Contents Acknowledgements .........................................................2 Introduction......................................................................3 Method of Study...............................................................4 Geologic Setting ..............................................................5 Stratigraphy......................................................................6 Lake Superior Group......................................................6 Munising Formation ................................................. 6 Trempealeau Formation .......................................... 7 Prairie du Chien Group ..................................................7 St. Peter Sandstone ................................................ 8 Black River Group ..........................................................8 Trenton Group................................................................8 Richmond Group ............................................................8 Utica Shale .............................................................. 8 Cataract Group...............................................................9 Structural Characteristics...............................................9 Rim Zone........................................................................9 Annular Depression......................................................10 Central Uplift.................................................................10 Microbreccia.................................................................10 Shock-Metamorphic Effects .........................................11 Iridium Analysis ............................................................12 Energy Requirement ....................................................12 Impact Origin of the Calvin 28 Age of the Structure ......................................................12 Cryptoexplosive Disturbance, Cass County, Michigan Evaluation of Possible Origins.....................................13 Volcanic Origins ...........................................................13 by Randall L. Milstein Energy Considerations .......................................... 13 Report of Investigation 28 Maars and Diatremes ............................................ 14 Calderas ................................................................ 15 Michigan Department of Natural Resources Intrusive Origins ...........................................................15 Geological Survey Division Diapiric Origins.............................................................15 1988 Solution Subsidence ....................................................16 REPRODUCTION of Geological Survey Division publications in whole Impact Origins ..............................................................16 or in part may be done and is encouraged if proper credit is given to Impact Structures .................................................. 16 the Michigan Department of Natural Resources, Geological Survey Division and where appropriate, the author. Summary......................................................................18 The author holds the registered copyright on the original form of this Conclusion .....................................................................19 work which was completed as partial requirements for a Master of Arts degree at the University of Northern Colorado. Report of Investigation 28 – Page 1 of 22 stages of this study. Bruce Bohor, United States Figures Geological Survey, provided additional sample 1 - Location of study area, Calvin 28 cryptoexplosive inspection for shock metamorphic features. The author disturbance, Cass County, Michigan. ...............................3 is grateful to Ronald J. DeHaas, President of Michigan 2 - Traverse Limestone structure contour map and line of Petroleum Geologists, Incorporated, for his many and geologic cross-section, Calvin 28 cryptoexplosive invaluable consultations about the structural nature of disturbance.......................................................................4 the study area. Special appreciation is extended to Kenneth D. Hopkins, K. Lee Shropshire, William D. 3 - The Michigan Basin and surrounding geologic structures ...6 Nesse, William H. Hoyt, and Richard D. Dietz, University 4 - Stratigraphic succession in Cass County, Michigan............6 of Northern Colorado for their critical review of this study. A very special thanks is extended to Lynda K. Jones for 5 - Generalized geologic cross-section and profile of Traverse Limestone tops, Calvin 28 cryptoexplosive disturbance. ..7 typing and preparation of the manuscript, and Tyrone Black for computer type setting and presentation. 6 - Bedrock Geology of Cass County, Michigan. ......................9 7 - Photographs illustrating microscopic breccia from the Calvin 28 cryptoexplosive disturbance: Lawson #1, 1016.5-1018 ABSTRACT m. ...................................................................................11 The Calvin 28 cryptoexplosive disturbance is an isolated, 8 - Location of selected Midwestern cryptoexplosive nearly circular subsurface structure of Late Ordovician disturbances. ..................................................................15 age in southwestern Michigan. The structure which is 9 - The Calvin 28 cryptoexplosive disturbance compared with defined by 107 test wells, is about 7.24 km in diameter stratigraphic uplift/diameter relationships for terrestrial and consists of a central dome, an annular depression impact structures ............................................................17 and an encircling anticlinal rim. Seismic and 10 - Photographs illustrating structural similarities between geophysical well log data confirm an intricate system of impact craters on other planets and the Calvin 28 faults and structural derangement exists within the cryptoexplosive disturbance. ..........................................18 structure. Deformation decreases with depth and distance from the structure. U.S.G.S. topographic maps and aerial imagery show the structure is reflected as a Tables surface topographic rise controlling local drainage. 1 - Comparative features of selected Midwestern Igneous or diapiric intrusion and solution collapse are cryptoexplosive disturbances. ........................................16 rejected as possible origins for the Calvin 28 structure on the basis of stratigraphic, structural and geophysical 2 - Classification of meteorite and astrobleme craters............17 evidence. A volcanic origin is rejected due to an absence of igneous material. Although shock- metamorphic features are unidentified, microbreccia ACKNOWLEDGMENTS occurs in deep wells that penetrate the structure. The author wishes to express his gratitude to the staff of Morphology and structural parameters suggest an the Michigan Geological Survey, and especially the impact origin. members of the Subsurface and Petroleum Geology Unit, for their help in data acquisition, many questions and helpful suggestions. Access to Devonian cores and unpublished gravity and magnetic data was made available by William B. Harrison, Western Michigan University. Review of proprietary seismic data was made possible with the kind permission of Robert Mannes, Mannes Oil Company and Alvin R. Hosking, Hosking Geophysical Corporation. Analysis and comparison of this data was done with the help of Steven Schaefer, Hosking Geophysical Corporation. Two separate neutron activation analyses for iridium were conducted by Mark Mitchell and Bruce Wilkerson, Division of Engineering Research, Michigan State University and Charles J. Orth, Los Alamos National Laboratory. Jerry Cowen, Department of Physics, Michigan State University, reviewed for accuracy the mathematical procedures used in the study, R. A. F. Grieve, Geological Survey of Canada, Mark J. Cintala, Lyndon B. Johnson Space Center and Frances B. Waranius, Lunar and Planetary Institute provided beneficial information and suggestions during the early Report of Investigation 28 – Page 2 of 22 1968a). None of the individual features listed can be INTRODUCTION relied on as an unfailing diagnostic tool in the eophysical data, geologic mapping, and the identification of a cryptoexplosive disturbance. Gextensive drilling of oil and gas test wells have The author has chosen to identify the Calvin Township delineated a subsurface structure in Calvin Township, structure as Calvin 28. The choice is based on the Cass County, Michigan, centered 1.4 km southwest of location of the first deep test well which identified the the village of Calvin Center (Figure 1 and Figure 2). The disturbance, the central positioning of the structure structure consists of a central domal uplift bounded by beneath Section 28 of Calvin Township and the name of an annular depression and an encircling anticline. The the most productive of three oil fields associated with the overall structure has a diameter of 7.24 km. feature. An exploratory well drilled through the central uplift The main purpose of this study is to provide a possible penetrated roughly 485 m of regional, although thinned, explanation for the origin of the Calvin 28 Paleozoic strata down to the Late Ordovician cryptoexplosive disturbance. Structure contour maps, Cincinnatian Series. Instead of encountering the Middle stratigraphic cross-sections, geophysical data, structural and Lower Ordovician stratigraphic sequence, the well comparisons, petrographic analyses, neutron-activation bore entered into a shortened section of the Lower analyses and