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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 By 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 under- laid 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.
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