Palaeoenvironmental Study of the Gurpi Formation at Farhadabad Section (Northeast of Kabirkuh) based on Palynofacies and Organic Geochemistry Analysis

The Gurpi Formation at Farhadabad section in west of Darehshahr, was studied in detail for environmental changes and water–level fluctuations via using organic matter contents of the rock samples. Depositional environment interpretation was done based on integration of evidences derived from organic geochemistry analysis (TOC, TIC and Aliphatic n-alkane (short, mid and long chain n-alkane to sum of n-alkane, TAR (terrigenous-aquatic ratio), CPI (Carbon Preference Index), ratio of C17 to C27 and Isoperinoids (pr/ph,pr+ph/nC17+nC18, pr/nC17and ph/nC18) and palynofacies data (Amorphous organic matter, marine palynomorphs and palynomaceral). Statistical studies on palynomorphs confirm the results gained from the organic geochemical analysis. High ratio of palynomaceral and high TAR and CPI and dominance of long chain n- alkane and low ratio of C17/C27 (C17 relative to C27) and high ratio of Pr/Ph (>1) indicate regression condition at the base of the Gurpi Formation, Lopha Member, top of Emam-Hassan Member and Cretaceous – Tertiary boundary. On the other hand high abundance of marine palynomorphs and transparent amorphous organic matter and unimodal n-alkane with a marked even over odd predominance (CPI <1) and TAR <1 and high C17/C27 (C17 relative to C27) and low ratio of pr/nC17<0.5 indicate transgressive condition in upper part of Globotruncanita stuartiformis biozones, black shales between Lopha and Emam- Hassan, upper part of Contusotruncana contusa biozones and at the base of purple shale of the Pabdeh Formation. Maximum flooding surface probably equates the 70 Ma MFS K175, 68 Ma MFS K180 and 58 Ma MFS Pg10 of Sharland et al., 2001 that is recorded in many parts of the Arabian Plate. The relative sea level curve resulted in form this study is well in accord with the existing eustasy curves especially for the Campanian- Maastrichtian boundary indicating effect of eustasy on relative sea- level changes during this period. Combining all these data, an open marine environment with terrestrial organic input under suboxic to anoxic conditions can be concluded for the depositional course of the Gurpi Formation.