Facies and Sedimentary Structures of the Kerur Formation, Badami Group, South India: Implications for Depositional environment

Abstract

Facies architecture and sedimentary structures, followed by petrographic characteristics of the rocks of the Kerur Formation of the Badami Group of Kaladgi-Badami basin exposed in the Deshnur area near Belgaum,   in South India is discussed in the present paper to understand their depositional environment. The Kerur Formation at Deshnur comprises of conglomerate, gritty sandstone- quartz arenite and shale. In the type area at Badami, it comprises of Kendur conglomerate, 90m thick sequence of arenite called “the temple arenite and the Halageri shale.  In the western part, at Deshnur, near Belgaum, Kerur Formation show  rhythmic variation in  conglomerate and gritty sandstone followed by medium to fine grained arenite and intercalated thin shale beds suggesting fluctuating-fluvial, beach facies and shallow water depositional condition. Features of fluvial sedimentation such as pebble beds; shallow water sedimentary features such as symmetrical ripple marks, cross bedding; beach facies like dune lamination and Aeolian dunes; and shallow marine facies such as intercalation of shale with sandy facies are observed. Features such as pebbles size, rounding, along with current bedding structures within the gritty layers between the pebble beds suggest fluvial discharge to an open sea. The thicker, gentler landward-cross strata, thinner, steeper seaward cross-strata in the herringbone cross stratification in the overlying arenite indicate the tidal modulation during their deposition. In thin section, the grit show well rounded while the quartz arenite show moderately rounded to well rounded, medium grained, texturally and compositionally matured characters suggesting sorting by winnowing action of waves, implying coastal environment of deposition. The facies Architecture, sedimentary structure like tidalites indicative of interaction of tides and current (including changes in their speed and direction) suggest tidal modulation of depositional environment. Well rounded, moderately assorted, vein quartz rich pebbles in alternate sandy- pebbly layers along with arkosic nature of the grits indicate periodic flashy floods dominated the surface hydrodynamics of transport during the early phase of sedimentation.