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New Horizons in Shale Sedimentology — How Experimental Advances Allow a New Look at the Rock Record


In recent years flume studies have enabled novel ways of interpreting shale successions by demonstrating that mud can be deposited from currents competent to move sand in bedload, that flocculated muds form ripples that migrate over the seabed, and that these processes result in finely laminated deposits. In the rock record, the latter were once associated with deep and stagnant environments, whereas now a more energetic and dynamic depositional history can be inferred. Comparable advances have been possible for the interpretation of carbonate muds and for shales with lenticular fabrics. A careful look at ancient shale fabrics also suggests that aside of evidence of unidirectional currents (bottom currents, tempestites, turbidites), there is a record of closely spaced multi-directional events that may record distal tidal influences as well as storm wave impingement on muddy sea bottoms. Initial experiments with a newly constructed mudflume system show that comparable structures can be produced in a tidal current regime in muddy sediments. Experimental work also suggests that in cold water settings, eroding muddy substrates should produce an abundance of transport prone sand-size mud aggregates. In an open ocean setting, such conditions should be conducive to mud transport at a larger scale, such as in the form of megaripples and mudwaves. Distribution of deep sea mudwaves in modern oceans, as well as rock record examples observed in the Ordovician Mazarn Shale of Arkansas and the Oligocene Bituminous Marl Formation of the eastern Carpathians in Romania, seem to validate that assumption. Ongoing flume studies of mud deposition are expanding to include multiple and variable flow histories, a wide spectrum of sediment compositions and grain size distributions, and the integration of organic matter (marine snow) into the depositional setup. Observations from these experiments will allow a close calibration of rock record textures to likely physical conditions at the time of deposition.