Mid-late Holocene coastal forcing variability as possible cause of alternating drift- and swash-aligned gravel beach ridges in the Atlantic province of Tierra del Fuego, Argentina

ORFORD, JULIAN; BUJALESKY, GUSTAVO GABRIEL AND MONTES, ALEJANDRO

Cologne

Congreso; 32nd Intenational Geographical Congress; 2012

The nature of drift-aligned and swash-aligned coastal accumulation forms have been considered for some decades, as being a product of changing longshore sediment supply: with sediment-rich supply forcing drift-aligned ridges and sediment-poor supply forcing swash-aligned ridges. In recent work on gravel dominated barriers around the mid-latitude North Atlantic (Orford et al, 1996, 2002, and 2007) the paraglacial reworking of limited point sources of glacigenic gravel sediments has been shown to account for the significant realisation over time of swash-aligned barriers, compared to decreasing presence of drift-aligned barriers. This is linked to diminishing sediment supply with, as a consequence, increasing cannibalisation of beach ridge sections, linking the transition from drift to swash-alignment. The questions remain as to i) whether this barrier transition is dominated by drift to swash status and ii) whether it is always a function of sediment supply? Is it feasible for forcing conditions (wave climate) to account for this transition, and thereby for a reverse transition to occur? These two questions have been considered in the context of a substantial late Holocene, mixed drift and swash aligned beach ridge series in the macro-tidal San Sebastian Bay, eastern Tierra del Fuego (Isla and Bujalesky, 2000). We consider through a 14C dated ridge sequence the controlling factors by which a prograded, interdigitated drift and swash-aligned gravel beach ridge series (still operating at present) may have been generated. Given the constancy of gravel-rich sediment supply (?), the possibilities of millennium periodic shifts in the interaction between Southern Hemisphere westerly winds (offshore) and incident northeasterly waves is considered as a possible generator of alternation of these characteristic coastal forms.