Great anomalies, usually regarded as “regional,” clutter gravity and magnetic maps. These anomalies are removed in the process of isolating residual features that are significant as basement and (or) sedimentary effects. A study of these deep crustal features on detailed regional maps of Oklahoma reveals that: (1) each large gravity anomaly has a corresponding large magnetic anomaly, and (2) the most probable depth values calculated from selected large magnetic and gravity anomalies, assuming the same source (dense rocks rich in magnetite), show a surface, well below the top of the crystalline basement, which has considerable relief. The relief includes pronounced lateral displacements along faults, all east-west in trend. The most appropriate name for this surface is the “Algoman surface,” the U. S. Geological Survey designation for the vast worldwide surface which reflects the great unconformity which followed the end of the Archean (early Precambrian). Prior to this time, it now appears, the earth had very little oxygen in its atmosphere, and minerals such as magnetite and uraninite, for example, escaped oxidation and were preserved below this Algoman surface. Widespread mapping of this interface has both tectonic and mining significance, because the positions of this surface near the present crystalline basement are commonly associated with mineralization. Speculatively, the folding of the “Archean” rocks and the relief of the Algoman surface may be in part a manifestation of great tidal movements prevalent if the orbit of the moon, projected back 2.5 billion years, was much closer to the earth.

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