Climate forcing of fine-grained deep-marine systems in an active tectonic setting: Middle Eocene, Ainsa Basin, Spanish Pyrenees

Abstract

A multi-proxy approach to understand environmental change in deep time was undertaken on about 150m of core from a 230m-long Middle Eocene core from the Ainsa basin, Spanish Pyrenees, representing deep-marine siliciclastic sediments, using detailed sedimentary logging, high-resolution multi-element XRF geochemistry, total organic carbon, and stable carbon isotopes. The Well A6 was drilled, as part of an industry–university consortium, through siltstones, fine-/very fine-grained sandstone turbidites, and hemipelagic structureless mudstones, that were deposited as overbank and off-axis deposits from a sandy submarine fan, and interfan deposits. For comparative mineralogy between the sandstone turbidites and siltstones, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were undertaken on selected samples. The sandstone turbidites show enrichment of detrital elements such as Si, Zr and Ti, that can be linked to greater quartz and heavy-mineral content compared with adjacent siltstones. Structureless hemipelagic mudstones comprise mainly clay minerals and carbonate. We interpret the sandstone turbidites as from hyperpycnal flows during high river sediment discharge, whilst hemipelagic mudstones resulted from the suspension fall-out of hypopycnal flows. Cyclostratigraphic analysis of the core reveals Milankovitch cyclicity at frequencies of ~0.03cycles/m (short eccentricity), ~0.09cycles/m (obliquity), ~0.15cycles/m (precession couplet) and ~0.19cycles/m (precession couplet). Orbital parameters appear to have controlled the cyclic delivery of coarser-grained sediment by turbidity currents. Two equally plausible depositional models, both as Milankovitch-driven, can explain the cyclical changes in the deep-marine sediments: (1) climatic cycles, with humid periods of enhanced chemical weathering, increased storminess and greater riverine run-off, leading to high sediment flux to the deep basin as sandstone turbidites; (2) climatic cycles, with cooler conditions linked to high-frequency small-scale eustatic sea-level fluctuations, with lowstand shelf-edge delta progradation, resulting in greater volumes of coarse detrital sediment to the seafloor by hyperpycnal flows. This study provides an insight into the likely depositional effects of orbitally-induced climate change on the nature and delivery of terrigenous sediment into deep-marine environments.