Scree conglomerate and its derivatives in the Upper Cretaceous Kallankurichchi Limestone, Ariyalur Group, Cauvery Basin, South India

Abstract

The repetitive influx of coarse clastics of mixed composition, siliciclastics and carbonates, locally common in the lower part of the Upper Cretaceous marine Kallankurichchi Limestone specifies a resurgence of tectonic unrest in the Cauvery rift basin, India. The basin-margin scree and its derivatives elicit diverse modes of emplacement and differ in many ways from denoting it only as basal conglomerate. The study meticulously reveals the depositional history of these basin-margin coarse clastics. The scree conglomerate bodies are wedge-shaped in appearance and often have flat, eroded tops. At places, their surfaces, tops and flanks, are encrusted with Inoceramus although internally, they are mostly unfossiliferous. They are clast-supported and extremely poorly sorted, having interstitial spaces filled by sand-sized grains at the basal part. The clasts can be traced into the underlying Sillakuddi Sandstone and the granitic basement. The clasts derived from the sandstone are angular and measure up to 60 cm in length, while the basement-derived clasts dominate the smaller (maximum diameter measured 5 cm) and more rounded population. The elongated clasts are chaotically arranged, even oriented sub-vertically, reclining on other clasts. The lowermost scree conglomerate has a sharp base, and the pebbles sunk into the underlying sandstone, although no discernible impact laminae wrapping their bottom were ever observed. Evidently, the scree fans were initiated under the sea, which permitted the slow sinking of assorted rock fragments dropped from above. The sporadic occurrence of marine fossils further corroborates this contention. The scree conglomerates at different stratigraphic levels transitioned laterally into conglomerates of mass flow origin and then to massive calcarenite, together forming wedge-shaped bodies. The initial alignment of clasts parallel to bedding transforms to a chaotic alignment representing the transition from internally sheared flow to debris flow and associated shapes. Matrix-supported fabric grades into massive calcarenite, suggesting gravity-driven transformation. Further down the wedge, the massive calcarenite turns into cross-stratified facies, making it evident that laminar flow turned turbulent in the course of body transformation of the sediment-driving flows. It can be presumed that these flows were triggered by subsidence, which resulted from renewed gaps in formation and led to the collapse of scree cones. Facies variability suggests scree deposits giving way downslope to debris flow and related deposits. Eventually, all the scree products pass laterally into the carbonate formation basinwards.