The study of mudstones has made considerable strides in recent years, aided by detailed outcrop studies, in-depth petrographic studies, and the infusion of experimental data on clay deposition. Instrumentation that has been especially helpful in that regard includes SEM’s with high resolution backscatter detectors and the introduction of real time color SEM-CL. Flume studies have shown that deposition of clays from currents is probably common, a finding that will most likely require reassessment of depositional models for mudstones.

In multiple studies shales have shown great facies variability.

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Shale facies can be differentiated on the basis of hand specimen and core descr ipt ions . Thei r ar rangement i s ordered and predictable. Through providing information about environment and processes, shale facies analysis also enables shale-based basin analysis. Aside from the secular vector of bioturbation, there seem to be no fundamental differences between shales of different ages.

Sequence Stratigraphy is possible in essentially “pure” shale successions, and distal shales may record substantial stratigraphic gaps due to intermittent erosion. In Devonian black shales of the eastern US a coherent sequence stratigraphic framework has emerged that can be linked to global sea level variations.

Detailed examination of sedimentary features in various source rocks shows that bioturbation in black shales tends to be much more prevalent (though subtle) than previously assumed, and that it is risky to rely on geochemical proxies for identification of water column anoxia. Sedimentological assessment, in combination with detailed petrographic study (Petscope, SEM), reveals many features such as pyrite grains, quartz grains, and carbonate cements that can shed new light on the accumulation history of a shale. Such features may for example signify extreme sediment starvation, reworked horizons, recycling of biogenic opal, and intermittent oxidation (burn-down) of bottom sediments.

Shales are complex rocks, that record an inter twined histor y of sedimentar y processes, sediment-microbe interactions, and multiple overprinted geochemical processes. For these reasons, the study of shales requires an integrated multi-scaled approach. Conclusions reached at the mm and micron scale have to hold up in the face of boundary conditions determined at the outcrop and basin scale.

Figure 1. The current flume lab at Indiana University with two racetrack flumes. Each flume is 10.5m x 3.5 m in size and has a paddle-belt drive. They have been specially designed for experiments with muddy suspensions and avoid the destruction of flocculated materials, a drawback of other recirculating flume designs.

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Figure 2. Experiments with our flumes have shown that clays flocculate under a wide range of conditions, and that these floccules travel largely in bedload at flow velocities between 15-35 cm/sand give rise to floccules ripples. Such a ripple can be seen in the center of the image, overlying a partially eroded clay bed from an earlier experiment. Flow is from left to right. The ripple is asymmetrical and has well developed slip face. For further information see SCIENCE, v. 318, p. 1760-1763 and GEOLOGY, v. 37, p. 483-486. The results of our flume research have serious implications for the interpretations of ancient mudstones, in particular with regard to the common assumption that muds can only accumulate in low energy environments.

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