New Mexico Geological Society Annual Spring Meeting — Abstracts

Quaternary eolian deposits provide one very important type of record of landscape evolution in the Petrified Forest National Park. The park is located in northeastern Arizona on I the periphery of the Tusayan dune field, one of the largest North American dune fields. Quaternary deposits were mapped to delineate the distribution, morphology, and soil characteristics of Quaternary deposits; this evidence, supplemented by other types of data, was used to interpret the Quaternary geologic history of the study area. The relative timing of eolian activity was constrained through characterization of stratigraphic relationships and degree of soil development in eolian deposits.

The landscapes of the study area are typical of the southern Colorado Plateau near the Little Colorado River. Where easily erodable mudstones of the Triassic Petrified Forest Formation are exposed, badlands develop; where the more resistant sandstone units outcrop,
mesas and buttes are formed. This region has experienced little structural deformation since deposition of the Triassic units. Development of the modern landscape is due largely to post-Pliocene fluvial incision. Evolution of this landscape in the Quaternary has been most strongly affected by climate change, and the landscape response to such change has been complex.

The location and stability of eolian landforms has been significantly affected by the hydrologic characteristics of Triassic, Pliocene, and Quaternary geologic materials. The study area contains three major drainage basins, Lithodendron Wash, Dry Wash, and Jim Camp Wash, which are characterized by distinct lithologies and distributions of eolian deposits. Lithodendron Wash is principally underlain by an easily erodable mudstone, eolian deposits are thin and uncommon. This basin, however, is surrounded by a thick mantle of eolian material containing well-developed soils. The mantle of eolian material is underlain by either Pliocene rocks or fluvial sediments. Dry Wash, with the most extensive outcrops of resistant sandstone, is characterized by common mesas and buttes largely devoid of eolian deposits. In contrast, its wide alluvial valleys contain abundant eolian deposits associated with moderately to weakly developed soils. The eolian deposits of Jim Camp Wash, which contains rocks intermediate between those of the other two basins, are similar to but less common than those in Dry Wash. Eolian deposits are common on the drainage divides and are associated with moderately to weakly developed soils; these deposits are typically underlain by fluvial sediments.

Age estimates for eolian deposits were determined through the careful integration of soils data, stratigraphic relationships, and landforms such as fans and pediments. The eolian deposits associated with well-developed soils are estimated to be of middle Pleistocene age, the eolian deposits with relatively moderate soil development are probably of early to middle Holocene age, and the eolian deposits which exhibit only very weakly developed soils have been deposited within the last millennia. The study area has experienced episodic eolian activity throughout the Quaternary and is very close to the threshold for eolian activity in the modem climate.