The southwest shore of the island of Dugi Otok is characterized by the unusual sample occurrence
of Mn-hydrated oxide mineralization in the form of botryoidally and globular, fine-laminated
concentric aggregates, crusting the Upper Cretaceous backshore limestones. The mineralization
consists mainly of Fe impoverished 10-Å tecto-manganate todorokite (enriched in Mg and transition
metals) accompanied by accessory MnO·OH. Todorokite chemical composition is typical of a
hydrogenous origin in an oxic shallow marine environment. According to our model, Mn has been
recurrently leached and mobilized from the Late Pleistocene sea floor sediments located around 50
nautical miles south of Dugi Otok. We propose a link between the Mn-geochemical anomaly of these
sediments and the alteration of distinct, spatial close pre-Holocene tephra, dispersed over the entire
recent Adriatic area.
Key words: 10-Å manganates, todorokite, hydrogenous origin, pre-Holocene tephra, Dugi Otok
island, Adriatic
INTRODUCTION
The island of Dugi Otok is a part of the
Adriatic carbonate platform and belongs to
the Dinaridic orogenic mountain chain. The
coastal outline of the orogenic topography
was ultimately reshaped by extensive sea
level rise during Holocene deglaciation (Fig.
1). The island consists entirely of gastropods
fosiliferous Upper Cretaceous limestone and
minor dolomite (MAMUŽIĆ et al., 1967) showing
various karstification morphology phenomena
(DŽAJA, 2003). The Mn-mineralization presented
by the predominant todorokite and accessory
