Formation and evolution of barrier coast under condition of sand scarcity; an example from the Polish coast of the southern Baltic sea

Abstract

The formation and evolutionary history of intracontinental, tideless sea barrier coasts is still poorly known in comparison to open-ocean and tidal sea barrier systems. This paper describes the morpho-geological structure of the barriers and its substrate along a 90-km section of the middle part of the Polish southern Baltic coast. The main goal of the study is a better insight into the evolution of the barriers in case of sand scarcity and relations to sea level changes. The study area was examined by analysis of the published and historical data from 120 borehole profiles supplemented by new 18 core drillings. In addition to lithological and biostratigraphical data, a large set of radiocarbon ages was acquired and analysed. The barriers during its whole middle and late Holocene history were retrogradational despite of varied geological and morphological features of its substrate. The average rate of barrier's retreat has shown considerable variation in a close relation to the rates of sea-level rise. During the period 8500-7500 yr b2k when the sea level rose by 6–5 mm/yr, the coast retreated ca. 4.6–3.5 m/yr. Between 7500 and 6000 yr b2k, the sea-level rise slowed down to ca. 1.7 mm/yr and the rate of coastline retreat fell to ca. 1.0 m/yr. During the last 6000 years, coastal retreat slowed down to ca. 0.3 m/yr as the average sea level rise was ca. 0.5 mm/yr. The permanent retrogradational history of the barrier coast, even in the case of sand availability in the barrier's substrate, is explained by the existence of a divergence zone of longshore currents and a related source zone for the alongshore sandy streams. The divergence zone is present within the study area not only today, but it has existed most probably also at least since the beginning of the late Holocene. As a result, the erosional trend (sand scarcity) dominates here, which contributes to the presence of retrogradational barriers.