THE PLIENSBACHIAN (EARLY JURASSIC) ANOXIC EVENT IN ASTURIAS, NORTHERNSPAIN - SANTA-MERA MEMBER, RODILES FORMATION

The Pliensbachian rhythmite of Asturias (Northern Spain) consists of a lternating layers of limestone and marl of postdepositional origin. Th e sediments are organized in shallowing upwards parasequences of varyi ng energy. Minor scale microcycles are found within the parasequences which probably represent the record of sudden high energy events alter nating with quiet hemipelagic or pelagic sedimentation. Organic petrog raphic and geochemical study of different marry samples throughout the sections did not show significant compositional differences. There ar e three laterally equivalent intervals with higher TOC and total sulph ur contents, higher concentration of aromatic compounds and heterocomp ounds, higher Pr/n-C-17 and Phy/n-C-18 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 low er part of the margaritatus zone. The uniformity of petrographic and q ualitative 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 prese rvation were likely responsible for the formation of the organic-rich intervals. Lower dissolved oxygen conditions probably produced a highe r preservation of both organic carbon and aromatic and heterocompounds , as indicated by the fluorescence properties and the infrared spectra of the extracts. (C) 1997 Elsevier Science Ltd.