Cretaceous (Maastrichtian) Vertisols of the Serra da Galga Member (Bauru Basin, Brazil): Implications for palaeoclimate and palaeoecology

Abstract

In this paper, we report the occurrence of Cretaceous (Maastrichtian) Vertisols from three profiles in the Serra da Galga Member (Marília Formation) of the Bauru Basin, Brazil, which developed as distributary fluvial deposits. Through integrated analysis of macroscopic and microscopic pedogenic features, clay mineralogy, whole-rock geochemistry, and ichnofossil assemblages, we assess the roles of climate, biota, and topography in shaping pedogenesis and organismal behavior under seasonal climatic regimes. The primary pedogenic processes identified include argiliturbation, clay translocation, calcification, and biological activity. Paleoclimatic reconstructions indicate mean annual precipitation ranging from 290 to 614 mm/year (mean: 504 ± 33 mm/year) and mean annual temperature between 10.6 °C and 12.2 °C (mean: 11.4 ± 0.62 °C). Köppen-based aridity indices suggest that all soil horizons formed under semiarid to subhumid conditions. The ichnoassemblages include invertebrate traces (Taenidium, Skolithos, and Beaconites) and plant-related structures (calcareous rhizoconcretions, rhizohalos, and calcified bulbilous structures), exhibiting low to moderate bioturbation degree. The correlation between ichnofossil distribution, bioturbation intensity, geochemical data, and pedogenic characteristics suggests that the Vertisols developed under distinct seasonal climatic conditions. These soils displayed moderate drainage, characterized by short-term rises in the water table within the floodplain, followed by prolonged dry periods. Soil moisture availability played a crucial role in regulating biotic activity throughout these seasonal fluctuations. The results obtained not only enhance the understanding of pedogenetic and paleoecological processes during the Maastrichtian in the Bauru Basin but also provide a model for interpreting Vertisol formation in semiarid environments at mid-latitudes. Furthermore, they may provide a useful basis for global paleoclimate reconstructions.