Integration of gamma ray spectrometry, magnetic susceptibility and calcareous nannofossils for interpreting environmental perturbations: An example from the Jenkyns Event (lower Toarcian) from South Iberian Palaeomargin (Median Subbetic, SE Spain)

摘要

The Pliensbachian-Toarcian sedimentary rocks of the Median Subbetic (Betic Cordillera, SE Spain) were deposited in the South Iberian Palaeomargin under hemipelagic conditions. In the Arroyo Mingarron section (SE Spain) these deposits are composed by a marl and marl and limestone alternation with abundant radiolarians. The application of magnetic susceptibility and gamma ray spectrometry to these deposits allows to interpret environmental changes related to the fluctuations in the continental influx of terrigenous and phytodetritus to a hemipelagic setting in the South Iberian Palaeomargin. The increase of terrigenous input was coincident with the negative carbon isotopic excursion (CIE) at the base of the NJT6 nannofossil Zone that correlates with the Jenkyns Event. The increase of Th and K content is related to enhanced detrital input, especially oxides and silicates, including clay minerals. The gamma ray signal derived from U is comparatively low with respect to K and Th. Nevertheless, the U content shows an increase in the base of NJT6 nannofossil Zone that indicates a relative increase of organic matter content coincident with the Jenkyns Event. Changes in the calcareous nannofossil assemblages at the beginning of the NJT6a nannofossil Subzone and, in particular, the sharp decrease in abundance of the species Mitrolitus jansae during the negative CIE of the Jenkyns Event are interpreted as related to sea-water stratification and a probable poor oxygenation making inhospitable the deep photic zone during this event.