The character and genesis of calcrete in Late Quaternary alluvial deposits, Gujarat, western India, and its bearing on the interpretation of ancient climates

Abstract

Late Quaternary deposits in Mainland Gujarat contain sediments deposited in subhumid and semi-arid climates. The 30–35 m succession shows the presence of Vertisols at the base and a red-bed horizon (ferric Calcisol when pedogenic in origin) that roughly bisects the succession. A widespread development of calcretes is observed throughout the succession. The various varieties of calcrete include pedogenic calcrete, groundwater calcrete, calcrete conglomerate (transported calcrete) and rhizogenic calcrete. Pedogenic calcrete nodules associated with Vertisols and the red-soil show marked differences in morphology, dimensions and the distribution of microscopic features. These differences arise due to contrasting climate-controlled physicochemical environs under which they formed. Pedogenic calcrete nodules associated with Vertisols acquire large (5–10 cm) dimensions and are characterised by either showing the presence of a nucleus of soil-matter or showing a dense micritic groundmass cut by thick sparitic veins. In contrast, calcrete nodules that formed in the red-soil are <3 cm in size and do not exhibit dense networks of sparitic veins. Calcretes associated with Vertisols and the ferric Calcisol also exhibit differences in the morphology of rhizoliths. These differences also show up in the distribution of microfabrics of calcretes. Grain coats are present in rhizogenic and pedogenic calcretes, but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes. Clotted micrite is present in all types except groundwater calcretes. Sparitic veins, however, are observed in each type, but are relatively less developed in groundwater calcretes. A similar distribution of displacive and replacive textures is also seen, although some grains in groundwater calcretes showed signs of corrosion. The Vertisol-associated calcretes represent a subhumid (500–700 mm) climate, whereas the red-soil calcretes suggest a semi-arid (100–500 mm) climate. Calcretes from the Vertisol association show a range in δ 13C composition constrained between −9‰ and −5‰, whereas the red-soil calcretes exhibit the whole spectrum of values from −9‰ to −1‰. Based on mineralogical associations, calcretes, usually taken to reflect semi-arid episodes in the Earth's history, may be classified further. Calcretes, when associated with sepiolite/palygorskite, suggest an arid climate (mean annual rainfall=50–100 mm), when associated with smectite, haematite and the absence of hydromorphism, a semi-arid climate (mean annual rainfall=100–500 mm), and when found in smectitic Vertisols, subhumid climates (mean annual rainfall=500–700 mm).