Abstract

Pilot Knob is an exhumed volcano of Cretaceous age, composed of 'serpentinized' pyroclastics and minor amounts of basalt in both intrusive and extrusive masses. The geology of Pilot Knob was re-examined, and gravity and magnetic observations made and interpreted, in order to present a complete picture of the feature itself, its history, its relation to the region and area surrounding it, and the resemblances between it and the serpentine plugs in the neighborhood, to which it is geologically related. Some of these plugs have been discovered by geophysical means, and some so discovered have produced oil; the application of gravity and magnetic data to such discoveries is analyzed.The extrusive masses are here reported for the first time, and other evidence is given for the age and volcanic nature of Pilot Knob. The observations reveal 1) strong gravity and magnetic anomalies over the central basalt mass, 2) a pattern of weaker anomalies probably caused by flows and dikes and suggesting that Pilot Knob is situated near the intersection of two sets of fractures, and 3) evidence that 'serpentinized' pyroclastics show weak magnetic anomalies and (in the local setting) no visible gravity anomalies.

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