Picture Page 5: Sculpting of Muddy Bottoms, Erosion Surfaces and Ripples

Sculpting of submarine mud bottoms by currents and waves may be reflected in the sedimentary record by erosion surfaces and related features. Examples shown on this page are from the Chattanooga Shale (Late Devonian) of Tennessee. Submarine erosion surfaces are a little known but probably quite common feature of shallow marine shale successions. Those of the Chattanooga Shale have been described in several publications (Schieber, J., 1994, Evidence for episodic high energy events and shallow water deposition in the Chattanooga Shale, Devonian, central Tennessee, U.S.A. Sedimentary Geology, v. 93, p. 193-208. Schieber, J., 1994, Paleoflow patterns and macroscopic sedimentary features in the Late Devonian Chattanooga Shale of Tennessee: Differences between the Appalachian Basin and the American Craton. Canadian Society of Petroleum Geologists, Memoir 17, p. 763-772). If you have seen shales with features resembling those shown below, send me a message ( schieber@uta.edu ). Photos are low resolution for faster loading. Click on the image to get a larger, high resolution picture.

Figure 5/1: Outcrop of Dowelltown Member of the Chattanooga Shale. Shows several inclined truncation surfaces within black shales. Note angular truncation of beds beneath truncation surfaces. Shales deposited after an erosive event are conformable to the underlying erosion surface. Their draping over these truncation surfaces produces a kind of pseudo cross-stratification. BU = basal unconformity of the Chattanooga Shale; T1 and T2 are truncation/erosion surfaces of probably submarine origin. I consider them expressions of sequence boundaries.
Figure 5/2: Outcrop of Gassaway Member of Chattanooga Shale. Shows again several inclined truncation/erosion surfaces. Note angular truncation of beds beneath truncation surfaces. E1 and E2 mark the erosion/truncation surfaces visible in this exposure. Surface E2 has already been traced over a large distance along the eastern Highland Rim (for about 150 km). The large lateral extent strongly suggests that E2 is a bona fide sequence boundary. Conodont studies to determine its age are under way.

Figure 5/3: Photo of polished slab showing details of an erosion surface as shown in Figs. 5/1 and 5/2. Erosion surface is overlain by a fine sandy to silty lag deposit (marked Sl) that is in turn overlain by black shales. Note how silt layers in underlying black shales show truncation and soft sediment deformation, and how shales above the erosion surface conformably overly it.
Figure 5/4: Photo of a bedding surface of black shale. Shows faintly visible parallel ripple crests (strike about 80 degrees relative to the vertical margin of photo, and are marked with r). There seems to be no silt or sand associated with these ripples, and their origin is as yet unclear. They do, however, suggest currents or waves interacting with the seabed.

 

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© Jürgen Schieber, UTA Department of Geology
Last updated: March 12, 2001.