Geotechnical Characterization and Parameter Correlation of Paleogene Formations in the Transylvanian Basin, Romania

The Transylvanian Basin is an intra-Carpathian sedimentary unit displaying complex tectonic and sedimentary evolution that started in the Late Cretaceous. This study presents a geotechnical characterization of three Paleogene lithostratigraphic units located in the northwestern part of the basin, i.e., Brebi, Mera, and Moigrad. These formations record the transition from marine carbonate facies to brackish and subsequently fluvial environments, controlled by tectonic uplifts, marine regressions, and fluctuations in sediment supply. A total of 583 soil samples were collected through geotechnical boreholes and analyzed in the laboratory according to EN ISO standards, assessing natural moisture content, bulk density, grain size distribution, Atterberg limits, carbonate content, unconfined compressive strength, and shear strength parameters. Characteristic values of these properties were determined based on probabilistic distributions. The analyzed formations exhibit well-differentiated lithological and geotechnical characteristics, primarily governed by the degree of plasticity and the presence of calcium carbonate. The Brebi Formation predominantly consists of medium-plasticity clays with highly to very highly carbonate content, indicating a partially cemented microstructure. The Mera Formation is mainly composed of high-plasticity clays having a variable content of carbonates, with frequent sandy intercalations, resulting in significant variability in mechanical properties. The Moigrad Formation consists of two distinct lithological complexes: a clay-rich complex composed of variably plastic calcareous clays spanning all four plasticity classes and a sandy unit made up of weakly cohesive sediments with a granular structure and locally developed carbonate microcementation.