Unusual Holocene alluvial record from Rio del Oso, Jemez Mountains, New Mexico: Paleoclimatic and archaeologic significance
Stephen A. Hall and Richard D. Periman
The lower Rio del Oso valley is characterized by a single Holocene terrace that is formed by 3 to 5 m of largely overbank fine quartz sand dated by 22 radiocarbon dates between 4610 and 640 14C yrs BP making this one of the best dated sequences in the region. The alluvium contains seven cumulic A horizon paleosols, 5 to 62 cm thick, a remarkable feature seldom seen in other alluvial records. The thickest paleosol “F” formed ca. 3000 to 2000 14C yrs BP and was a valley-wide floodplain soil while the other paleosols were only local in extent. The paleosols are dark brown to very dark grayish brown in contrast to the light to pale brown nonpaleosol alluvium. The paleosols are characterized by 0.3 to 0.5% organic carbon and 8 to 9% silt, the silt originating in part by atmospheric dust influx. The alluvial sedimentation rate is 0.075 cm per calendar year, r = 0.98. The Rio del Oso record of deposition and erosion correlates with other records on the Pajarito Plateau that in turn contrast sharply with regional sequences. Specifically, southwest-southern plains alluvial valleys were trenched 1000 14C yrs BP while the Oso and Pajarito-area valleys were not and alluviation continued to 600 14C yrs BP. Since regional downcutting was a response to a shift from moist to dry climate and the Oso and other Jemez area streams were evidently not significantly influenced by this episode of climate change, the Jemez Mountains may be providing a local mesic environment, resulting in local alluvial sequences that differ from those in the greater Southwest. Several in situ prehistoric features were observed at 2 to 4 m depth in the alluvium and are dated about cal BC 3500 to BC 800. After ca. 600 14C yrs BP, the Oso valley floodplain was incised and the broad floodplain was largely removed by erosion; the Holocene terrace is a remnant of that surface. Late prehistoric agriculture on the Oso valley floodplain, whether or not irrigated, would have been dramatically changed by incision and erosion of the floodplain.
- Hall, Stephen A.; Periman, Richard D., 2007, Unusual Holocene alluvial record from Rio del Oso, Jemez Mountains, New Mexico: Paleoclimatic and archaeologic significance, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 459-468.