Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
A Reevaluation of Tidally Deposited “Mud Drapes”: Criteria for Recognizing Dynamically Deposited Fluid-Mud Layers
(1) Geological Sciences and Geological Engineering, Queen's University, Kingston, ON, Canada.
Tidally deposited mudstone layers are typically treated as slack-water deposits that reflect the tidal rhythm. Recent experimental work shows, however, that high near-bed suspended-sediment concentrations (i.e., “fluid muds”) can produce mud layers under conditions where the velocity exceeds the threshold of mud erosion.
Mudstone layers in the tide-dominated Bluesky Formation have enormously variable sedimentary characteristics (e.g., thickness, internal stratification and bounding contacts) that reflect a wide range of suspended-sediment concentrations and flow velocities. Four recurring mudstone types are present, each of which is interpreted to have formed under distinct depositional conditions: (1) “classic” mud drapes generated by slow settling from suspension form in tidal environments where SSC values are less than 1 gL-1; (2) cross-stratified mudstone layers deposited by turbulent or transitional turbulent flows with appreciable current speeds (> 0.2 ms-1) and moderate SSCs (1-10 gL-1); (3) horizontally laminated mudstone layers formed under conditions of transitional plug flow with moderate to high SSCs (1-100 gL-1) and appreciable currents (> 0.2 ms-1); and (4) thick mudstone lamina and beds (> 2 mm thick), with no internal lamination and common soft-sediment deformation, deposited by unstable plug flow or quasi-laminar plug flow, with moderate to high SSCs (1-1000 gL-1) and current speeds ranging from slack water to 1-2 ms-1.
Mud layers in tidal successions provide a wealth of information regarding depositional environments because their characteristics are sensitive to the SSC value at the time of deposition. From modern coastal environments, we know that high SSC values are commonly associated with tidal dominance and, more specifically, with tidal-channel bases and proximity to the turbidity maximum. Our ability to recognise high-SSC mud deposits is also critical for reconstructing tidal rhythms from sand-mud thickness alternations since fluid-mud deposits may form over significant portions of the tidal cycle (i.e., not only at slack water) and perhaps over multiple tidal cycles if fluid mud layers persist.