Sedimentary and chemostratigraphic record of climatic cycles in Lower Pliensbachian marl–limestone platform successions of Asturias (North Spain)

A combined sedimentological, lithological and chemostratigraphical (Mg/Ca, δ13C, δ18O) analysis of the Lower Pliensbachian marl–limestone platform successions exposed along the Asturias coastline (northern Spain) has resulted in the characterization of high-frequency cycles. The highest-order sedimentary cycles (i.e. elementary cycles) are centimeter- to deciemeter-thick alternations of bioclastic and muddy laminated/burrowed facies, which do not match the marl–limestone couplets. They encompass three sedimentary stages: deposition from storm-density currents (bioclastic facies), dominant lateral advection of continental terrigenous mud accumulated on to an oxygen-deficient seafloor (laminated facies), and recovery of bottom oxygenation involving the burrowing of laminated sediments (burrowed facies). The close match between the number of elementary cycles recorded during the Jamesoni Subzone in Asturias and Yorkshire (Northern England) gives support to the idea of the influence of a regional climatic factor (i.e. millennial-scale cyclicity).Decimeter- to meter-scale cycles formed by bundles of elementary cycles are thought to record orbitally driven climatic changes (precession or obliquity, depending on the time calibration considered). Lower hemicycles of bundles are dominated by marls/calcareous mudstones, with decreasing burrowing and eventual preservation of laminated facies. They formed during humid periods, which controlled an increase in freshwater and terrigenous input to the platform and quasi-estuarine circulation promoting bottom-anoxia. Upper hemicycles of bundles are dominated by burrowed and bioclastic limestones, thought to be formed under arid conditions with anti-estuarine circulation and an increase of shallow carbonate production and offshore resedimentation. Chemostratigraphic data from belemnites recorded in the muddy laminated and burrowed facies indicate that significant concomitant shifts in δ13C and δ18O occurred during the lower hemicycles of bundles (i.e., humid periods). Isotopic shifts are interpreted as reflecting changes in the balance between the proximity of the terrestrial sources, the local incursion of deeper cooler waters, the storage of organic matter within sediments, and the re-cycling of organic matter, due to long-term relative sea-level rise.

► Millennial-scale cycles are centimeter- to decimeter-thick alternations of bioclastic/muddy facies.
► Marl–limestone couplets are mostly diagenetic in origin.
► Facies, lithology and C and O isotopes reflect decimeter- to meter-scale orbital cycles.
► Marly dominated hemicycles with decreasing burrowing formed during humid periods.
► Hemicycles dominated by burrowed limestones related to arid periods.