Contributions to the ichnology and ichnofabrics of deep marine systems.

摘要

The new data presented in this thesis demonstrates that detailed palaeoenvironmental reconstructions of deep marine and turbidite settings are improved with the integration of ichnofabric analysis. Facies analysis was integrated with ichnofabric analysis in the Eocene/Oligocene sand-rich turbidites of the Gres d'Annot Basin, SE France. Bed-by-bed logging demonstrated that ichnofabric analysis can be used as a high resolution tool for reconstructing depositional palaeoenvironments and their variability in deep-water systems. The Ophiomorpha rudis ichnofabric association (analogous to the Ophiomorpha rudis ichnosubfacies of the Nereites ichnofacies) are present in deposits of high current energy settings and low nutrient levels like channelized sands and thick-bedded turbidites in the Grès d'Annot Basin. The nature of the Gres d'Annot Basin as a thrust sheet-top basin confined and channelled the infilling turbidites. This confining of flows suppressed the deposition of thin-bedded and heterolithic turbidites which allow the preservation of a diverse ichnofauna. Low ichnodiversity (restricted to ichnospecies of Ophiomorpha and Thalassinoides ) result from a combination of nutrient stress and the confined nature of the Gres d'Annot Basin. When thin-bedded heterolithic turbidites are preserved, however, they contain a higher ichnodiversity, density, and abundance of trace fossils than sand-rich facies. The Ophiomorpha annulata--Planolites and Phycosiphon-Ophiomorpha rudis ichnofabrics represent the activity of vagile, deposit feeding and farming organisms in quiescent basin slope settings and channel fill facies where turbidites of low erosive power allowed the preservation of a greater number of ichnogenera.;The Upper Cretaceous Wyandot Formation, offshore Nova Scotia, is an autochthonous carbonate reservoir facies. This pelagic system was used to demonstrate that ichnofabric analysis can be successfully employed across a range of deep-water palaeoenvironments. The Wyandot Formation is divided into two lithofacies: chalk and marlstone. Ichnofabric analysis showed that the Chondrites-Thalassinoides-Zoophycos ichnofabric dominates both lithofacies. High densities and abundances of ichnotaxa in this ichnofabric, in conjunction with a highly bioturbated mixed layer (BI 5-6), imply a well oxygenated sediment-water interface. Environmental perturbations are interpreted through changes to Chondrites-dominated ichnofabrics. In the Wyandot Formation this resulted from increased sedimented organic matter, causing the redox front to rise in the sediment, leading to porewater anoxia, and the exclusion of all but the stress-tolerant Chondrites tracemaker. (Abstract shortened by UMI.);Ophiomorpha is commonly observed at sandstone-mudstone interfaces in turbidite systems, including the Gres d'Annot Basin. This paper presents evidence that interface trace fossils represent a deep-sediment farming strategy in deep-water settings with data obtained using mesocosm experiments, CT imagery, and geochemical analyses. The modem thalassinid shrimp Neotrypaea californiensis constructs interface burrows morphologically identical to Ophiomorpha rudis as observed in the Gres d'Annot (and other) systems. Mesocosms were mapped in three dimensions with CT imagery showing that burrows were preferentially constructed at sand-mud interfaces. The characteristic burrow-lining pellets of Ophiomorpha are traditionally considered to be a structural reinforcement to prevent burrow collapse. Our studies have shown that this might not be true in all cases as N. californiensis was observed feeding from the pellets indicating they might be a nutritional resource. Geochemical analyses showed higher total organic carbon values in burrow linings and pellets compared to unbioturbated samples from the same mesocosm. Observation showed that N. californiensis selected organic carbon-rich grains to make the pellets while higher TOC values indicate organic enrichment with burrow walls having an increased bacterial biomass (with bacterially produced dissolved organic carbon). Stable isotope analysis showed that the pellets where generally enriched in 13C compared to the isotopically lighter host sediment. This is interpreted as aerobic microbial fractionation. By bioirrigating their burrows N. californiensis (and other ancient Ophiomorpha tracemakers) encouraged the growth of aerobic microbial populations in burrow-lining pellets which they used as a nutritional resource, utilizing microbial dissolved organic carbon. This new data provides a model for the abundance of trace fossils at sandstone-mudstone interfaces in turbidites systems which had hitherto been unexplained.