Anoxia and fluctuating climate recorded from the Devonian–Carboniferous black shales, Tethys Himalaya, India: a multi-proxy approach

Abstract

Due to inaccessibility and tough conditions (high-altitude low-oxygen region) very less information is available about the Indian Tethys Himalayan sequences, particularly geochemical data are scarce. An attempt is made in the present study to evaluate the sediment geochemistry and paleo-redox conditions of Devonian–Carboniferous black shales from the Spiti region, Tethys Himalaya, using multi proxy approach. Besides black shales, this paper also provides the first in-depth study of the geochemistry of phosphatic nodules (hosted by the black shales) that will allow primary and secondary processes to be understood. Geochemical investigations, particularly redox-sensitive trace element ratios (for e.g. V/(V + Ni) > 0.5; Ni/Co = 4.3–7.2; V/Cr > 2 and positive Ce anomalies), suggest deposition of these black shales under sub-oxic to anoxic marine conditions, thus recording an ocean anoxic event in the region. These ocean anoxic events of Devonian–Carboniferous Spiti black shales, which possess distinctive geochemical signatures, can very well be correlated with global events such as Hangenberg Events and the Lower Alum Shale Event. The span of the chemical index of alteration (CIA) values (54–78), variation of ∆4Si% parameter (82–3) and C values (0.1–0.6) recorded in the black shales strongly indicate varying degrees of weathering conditions in the source area, which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in Devonian–Carboniferous period in the Spiti region. The distinct geochemical characteristics such as Y, Sr and rare earth element (REE) enrichments (particularly middle REE enrichments with a “hat-shaped” pattern) with high P2O5 contents (up to 14 wt%) distinguish the nodules from the host rocks (black shales). SEM (scanning electron microscope) results, supplemented by XRD (X-ray diffraction) studies of the nodules, show the dominance of calcium, silica and phosphatic phases. The diversity of clay minerals, significant variations in mineralogical composition and lack of any continuous vertical trend in the nodules suggest a relatively low impact on the primary environmental signal by burial diagenesis.