Magnetic investigation in the vicinity of Derby, Adams County, Colorado, U.S.A.

Abstract

During the summer and fall of 1967, vertical magnetite· intensity surveys were conducted in two areas adjacent to the Rocky Mountain Arsenal well No .1, northeast of Denver, Adams County, Colorado. The surveys are described as the Manila and Eastlake areas, both of which are covered with alluvium material; the age of the underlying rocks ranges from Precambrian to Cenozoic; the buried basement metamorphic rocks are assumed to be complexly folded and faulted. The depth of the basement rocks, based on the stratigraphic logs of Arsenal well No. 1, was used as a basis for quantitative calculations of the magnetic anomalies in the areas, but the interpretation of magnetic data was difficult because of lack of sufficient drill-hole information and of variation of basement rock magnetic properties.
There is no magnetic evidence of major vertical fault displacements in the two areas; hence, the only faults in the Eastlake arc that could possibly cause tremors are the horizontal slide-strike, faults accompanied by minor vertical displacements. Amplitudes of computed and observed anomalies in both areas are in agreement at a basement depth of 11,500 to 13,000 feet. The interpretation so arrived al in the Manila area could be objectionable, but the vertical ground-magnetic patterns arc in accordance to the general aeromagnetic patterns.
The steep magnetic gradient at the Eastlake area is probably due to a magnetic susceptibility contrast of about 1,350 x 10-₆ cgs, unit between a 4-mile block of sheared rock and amphiobolitic gneisses. This rock interface may be the eastern extension of the east-west striking formations which crop out on the Front Range to the west. The causes of magnetic anomalies in the Denver basin arc varied, therefore extension of the results lo other areas in the same basin is not possible.