Diagenetic and environmental control of the clay mineralogy, organic matter and stable isotopes (C, O) of Jurassic (Pliensbachian-lowermost Toarcian) sediments of the Rodiles section (Asturian Basin, Northern Spain). (May 2020)

• Record Type: Journal Article
• Authors: Deconinck, Jean-François; Gómez, Juan Jose; Baudin, François; Biscay, Hugo; Bruneau, Ludovic; Cocquerez, Théophile; Mathieu, Olivier; Pellenard, Pierre; Santoni, Anne-Lise
• Is Part Of: Marine and petroleum geology. Volume 115(2020)
• Publication Date: 2020-05
• Subjects: Clay minerals -- Early Jurassic -- Iberian peninsula -- Organic matter -- Paleoclimate -- Pliensbachian -- Stable isotopes; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2020.104286
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Clay mineralogical and geochemical analyses (δ 18 Ocarb, and paired carbonate and organic matter carbon isotope), completed by the characterization of organic matter, have been performed on the Pliensbachian sediments exposed on the Rodiles section (Asturian Basin, Northern Spain). The objectives were to precise the Pliensbachian climate evolution at a relatively low paleolatitude of the Tethyan domain by comparison with previously published high-resolution clay mineralogical and geochemical data from the NW Tethyan domain (e.g. Paris Basin and Cardigan Bay Basin, West Wales). The clay mineral assemblages are dominantly composed of illite and illite-smectite mixed-layers (I–S) associated with minor proportions of chlorite and kaolinite. Smectites are not identified what may result either to their absence at the time of sedimentation or to their progressive illitization due to the burial diagenesis. Rock-Eval data show that organic matter reaching more than 5% in black-shale horizons is thermally mature with Tmax comprised between 444 and 454 °C indicating that the section is in the oil window and that the burial temperature reached or exceeded 80–90 °C. The low δ 18 Obulk values comprised between −3 and −6‰ clearly results from a diagenetic influence (thermal effect due to burial) but their fluctuations compared with δ 18 O measured on well-preserved belemnites guards show that the overall trend is however preserved. By comparison with higher latitude sedimentary basins, the most striking feature is the very small proportions of kaolinite (generally less than 5% while the proportions of this mineral reach 40% at higher paleolatitudes), except at the Pliensbachian/Toarcian transition, where the proportion of kaolinite increases significantly. Clay minerals likely originated from the erosion of the Iberian Massif located to the South of the Asturian Basin in a semi-arid climatic belt. This may explain the scarcity of kaolinite by comparison with sedimentary basins located at higher latitude influenced by more humid climate. The increasing proportions of kaolinite by the end of the Pliensbachian suggest southward expansion of the humid belt as a consequence of cooler temperatures and the establishment of steeper latitudinal gradients of temperature likely resulting from the formation of restricted polar ice cap. The main δ 13 C (on both 13 Cinorg and 13 Corg ) Pliensbachian excursions including the negative excursion of the Sinemurian/Pliensbachian Boundary Event (SPBE) and the Late Pliensbachian positive excursion are well recognized. Highlights: Clay mineralogy and organic matter maturity indicate that the Rodiles section reached the oil window and a maximum burial temperature around 80–90 °C. Despite the diagenetic influence, the scarcity of kaolinite suggest that a semi-arid climate prevailed over the Iberian massif. Southward shift of the humid belt due to steeper latitudinal gradients of temperature by end of the Pliensbachian.