Environmental changes in the South-Iberian Palaeomargin from the latest Pliensbachian to middle Toarcian (Early Jurassic) based on mineralogical data and geochemical proxies

Abstract

The Arroyo Mingarrón section from the Median Subbetic (Betic Cordillera, South Spain) consists of hemipelagic sediments, including marls, limestones, and condensed limestones of the uppermost Pliensbachian to middle Toarcian (NJT5a to NJT7c nannofossil subzones). Mineralogical and geochemical analyses suggest that environmental conditions in this part of the Western Tethys were predominantly arid. The dominance of illite and quartz indicates physical weathering and arid conditions in the emerged lands surrounding the South-Iberian Palaeomargin.

This section includes the negative carbon isotopic excursion (CIE) that characterizes the Jenkyns Event located in the boundary between NJT5 and NJT6 zones. In Arroyo Mingarrón section, the Jenkyns Event is characterized by a decrease in carbonate content and enhanced detrital input of siliciclastics. The decrease in carbonate productivity during the event is evidenced by changes in the composition of the calcareous nannofossil assemblages with abundance and diversity decrease, as well as reduced size of Schizosphaerella. Anoxic conditions were not developed at the sea bottom considering the record of trace fossils and microfossils at the base of the NJT6 Zone, as well as the low total organic carbon (< 0.3 wt%). High proportions of illite, the decrease in plagioclase index of alteration (PIA) and the increase in index of compositional variability (ICV) indicate relatively more arid conditions during the Jenkyns Event. The lower part of the middle Toarcian shows the evolution of the studied sector from a subsident area to a pelagic swell with increasing carbonate content and progressively more condensed facies toward the top. The record of kaolinite and increased values of weathering proxies (CIA and PIA) and the fluvial detrital proxy K/Al points to a relative increase in humidity and weathering. The development of the pelagic swell in the upper part of the middle Toarcian is parallel to a return to enhanced dry climatic conditions as indicated by the decrease in clay content with the absence of kaolinite, the decrease of in fluvial detrital proxy K/Al, and the decrease in weathering proxies (PIA and C-value).