Gravity and bedrock geology study of East-Central Minnesota
Carlson, David Roy
MetadataShow full item record
A detailed gravity survey of east-central Minnesota has permitted a more precise definition of the geology associated with the west flank of the Midcontinent Gravity High. The Douglas Fault, recognized in previous studies, is outlined by a belt of steep gravity gradients. The Pine Fault, which bifurcates from the Douglas Fault, is also associated with steep gravity gradients. Two positive Bouguer anomalies near Onamia may be caused by high density intrusions similar to the basic intrusions near Freedham. Two intrusions, previously mapped as basic rocks are represented by a negative Bouguer anomaly and a positive anomaly. Further investigations are needed to determine the cause of the apparent low density contrast indicated by the negative anomaly. Theoretical models are used to define Bouguer profiles crossing the Douglas and Pine Faults. The models show the Keweenawan sedimentary formations thicken to the south and west of the Douglas Fault. The Douglas Fault is also overlain by Keweenawan formations in the south. Depths of 5.0 kilometers in the south and 5.5 kilometers in the north along the Douglas Fault are revealed as the maximum throw of the models across the Douglas Fault. Rocks having densities similar to the Keweenawan volcanics are presumed to lie beneath the Keweenawan sediments.