Geology and regional significance of the Sarnoo Hills, eastern rift margin of the Barmer Basin, NW India

Abstract

The Barmer Basin is a poorly understood rift basin in Rajasthan, northwest India. Exposures in the Sarnoo Hills, situated along the central eastern rift margin of the Barmer Basin, reveal a sedimentary succession that accumulated prior to the main Barmer Basin rift event, and a rift-oblique fault network that displays unusual geometries and characteristics. Here, we present a comprehensive study of Lower Cretaceous sedimentology on the basin margin, along with a detailed investigation of riftoblique faults that are exposed nowhere else in the region and provide critical insights into Barmer Basin evolution. Lower Cretaceous sediments were deposited within a rapidly subsiding alluvial plain fluvial system. Subsequent to deposition, the evolving Sarnoo Hills fault network was affected by structural inheritance during an early, previously unrecognised, rift-oblique extensional event attributed to transtension between India and Madagascar, and formed a juvenile fault network within the immediate rift-margin footwall. Ghaggar-Hakra Formation deposition may have been triggered by early rifting which tectonically destabilised the Marwar Craton prior to the main northeast-southwest Barmer Basin rift event. The identification of early rifting in the Barmer Basin demonstrates that regional extension and the associated rift systems were established throughout northwest India prior to the main phase of Deccan eruptions. Inheritance of early oblique fault systems within the evolving Barmer Basin provides a robust explanation for poorly understood structural complications interpreted in the subsurface throughout the rift. Critically, the presence of syn-rift sedimentary successions within older oblique rift systems obscured beneath the present-day Barmer Basin has significant implications for hydrocarbon exploration. INTRODUCTIONDespite being a significant structure within the West Indian Rift System (Fig. 1a), and an important hydrocarbon province (Dolson et al., 2015), the structural geology and evolution of the Barmer Basin rift remains relatively poorly understood. The current understanding of rift evolution is based predominantly upon subsurface data sets, where an abundance of complex structures and rift-oblique faults are variably imaged on seismic data throughout the rift (Bladon et al., 2015; Fig. 1b). However, the origin of these structures remains elusive, and limited exposure of the structure and stratigraphy of the rift at outcrop has restricted outcrop-based studies. This work characterises a rift-oblique fault network that is apparent nowhere else in the region, and is exposed on the eastern rift margin of the Barmer Basin, in the Sarnoo Hills. Detailed sedimentological and structural analyses are used to clarify the origin of similar rift-oblique faults imaged in the Barmer Basin subsurface, and subsequently to place the findings within the wider context of northwest India. Sediments were deposited in a maturing fluvial system within a rapidly subsiding continental alluvial plain environment. The expos...

Sedimentology and the facies architecture of the Ghaggar‐Hakra Formation, Barmer Basin, India: Implications for early Cretaceous deposition on the north‐western Indian Plate margin

“…The Barmer and Cambay basins, together with the Sanchor sub‐Basin that separates them, form a present‐day generally north‐northwest tending rift “arm” of the WIRS extending up to 600 km into north‐western India from the Gulf of Khambhat (Figure a). The Barmer Basin is a 200 km long, <40 km wide, 6 km deep, failed continental rift (Biswas, ; Bladon, Burley, et al., ; Compton, ; Dolson et al., ) (Figure b) containing continental sediments (Figure a; Compton, ; Dolson et al., ; Sisodia & Singh, ). A comparable rift system, generally known as the Thar or “Lower Indus” rift (Ahmad & Amad, ; Zaigham, Ahmad, & Hisam, ; Zaigham & Mallick, ) and specifically including the Panno‐Aqil Graben, extends northwards into Pakistan south of the Mari‐Kandkot High and east of the Jacobabad High (Ahmad & Amad, ) and probably represents the continuation of the WIRS (Figure a).…”

“…Basement rocks of the Precambrian Malani Igneous Suite are unconformably overlain by clastic fluvial sediments of the Jurassic Lathi Formation and the Lower Cretaceous Ghaggar‐Hakra Formation (Figure a). Eastwards the Ghaggar‐Hakra Formation onlaps the Karentia Volcanic Formation (Figure a), consisting predominantly of microcrystalline and plagioclase‐phyric basalts (Roy & Jokhar, ) which erupted in response to initial rifting between east and west Gondwana (Bladon, Burley, et al., ; Reeves & De‐Wit, ; Storey et al., ). Previous authors have ascribed all three of these formations to a “pre‐rift succession” (Biswas, ; Compton, ; Roy & Jokhar, ).…”

“…In the Sarnoo Hills, a succession of fluvial sediments unconformably overlay volcanic rocks of the Karentia Volcanic Formation and, in places, the Malani Igneous Suite (Figure b; Baksi & Naskar, ; Compton, ; Mishra et al., ; Sisodia & Singh, ). The formation is ~100 m thick at outcrop (Mishra et al., ) and comprises three distinct, quartz‐rich, fluvial channel belt sandstone packages separated by ~30 m of variegated white and red coloured, horizontally laminated and cross‐laminated sands, with rhizoliths and soft‐sediment deformation structures (Bladon, Burley, et al., ), attributed to deposition on a fluvial floodplain (Bladon, Burley, et al., Bladon, Clarke, et al., ). The sandstone packages have been assigned informal lithostratigraphical status and named (in ascending order): the Darjaniyon‐ki Dhani, Sarnoo and Nosar sandstones (Figure b; Bladon, Burley, et al., Bladon, Clarke, et al., ).…”

“…= Bariyada Member (Compton, ; Dolson et al., Sisodia & Singh, ; Tabaei & Singh, ; Tripathi et al., ); (b) generalized vertical section of the Ghaggar‐Hakra Formation, displaying the separate lithostratigraphically informal sandstone units recognized by previous authors (adapted from Bladon, Burley, et al. )…”

“…Recently, an important earlier northwest–southeast extensional event has been recognized, preserved in structural geometries exposed on the basin margins and overprinted by the perceived main Palaeogene rifting (Bladon, Clarke, & Burley, ; Dasgupta & Mukherjee, ). Fluvial sediments preserved on the rift margin are ascribed to the Lower Cretaceous Ghaggar‐Hakra Formation and rift margin geometries suggest that they were deposited contemporaneously with the pre‐Palaeogene normal faulting (Bladon, Burley, Clarke, & Beaumont, ; Bladon, Clarke, et al., ). Although the Ghaggar‐Hakra Formation occurs in relatively small outcrops around Sarnoo, they are regionally significant as they provide excellent exposures of the early Cretaceous sediments which document intraplate continental deposition during the early break‐up of India from Gondwana.…”