Special Publication 2008-2 G

Geomagnetic Results , Secular Variation, and Archaeomagnetic Chronology I Oklahoma Geological Survey Special Publication 2008-2 G. RANDY KELLER, Interim Director ISSN 0275-0929 Geomagnetic Results, Secular Variation, and Archaeomagnetic Chronology, United States and Mesoamerica, including Archaeomagnetic Data and Time Assignments Robert L. Dubois, Emeritus1 School of Geology and Geophysics University of Oklahoma Norman I-the inclination of the magnetic field D-the declination of the magnetic field East I D X H Down F North F-the vector direction of the magnetic field Sponsored by: Oklahoma Geological Survey The University of Oklahoma Norman 2008 II Robert L. DuBois, Emeritus1 SPECIAL PUBLICATION SERIES The Oklahoma Geological Survey’s Special Publication series is designed to bring timely geologic information to the public quickly and economically. Review and editing of this material has been minimized in order to expedite publication, and author-prepared illus- trations have been used throughout. This publication, printed by the Oklahoma Geological Survey, is issued by the Oklahoma Geological Survey as authorized by Title 70, Oklahoma Statutes, 1981, Sections 231–238. 100 copies have been prepared for distribution at a cost of $1,751.00 to the taxpayers of the State of Okla homa. Copies have been deposited with the Publications Clearinghouse of the Oklahoma Department of Libraries. Geomagnetic Results , Secular Variation, and Archaeomagnetic Chronology III CONTENTS Abstract 1 Introduction 1 Procedures and Initial Laboratory Study 3 Geographic Location of Sample Sites 8 Archaeomagnetic Data Tables 8 Archaeomagnetic Results 8 The Southwest (United States) 9 Nonaxial Field, the Southwest 10 The Midcontinent 11 North America, Samples Older Than A.D. 500 12 Mesoamerica 13 South America 13 Archaeomagnetic Chronology 13 Discussion of Archaeomagnetic Results 14 Geomagnetic Declination and Inclination for Lat 32°–40°N., Long 80°–114°W., North America, A.D. 200–1965. 15 Summary 16 Acknowledgments 17 References Cited 19 Appendix: Explanation of the Archaeomagnetic Data Tables 21 Appendix 1. Northern Arizona and Adjacent Areas 23 Appendix 2. Southern Arizona 43 Appendix 3. Northwestern New Mexico (excluding Chaco Canyon) and Adjacent Areas 66 Appendix 4. Chaco Canyon, Northwestern New Mexico, with Explanatory Text by Thomas C. Windes 81 Appendix 5. Northeastern New Mexico and Adjacent Areas 165 Appendix 6. Southern New Mexico and Adjacent Areas 199 Appendix 7. The Western Midcontinent 205 Appendix 8. The Northeastern Midcontinent 217 Appendix 9. The Southeastern Midcontinent 214 Appendix 10. Mesoamerica 275 Appendix 11. South America 307 Appendix 12. Miscellaneous Sites 315 IV Robert L. DuBois, Emeritus1 TABLES Table 1.—Major Structural Provinces.......................................................................................... 319 FIGURES Figure 1 319 Figure 2 319 Figure 3 320 Figure 4 320 Figure 5 320 Figure 6 320 Figure 7 321 Figure 8 321 Figure 9 321 Figure 10 322 Figure 11 322 Figure 12 322 Figure 13 322 Figure 14 323 Figure 15 323 Figure 16 323 Figure 17 323 Figure 18 324 Figure 19 324 Figure 20 325 Figure 21 325 Figure 22 325 Figure 23 326 Figure 24. 326 Figure 25 326 Figure 26 326 Figure 27 327 Geomagnetic Results , Secular Variation, and Archaeomagnetic Chronology V Figure 28 327 Figure 29 327 Figure 30 328 Figure 31 328 Figure 32 329 Figure 33 329 Figure 34 329 Figure 35 330 Figure 36 330 Figure 37 331 Figure 38 331 Figure 39 331 Figure 40 332 Figure 41 332 Figure 42 332 Figure 43 332 Figure 44 333 Figure 45 333 Figure 46 333 Figure 47 333 Figure 48 334 Figure 49 334 Figure 50 334 Figure 51 335 Figure 52 335 Figure 53 336 Figure 54 336 Figure 55 337 Figure 56 337 Figure 57 338 Figure 58 338 Figure 59 339 Figure 60 339 Figure 61. 339 Figure 62 340 Figure 63 340 Figure 64 341 Figure 65 341 Figure 66 342 VI Robert L. DuBois, Emeritus1 Figure 67 342 Figure 68 342 Figure 69 343 Figure 70 343 Figure 71 344 Figure 72 344 Figure 73 344 Figure 74 345 Figure 75 345 Figure 76 345 Figure 77 345 Figure 78 345 Figure 79 345 Figure 80 345 Figure 81 345 Figure 82 346 Figure 83 346 Figure 84 346 Figure 85 346 Figure 86 346 Figure 87 346 Figure 88 346 Figure 89 346 Figure 90 347 Figure 91 347 Figure 92 347 Figure 93 347 Figure 94 347 Figure 95 347 Figure 96 347 Figure 97 347 Figure 98 348 Figure 99 348 Figure 100. 348 Figure 102 348 Figure 103 348 Figure 104 348 Figure 105 348 Figure 106 349 Geomagnetic Results , Secular Variation, and Archaeomagnetic Chronology VII Figure 107 349 Figure 108 349 Figure 109 349 Figure 110 349 Figure 111 349 Figure 112 349 Figure 113 349 Figure 114 350 Figure 115 350 Figure 116 350 Figure 117 350 Figure 118 350 Figure 119 350 Figure 120 350 Figure 121 350 Figure 122 351 Figure 123 351 Figure 124 351 Figure 125 351 Figure 126 351 Figure 127 351 Figure 128 351 Figure 129 351 Figure 130 352 Figure 131 352 Figure 132 352 Figure 133 352 Figure 134 352 Figure 135 352 Figure 136 352 Figure 137 352 VIII Robert L. DuBois, Emeritus1 Geomagnetic Results , Secular Variation, and Archaeomagnetic Chronology 1 ABSTRACT INTRODUCTION Secular Variation Magnetic measurements of ancient materials reveal Geophysicists have long been interested in secular that the earth’s magnetic field for the last 10,000 yrs— variation in the earth’s magnetic field and how it relates changes in direction, declination and inclination, and in- to changes in physical and thermal features of the earth’s tensity. The archaeomagnetic results are based on ~2,000 interior. However, studies have been few. Many obser- fired features sampled in the Midcontinent, the American vatories around the globe have reported direct measure- Southwest, Mesoamerica, and a few other areas. Data ments of the geomagnetic field for the last few hundred were plotted as archaeomagnetic poles grouped by area years: Bauer (1895) recorded early data from London, and time range, and then were drawn as average magnet- Paris, Rome, and elsewhere. Malin and Bullard (1981) re- ic polar curves. Average values of declination and inclina- viewed data concerning the direction of the magnetic field tion enable comparison of data at various times and areas. from 1570 to 1975 at London. The number of records de- The archaeomagnetic polar curves have a cloverleaf pat- cline with age of observation, and continuous records are tern circumscribing the earth’s axis of rotation and forming scarce. loops that extend outward from the axis between lat 75ºN. Archaeomagnetism yields information about direction and lat 90ºN. (the north geographic pole). An average pole and intensity of the field during thousands of years in the position for the Southwest data set is calculated to be near past. Research involves measuring magnetism in baked- the north geographic pole or axis of rotation but displaced clay samples collected as oriented pieces from archaeo- to the near side. logical sites. Given reliable assignment of age, it provides Data for the Southwest show maximum extensions historical details of secular variations. for the polar curve about A.D. 500, 850, 1100, 1350, and Secular variation includes changes in direction, dec- approximately 1850. Between A.D. 850 and A.D. 1100, lination, and inclination. Direct measurements of the geo- a large change in polar position occurred—~23° of sub- magnetic field suggest that these changes range from a tended angle—a shift of perhaps 1° every 6.5 yrs. During few tenths of a degree over 1 to 10 yrs, to a few degrees the same time, declination changed from 10° E to 15° W; over decades; changes in direction of 10º–30º over a few change in inclination was 43° to 64°. Geomagnetic inten- hundred years have been recorded. Changes in the di- sity ranged from 1.4 to 0.75 times that of the present day. rection of the geomagnetic field may reflect changes in A field intensity of 1 (i.e., the intensity we know today) earth’s fluid core or solid inner core, or at the core-man- coincided with times when the ancient geomagnetic field tle boundary. Electrical features of the inner core and at aligned with the earth’s axis of rotation or nearly so. Assum- the core-mantle boundary may be involved. Changes in ing an intensity of 1 and the archaeomagnetic pole at lat thermal properties of the fluid core are important in gen- 90ºN., and using archaeomagnetic field-intensity data and erating the magnetic field, but perturbations and time and the average archaeomagnetic pole positions (the direction frequency considerations are most important in bringing and intensity of that part of the field left after subtracting about major changes in the field, shifts in intensity, and the axial part) permits calculation of a nonaxial compo- reversals. Smaller changes in the magnetic field may have nent. That part of the field varies in intensity from +0.5 to extraterrestrial causes—notably the effects of solar wind −0.2 times that of a field of 1 and in direction from essen- on earth’s magnetosphere. tially north polar to south polar—lat 90ºN. to lat 90ºS. Archaeomagnetic data for the southwestern United Results for the Midcontinent resemble those from the States presented here indicate changes in declination and Southwest. From a special group of results for times be- inclination of ~10º in 100–300 yrs, ~20º in 500 yrs, and fore A.D. 500, a selected set from 7800 B.C. to 6300 B.C. ~30º in ~1,350 yrs. These variations at the surface rep- yields polar curves extending from the axis of rotation al- resent changes in the earth’s interior.

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