Petrography and geochemistry of Ahwaz Sandstone Member of Asmari Formation, Zagros, Iran: implications on provenance and tectonic setting

Abstract

An integrated petrographic and geochemical study of the Ahwaz Sandstone Member of Oligocene- Miocene age, Asmari Formation in Zagros, southwest Iran, was carried out to infer their provenance and tectonic setting. This study is based on the analysis of core samples from three subsurface sections (wells Az-85, Az-11, and Az-89) in Ahwaz oil field in the Dezful embayment subzone.

On the basis of the framework composition (point counting) and whole-rock geochemistry (major elements), the sandstones are classified as quartzarenite, sublitharenite, and subarkose types. Petro- graphic studies reveal that these sandstones contain quartz, feldspars and fragments of sedimentary and metasedimentary rocks. The modal analysis data of 50 collected (medium size and well sorted) samples, imply a recycled orogen and craton interior tectonic provenance. Moreover, petrographic point count data indicate quartz-rich sedimentary (recycled), middle to high-grade metamorphic, and plutonic par- ent rocks for the Ahwaz Sandstone. Tectonic setting discrimination diagrams based on major elements suggest a quartzose sedimentary provenance in a passive continental margin. As indicated by the CIW´ index (chemical index of weathering) of the Ahwaz Sandstone (average value of 67) their source area underwent “intense” recycling but “moderate” degree of chemical weathering. The petrography and geochemistry results are consistent with a semiarid climate and low-relief highlands.