The role of geochemical and coal petrographic analyses to determine the depositional sets of Visean coal and shale sequence in Um Thora-Abu Hish area, Central Sinai, Egypt

Abstract

The Visean time at the Sinai Peninsula is characterized by the deposition of two conformable formations of a siliciclastic (Abu-Thora) resting upon a calcareous (Um-Bogma). The Abu-Thora Formation (Fm) is informally divided into three rock units that comprise a lower and upper sandstone sandwiched with a middle coal and shale sequence. A combination of paleoenvironmental indices and ratios from petrographic and geochemical analyses of the middle sequence succeed to build up the depositional paleoenvironment and may apply to any other coal and shale sequence, especially in coal mine regions. Geochemical analysis reveals that the studied shale strata are not homogenous and can be subdivided into four units from the base to the top. The lower shale unit at the base is 4m in thickness and composed of kaolinite-rich shale (10–77%; nacrite mineral). The second is the middle shale unit of 5.10m in thickness composed of Fe-shale (Fe2O3; 2.04–16.45 wt%). The third is the coal unit of 0.55m in thickness comprising coal seam, and the fourth is the upper shale unit of 6m in thickness composed of wacke (SiO2; 71.63–72.83 wt%). X-ray diffraction (XRD), x-ray fluorescence (XRF) plus mineralogical analyses are applied on the first, second, and fourth units of almost pure shale, whilst the third unit of almost pure coal is subjected to a petrographic analysis instead to illustrate the depositional site and type of vegetation where coal was formed. The palaeoclimatology based on parameters of CIW, C-value, Rb/Sr, and Sr/Cu refers to the deposition of shale initiated at a warm-humid and then to a hot semi-arid climate. The precursor rock of the studied shale is a basaltic and intermediate igneous rock. The shale is of mature components subjected to intense weathering processes based on CIA and ICV of 91.71 and 0.34, respectively. Furthermore, discriminant functions F-1 and F-2 reveal the shale components came also from quartzose sediments that promoted a multi-source origin that was gathered from both preexisting mafic and sedimentary provenances. The accumulation and deposition took place at a marine anoxic shale-carbonate of the active continental margin and oceanic island arc tectonic setting. Maceral composition of the coal unit reveals a depositional setting of a coal-forming mire system that occurred at a paralic aquatic margin setting involving a dominated reed herbaceous vegetation in a hydrological water-logged of the limnic marsh.