Physical, chemical and biological changes at geological boundaries: causes, consequences and clues based on the study of Indian sections

Abstract

Significant and abrupt biological changes are known to occur at the geologic boundaries and are invariably accompanied by some specific physical and chemical changes in the sedimentary deposits. At the Cretaceous-Tertiary Boundary (KTB), for example, shocked minerals, nickel-rich spinels, anomalously high iridium concentration and a variety of other physical and chemical markers have been found. It is generally believed that the biological changes are a consequence of physico-chemical processes which are triggered by some high energy, physical events such as a bolide impact, supernova explosion in the vicinity of the Earth or volcanic episodes. To understand the cause and nature of these physico-chemical processes, a high resolution stratigraphic analysis of physical, chemical, geological, environmental and biological markers of stratigraphic boundaries has been made over the past few decades.

In this article, we summarise the main features of some important bioevent horizons based on typical stratigraphic sequences and then dwell upon various processes which could possibly be responsible for them. The Cretaceous-Tertiary and Permian-Triassic boundaries are discussed in some detail. The clues obtained from the study of stratigraphic sections from India are used to build possible scenarios prevailing at these bioevents.