Micromorphological studies of the complex early Oligocene Himalayan foreland palaeosols in relation to Asian monsoon climate

Abstract

The Cenozoic Himalayan orogeny not only modified Asia's climate but also affected global climate. One of its significant outcomes is the inception of Asian monsoon climate. Though researchers have shown that changes in Himalaya and Tibetan Plateau elevations control its evolution, however reached no consensus on pre-Miocene timing of monsoon climate inception, perhaps due only limited studies in pre-Miocene Himalayan sediments. Another possibility is that we have restricted our studies in deducing a relationship between Himalayan tectonic upheavals and monsoon climate. However, priority should be to first explore any palaeomonsoon signatures since deposition of Himalayan Foreland Basin (HFB) sediments because some researchers have reported palaeomonsoon signatures during Eocene-Oligocene time. Here we report another such study from early Oligocene Dagshai palaeosols of Subathu sub-basin, India. Detailed morphological and micromorphological studies revealed abundant presence of matrix and intrusive pedofeatures in mature palaeosol profiles. Most striking evidence is the presence of FeMn oxide and carbonate pedofeatures together. The concretions present in the thick Bk horizons having diffuse boundaries show intermixing and/or alternation of calcite and iron oxides. Also occurrence of pedogenic carbonate nodules is not restricted in Bk horizon as is observed in present monsoon dominant regions and nodules are impure, dense and largely micritic, similar to those documented in Indo-Gangetic Holocene soils. Further, most common FeMn oxide nodules are typical, impregnative and associated with granostriated fabric which is indicative of soils formed under seasonal wet and dry cycles. Furthermore, occurrence of illuvial coatings and carbonate together in the B-horizon indicate prevalence of sufficient wet and dry cycles respectively. All these features indicate presence of seasonal climatic conditions and therefore monsoon conditions prevailed during early Oligocene time which is contrary to the most common known Late Miocene age of Indian Monsoon inception.