The Cretaceous of NW India is poorly known from sparse outcrops in Rajasthan and Gujarat. Here, we describe the stratigraphy and sedimentology of outcrops of the Ghaggar-Hakra Formation of probable Lower Cretaceous age from the Sarnoo Hills, eastern Barmer Basin, Rajasthan and correlate them with equivalent sediments in core from the subsurface of the Barmer Basin.
At outcrop the Ghaggar-Hakra Formation contains three fluvial sandstone sequences of varying depositional type and geometry interbedded with associated floodplain deposits. At the base of the exposure the Darjaniyon-ki Dhani Sandstone is composed of compositionally mature, granule-grade quartzitic conglomerates that form braid bars. The deposits represent a poorly-developed braided system. Subsequently, the Sarnoo Sandstone constitutes medium- to very coarse-grained, cross-bedded sandstones, that fine upwards into fine-grained rippled and laminated sandstones, forming in-channel bars and point bars, implying the development of a meandering system. Lastly, capping the exposed sequence, the Nosar Member is composed of very coarse- to medium-grained, planar and trough cross-bedded quartz-arenites. These deposits represent in-channel dunes that display evidence of braiding, indicating the establishment of a well-developed braided system. The intervening mudrocks are characteristic of floodplain deposits, which are mottled, with vertical fractures, soil slickenlines and a pedogenic nature.
Within the core the sequence differs from outcrop as there are two separate environments a braided system with associated floodplain deposits and a lacustrine system. The braided river comprises of very coarse- to fine-grained cross-bedded and rippled sandstones. The associated floodplain deposits are mottled, rooted, fractured and have rizoliths within. The detrital composition of the braidplain sandstones does not vary greatly from the outcrop as the minerals are quartz, lithics, heavy minerals and clays. However, the authigenic minerals do vary, as in the subsurface we see kaolinite clays, quartz overgrowths, siderite, pyrite and chlorite cements, whereas at the surface there are quartz overgrowths, calcite, dolomite and haematite cements with kaolinite clays. It is likely that the braided river in core is the same sequence as seen at outcrop.
We interpret the successions here into a single fluvial system that has been affected by regional tectonics from the separation of Madagascar (Aptian) from the Indian continent and the continent drifting northwards. This heavily influenced the localised tectonics, which alludes to why there is a significant change in fluvial style from the Sarnoo Sandstone (meandering) to the Nosar Member (braided).