Morphology of the last subaerial unconformity on a shelf: insights into transgressive ravinement and incised valley occurrence in the Gulf of Cádiz

Abstract

The main aim of this study is to explore the spatial patterns of the shelf-scale erosional unconformity related to the last glacial maximum (LGM), particularly in terms of the role of underlying geology and the presumed primary influence of sea-level changes. This involved a detailed mapping of the most recent and widespread erosional shelf surface in a sector of the northern margin of the Gulf of Cádiz (northeast Atlantic Ocean) located adjacent to a major fluvial source. A dense network of high-resolution seismic profiles collected in the 1990s and 2013 off the Guadiana River revealed two distinct geomorphological domains on the LGM shelf-scale subaerial surface. The outer domain exhibits a widespread occurrence of erosional truncations, with a rugged, erosional pattern over the most distal shelf setting that evolves landward into a planar unconformity. The inner domain is more extensive and is characterized by the common occurrence of highly reflective, localized mounded seismic facies that laterally evolve into an irregular surface and in places may develop a channelized morphology. Significant fluvial incision is limited to a major straight valley and a secondary distributary channel. A distinct partition of the lowstand surface is documented, and attributed to a well-marked lithological change. A coarsegrained inner shelf comprises underlying lithified coastal deposits, whereas a fine-grained outer shelf is regarded as the uppermost expression of regressive prodeltaic wedges. The influence of regional indurated surfaces is also expressed in (1) the pattern of erosion, this being more patchy on the inner shelf due to lateral changes of erodibility, whereas on the outer shelf it shows laterally continuous bands, owing to different modes of transgressive ravinement; (2) the spatial and temporal variability of fluvial incision. Inner shelf armoring by indurated deposits prevents reoccupation of previously incised valleys.

Sediment characteristics and internal architecture of offshore sand ridges on a tideless continental shelf (western Mediterranean)

“…At present, the source of the sandy sediment should be limited to the winnowing of coarse deposits associated with the erosion of this surface. The highly reflective mounded morphologies, as identified in the ravinement surface, are similar to those described on other continental shelves, such as the US Atlantic shelf (Goff et al, 2005; McHugh et al, 2010), in the English Channel (Mellett et al, 2012; Mellett and Plater, 2018), on the South African shelf (Pretorius et al, 2016), in the Gulf of Cádiz (Lobo et al, 2018) and in the North Sea (Emery et al, 2019). On the US Atlantic shelf, irregular highs with chaotic reflections have been related to coarse‐grained, armoured shallow‐water deposits (Goff et al, 2005) or coastal headlands (Schwab et al, 2014).…”