Abstract of Holocene Stream Aggradation and degradation, Pajarito Plateau, northern New Mexico presentation

Reneau, Steven; Gardner, Jamie; Kelson, Keith; Hemphill-Haley, Mark

Radiocarbon dates obtained from three canyons on the Pajarito Plateau, northern New Mexico, document alternating periods of Holocene stream aggradation and degradation that are, at least in part, synchronous between canyons. Four or more Holocene terraces are present in Frijoles Canyon, which has been rapidly incising into easily eroded, non-welded 1.1 Ma Bandelier Tuff. The oldest of these is an extensive fill terrace that is uncertain by up to 13 m of stream gravels. This fill terrace records the most significant period of Holocene aggradation in Frijoles Canyon, and a date d 6.2 ka from basal slopewash burying the stream sediments provides a minimum limiting age for this terrace. After 6.2 ka, temporary pauses in channel incision are recorded by strath terraces dated at ca. 4.6 and 1.6 ka, and an intermediate undated strath terrace. Age data obtained from Rendija and Cabra Canyons, 13 km to the north, indicate periods of stream aggradation similar in age to those in Frijoles Canyon. A minimum of two Holocene terraces are present in Rendija Canyon, and dates of 6.1-6.3 ka from stream sediments beneath the higher terrace and from overlying slopewash correlate it with the major fill terrace in Frijoles Canyon. In Cabra Canyon, which has a smaller drainage area than Frijoles or Rendija Canyons, the unchanneled valley floor is underlain by more than 7 m of unconsolidated sediments, and four or more unconformable sedimentary packages were exposed in a 4-m deep trench. The lowest exposed Holocene sediments yielded an age of ca. 5.7 ka, and three overlying units that back-fill paleo-channels yielded ages of 3.5-3.8, 1.6, and 0.3 ka. In all three canyons the modem floodplains are aggrading, and radiocarbon dates of 0-0.4 ka from the uppermost sediments of the valley floors suggest these streams have been near present level for several hundred years. The similar ages obtained from stream deposits in Pajarito Plateau drainages suggest a climatic control for at least some of the aggradation events. Similarity in ages of radiocarbon dated valley fills in Chaco Canyon (Hall, 1977), 150 km to the west, provides further evidence for a regional control on Holocene stream aggradation in northern New Mexico. Episodes of Holocene stream aggradation and floodplain development on the Pajarito Plateau may primarily occur during relatively dry climatic periods, followed by channel incision accompanying larger floods in wetter periods, as proposed by Love (1977) for the Holocene sequence in Chaco Canyon, although variations in sediment supply may also be important.