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Miscellaneous Sedimentary Features in Shales

Miscellaneous Sedimentary Features in Shales

Shown features are from the Newland Formation and the Prichard Formation, both from the Mid-Proterozoic Belt basin. Newland Formation shales were deposited in comparatively shallow water, and those of the Prichard Formation in comparatively deep water. For further information consult a publication from 1990.
Schieber, J., 1990, Sedimentary structures, textures, and depositional settings of shales from the Lower Belt Supergroup, Mid-Proterozoic, Montana, U.S.A.: In: Microstructure of Fine-Grained Sediments: From Mud to Shale, (eds. R.H. Bennett, W.R. Bryant, M.H. Hulbert), p. 101-108, Springer Verlag. Link

Photomicrograph of a shale with suspended coarse sand grains (Newland Formation). Material such as this may form lenses within gray dolomitic shales. To have a clay matrix in what appears to be a migrating bedform (starved ripple?) is puzzling. The clay particles of the matrix and the sand grains are not hydraulic equivalents, so how did they get deposited together in these lenses? One possible explanation could be that we started out with clay aggregates or rip-up clasts that traveled together with the sand grains, and that the margins of shale clasts became indistinguishable due to compaction and diagenetic overprint.

Photomicrograph of gray dolomitic shale (Newland Formation). Shows faint internal banding due to variations in dolomite content (dolomite rich bands are lighter). The lighter (dolomitic) bands have been interpreted as distal mud tempestites.

Schieber, J., 1989, Facies and origin of shales from the Mid-Proterozoic Newland Formation, Belt basin, Montana, U.S.A.: Sedimentology, v. 36, p. 203-219. Link