Picture Page 2: Microbial Mat Features in Proterozoic Shales

Sedimentary features suggestive of mud bottom colonization by microbial mats were recognized in the Mid-Proterozoic Newland Formation of Montana. A detailed description can be found in a publication from 1986 (Schieber, J., 1986, The possible role of benthic microbial mats during the formation of carbonaceous shales in shallow Proterozoic basins: Sedimentology, v. 33, p. 521-536). There is no apparent reason why microbial mats should not also have colonized mud bottoms of other Precambrian basins, as well as those of Phanerozoic basins under certain circumstances. 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 2/1: Drill core specimen of "striped shale" (Newland Formation). The dark (black) layers are interpreted as microbial mat deposits. The lighter layers may show grading with silt laminae and lenses at the base and clays at the top. These graded "silt/mud couplets" are interpreted as storm deposits.
Figure 2/2: Photomicrograph of "striped shale" (Newland Formation). Shows interbedded carbonaceous shale (dark) and graded silt/mud couplets (basal silt = S; clay drapes = Cl). The irregular carbonaceous laminae are considered internal laminae of microbial mats. Scale bar is 0.5 mm long.

Figure 2/3: Photomicrograph of carbonaceous flakes (marked with A) in clayey shale bed (Newland Formation). Flakes are contorted and contain more silt than the surrounding shale matrix. This suggests cohesive behavior during transport and trapping of silt grains on microbial films. Scale bar is 0.5 mm long.
Figure 2/4: Photomicrograph of pyrite mineralized "striped shale" (Newland Formation). The laminated pyrite bed corresponds to carbonaceous microbial mat layers in Fig. 2/2. It is overlain by a silt bed (marked S) that has load casts at the base. Pyritic "striped shale" is a special facies of "striped shale", and probably formed due to colloidal iron input along the coastline (Schieber, J., 1989, Pyrite mineralization in microbial mats from the Mid-Proterozoic Newland Formation, Belt Supergroup, Montana, U.S.A.: Sedimentary Geology, v. 64, p. 79-90; Schieber, J., 1990, Pyritic shales and microbial mats: Significant factors in the genesis of stratiform Pb-Zn deposits of the Proterozoic? Mineralium Deposita, v. 25, p. 7-14).

 

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