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Sampling of the several type sections of the Tesuque Formation, lower Santa Fe Group, exposed near Espanola, New Mexico, has yielded a composite magnetic-polarity stratigraphy in which the 760-ft (230 m), fine-grained, tuffaceous middle (Skull Ridge) Member is characterized by a long, uninterrupted interval of reversed polarity; approximately 900 ft (275 m) of the coarser grained upper (Pojoaque) member is characterized by rocks predominantly of normal polarity. Laboratory results from AF demagnetization in fields up to 200 oe, and from the acquisition and removal of IRM (often of multiple specimens cut from single samples), indicate that the magnetic signals in these rocks, probably borne by detrital magnetite, are a record of the geomagnetic field at or near the time of deposition. The stratigraphic utility of the results is confirmed by replication in geographically separate, lithostratigraphically equivalent sections in different fault blocks. Within the context of the medial to late Miocene time span long acknowledged for this part of the Santa Fe Group, this composite polarity stratigraphy appears to correlate best with the standard Tertiary polarity time scale as follows: fossiliferous Nambe Member: early Epoch 16; Skull Ridge Member: late Epoch 16 and early Epoch 15; Pojoaque Member: mid-Epoch 15 and younger. These results indicate an age for at least the Pojoaque Member significantly younger (by about 3 to 4 m.y.) than fission-track ages recently reported for ash beds in the type Pojoaque section.
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