Himalayan Geology, Vol. 41 (2), 2020, pp. 157-170, Printed in India
Late Miocene Paleoclimatic Variability in the Himalayan Foreland Basin, India: Records of prounounced arid phase amidst wetter phase around 9.1 Ma
SUBHAJIT SINHA
Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata-700019, India
Email subho.ecstasy@gmail.com
Abstract: The proxies of climatic turmoil are explicitly preserved in the sediments of the Himalayan Foreland Basin referred to as the Siwalik Group. In this study, oxygen and carbon isotopic ratios of pedogenic carbonate nodules were examined together with the adjunct clay mineralogy, geochemistry and sedimentological attributes as proxies to reveal the paleoclimatic trends and fluctuations during the most crucial time interval 12.7 to 8 Ma of earth's climatic development.
Oxygen isotopic data from the Ravi re-entrant in Himachal Himalaya reveal an overall wet climate corresponding to intense precipitation. However, a prominent phase of δ18O enrichment of the order of >4.5‰ at 9.1 Ma is observed suggesting fractionation of the heavy isotope relative to the lighter one due to excessive soil water evaporation. This arid phase is also reported from other sections studied in Nepal and Pakistan on either end but not discernable in the adjacent Kangra sub-basin. Another but rather weaker phase of mild aridity is also discernable at around 11.5 Ma.
The temporal variation trend of the oxygen isotope data suggests gradual transition towards more aridity with 18O values approaching present-day values. However, this trend was perturbed intermittently by phases of rainfall intensifications. The neoform illite polytype (Poorly Crystallized Illite, PCI) abundance is also synchronous with the 18O variation with more abundance in arid conditions compared to otherwise wet conditions while the detrital illites (Well Crystallized Illite, WCI) content decreases. The period of aridity at 9.1 Ma is further supported by sedimentological attributes like a relatively high sedimentation rate, thicker mudstone deposits and the appearance of a compound calcisol profile indicative of a dry sub-tropical climate.
Keywords: Himalayan foreland basin, oxygen isotope, clay mineralogy, aridity, Siwalik.
