Landscape changes, terraces and deposits of the river Pulvar (Fars, Iran): new perspectives and new chronology derived from OSL dating
Résumé
This work is implemented in the frame of an Iranian-French archaeological project focusing on the area of Pasargades, an Achaemenid capital founded in the middle of the 1st mill. BC, located in an intermountaneous flooding plain in the Zagros. The goal of the project is to understand the regional human occupation, consequently deeper study of the large-scale Holocene landscape dynamics is necessary. This project is implemented under the agreement of the of the Iranian Centre for Archaeological Research (ICAR) with the support the Pasargadae World Heritage Site Office, the Shahid Beheshti University, the French National Agency for Research (ANR), and the French Foreign Ministry.
Critical results concerning landscape changes in mountain environment has been produced. Especially for the Holocene river dynamics, three main phases have been characterized corresponding to three alluvial terraces (T1 to T3) mapped and analyzed over the whole studied area.
A large scale phase of alluvial deposits occured from the beginning of the Holocene until middle or even recent Holocene. It is characterized by a thick fine sediment layer visible throughout the region which fills plains and valleys. This deposit was cuted as terrace 15 meters up to the river (T1). The controlling factors of this accumulation appears to be climate and geological structure. Holocene climatic optimum (8000-5000 BP) increased the erosion of previous levels and especially sandy-marly quaternary deposits. This would explain both the high carbonate content and the fineness of the particles, because the quaternary deposits consist partly on the same type of sediment. Furthermore the increase of the precipitations and the vegetation covering could explain the strong cutting of the T1 terrace. An Achaemenid dam built over the T1 terrace gives a relative date to this phenomena (at least 500 BC). OSL dating (results to be received during the first 2019 semester) will precise this event.
The thickness and granularity of the sediments in T1 are stricking, in such an environment. The steepness of the side slopes, the former Pleistocene deposits and the presence of intermountain plains downstream of the studied section (Tang-e-Bolaghi gorge) could have played a decisive role.
The second terrace (T2) is linked to reactivation of the alluvial dynamic, with coarser deposit in a context that seems relatively wet. The obtained C14 dates refer to the Islamic period as also demonstrated by a ceramic sherd found in a sequence in the plain of Pasargades.
Third terrace (T3) is less visible but still present as benches along the present river bed sides. It shows the sub-actual river dynamics, in elevation perhaps due to the excessive puncture of alluvium in the Pulvar bed.
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