Lithofacies associations and stable isotopes of palustrine and calcrete carbonates: examples from an Indian Maastrichtian regolith

Abstract

Detailed information on semi-arid, palustrine carbonate-calcrete lithofacies associations in a sheetwash-dominated regolith setting is sparse. This is addressed by studying the Lower Limestone of the Lameta Beds, a well-exposed Maastrichtian regolith in central India. The general vertical lithofacies assemblage for this unit comprises: (a) basal calcareous siltstones and marls with charophytes, ostracods and gastropods; (b) buff micritic limestones associated in their upper parts with calcretized fissure-fill sandstones; (c) sheetwash as fissure-fill diamictites and thin pebbly sheets, locally developed over a few metres; and (d) sandy, nodular, brecciated and pisolitic calcretes at the top. The sequence is 'regressive', with upsection filling of topographic lows by increased sheetwash. Lateral lithofacies change is marked, but there are no permanent open-water lake deposits. In topographic lows close to the water table, marshy palustrine or groundwater calcretes formed, whereas on better drained highs, brecciation and calcretization occurred. Prolonged exposure is implied, suggesting that shrinkage was the main cause of brecciation. Evidence for rhizobrecciation and other biological calcrete fabrics is sparse, contrasting with the emphasis on root-related brecciation in many studies of palustrine lithofacies. Stable isotope (δ¹⁸O and δ¹³C) values are consistent with the palustrine limestones being fed from meteoric-derived groundwater with a strong input of soil-zone carbon. There is overlap of both δ¹⁸O and δ¹³C values from the various palustrine and calcrete fabrics co-occurring at outcrop. This suggests that, in groundwater-supported wetlands, conversion from palustrine carbonate to calcrete need not show isotopic expression, as the groundwater source and input of soil-zone carbon are essentially unchanged. Cretaceous-Tertiary δ¹⁸O and δ¹³C values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands. Hydrologically closed lakes and marine-influenced water bodies tend to result in low negative palustrine δ¹⁸O and δ¹³C values. During brecciation and calcretization, the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland. Marked biological activity (e.g. rhizobrecciation or root mat development) during calcretization may lower δ¹³C values where C₃ plants are abundant but, in shrinkage-dominated systems, δ¹³C values will be largely inherited from the palustrine limestones.