Formation and preservation of an overstepped segmented lagoon complex on a high‐energy continental shelf

Abstract

The duration of shoreline occupation at a given sea-level, coastal response to sea-level change and the controls on preservation of various shoreline elements can be recognized by detailed examination of submerged shorelines on the continental shelf. Using bathymetric and seismic observations, this article documents the evolution and preservation of an incised valley and lithified barrier complex between À65 m and À50 m mean sea-level on a wave-dominated continental shelf. The barrier complex is preserved as a series of aeolianite or beachrock ridges backed by laterally extensive backbarrier sediments. The ridges include prograded cuspate lagoonal shoreline features similar to those found in contemporary lagoons. The incised valley trends shore-parallel behind the barrier complex and records an early phase of valley filling, followed by a phase of extensive lagoonal sedimentation beyond the margins of the incised bedrock valley. Sea-level stability at the outer barrier position (ca À65 m) enabled accumulation of a substantial coastal barrier that remained intact during a phase of subsequent slow sealevel rise to À58 m when the lagoon formed. These lagoonal sediments are stripped seawards by bay ravinement processes which caused the formation of several prograded marginal cuspate features. An abrupt rise in sea-level to À40 m, correlated with melt-water pulse 1B, enabled the preservation of thick lagoonal sediments at the top of the incised valley fill and preservation on the sea bed of the cemented core of the barriers. This situation is unique to subtropical coastlines where early diagenesis is possible. The overlying sandy sediment from the uncemented upper portion of the barriers is dispersed by ravinement, partly burying the ridges and protecting the underlying sediments. The high degree of barrier or shoreline preservation is attributed to rapid overstepping of the shoreline, early cementation in favourable climatic conditions and the protection of the barrier cores by sand sheet draping.

Early cementation and accommodation space dictate the evolution of an overstepping barrier system during the Holocene

“…12A). The presence and the role of paleovalleys have also been reported for other sites (Mellett et al, 2012;Green et al, 2013): they were considered a conduit for sediment transport during sealevel lowstand and subjected to backstepping infilling during transgression. In our case, the paleovalley of the Tirso river created the accommodation space for the sediment deposition.…”

“…The same authors highlighted that this relatively low rate of sea-level rise should be in contrast with barrier preservation and that other factors (i.e. antecedent topography), may be of primary importance in barrier evolution (Mellett et al, 2012;Green et al, 2013;Lorenzo-Trueba and Ashton, 2014).…”

Seismic architecture, stratigraphy and evolution of a subtropical marine embayment: Maputo Bay, Mozambique

“…Deposits similar in both seismic configuration and morphology are found in a variety of depths on the South African continental shelf and have been identified as aeolianite remnants that have escaped complete erosion by virtue of their cementation (e.g. Martin and Flemming, 1986;Green et al, 2013aGreen et al, , 2014.…”

“…Cooper et al, 2013). Green et al (2013a) describe a similar shelf-hosted lagoonal sequence that onlaps a cemented barrier system and various studies of the larger coastal waterbodies of South Africa (e.g. Wright et al, 2000;Mkhize, Fig.…”

Late Quaternary evolution and sea-level history of a glaciated marine embayment, Bantry Bay, SW Ireland

“…Such abrupt RSL rise, linked to MWP-1A, would be expected to have caused shoreline overstepping, and preservation of the former coastal sediments (Green et al, 2013;. Although this appears to be the case in Bantry Bay (preservation of lowstand shoreline complex), it is not evident in the long profile of the ravinement surface.…”