New Mexico Geological Society Annual Spring Meeting — Abstracts

A land-based geomagnetic survey was conducted over an alluvial fan complex at the northwestern end of the Sandia Mts. on the eastern border of the Rio Grande Rift. Local total-field magnetic anomalies were found. These anomalies correspond to the topographic relief associated with the major arroyos draining the fan.

Running data lines perpendicular to the drainage direction gave data show magnetic highs neclrs the arroyo edges and magnetic lows in the channel bottom. The data range of these anomalies was as great as 200 gammas, while maximum background noise was under 30 gammas.

These anomalies were modeled using a "Talwani" (magnetics) computer code. The computer generated data agreed extremely well with the measured field data. The model found the anomalies were due to the misstng material represented by the cross sectional area of the arroyo (i.e NRM of the fan) and were not due to the surficial deposits (i.e. heavy minerals) in the main channel of the arroyo.

Further analysis of the data/model shows a correspondence between down-fan location and strength of magnetization. Variation of the deposition environment (i.e. decreasing flow regime down-fan) is believed to account for this magnetization variation. In retrospect, land-based magnetic surveying has been found to be a tool in determining depositional character.