Depositional environment of the late Paleocene-early Eocene Sinjar Formation, Iraq: Implications from facies analysis, mineralogical and geochemical proxies

Abstract

Reconstruction of the depositional environment of the Paleocene-Eocene Sinjar Formation from two outcrop sections in northwestern and northeastern Iraq has been evaluated using the traditional petrographic and facies analysis supported by X-ray diffraction and scanning electron microscopy with a focus on the Paleocene-Eocene (P-E) transition boundary. To this end, major and trace elemental geochemistry was conducted and various paleoenvironmental proxies for the paleoredox, paleoclimate, paleosalinity and paleoproductivity were determined in order to evaluate the changes in widely acknowledged environmental and climatic indicators and the elemental enrichment/depletion across the P-E boundary. The redox-sensitive trace element enrichment and the ir ratios (V/V + Ni, V/Cr, and U/Th) indicate that normal oxygenated circumstances prevailed during the late Paleocene deposition, and that anoxic conditions and a gradual commencement of oxygen depletion occurred during the early Eocene deposition. The coeval increase in the P₂O₅ content, P/Ti, and P/Al ratios in the Eocene sediments suggests an increase in nutrients and primary productivity due to the effect of upwelling currents during early Eocene. The conditions can be verified by observing a small change in salinity levels from low to high across the P-E boundary, which can be indicated by the Sr/Ba ratios. In addition, certain minerals such as Mg-calcite, dolomite, and palygorskite are commonly present, and paleoclimatic changes can be observed across the P-E transition from arid to semiarid and then to humid conditions, which can be recorded from C-values, Sr-Cu, Rb/Sr ratios, and clay mineralogy. These conditions were noted in the Sinjar Formation, which is made up of many microfacies such as lime mudstone, wackstone, packstone, grainstone and boundstone. These microfacies were deposited in a shallow marine environment that extended from tidal flats to reef slopes, with a developed reef environment that included back reef, reef core, and fore reef environments.

Keywords: Depositional environment; Facies analysis; Iraq; Paleoclimate; Paleoproductivity; Paleoredox; Paleosalinity; P–E boundary; Sinjar Formation.