Abstract

Thirty-four K-Ar ages and supporting paleomagnetic measurements from flows of the Cima volcanic field provide a detailed volcanic history and a temporal basis for analysis of evolving landforms. The Cima field has undergone three periods of volcanic activity, spanning late Miocene through latest Pleistocene time: (1) 7.6 to 6.5, (2) 4.5 to 3.6, and (3) 1.0 to at least 0.015 m.y. ago. These precisely dated sequences afford a unique opportunity to quantify the long-term effects of pedogenesis and erosion on volcanic landforms in an arid environment. Morphologic and pedologtc data indicate that the most stable geomorphic surfaces in the field occur on flows between 0.25 and 0.75 m.y. old. Younger flows are dominated by eolian aggradation and outcrop rubbling, and older flows are dominated by surface runoff and fluvial dissection. A process-response model involving progressive pedogenesis and a subsequent shift from infiltration to surface runoff is proposed to explain this temporal variation in land-surface stability.

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