Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A.

Abstract

Within the strongly shale dominated sediment fill of the Mid-Proterozoic Belt basin a large variety of shale types can be distinguished. Sedimentological investigations of several formations have yielded valuable data about lateral associations of shale types as well as about the principal mechanisms of shale deposition. Depositional environments of shales in the Belt basin range from red bed mudstones of ancient flood plains to deep water mudstones in a turbidite setting. Graded silt/mud couplets are ubiquitous in most shales of the Belt basin. However, they differ in detail and can be related to a variety of depositional processes, such as low-density turbidity currents, sheet floods, storms, and wave reworking. In several formations there is evidence that microbial mats colonized the sediment surface and probably protected the sediment surface from erosion. Shales from comparable Phanerozoic settings generally lack the silt/mud couplets that are so commonly observed in shales from the Belt basin and other Proterozoic epicontinental basins. This difference probably reflects the proliferation of bioturbating organisms in the Phanerozoic. The absence of benthic microbial mats in Phanerozoic shales is probably due to the evolution of metazoan grazers towards the end of the Proterozoic.