AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Unbioturbated Marine Mudstones: Environmental Stress or Rapid Deposition? A Worked Example from the Ordovician Beach Formation, Newfoundland, Canada
(1) Earth Science, Memorial University, St. John's, NF, Canada.
Sedimentary successions containing unbioturbated marine mudstones are commonly interpreted as being deposited by particle settling from suspension under oxygen-depleted bottom-water conditions. However, modern research demonstrates that beside suspension settling, combined flows can transport and deposit mud quickly in the aftermath of storm events or during high river discharge. Textural evidence indicating combined flows is difficult to provide for the Phanerozoic rock record, because primary fabrics are usually overprinted by bioturbation. Within this study we have studied Lower Ordovician, storm-dominated, shoreface sediments of Bell Island that exhibit well-bioturbated, interbedded sand- and mudstones with intercalated, unbioturbated mudstones. We examine the most likely controls on deposition of unbioturbated mudstones through integration of sedimentological and geochemical datasets at a range of different scales. We test the hypothesis that some of the unbioturbated mudstones were deposited as combined flows. Critical features in unbioturbated mudstones include well-developed, low-angle cross stratification and thin (<1 mm), stacked beds with erosional tops and bases. Microscopic bioturbation in the form of very small (mm-sized) Planolites burrows is common in mud- and siltstones previously described as unbioturbated. Mudstones contain 0.5 % TOC in average, with peak values of up to 3.4% TOC. Size-frequency distributions of framboidal pyrite reveal a fully-oxygenated water column close to the sediment-water interface during mud deposition. Sedimentary provenance analysis (using Rare Earth Elements) of mudstones reveals that all examined mudstones are either from the same source, or have undergone the same mixing process before deposition. Given: 1) the distal location of the exposure with respect to a potential fluvial source; 2) the fully oxygenated state of the water column; and 3) the evidence for bottom currents, we conclude that at least some of the unbioturbated muds were deposited as wave-enhanced fluid mud flows. A careful, integrated, study of other ancient mudstone successions is needed in order to assess the importance of similar sedimentary products originating from advective sediment transport throughout the fine-grained sedimentary record.