Early Oligocene paleosols of the Dagshai Formation, India: A record of the oldest tropical weathering in the Himalayan foreland

Abstract

This study reports paleopedological features of the fossil soils that formed during the earliest phase of continental sedimentation in the Himalayan foreland. The fluvial sequence of the Dagshai Formation (31.6±3.9Ma to 30.3±3.9Ma) exposed along the Koshaliya River, NW Himalaya, contains four pedofacies (named Pedofacies A–D) of ferruginous paleosol sequences contained within overbank sediments. The Dagshai Formation unconformably overlies the marine Subathu Formation. Pedofacies A consists of 3–4 well-developed ferruginous paleosols overlain by gray sandstone beds. Pedofacies B–D are marked by a progressive decrease in pedogenesis. These paleosols occur as 0.5m to 1.5m thick Bw/Bt/Btk/Bk/Bss horizons that are marked by extensive development of rhizoliths, pedogenic carbonate, and iron-rich clay pedofeatures that correspond to modern Entisols, Inceptisols, Alfisols and Vertisols. Based on early Oligocene paleogeographic position of the northward-drifting Indian Plate, it is inferred that these paleosols were formed at ~18°N paleolatitude in the Dagshai sub-basin in the Himalayan foreland. Micromorphology, geochemical analyses, weathering indices, and stable isotope composition of paleosols indicate tropical climate (paleoprecipitation of 947–1256mm and paleotemperature of ~25°C) with an initial phase of monsoonal conditions during pedogenesis. These paleoclimatic conditions favored C3 paleovegetation immediately after the transition from greenhouse to icehouse conditions.