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Outcrop Evidence of Previously Unrecognised Extensional Tectonics in Northwest India

Abstract

Several giant oilfields were discovered in the Barmer Basin in 2004, predominantly situated within the crests of faulted blocks, and the basin is now an established oil and gas producing province (7.3 BBL of STOIIP, 200,000bopd production). However, the extent and geometry of many fault-blocks within the rift are controlled by poorly understood rift-oblique faults that are variably imaged throughout the subsurface. Here we present a study of Lower Cretaceous sedimentology that accumulated prior to the main rifting event in the Barmer Basin, exposed along the eastern rift margin in the Sarnoo Hills, along with a detailed investigation of a rift-oblique fault network that is exposed nowhere else in the region. The findings provide critical insights into the structural evolution of the Barmer Basin and regional tectonic processes hitherto unrecognised. Lower Cretaceous sediments were deposited within an alluvial plain fluvial system. The high proportion of floodplain mudstones and siltstones preserved within the fluvial succession, and the lack of evidence for long-term floodplain stability, suggest aggradation of the floodplain, possibly due to rapid subsidence or a high sediment supply. Subsequent to deposition, brittle deformational structures accommodated northwest-southeast extension, highly oblique within the north-northwest trending Barmer Basin and previously unrecognised in northwest India. Despite the pre-rift tectono-stratigraphical relationship between the sedimentary succession and the fault network exposed at outcrop, the sedimentology suggests deposition was triggered by the onset of rapid subsidence in the Barmer region during the Lower Cretaceous, and is likely a manifestation of the rift-oblique (≈ NW-SE) extensional tectonics exposed. The identification of Lower Cretaceous, rift-oblique extension that pre-dated the main rifting event in the Barmer Basin during the Paleogene, indicates the present day structural architecture of the Barmer Basin resulted from two, superimposed, non-coaxial extensional events, and elucidates poorly understood rift-oblique faults interpreted in the subsurface throughout the rift. Integration of the findings with the currently understood regional tectonic framework suggest northwest-southeast orientated Lower Cretaceous extension is an intra-continental manifestation of transtension between the Greater Indian and Madagascan continents during Gondwana fragmentation.