Al-Taee Noor T, Al-Juboury Ali I, Ghafor Imad M, Zanoni Giovanni, Rowe Harry
Geology Department, College of Science, Mosul University, Iraq.
Al-Kitab University, Kirkuk, 36015, Iraq.
Heliyon. 2024 Feb 10;10(4):e25657. doi: 10.1016/j.heliyon.2024.e25657. eCollection 2024 Feb 29.
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 PO 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.
利用传统的岩石学和相分析，并辅以X射线衍射和扫描电子显微镜，对伊拉克西北部和东北部两个露头剖面的古新世-始新世辛贾尔组沉积环境进行了重建，重点关注古新世-始新世（P-E）过渡边界。为此，进行了主量和微量元素地球化学分析，并确定了古氧化还原、古气候、古盐度和古生产力的各种古环境指标，以评估广泛认可的环境和气候指标的变化以及P-E边界处元素的富集/贫化情况。氧化还原敏感微量元素的富集以及Ir比值（V/V + Ni、V/Cr和U/Th）表明，在古新世晚期沉积期间普遍存在正常的氧化环境，而在始新世早期沉积期间出现了缺氧条件和氧含量逐渐减少的情况。始新世沉积物中同时出现的PO含量、P/Ti和P/Al比值增加，表明由于始新世早期上升流的影响，营养物质和初级生产力有所增加。这种情况可以通过观察P-E边界处盐度水平从低到高的微小变化得到证实，Sr/Ba比值可以表明这一点。此外，常见某些矿物，如镁方解石、白云石和坡缕石，并且从P-E过渡期间可以观察到古气候变化，从干旱到半干旱再到潮湿条件，这可以从C值、Sr-Cu、Rb/Sr比值和粘土矿物学中记录下来。这些情况在由许多微相组成的辛贾尔组中被注意到，这些微相包括灰质泥岩、粒屑泥岩、颗粒灰岩、粒状灰岩和粘结灰岩。这些微相沉积在一个从潮坪延伸到礁坡的浅海环境中，有一个发育良好的礁环境，包括礁后、礁核和礁前环境。
