NEW INSIGHTS INTO THE MAGNETIC PROPERTIES OF THE OPPELO BRECCIA IN ARKANSAS

The Oppelo breccia crops out on the Payne Farm at Oppelo, Arkansas, situated in the Arkansas Valley (eastern Arkoma basin). It is one of numerous, small, apparently random and poorly exposed outcrops and well discoveries of carbonatite breccias and lamprophyres that occur within the valley from Morrilton to Perryville. These rocks are dated at 96 Ma on K-Ar of phlogopite. The breccia intrudes shale and sandstone host rock of the Pennsylvanian Atoka Formation.

A high-resolution magnetic survey was conducted of the area for the purpose of defining the magnetic character of the Oppelo breccia. The data was collected utilizing a land-based cesium magnetometer. The survey was conducted along sixteen east-west trending profiles, ranging in length between 230 meters to about 100 meters. The magnetometer was set to collect ten samples per second which produced 16, 164 data points, in total. Analysis of the raw data indicates a strong magnetic anomaly that correlates with the location of the exposed breccia. The anomaly seems to be restricted to the west-southwest quadrant of the surveyed area. The strongest of these anomalies have values of more than 10% over the threshold magnetic level.

The partially completed magnetic survey suggests that the main structure may be cylindrical or pipe-like in shape. Furthermore, the data suggests that the main body is structurally complex. Two- and three-dimensional representation of the data indicates small but very strong anomalies possibly produced by cylindrical-shaped bodies, which are very close to the surface. At least on of these bodies indicates an inclined orientation. Data also indicates north-south trending dike-like structures paralleling a prominent joint set in the region. Although data is limited, analysis indicates no strong anomalies exist to the north and east of the surveyed area. Our current data is limited in defining the southern and western boundaries of the structure, however it delineates a quarter-circle boundary to the north and east. Future plans will be to extend the survey to the south and west to identify those boundaries.