AEROMAGNETIC MAP OF THE COLORADO FRONT RANGE

The Earth has a magnetic field caused by an electric dynamo in its core. Superimposed upon the main field, which approximates the dipole field of a bar magnet, is an anomaly field caused by magnetic sources, primarily magnetic rocks in the Earth's crust. Aeromagnetic maps are made by flying an airplane or helicopter in a grid pattern over the Earth's surface, recording the data on an onboard computer, and processing the flight line data into gridded raster maps.

The colored region is an aeromagnetic map¹ of the Front Range and adjacent Great Plains, Colorado. In the mountains it was flown by a helicopter flying along east-west flight lines spaced 1.6 km apart. In the plains it was flown by a fixed-wing aircraft along east-west flight lines spaced 5 km apart. Mean terrain clearance was 120m.

In the western part of the map, where bedrock is exposed, the magnetic highs are caused by magnetic granitic rocks, such as the Longs Peak-Saint Vrain and Mt. Evans batholiths. The magnetic anomalies that form the deer's head are caused by magnetic phases of the Mt. Evans batholith.

Over the exposed magnetic bedrock of the Front Range, magnetic highs contain both long- and short-wavelength information, indicating sources that extend from the surface to great depth. A prominent magnetic low over the Pikes Peak batholith is reported to be caused by reversely magnetized rocks². This hypothesis should be tested by measuring magnetic properties of the rocks in the field.

In the eastern third (approximately) of the map, the magnetic highs lack short-wavelength detail. Conventionally, long-wavelength magnetic anomalies over basins are attributed to magnetic igneous rocks in the crystalline basement, buried equivalents to the uplifted and eroded plutons exposed in the Front Range. Short-wavelength magnetic anomalies are attenuated by distance from the buried sources, so only broad features of the magnetic field are seen at the surface. See if you can find the boundary between the bedrock upland and the Great Plains.

Notice that many of the magnetic anomalies in the plains occur adjacent to magnetic bedrock in the Front Range. Is it possible that some of the magnetic anomalies in the plains are caused by magnetic sediments eroded from magnetic plutons in the Front Range? Magnetic susceptibility transfers directly by erosion and deposition-magnetite eroded from the pluton is redeposited as black sand in the adjacent basin. Could you tell the difference between a magnetic high caused by a broad sedimentary deposit and that caused by a deeply buried batholith? If the Pikes Peak batholith is reversely magnetized, would black sand eroded from the batholith cause a magnetic high or a magnetic low?

These are some of the questions that we love to work on at GeoPeregrino. We interpret data in the office, formulating hypotheses that we then test by geologic fieldwork. Hence our emphasis on the "Field Geologic Interpretation of Aeromagnetic Maps."

John W. Cady, Consulting Geophysicist