NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES Geology of the Eastern Plains of San Agustin and Upper Alamosa Creek Daniel J. Koning and Alex Rinehart Open File Report 611 June 2021 New Mexico Bureau of Geology and Mineral Resources A Research Division of New Mexico Institute of Mining and Technology Socorro, NM 87801 (575) 835 5490 Fax (575) 835 6333 www.geoinfo.nmt.edu Geology of the Eastern Plains of San Agustin and Upper Alamosa Creek Daniel J. Koning and Alex Rinehart Open File Report 611 June 2021 New Mexico Bureau of Geology and Mineral Resources NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 611 PROJECT FUNDING Funding for this project has been collaborative, with support from the New Mexico Bureau of Geology and Mineral Resources, Aquifer Mapping Program of the NM Bureau of Geology and Mineral Resources, Healy Foundation, Office of the State Engineer, and USGS National Cooperative Geologic Mapping Program (STATEMAP). CONVERSION FACTORS ABBREVIATIONS ft foot, feet Multiply By To obtain km kilometer, kilometers m meter, meters miles 1.609 kilometers mi mile, miles Ma Millions of years in age, mega anum feet 0.3048 meters LA Very Large Array radio telescope observatory inches 2.54 centimeters SA San Agustin (Fahrenheit 32) 0.556 Celsius DISCLAIMER The reports and data provided here are intended to aid in the understanding of the geologic and hydrologic resources of New Mexico. However, there are limitations for all data, particularly when subsurface interpretation is performed, or when data are aggregated that may have been collected at different times, by different agencies or people, and for differ- ent purposes. The information and results provided are also dynamic and may change over time. Users of these data and interpretations should exercise caution, and site-specific conditions should always be verified. These materials are not to be used for legally binding decisions. Any opinions expressed do not necessarily reflect the official position of the New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, or the State of New Mexico. Although every effort is made to present current and accurate information, data is provided without guarantee of any kind. The data are provided “as is,” and the New Mexico Bureau of Geology and Mineral Resources assumes no responsibility for errors or omissions. No warranty, expressed or implied, is made regarding the accuracy or utility of the data for general or scientific purposes. The user assumes the entire risk associated with the use of these data. The New Mexico Bureau of Geology and Mineral Resources shall not be held liable for any use or misuse of the data described and/or contained herein. The user bears all responsibility in determining whether these data are fit for the user’s intended use. The views and conclusions are those of the authors, and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the State of New Mexico. Cover: Photograph of a dish-shaped antenna of the Very Large Array radio telescope observatory, situated in the eastern Plains of San Agustin. View is to the southeast; the forested, western part of Mount Withington lies in the background. The shaded slope in the grasslands, just beyond and right of the antenna, is an eroded fault scarp created by repeated earthquake events along a north-south, west-down fault—Photo by Daniel Koning. GEOLOGY OF THE PLAINS OF SAN AGUSTIN CONTENTS Executive Summary ...................................................................... 1 III. Conclusions ............................................................................. 63 I. Introduction .............................................................................. 4 References ......................................................................................................................... 64 Purpose ............................................................................................... 4 Previous Work .............................................................................. 4 Acknowledgments .............................................................................................. 69 Geographic and Topographic Setting ..................... 4 Plains of San Agustin ................................................ 6 Alamosa Creek ............................................................... 6 Figures Vegetation ........................................................................................ 7 1. Geographic and topographic features of the Climate ................................................................................................ 8 Plains of San Agustin and Alamosa Creek .......... 3 2. Photographs of the San Agustin Plains ............... 5 ............................................................................................ 9 II. Geology 3. Geographic and topographic features of Structure ............................................................................................ 9 the eastern Plains of Agustin and upper Upper Alamosa basin ............................................... 9 Alamosa basin ................................................................................ 7 East San Agustin basin ......................................... 12 4. Three-dimensional model of the geologic Calderas ............................................................................. 15 structure under the Plains of San Agustin .......... 8 Stratigraphy Background ................................................ 15 5. Geologic map of the C-N embayment and upper Stratigraphy of Bedrock ................................................... 20 Alamosa Creek ..................................................................... 10 Middle Eocene sedimentary rocks ............ 20 6. Geologic map and well locations of the Late Eocene and early–late Oligocene East San Agustin basin ................................................... 11 volcanic and volcaniclastic strata ........ 21 7. Schematic illustration of a full graben Stratigraphy of Neogene Basin Fill ........................ 34 and half graben ..................................................................... 12 Upper Alamosa Basin ............................................ 35 8. A west-east cross-section across the East San Agustin Basin ......................................... 37 Winston graben ..................................................................... 13 Basin fill in the North graben and under 9. Gravity station locations in and the VLA ....................................................................... 39 near the study area ............................................................. 14 Stratigraphic relations 10. Map of the southern C-N embayment and and paleogeography ............................................ 56 Upper Alamosa basin showing the Bouguer Implications for groundwater anomaly gravity data ....................................................... 18 management ............................................................ 57 11. Map of the East San Agustin basin showing Hydrostratigraphic units ................................................. 57 Bouguer anomaly gravity data ................................ 19 Hydrostratigraphic units in 12. Illustration of the stratigraphic units of the the volcanic bedrock ....................................... 58 Southland Royalty SA No. 1 well ........................ 20 Eolianite hydrostratigraphic unit ............... 61 13. Illustration of volcanic calderas ............................. 21 Santa Fe Group basin fill 14. Illustration of various types of bedding ......... 22 hydrostratigraphic unit ................................. 61 15. Illustration of the geologic time scale ............... 23 Coarse valley fill hydrostratigraphic 16. Schematic figure illustrating stratigraphic unit ................................................................................... 62 terms and concepts ............................................................ 24 17. Hierarchal system of categorizing units of rocks ........................................................................... 24 NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES: OFR 611 18. Stratigraphic correlation diagram illustrating stratigraphic units in volcanic bedrock ........... 25 Appendices 19A. Cross-sections A-A' and B-B' across the East San Agustin basin ................................................... 26 19B. Cross-section D-D' that trends SSE from 1. Appendix 1. Descriptions of geologic units depicted in Plate 1. .............................................................. 12 the Datil Mountains ......................................................... 27 2. Appendix 2. Summary of rock units that 19C. Geologic unit colors for cross sections ............ 28 20. Schematic diagram illustrating underlie the Mogollon-Datil volcanic field (Proterozoic, Paleozoic, Mesozoic, and stratigraphic relations ..................................................... 29 21. Photographs of various volcaniclastic early Cenozoic eras). ............................................................. 5 3. Appendix 3. Descriptions of well cuttings and sedimentary rocks of the Spears Group ......... 31 22. Photographs of ignimbrites of the wireline notes for the Southland Royalty SA #1 Agustine Test Well. ....................................................................7 Datil and Mogollon Groups ..................................... 32 23. Features related