Geochemistry of Lower Palaeogene bauxites – a unique signature for the tectonostratigraphic evolution of part of the Croatian Karst Zoran Peh and Erli Kovačević Galović

The origin and emplacement of Lower Paleogene Bauxites discussed here relies on recent perceptions concerning the development of the collision-induced diachronous discontinuity surfaces in certain parts of the Adriatic-Dinaric Carbonate Platform (ADCP), and in the Istrian Peninsula in particular. This process, related to forebulge uplift during the initial stages of the orogeny, is amply recorded in the geochemical signature of related Istrian Lower Paleogene bauxites (ILPB) as was shown in previous investigations. As their self-evident extension, due attention is paid here to the effects of a flexural bulge in another part of the platform with its own suite of Lower Paleogene bauxite deposits (LPB), notably in its Dinaridic unit. Explication of the typical patterns of bauxite formation is grounded in the development of discriminant function models based on the Compositional Data (CoDa) analysis of geochemical data. This method provides the solid ground for separation of the various a priori defined bauxite groups deposited in the different subaerially exposed platform palaeoenvironments. In the final analysis, the discriminant model characteristic for the westernmost compartment of the Adriatic SW Unit (Istrian Karst) is cross-compared with the analogous model of the entire ADCP taken from earlier investigation but reconsidered within the CoDa framework. It is done with the purpose of assessing the geochemical correspondence between the two event-markers (ILPB and LPB) and clarifying the nature of the pertinent tectonostratigraphic constraints. The computer models show that the forebulge unconformity is not a widespread phenomenon, being distinctly recognized only in the Istrian part of the ADCP. The most recent investigations of bauxites in Croatia had addressed the root causes of geochemical variations observed in the bauxite deposits of Lower Palaeogene age (LPB) from the Croatian karst (KOVAČEVIĆ GALOVIĆ et al., 2012) (Fig. 1), focusing especially on the Istrian Peninsula due to its specific geodynamic evolution through Cretaceous to Palaeogene times (PEH & KOVAČEVIĆ GALOVIĆ, 2014). These studies attempted to explain regular variations in bauxite geochemistry both within regional (ADCP) and subregional (Istrian Peninsula) contexts highlighting changes of depositional and diagenetic characteristics during subaerial exposure of the platform. Although Istrian Lower Palaeogene bauxites (ILPB) accommodate the overall geochemical trend (marked primarily by the southeastward increase in chromium content), their stratigraphic position at the regional K–Pg unconformity occurring in the Adriatic part of the platform (Adriatic domain, sensu KORBAR, 2009) must be treated with due caution because of the implicit possibility of a unique geochemical signature resulting from tectonostratigraphic constraints. Essentially, the implied peculiarity of the ILPB revolves around the major hiatus between the Cretaceous and Palaeogene carbonate successions, which unlike the more south-eastern portions of the ADCP, lasted seemingly longer in Istria (e.g. VELIĆ et al., 1995; MATIČEC et al., 1996; TIŠLJAR et al., 1998; VLAHOVIĆ et al., 2005; OTONIČAR, 2007, 2008; KORBAR, 2009; CVETKO TEŠOVIĆ et al., 2011). As a result, the ILPB are underlain by carbonate rocks characterized by a wide age interval (from Albian to Santonian-Coniacian) suggesting the predominance of tectonic over eustatic controls due to differential