GEOLOGICAL SURVEY CIRCULAR 489

A MAGNETIC ANOMALY OF POSSIBLE 

ECONOMIC SIGNIFICANCE IN 

SOUTHEASTERN MINNESOTA

Prepared in cooperation with the Minnesota Geological Survey
A MAGNETIC ANOMALY OF POSSIBLE ECONOMIC 

SIGNIFICANCE IN SOUTHEASTERN MINNESOTA

By Isidore Zietz

Prepared in cooperation with the Minnesota Geological Survey
United States Department of the Interior

STEWART L. UDALL, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on application to the U.S. Geological Survey, Washington, D. C. 20242

1964
A MAGNETIC ANOMALY OF POSSIBLE ECONOMIC 

SIGNIFICANCE IN SOUTHEASTERN MINNESOTA

By Isidore Zietz

ABSTRACT

An aeroniagnetic survey in southeastern Minnesota by the U. S. Geological Survey in cooperation with 

the State of Minnesota has revealed a high-amplitude, 

linear, and narrow magnetic feature that suggests a 

possible source of Precambrian iron-formation of 

economic value.

For the past few years the U. S. Geological Survey 

has been conducting detailed geophysical studies of the 

midcontinent gravity anomaly a broad, high-amplitude feature that extends from Lake Superior through 

the States of Minnesota, Iowa, Nebraska, and part of 

Kansas. As part of this study an aeromagnetic survey 

of the southern part of the State was made in cooperation with the State of Minnesota during the summer of 

1963, in which a linear high-amplitude anomaly of the 

order of 4,000 gammas was discovered. Because of the 

high amplitude, the linearity, and the narrowness of 

the magnetic feature, it is believed the source may be 

Precambrian iron-for mat ion of possible economic 

value.

The anomalous area is in Fillmore County, approximately between the towns of Lanesboro and 

Peterson in the extreme southeastern part of the State. 

(See figures 1 and 2.)

At the site of the anomaly, Cambrian sedimentary 

rocks occur in the valley of the Root River, and Ordovician rocks (nearly flat lying) mantle the upland areas. 

The uplands are largely covered by glacial deposits, 

which are relatively thin (Paul K. Sims, written communication, 1964). Depths to the Precambrian are 

estimated to range from 500 feet to 1,000 feet below 

the surface.

The aeroniagnetic map shown in figure 2 was 

compiled from continuous magnetic profiles made 

along east-west flight lines 1,000 feet above ground, 

and spaced approximately 1 mile apart. Contour intervals of 20, 100, and 500 gammas were used depending 

on the intensity. The instrument for the survey was a 

flux-gate type magnetometer (AN/ASQ-3A) which 

measures total-field variations.

The contour map displays variations in magnetic 

pattern which are typical of shallow Precambrian 

rocks. Anomalies of the order of 1,000 gammas are 

shown along the east and west edges of the map. The

outstanding feature is the previously mentioned linear 

positive anomaly that trends northeast and reaches a 

peak of 3,960 gammas. The positive anomaly is contoured from data on four consecutive profiles, but only 

two show high amplitudes. The high-amplitude anomalies along traverses 1 and 2 are shown in figure 3. 

Depth calculations suggest that the source of the 

anomaly lies about 1,000 feet below the surface. Assuming a dikelike source and magnetization resulting 

entirely from induction in the earth's field, several 

calculations were made in an attempt to fit the magnetic profile taken along the line AA' (see figs. 2 and 

4), considered to be a typical cross-section of the 

magnetic anomaly. Comparisons are shown between 

observed and computed profiles. The fixed parameters 

used were (a) distance from detector to source of 

2,000 ft; width of dike of 5,000ft; dip of dike of 75°, 90°, 

105°, and 120°, as shown. The best fit occurs when the 

dike is vertical or dips 75° to the southwest. For these 

cases, the susceptibility, k, is computed to be 0.016 

c.g.s. units, and is comparable to k = 0.02+ calculated 

by Bath (1962) for the relatively unmetamorphosed 

iron-formation of the Main Mes'abi district in Minnesota where the induced magnetization was most likely 

the dominant magnetization. If the dominant magnetization for the anomaly in Fillmore County were 

remanent rather than induced, the economic importance of the anomaly would be greatly reduced.

This anomaly seems sufficiently promising to 

warrant further geologic and geophysical investigation. 

Detailed ground magnetic and electrical studies would 

be useful to delineate the feature. In the final analysis, 

however, the presence of iron-formation can be determined only by the drill.

REFERENCES

Bath, Gordon D., 1962, Magnetic anomalies and magnetizations of the Biwabik iron-formation, Mesabi 

area, Minnesota: Geophysics, v. 27, no. 5, p. 627- 

650.

Craddock, Campbell, Thiel, E. C., Gross, Barton, 

1963, A gravity investigation of the Precambrian 

of southeastern Minnesota and western Wisconsin: 

Jour. Geophys. Research, v. 68, no. 21, p. 6015- 

6032.
9Z°00' 

R. II W, R. 10 W. R. 9 W. R.6 W. R.7 

5 MILES

Figure 1. Topographic map of parts of Fillmore and Houston Counties, Minnesota.
92°00 

92*00

5 MILES

Figure 2. Aeromagnetic map of parts of Fillmore and Houston Counties, Minnesota. Values are total magnetic intens

Contour interval is 20, 100, and 500 gammas. Flight level is 1,000 feet above the ground. Magnetic surve

P. W. Philbin. Compilation was made by Francis Gilbert.
Line

Line 2

92°

30'

R.II W. R.IO W. R.9 W. R.8 W. 

Figure 3. Aeromagnetic profiles for parts of Fillmore and Houston Counties, 
4000

3000

(/>

4

z

Z 2000 

o

z 1000

_i 

£ 

g

-1000

NW

 OBSERVED

 COMPUTED

10 20 30

NW

10

SE NW SE IM

20 30 40 0 10 

THOUSANDS OF FEET

20 30 0 

OBSERVATION LEVEL

*= MAGNETIC SUSCEPTIBILITY 

w=5000 FEET

"IT

k sO.0162 

Figure 4. Comparison of observed and computed profiles for line A-A 1 in figure 2. Calcul