Palynological Evidence for Miocene Age of Abiquiu Tuff, North-Central New Mexico: ABSTRACT

Abstract

The Abiquiu Tuff crops out at the north end of the Nacimiento uplift and in the western part of the Espanola basin in north-central New Mexico. No paleontologic or radiometric age dates have been published for this unit, but regional correlations based on lithology and stratigraphic position suggest a Miocene age. At the south end of the Nacimiento uplift are extensive outcrops of the Zia sand of early to middle Miocene age based on vertebrate fossils. Between the outcrops of these two units are scattered exposures tentatively identified as Abiquiu(?) formation. To determine if the Abiquiu Tuff, Zia sand, and Abiquiu(?) formation are stratigraphically equivalent, sections of each were analyzed for pollen content. The Abiquiu Tuff near the village of Abiquiu, the Zia sand at its type locality near Zia Pueblo, and an outcrop in the Nacimiento uplift near Gilman were sampled at 30-ft (9.3 m) intervals. Most samples were barren, but sufficient pollen was recovered to indicate a Miocene age for all of the outcrops. Pollen from the Abiquiu Tuff and Zia sand are similar to pollen from the Miocene Split Rock Formation of Wyoming and Brown's Canyon Formation of Chaffee County, Colorado. The Zia sand and Abiquiu Tuff are exposed along the west margin of the Rio Grande rift. Extensional faulting that cuts these formations cannot have occurred earlier than late Miocene. The Abiquiu(?) formation at Gilman rests against Precambrian rocks along the Sierrita fault, the major bounding fault between the south part of the Nacimiento uplift and the Rio Grande rift. The juxtaposition of Miocene and Precambrian rocks along this fault indicates that 530 to 685 m of movement associated with rifting and uplift occurred in post-middle Miocene time. End_of_Article - Last_Page 558------------