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In this study, we report for the first time, the occurrence of pothole features and the processes of their development on the bedrocks of Ghaghghar River, Son valley, India. Pothole geometries were identified and measured from the middle reaches of Ghaghghar River, where it is flowing exclusively over the Rohtas limestone (a Meso-Proterozoic age) of Vindhyan Supergroup, near to the Gurma village. On the basis of field survey and remote sensing study, it has been ascertained that potholes developed in the river bed were only related to local geological and geomorphological conditions of a stream. They are mainly developed due to abrupt slope change along the river course and usually associated with intersect fractures, joints, and veins in the bedrock. The knick point of the river profile also confirms the abrupt slope change of the river in Gurma region. The river changes its course from E-W to N-S due to Markundi fault (ENE-WSW) occurring in the study area and causes the tectonic disturbances to support the development of potholes. The formations of potholes on bedrock of Ghaghghar River were generally governed by abrasion of rock fragments during high-flow regime. The geometrical dimensions of the potholes are controlled by the joints, fractures, and veins developed in bedrock of the river. The veins are typically calcitic in nature and the orientations of long axis of potholes are analogous to the trend direction of the vein. The collapse bedrocks suggest that a pothole can be formed within a short period, but cannot be fully developed and maintained for a long time in a strong incision of riverbed.

The Rohtas Limestone of Semri Group is conformably overlain by Lower Kaimur Group of Vindhyan Supergroup in the Son Valley, Central India. Lower Kaimur Group comprising Sasaram Sandstone, Ghurma Shale and Markundi Sandstone are dominantly quartz arenite in composition. While, Rohtas limestones are micritic carbonates with considerable input of detrital grains/rock fragments. Major oxide data shows that Lower Kaimur sandstone have SiO = 92 wt % while limestone exhibit SiO = 9.52 wt %, CaO = 47 wt %. Limestones exhibit Sr = 352 ppm, Zr = 20 ppm, while Lower Kaimur sandstones are enriched in Zr = 400 ppm, due to the presence of heavy minerals like zircon. The average limestone and sandstone shows a fractionated LREE pattern and a nearly flat HREE pattern with a small negative Eu anomaly. REE patterns and abundance of limestone are quite similar and comparable to sandstones from Lower Kaimur than to the pure micritic limestone. This indicates that the REE pattern and abundance of Rohtas limestone have been controlled by detrital components possibly derived from the similar source as that for Lower Kaimur sandstones. Apart from the similarity in the REE patterns of limestone and sandstone, the trace element ratios such as Ti/Zr, La/Sc, Th/Sc, Th/Co, Th/U, Zr/Sc, Ba/Sr, La/Th, La/Th, Zr/Hf, [La/Yb], [Ce/Yb], [Gd/Yb], [Sm/Nd], [La/Lu] and Eu/Eu* strongly support the above inference. The ubiquitously comparable REE patterns and incompatible elemental ratios of Rohtas limestones and Lower Kaimur sandstones indicate their derivation from a similar felsic source. The geochemical signatures suggest that the Bundelkhand Gneissic Complex had significantly contributed sediments to the Vindhyan basin prior and during the deposition of Lower Kaimur Group. [ABSTRACT FROM AUTHOR]

Abstract: Paleogeography and process-related facies analysis of the shelf-originated Rohtas Limestone of early Mesoproterozoic age has been attempted. Lithology, structure, microfabric and TOC are the mainstay of facies classification. The Rohtas Limestone consists of seven facies; out of which the organic-rich black shale and the crinkle laminated limestone facies dominates the basal part, reveal distinctive fabric of microbial mat origin and represents outer shelf products. With gradual shoaling, the crinkle laminated limestone facies gains importance and locally turns into the nodular limestone facies through early diagenetic modification. The grey shale facies, poor in organic content, replaces them further upward. The other three facies, viz., the thinly laminated heterolithic facies, typified by close alternations between black shale and limestone, the planar laminated limestone facies and the wavy laminated limestone facies alternate with the grey shale facies, progressively dominating the top part of the succession. While the heterolithic facies records deposition across the paleochemocline, the planar laminated and the wavy laminated limestone facies document deposition in shallow agitated shelf. Confined between a condensed zone below and an unconformity above, the overall coarsening upward Rohtas Limestone records development of a HST. [Copyright &y& Elsevier]

The Proterozoic Rohtas Limestone succession in the Vindhyan basin, central India depicts a transition from outer shelf to inner shelf depositional setting. The succession comprises of seven facies viz., black shale, crinkly-laminated limestone, nodular limestone, thinly-laminated heterolithic, grey shale, plane-laminated limestone and wavy-laminated limestone. From field and petrographic features the nodular limestone is identified as early diagenetic product of crinkly-laminated limestone. δC and δO values of the limestone bear essentially the 'primary' seawater composition of late Paleoproterozoic to early Mesoproterozoic time and rules out any postdepositional resetting. δC and δO compositions of the nodular and non-nodular portions when analysed separately reveals that the nodular portions are invariably enriched in heavier oxygen and carbon isotopes. Enrichment in oxygen and carbon isotope values in the same nodular limestone, although negligible, suggest preferential cementation within the nodular portion. Nodules were possibly generated by dissolution of carbonate sediments and subsequent reprecipitation of carbonate cements close to the sea floor. [ABSTRACT FROM AUTHOR]