Magnetic stratigraphy applied to synsedimentary slumps, turbidites, and basin analysis: The Scaglia Limestone at Furlo (Italy)

Abstract

The previously studied Italian Scaglia sections at Gubbio and Moria consist entirely of pelagic limestone and marl. By contrast, the Furlo section is characterized by synsedimentary slump folds and slide masses in the lower portion (Cenomanian-Coniacian) and by intercalation of white calcarenitic turbidites in the upper part (Campanian-Paleocene). Demagnetization to 20 mT and 350 °C or higher reveals a single, stable direction after removal of an initial low-coercivity component. Despite the stratigraphic complications, the pattern of polarity zones clearly matches Gubbio and the sea-floor anomalies. The abrupt faunal break at the end of the Cretaceous was located, and foram zonation shows that the very base of the Paleocene is missing. Sedimentation-rate determinations based on the magnetic polarity zonation have clarified the origin of stratigraphic complications in the Furlo Paleocene. Most samples were taken from pelagic beds, but ten samples from calcarenitic turbidites yielded directions indistinguishable from those of adjacent pelagic beds. Detailed sampling of a turbidite 33 cm thick confirmed that this lithology is an accurate recorder of the geomagnetic field. Synsedimentary slumps range from coherent slide blocks with undeformed bedding to smoothly curving folds indicating flow of very soft sediment. Coherent slide blocks show declinations rotated by tens of degrees away from the typical regional direction, but their inclinations are unaffected. Soft-sediment slump folds yield typical regional directions, indicating resetting of the remanent magnetization after deformation. Using stratigraphic, structural, and paleomagnetic data, it is possible to identify, in the lower part of the section, a basal nonslumped unit overlain by seven slump units with various characteristics, inter-layered with four units of pelagic sediments which were deposited on older slump units but which themselves were not affected by synsedimentary slumping. This paper emphasizes the value of paleomagnetic stratigraphy as a tool in detailed studies of sedimentology, stratigraphy, and basin analysis.