Geochemistry of Lower Palaeogene bauxites – unique signature of the tectonostratigraphic evolution of a part of the Croatian Karst

Abstract

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.

Geochemical records in subaerial exposure environments in Croatia using discriminant function analysis of bauxite data

“…This method provides solid ground for the separation of the various a priori defined bauxite groups deposited in the changing, subaerially exposed, ADCP paleoenvironments from the earliest (Upper Triassic) to the latest (Miocene) periods of bauxite formation in accordance with their geochemical signature. In a sense, this study is a continuation of earlier investigations performed more locally with focus on the single (K/Pg) unconformity at the same platform (Kovačević Peh and Kovačević Galović 2014;Peh and Kovačević Galović 2016) which have been launched with the purpose of initiating the regional correlation studies within the entire area of the Croatian Karst Dinarides. This study presents a step further, trying to elucidate the patterns of bauxite formation through time as defined by eight successive regional unconformities ranging from Triassic to Neogene times.…”

“…The basic premises of the recent investigations of bauxites in Croatia converged on geochemical variations detected in the bauxite deposits of the Lower Paleogene age (LPB) from the Croatian karst (Kovačević . They employed statistical tools (discriminant function analysis), with special reference to the Istrian Peninsula because of its distinct geodynamic evolution through Cretaceous to Paleogene times (Peh and Kovačević Galović, 2014) and its relations to the rest of the ADCP in light of the ergodic hypothesis (swapping spatial for temporal variability) via comparison between two statistical (discriminant) models (Peh and Kovačević Galović, 2016). The latest study addressed the issues of the bauxite formation during the Miocene climatic optimum, based mostly on highprecision zircon geochronology (Brlek et al, 2021).…”

Discrimination of topsoil environments in a karst landscape: an outcome of a geochemical mapping campaign

“…In addition, data processed in this way can develop a mapping quality that explains the relationship among the original variables more clearly. The aims and principles of DFA are described in detail elsewhere (e.g., [18,20,61]) and have been explained repeatedly by the present authors in various geochemical and environmental studies [23,26,28,34,49,50,52,53,65]. It suffices to say in this paper that DFA is a multivariate method that is particularly effective in pursuing the major sources of between-group differences which, in this study, derive their origin from the accumulation of heavy metals and possibly harmful elements (PHE) in karst soils.…”

“…To this purpose, the topsoils collected during the multi-year geochemical mapping campaign (GAC) are examined in light of the various geological and environmental criteria [40-42, 51, 58, 59]. These criteria, as in other geochemical studies on similar problems in the area [34,52,53], are exploited in this study as the most revealing avenues through which the processes mentioned above can be most effectively understood. The criteria are autonomous with regard to the soil geochemistry such as the geological (lithological) setting, description of land use, soil types, or geographical position (with climate implications), which provide the most efficient means of a priori arrangement of the soil samples into a number of coherent and exhaustive statistical groups.…”

Geological and structural map of the southeastern Pag Island, Croatia: field constraints on the Cretaceous - Eocene evolution of the Dinarides foreland

“…Relative sea level changes occurred due to the interplay between global eustatic variations and synsedimentary tectonics during the polyphasic convergence between Eurasia and Adria plates, which began in Late Jurassic times (Aubouin et al, 1970;Schmid et al, 2008) and is still active (Kastelic and Carafa, 2012;Kastelic et al, 2013). With the propagation of the orogenic front to the SW, the AdCP underwent dissection and emersion during a major event recorded by a regional unconformity surface, locally bearing bauxite (Vlahović et al, 2005;Kovačević Galović et al, 2012;Peh and Kovačević Galović, 2016). Sedimentation resumed in Early Eocene times, when the Northern Dalmatia constituted the foreland basin of the Dinarides.…”