A comparative study of the paleoclimate, paleosalinity and paleoredox conditions of Lower Jurassic-Lower Cretaceous sediments in northeastern Iraq

Abstract

The findings of this study are considered to be of great importance given their singularity in northeastern (NE) Iraq, as the first broader study that discusses various paleoenviromental proxies (i.e., paleoclimate, paleosalinity and paleoredox proxies) which were predominant during the Early Jurassic-Early Cretaceous period of NE Iraq. To this end, a comparative study of the mineralogical and geochemical features of the Sarki, Sehkanyian, Sargelu, Naokelekan, Barsarin and Chia Gara formations from the Early Jurassic-Early Cretaceous-age Ranya section, was carried out together with a study of the same formations from the Warte section (both sections are located in NE Iraq). Petrographic and microfacies analyses show that all six formations in both sections were deposited in tidal flat (supratidal), restricted lagoons and shallow marine to bathyal depositional environments. The paleoclimate proxies (Ca, Al, Sr/Ba, Rb/Sr and Sr/Cu) and paleosalinity proxies (Sr, Ca, Al and Sr/Ba) together with oxygen isotopes, suggest that the climate was predominantly hot and arid during the period of deposition, contrasting with periods of warm and humid climates during the deposition of the Late Jurassic-Early Cretaceous-age Chia Gara Formation. This climatic change was revealed by a shift in clay mineralogy, from a dominance of illite to increasing amounts (and even dominance) of kaolinite marking the change to more humid conditions within the Late Jurassic-Early Cretaceous period. Geochemical proxies for redox conditions (U/Th, V/Cr, Ni/Co, V/Sc and V/(V+Ni) indicate that deposition of the sediments in the examined areas occurred under anoxic (or reduced oxygen) conditions. This difference in paleoclimate, paleosalinity and paleoredox conditions is likely to be controlled by tectonic activity in the region of northern Iraq, with the development of small silled sub-basins within the broader Zagros Basin/Zagros Fold Belt.