Optical dating of late Quaternary carbonate sequences of Saurashtra, western India

Abstract

Bioclastic carbonate deposits that formed because of a combination of nearshore marine, fluvial, and aeolian processes, occur along the Saurashtra coast and in the adjacent interior regions of western India. Whether these carbonates formed by marine or aeolian processes has been debated for many decades. The presence of these deposits inland poses questions as to whether they are climate controlled or attributable to postdepositional tectonic uplift. In particular, the debate centres on chronologic issues including (1) appropriate sampling strategies and (2) the use of 230Th/234U and 14C ages on the bulk carbonates. Using traces (<1%) of quartz grains trapped in carbonate matrices, optically stimulated luminescence (OSL) dating of quartz grains, deposited along with the carbonate grains, provides ages for the most recent deposition events. The OSL ages range from >165 to 44 ka for the shell limestones, 75–17 ka for the fluvially reworked sheet deposits, and 80–11 ka for miliolites deposited by aeolian processes. These are younger than the 230Th/234U and 14C ages and suggest that the inland carbonate deposits were reworked from older carbonate sediments that were transported during more arid phases.

Sedimentological analysis of a Late Quaternary coastal dune system: An example from Gopnath, south‐east Saurashtra, Western India

“…These authors also Costa, 2015). The uppermost aeolianite unit in Gopnath bears an OSL age of 156 AE 14 ka (after Sharma et al, 2017). identified phreatophytic rhizoliths, freshwater gastropods and fossilized bones from terra rossa horizons of Gopnath. Khadkikar (2004) attributed the formation of terra rossa like soils to periods of increased rainfall leading to weathering, while prolonged periods of reduced rainfall but strong winds favoured the formation of parabolic dunes which deposited the aeolianites.…”

“…(B) Composite succession of coastal carbonate aeolianites and palaeosol horizons (PLSL1 to PLSL3) in Gopnath and adjoining areas (modified after Costa, ). The uppermost aeolianite unit in Gopnath bears an OSL age of 156 ± 14 ka (after Sharma et al ., ).…”

“…Considering the significance of Saurashtra Miliolites as a Late Quaternary sedimentary archive, a detailed sedimentological analysis is indispensable for unravelling and understanding the coastal dune building processes and controls that would augment the study of similar deposits in the Afro-Asian monsoonal belt in particular and the coastal belts in other parts of the world in general. This paper presents a case study of a a Late Quaternary coastal dune system from Gopnath in south-east Saurashtra which represents the oldest known Quaternary coastal aeolianite sequence in western India, deposited during the glacial epoch at ca 156 ka [Marine Isotope Stage (MIS) 6] (Sharma et al, 2017). It aims at bridging the gap in understanding of the Saurashtra Miliolites from the viewpoint of aeolian depositional components, dune building processes and controls on sedimentation.…”

“…This was later lithostratigraphically designated to the Miliolite Formation and Chaya Formation which were further subdivided into members (Mathur, ; Bhatt, ). The OSL ages of the coastal Miliolites in Saurashtra range from >156 ka (Gopnath in the east) to 45 ka (Makanpur in the west), with the units becoming younger in age on moving from east to west along the Saurashtra coast (Sharma et al ., ) covering a stretch of nearly 400 km (Fig. ).…”

Late‐Quaternary miliolite (biogenic carbonate) deposits and their implications for sea‐level fluctuations and climatic variability

“…In the initial attempt by radiocarbon (Agrawal et al, 1978;Gupta, 1991) and thorium-230/uranium-234 ( 230 Th/ 234 U) dating techniques (Baskaran, 1985;Ku et al, 1979;Somayajulu, 1993), the depositional phases clustered at 70-50 ka, 75-15 ka and 210-114 ka (Baskaran et al, 1989). The applicability of the dating techniques on continental carbonate rocks was questioned because of the problem of post-depositional diagenetic alteration which can result in either overestimation or underestimation of the real age of deposition (Brückner, 1986;Gupta, 1991;Sharma et al, 2017). In addition to this, the Electron Spin Resonance (ESR) technique was also used on limited samples (molluscs and coral fragments) from the Saurashtra miliolites which yielded ages between 75 ka BP to 156 ka BP.…”