. Evidently a HCO3/CI ratio of less than 2 indicates contamination by saline waver.____MM*

Sulfate - Sulfate in the study area is generally in low concentrations. The distribution of sulfate in the 50-150 feet zone 13 shown on Figure 37* and only in the extreme western part are sulfate concentrations more than a few ppm. The trilinear graphs (Figures 29-32) show that practically all analyses have less than 10 percent SO4, a few have between 10-20 percent SO4, and only one analysis from the western part of the area has more than 20 percent SO4. The major portion of the sediments of the area are from granitic rocks, and these are not a source of SO4 (Hera, 1959* p. 100). Furthermore, the sediments are not known to contain gypsum or other sulfate minerals. The So^ contours on Figure 37 indicate the existence of a source of sulfate in the extreme western part. The area west of the San Joaquin River is known to be characterized by waters with a considerable SO4 content (Davis, and others, 1957* p. 355). With a few exceptions in the western part of the area most SO4 in the ground water comes from precipitation and imported irrigation water. 197

Sulfate was determined by a rather accurate method for most of the Stanford analyses (Davis and Hall, 1959* p. 26). The values for 40 analyses of normal ground water range from 0-27 ppm with an average of 6 ppm. Of the 40 analyses 17 contain less than 0.5 ppm. The range for 33 Twining and other analyses of normal ground water is 2-30 ppm with an average of 11 ppm. These slightly higher values may be due in part to a difference in analytical techniques. Also, the Twining analyses are mainly from the western half of the area where SO4 is a little higher. In the western part of the area a few analyses have sulfate concentrations somewhat in excess of 30 ppm with two values at 77 and 165 ppm. The alkali pond water has about 137 ppm of SO4. The saline waters have a low sulfate content indicating sulfate reduction. Precipitation and imported irrigation water contain about 0.5 to 1.5 ppm of SO^ and have about the same content of CI. If no chemical changes occur SO^ should be present in normal ground water to the same extent as CI. This is true only for a few analyses, mainly in the Oakdale area. The normal ground water of the Oakdale area is a mixture of rainwater and excellent quality irrigation water with a low dissolved solids content. Few changes have taken place in the water. In the rest of the area, SO^ content is usually lower than CI. The loss of SO4 can most readily be accounted for by reduction due to bacterial action 198 primarily in the soil (Hem, 1959* P. 223-224). The SOjfj/Cl ratio is a good index for sulfate reduction in normal ground water and has some use in tracing contamination by saline water. Values of this ratio for some of the waters are listed in Table 10. The high stream value Is for Dry Creek. The value at the mouth of the Stanislaus is 1.1 whereas those from the Tuolumne below the gas wells are about .035. The ratios Indicate that values less than 1 show some sulfate reduction and values less than .01 probably show pronounced sulfate

Chloride - Chloride is nearly always present in natural waters. As discussed previously chloride seems relatively unaffected by precipitation or lon exchange and probably Is not removed from solution by ordinary chemical processes. Therefore, the investigation of chloride has proved extremely useful in studying the chemical character of water in the study area. In fact, chloride content is a direct index to the quality of water. The sources and distribution of chloride in the study area have already been discussed (Davis and Hall, 1959* p. 40-52). The only sources of significant amounts of chloride in ground water are the atmosphere and contamination by saline water. Chloride content may also be increased by concentration by evaporation. 199

The distribution of chloride is shown in Plate 4 and Figures 38-41 for depths from near land surface down to about 1000 feet. Some information is also given on Figure 36. In general, normal ground waters have less than 25 ppm In the eastern half of the area and less than 50 ppm in the western half. Therefore, chloride content in excess of 50 ppm indicates either contamination by saline water or pronounced concentration by evaporation. Chloride in excess of 100 ppm on Plate 4 and Figures 39 and 40 probably indicates contamination by saline water. Values In excess of 1000 ppm on Figures 40 and 4l indicate the presence of saline water. Chloride values in excess of 50 ppm on Figure 38 indicate concentration by evaporation A comparison of Figure 38 and Plate 4 gives evidence for some shallow waters in the western part of the area being more concentrated than underlying waters. The isochlors for the 50-150 feet zone around Modesto are shown on Figure 39. Some of the values between 50-100 ppm along the Tuolumne River at Modesto probably are due to ground-water recharge from high chloride water of the Tuolumne River (see section on streams). The shape of the isochlors and their resemblance to the water level contours of Figure 24 show also that chloride is closely related to ground water pumpage. As large amounts of normal ground water have been removed from the cone of depression the decreased head probably 200

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<2 o piVER OA DALE T. 2 S. 4 o > ' o I f\-rA /-"N. J o O o °o o o A O o ~ \.C o v -ioo0 LESS - Ifty LTHAN 50Ppi T3S AN 50 Vt* PPM /\. MODESTO *^ / I too^ FIGURE 41 tV>A CHLORIDE CONTENT \ OF WATER FROM WELLS / IN THE MEHRTEN FORMATION / 0-o / o WELL SAMPLED TURLOCK / a ELECTRIC LOG D SCALE 'h/7 / 5 0 5 MILES o9^ 205 has allowed saline water to move upward, and deeper wells now pump normal ground water contaminated by saline water.

Silica - Silica is a common constituent in most natural waters. The maximum solubility of silica is about 100 ppm within the normal ranges of pH and temperature found in the area (Krauskopf, 1956), although few of the waters attain saturation with respect to silica. Determina- tions of silica were made for 10 analyses during the Stanford project, and the range was 34-50 ppm with an average of 36 ppm. Forty-nine silica determinations were made by Twining and others with a range of 22-74 ppm and an average of 47 ppm. The saline water from gas wells has a range of silica of 57-102 ppm. The silica values obtained in the study area are somewhat higher than those found in most of the United States (Davis and Hall, 1959* p. 39). The reasons for this higher silica content are not known. The area does have granitic sediments with a large content of quartz and silicate minerals. Also, ground water temperatures are relatively high (18-24°Cfor ground water, and up to 36°Cfor saline water). Silica is taken into solution during weathering of silica-bearing minerals in the soil and underlying materials. This weathering probably releases trace amounts of boron and fluoride and larger amounts of Ca, Mg, and Na, and possibly minor amounts of K. The presence of a silica hardpan in the San Joaquin soil which is found over much of the Riverbank Formation 206

Indicates that silica is being transported and redeposited in the soil, At present little is known about the origin of silica hardpans (Soil Survey Staff, 1951* p. 241-242).

Minor Constituents - Boron was not determined in the Stanford analyses, but a number of determinations are available for Twining and other analyses. Data for 50 Twining analyses give a range for boron of less than 0.01 to 0.2 ppm with an average of 0.07 ppm. Most of the determinations are for normal ground water and, with one exception the waters have less than about 600 ppm total dissolved solids. The B/Cl ratio is only useful for tracing CI because boron changes so little in the analyses. The boron content and B/Cl ratio for saline waters have been discussed in the section on saline water. The saline waters have a boron range of .33 to 5.4 ppm with an average of 1.6 ppm and a median of 0.74 ppm. Hardened saline water has about 0.08 to 0.2 ppm boron. These results show that saline water has more boron than does normal ground water. The sources of boron in the ground waters of the study area may be minerals in the sediments such as tourmaline (Hem, 1959* p. 120-121). Imported irrigation water has trace amounts of boron (Plumb, and others, 1956 and 1957)* and rainfall may have some boron. The low concentration of boron in the ground water is probably due to the limited 207 availability of boron in sediments of the area. The saline waters most likely obtained their boron from sea water trapped in marine sediments. The concentration of nitrate was determined in 28 Stanford analyses. The range is 5-68 ppm with an average of 14 ppm and a median of 10 ppm, and three analyses have less than 5 ppm. The saline waters have from .2-2.9 PPn* nitrate. The sediments of the area are not a source of nitrate, although some may form by bacterial action in the soil. The larger part of the nitrate in the ground water probably comes from liquid ammonia fertilizer (Hem,

1959* P. 116-117). The nitrate in saline water may represent either contribution from shallow ground water or may have come from ancient sea water. Iron Is commonly an Important constituent of water. It can readily be detected in minor amounts either visually or by taste. In the study area, however, Iron is not present In ground water except In trace amounts with a few exceptions close to the San Joaquin River. Only a few determinations of Fe were made during the Stanford project, and one value of 0.1 ppm was obtained from a well close to the San Joaquin River in the study area. The chemistry of iron in water shows that within the range of pH (7.0-8.0) and bicarbonate (80-400 ppm) found in most water of the area iron will be present in concentrations of less than 1ppm (Schoeller, 1956, Figure 31). The Twining 208 analyses report Iron and aluminum together, and values may be as high as 4or 5 ppm. Inasmuch as Al Is probably present to the extent of only 1 ppm or less (Hem, 1959, p. 68-70), these values seem high. Determinations of fluoride were made in 30 analyses during the Stanford project in the study area. Of the determinations 24 have less than .1 ppm and the remaining 6 have a range of .1-.2 ppm. Fluoride determinations were made for 7 gas wells by the U. S. Geological Survey and 6 have less than 0.1 ppm and one has .4 ppm. The sources of fluoride in ground water of the study area may be such minerals In the sediments as mica and hornblende. Slight amounts of fluoride are present also in imported surface water (Plumb, and others, 1956 and 1957). Trace amounts may be present In precipitation. The very small amounts of F are probably due both to relative insolubility of fluoride in the presence of calcium (Hem, 1959* p. 112-113) and to the relative unavailability of fluoride in sediments. The low fluoride in saline water indicates that fluoride has been lost during the change from sea water to connate water. This loss might be explained by precipitation caused by the increase of Ca during natural hardening or else by exchange for OH in clay minerals and micas. Bromide determinations were made for saline water from two gas wells, and in the section on saline water the suggestion was made that the bromide came from ancient sea 209 water. Nine bromide determinations during the Stanford project range from 0,03 ppm for shallow well water to

215 PP*n for alkali pond water and Indicate that some bromide is present in most waters of the area (Davis and Hall, 1959* p. 37). REFI-RENCES CITED

Allen, V. T,, 1929* The lone formation of California: Univ. of Calif., Bull, of Dept. Geol. Sci., v. 18, p. 347-448. Allen, V. T., 1941, Eocene anauxite clays and sands in the Coast Range of California: Geol. Soc. Amer. Bull., v. 52, p. 271-294. Anonymous, 1943* Tabulated data on wells drilled outside of the principal oil and gas fields in Geologic formations and economic development 5Y the oil and gas fields of California, cd. by 0. P. Jenkins: Calif. Div. Mines, Bull. 118, p. 636-664. Anonymous, 1957a, Study of water needs for Stanislaus County: Stanislaus County Planning Commission, Martin L. Schueller, Director of Planning. Anonymous, 1957b, Water quality investigations, report to State Water Polution Control Board on periodic stream sampling program: Calif. Depart. Water Resources, mimeo. report issued monthly. Arkley, R. J., 1954, Soils of eastern Merced County, California: Univ. of Calif., Agri. Exp. Sta., Soil Survey No. 11. Arkley, R. J., 1959* Soils of eastern Stanislaus County, California: Univ. of Calif., Agri. Sxp. Sta., Soil Survey No. 13. Axelrod, D. 1., 1944a, The Pliocene sequence in Central California in Pliocene floras of California, cd. by R. W. Chancy: Carnegie Inst. Wash., Publ. 553* p. 217-220. Axelrod, D. 1.. 1944b, Oakdale flora in Pliocene floras of California, cd., by R. W. CEaney: Carnegie Inst. Wash., Publ. 553* p. 147-167. Axelrod, D. 1., 1956, Mid-Pliocene floras from west- central Nevada: Univ. of Calif., Publ. in Geol. Sci., v. 33* P. 1-322.

210 211

Axelrod, D. 1., 1957* late Tertiary floras and the Sierra Nevada uplift: Geol. Soc. Amer. Bull., v. 68, p. 19-46. Barbat, W, F., and J. Galloway, 1934, San Joaquin clay, California: Amer. Assoc. Petrol. Geol. Bull., v. 18, p. 476-499. Becker, G. F., 1885, Notes on the stratigraphy of California: U. S. Geol. Surv., Bull. 19. Bryan, Kirk, 1923* Geology and ground-water resources of Sacramento Valley, California: U. S. Geol. Survey, Water-Supply Paper 495. Carroll, Dorothy, 1959* lon exchange in clay and other minerals: Geol. Soc. Amer. Bull., v. 70, p. 749- 780. Carter, G. F., 1956, On soil color and time: Southwestern Jour, of Anthropology, v. 12, p. 295-324. Carter, G. F., 1957* Pleistocene man at San Diego: The Johns Hopkins Press, Baltimore. Chebotarev, I. 1., 1955* Metamorphism of natural waters in the crust of weathering: Geochim. et Cosmochim. Acta, v. 8, p. 22-48, 137-170, 198-212. Church, H. V., Jr., and others, 1958, Correlation sections longitudinally north-south thru central San Joaquin Valley from Rio Vista thru Riverdale (10 north) and Riverdale thru Tejon Ranch area: Amer. Assoc. Petrol. Geol., Pacific Section. Correns, C. W., 1956, The geochemistry of the halogens in Physics and Chemistry of the Earth, cd. by H7 H, Ahrens, K. Rankama, and S. K. Runcorn: v. 1, p. 181-233, Pergamon Press, London. Curtis, G. H., 1954, Mode of origin of pyroclastlc debris in the Mehrten formation of the Sierra Nevada: Univ. of Calif., Publ. in Geol. Sci,, v. 29, p. 453-502. Curtis, G. H., and others, 1958, Age determinations of some granitic rocks in California by the potassium- argon method: Calif. Div. Mines, Spec. Rep. 54. 212

Davis, G. H., and others, 1957* Ground-water conditions and storage capacity in the San Joaquin Valley, California: U. S. Geol. Surv., Sacramento, open file report (Note: report was published in 1959 as U. S. Geol. Surv., Water-Supply Paper 1469). Davis, S. N., and F. R. Hall, 1959* Water quality of eastern Stanislaus and northern Merced Counties, California: Stanford Univ. Publ. Geol. Sci., v. 6,

Durham, J. W., 1954, Marine Cenozoic of Southern California Geology of Southern California, cd. by R. H. Johns: Calif. Div. Mines, Bull. 170, Chap. 111, §inP. 23-31. Durrell, C., 1944, Andesite breccia dikes near Blairsden, California: Geol. Soc. Amer. Bull., v. 55* p. 255-

Sric, J. H., and others, 1955* Geology and mineral deposits of the Angels Camp and Sonora quadrangles, Calaveras and Tuolumne Counties, California: Calif. Div, Mines, Spec. Rept. 4l Eriksson, Erik, 1952, Composition of atmospheric precipitation: Tellus, v. 4, p. 215-232, 280-303. Foster, M. D., 1942, Base-exchange and sulfate reduction salty ground waters along Atlantic and Gulf Coasts: Amer. Assoc. Petrol. Geol. Bull,, v. 26, ■inp. 838-851. Foster, M. D., 1950, The origin of high sodium bicarbonate waters in the Atlantic and Gulf Coastal Plains: Geochim. et Cosmochim. Acta, v. 1, p. 33-48.— Frink, J. W., and H, A. Kues, 1954, Corcoran clay a Pleistocene lacustrine deposit in the San Joaquin Valley, California: Amer. Assoc. Petrol. Geol. Bull., v. 38, p. 2353-2371. Goddard, E. N., and others, 1948, Rock color chart: National Research Council. Hera, J. D., 1959, Study and interpretation of the chemical characteristics of natural water: U, S. Geol. Surv., Water-Supply Paper 1473. Hoots, H. W., and others, 1954, Geological summary of the San Joaquin Valley, California in Geology of Southern Calif., cd. by R. H. JoEns: Calif. Div. Mines Bull. 170, Chap. 11, p. 113-129. 213

Howard, A. D., 1951, Development of the landscape of the San Francisco Bay counties: Calif. Div. Mines Bull. 154, p. 95-106. Huey, A. 8., 1948, Geology of the Tesla quadrangle, California: Calif. Div. Mines, Bull. 140. Jennings, C. W., and E. W. Hart, 1956, Exploratory wells drilled outside oil and gas fields in California to r December 31* 1953* Calif. Div. Mines, Spec. Rept. 45. Jenny, Hans, 1941, Factors of soil formation: p. 1-20, McGraw-Hill, New York. Junge, C. E., 1958a, Atmospheric chemistry in Advances in Geophysics, cd. by H. E. Landsberg and J. Van Mieghem: v. 4, Academic Press, New York. Junge, C. £ 1958b, The distribution of ammonia and nitrate in rainwater over the United States: Trans. Amer. Geophys. Union, v. 39* p. 241-248. Junge, C, E., and P. E. Gustafson, 1957* On the distribution of sea salt over the United States and Its removal by precipitation: Tellus, v. 9, p. 164-173. Krauskopf, K. 8,, 1956, Dissolution and precipitation of silica at low temperatures: Geochim. et Cosmochlm. Acta, v. 10, p. 1-26. Lerbekmo, J. F., 1957, Authigenic montmorlllonoid cement in andesitic sandstones of central California: Jour. Sed. Petrol., v. 27* p. 298-305. Lindgren, W., 1894, Description of the gold belt; description of the Sacramento sheet (California): U. S. Geol. Surv., Geol. Atlas, Folio 5. Lindgren, W., 1911* The Tertiary gravels of the Sierra Nevada of California: U. S. Geol. Surv., Prof. Paper 73. Loewengart, S., 1958, Geochemistry of waters in northern and central Israel and the origin of their salts: Bull. Research Council of Israel, Section G, Geo-Sciences, v. 7G, p. 176-205. Louderback, G. D., 1924, Period of scarp production in the Great Basin: Univ. of Calif., Bull, of Depart. Geol. Sci., v. 15* P. 1-44. 214

McNealy, D. D., and others, 1956, Interim report on Lower San Joaquin Valley water quality investigation: Calif. Dept. Water Resources. Mendenhall, W. C, 1908, Preliminary report on the ground water of San Joaquin Valley, California: U. S, Geol. Surv., Water-Supply Paper 222. Mendenhall, W. C., and others, 1916, Ground water in San Joaquin Valley, California: U. S. Geol. Surv., Water-Supply Paper 398. Merriam, C. W., and F. E. Turner, 1937* The Capay middle Eocene of Northern California: Univ. of Calif., Bull, of Depart. Geol. Sci., v. 24, p. 91-114. Oakeshott, G. 8., and others, 1952, Exploratory wells drilled outside of oil and gas fields in California to December 31, 1950: Calif. Div, Mines, Spec. !t»; Rept. 23. Piper, A. M., 1944, A graphic procedure In the geochemical interpretation of water analyses: Trans. Amer. Geophys. Union, v. 25, p. 91^-923 (revised version mimeo. in 1953 by U. S. Geol. Surv., Ground Water Branch as Ground Water Notes, Geochemistry, No. 12) Piper, A. M., and others, 1939* Geology and ground-water hydrology of the Mokelumne area, California: U. S. Geol. Surv., Water-Supply Paper 780. Piper, A. M., and others, 1953* Native and contaminated ground waters in the Long Beach-Santa Ana area, California: U. S. Geol. Surv., Water-Supply Paper

Plumb, C. 1,, and others, 1956, Quality of surface waters in California, 1951-54: Calif. Depart. Water Resources, Water Quality Investigations Report 15. Plumb, C. K.j and others, 1957* Quality of surface waters in California, 1955-56: Calif. Depart. Water Resources, Bull. No. 65. Ransome, F. L., 1898, Some lava flows of the western slope of the Sierra Nevada, California: U. S. Geol. Surv., Bull. 89. Reiche, Parry, 1950, Geology of part of the Delta-Mendota Canal near Tracy, California: Calif. Div. Mines, Spec. Rept. 2. 215

Revelle, R,, 1941, Criteria for the recognition of sea water in ground waters: Trans. Amer. Geophys. Union, v. 21, p. 593-597. Savage, D. E., 1955* Nonmarine lower Pliocene sediments in California: Univ. of Calif., Publ. in Geol. Sci., v. 31* P. 1-26. Schoeller, H., 1955, Geochimie dcs eaus souterraines application aux eaux gisements de petrole: extrait de la Revue de L!lnstitut Francais dv Petrole et Annales dcs Combustibles liquides, March, April, June, July, and August, 1955. Shaw, C. F,, 1928, Profile development and the relationship of soils in California: Proc, and Papers of Ist Intematl. Cong, of Soil Sci., v. TV, p. 291-317. Slater, W. R., 1957* Letter to J. Allen Hall, Supt. Banta-Carbana Irrigation District concerning chloride from gas wells along Tuolumne River: Calif. Dept. Water Resources, unpublished. Soil Survey Staff, 1951* Soil survey manual: U. S. Dept. Agri., Handbook No. 18. Steams, H. T., and others, 1930, Geolo^r and water resources of the Mokelumne area, California: U. S. Geol. Surv., Water-Supply Paper 619. Stirton, R. A., 1939, Cenozolc mammal remains from the San Francisco Bay Region: Univ. of Calif., Bull, of Dept. Geol. Sci., v. 24, p. 339-410. Stirton, R. A., 1951* Principles in correlation and their application to later Cenozic holarctic continental mammalian faunas: Report of 18th session Intematl. Geol. Cong., London, 1948, part XI, p. 74-84. Stirton, R, A., and H. F. Goeriz, 1942, Fossil vertebrates from the superjacent deposits near Knights Ferry, California: Univ, of Calif., Publ. in Geol. Sci., v. 26, p. 447-472. Storie, R, E., and Harradine, F., 1958, Soils of California: Soil Science, v. 85, p. 207-227. Taliaferro, N. L., 1943, Manganese deposits of the Sierra Nevada, their genesis and metamorphism In Manganese in California, cd. by 0. P. Jenkins: Calif. Div. Mines, Bull. 125* p. 280-286. 216

Taliaferro, N. L.,and A. J. Solari, 1949, Geologic map of the Copperopolis quadrangle, California: Calif, Div. Mines, Bull. 145. Thorp, J., and others, 1951* Some post-Pliocene buried soils of central United States: Jour. Soil Sci., v. 2, p. 1-19. Turner, H. W,, 1894, The rocks of the Sierra Nevada: U. S. Geol. Surv., l4th Ann. Rept., pt. 11, p. 435-495. Turner, H. W., 1896, Further contributions to the geology of the Sierra Nevada: U. S. Geol. Surv., 17th Ann. Rept. pt. I,p. 521-762. Turner, H. W., and F. L. Ransome, 1897, Sonora Folio: U. S. Geol. Surv., Geol. Atlas of U. S., Folio 41. Vanderhoof, V. L., 1933* A skull of Pllohlppus tantalus from the later Tertiary of theASierran foothills of California: Univ. of Calif., Bull, of Dept, Geol. Sci., v, 23, p. 183-193. Van Winkle, W., and F. M, Eaton, 1910, The quality of the surface waters of California: U. S. Geol. Surv., Water-Supply Paper 237. Watts, W. L., 1890a, Stanislaus County,* 10th Ann. Rept. of the State Mineralogist, Calif. State Mining Bureau, p. 680-690. Watts, W, L., 1890b, Merced County: 10th Ann. Rept. of the State Mineralogist, Calif. State Mining Bureau, p. 323-331. Weather Bureau, 1958a, Climatologic data, California, Annual Summary, 1957? U. S. Dept. Coram., Weather Bureau, v. LXI, No. 13. — Weather Bureau, 1958b, Summary of the United States supplement for 1931-1952 (California): U. S. Dept. Comm., Weather Bureau, Climatography of the U. S., No. 11-4. Wells, J. V. 8., and others, 1959* Surface water supply of the United States, 1956j Part 11, Pacific Slope Basins In California: U. S. Geol. Surv., Water-Supply Paper 1445, p. 315-320. White, D, E., 1957* Magmatic, connate, and metamorphic waters: Geol. Soc. Amer. Bull., v. 68, p. 1659- 1682. 217

Wilcox, L. V., 1955* Classification and use of irrigation waters: U, S. Dept. Agri., Cir. No, 969. Woodring, W, P., and others, 1940, Geology of the Kettleman Hills oil field, California: U. S. Geol. Surv., Prof. Paper 195. Yaalon, D. H., 1958, Studies on the effect of saline irrigation water on calcareous soils j 11, The behavior of calcium carbonate: Bull. Research Council of Israel, Section G, Geo-Sciences, v. 7G, p. 115-122. APPENDIX A

MEASURED SECTIONS 219

Section 2-9-26 X Riverbank Formation

NW £ of NE £ of section 26, Township 2 S, Range 9 E, Riverbank quadrangle. Exposures in south bluff of Stanislaus River just north of intersection of Jackson and Topeka

Streets in Riverbank, (Note: sand sized material consists mainly of quartz, feldspars, lesser amounts of biotlte, and minor amounts of heavy minerals such as magnetite and hornblende, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) This is the type section of the formation. Top of bank (about 4 feet below road level) Thickness (Elevation is 135 feet) feet Dune Sand * Sand, fine-grained, yellowish brown (damp). Top of is somewhat indurated with some medium-grained sand. See Appendix C, Table 4, and Fig. 10. 9. Thickness of Dune Sand Riverbank Formation 2, Silt, micaceous, yellowish gray to yellowish brown (damp). Rather hard. 3.5 3, Sand, medium- to coarse-grained, pebbly, yellowish brown (damp). Rather hard. Bottom part of unit 2 and this unit make up the hard layer that forms the hard pan of the Riverbank soils. Base of this hard layer can be traced by small caves and openings. 2.5 4. Sand, medium- to coarse-grained, pebbly, yellowish brown (damp). 1.0 5. Sand, fine- to medium-trained, yellowish brown (damp). 1.0 220 I (Riverbank, continued) 6. Sand, medium- to coarse-grained, scattered pebbles, reddish brown (damp). 7. Sand, fine- to medium-grained with coarser

8. Silt or clay, micaceous, brown to gray (damp), 9. Poor exposure. Seems to be sand, fine-grained, scattered pebbles, yellowish brown to yellowish

10. Sand, medium- to coarse-grained, scattered pebbles, cross bedded, yellowish brown to yellowish gray with iron stains. Has a few clayey cemented layers. See Appendix C, Table 4, and Fig. 10. 11. Poor exposure. Sand, fine- to medium-grained above and medium-grained below, yellowish brown (damp). See Appendix C, Table 4, and Fig. 10. 12. Sand, medium- to coarse-grained, scattered pebbles, clay matrix, yellowish brown. Seems to be in place but might be slumped. 13. Concealed. Thickness of Riverbank Bottom (top of lower terrace) Thickness of measured section 221

Section 2-10-1 H and 2-11-6 E & M Turlock Lake Formation and Mehrten Formation

SE £ of NE Iof section 1, Township 2 S, Range 10 E, and SW iof NW £ and NW f of SW £ of section 6, Township 2 S, Range 11 E, Oakdale quadrangle. Series of road cuts on Rodden Road down north bluff of Stanislaus River. It seems likely that the post-Mehrten channel fills intermediate in elevation between the top and bottom of the section belong to the Riverbank formation. (Note: all colors are for dry material unless otherwise stated.) Top of road cut (above highway) (Elevation is 240 feet) Thickness feet Turlock Lake Formation 1. Soil, sandy, brown. 1.0 2. Sand, quartz, feldspar, biotite, etc, medium- grained, light brown, cross bedded, 8,0 3. Sand, quartz, feldspar, biotite, fine-grained, light brown, cross bedded. 2.0 Total Turlock Lake 11.0 Mehrten Formation 4. Clay, reddish. 0.3 5, Clay, red to dark pink, blochy. 2.5 6. Sand, andesitic, fine-grained to silty, brownish gray to pink, hard. (Note: units 1-5 and the upper 2.7 feet of unit 6 were measured In a large road cut; the lower 3.5 feet of unit 6 and the rest of the section were measured along the road.) 222

(2-10-1 H and 2-11-6 E & M, continued) Thickness feet 7. Sand, quartz, feldspar, biotite, medium-grained, dirty brown, hard. Post-Mehrten channel fill. 16.0 8, Sand, quartz, feldspar, biotite, fine-grained, dirty brown, hard, Post-Mehrten channel fill. 2.0 9. Sand, andesitic, fine-grained, gray to black, hard. 2.5 10. Silt to clay, gray to pink, hard. 3.5 11. Sand, andesitic, fine-grained, blue-black, hard. 1.0 12. Sand, andesitic, fine-grained to silty, gray to black with layers of pink to gray clay. 7.5 13. Silt, andesitic, gray with some pink clay. 1.0 14. Sand, andesitic, fine- to medium-grained, nearly black, cross bedded. Sedimentary structures seem disturbed. 1.0 15. Sand, andesitic, fine- to medium-grained, interbedded in brownish and grayish layers. 3.0 16. Clay, pink. 1.0 17. Silt, quartz, feldspar, biotite, gray with brown streaks. Post-Mehrten channel fill. 3.0 18. Sand, quartz, feldspar, biotite, fine-grained, light brown. Post-Mehrten channel fill. 4,0 19. Clay, pink. 0.5 20. Sand, quartz, feldspar, biotite, fine-grained, pebbly, brown with some pink clay. Post-Mehrten channel fill. 3.0 21. Sand, quartz, feldspar, biotite, fine- to medium- grained, light brown alternating In places with a silt, quartz, feldspar, biotite, gray. Post-Mehrten channel fill. 3.5 ■C»* fc»

(2-10-1 H and 2-11-6 E & M, continued)

22. Silt, quartz, feldspar, biotite, ggrayish to brownish (pinkish when wet) with layers of sand, fine-grained, gray with a similar mineral composition. Post-Mehrten channel fill. 23. Silt to clay, andesitic, gray to brown, grading up into sand, andesitic, fine-grained. 24. Silt, quartz, feldspar, biotite, gray to brown (gray when damp). Post-Mehrten channel fill. 25. Silt, gray, hard. 26. Concealed. 27. Sand, andesitic, medium-grained, brown to gray. 28. Clay, pink, hard. 29. Sand, andesitic, fine-grained, gray, ashy. 30. Clay, pink, ashy, hard. 31. Concealed. 32. Soil, cobbly and pebbly. Road intersects hillside soil profile. 33. Conglomerate with matrix of sand, andesitic. Similar to unit 37. 34, Sand, andesitic, medium-grained, gray to brown

with layers of coarser grains and pebbles cross bedded. Lenticular unit and wedges out to the east. 35. Conglomerate with matrix of sand, andesitic. Similar to unit 37. 36. Concealed. 224

(2-10-1 H and 2-11-6 E & M, continued) Thickness feet 37. Conglomerate with matrix of sand, andesitic, fine- to medium-grained, gray to reddish brown. Material up to 6 Inches in diameter, but mostly around 2 Inches. Rock types are andesitic and 4.5 38. Sand, andesitic, fine-grained, gray. 4.0 39. Conglomerate up to 6 inches in diameter but mostly less than 3 inches in matrix of sand, andesitic, medium-grained, gray to reddish. Rock types Include andesitic, metamorphics, and quartz. This unit is channeled into unit 40, but channeling does not represent a long time interval. 1.0 40, Sand, andesitic, fine- to medium-grained, gray. 1.0 41. Concealed, but exposures to north show signs of Mehrten sand and conglomerate. 5.5 Total Mehrten (including portion concealed by post channels) Mehrten w 129.5 Total section 140.5 (Note: the topographic map only allows 120 feet for the measured section. A careful field check did not reveal the cause of the dlscrepency.) 225

Section 2-10-10 B, G, & H Modesto Formation (?) and Riverbank Formation

SE iof NE £, SW iof NE £, and NW $ of NE £ of section 10, Township 2 S, Range 10 E, Oakdale quadrangle. Exposures along east side of State Highway 120 on north side of Stanislaus River just north of Oakdale. (Note: the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated,) Thickness Top of road grade (Elevation is 180 feet) feet Riverbank Formation 1. Sand, medium- to coarse-grained, pebbly, brown, cross bedded. (Note: upper 3.0 feet of this unit is above road level. The rest of the section is along the road.) 6.0 2. Sand, medium- to coarse-grained, pebbly, brown, hard. 2.0 3, Sand, fine- to medium-grained, tan to light brown, cross bedded. 5.5 4. Sand, medium-grained, tan to reddish brown. 5.0 5. Sand, medium- to coarse-grained, pebbly, tan to reddish brown, cross bedded. Some layers are very pebbly. 6.5 6. Sand, fine-grained, some coarser grains, tan to brownish, hard. 4.5 226

(2-10-10 B, G, & H, continued)

7. Sand, fine- to coarse-grained, pebbly, light brown (5 YR 6/4) cross bedded. This unit is hard, oxidized, and has vertical jointing indicative of an old B horizon. This unit goes on below unit 8 which is younger and which laps up onto unit 7. The remainder of the section (units 8-10) are younger and may represent the Modesto Formation. 8. Silt, gray (pinkish brown when wet) with brown lamina. Becomes coarser in upper few feet. Hard and oxidized where it laps onto unit 7 with grayish orange color (10 YR 7/4). 9. Sand, fine-grained to silty, gray, interbedded with sand, fine-grained, brown with coarser grained layers. Whole unit is cross bedded. 10. Sand, medium-grained, brown, cross bedded with coarser grained layers. Bottom (road level about 250 feet north of bridge) Total section 227

Section 2-10-27 M Riverbank Formation

NW $ of SW iof section 27* Township 2 S, Range 10 E, Waterford quadrangle. Road cut along east side of Bentley Road just south of Peterson Road and north of aqueduct crossing. (Note: sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bank (Elevation is 180 feet) feet 1. Concealed. Quartz and metamorphic pebbles lying on ground. 0,3 2. Soil, sandy, medium-grained, light brown (10 YR 4/2), some coarser quartz grains. Crumbly. 0.3 3. Sand, medium-grained, light brown, coarser quartz grains. Hard. 0.8 4. Sand, fine- to medium-grained, reddish (5 YR 5/6), set in a finer grained ground mass. Hard. 0.3 5, Sand, medium-grained, reddish. Coarser quartz grains and rounded rock fragments. 0.4 6. Sand, medium-grained, brown to tannish (7j YR 6/4), cross bedded. Has coarser grains and small, rounded pebbles of metamorphics, volcanics, and a few weathered granitic types. Has areas of light gray to light brown silt up to 3 inches by l£ Inches. Hard. 2.0 228

(2-10-27 M, continued)

7. Sand, fine- to medium-grained, brown to grayish. Has some coarser grains. Road level Total section 229

Section 2-11-4 G Mehrten Formation

SW £ of NE |of section 4, Township 2 S, Range 11 E, Oakdale quadrangle. Exposures in large road cut on Rodden Road about £ mile northwest of Orange Blossom Bridge. Section measured near middle of the exposure. At both ends of the exposure there are channel fills of post-Mehrten material. The western channel is 20-30 feet deep and several hundred feet wide, and the eastern channel is about the same size. The channel bases are just above road level. From the elevation of the channels and the character of their fills it Is thought that they are Modesto formation, (Note:

all colors are for dry material unless otherwise stated,) Thickness Top of bank (top of road cut)(Elevation is 210 feet) feet 1. Soil, sandy, brown. 2.0 2. Gravel, rounded, up to ten inches diameter but most less than 6 inches. Rock types include quartz, metamorphics, andesite, granite, quartzite, and

rhyollte. Matrix is andesitic sand with some quartz,

feldspar, and biotite. Over all color is yellowish brown. A good example of Mehrten transition. 7.0 3. Silt to clay, brown. 5.0 4, Sand, andesitic, fine- to medium-grained, reddish brown, cross bedded. Color seems to be from an oxidized coating. 5.0 5. Sand, andesitic, fine- to medium-grained, alter-

nating layers of red and gray, cross bedded. 2.0 230 Thickness (2-11-4 G, continued) feet 6. Clay, reddish. 2,0 7. Sand, andesitic, fine-grained, gray. Seems to have

1,0

8. Clay, reddish. 0.5 9. Sand, andesitic, fine-grained, gray. Has a few biotite flakes. 1.0 10. Silt to clay, reddish with red stains. Has a few biotite flakes. Has organic material. 0.5 11. Silt to clay In alternating beds about 2 feet thick of pink and brown. Pink clay Is quite conspicuous when damp. 15.0 12, Clay to silt, spheroidal, brownish gray. Ressembles tuff as it has feldspar laths across grains and It has white ash in it. " 13. Sand, fine-grained to silty, spheroidal (less than 1 inch up to 1 foot in diameter), grayish brown. Has some layers of hard, pink clay with black glass shards. Clay has black and red stains. Near top of unit there Is a more continuous clay layer with fossil 7.5 14. Conglomerate, pebbles, with a matrix of sand, andesitic, fine- to medium-grained, grayish brown. Coarse material is as large as 4 x 3 x 2 inches but i3commonly less than 2 inches in diameter. Coarser material Is predominatly andesitic varying from hard

lava to scoria but with 3ome metamorphics present. 7.0 Bottom (road level) Total section 65.5 231

Section 2-11-5 R Modesto Formation and Mehrten Formation

SE |of SE £ of section 5* Township 2 S, Range 11 E, Oakdale quadrangle. Exposures In private farm road down south bank of Stanislaus River. Location is just west of OID lateral near north end of Wamble Road. (Note: This section clearly shows the strath terrace cut on the Mehrten Formation. The terrace deposits are thought to belong to the Modesto Formation based on projection of the Modesto surface profile. The strath terrace can be observed all the way from Orange Blossom Bridge into Oakdale.) Thickness Top of bank (terrace) (Elevation is 160 feet) feet Modesto Formation 1. Gravel with cobbles and sand matrix. Sand consists of medium- to coarse-grained, angular to subangular grains of quartz, feldspars, biotite, and dark minerals. Coarser material includes such rock types as granodiorite, granite, quartz, andesite, iand metamorphics. Exposure is covered with vegetation. 7 Total Modesto 7 Mehrten Formation 2. Silt to clay, pinkish gray (5 YR 8/1) 2 3. Sand, andesitic, minor quartz, feldspar, and biotite, fine-grained, pinkish to brown. 7 4, Concealed. 14 Bottom (top of flood plain) Total Mehrten 23 Total section 30 232

Section 2-11-21 B Turlock Lake Formation

NW iof NE ( of section 21, Township 2 S, Range 11 E, Oakdale quadrangle. Most northerly of three sections measured south of Wamble Road in new cuts from land leveling. (Note: the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bank (Elevation is 240 feet) feet 1. Concealed. 1.0 2, Soil, sandy, brown. 0.5 3. Concealed. 1.3 4. Sand, medium-grained,tan. Hard. 0.2

5. Sand, medium-grained, tan to brown, well sorted. w* f 6. Silt, gray with streaks of sand, fine-grained. Grades downward into sand, fine-grained, brown to 0.9

Fi vUiik> KidJ.tjLi. 0.7 8. Silt, gray interlayered with sand, fine-grained, brown. Damp. 1.3 "ttom (land leveling) Total section 6.6 233

Section 2-11-21 G Turlock Lake Formation

SW iof NE £ of section 21, Township 2 S, Range 11 E, Waterford quadrangle. Middle of three sections measured south of Wamble road in new cuts from land leveling. On hill top just west of stockyards. (Note: the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bank (and hill)(Elevation is 260 feet) feet 1. Soil, sandy, brown, pebbly.

2. Similar to unit 1 but harder. X i_5 3. Silt gray, hard with streaks of brown sand. 1.3 4. Sand, medium- to coarse-grained, brown, cross bedded, with some granitic and volcanic pebbles. Semi-Indurated. Has black magnetite streaks on 3.7 Bottom (land leveling) Total section 8.5 234

Section 2-11-21 X Turlock Lake Formation

NW iof SE iof section 21, Township 2 S, Range 11 E, Waterford quadrangle. Most southerly of three sections measured south of Wamble road in new land leveling cuts. Just north of OID South Main Canal. (Note: the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bank (Elevation is 250 feet) feet 1. Soil, sandy, reddish brown, pebbly. 3.0 2. Sand, medium- to coarse-grained, brown to tan, pebbly, cross bedded. Pebbles are of quartz, volcanic, and granitic rocks and tend to be layered. Has streaks of fine-grained gray sand that resembles Mehrten sand. Magnetite streaks on some bedding planes. Has a few beds of fine grained sand. 4.5 Bottom (land leveling) Total section 7.5 235

Section 2-11-29 P Turlock Lake Formation

SE 4 of SW iof section 29* Township 2 S, Range 11 E, Waterford quadrangle. Exposure in silage pit just east of bend in Bond Road. Thickness Top of bank (Elevation is 200 feet) feet 1. Soil, pebbly to cobbly, brown. Includes debris from pit. Large number of rhyollte cobbles lying on surface and in the soil in the whole vicinity. 4.0 2. Sand, medium- to coarse-grained, quartz, feldspar, biotite, dark grains, angular to subangular, light brown to grayish. Dry. Has many volcanic, granitic,

and metamorphic pebbles and cobbles (see pebble count In Table 3of the text). Unit is cross bedded and lenticular. Granitic pebbles and cobbles are mainly deeply weathered. 3.0 Bottom of pit Total section 7.0 236

Section 3-10-27 B Riverbank Formation

NW iof NE iof section 27* Township 3 S, Range 10 E, Waterford quadrangle. Exposures in private farm road down south bank of Dry Creek. (Note: sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bank (Elevation is 125 feet) feet 1. Concealed. Sandy. 3.0 2. Soil, sandy, medium-grained, brown, compact but crumbly. This part of the section cuts across a hillside soil profile. 8.5 3. Sand, medium- to coarse-grained, brown to grayish, very pebbly. Somewhat Indurated. Grades upward 4.5 4. Sand, coarse-grained, brown to grayish. Has volcanic, metamorphic, and granitic pebbles (see pebble count in Table 3of text). Has streaks and small areas of finer-grained material down to silt size. Hard. 2.0 5. Silt to clay, pink with fine-grained sand. Has reddish to brownish streaks. 0.4 6. Sand, fine-grained to silty, brown. Has streaks of gray silt. Hard. 2.7 7. Silt to clay, brownish to grayish, some fine-grained sand. Has yellowish to reddish streaks. 2.3 237 Thickness (3-10-27 B, continued) feet 8. Sand, medium- to coarse-grained, brown. Hard. 0.3 9. Silt, gray to pink, some biotite flakes. 0.2 10. Sand, fine-grained, brown. Hard. 0.3 11. Sand, coarse-grained, dark brown, (damp). 0.3 12. Sand, medium-grained, dark brown (damp). Has some pebbles. 0.5 13. Sand, medium-grained, light brown, coarser quartz grains and pebbles. Has streaks of finer grained reddish sand. 0.7 14. Sand, fine-grained, light brown. 0.5 15. Sand, fine-grained, brown. Has streaks of coarser sand. Hard. 0.5 16. Silt, gray, with coarser grains. Hard. Has what seem to be old worm holes or roots filled with brown sand. Might be part of an old soil horizon. 0.3 17. Sand, medium- to coarse-grained,, Damp. 0.1 18. Sand, fine-grained. Damp. 0.3 19. Silt, brown (damp). 0.1 20. Sand, medium- to coarse-grained, brown, some pebbles. 0.8 21. Silt, gray. 0.3 22. Sand, medium- to coarse-grained, gray to brown. Has some dark rock fragments. 0.2 23. Sand, medium-grained, brown (damp) interbedded with silt, gray. 0.3 24. Silt, brown (damp). 0.2 25. Sand, medium-grained, very light brown to gray (damp) with coarser quartz grains and pebbles. 0.4 238

(3-10-27 B, continued) 26, Sand, medium-grained, tan (damp). 27. Sand, fine- to medium-grained, light brown to tan. Bottom of hill (top of flood plain) Total section 239

Section 3-11-4 B Turlock Lake Formation and Mehrten Formation

NW iof NE £ of section 4, Township 3 S, Range 11 E, Waterford quadrangle. Exposures in road cut on south side of Clarlbell Road about 45 feet west of steel power pole. (Note: in Turlock Lake Formation the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bank (Elevation is 233 feet) feet Turlock Lake Formation 1. Soil, sandy, fine-grained, brown or reddish brown with some coarser quartz grains and pebbles of quartz and volcanics. 0.9 2. Sand, fine-grained, reddish brown with a few pebbles. Hard. 0.5 3. Sand, fine-grained, tan (7| YR 5/5) with a few coarser grains and considerable biotite. Semi-

_- " _} Total Turlock Lake 3.7 Mehrten Formation 4. Sand, andesitic, fine-grained to silty, light gray (10 YR 7/2) with some quartz, feldspar, and biotite. This unit Is somewhat transitional to Turlock Lake. 2.0 Road level Total Mehrten 2.0 Total section 5.7 240

Section 3-11-33 E & F Modesto Formation and Riverbank Formation

SE £ of NW iand SW £ of NW £ of section 33* Township 3S, Range 11 E, Waterford quadrangle. Exposures in road cuts down north bluff of the Tuolumne River starting from the corner of Western and Riverside In Waterford. (Note: Driller*s log 3-11-33 Ml Appendix B is on the south bank of the Tuolumne opposite the lower end of the section.) (Note: the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) Thickness Top of bluff (Elevation is 160 feet) feet Modesto Formation 1. Soil, sandy, fine- to medium-grained, brown. Soil profile is not well developed and is typical of soil developed on the Modesto Formation. Has some unweathered 4.5 Total Modesto 4.5 Riverbank Formation 2. Sand, medium-grained set in a hard light brown (5 YR 6/4) ground mass. Has vertical jointing and columnar structure. Has white coating and dark stains. This is buried B horizon at top of Riverbank Formation. 3.5 241

Thickness (3-11-33 E & F, continued) feet 3. Sand, medium- to coarse-grained, pebbly, brown,

cross bedded, with streaks of brown silt or clay. Hard. Pebbles are metamorphlcs and granitic. Granitic material somewhat weathered but Intact. 8.0 4, Silt, gray with layers of sand, fine-grained and a few layers of coarser material. 6.0 5. Silt, gray interlayered with sand, fine-grained, gray. Silt has layers of sand, medium-grained,

pebbly, brown. Whole unit displays complex lenslng and cross bedding. 4.5 6, Clay, grayish to pink, hard. i.o 7. Sand, medium-grained, brown and gray, interlayered

with silt, gray and sand, fine-grained, brown and gray. Whole unit displays complex lenslng and cross bedding. See Appendix C, Fig. 8 and Table 4 for more detailed description of this unit. 5.0 8. Sand, medium-grained, brown. 0.5 9. Silt, gray (pink to dark brown when damp) iron 3tained interbedded with sand, fine-grained, gray. 13.0 10. Sand, medium-grained, brown with streaks of silt

or clay, brown. l»v 11. Sand, fine- to medium-grained, alternating beds of gray and rather dark brown with iron stains, cross bedded. Has a few gray silt layers. 2.0 12. Silt, gray (dark brown to pink when damp), hard with layers of sand, fine-grained, reddish brown, iron stained. 4.5 242 Thickness (3-H-33 S & F, continued) feet 13. Sand, fine-grained, light brown, cross bedded. 0.5 14. Silt, gray with brown streaks, hard. Has thin beds of sand, fine-grained, brown and gray. Whole unit is cross bedded. 5.0 15. Concealed, 3.0 16. Gravel, rounded, oblate and discoidal, up to 6 Inches X 2 inches, cobbly with matrix of sand, mediujn-grained. Rock types include gabbro (?), quartzite, metamorphics, volcanlcs (rare) and granite (rare). 0.5 17. Sand, medium-grained, pebbly, cobbly, brown to gray with a few silt streaks. 5.5 Total Riverbank 64.0 Younger Alluvium 18. Soil, sandy, fine-grained, pebbly, small cobbles, dirty brown. Section cuts across a sloping soil profile which conceals underlying material. This unit marks top of younger probably Recent alluvium. 10.0

19, Concealed. j!-£ m 20. Sand, medium-grained, pebbly, brown with gray silt

streaks, cross bedded. _-.vJ 21. Soil, sandy, pebbly, with streaks of gray silt. Soil is poorly developed. This Is top of present or very recent flood plain. 5.5 22. Concealed, but seems similar to unit 23. 5.5 23. Sand, fine- to medium-grained, brown to light gray. 5.5 243

Thickness (3-H-33 E & F, continued) feet 24, Gravel, cobbly. Badly disturbed by gravel pit operations. 4.0 Bottom (river level at gravel pit) Total younger alluvium 34,5 Total section 103.0 (Note: topographic sheet only allows about 90 feet for this section. Some of difference may be due to low river level and gravel pit operations.) 244

Section 3-13-31 X, L, P, & Q Turlock Lake Formation

NW £ of SE £, SW iof SE £, SE £ of SW i, NE £ of SW i of section 31* Township 3 S, Range 13 E, Cooperstown quadrangle. Series of road cuts along asphalt road in Turlock Lake State Park leading up hill from Park Rangers' quarters. (Note: in Turlock Lake Formation the sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of hea^y minerals such as magnetite and hornblende, and this will not be repeated for each description. All colors are of dry material unless otherwise stated.) This is the type section of the formation. Thickness Road level at top of road grade (Elevation is 310 feet) feet Turlock Lake Formation (unit 1 Is top of section, unit 10 is base of formation.) 1. Sand, fine- to medium-grained, with a few pebbles, sand stained brown. unit forms west bank of old channel; unit 2 is the channel fill. The channel formation and filling were nearly contemporaneous and do not represent a long time spanj" 4.5 2. Sand, fine- to medium-grained brown and gray, and silt, brown to pink. Sand is interlayered with silt in flat beds, and there are cross bedded sand lenses in silt. Sand may be iron stained. Gray sand has magnetite, /phit has sedimentary structures that might be flow rolls of silt or clay and plates of fine- to medium-grained gray sand at irregular angles^7 See Appendix C, Table 2, and Fig, 7. 7.0 245 Th (3-13-31 X, L, P, ft Q, continued) 3. Sand, fine-grained to silty, brown to gray with scattered pebbles.

4, Sand, medium-grained reddish brown interlayered with clay, sllty, brown. Has a few pebbles. 5. Sand, medium- to very coarse-grained, yellowish brown and gravel. Somewhat similar to unit 6 below, but layers are flat with less gravel. See Appendix C, Table 2, and Fig. 7, 6. Gravel, cobbles up to 5 Inches in diameter, cross bedded (tabular), a gravel layer cuts across the top of the whole unit. Cross bedding dips about 35 toward the west. Individual layers are up to 1.7 feet thick and vary from cobbles below to sand above giving the appearance of graded bedding. See Table 3 for pebble count. 7. Somewhat concealed but seems similar to unit 8 below. Has clay blebs that are similar to gray Mehrten but

8. Similar to unit 9 below but has more fine-grained material and has coarser material in more distinct

Iffli"*_rasit"_^ fit

9. Gravel, cobbles up to 5 inches in length, sand fine- to coarse-grained, with a few pebbles, and silt to clay, reddish brown. Unit is poorly sorted. Sand and finer-grained material are deeply oxidized and form a dense, compact mass which serves as a matrix for the coarser material. Most of the coarser

material Is well rounded and consists of igneous, 246 Thickness (3-13-31 X, L, P, & Q, continued) feet metamorphic, and volcanic rocks. All coarser- grained material Is deeply weathered, and even

quartz and quartzite break fairly easily. Weathering has obscured sedimentary features. Throughout the unit there are pieces of silt and clay up to 8 inches in diameter. 9.5 10. Sand, fine- to coarse-grained, silt and clay, pink to brown, and some gravel with scattered cobbles and pebbles. Unit is poorly sorted. Gravel and cobbles may have worked down from above. Unit is deeply weathered and oxidized. Fine-grained material at base is grayish and can not be distinguished in the field from typical Mehrten. 1.5 Turlock Lake thickness 51.0 Mehrten Formation (unit 11 is top, bottom not exposed) 11. Sand, andesitic, fine-grained, pink (damp). Has increasing amounts of biotite, quartz, and feldspar toward top. Top 0.5 feet is a light brown to white clay with plant remains. Contact between Turlock Lake and Mehrten probably occurs near top of this 4.0 12. Concealed; traces of fine-grained andesitic material. 5.5 13. Concealed, 0.5 14. Sand, andesitic, fine-grained grading to silt or clay, grayish pink, and pale red. 2.0 15. Concealed. 3.5 16. Sand, andesitic, fine-grained to silty, gray to grayish pink. 4.5 247 Thickness (3-13-31 X, L, P, & Q, continued) feet 17. Concealed. 5.0 18. Similar to unit 19 below, but has some brown silt

19. Similar to unit 20 below, but Is finer-grained. 2.5 20. Sand, andesitic, fine- to medium-grained, bluish gray. Has ash, rock fragments, and glass (?). Rather light weight and not too well consolidated. 4,8 21. Concealed. 14.7 Bottom of hill Thickness of Mehrten 48.0 Thickness of measured section 99.0 248

Section 4-9-2 J Modesto Formation

HE iof SE .of section 2, Town.hip 4 S, Range 9 E, Ceres quadrangle. Exposures in private road down south bluff of Tuolumne River just west of highway bridge. (Note: sand sized material consists mainly of quartz, feldspars, lesser amounts of biotite, and minor amounts of heavy minerals such as magnetite and hornblende, and this will not be repeated for each description. All colors are for dry material unless otherwise stated.) This is the type section. Thickness Top of terrace (Elevation is 100 feet) feet 1. Sandy, crumbly soil that has been disturbed by road building. 2.5 2. Disturbed blocky B horizon; somewhat compact, but not deeply weathered or oxidized. 2.0 3. Silt, gray with sand layers. 3.0

4. Sand, medium-grained, light yellowish brown. 1"«? 5. Silt and some sand, fine-grained, gray to yellowish brown, compact. Iron stained laminae In the silt. 2.0 6. Sand, medium-grained yellowish brown. 0.5 7. Silt and sand, fine-grained, silt is gray to yellowish brown, compact. Iron stained laminae in the silt. 2.0 8. Silt and sand, fine- to coarse-grained, complexly Interlayered and cross bedded, gray to yellowish brown with some iron stains. Sand has magnetite on some layers. Some iron stained layers cut across

bedding. Has spherical bands of iron stains. Unit 249 Thickness (4-9_2 j. continued) feet tend to be finer-grained at bottom and coarser- grained at top. See Appendix C, Table 5, and 2.5 9. Silt, some sand, fine-grained, silt is powdery and gray. Extremely thin laminae shown by reddish color. Spherical bands of iron stained fine-grained sand around silt cores. - 2.5 10. Silt, complex interlayerlng of £ iinch laminae of a blocky clay like material. Probably represents a buried immature soil. 1.0 11. Silt, powdery, gray with iron stains. Has a few lenses of sand, fine- to medium-grained, cross bedded, gray and yellowish brown and Iron stained. 5.0 12. Sand fine- to medium-grained, gray to reddish brown complexly interbedded and cross bedded with gray silt. Iron stains on cross bedding. 2.0 13. Sand, medium- to coarse-grained, cross bedded with magnetite and iron stains on layers, light gray. See Appendix C, Table 5, and Fig. 11. 0.5 14. Silt and sand, medlum-gralned, gray. 3.0 15. Silt at top grading down into sand, medium-grained, light brown to gray at bottom. 3.5 16. Sand, medium-grained, cross bedded. 2.0 17. Sand, medium-grained, cross bedded, interlayered black and light yellowish brown. Black color is a stain and not magnetite. See Appendix C, Table 5, and Fig. 11. 0.5 250 Thickness (4-9-2 J, continued)

18, Sand, medium-grained, cross bedded, brown to

reddish brown. Has silt blebs. _» .U 19. Concealed, but seems sandy. 3.0 20. Concealed. 5.5 21. Concealed, but there Is gray silt in the bank to one side. 5.5 Bottom (top of flood plain) Thickness of measured

section 52,0 APPENDIX B

WELL LOGS APPENDIX B

The well logs In this appendix were taken from driller's records and in so far as possible they have not been altered from the original records. The logs were not put into standardized form due to difficulties in under- standing individual driller's terms and In reconciling terminology used by different drillers. The only exception is well 3-7-21 HI which is based on a detailed sample description. The following observations can be made about some terms used by various drillers: 1. Hard set and set sand usually refer to more resistant sand layers and beds. 2, Shaley and shale usually refer to fine-grained sand and silt. 3, Black sand usually refers to andesitic sands of the Mehrten Formation, 253

Well 2-9-25 Gl Riverbank Water Company

(Drilled by Hennings Brothers. Altitude, 140 feet. On Riverbank surface. Suggested depths to contacts: 100 feet to top of Turlock Lake Formation, 290 feet to top of Mehrten Formation and hole bottomed in Mehrten.)

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Top soil 2 2 Brown clay 8 154 Hard pan 2 4 Set sand gravel 11 165 Brown sandy clay 4 8 Brown-red clay 5 170 Sand and gravel 8 16 Shale 2 172 Hard clay 8 24 Set sand 8 174 Hard set and rocks 12 36 Shale 176 Coarse sand 20 56 Red clay 2 178 Set sand some gravel 4 60 Black sand set 10 IEE Gravel 3 63 Red shale 1 189 Silt clay 2 65 Set sand red 6 195 Sand (water) 10 75 Rocks 23 218

Gravel set 5 OU w_.djf 240 Brown clay 3 83 Brown shale 257 Brown shale clay 2 85 Sand and gravel 1 258 Rocks 7 92 Brown shale 7 265 Hard gray clay 5 97 Black sand 9 270 Set sand and gravel 3 100 Brown shale 40 310 Silt clay 16 116 Set muddy sand 10 320 Set sand 4 120 Clay brown 7 327 254

(Well 2-9-25 Gl, continued) Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Brown clay 4 124 Brown shale 3 330

Set sand 1 128 Clay 20 350 Shaley brown clay 12 140 Set sand dirty 7 357 Set sand gravel 6 146 Black sand and gravel 8 365 255

Well 2-10-11 Nl Oakdale Irrigation District

(Drilled by Howk. Altitude, 155 feet. On Modesto surface. Suggested depths to contacts: 55 feet to top of Mehrten Formation, and hole bottomed In Mehrten.) (Not known whether there is Turlock Lake Formation present. Probably there Is

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Top soil 4 4 Large gravel 2 125 Cobble stone 2 6 Tight gravel and clay 5 130 White sand Clay and gravel 11 141 Large gravel Brown clay and Small gravel and gravel 13 154 clay Medium loose gravel, Medium gravel brown sand 10 164 Boulders 28 Brown clay and gravel 6 170 White sand, small gravel 19 57 Medium gravel, fair 5 175 Medium sized Tight gravel and gravel 3 60 clay, poor 7 182

Hard boulders 4 64 Clay limestone streaks 4 186 Hard clay, limestone 11 75 Tight gravel and clay 2 188 Clay and gravel 89 Loose gravel and Medium gravel, sand, good 10 198 fair 96 Tight gravel and Large gravel I 98 clay 5 203 Hard clay 10 108 Loose gravel and sand, good 5 208 Tight gravel, poor 4 112 Clay and gravel 3 211 Clay and gravel 11 123 Clay 215 256

(Well 2-10-11 Nl, continued) Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Gravel, fair 4 219 Medium gravel, tight streaks 14 303 Clay and gravel, poor 5 224 Loose boulders 7 310 Gravel and sand, Large loose gravel fair 3 227 black sand 318 Gravel and clay 3 230 Tight gravel 5 323 Gravel, fair 4 234 Medium loose gravel, fair 8 331 Clay and gravel 6 240 Large loose gravel, Gravel, good 3 243 good 23 354 Clay and gravel 247 Tight gravel, free streaks 11 365 Large loose gravel, good M 254 Black boulders 6 371 Large gravel, good 6 260 Large black gravel, loose sand 16 387 Medium gravel, fair 6 266 Gravel, small clay streaks S 393 Clay streaked gravel 16 282 Medium gravel, sand fair 7 400 Large loose gravel 7 289 257

Well 2-10-31 N3 U. S. Army

(Drilled by Western Drilling Company. Altitude, 131 feet. On Riverbank surface. Electric log available. Suggested depths to contacts: 91 feet to top of Turlock Lake Formation, 28l feet to top of Mehrten Formation, and hole bottomed in Mehrten.) Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Surface 3011 4 4 Hard clay and gravel 385 Hard pan 4 8 Sandy clay 7 392 Sandy clay and sand, hard streaks 37 45 Sand 13 405 Hard sand with soft Clay and gravel 43 448 streaks 59 104 Cemented clay and Hard sand and free gravel 24 472 streaks 116 220 Extra hard cemented Hard sand, rough clay and gravel 21 493 (water) 7 227 Hard sand rough 27 520 Cemented sandy clay and gravel 15 242 Sandy clay 6 520 Cemented clay and Sandy clay and gravel 36 278 gravel 16 542 Sandy clay and Sandy clay, gravel gravel 49 327 and sand streaks, 75 617 Coarse gravel, some clay, free 12 339 Hard clay and gravel 45 662 Sandy clay 9 348 Sand 15 677 Gravel and clay, free 14 362 Sandy clay and sand streaks, free 15 692 Brown clay and sand 18 710 258

Well 2-11-8 Xl Milton F. Parker

(Drilled by Stockton Armature and Motor Works. Altitude, 230 feet. On Riverbank surface. Suggested depths to contacts: 30 feet to top of Mehrten Formation and hole bottomed in Mehrten.) Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Sand and gravel 45 45 Sandstone 15 330 Cobble stones 13 58 Clay 35 365 Brown clay and Sand and gravel 10 375 gravel 17 75 Clay 10 385 Rotten sand and gravel 10 85 Cemented gravel 20 405 Gray shale 10 95 Cemented sand 15 420 Pink shale 5 100 Sand and gravel 15 435 Volcanic ash 25 125 Sandy clay 20 455 Brown sandstone 35 160 Cemented sand b 460

Sandstone soft 10 170 Sand and gravel J 465 15 185 Clay 25 490 Sandstone 30 215 Water sand hard 15 505 Sand 15 230 Cemented gravel 15 520 Sandstone 20 250 Hard sandstone 15 535 Boulders, sand, and Hard sand water 10 545 gravel 10 260 Sandstone 15 560 Cemented gravel 25 285 Clay 15 575 Clay 10 295 Sand and cemented Sandstone 10 305 gravel 15 590 Clay 10 315 S. clay 10 600 259

(Well 2-11-8 Xl, continued) Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Black sand and Brown shale, hard 15 660 gravel 15 615 Brown clay 10 670 White clay 5 620 Black sand and Brown shale gravel 10 680 boulders 15 635 Pea gravel and Sandstone 10 645 black sand 693 260

Well 2-11-29 Ml Union Improvement District #30

(Drilled by Stockton Armature and Motor Works. Altitude, 202 feet. On Turlock Lake surface. Suggested depths to contacts: 52 feet to top of Mehrten Formation and hole bottomed In Mehrten. ) Thi.cle- Dept;h Thick- Depth escription ness ft. ft. Description ness ft. ft.

Soil 3 3 Black sand .gravel 1/3" 3 327 H. P. brown 9 12 Brown sandstone 95 422 Clay reddish brown 20 32 Cemented sand and Sand and gravel 1" 3 35 gravel 3H3 H 7 429 Brown clay 3 38 Gray clay 3 432 Sand, gravel, Brown sandstone 72 504 rocks 9" 21.5 59.5 Black sand, fine Clay, brown 3.5 63 and coarse 4 508 Brown sandstone 25 88 Hard pink sandy clay 6 514 Cemented rocks 10" 40 128 Brown sandstone 74 588 Clay, red 2 130 Brown clay 4 592 Sandstone, brown 15 145 Brown sandstone 34 626 Sandy brown clay 8 153 Black sand 2 628 Brown sandstone 122 275 Brown sandstone 35 663 Soft grey sandstone 11 286 Black sand 7 670 Soft grey-black sandstone 38 324 "Slley sand brown" 5 675 Brown sandstone 5 680 261

Well 3-7-21 HI Mapes Ranch

(Drilled by Hennings Brothers. Altitude, 36 feet. On Recent alluvium. Electric log available. Suggested depths to contacts: 36 feet to top of Modesto Formation, lib feet to top of River- bank Formation, 190 feet to possible trace of blue clay, and 330 feet to top of Turlock Lake Formation. Hole bottomed in Turlock Lake. This is a testhole known as Stanford #1, and the log Is based on the drillers log and a detailed sample description by Keith A. Kvenvolden of Stanford University,) Thick- Depth Description ness ft. ft.

Soil,, sandy, no feldspar fine- to medium-grained, light tan to gray, and clay, silty, gray. Large number of yellow-brown limonite concretions. 5 5 Hard pan 1 6 Clay, gray with micaceous and carbonaceous fragments. 3 9 Sand, very coarse, tan with pebbles, subanguiar to subrounded. Some silty to medium-grained tan to gray sand. 1 10

Clay, slightly silty, micaceous, tan-gray. 9 1y Sand, fine- to medium-grained, tan and clay, silty,

Sand, fine-grained to pebbly, light gray. Pebbles have 4 mm average diameter and are subrounded and dark. Some woody organic matter. 0.5 40 Sand, fine-grained to pebbly, light gray and sand, very coarse to gravel, subrounded, brown-gray. Coarser material includes schist, quartzite, and other dark rocks, such as andesite. 17 57 262

(Well 3-7-21 HI, continued) Thick- Depth Description ness ft. ft.

Clay, very silty, micaceous, tan. Some fine- to very coarse-grained light gray sand and coarse-grained to pebbly, dark subrounded sand and gravel. Minor organic material. 7 64 Clay, very silty, micaceous, gray and sand, fine- to coarse-grained, light gray. 2 66 Sand, fine- to very coarse-grained, considerable feldspar, gray to tan, and clay, silty, micaceous, tan. 8 74 Sand, medium- to coarse-grained, pebbly, tan. Coarser material darker color and subrounded Some organic material. Minor clay. 86 Clay, tan. Some very fine- to coarse-grained tan to gray sand. Minor organic material. 8 94 Sand, coarse- to very coarse-grained, tan to gray.

Minor tan clay. -2 97 Clay, silty, micaceous, some limonite concretions, gray and tan. Some coarse- to very coarse- grained tan to gray sand. 4 101 Sand, medium- to very coarse-grained, subangular to subrounded, tan and buff. Coarse grains may include metamorphics particularly quartzite. Minor tan and buff clay. 14 115 Clay, silty, micaceous, carbonaceous, tan-gray. Some fine- to medium-grained grayish sand. 18 133 263 (Well 3-7-21 HI, continued)

Thick- Depth Description ness ft. ft.

Sand, medium- to coarse-grained, a few pebbles, tan. Minor reddish, tan, and gray silty clay. 25 158 Clay, sandy, micaceous, tan and blue-gray, and sand, fine- to very coarse-grained, pebbly. Coarser grains mainly quartz and quartzite. 3 161 Sand, fine- to very coarse-grained, pebbly, gray. Minor gray and tan clay. 19 180 Clay, micaceous, light gray. Some fine- to coarse-grained-pebbly, gray sand. Minor organic material. 2 182 Clay, silty to nearly pure, micaceous, gray and sand, fine- to coarse-grained, pebbly, gray. Minor organic material. 8 190 Clay, silty, gray and sand, very fine- to fine- grained, argillaceous, gray. Minor organic material. 6 196 Clay, silty, carbonaceous, buff-gray. Some fine-

201t— \J -X Sand, medium- to very coarse-grained, some pebbles, subangular to subrounded, tan. Coarse fragments include quartzite and schist. 24 225 Gravel, mostly pebbles, subrounded, dark colored

and sand, medium- to very coarse-grained, tan. Many pebbles broken probably by drilling.

Pebbles are dominantly chert, schist, 264 (Well 3-7-21 HI, continued) Thick- Depth Description ness ft. ft.

metagraywacke, and gneiss (?) with one possible igneous pebble. Some tan clay. 25 250 Gravel, rather coarse with maximum average diameter of 20 mm, subrounded to rounded, dark colored. Pebbles mostly dark quartzite with some chert and quartz and possibly some granitic material. Some fine- to very coarse- grained tan sand and silty tan clay. 5 255 Clay, sandy, micaceous, blue-gray and sand flne- to very coarse-grained, tan. 3 258 Gravel, subangular to rounded. Pebbles include quartzite, quartz, and quartz gneiss. Some sand. 3 20l Clay, silty to sandy, slightly micaceous, blue-gray. Minor fine- to medium-grained, tan-gray sand. Unit is rather soft. 21 282 Clay, silty to sandy, slightly micaceous, blue- gray. Minor coarse-grained tan sand. Unit 299 Clay, silty to sandy, slightly micaceous, blue- gray and some green-gray. Minor fine-grained gray sand. Unit is soft. 31 330 Clay, silty to sandy, slightly micaceous, blue- gray. 32 362 265

Well 3-7-29 Jl Faith Ranch

(Drilled by Howk. Altitude, 25 feet. On Recent alluvial plain. Suggested depths to contacts: 40 feet to top of Modesto Formation, 115 feet to top of Riverbank Formation, 193 feet to top of blue clay, and about 340 feet to top of Turlock Lake Formation,) Thick- Depth Thick- Dept escription ness ft. ft. Description ness ft. ft.

Top soil and Coarse sand and sediment 18 18 gravel 5 221 Small black sand 20 38 Medium gravel and sand 243 Coarse sand 80 118 Large gravel and Hard clay sand 17 260 Coarse sand, small Boulders 262 gravel 36 165 Clay 40 302 Sticky clay 23 188 Large gravel and Coarse sand 5 193 sand 31 njr«#4ril Soft blue clay 15 208 Hard clay and gravel 343 Coarse sand 8 216 266

Well 3-8-18 Di Modesto Irrigation District

(Driller unknown. Altitudes 45 feet. On Modesto surface Well known as Basso Pump well number 2. Suggested depths to contacts: 100 feet to top of Riverbank Formation, 190 feet to top of blue clay, hole bottomed in blue clay.)

Thi.ck- Depth Thick- Depth scription ness ft. ft. Descriptio ness ft. ft.

Soil 5 5 Soft clay 17 117 Hard pan 40 45 White clay 73 190 Sand 40 85 Blue clay 10 200 3ravel 100 267

Well 3-8-24 Bl State of California

(Drilled by George M. Clark and F, De La Grange, Altitude, 75 feet. On Modesto surface. Suggested depths to contacts: 72 feet to top of Riverbank Formation, 187 feet to top of blue clay, and hole bottomed in Riverbank.)

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Sand 18 Hard sand 1 109 Clay 36 Fine clay 16 125 Sand 39 Shaley clay 13 138 Clay 45 Sandstone 1 139 Sand 49 Sand 26 165 Clay 52 Clay 7 172 Sand 72 Sand 8 180 Red Hd. clay 90 Clay, brown 7 187 Sand 93 Clay, blue 35 222 Sandy clay 15 108 Sand 226 268

Well 3-9-6 Xl Couture Farms

(Drilled by Green and Berry. Altitude, 92 feet. On Modesto surface. Suggested depths to contacts: 52 feet to top of Riverbank Formation, 250 feet to top of Turlock Lake Formation, and hole bottomed in Turlock Lake.)

Thick- Depth Thi,ck- Dept escrlption ness ft. ft. Description ness ft. ft.

Sand andi soil 16 16 Red shale 125 260 Sandy clay 9 25 Clay 40 300 Water■ sand 25 50 Red rock; 25 325 Gray shale 56 Red clay, sandy 75 400 Gray sandy clay 100 Red clay 130 530 Blue clay 35 135 Blue clay 117 647 269

Well 3-9-25 Ai McClure Ranches

(Drilled by Green and Berry. Altitude, 110 feet. On Modesto surface. Suggested depths to contacts: 20 feet to top of Riverbank Formation, 220 feet to top of Turlock Lake Formation, and hole bottomed In Turlock Lake.)

Thi,ck- Depth Thick- Depth escription ness ft. ft. Description ness ft. ft.

Top soil 5 5 Sand and rock 5 170 Hard pan 4 9 Sandy clay 8 178 Pack sand 6 15 Hard set sand 4 182 Sandy clay hard. set 13 28 Sandy clay 16 198 Clay sand hard set 66 94 Hard sandy clay 4 202 Muddy clay 4 98 Sandy clay 46 248 Sandy clay 19 117 Hard set sand 5 253 Fine sand 5 122 Clay and streaks of sand 292 Sandy clay 6 128 Sticlcy clay 16 308 Hard set sand 9 137 o<*na 7 315 Muddy sand 23 160 3 320 Hard set 2 162 Hard set and shale 18 338 Fine sand 165 270

Well 3-9-29 Dl City of Modesto

(Driller unknown. Altitude, 84 feet. On Modesto surface. Well known as number 8. Suggested depths to contacts: 84 feet to top of Riverbank Formation, 170 feet to top of blue clay, 290 feet to top of Turlock Lake Formation, and hole bottomed

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

>" W -A-«_-. 5 5 Sand and clay 2 218 Hard pan 2 7 Sandy clay 6 224 Sandy soil 15 22 Soft clay Sandy hard pan 35 57 Sand Hard pan 62 Sand clay Sand 6 68 Blue clay Hard pan 4 72 Soft white clay and sand 27 297 Sand 4 76 Red hard pan and Hard pan 13 89 sand 27 324 Sand I 90 Sand and clay 15 339 Soft clay 10 100 Hard clay ->, 341 Sandy hard pan 16 116 Hardpan 27 368 White joint clay 24 140 Coarse sand 2 370 21 16l Hard pan 18 388 Clay 9 170 Hard pan 56 444 Blue clay 190 Blue sand ? ? Sand and clay 26 216 271

Well 3-10-1 Gl M. S. De Simas

(Driller unknown. Altitude, 182 feet. On Turlock Lake surface. Suggested depths to contacts: 82 feet to top of Mehrten Formation and hole bottomed in Mehrten.)

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Hard pan 9 9 Brown clay 15 280 Sand 27 36 Red shale 15 295 Hard clay 6 42 Brown clay 5 300 Sand and clay 4 46 Hard brown sand 10 310 Hard shale 15 6l Black sand gravel 5 315

Clay 6 67 Red shale 0 320 Rocks 15 82 Red clay 5 325 Shale, red 48 130 Brown sand, hard 5 330 Clay, gray 30 160 Red shale 5 335 Reddish shale 60 220 Black sand 4 339 Black sand and Red shale 61 400 gravel, hard set 30 250 Black sand, shale 7 407 Shale 10 260 Brown shale 28 435 Red clay 5 265 272

Well 3-10-26 Jl Rude

(Drilled by Hennlngs Brothers. Altitude, 137 feet. On Modesto surface. Suggested depths to contacts: 10 feet to top of Riverbank Formation, depth to top of Turlock Lake Is unknown, hole probably bottomed In Turlock Lake.)

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Top soil 3 3 Clay and sand streaks 6 89Q.-1 Clay and shale 5 0 Red sandy clay 11 100 Sand 5 13 Sand 3 103 Clay 12 25 Clay and sand Sand 3 33 streaks 4 107 Clay 24 57 Red clay and shale 23 130 Fine sand 60 Sand fine 4 134 Clay and shale 7 67 Clay and shale 6 140 Red sandy clay 6 73 Clay hard shale 15 155 Sand and shale 7 80 Black sand and gravel 15 170 Sand 3 83 273

Well 3-11-28 Fl Robert Thompson

(Drilled by Hennings Brothers. Altitude, 162 feet. On Modesto surface. Suggested depths to contacts: 5 feet to top of Riverbank Formation, 70 feet to top of Mehrten Formation, and hole bottomed in Mehrten. It is not known -whether Turlock Lake Formation is present.)

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft. ft.

Top soil Clay 1 44 Sand Rock and gravel 5 49 Clay Sanded clay, hard 21 70 Sand Black sand 3 73 Clay Sanded clay hard 18 91 Sand Shale 13 104 Gravel Sanded clay hard pink 79 183 Clay Black sand 3 186 Rock and gravel 43 Sanded clay hard 11 197 274

well 3-11-33 Ml

Putnam Sand & Gravel

(Drilled by Hennings Brothers. Altitude, 85 feet. On Recent flood plain. Suggested depths to contacts: 26 feet to Riverbank or Turlock Lake Formations, 50 feet to top of Mehrten Formation, and hole bottomed in Mehrten Formation.)

Thick- Depth Thick Depth Description ness ft. ft, Description ness ft, ft.

Top soil 2.5 2.5 Hard shale 56 Black sand 4.5 7 Black sand I 64 Rocks 19 26 Shale gray 18 82 Blue clay 1 27 Brown shale 10 92 Rocks 10 37 Black shale 28 120 Red clay 2 39 Fine sand black 10 130 Shale brown 11 50 Black sand 13 143 Black sand 5 55 Black shale 145 275

Well 4-8-2 CI Peter Yap

(Drilled by Hennings Brothers. Altitude, 67 feet. On Modesto surface. Electric log available. Suggested depths to contacts: 92 feet to top of Riverbank Formation, 167 feet to top of blue clay, and hole bottomed In Riverbank.)

Thick- Depth Thick- Depth Description ness ft. ft. Description ness ft, ft.

Top soil 3 3 Clay o 76 Hard 1 4 Sand c 10 86 Sand 9 13 Clay o 92 Sanded clay and Sand c 10 102 shale sk 15 28 Soft brown clay 5 107 Sand c 9 37 Fair clay 3 115 Sanded clay 3 Sand c 130 Clay 10 50 Clay soft and Sand c 6 56 sand sk 5 135 Clay soft 3 59 Sand c 3 138 Sand c 5 64 Clay 16 154 Clay 2 66 Blue clay 43 197 Shale clay 4 70 Blue clay soft 15 212

"c" presumably means coarse "shale" apparently means hard silt "sk" probably means streak 276

Laboratory observations on 9-16-22 B U. S. Bureau of Reclamation

Total depth 831. Eleva. 268 », Cored from 5-80', 120-130', 160-246.5', 300-310',* 350-360», 400-530', 570-577* 630-635* 680-690', 720-730 and 760-780.

Samples Examined Classifications and Geologic Remarks (depth) - 17.0 S.m.a.* ill-sorted fine sand Recent/sub-recent 63,0 S.m,a. ill-sorted fine sand Chowchilla R. 126,0 Silty clay. No diatoms. alluvium Montmovillonltic

165.6 Weathered gravel. Opaline PALEOSOLIC ZONE coatings, Sierran mtmphc. Weathered sierran 176.0 Clayey paleosol gravels and sands 187.9 S.m.a, weathered ill-sorted with well-developed coarse sand firmly-cemented 223.0 S.m.a. weathered 111-sorted non-calcareous "clay- fine sand ironpan" soil 225-235 Gravel. Pebbles about 10% text. structure andesite 301.5 Silty paleosol 307.6 Silty paleosol 355.0 Clayey paleosol 403.0 Clayey paleosol Color change 414.5 Andesite-tuff, 111-sorted fine "MEHRTEN FM" Dark sand grey to olive gray 449.0 Andesite and Rhyolitic ill- fluviatile crystal- sorted fine sand llthic-vitrlc pyroxene 464.0 Andesite-tuff, ill-sorted fine andesite tuff with sand much rhyolitic 493.1 Andesite-tuff, ill-sorted fine pumice and glass in sand 449.0 and appreciable 497.7 Andesite-tuff, ill-sorted coarse amounts in 414,5 and sand 464.0. Very minor 516.8 Andesite-tuff, ill-sorted fine S.m.a. Variably sand uncemented (sil.). Color change: Dark- greenish grey at 573.5 and below.

"ft S.m.a. ■ Sierran micaceous arkose 277

(Lab. observations on 9-16-22 B, continued) Samples Examined Classifications and Geologic Remarks (depth)

573.5 Silty clay, sparsely PLIOCENE MARINE, diatomaceous Coscinodiscus sp. 630.5 Clay predominant diatom 720.5 Sandy silt (stone) (see Lohman, U,5.G,3. 722.3 Clay, calcareous, diatomaceous P.P. 189-C) 766.0 Sandy clay, diatomaceous Glaucophane conspicuous 775.0 Silty clay, calcareous, in 766.0. Serai- diatomaceous lithified, transitional to claystone and siltstone. APPENDIX C

SIEVE ANALYSES 279

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m CN CO in to in CO CO CO ON CN © vo CO CO CO c- If to co in CN in in rH rH rH rH r-rt CN to rH CN CO to co I— t in in o >—i CN r-i CN © o o © O o © o o © © o o o © © o © d O © o

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vo to o o © in o Os o O to CO in co to o in co in as rH H \ VO in CN CO to CN O rH CO o o iH in o to o iH o in d rH CO V

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N in to in o o to in rH rH H H H H -H -H -fl -H -H -H CO © rH VO in rH rH VO CO VO CN O H © CM H © co e4 rH CO CN CN CN to H H I ! H ■ H -H H H -H ■ 4H H i H I CO CM co rH to H H ■ i i i CM 1 H H H H H t H H ■ z CX3 p CO +H i i i i i VO I I I I H H i CO vo CM CD -a ■ o CN CN a 0) H-> d w M J3 hQ A o o CD CD CD ID o J J > CO 03 CO CO Chl U O W J_> 4J Uh CO CQ v h h o -o "rl CD CD CD CD CD 4) >CD >ID CD >CD > -a -O T3 T3 v v rC A Oh iCO ICO COa CO O o o O O O O ■H "H "H "H "rt ID CD CD CD s 09 co CO CO < ■ s CC CC CC CC ■ S a CN in cv 4-> N ■H "H 1 rS a "H c? c? d a -o -o z o rH i i i— l CO t— t-l r-i CO a H oo l—l "-rt 1 -a a tj -J 4J it o o v 9Oh ■H 4-> "H "H T3 Ob < "H ■H "H C a. «" —cb rS rP I rJ O I 03 CO o CO 2" CO cv o U a a td W o U "rH CO CU bg o M co rH co MrH i-H ON rH X Cv U 00 "-a rHQ MVO rH CM CO CO CN CO in to to V V *C £ CN oo «c vo co *j CN CN 5CN CN I . c oo ON t— CO rH 00 ON Os ON ON t-rt Os ON I— l rH i-rt i— l rH rH rH i-H rH H H rH I I I I I i I I I I I I I I I I I CM CN CO ro VO CN <# CM CM CN CO CO CO CO rH i— l CN CM r-i CM co APPENDIX D

SELECTED CHEMICAL ANALYSES 281

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