New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jemez River, a tributary ofthe Rio Grande, heads in the Jemez and Nacimiento Mountains of north-central New Mexico. Near its confluence with the Rio Guadalupe, the Jemez River is a medium to high order straight perennial bouldery mountain stream. Qt1 and Qt2 are mid-Pleistocene age fill terraces at this locality. Lateral extent, regional correlation based upon ash and elevation above grade, and fossil gastropod assemblages suggest the fills of Qt1 and Qt2 are ofclimatic origin. The alluvium beneath Qt1 and Qt2 record identical cycles of climatically-driven incision, equilibrium, aggradation, equilibrium and incision. Incision occurs during interglacial-glacial transitions, equilibrium during interglacial and glacial conditions, and aggradation during glacial-interglacial transitions. Evidence for this hypothesis comes from fill stratigraphy, the presence of Lava Creek B ash, paleoecological inferences, and by analogy with the modem floodplain
of the Jemez River.

The fluvial stratigraphy ofQt1 and Qt2 fills is similar. Both terrace deposits lie on bedrock. This implies incision prior to alluviation. The basal member of each fill is a 3-4 meter thick axial-river boulder bed in planar contact with bedrock. This layer is a buried strath and represents a time of fluvial equilibrium. Overlying the boulders is a thick (3-10 m) sequence offiner channel gravels, channel sands and overbank sediments. This layer represents the valley filling interval. Fossil gastropods found within the overbank fines have living counterparts in the Jemez Mountains today but only at significantly higher (and wetter) elevations. Paleoecologic inferences drawn from these assemblages and from the discovery of Lava Creek B ash in the overbank sediments of Qt1 fill, which at an age of 620 ka falls between marine oxygen isotope stages 16 and 15, support a hypothesis that aggradation occurred during glacial-interglacial transitions. The finer fills are capped
by boulder layers similar to those beds found near the bases ofthe terrace deposits and are interpreted to represent another strath. At present, in the vicinity ofthe confluence, the Jemez River and its principal tributaries are cutting and depositing similar boulder-capped
straths on alluvium. By analogy, the gravel caps of Qt1 and Qt2 formed during interglacial periods. Terrace formation by incision would have followed the interglacial equilibrium period, probably an interglacial-glacial transition, a time when stream power was increasing and sediment supply was relatively low. The strath at the base of the fill terraces, therefore, represents equilibrium conditions that existed during glacial episodes.

This climatically-driven fluvial cycle hypothesis may explain the more complex and poorly exposed stratigraphy ofyounger Qt3 and Qt4 terraces. It does successfully predict the stratigraphy beneath the modem floodplain which, from well logs, includes the presumed Late Wisconsin strath at the fil1fbedrock interface and the overlying Late Wisconsin-Holocene transition alluvial fill.