Calcretes in the Thar desert: Genesis, chronology and palaeoenvironment

• Published in: Academy Proceedings in Earth and Planetary Sciences 2004-09, Vol.113 (3), p.473-515
• Main Authors: DHIR, R. P, TANDON, S. K, SAREEN, B. K, RAMESH, R, RAO, T. K. G, KAILATH, A. J, SHARMA, N
• Format: Article
• Language: English
• Subjects: Accretion; Aggradation; Alluvial plains; Arid climates; Aridity; Atmospheric particulates; Calcrete; Carbonates; Cathodoluminescence; Cementation; Chlorite; Chronology; Climate; Dating; Deserts; Dunes; Earth sciences; Earth, ocean, space; Electron paramagnetic resonance; Electron spin; Electron spin resonance; Eolian dust; Eolian sands; Exact sciences and technology; Foothills; Geology; Groundwater; Groundwater recession; Hiatuses; Mica; Morphology; Nodules; Palygorskite; Pleistocene; Quaternary; Rain; Rain water; Regolith; Sand; Sandstone; Schists; Sedimentary rocks; Sediments; Semiarid climates; Spin resonance; Stable isotopes
• ISSN: 0253-4126; 0973-774X
• DOI: 10.1007/BF02716737

The calcretes in the Thar desert occur in a variety of settings, including the piedmonts, sheetwash aggraded plains; and this study adds calcretes in regolith and colluvio-alluvial plains to the group of settings in which calcretes occur in the region. Field logs, morphological details and analytical data such as petrographic, cathodoluminescence and geochemical characteristics are described along with a discussion on their implications. Sand dunes and sandy plains dating to < 20 ka have weakly developed calcretes. The better-developed calcrete horizons occur in piedmonts, interdunes or in areas that have sufficient groundwater. Deep sections in the region show phases of calcrete development in aeolian sand aggradation at 150, 100, 60 and 27-14 ka. The extensive sheetwash plains have mature calcretes and date to mid-Pleistocene. Our studies indicate that these calcretes represent a hybrid process, where carbonate enrichment of the originally calcareous host occurred due to periodically raised groundwaters, and its differentiation into nodules occurred under subaerial environment i.e., after recession of groundwater. Deep sections also show a stack of discrete calcretes that developed in individual aggradation episodes with hiatuses as indicated by ESR dating results. Nodules display a multiplicity of carbonate precipi tation events and internal reorganization of calcitic groundmass. The process is accompanied by degradation and transformation of unstable minerals, particularly clays and with a neosynthesis of palygorskite. The ancient calcretes are dated from the beginning of the Quaternary to 600 ka and show more evolved morphologies marked by brecciation, dissolution, laminar growth on brecciated surfaces, pisolites and several generations of re-cementation. Mica/chlorite schists and such other rocks are particularly vulnerable to replacement by carbonate. In an extreme case, replacement of quartzose sandstone was observed also. The presence of stretches of alluvio-colluvial plains in an area presently devoid of drainage bespeaks of occasional high-energy fluvial regime, under a semi-arid climate. The mid-Pleistocene period saw a shift towards more arid climate and this facilitated sheetwash aggradation. Finally, during the late Pleistocene, aggradation of aeolian sands indicated a progressively drier climate. However, this does not find its reflection in stable isotope data. The amount of carbonate in the form of calcretes is substantial. The present st