SOIL-STRATIGRAPHIC FRAMEWORK, TIMING OF RECENT EOLIAN EVENTS, AND PRESERVATION OF ARCHAEOLOGICAL SITES ON THE PAJARITO PLATEAU, NEW MEXICO

Stratigraphic relationships and soil characteristics provide a geomorphic framework for Coalition Period to Classic Period Ancestral Puebloan, Archaic, and Paleoindian sites on the Pajarito Plateau near Los Alamos, New Mexico. The record of eolian and colluvial deposition on mesa tops and within canyons indicates periods of widespread deposition during the latest Holocene (generally < 1 ka deposits) and during the late Pleistocene to early Holocene. Ancestral Puebloan sites are partially buried, typically by eolian deposits on mesa top settings or by colluvial and/or eolian deposits in canyon settings, and are generally in good archaeological context. It is inferred that 15 to 20 cm of eolian deposition occurred sometime after the Middle Coalition Period but prior to the Classic Period (i.e., ca. 1250 to 1325 AD), and in many cases Coalition and Classic Period sites can be differentiated based on soil stratigraphic relationships. Mesa top sites are underlain by 0-1.5 m of Pleistocene and Holocene deposits overlying the 1.22 Ma Bandelier Tuff, recording a sequence of discontinuous, truncated late Pleistocene through mid to late Holocene soils that represent episodic eolian deposition and soil formation followed by erosion. In canyon settings, early to middle Holocene deposits are less extensively preserved, except in some canyon bottoms, recording net erosion during the Holocene across most of the landscape. Although the older deposits are typically buried in mesa top settings, some Archaic and Paleoindian sites have been exposed in excavations in good archaeological context. Early to middle Holocene colluvial deposits are sometimes preserved as swale- and valley-fill deposits, or in aggrading toe-slope settings, locally preserving Archaic sites in canyons. The archaeological context of artifacts at Archaic sites can be affected by colluvial transport and extensive bioturbation. The relative paucity of pre-1 ka Holocene deposits, absent from many locations, likely skews the archaeological record against preservation of Archaic sites. Although few sites have been recorded, the presence of late Pleistocene to early Holocene deposits in many geomorphic settings indicates that the proper conditions exist for preservation of Paleoindian sites on the Pajarito Plateau.