Early Turonian pelagic sedimentation at Moria (Umbria-Marche, Italy): Primary and diagenetic controls on lithological oscillations

Abstract

The Upper Cretaceous of the Umbria-Marche Basin (Italy) consists of an alternation of calcareous and siliceous fine-grained sedimentary rocks in which the calcareous lithologies are predominant. The Cenomanian succession is dominated by white, micritic limestones and contains a black shale horizon (Bonarelli Level) in its uppermost part which is the regional expression of oceanic anoxic event 2. In the Turonian, the rocks gradually become red upwards and are referred to as Cretaceous oceanic red beds. The lithological variability in the succession reflects the local response of sedimentary systems to varying local and global environmental conditions. In this study, local processes behind the transition from black shale to red bed deposition and, in particular, behind lithological oscillations within the lowermost Turonian are studied and placed in their global context. Sediment colour, composition, geochemistry and nannofossil content were studied using samples of the Scaglia Formation at Moria (Umbria-Marche Basin). The results show that, following the Cenomanian/ Turonian boundary, terrigenous input decreased and its composition was different in calcareous and siliceous oozes. Organic productivity was high during the deposition of siliceous ooze due to upwelling and varied on orbital timescales. The alternation between the siliceous and calcareous ooze was further controlled by carbonate dissolution. Small fluctuations in the calcite compensation depth (CCD) had a significant impact on sediment composition because depositional depth was between the lysocline and the CCD. Diagenesis resulted in the segregation of carbonate and silica, in the concentration of terrigenous material in smarl interbeds and in the enhancement of bedding. After the earliest Turonian, ocean circulation patterns changed, seawater temperatures declined slowly and seawater became better oxygenated. Productivity and terrigenous input also declined and the resulting combination of oxygen demand in the sediments and sedimentation rates favoured the formation of authigenic haematite in the sediments which led to their reddening.