Controls on the evolution of submarine canyons in steep continental slopes: geomorphological insights from Palar Basin, southeastern margin of India

Abstract

Palar Basin in India’s southeastern margin is an intracratonic rift basin characterized by a very steep continental slope (gradient, 3°–78°). Recently acquired high-resolution swath bathymetry data from the region documented various geomorphic features on the slope of the Palar Basin, and canyons are most prominent among them. Twenty submarine canyons, grouped as the Palar Canyon System (PCS), have been mapped and identified for the first time in this study. The geological element that controls PCS’s development presents a morphological framework for canyons developed in extremely steep continental slopes, which are rare on continental margins elsewhere. In contrast to many other submarine canyons, the short and low sinuosity canyons in the Palar Basin (1) traverse a large fault (escarpment) exposed up to 1300 m high, (2) erode into a relatively steep margin, and (3) create downstream features akin to the subaerial origin. Because of these unusual characteristics, the environment presents a wide range of possibilities to discuss the processes involved in their origin, evolution, the controlling factors, and sedimentary activity. The present-day PCS architecture implies that the early stages of canyon formation were shaped by bottom–up retrogressive landslides initiated from the disintegrative failures in the oversteepened slope. The evolution of PCS corresponds to the orientation and relative displacement of the fault escarpment, where the movement of the fault generates canyon activity in response to maintaining slope equilibrium. In addition to the fault movement, downward eroding gravity flows and failures along the flanks aid in the canyon shaping processes. For canyons truncated by a large fault, we propose an upslope/bottom–up model of canyon formation that initiates near the steep fault escarpment and incises downslope and propagates upslope to breach the shelf edge. PCS also comprises ellipsoidal depressions and asymmetric bedforms at the downstream reaches of the canyons, classified as “plunge pools” and “mega dunes,” respectively. Seven plunge pools (> 65 m deep) formed at the mouth of some of the canyons are interpreted to reflect the impact of confined high momentum gravity flows, whereas the wave train of mega dunes extending for 10 km opposite to the mouth of a canyon points to the activity of repeated hydraulic jumps in the region.