Evolution of Late Cretaceous Si cycling reflected in the formation of siliceous nodules (flints and cherts)

Abstract

During the Late Cretaceous, the dissolved silicon (DSi) concentration in the marine, pelagic environment of the epicontinental European Basin was controlled by the oceanic influx of DSi-enriched water. The inflow of oceanic water masses caused parts of the shelf basin that were influenced by them to reach DSi concentrations sufficient for opal-CT formation, leading to the abundant growth of siliceous sponges and the formation of carbonate–siliceous rocks (opoka). In the part of the basin, which was shielded from such oceanic influence, the DSi concentration was depleted (below the level necessary for quartz crystallisation) and pure carbonate sedimentation (chalk) occurred. In this study, the petrography, mineralogy, and microtextures of siliceous nodules (flints and cherts), which occur as abundant horizons within Campanian deposits of the European Basin, revealed events during which the DSi concentration was significantly elevated in both parts of the basin, leading to the crystallisation of silica polymorphs in the form of flint and chert nodules. The formation of siliceous nodule horizons was related to spatial and temporal fluctuations in the Campanian Si cycle triggered by large-scale hydrothermal and volcanic activity associated with Late Cretaceous the rearrangement of major plate boundaries in the Atlantic and Tethys Oceans. During such processes, a vast amount of DSi would have been released into the seawater and transported to the European Basin via oceanic inflow. The DSi-enriched seawater concentration in this shelf basin did not increase biosiliceous productivity, but instead triggered the abiotic precipitation of siliceous nodules during early diagenesis. The proposed model of an abiotic (volcano/hydrothermal) control for the formation of these nodules agrees with the existing literature on quartz precipitation hypothesis and counter to the widely discussed maturation hypothesis. Moreover, this proposed model suggests that, despite the gradual decline in seawater DSi concentrations throughout Earth's history, disturbances to the global geodynamic system caused by major plate boundary rearrangements governed the fluctuations of DSi concentration in the Cretaceous and periodically allowed for the formation of siliceous nodules. This study is the most exhaustive to-date, comparing to datasets from all previous petrographic, mineralogical and microtextural research and comprises 67 horizons of siliceous nodules form various parts of the Campanian European Basin. • The DSi concentration in Campanian European Basin were controlled by oceanic inflows. • The maturation and replacement hypotheses does not explain the origin of Campanian siliceous nodules. • The siliceous nodules record an oceanic inflows of DSi of hydrothermal/volcanic origin. • The siliceous nodules were formed in a abiotically during early diagenesis.