Quantitative palynological studies for paleoecology and sequence stratigraphy of purple shale in Malekshahi section

Zagros sedimentary basin is one of the most important geological areas based on hydrocarbon potential in the world (Alavi, 2004) located between central Iran and the Arabian plate (Talebian & Jackson, 2004). Purple shale is one of the informal members at the base of the Pabdeh Formation in the Zagros basin (James & Wynd, 1965). This member in the Arabian plate was correlated to Rus and upper parts of Umm er Radhuma Formations (Ziegler 2001). We aimed to study the paleoenvironment and sequence stratigraphy of a section in the SouthEast of Malek-Shahi city, Ilam Province (West of Iran).

Purple shale member in this section is composed of three lithological parts (lower purple shale, middle gray shale, and upper purple shale). One hundred and eleven palynological slides related to the 37 rock samples have been prepared and studied. In sample preparation, we used HCl, HF, and ZnCl2 to extract palynological particles from carbonate and silicate minerals. All palynological slides were studied using a transmitted microscope. Three hundred and fifty palynological particles (including palynomorphs, structureless organic matter and phytoclasts) per one slide have been counted and relative abundance graph lines and charts including a palynological chart (Tyson 1993) have been plotted.

Palynofacies analysis and triangle chart can be used in paleoenvironmental reconstruction (Tyson, 1993). Plotting data on Tyson chart show four palynofacies areas including VI, VII, VIII, and IX. Most data show a high abundance of SOM (Palynofacies type IX). Based on this fact we can propose a distal suboxic-anoxic basin as the dominant paleoenvironmental conditions for this strata. Pridinioid/Gonialacoid ration in dinocyst assemblages can be used in productivity rate and upwelling current (Pross et al. 2006; Radi et al. 2006). Also, we used paleoecological index taxa for the reconstruction of paleoenvironment conditions including temperature, light, and energy current. We used cool water/warm water taxa ratio considering cool water taxa (Achomosphaera, Glaphyrocysta, Spiniferites) and warm water taxa (Areoligera, Exochosphaeridium) and other factors in this study based on Helenes and Tellez-Duarte 2002; Pross et al. 2006; Tyson 1993; Oboh & and Villiers 2003; Pross & and Schmiedl 2002; Stover 1996; Travers 2007; Gasemi-Nejad 2001). Palynological data and index dinocyst taxa can be used also in sequence stratigraphy studies (Carvalho, 2006; Robertson, 1993; Habib et al. 1992). One stratigraphic sequence has been identified based on palynological data in this study. 

Palynological investigations have been carried out for the reconstruction of the paleoenvironment and sequence stratigraphic of purple shale in the Zagros Basin. Based on this study, distal suboxic-anoxic basin, oligotrophic, low productivity and cool conditions have been proposed as the main paleoenvironmental conditions for this strata. Moreover, one stratigraphic sequence has been identified based on palynological data in these strata that shows a deepening upward trend in this section.