Contributions to the ichnology and ichnofabrics of deep marine systems

Phillips, Christopher (2011) Contributions to the ichnology and ichnofabrics of deep marine systems. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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. This thesis is divided into three papers; each using ichnofabric analysis in end-member sedimentological, deep-water systems. In addition, neoichnological research conducted as part of this project links modern and ancient tracemaker behaviour to provide a new model for a commonly observed trend in trace fossil distribution in deep-water settings. -- Facies analysis was integrated with ichnofabric analysis in the Eocene/Oligocene sand-rich turbidites of the Grés 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 Grés d'Annot Basin. The nature of the Grés 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 Grés 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. This paper is the first ichnologically-based study of Grés d'Annot Basin and demonstrates the utility of ichnofabrics in reconstructing palaeoenvironments and changes in depositional conditions. -- Ophiomorpha is commonly observed at sandstone-mudstone interfaces in turbidite systems, including the Grés 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 modern thalassinid shrimp Neotrypaea californiensis constructs interface burrows morphologically identical to Ophiomorpha rudis as observed in the Grés 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. -- 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. This paper successfully tracks changes in the depositional environment of the Wyandot Formation through changes in ichnofabric. A petrographic study of the Wyandot Formation reveals textural and mineralogical differences between burrow fills and host sediment. It is considered that deposit feeding endobenthic organisms changed the clay mineral assemblages of the sediment by introducing authigenic kaolinite produced during sediment digestion. Burrow fills have a more diverse clay mineral assemblage with a 33% enrichment of kaolinite compared to the host sediment. This is interpreted to be the result of biological weathering and low temperature authigenesis in the digestive systems of the endofauna. This process introduces mineralogical heterogeneity into the Wyandot Formation reservoir facies. This work improves facies analysis and reservoir characterization in an understudied carbonate system. -- The work on the Grés d'Annot Basin and the Wyandot Formation presented in this thesis are the first ichnologically orientated studies of these deep-water systems. These data highlight the significance of trace fossil studies in deep-water systems previously understudied with respect to animal-sediment interactions. This thesis also shows that neoichnological research is a useful approach to understanding ancient tracemaker behaviours which can be used to identify palaeoenvironmental conditions. These papers, individually and cumulatively, demonstrate that ichnofabric analysis can be successfully integrated alongside facies analysis in deep-water systems. The trace fossil record can be used to identify changes in depositional and environmental conditions, as well as identifying physiologically stressed conditions on the ancient deep seafloor.

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