Geochronology of sediments as a tool to identify lost geological features - a case study from the Mesozoic sedimentary succession of the Kutch Basin, western India

Sedimentary successions capture the history of geological features and events observable on present-day earth surface as well as those exposed earlier but currently buried or lost to erosion. The Mesozoic rocks in the Kutch Basin (western India) deposited between Middle Jurassic and Early Cretaceous reveal interesting provenance information on lost orogens and buried basins. The southwesterly sediment transport direction indicates north and northwest of the Indian subcontinent as the source area. Detrital zircon and monazite U-Th-Pb geochronology identify dominant sediment input from source rocks equivalent to the late Neoproterozoic Pan-African orogeny (500–650 Ma) along with substantial input from those equivalent to the Cambro-Ordovician Bhimphedian (aka Kurgiakh) (400–500 Ma) orogeny. All other contributing source rocks (ranging from 700 Ma to 3300 Ma) are traceable to the source area following the sediment transport direction. However, outcrops of crystalline rocks with zircon and monazite ages corresponding to the dominant age components are virtually lacking. Rocks equivalent to the Pan-African orogeny are found only as sparse isolated outcrops in the source area. In contrast, this orogeny is well reported from the southern granulite terrain (India), Madagascar, Seychelles and Eastern Africa. Therefore, considering the position of continents during the Mesozoic and the predominance of a 500–650 Ma sediment source in the Kutch Basin, the Pan-African orogenic belt possibly extends to north and north-western India. The current dearth of these outcrops suggests extensive erosion during the Mesozoic greenhouse climate and/or burial under the Deccan Flood Basalts. The other dominant source (400–500 Ma), equivalent to the Bhimphedian orogeny, currently reported as isolated outcrops in the Himalayan-fold-thrust belt (northern India) might have been disturbed and buried by thrusting during the Cenozoic Himalayan orogeny. This study also reveals a large gap of nearly 280 Ma between the youngest detrital zircon (458 Ma) and the depositional age (~170 Ma). This gap may be explained by (i) input of recycled sediment from an older basin, and/or (ii) absence of younger metamorphic events in the source area. The evidences of sediment recycling from thin-section petrography and ultra-stable heavy mineral assemblages (dominated by zircon, rutile and tourmaline) suggest the possibility of a so far unknown (buried or completely eroded) sedimentary basin older than the Kutch Basin. The on-going study of detrital rutile grains in these sediments may provide an alternative explanation for the 280 Ma gap by revealing lower temperature metamorphic events that are not recorded by U-Th-Pb ages of zircon and monazite.