Environmental changes in the lower Mirna River valley (Istria, Croatia) during Upper Holocene

Abstract

Sedimentological, macro- and micropaleontological analyses on 3 hand augering cores were carried out in the terminal sector of the Mirna River, in order to study the depositional facies and the environmental evolution. The cores allowed investigating the subsoil down to 13 m of depth, sampling a fairly continuous depositional sequence of estuarine, lagoon and alluvial sediments. The Holocene marine transgression entered upstream the present-day coast for at least 8 km, while in the last 7 ka it was followed by the progradation of Mirna delta. The protected coast offered by the lower valley and the strong input of fresh water led to the presence of a brackish microfauna even in front of the river mouth. The oldest sediments in the cores were characterized by dominance of Ammonia beccarii and significant proportions of Elphidium spp. and miliolids suggesting estuarine origin. Foraminiferal assemblage in overlying sediments became less diverse, relative abundance of Elphidium spp. and miliolids dropped, implying shift to transitional environment (inner lagoon facies, Bb). Sediments originating in hyposaline marshes (facies Ba) had the lowest foraminiferal species diversity index (A. beccarii predominated over Trochammina inflata and Haynesina sp.) Since late-Antiquity a significant alluvial matter input led to the deposition of several meters of silty clay sediments that in the core M3 are thicker than 9 m. The sedimentary supply has been partly increased by deforestation carried out in the Mirna catchment area that was particularly severe from the 15th to the 19th century and fed the fluvial system with large amount of material, allowing the fast progradation of the delta. This study highlights the potential role of the use of hand augering in sampling and describing the subsoil in reconstructing the geomorphological evolution of the area and supporting the study of past relative sea levels, climate changes, and anthropogenic activities that occurred during the Holocene.

Keywords

• sedimentary facies, lagoon, estuary, foraminifers, radiocarbon age, Adriatic Sea