Quaternary history of the western Espanola Basin, New Mexico
David P. Dethier and Steven L. Reneau
Quaternary deposits and landforms are exposed widely in the Espanola Basin, where base level has fallen >150 m since early Pleistocene time and drainages have eroded weak Cenozoic rocks, stranding ancestral piedmont and axial channel deposits beneath terraces that flank the Rio Grande valley. Extended periods of high flow on drainages that carried pluvial and glacial runoff probably produced middle and late Quaternary incision. The Bandelier Tuff (1.6 and 1.2 Ma), Lava Creek B ash (0.62 Ma) and the El Cajete pumice ( 50-60 ka) provide local age control, supplemented by ages estimated from amino-acid ratios in gastropods and by 14C ages. The Pajarito, Rendija Canyon and Guaje Mountain faults displace Pleistocene deposits; the latter faults were probably active in the Holocene. The NE-trending Embudo fault zone has been the northern structural margin of the Espanola Basin since middle Miocene time, but it is difficult to demonstrate Quaternary slip along the zone. Quaternary fluvial deposits consist of axial-channel gravels and thin overbank deposits covered by coarse sand and gravel delivered by ancestral alluvial fans. Sequences of lithologically similar deposits are preserved at heights from more than 170 m to 15 m above the modern floodplain between Abiquiu and Otowi Bridge. These sequences suggest that alluvial fans expand across the Rio Grande floodplain during transitions from pluvial to interpluvial climate and that deposits are preserved when the river cuts down during the next pluvial period. Holocene cut-andfill sequences arc preserved along some tributaries to the Rio Grande, but their climatic significance is uncertain. Incision along the Rio Grande in upper White Rock Canyon generated massive slumps beginning in middle Pleistocene time as the river cut through weak rocks of the Santa Fe Group, the Puye Formation and Pliocene lacustrine deposits. Reactivation of toe areas of some slumps dammed the Rio Grande at >43 ka and at least four times between 18 and 12 ka, producing lakes as deep as 60 m. Landslide motion and lake age in White Rock Canyon coincide with the glacial maximum and final pluvial pulses of the latest Pleistocene, suggesting that climate change destabilized older slumps through toe incision or by greater pore pressures from increased groundwater recharge. The Rio Grande flowed at its modern grade before about 12 ka.
- Dethier, David P.; Reneau, Steven L., 1995, Quaternary history of the western Espanola Basin, New Mexico, in: Geology of the Sante Fe Region, Bauer, Paul W.; Kues, Barry S.; Dunbar, Nelia W.; Karlstrom, K. E.; Harrison, Bruce, New Mexico Geological Society, Guidebook, 46th Field Conference, pp. 289-298.