Paleoenvironmental implications of interbasaltic volcaniclastic sediments within Late Cretaceous Deccan volcanics, India

Abstract

The Deccan volcanic province is extensively studied for the occurrence of the K–Pg boundary, Cretaceous mass extinction, associated climate change and biotic crises. The Late Cretaceous volcaniclastic sediments occurring in between the lava flow, known as bole beds, are often overlooked. These bole beds, however, are useful to understand the paleoenvironmental conditions during non-eruptive phases of Deccan volcanism. A comprehensive study involving field, petrography and mineralogical investigations focuses on a few cm to a few m-thick bole beds of variable lateral continuity.Field observations of bole beds reveal red to green colour and variable geometry i.e., massive, bedded, lensoid etc. Petrographic study reveals differences in textural and mineralogical characteristics of red and green boles. Red boles show an incipient to moderate degree of pedogenesis. The incipiently pedogenised red boles show the dominance of volcanic lithic fragments (lathwork, microlitic and vitric), scoria, plagioclase, and pyroxene. The moderately pedogenised red boles, show dominance of altered basaltic clasts (100 μm to 5 mm), iron glaebules, alteromorph and oxidized fragments set in the fine-grained clayey groundmass. The green bole, however, shows poor pedogenesis, with predominance of volcaniclastic fragments, plagioclase, pyroxene and opaques. The volcanic glasses and minerals in green boles are thoroughly replaced by green clay. X-ray diffraction study shows presence of hematite and 14 Å smectite in red bole. The green bole, however, contains 10 Å celadonite and 14 Å smectite as major clay minerals. FTIR and VNIR spectroscopy further confirm the slight difference in composition of smectite associated with bole beds, i.e., Fe, Al-rich smectite in red boles and Fe, Mg-rich smectite in green boles. Although the original material for forming red and green boles is basalt-derived, the distribution pattern of green boles is not as extensive as red boles. This indicates difference in the paleoenvironment of formation of the two bole beds. The study of red bole suggests its formation in oxic, subaerial conditions. The relatively thick red bole units (up to a few m), showing gradational contact with lower basalt and distinct pedogenic features, mark prominent breaks in Deccan eruptions. However, the cm-scaled red bole units, showing uneven boundary with lower basalt, and containing abundant volcaniclastic fragments, indicate minor breaks in volcanism. Green boles, on the contrary, indicate confined suboxic conditions in local depressions on the Deccan lava flow.