Abstract

Several of the type sections of the Tesuque Formation, lower Santa Fe Group, exposed near Española, New Mexico, have yielded a composite magnetic-polarity stratigraphy in which the 230-m, fine-grained, tuffaceous middle (Skull Ridge) member is characterized by a long zone of uninterrupted reversed polarity; approximately 300 m of the coarser-grained, upper (Pojoaque) member is characterized by rocks predominantly of normal polarity. Results from AF demagnetization in fields up to 300 oe and from the acquisition and removal of IRM indicate that the more stable magnetic remanence in these rocks is probably borne by detrital magnetite and therefore can be taken as 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 constraints of a middle to late Miocene age long acknowledged for this part of the Santa Fe Group, this composite polarity stratigraphy appears best to correlate with the standard Tertiary polarity time scale as follows: fossiliferous Nambé Member, early Epoch 16; Skull Ridge Member, late Epoch 16 to early Epoch 15; Pojoaque Member, mid Epoch 15 to as young as early Epoch 12. The results indicate an age for at least part of the Pojoaque Member significantly older (by about 1.5 to 3.5 m.y.) than fission-track ages recently reported for ash beds in its type section. They are, however, more consistent with age assignments of these sediments based on placement of their contained mammalian faunas in the North American Land Mammal Age chronology.

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