Stewart Mountain Terrace: a New Salt River Terrace

Stream terraces of the Salt River form the interpretive backbone of Plio-Pleistocene landscape evolution of central Arizona, because they represent the base level of all tributary streams. This paper presents a new addition to T.L. Péwé’s Salt River Terrace sequence (in decreasing topographic position and age: Sawik, Mesa, Blue Point, and Lehi) that has been unrefined for the last 30 years. The existence of an older, higher terrace was predicted by research suggesting that the lower Salt River originated by lake overflow from an ancestral Pliocene lake in the Tonto Basin. Field reconnaissance, aerial photo interpretation, and sedimentological analysis revealed this terrace on the north side of the Salt River, named here the Stewart Mountain Terrace (SMT). Where exposed, the fluvial sediments of SMT overlay Tertiary basin fill unconformably. SMT sediments are characterized by ~50 m thick fluvial gravels found more than 70 m above remnants of the Sawik Terrace. Although the gravels are distinctly Salt River in origin, Stewart Mountain gravels differ from the lower and younger Salt River Terraces. The clast sizes are much larger on average and host a significantly different lithology. Because of these differences the SMT has profound implications for the understanding of regional drainage reorganization after basin and range extension. The existence of this terrace and its distinct gravels are consistent with, but do not prove, a lake overflow mechanism for the initiation of through flowing drainage in the Salt River Valley.