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(<20Q DEPARTMENT OF THE INTERIOR DHITED STATES GEOLOGICAL SURVEY INTERPRETATION OF AEROMAGNETIC ANOMALIES IN SOUTHEASTERN MISSOURI By John W. Allingham A study of the relations of aeromagnetic anomalies to the Precambrian igneous geology and related mineral deposits Prepared in cooperation with the Missouri Geological Survey Open-file Report 76-868 1976 \ DEC 2 01975 j This report is preliminary and has not been edited or : reviewed for confromity with U.S. Geological Survey standards or nomenclature. "* -. This report is the first draft of a manuscript which was written in 1966 and intended for more formal publication. It is released now (1976) in its original form to make the data available to the public. CONTENTS PAGE Abstract..................................................... 1 Introduction................................................. A Geological and geophysical investigations...............j 10 . Acknowledgments......................................... ' 16 \ Physiography............................................ 18 Interpretative problems and results..................... 28 Geologic setting............................................. 33 Igneous and sedimentary rocks........................... 41 Volcanic rocks..................................... 44 Granitic rocks..................................... 47 Mafic rocks........................................ 49 Sedimentary rocks.................................. 50 Laraotte Sandstone............................. 50a Bonneterre Dolomite........................... 51 Davis Formation............................... 54 Derby-Doerun Dolomite......................... 54a Potosi Dolomite............................... 55 Eminence Dolomite............................. 57 Magnetic properties of the rocks................... 59 Measurements.................................. 61 Susceptibility................................ 62 Remanent magnetism............................ 70- CONTENTS (Continued) . PAGE Structure.......................^..............«........ 75 Faults............................................. 82 Joints............................................. 86a Mineral deposits........................................ 87 Classification..................................... 93 Structural control................................. 94 Domal and anticlinal structures............... 94 Ridge and basin structures.................... 97 Fractures.......................................... 1QO Aeroraagnetic survey. .....................:................... 103 Collection and compilation of data...................... 109 Field operations................................... 109 Compilation of field data.......................... 110 Accuracy of position.................._.,......... 110 Total-intensity cap..................................... 113 Aeromagnetic anomalies....................................... 118 Interpretative techniques............................... 120 Data and calculations. .I................................. 130 Characteristic anomalies.....4.......................... 132 CONTENTS (Continued) PAGE Rock relations..................................*...... 152 Structural relations................................... 158 Topographic anomalies............................. 160 Roof pendants of volcanic rocks..................« 169 '. Faults and basins.................................. 176 Relations to mineral deposits.......................... 185 Iron deposits..................................... 186 Pea Ridge anomaly............................ 187 ' ^i. Cottoner Mountain anomaly.................... 195 Kratz Spring anomaly......................... 200 Shepherd Kountain anomaly.................... 201 Iron Mountain anomaly........................ 207 . Lead and zinc deposits..,........................., 208 Fotosi area.................................. 210 The eastern mining area...................... 221 Irondale anomaly............................. 223 Bonne Terre area............................. 225 Anomalies and mines.......................... 229 Analysis of anomalies........................ 233 Detailed aeromagnetic data................... 245 Relations to regional features......................... 249 Sources of volcanic rocks......................... 250 Belle anomaly..................................... 252 Ironton anomaly................................... 257 Indian Creek anomaly.............................. 260 Richwoods anomaly.................................. 265 CONTENTS (Continued) . PACE Avon anomaly........................................ 272 Gabbroic rocks..................................... 275 -_f " ~"- " * "- - .-.- - . - - - _- .._ .._._. ._ , .__ .. -^.»_. .^ ,__ __Subsurface structure and geophysical data............... 277. Resume and conclusions....................................... 291 References..................................*.».............. 294 TABLES » PAGE Table 1. Average susceptibility of rocks from the St. Francois Mountains.............^.............. " 1p"60 ,.,.-- ; Table 2. Depths to buried Precacabrian basement es- timated from aeromagnetic data.................... 124 Table 3. Correlation of Precainbrian topography and . : lowanplitude magnetic anomalies................... 167 Table 4. High-amplitude anomalies related to possible mineralized volcanic structures.......... 251 Table 5. Ring-like magnetic anomalies in Missouri.......... 256 ILLUSTRATIONS PAGE Figure 1. Index map of Missouri showing exposed Precambrian igneous rocks and major structural features........ 19 2. Geologic map of the St. Francois Mountains......... 34 2a. Aeromagnetic and geologic map of the lead mining district of southeastern Missouri.................. at enc _ v 3. Total-intensity aeromagnetic map of the St. Francois Mountains.................................. 35 4. Generalized geologic section on the flanks of the St. Francois >kmntains.. ........................... 40 5. Stratigraphic section of Precambrian and Cambrian rocks in the Bonne Terra area, Missouri............ 43 6. Plot of magnetite vs magnetic susceptibility of rocks from the St. Francois Mountains.............. 64 7. Range of magnetic susceptibility of rocks from the St. Francois Mountains...... ..'................. 66 8. Susceptibilities of granitic rocks in the St. Francois Mountains......,..*................... 68 9. Susceptibilities of volcanic rocks in the St. Francois Mountains................................. 69 10. Remanent icocents of samples of rhyolite near Stouts Creek before and after partial demag netization. ........................................ 72 11. Random scatter of remanent vectors of granitic rocks from the St. Francois Mountains.............. 74 ILLUSTRATIONS (Continued) PAGE 12. Relations between sedimentary and basement structures ......................................... 95 i ! ' ' . \ 14. Use of Polar charts for two- and three dimensional analyses of aeromagnetic anomalies................. 121 15. Diagram showing the parameters used to compute the displacement of normal faults in basement rocks...........................................J.. 126 16. Characteristic magnetic profiles across vertical fault zones......................................... 134 17. Width of the Sinnas Mountain fault zone estimated from aeromagnetic data................... 135 18. Characteristic total-intensity magnetic profiles over normal faults...................................... 136 19. Aeromagnetic interpretation of the Palmer fault zone............................................... 137 20. Magnetic effect of inclined contacts between granite and volcanic rocks......................... 139 21. Characteristic total-intensity magnetic profiles in the magnetic meridian over vertical contacts shotting the effects of varied rock susceptibilities. 140 ILLUSTRATIONS (Continued) PAGE 22. Characteristic effects of ends of volcanic flows at different thicknesses and depths................ 141 23. Aeromagnetic profiles over contacts between granitic and volcanic rocks near Flatwoods area. ................................................ 142 24. Aeroiaagnetic profiles over volcanic flows near Minimum and Criswell^,Missouri................ 143 25. Prismatic ciodel used to approximate aeromagnetic anoisalies of the type neqr Shiiley, Missouri....... 145 26. Interpretation of the high-amplitude anomaly near Shirley, Missouri............................. 146 27. Magnetic profiles over cylindrical models of infinite depth extent.............................. 147 28. Aerotaagnetic anomaly associated with a plug-dike feature near Pyatt................................. 148 29. Magnetic profiles over erosional areas in volcanic flows..................................... 150 30. Aeroraagnetic anomaly at Cedar Creek caused by an erosional discontinuity in volcanic flows........... 151 31. Aeroaagnetic and geologic map of part of southeastern Missouri showing contrasting aeromagnetic patterns over granitic and volcanic rocks.......... ILLUSTRATIONS (Continued) PAGE 32. Subsurface structure and lithology deduced frora aeroinagnetic profiles over the St. Francois Mountains.......................................... 155 33. Topography of a granite knob showing the irregular ., three-dimensional raodel used in computations........ 161 3^ _ The magnetic effect of topographic relief by !~'~~ "i three-dimensional techniques........................ 162 35. Magnetic profiles computed from two-dimensional models showing the effect of topography............. 166 36. The magnetic expression of