Depositional canyon heads at the edge of narrow and tectonically steepened continental shelves: Comparing geomorphic elements, processes and facies in modern and outcrop examples

Marine geology data show that canyon heads can be the site of depositional processes and furnish the details of the geometry of their geomorphic elements. Canyon heads are usually floored by sediment with a prevailing coarse-grained nature and their sampling is very difficult thus preventing the characterization of the facies of their infill. Lithological and facies information is however available through outcrop studies. In this paper, we integrate modern seafloor and outcrop data to characterize the architecture of depositional canyon heads in tectonically active continental margins with a narrow shelf. The modern examples are located along the northeastern Sicilian margin (Milazzo and Niceto canyon-head systems), whereas the ancient one, Pliocene in age, is located onshore, along the Ligurian coast (Ventimiglia canyon-head system). The modern Milazzo canyon head is located at the coastline and has a steep slope. The flanking deposits are equivalent to the oldest Gilbert delta foresets of the Ventimiglia canyon head as the delta progrades directly into the upper slope. A deeply entrenched channel and a large chute mark the mostly erosional area directly facing the mouths of the rivers that enter the Milazzo canyon head. Laterally, the upper part of the foresets slope is characterized by a bulge with swales and ridges topography. These geomorphic elements are interpreted to be formed by debris flows and turbidity currents as suggested by the upper part of the Ventimiglia foresets, where chaotic, massive or graded deposits are observed. The swales and ridges topography gradually disapper downslope; this area is dominated by turbulent processes due to flow expansion from a confined to a more unconfined setting. An high turbulent flow environment is also confirmed by the formation of plunge pools, due to hydraulic jump, at the foresets-toesets transition. Similar features in the outcrop are infilled by bedsets facies with grain-size coarser than the eroded surroundings. The modern Niceto canyon head is connected landwards to a delta system that stretches across a 1 km wide shelf. Channelized delta distributaries are similar to the topset strata of the youngest deltas of the Ventimiglia canyon head. Channels are up to 200 m wide and 50 m deep and show axial bedform trains confirming that large scale trough cross beds are an important facies of channel infill. Wave reworking in the upper part of the topsets is also suggested by both the modern and outcrop data. Our work shows how the nature of the infill and the depositional processes at the canyon heads are dependent on the shelf width and can give hints on the degree of evolution of the canyon itself. In addition, our work shows how the location and character of river entry points is important in dictating lateral facies variations within canyon heads.