Genesis of the Rovinj-1 bauxite deposit (Istria, Croatia): Record of palaeoclimatic trends and palaeoenvironmental changes during the latest Jurassic of the Adriatic Carbonate Platform

Bauxites are an excellent indicator of tropical palaeoclimate and subaerial exposure in the geologic record, where the study of their structures, textures, mineralogy, geochemistry, and cover deposits can provide invaluable insight into the palaeoclimatic and palaeoenvironmental conditions during their formation. The present study focuses on these aspects of the bauxite and its cover from the Rovinj-1 deposit with the aim of reconstructing the evolution of the palaeoclimate and palaeoenvironments during its formation.

The Rovinj-1 deposit formed during the early Kimmeridgian to late Tithonian subaerial exposure phase on the Istrian part of the Adriatic Carbonate Platform (AdCP). The bauxite formation can be divided into two phases: (1) the formation of pelitomorphic bauxite under humid tropical climate and (2) the erosion and redeposition of bauxite at the end of its formation due to climatic aridification. The climate aridification also led to increased iron oxide formation, which scavenged numerous trace elements, and especially light and middle rare earth elements, causing the increase in the upper part of the deposit. The upper part also exhibits negative Ce anomalies, while the lower part of displays high Ce anomalies. The bauxite formation was followed by an internal transgression during which a karstic lake formed above the bauxite. As the transgression progressed, the alternation of limestones and clays was deposited under anoxic to euxinic conditions in the early stages, and equivocal to oxic conditions towards the end of the transgression, changing lake conditions from restricted to open conditions and increasing marine influence as indicated by the formation of glauconite. This sequence was interpreted as a blue hole sequence. The clays/marls of the cover sequence also reflect climatic aridification as they show a gradual increase in mixed-layer illite–smectite and illite content and a decrease in kaolinite content. The end of this regional subaerial exposure phase and the restoration of full-marine carbonate deposition is marked by the deposition of the Kirmenjak unit covering the bauxite deposit. Overall, this study enabled a detailed palaeoclimatic and palaeoenvironmental reconstruction of the Istrian part of the AdCP, expanding the current knowledge of the conditions at the AdCP during the Late Jurassic.