A Paleogene mixed carbonate-siliciclastic system in the western Tethys: spectral gamma-ray as a tool for the reconstruction of paleoclimate and transgressive-regressive cycles

Abstract

The mixed carbonate-siliciclastic ramps of the Dauphinois-Provençal domain in northwest Italy are characterized by a common development process during the Paleogene. Spectral gamma-ray (SGR) analysis indicates the concentration of potassium (K), uranium (U), and thorium (Th) in the total gamma-ray signal, permitting the identification of subtle stratigraphic compositional trends, sedimentary minerals, paleoclimatic conditions, and the individualization of lithofacies characters through the stacking pattern cycles. This study integrates known lithological and fossil content analysis of the section with new independent data from the spectral gamma-ray (SGR) to explore the paleoclimate and stacking patterns for defining large-scale depositional cycles along a Paleogene succession in the southwestern Alps.The differences between the outcropping Microcodium Formation and the Capo Mortola Calcarenite Formation are indicated by the SGR, conventionally shown on a linear scale as American Petroleum Institute (API) units, i.e., Th/U, and Th/K ratios. Our data indicate lithofacies associated with normal marine to continental conditions and the presence of abundant smectite, rather than kaolinite, suggesting that the prevailing climate was characterized by prolonged dry periods. Furthermore, the SGR (API) reconstruction of the overall short cycles reflects a progradational and/or retrogradational stacking geometry that accumulated throughout the whole succession. However, the use of the derivative trend analysis (DTA) of the SGR indicates the establishment of long cycles in both formations. The short cycles observed in the Capo Mortola Calcarenite Formation are of particular importance because they characterize the marine transgression of the middle-upper Eocene, transgressive upon the Upper Cretaceous beds, and permit pinpointing the onset of the drowning ramp, which can be correlated regionally with sections spanning from the French-Italian Maritime Alps to the France-Switzerland border.