Depositional facies and Hohokam settlement patterns on Holocene alluvial fans, north Tucson basin, Arizona

Abstract

The distribution of depositional facies on nine Holocene alluvial fans of varying dimensions is used to evaluate prehistoric Hohokam agricultural settlement patterns. Two facies are recognized: channel gravelly sand facies and overbank silty sand facies. No debris flow deposits occur. The channel facies is characterized by relatively well sorted stratified sands and gravels with common heavy mineral laminations. Overbank facies deposits are massive and very poorly sorted due to heavy bioturbation. Lithostratigraphic profiles from backhoe trenches and sediment size analysis document headward migration of depositional facies which results in fining upward sequences. Each sequence is a channel fan lobe with an underlying coarse—grained channel sand which fines to overbank silty sands. Lateral and vertical variations in facies distributions show that depositional processes are affected by drainage basin area and distance from fan head. Gravelly channel sands dominate at the headward portions of the fan and are more pervasive on larger fans; overbank silty sands are ubiquitous at fan toes and approach closer to the fan head of smaller alluvial fans. When depositional facies are considered as records of water flow over an alluvial surface, the farming potential of each fan can be analyzed. Most Hohokam agricultural settlements occur on surfaces dominated by overbank deposits where water evenly wets the entire fan and the need for water diversion or labor intensive irrigation is avoided. Depositional models of alluvial fan sedimentation provide the basis for understanding Hohokam settlement patterns on alluvial surfaces.