The Magnetic Properties of the Oppelo Breccia Revealed

The Oppelo breccia is one of the few carbonatite breccias that occur in the Arkansas valley (eastern Arkoma basin). This breccia outcrops on the Payne Farm at Oppelo, Arkansas. Numerous, small, apparently random and poorly exposed outcrops and well discoveries of carbonatite breccias and lamprophyres occur within the valley from Morrilton to Perryville. The Oppelo breccia is Cretaceous age and dated at 96 Ma on K-Ar phlogopite. Shale and sandstone of the Pennsylvanian Atoka formation is the host rock intruded by the breccia.

A high-resolution survey, using a cesium land base magnetometer was conducted in 2001 to study the Oppelo breccia that was described by Croneis and Billings (1927). This type of magnetometer is used to measure the total magnetic field of the Earth. The current work is expansion of the 2001 survey to cover the larger area to include the anomalous body.

A high-resolution ground magnetic survey was conducted in 2008 with the objective of studying the morphology and the geologic setting of the Oppelo breccia. The magnetic data was collected from continuous profiling of 28 profiles, each at 300 meters length, from an east-west direction using a ten-meter spacing. This will give an area of 300x300 meters that was covered by the survey. Two and three dimensional imaging reveals 10% over the threshold anomaly of the relatively small exposure of the breccia. The detailed analysis of the magnetic data indicates that the breccia body is much larger than the surface exposure. The data also indicates that the shape of the anomalous material is elliptical, with the long axis extent in the east-west direction. The detail of the magnetic anomaly is not uniform. It is rather more complex with many areas that indicate pipe-like structure. The main anomaly is restricted within a 120x250 meter area.