Swaley cross-stratification in medium to coarse sandstone produced by oscillatory and combined flows: examples from the Proterozoic Kansapathar Formation, Chhattisgarh Basin, M.P., India

Abstract

Abstract The Proterozoic Kansapathar Formation is the topmost unit of the siliciclastic Chandarpur Group of rocks which records a fluvial to marine transition and comprises a fining-upwards transgressive succession overlain by a coarsening-upwards prograding succession. The Kansapathar Formation around Panduka, M.P., is composed of medium- to coarse-grained, well sorted quartzarenite and is replete with swaley cross-strata (SCS), trough cross-strata and other structures of intermediate morphology. In laterally continuous exposures SCS are more common in the northeastern part of the study area, whereas trough cross-strata are more dominant in the southwestern part. We have recognised the SCS on the basis of the stratal geometry, originally described by Leckie and Walker (1982) . SCS occur as nested sets of elliptical swales which are commonly symmetrically filled. The trough cross-strata are variable in scales and dominantly SW-directed. The major axes of swales show a strong (NNW–SSE) preferred orientation orthogonal to the orientation of trough cross-strata and at a low angle to the shoreline orientation determined from the megaripple orientation in the immediately underlying unit. SCS described herein, in contrast to many examples reported from elsewhere, formed in medium to coarse sand and the maximum inclinations of their lower bounding surfaces and of the infilling laminae are greater than those of fine-grained hummocky cross-strata (HCS) and SCS. This is explicable since medium- to coarse-grained sediments did not experience significant suspension transport and, thus, the development of low-angle lamination was suppressed. The symmetrically-filled swales do not show any evidence of lateral migration and might have been formed by bidirectional oscillatory flow. The trough cross-strata could have been formed by translatory current-dominated combined flow. The spectrum of structures may owe their origin to symmetrical oscillatory flow transforming into asymmetric flow during onshore propagation of waves. This is expected to be common in shoreface environment. The spatial variation in relative frequency of occurrence of various types of cross-strata is also consistent with the palaeogeographic reconstruction of the study area. The trough cross-strata seem to be more common towards the constantly shifting sands of nearshore. Thus the relative frequency of trough cross-strata and SCS has been used as a clue to determine relative proximity to shoreline.