Lipid and mineralogical composition of the Cretaceous black shale deposits of the Fardes Formation (southern Iberian Paleomargin, Betic Cordillera, south Spain)

Abstract

“Black shale”-type facies (1.3–1.5% TOC) registering the global Cenomanian-Turonian boundary oceanic anoxic event (CTBE) in a deep trough on the southern Iberian Paleomargin (Fardes Formation, Betic Cordillera) have been analyzed for their lipid composition and bulk clay mineralogy. The bulk mineralogy is composed of phyllosilicates (70–80%), quartz (10–30%) and feldspar (<5%). Clinoptilolite, opal CT, calcite, dolomite, pyrite, barite and halite are present in minor amounts. Dioctahedral smectite (50–85%) is the dominant phase among clays, concurrently with palygorskite (10–40%), illite (10–30%) and kaolinite/chlorite (<5%). These mineral associations and other inorganic geochemical features reflect the importance of oceanic bottom circulation for smectite particle transport coming from other neighbouring oceanic realms. The study of the solvent-extractable organic matter has shown that these black shales result from anoxic depositional events with high microbial reworking of autochthonous and allochthonous inputs. Distributions of C 21C 27 n-alkanes without odd-to-even carbon number preference occurring along with C 25C 28 iso- and anteiso-alkanes are found in all shales. Other microbial lipids are more specific of each black shale type. Hopanoid components, namely hopanoid acids, are more abundant in the sediments of the Cenomanian-Turonian boundary (CTB) whereas the underlying Cenomanian hemipelagite layers contain a number of acyclic compounds such as phytol-derived isoprenoid acids, linear methyl ketones, 5,6,10,14-trimethylpentadecan-2-one and C 23C 26 (ω-13) oxofatty acids, representing detrital contributions of microbially oxidized lipids. Archebacterial inputs are re presented by significant amounts of 2,6,10,14,18,22-hexamethyltetracosane in the CTB shales and by dimethylgeranylbenzene and dimethylfarnesylbenzene, in the underlying Cenomanian hemipelagite sections; the latter isoprenoid components are tentatively identified for the first time. Higher-plant components are also found. Suberin acids, namely C 22 and C 24 mono- and α,ω-dicarboxylic are abundant in the CTB shales and dammarane is a major component of the Cenomanian sediments.