Abstract

Both fluvial terraces and pediment-terraces (defined as pediment-like surfaces cut in soft rock) have been observed to occur together, within the same drainage basins, throughout the Southwest. Despite their significance for resolving the genesis of land forms and the late Cenozoic history of the Southwest, few detailed studies regarding them have been reported. They are commonly explained as the product of tectonic uplift during the Pleistocene. Some have attributed terraces and pediment-terraces to climatic changes, but few agree on the specific causes. No prior study explicitly treats the mutual occurrence of terraces and pediment-terraces.

In the Tonto Basin of central Arizona are seven distinct terraces and five pediment-terraces that range from 20 to nearly 1,000 ft above the present flood plain of Tonto Creek. These two types of surfaces do not grade into one another, but are products of different erosional episodes.

Tectonic hypotheses do not account for either the regional distribution of pediment-terraces or their alternation with fluvial terraces. Therefore we favor a climatic origin, and suggest that pediment-terraces are products of arid episodes and terraces are products of more humid episodes. They probably correlate with interpluvials and pluvials of the Pleistocene.

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