ABSTRACT Green marine clays preserve critical signatures of the palaeoclimatic and palaeo‐oceanographic conditions during the climatically dynamic Jurassic Period. This study focuses on the authigenic green clays formed in the Neo‐Tethyan domain within the marine shelf of the Late Jurassic Middle Member in Spiti Shale Formation, consisting of black shale, green sandstone, grey shale, and heterolithic facies. The authigenic green clay in green sandstone typically occurs as faecal pellets and infills within bioclasts. These green clays, formed by the alteration of kaolinite and Al‐smectite substrates, show high contents of Al 2 O 3 (> 20 wt%) and K 2 O (> 6 wt%), with low Fe 2 O 3 (< 10 wt%). The 001, 002, and 003 peaks in XRD, along with the 10 Å spacing in 16‐layer laths in TEM, infer the glauconite mineralogy, with smectite interstratification. The high K 2 O content and the rosette micro‐texture of the grains indicate evolved glauconite, while their high Al 2 O 3 content confirms Al‐glauconites. Negative correlation of Al and Fe in the octahedral site suggests Fe has substituted Al in the octahedral site. Al glauconite with kaolinite substrate suggests extreme continental weathering in humid climatic conditions. Restricted Fe 2+ mobilisation in shallow marine conditions results in a low TFe 2 O 3 content in the glauconite. Although similar high‐Al glauconite has been reported in sedimentary deposits corresponding to warm climatic conditions, the Spiti glauconite is the first such report from the Late Jurassic time. Negative δ 13 C org values further suggest upwelling and increased primary productivity, leading to low oxygen conditions. The transition from black shale to glauconitic green sandstones reflects a shift of depositional conditions from anoxic to sub‐oxic within the Late Jurassic succession.
Mandal et al. (Thu,) studied this question.
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