Numerical Analysis of Sedimentary Components: A Key for Interpretation of Macroscopic and Microscopic Features in Ammonitico Rosso Facies (Uppermost Jurassic-lowermost Cretaceous)

Abstract

ABSTRACT The relationship between macroscopic and microscopic features in the widespread Tethyan ammonitico rosso facies, traditionally considered comparatively uniform, is analyzed for the first time using the discriminatory power of numerical analysis. This methodology has been applied to selected sedimentary components in four macroscopically distinguished type facies and 91 thin sections studied in three uppermost Jurassic (Tithonian)-lowermost Cretaceous (Lower-Middle Berriasian) sections from the Sierra Norte (Mallorca, Spain). Cluster analysis (17 variables) and principal-component analysis (7 variables) reveal microfacies grouping by stratigraphy and depositional conditions. Without resedimentation, high values of the Dlink/Dmax ratio in cluster analysis are inter reted as caused by synsedimentary and early diagenetic dissolution phenomena, bioturbation, and time-averaging effects related to low sedimentation rates. In addition, the resulting information provided by principal-component analysis (Components 1, 2, and 3 explaining 75% of variance) emphasizes subtle depositional trends (differential fabrics) in such a homogeneous facies by skeletal composition, in which winnowing and slow sedimentation were relevant. Biological, taphonomic, and sedimentary processes affected significant variables, explaining 85% of the total variance.