Clay minerals from rock salt of Bahadur Khel Formation, Eocene, Pakistan

Abstract

According to studies of the pelitic fraction of the water-insoluble residue of 10 samples of Eocene rock salt of the Bahadur Khel Formation (Pakistan), it was determined that the clay minerals association contains swelling chlorite, chlorite-smectite, illite and kaolinite; chlorite was identified in three samples. Non-clay minerals are represented by quartz, dolomite, less often – magnesite; one sample contains impurities of both carbonates. Swelling chlorite, chlorite and mixed-layer minerals are trioctahedral, and illite and kaolinite are dioctahedral. All identified clay minerals, with the exception of kaolinite, are authigenic. The presence of swelling chlorite in Eocene rock salt is probably caused by changes in the concentration of brines in the basin against the background of complex geological processes of this era (climate change from thermal maximum to global cooling, changes in water circulation in oceans, changes in isotopic composition of carbonates). The association of clay minerals of Eocene rock salt, taking into account the peculiarities of its composition and the presence of swelling chlorite in it, we attributed to that formed during the SO4-rich seawater chemical type. This is also confirmed by two finds of swelling chlorite in the Triassic evaporites (rock salt of the Western Moroccan Basin, Midland marl) described in the literature, which are known to have been deposited from SO4-rich seawater. The presence of kaolinite in almost all samples is caused by its largest accumulation in sediments of this time period – terrigenous kaolinite came in large quantities from dry land and did not transform even at the stage halite precipitation. Full Text Keywords clay minerals, swelling chlorite, rock salt, Eocene, Bahadur Khel Salt, Pakistan.