The Pliensbachian (Early Jurassic) “anoxic” event in Asturias, northern Spain: Santa Mera Member, Rodiles Formation

The Pliensbachian rhythmite of Asturias (Northern Spain) consists of alternating layers of limestone and marl of postdepositional origin. The sediments are organized in shallowing upwards parasequences of varying energy. Minor scale microcycles are found within the parasequences which probably represent the record of sudden high energy events alternating with quiet hemipelagic or pelagic sedimentation. Organic petrographic and geochemical study of different marly samples throughout the sections did not show significant compositional differences. There are three laterally equivalent intervals with higher TOC and total sulphur contents, higher concentration of aromatic compounds and heterocompounds, higher Pr/n-C17 and Phy/n-C18 ratios, and lower Pr/Phy ratio and CPI values. These intervals correspond to the end of the jamesoni zone, the boundary of the ibex and davoei zones and especially the lower part of the margaritatus zone. The uniformity of petrographic and qualitative chemical composition throughout the sections indicates that the nature of the organic matter supply did not change significantly during sedimentation, implying that changes in the conditions of preservation were likely responsible for the formation of the organic-rich intervals. Lower dissolved oxygen conditions probably produced a higher preservation of both organic carbon and aromatic and hetero-compounds, as indicated by the fluorescence properties and the infrared spectra of the extracts.