Palaeoenvironmental conditions of hardgrounds formation in the Late Turonian-Coniacian of Mangyshlak Mountains, Western Kazakhstan

Abstract

A carbon and oxygen isotope stratigraphic profile has been made, for the first time, through the Late Turonian-Coniacian sedimentary sections containing regionally widespread firm- and hardgrounds of Mangyshlak Mountains, western Kazakhstan. Generally, Turonian and Coniacian time has been considered as a transitional stage between two Oceanic Anoxic Events (OAE), because of the peculiar pattern of variation of the δ13C and δ18O values. Unfortunately, there is no such record in the sections we examined, thus the Mangyshlak Sea behaved uniquely compared to the majority of seas and oceans at that time. The process of hardground formation is polygenetic but involved stopping deposition of calcium carbonate and initiation of the hardground over the large area of the sea floor. Normal causes of cessation of calcium carbonate seem unlikely and the expected drastic changes of hydrochemistry of the bottom waters cannot be detected in any of the minerals within the hardground sediments. Also, changes in climate, if there were any, are very difficult to estimate. Moreover, winnowing of the carbonate sediment is also not detectable from the characteristics of the hardground surface. Because the sedimentary sequence containing that regional hardground formation is transgressive, the most plausible reason for cessation and deposition of calcium carbonate is acceleration of the transgression. This might release sufficient amounts of carbon dioxide and bicarbonate to slow precipitation and deposition of calcium carbonate. Also, the greater distance from the shore might have reduced the supply of nutrients which decelerated photosynthetic activity, which in turn decreased consumption of carbon dioxide, thus enhancing precipitation of calcium carbonate. Some additional winnowing of calcium carbonate sediments would have helped in subsequent development of the hardground. Finally, deceleration of the transgression renewed precipitation and deposition of calcium carbonate.