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

Abstract

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, especially light and middle rare earth elements, causing their increase in the upper part of the deposit. The upper part also exhibits negative Ce anomalies, while the lower part displays high Ce anomalies. The bauxite formation was followed by an internal transgression during which a karstic lake formed above the bauxite. At the beggining of the transgression, the deposition of the alternation of limestones and clays started under anoxic to euxinic conditions, and equivocal to oxic conditions towards the end of the transgression, indicating the shift in lake conditions from restricted to open which is also supported by the presence of glauconite as this reflects the increasing marine influence. 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 limestones of 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.