High-energy seafloor processes and biological reworking as first-order controls on mudstone composition and geochemistry

Harazim, D. (2014) High-energy seafloor processes and biological reworking as first-order controls on mudstone composition and geochemistry. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This PhD research project aims to develop a better understanding of how physical seafloor reworking and macrofaunal bioturbation control the lithofacies variability of mudstones deposited under high-energy seafloor conditions. To address this aim, bioturbated and unbioturbated mudstones from two natural laboratories in Newfoundland, Canada and Baja California, Mexico were investigated. Sections of interest were logged at a cm-scale and organic and inorganic geochemical measurements were performed on bioturbated and unbioturbated mudstones within both successions. To improve the fidelity of paleoenvironmental reconstructions these geochemical measurements were combined with high-quality sedimentological and ichnological datasets at a range of length scales. Unbioturbated mudstones in the Early Ordovician Bell Island Group, Newfoundland, previously reported to have been deposited under anoxic conditions, instead, most likely originated as hyperpycnal flows and wave-enhanced sediment gravity flows. The proximity to a fluvial source and residence time of rock components in the near-surface zone are interpreted to be the primary control on the compositional heterogeneity of mudstones within this Ordovician mud-dominated shoreface paleoenvironment. Following this, high-energy seafloor conditions are a more realistic explanation for the high presence of unbioturbated mudstones in the heterolithic Bell Island Group. Additionally, the formation mechanism of shrinkage (‘synaeresis’) cracks, which are sedimentological prime indicators for salinity fluctuations in marginal-marine environments, has been re-evaluated. Sediment cracking is proposed to form as an exclusively intrastratal process, independent of fluctuations of pore water salinity. Spatial rheological inhomogeneities associated with microbial mat decay shortly after burial are proposed to produce intrastratal shrinkage cracks. The effect of bioturbation on the geochemical variability of mudstones has been investigated within mudstones and finegrained sandstones of the Rosario Formation, Mexico. The spatial distribution of organic carbon and redox-sensitive trace elements is controlled by the feeding activity of grainsize selective vermiform animals and associated in-vivo alteration of weatheringsusceptible minerals. The reactivity of organic carbon is proposed to be a critical variable controlling pathways of diagenesis in bioturbated mudstones. It is imperative that paleoenvironmental analyses consider the long-term effects of bioturbation and highenergy seafloor processes to fully understand the compositional variability of mudstones within a basin-wide context.

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