Chemostratigraphical characterisation of lower Silurian black shales from the Formigoso Formation (southern Cantabrian Mountains, Spain)

Abstract

This thesis focuses on the development of the black shales belonging to the lower Silurian Formigoso Formation (Fm.) situated within the Cantabrian Fold Belt, Northern Spain. The geochemical data obtained by the use of an energy-dispersive Niton XL3t XRF analyser under controlled laboratory conditions is comparable to that of conventional wavelength dispersive XRF analysers. The in-situ field analysis resulted in similar geochemical signals and elemental concentrations to that of the laboratory analysed samples. The high-resolution geochemical database, consisting of 4148 readings of the Formigoso Fm. was used to characterise the shales of the Formigoso Fm. relative to a number of international shale standards. Following this geochemical characterisation, a number of palaeoenvironmental proxies for [1] anoxia reconstructions, [2] palaeosalinity, [3] palaeohumidity, [4] weathering indices, and finally [5] bioproductivity were utilised. The combined outcome of these proxies together with field-based sedimentological observations led to a detailed reconstruction of the environment that prevailed during the deposition of the Bernesga Mb. black shales. A generic model is developed for the formation of these organically enriched sediments. This model is subsequently compared to previously published Silurian, black ‘hot’ shale models. The major parameters controlling the development of the Cantabrian black shale deposits were mainly the presence of a pronounced palaeorelief, which was generated by a combination of tectonic pulses related to the widening of the Rheic Ocean to the north and glaciogenic processes of the Hirnantian ice masses to the south. Orbital parameters or ‘deep time cycles’, mainly Obliquity and Eccentricity in combination with tectonic pulses led to high resolution sea-level oscillations and consequent cyclic behaviour of the redox elements recorded within the ‘hot’ shale deposits of the southern Cantabrian Basin. Additionally, 209 measurements of 76 international and inter-laboratory standards were performed to formulate new equations for the precise calibration of major and trace elements