185 / 2022-02-28 15:46:16

Paleoenvironmental implication of interbasaltic bole beds between lava flow of late Cretaceous Deccan volcanic province, India and their implication as Martian analogue

Deccan volcanic province,Interbasaltic bole beds,Pedogenesis,Volcaniclastic,Authigenic clay mineral

The interbasaltic bole beds of late Cretaceous Deccan volcanic province provide a unique opportunity to study the mineralogical and petrographic characteristics of authigenic mineral formation. This study presents physical, petrographical, and mineralogical investigations of interbasaltic red and green bole beds within the late Cretaceous western and central Deccan volcanic province, India to understand the paleoenvironmental condition of formation. Although several studies provided geochemical characteristics of such a critical unit (i.e., interbasaltic bole beds), it lacks very detailed sedimentological and mineralogical studies, which are the mainstay of present work. The field study reveals that the interbasaltic bole beds vary in thickness (ranging from a few centimetres to meters), colour (red to green), and lateral extent. They occur either as fine laminations or as thick beds having sharp contact with upper basalt and gradational or irregular contact with lower basalt. The boles having gradational contact with lower basalt receive a significant contribution of constituents from lower basalt. The petrographic investigation reveals that red boles are volcaniclastic deposits with varying degrees of pedogenesis, ranging from incipiently developed soil to moderately developed soil. The pedogenised red bole horizons show subangular to granular peds, altered coarse and fine fractions, undifferentiated ferruginous micromass with b-striations, and voids filled with secondary minerals (e.g., zeolite). However, the weakly pedogenised volcaniclastic boles contain altered basaltic fragments, volcanic glasses (brown and orange colour) of variable vesicularity and varying degrees of alteration, scoria, authigenic clay (smectite), zircon and secondary minerals (e.g., iron oxide, zeolite). The red colour of the bole bed is due to the presence of iron oxide, i.e., hematite. The petrographic character of green coloured bole bed shows presence of volcanic lithic fragments, green authigenic clay, plagioclase as primary constituents and red authigenic clay, pyroxene, apatite and opaques (Fe-Ti oxides) as minor constituents. Secondary zeolite occurs within void spaces and as replacing plagioclase at few locations. The green bole shows incipient development of soil with some signature of bioturbation and faunal activity in volcaniclastic deposits. The XRD (random and oriented mount), visible Near-infrared spectroscopy (0.35–2.5 µm), and FTIR spectroscopy (400–4000 cm^-1) indicate the presence of Al-smectite, i.e., montmorillonite and iron oxide in red boles, and celadonite (green authigenic clay) and Fe-smectite (red authigenic clay) in green boles. In reflectance spectra of VNIR spectroscopy, the red bole shows presence of 2.21 µm, 2.24 µm and 2.29 µm absorption features, indicating a mixture of Al-smectite (Montmorillonite) and Fe smectite as the clay mineral constituents. Whereas, green boles, shows the characteristic absorptions of approximately equal strengths at ~2.30 μm (Fe³⁺Fe³⁺–OH combination) and 2.35 μm (Mg₃–OH combination) and a strong slope between 1.00 μm and ~2.10 μm indicating presence of celadonite as major clay constituent. The clay mineralogy was further confirmed by the XRD data and FTIR spectroscopy. Thus, red boles indicate formation under oxic condition, and green boles indicate a waterlogged or anoxic condition of formation during a lull in Deccan volcanism. The pedogenised red bole are result of prolonged sub-aerial weathering of flow top basalt, indicating significant hiatus in Deccan volcanism.

The authigenic clay minerals are a sensitive indicator of the geochemical environment of deposition. Thus, the temporal and spatial distribution of authigenic clay minerals in bole beds becomes an important record of paleoenvironmental and paleoclimatic conditions during break-in Deccan volcanism. The presence of Al–Fe–Mg smectite and K, Fe mica (celadonite) on the Martian surface makes the study of interbasaltic bole beds important and interesting, as they can be used to understand the aqueous geochemical condition at the time of formation of these minerals on Noachian terrain of Mars.