Clay minerals associations of the Upper Neoproterozoic-Lower Cambrian marls of the Salt Range Formation, Pakistan

The peculiarities of the mineral composition of the pelitic fraction of marls of the Upper Neoproterozoic- Lower Cambrian Salt Range Formation are considered as a reflection of the influence of regional and global factors on the formation of clay minerals. The pelitic fraction of 53 marl samples of Salt Range Formation taken in the Salt Range in the Khewra Gorge was studied: from the Sahwal Marl Member (48 samples), the Bhandar Kas Gypsum Member (2 samples), and the upper part of the Billianwala Salt Member (3 samples). According to a set of analyzes (X-ray diffraction (XRD), scanning electron microscopy (SEM) with energydispersive X-ray microanalysis (EDX)), the determined association of clay minerals is represented by illite, chlorite, corrensite, chlorite-corrensite, chlorite-smectite, smectite, illite-smectite and in some samples also defective chlorite and defective corrensite occurs. Smectite, chlorite, and mixed-layer chlorite-smectite are magnesian trioctahedral minerals, illite is ferruginous dioctahedral, indicating their authigenic origin. A significant amount of labile minerals and phases in the associations is caused by a combination of the effects of contemporary volcanism, low brine concentration of the evaporite basin, and the presence of organic matter, mainly bitumen. Volcanic activity together with low brine concentration contributed to the formation of labile clay minerals and mixed-layer phases, and their interaction with organic compounds slowed down the processes of aggradation transformation. The increased magnesium content and the presence of magnesian clay minerals in the pelitic fraction of the studied deposits are characteristic of evaporite deposits formed from SO4-rich seawater type, which is consistent with the sulfate seawater type in the Neoproterozoic.