Neoproterozoic cave temple arenite of badami basin- paleoenvironmental and petrological implications

Abstract

Meso- to Neo-Proterozoic Kaladgi Basin,located on the north-western fringe of the western Dharwarcraton,is comprised of deformed, older Bagalkot Group and the overlying undeformed, younger Badami Group, exposed mainly in the western and central portion of the Basin. The terrestrial sedimentary units overlying the Chitradurga Schist Belt and/or Peninsular Gneiss in the southeastern margin of the western portion of the Kaladgi Basin, is characterised by multistoried conglomerate and sandstone bodies, belonging to the basal KendurConglomerate Member and Cave Temple AreniteMember of the Badami Group. The entire succession is made up of threevalley fills, the lower one of which is studied in detail in this occasion.Amply preserved primary structures though, enable one to apply state-of-the-art facies analysis scheme for the lower valley fill, which revealed ten genetic facies, combined in to three distinct facies associations. The first of which represents alluvial fan association, while the other two are chiefly fluvial, braided in pattern, but with gradually increasing flow durabilities within channels, thus representing ephemeral to perennial nature of the fluvial system, respectively. Apparently the fan association developed locally, along the high slope areas near the basin margin, followed by the fluvial associations vertically, as the depositional slope reduced due to basin filling. Nonetheless, ephemeral fluvial association I demands higher slope than the perennial fluvial association II. Sequence architectural analysis revealed five vertically juxtaposed channel belts within the lower valley fill. The emplacement of one valley fill over another undoubtedly requires sufficient tectonic acitvities to generate new accommodation space, while vertical juxtaposition of channel belts, one above another, can also be prompted by climatic variationsTo unravel the controlling factor, in case of the lower valley filling, recourse to the petrographic analysis are taken. The lower valley fill characteristically possessarkosic to subarkosic nature of their constituent sandstone bodies. The preservation of feldspar in sandstone requires either extreme climatic condition or high slope, leading to short transportation and quick dumping of the sediments. Detailed petrographic studies revealed a positive correlation between grain size and the feldspar content, along with a systematicdecrease in feldspar content both within as well as across channel belt, from bottom to top. This signifies tectonics generated slope as the prime mover in controlling the feldspar preservation within the lower valley fill.The QFR plot of these sandstone bodies also supported their deposition in a transitional continental area, which conforms well with the intracratonic rift origin of the basin. The overall fining upward trend of each channel fill accords well with the general aggradational nature of the fluvial system, leading to the gradual filling and concomitant reduction in the depositional slope. Overall, the multistoried lower valley fillsuccession in the western part of the Badami basin margin, documents gradual filling of the valleyby fluvial sedimentation, frequently disrupted by the tectonic movements leading to rejuvenation of depositional slope.