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Millimeter-Sized Bioturbating Animals Control the Mobility of Redox-Sensitive Trace Elements in Organic-Rich Mudstone

Abstract

Bulk-rock geochemical interpretation of organic-rich mudstones (e.g. black shales) is fundamental to geological models of hydrocarbon systems and ancient oceanic anoxic events. High-sensitivity two-dimensional elemental maps acquired via Synchrotron Rapid Scanning X-ray Fluorescence (SRS-XRF) identify for the first time that the partitioning of redox-sensitive trace elements in fine-grained siliciclastic rocks is systematically controlled by millimeter-sized vermiform organisms. Detailed petrographic analysis of silt- and claystones from the Late Cretaceous Rosario Formation (Mexico) reveal that producers of phycosiphoniform burrows selectively sort silt- and clay-sized components smaller than 40 μm, producing a spatially well-connected, high-porosity (20-30%) burrow halo. The burrow core is composed of a dense organo-clay matrix critically enriched in TOC (>1.5%) and some redox-sensitive elements (i.e. Fe, V, Cr, Mn, Co, Ni, Cu and As). Systematic differences in organic carbon quality between bioturbated and unbioturbated sediment (?Corg ∼0.6‰) provide evidence that fecal matter has been modified by burrowing organisms. The relative enrichment of redox-sensitive elements within the burrow core does not correlate with significant neo-formation of early diagenetic pyrite (via trace metal pyritization), but is best explained by physical concentration of clay- and silt-sized components. A measured loss (∼ −15%) of the high ionic radius elements Sr and Ba from both burrow halo and core cannot be explained by physical re-distribution of silt- and clay-sized rock components during foraging and feeding. Instead, this imbalance is most likely associated with the release of Sr and Ba to pore waters during biological (in-vivo) weathering of silt to clay-sized lithic components and also feldspar. Combined SEM, XRD and FTIR analyses reveal significant amounts of neo-formed smectitic clay and poorly-cristalline, non-identifiable clay-sized material in the burrow core. Phycosiphoniforms represent one of the most common Mesozoic and Cenozoic black shale trace fossils, which in certain cases can dominate an entire rock fabric. The intimate relationship between sediment ingestion and incipient, penecontemporaneous dissolution of the original mineralogy can change the bulk geochemistry of fine-grained sedimentary rocks prior to burial and should be considered in paleoenvironmental reconstructions and in the evaluation of potential organic-rich mudstones.