U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1474 Prepared in cooperation with the Arkansas Geological Commission Bouguer Gravity of Arkansas By JOHN D. HENDRICKS U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1474 Prepared in cooperation with the Arkansas Geological Commission A description and analysis of the Bouguer gravity anomalies of Arkansas, with emphasis on tectonic and igneous history of the region UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in-Publication Data Hendricks, John D. Bouguer gravity of Arkansas. (U.S. Geological Survey professional paper ; 1474) "Prepared in cooperation with the Arkansas Geological Commission." Bibliography: p. Supt. of Docs, no.: I 19.16:1474 1. Gravity Arkansas. I. Arkansas Geological Commission. II. Title. III. Series. QB337.5.U5H46 1988 526'.709767 87-600212 For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 CONTENTS Abstract ........................................... 1 Gravity anomalies resulting from Phanerozoic igneous activity 17 Introduction ....................................... 1 Geophysical anomalies associated with exposed igneous Acknowledgments ................................. 1 rocks ..................................... 18 Physiographic divisions and gravity patterns ............ 2 Little Rock complex .......................... 18 Ozark Plateaus .................................. 2 Magnet Cove Complex ........................ 20 General geology .............................. 2 Potash Sulphur Springs intrusive body .......... 21 Gravity anomalies of the Arkansas Ozarks ....... 3 Peridotite of Pike County or kimberlite at Murfreesboro 21 Gravity anomalies and faulting ................. 4 Geophysical anomalies associated with igneous rocks in Ouachita Mountains and Arkansas Valley ........... 4 drill holes ................................. 21 Stratigraphy ................................ 4 Peridotite near Rison in Cleveland County ....... 21 Bouguer gravity of the Ouachita Mountains- Dumas anomaly .............................. 23 Arkansas Valley ............................ 6 Diabase(?) in Drew County ..................... 23 Tectonic model for the Ouachita system ......... 7 Chicot and Ashley Counties anomaly ............ 23 Mississippi embayment ........................... 13 Covington and Millington, Tennessee ............ 24 Geologic history ............................. 13 Suspected intrusive bodies ........................ 24 Tectonic history of the Mississippi embayment ... 14 Associated anomalies near Rison ............... 24 Gravity modeling ............................ 14 Newport anomaly ............................ 24 Ouachita structure at depth in the Mississippi Jonesboro, Marmaduke, Sedgwick, and Walnut Ridge 27 embayment ................................ 15 Piggott "Low" ............................... 27 West Gulf Coastal Plain .......................... 16 Relation of igneous intrusions to proposed tectonic model 27 Stratigraphy ................................ 16 Summary .......................................... 28 Gravity patterns ............................. 17 References cited ..................................... 29 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Map showing Bouguer gravity and generalized geology of Arkansas. 2. Map showing interpreted Bouguer anomalies and tectonic elements of Arkansas. FIGURE 1. Histogram of implied sedimentation rates for exposed Paleozoic rocks in the Ouachita Mountains .............. 5 2. Regional Bouguer gravity map of the southeastern United States ......................................... 6 3. Graph and cross section of two-dimensional gravity model in western Arkansas ............................. 7 4. Maps and cross sections showing hypothetical tectonic evolution of the southern North American continent from late Precambrian to present .......................................................................... 9 5-8. Graphs and cross sections showing: 5. Two-dimensional gravity model from Yellville, Arkansas, to Scottsboro, Alabama ...................... 15 6. Gravity and magnetic model of the igneous complex at Little Rock .................................. 19 7. Gravity and magnetic model of the Magnet Cove, Arkansas, area .................................... 22 8. Gravity and magnetic model of the Newport, Arkansas, area ........................................ 25 9. Map of the Newport, Arkansas, area showing distribution of the hypothetical +0.2 g/cm3 body from figure 8 .... 26 10. Cross sections (north-south and east-west) from figure 9 .................................................. 27 TABLES Page TABLE 1. Drill holes referred to in text, with depths to Precambrian or igneous rocks ................................. 2 2. Physical properties of units used in gravity and magnetic models .......................................... 3 ill BOUGUER GRAVITY OF ARKANSAS By JOHN D. HENDRICKS ABSTRACT stations (ACIC, 1973) at Little Rock and Bald Knob, Ark., through a system of 41 supplemental base The Bouguer gravity anomaly patterns in Arkansas correspond very stations. Station density is highest in the east and well with the physiographic provinces and sections of the State. These provinces and sections are, in turn, defined by the limits of separate southern parts of the State, averaging about 1 station/ tectonic events that span the range in time from late Precambrian 6 km2, while in the northern and western parts of the to Tertiary. Bouguer gravity values range from +37 to -107 mGal State, it averages 1 station/20 km2. On plate 1 a rock within the State. The highest values are associated with ultrabasic density of 2.67 g/cm3 was assumed for the Bouguer alkalic igneous intrusions and the lowest values correspond to the correction. Ouachita Mountains and Arkansas Valley, which were formed dur­ ing the Ouachita orogeny. Variation in seismically determined crustal Corrections for instrumental drift, elevation changes, thickness from north to south is also reflected in the gravity anoma­ latitude differences, and Earth curvature were applied ly; that is, the higher gravity values in the southern part of the State to all readings. The resulting simple Bouguer gravity reflect a thinner crust than that in the northern part. values are accurate to greater than 0.5 mGal. Terrain Speculation on the tectonic history of the region has led to the pro­ corrections were not applied to these observations but posal of a model in which a series of tensional and compressional events (Wilson cycles) have formed the major structural features. These range from a maximum of about 4.0 mGal for stations elements, in turn, controlled the location of magmatic events. nearest major topographic features in the Ozark Regional profiles are constructed that will satisfy both the proposed Plateaus and Ouachita Mountains regions to 0 mGal tectonic model and observed gravity anomalies. Isolated gravity and in the Mississippi embayment. None of the gravity magnetic anomalies, thought to result from the presence of subsur­ anomalies discussed in this report result from the ef­ face igneous bodies, are also modeled and provide an estimate of the shape, density, and magnetic susceptibility of these bodies. fect of terrain differences. Published gravity maps that include all or parts of Arkansas, but are not included on plate 1, are those of INTRODUCTION Malamphy and Vallely (1944), Lyons (1961), Woollard and Joesting (1964), Phelan (1969), and the Society of Gravity measurements obtained within the State of Exploration Geophysicists (1982). In addition to the Arkansas have been compiled and plotted as a contour gravity maps, a total field aeromagnetic map covering map of the simple Bouguer anomaly (Hendricks and northeast and central Arkansas (Hildenbrand and others, 1981). The purpose of this report is to interpret others, 1981) is available, and data from that map are the major anomalies shown on this map and to relate used in this report to supplement interpretation of the anomalous patterns and trends to tectonic events. gravity anomalies. The Bouguer gravity map of Arkansas (pi. 1) is a direct reduction of the map of Hendricks and others (1981), which represents the compilation of more than ACKNOWLEDGMENTS 17,000 measurements obtained by the U.S. Geological This report was prepared in cooperation with the Survey and Brown Geophysical Company.1 The near­ Arkansas Geological Commission. I wish to thank Nor­ ly 14,000 gravity observations made by the U.S. man F. Williams, Director of the Commission, and all Geological Survey for this report were obtained from staff members who provided laboratory, field, and 1973 to 1978. Readings were made on points of known logistical support for collection of the data presented elevation and position. The gravity meters used have herein. G.R. Keller, University of Texas at El Paso, pro­ accuracies greater than 0.1 mGal (milligal). All readings vided supplemental gravity data which allowed a were tied to the world relative gravity reference network composite gravity map of Arkansas to be made. The following U.S. Geological Survey personnel assisted in Any use of trade names is for descriptive purposes only and does not im­ production of the Bouguer gravity map of Arkansas: ply endorsement by the U.S. Geological Survey. Boyd R. Haley, who directed and provided critical 1 BOUGUER GRAVITY OF