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State of South Dakota M STATE OF SOUTH DAKOTA M. Michael Rounds, Governor DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES Steven M. Pirner, Secretary DIVISION OF FINANCIAL AND TECHNICAL ASSISTANCE David Templeton, Director GEOLOGICAL SURVEY Derric L. Iles, State Geologist REPORT OF INVESTIGATIONS 116 DRILLING OF AN AEROMAGNETIC ANOMALY IN SOUTHEASTERN SOUTH DAKOTA: RESULTS FROM ANALYSIS OF PALEOZOIC AND PRECAMBRIAN CORE by KELLI A. MCCORMICK Akeley-Lawrence Science Center University of South Dakota Vermillion, South Dakota 2005 CONTENTS Page INTRODUCTION ................................................................................................................... 1 Background ....................................................................................................................... 1 Purpose of this study ......................................................................................................... 1 Previous investigations ..................................................................................................... 2 Drilling ...................................................................................................................... 2 Geophysics ................................................................................................................ 2 Methods of this study ........................................................................................................ 2 RESULTS FROM ANALYSIS OF THE PALEOZOIC ROCKS ...................................... 4 Devonian strata ................................................................................................................. 4 Ordovician strata ............................................................................................................... 5 Galena Group ............................................................................................................ 5 Platteville Formation ................................................................................................. 7 Glenwood Formation ................................................................................................ 7 St. Peter Sandstone .................................................................................................... 7 Cambrian strata ................................................................................................................. 8 Davis Formation ........................................................................................................ 8 Wonewoc Sandstone ................................................................................................. 9 Bonneterre and Eau Claire Formations ..................................................................... 10 Mt. Simon Sandstone ................................................................................................ 11 RESULTS FROM ANALYSIS OF THE PRECAMBRIAN ROCKS ............................... 12 Saprolite ............................................................................................................................ 12 Precambrian metagabbro .................................................................................................. 13 i RESULTS FROM ANALYSIS OF THE PRECAMBRIAN ROCKS – continued Page Weathered basement rock ......................................................................................... 13 Unweathered basement rock ..................................................................................... 14 Generalized thin section descriptions ............................................................... 14 Geochemistry .................................................................................................... 15 Neodymium model age and zircon age date ..................................................... 15 WATER CHEMISTRY RESULTS ...................................................................................... 15 AEROMAGNETIC DATA AND MODELING RESULTS ................................................ 16 ACKNOWLEDGEMENTS ................................................................................................... 17 REFERENCES CITED .......................................................................................................... 17 FIGURES 1. Aeromagnetic data from a flight line along the Missouri River from Yankton, South Dakota, to Ponca State Park, Nebraska .......................... External File 2. Photograph of coring through the flowing formation ........................................ External File 3. Graphic log of rocks intersected in test hole R20-2002-01 ............................... External File 4. A structure contour map of the Paleozoic surface in Union County, South Dakota ............................................................................ External File 5. A structure contour map of the Precambrian surface in Union County, South Dakota ............................................................................ External File 6. Geophysical log showing single point resistivity and natural gamma signatures of Paleozoic and Precambrian rocks in test hole R20-2002-01 ....... External File 7. An exterior mold of a brachiopod (an indeterminate cyrtinid) of probably Devonian age in dolostone ............................................................. External File 8. The block 6 aeromagnetic anomaly located 4 miles south of Elk Point, South Dakota .................................................................................... External File 9. Profile along line 5 of the block 6 aeromagnetic anomaly ................................ External File ii FIGURES – continued 10. A 2½-dimensional magnetic model of the block 6 aeormagnetic anomaly ....................................................................................... External File TABLES 1. A generalized correlation of Devonian through Cambrian formations between selected South Dakota, Iowa, and Nebraska test holes ....................... External File 2. Results from biostratigraphic analyses of trilobite and brachiopod fragments in Cambrian strata ............................................................................. External File 3. Oxide and element geochemistry of the Precambrian metagabbro intersected in test hole R20-2002-01 ................................................................. External File 4. CIPW normative mineral calculations using a Fe2O3/FeO ratio of 0.2 ............. External File 5. Water quality of two Union County aquifers and two water samples from test hole R20-2002-01 ............................................................................... External File APPENDICES A. Specifications of the aeromagnetic survey and method of modeling the aeromagnetic data ........................................................................ External File B. Stratigraphic column of the R20-2002-01 core and correlation with cores from Iowa and Kansas .............................................................................. External File iii INTRODUCTION Background An aeromagnetic survey was flown along the Missouri River from Yankton, South Dakota, to Ponca State Park, Nebraska (fig. 1), during 2002 as part of a project funded by the U.S. Army Corps of Engineers and conducted by the Archaeology Laboratory (principal investigator, Brian Molyneaux) at the University of South Dakota. The purpose of that project was to identify possible steamboat wrecks and other cultural artifacts along the river. The hope was that iron, such as that used in the steamboat boilers, would be large enough to be detected by an airborne survey. The aeromagnetic geophysical survey is commonly used in the geosciences to investigate very old rocks in the subsurface called basement rocks. These basement rocks can be buried very deeply below land surface (thousands of feet in some cases). Despite the depth of burial, magnetic minerals, principally magnetite, present in these rocks can be detected in these surveys. Magnetic minerals in very high concentrations may constitute or otherwise be associated with economic mineral deposits. Thus, rocks that are anomalously magnetic are often of interest to the mineral extraction industry. The Archaeology Laboratory and U.S. Army Corps of Engineers shared this new aeromagnetic data set with the Geological Survey Program, South Dakota Department of Environment and Natural Resources. Parameters of the flight, grid spacing, and elevation are presented in appendix A. A company named ENW Services of Golden, Colorado, offered to model the basement anomalies in for the Geological Survey Program. Several aeromagnetic “highs” were evident on this map (fig. 1). One of the identified anomalies, an ovoid-shaped, compound anomaly with an amplitude of >800 nT, was identified south of Elk Point, South Dakota. Purpose of this Study The investigation of all subsurface rocks is important to the Geological Survey Program in order to better assess the mineral and water resources of the state. For this reason, the Geological Survey Program drilled at the Elk Point aeromagnetic anomaly (block 6 anomaly; fig. 1). Core from 411 to 1,035 feet below land surface was collected. Water samples were collected from two different bedrock units underlying the Cretaceous Dakota Formation. Some of these same rocks are utilized as aquifers in other areas of the Midwest. Several researchers from academia and industry have shown interest in the core collected from this hole. Several core samples have been sent out for analysis (chemical, petrographic, biostratigraphic,
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