Morphostratigraphy and evolution of the quaternary 'red sands' near Bhimunipatnam, east coast of India

Field investigations in the intensely gullied coastal 'red sands' near Bhimunipatnam along the east coast of India in northern coastal Andhra Pradesh revealed four distinctly different sand units namely, (i) the lower coarse to medium grained yellow sand, (ii) medium to fine grained reddish-brown concretion bearing sand, (iii) fine-grained brick-red sand and (iv) the upper fine-grained light yellow sand, over variable thickness up to 30 m. In addition, the study brought to light some of the hitherto unreported duricrust layers and pebble beds that occur in between and within these sandy horizons, and the knick points and minor but significant waterfalls caused by these layers at various locations along the gully courses. The nature of the materials of the different sand units and the positions of duricrust and pebble beds suggest a polygenetic and multi-cyclic evolution involving fluvial, fluvio-marine and acolian processes, complicated by sea level changes, weathering and neotectonics. Geomorphic evidences like a wave-cut terrace, a valley-in-valley form and a spring in a gully stream that appear to be oriented along one of the major lineaments that criss-cross the area, besides the presence of pebble beds now even at 38 to 42 m elevations much above the bed levels of their possible source streams nearby, suggest tectonic upliftment of the area probably during the later stages of deposition in the late Pleistocene to Holocene epochs. The main objective of the paper is to present a comprehensive evolutionary picture of the various Quaternary sediment units along with the geomorphic history of the area.