Early Jurassic Kota Limestone: Record of a carbonate wetland in a continental rift basin of India

The preserved thickness of the sediments accumulated in the Pranhita-Godavari rift basin, central India, between the Permo-Carboniferous and the Early Cretaceous is about 5 km. This dominantly siliciclastic succession was... [ view full abstract ]

The preserved thickness of the sediments accumulated in the Pranhita-Godavari rift basin, central India, between the Permo-Carboniferous and the Early Cretaceous is about 5 km. This dominantly siliciclastic succession was deposited mainly in different types of fluvial environments. About 20-30m thick, Early Jurassic limestone interval (Kota Limestone) occurs in the upper part of this siliciclastic succession. The remains of freshwater fishes, ostracodes, estherids, coprolites as well as ichnofossils (burrows and dinosaur footprints) indicate that these limestones were deposited in a freshwater environment. This work describes different carbonate facies of the Kota Limestone and characterizes the depositional environment.

Ten carbonate facies (F1-F10) are identified and grouped into three assemblages (i.e., A, B and C). Assemblage A comprises: F1) interstratified shale-carbonate, F2) micritic-micrite laminites, F3) sparite-micrite laminites and F4) bioclastic laminites. This assemblage represents an environment where carbonate accumulates both as precipitates and clastics in a shallow body of standing water, with concomitant siliciclastic input. Assemblage B comprises: F5) mottled and nodular limestone, F6) brecciated limestone, F7) massive limestone, and F8) carbonate root mat. The massive limestone (F7) includes: 7a) microbial limestones, 7b) massive limestones with profuse small plant detritus. Assemblage B that includes a number subaerial weathering features represents a palustrine environment closely related to the shallow aquatic environment represented by assemblage A.

Assemblage C represents sediment gravity flows and comprises two types of intraformational conglomerates: F9) oncoidal rudstone and F10) angular clast-bearing conglomerate. The assemblage (C) occurs intimately associated with both massive and laminated limestones.

In most cases shales of F1 gradually pass upward into laminated carbonate facies (F2, 3, 4), which in turn is overlain by different facies of assemblage B constituting shallowing upward packages (70 – 120cm thick) representing a complete cycle of subaqueous emplacement of siliciclastics, followed by carbonate clastics and subsequent modifications through diagenesis due to subaerial exposure and groundwater influence. Repeated vertical stacking of these packages, comprises a major part of the Kota Limestone.

A high proportion of palustrine facies and its repetitive occurrence in the succession, limited development of the laminites, the presence of shallow, benthic fauna (ostracodes and estherids) within the laminites and the intimate association of root mats with shales and laminites together suggest that these waterbodies were probably not very deep. As the signatures of deep lake-bottom conditions are virtually absent the environment of deposition was somewhat similar to a modern freshwater carbonate wetland.