Geoarchaeology of Alluvial Fans:A Case Study from Jajroud and Hajiarab Alluvial Fans in Iran

Introduction
Alluvial fans in Iran portray the extensive records of occupation by prehistoric human. In fact, alluvial fan had supplied good condition for the settlement of prehistoric human. Settlement pattern and human-environment interactions over this landform have been depended to the kind of sediments and distribution of braided rivers on the fans. Shifts in settlement patterns over these landforms are a subject that many archaeologist and geoarchaeologist have studied and researched it. Alluvial fan always becomes a landform that attracts human as a location for living. Fresh water and appropriate soil for drinking, cultivation, making pottery, making mud-brick and other activities are easily available for settlers without a need to high technology. In foot hill of Alborz and Zagros Mountains there are numerous alluvial fans and prehistoric settlements as well as many new towns and villages which are located over these landforms.

Methodology
The study area is located in Tehran and Qazvin plains, on the Jajroud and Hajiarab alluvial fans. Jajroud alluvial fan is located at the south west of Tehran. There are many prehistoric mound on this fan that belongs to Neolithic to Iron Age. Also in Hajiarab alluvial fan there are three sites that form a cluster that putative to Sagzabad Cluster including Tepe Zagheh, Tepe Ghabristan and Tepe Sagzabad located two kilometer for from each other. In this research SRTM images were used for producing the map of mounds position. For sediment analysis three trenches near to Sagzabad cluster were dug up to 10 meter deep. Over the Jajroud alluvial fan we considered artificial trench that was made for mud-brick inquiries as well as information of previous excavation and digging the new trench. In order to find new and old braided river in alluvial fan we used aerial photo of 1:55000 (1935) and 1:20000 (1967) as well as IRS satellite images (year 2008). For preparation of graphic logs and different maps and figures, ArcGIS and Freehand soft wares were used for drawing and manipulating the maps and figures.

Results and Discussion
Results showed that in both alluvial fans, tells were located at the middle and distal of the fans, In fact fine sediment in lower part of fans is a good source for pottery and cultivation and making mud brick. In Kahrizak area, lots of Kilns for making the pottery showed vast industrial activities because of appropriate soil and water. Sedimentary records on graphic logs show a very thick of fine sediments that show a sheet flow with low energy in these sites. Also in both of two fans there was fluvial sediment with coarser sediment existed near the settlements tell. It’s obvious that in Qazvin plain these fluvial sediments are coarser with higher energy perhaps because of existing torrential rain and short distance between basin and alluvial fan. In Qazvin plain fluvial and coarse sediments in 5.5 meter deep showed a high energy fluvial system. If we accepted the sedimentation rate that mentioned by Schmidt (Schmidt et al, 2011) we reach to date of abandonment of settlement (Ghabristan). Therefore we concluded that the fluvial environmental condition forced people to leave the tell. Also fan head trenching by tectonic activities and climatic changes keeps the river 12 to 15 meter deep in Jajroud alluvial fan and 8 to 9 meter deep in Hajiarab alluvial fan. It finally led people to establish their tells in the lower part of alluvial fans. In fact prehistoric human couldn’t bring up water from such deep trench and they selected the lower part of fans for living. Interpretation of aerial photos also showed the changing and migration of braided river in both two fans especially Sagzabad cluster.

Conclusion
In fact interdisciplinary approaches in archaeological research have a major role to determination of human-environmental interactions and social-economic activities of prehistoric people. In this research we tried to establish a framework for assessing the effect of alluvial fans on distribution pattern of settlements in Tehran and Qazvin plains. The study showed that in spite of this fact that these landforms create good condition for site formation but in some cases a different condition was shown due to active dynamism of themselves that led the settlement to shift over the fans. In fact High energy fluvial environment sometimes treated the settlement mound in alluvial fans. Future studies with precise sedimentology and chronological data could reveal more information about for mation of settlement and even subsistence practices as well as socio-economic and cultural situations in prehistoric era.