New Mexico Geological Society Annual Spring Meeting — Abstracts

Little is known of the early development of the northwestern Albuquerque basin. OligoMiocene deposits exposed in the southwestern Jemez Mts. that have been previously assigned to the Abiquiu Fm. provide important information on the initial development of the Rio Grande rift in central New Mexico. The type area of the Abiquiu Fm, in the adjacent Española basin, contains three members: a lower, nonvolcanic member containing abundant Proterozoic granite and quartzite derived from the Tusas Mts, and to a lesser extent, the Sierra Nacimiento; a middle (Pedernal) member of chalcedony and chert; and an upper volcaniclastic sandstone member containing clasts of rhyolitic Amalia Tuff. Ongoing studies of the SW Jemez Mts. recognize two units underlying the ~19-22 Ma Piedra Parada Member of the Miocene Zia (Sand) Fm. These two units differ from the type Abiquiu section, so pending additional study, assignment to the Abiquiu Fm is not recommended. The lower volcaniclastic unit (LVU) is exposed along the hanging wall of the Jemez fault, west of Cañon and Gilman, where it consists of poorly sorted, volcanic-bearing sediments interpreted as debris flow facies. LVU clasts are rounded, up to 15-cm in diameter, of intermediate, basaltic, and rhyolitic composition, and were derived from an as yet undetermined source. This unit also contains minor Proterozoic and Permian clasts. Most LVU gravel contains plagioclase and pyroxene phenocrysts; fewer clasts contain hornblende, and sparser clasts contain quartz phenocrysts. The upper unit is white to tan, medium-grained sandstone that is alternately well cemented with silica and poorly cemented and is interlayered with volcanic ash. This upper unit is similar to upper member Abiquiu sediments to the north, but lacks 25 Ma Amalia Tuff. A biotite-rich ash bed from the upper unit in Cañon de la Cañada yields a ⁴⁰Ar/³⁹Ar age of 20.61 ± 0.07 Ma, younger than the 23-25 Ma determined for stratigraphically similar strata to the north. The LVU is 60 m thick near Cañon, thins to 40 m at Gilman, and appears to have filled a rugged paleotopography developed adjacent to the Jemez fault. To the south and west, on the footwall of the San Ysidro fault, a lag of Oligocene tuff clasts mark the Zia-Galisteo Fm boundary, suggesting that volcaniclastic deposits might have been more widespread than at present, and that some of the deformation along western rift-boundary faults occurred prior to Zia Fm deposition. The basal unit thins abruptly to the north and east and is generally represented by fluvial and debris flow deposits <1 m thick that contain only granitoid and volcanic clasts and local 1-m thick limestone beds. Possible sources include the San Juan, Mogollon-Datil, or OrtizEspinaso volcanic fields. Paleocurrent directions recorded by imbricated cobbles (n=28) in the lower volcaniclastic unit at one site near Cañon suggest flow toward the northwest. Paleocurrent data and the northward decrease in thickness suggest sources to the southeast. Available subsurface data from deep oil-test wells in the Albuquerque basin indicates a thick succession of volcanic-bearing sand and mud between the Zia and Popotosa fms and subjacent Galisteo Fm. The lack of coarse volcaniclastic deposits in the subsurface does not support a Mogollon-Datil source. The lack of Amalia Tuff clasts in the section does not support structural or depositional linkage between the Española and Albuquerque basins during late Oligocene to Miocene time.