Belt Basin

Belt Basin Shales

The Belt Basin of western Montana, northern Idaho, and British Columbia/Alberta cotains a thick succession (possibly as much as 20 km) of Mid-Proterozoic terrigenous clastics and carbonates. J. Schieber has worked on the Belt Basin in the course of his dissertation and intermittently in the years since then. The Belt Basin contains an abundance of shales, and some of them were the subject of detailed study. A basic approach to shale studies is facies analysis, and the following slides summarize shale facies in the eastern portion of the Belt Basin.

Reference:

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. download PDF file (left click on link and click on "Save as"...)

From study of individual stratigraphic slices in the Newland Formation of the eastern Belt Basin, a model of lateral association of shale facies types can be constructed.
The Newland Formation and associated units were deposited in an east-west trending half-graben.

A geological block diagram from an IU Shale Research study shows a cross-section of the Proterozoic Helena Embayment half-graben, illustrating a variety of shale and carbonate sedimentary facies and their depositional environments.

Silty shales occur in nearshore settings, recording wave and current reworking of the substrate.

A scientific diagram from the Belt Basin Study shows two depositional models for nearshore and shallow water shales in the Helena Embayment, including block diagrams and a photograph of a rock sample.

In shallow offshore settings low angel truncations between shale beds are common (frequent erosive events. Thin deformed carbonaceous flakes are a common component. They are the eroded and buried fragments of benthic microbial mats.

A scientific diagram from the Belt Basin Study shows a depositional model for a shallow offshore carbonaceous swirl shale facies, with a photograph of a rock sample and a microscopic image of the rock texture.

Further offshore (erosive events infrequent) benthic microbial mats attain greater thickness and are interbedded with distal muddy tempestites (storm deposits).

A scientific diagram from the Belt Basin Study shows a depositional model for offshore microbial mats that formed striped shale, with a corresponding block diagram, microscope image, and a photograph of the rock.

Shales with thick benthic microbial mats have been described as "striped shales". Their recognition in various other Mid-Proterozoic successions indicates that offshore benthic mats were a vital part of Proterozoic ecology and important for the formation of carbonaceous shales and potential source rocks.

A scientific diagram from the Belt Basin Study illustrates the formation of striped shales and the role of microbial mats in Proterozoic carbonaceous shales, with block diagrams showing mat colonization, erosion, and layering, along with a microscopic view of the rock.

Further offshore deposition of calcareous muds may "overpower" benthic microbial mat growth. Whereas nearshore benthic mats may have been photosynthetic, deeper water offshore mats could quite possibly have been chemosynthetic, utilizing oxidation of hydrogen sulfide (from organic matter decomposition) as an energy source.

A scientific diagram from the Belt Basin Study shows the offshore depositional model for Gray Dolomitic Shale and microbial mats, with corresponding block diagrams and photographs of the rock.

Reference for above:

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. download PDF file (left click on link and click on "Save as"...)

In the central and deepest portion of the east-west trending trough, background sedimentation (terrigenous input from the southern faulted margin) is too frequent to allow establishment of benthic microbial mats. Organic matter mostly mat fragments washed in from shallower parts of the basin.

A scientific diagram from the Belt Basin Study shows the basinal depositional model for Carbonaceous Streak Shale, with corresponding block diagrams and a microscopic image of the rock texture.

Shale facies types in the eastern Belt Basin as a function of water depth/proximality and sediment input.

A scientific diagram from the Belt Basin Study shows the controls on shale facies in the Helena Embayment, including a block diagram illustrating the transition from nearshore to basinal facies and a chart summarizing the impact of water depth and sediment type.

A similar proximality-based approach can be employed for the red-green-gray shale successions of the western Belt Basin, such as the Missoula Group.

A scientific diagram from the Belt Basin Study shows the sedimentological models for terrigeneous clastic deposition in a shallow basin, illustrating different shale facies from the sea margin to the basinal environment.

Reference for above:

Schieber, J., 1990, Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A.: Sedimentary Geology, v. 69, p. 297-312. Associated PDF

For the basal portion of the western Belt Basin (Prichard Formation), sedimentary features suggest a mix of fluvial forcing, gravity driven deposition, bottom currents, and eolian input.

A scientific diagram from the Belt Basin Study shows the sedimentological models for the Prichard Formation, illustrating various shale and siltstone features and their origins from different sedimentary processes in a deep basin.

Reference for above:

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. Associated PDF

Precambrian shales are unaffected by bioturbation, reflecting mainly physical processes of transport and deposition and superimposed effects of microbial mat colonization of the sediment surface. They were therefore the starting point for subsequent studies of younger shales where bioturbation overprinted the original physically based shale fabric to various degrees.

A presentation slide titled BELT BASIN STUDY - CONCLUSIONS lists five key findings, including that shale facies can be differentiated, provide environmental information, and are useful for basin analysis, and that microbial mats are an important factor in the deposition of terrigenous clastics.