Sedimentary Geology and Stromatolites of the Middle Proterozoic Belt Supergroup, Glacier National Park, Montana

Abstract

Abstract The Belt Supergroup is a thick, dominantly fine-grained sequence of Middle Proterozoic strata occurring in western Montana, northern Idaho, and parts of Washington state, Alberta, and British Columbia. The sequence in Glacier National Park is located along the northeastern part of present exposures of the Belt Supergroup; it is ∼ 2.9 km thick, extremely well exposed, and for the most part structurally simple. Although it was subjected to lowermost greenschist-facies metamorphism, primary sedimentary structures are exceptionally well preserved. Subtidal, intertidal, alluvial and possibly deltaic depositional environments appear to be represented in the Belt sequence in Glacier National Park. The lowermost unit, the Altyn Limestone, is not entirely exposed in the park. A partial section, ∼ 150 m thick, consists of impure dolostones deposited largely in shallow subtidal and intertidal settings. This carbonate unit is overlain by terrigenous strata of the Appekunny and Grinnell Argillites. The Appekunny Argillite is ∼ 700 m thick, consists largely of green-colored, fine-grained terrigenous material and appears to have been deposited predominantly in offshore and/or deltaic settings. The overlying Grinnell Argillite is 605 m thick and consists of red-colored terrigenous material deposited largely on an alluvial plain. The overlying Siyeh Limestone is 780 m thick and consists largely of impure dolostones and dolomitic limestones deposited in shallow subtidal and intertidal settings. Overlying the Siyeh Limestone is the 385 m thick Snowslip Formation, which consists of slightly dolomitic, predominantly fine-grained terrigenous strata deposited largely in intertidal settings. The overlying Shepard Formation is not exposed in its entirety in the central part of Glacier National Park. A 270 m thick section, which excludes the uppermost part of the formation, consists of impure dolostones and argillites, and appears to have been deposited in subtidal and intertidal settings. Stromatolites are abundant, diverse and well preserved in Glacier National Park, with mound-shaped forms and columnar forms of the group Baicalia occurring in the Altyn Limestone and Siyeh Limestone, and mound-shaped stromatolite-like structures occurring in the Snowslip and Shepard Formations. Particularly prominent is a 24–32 m thick stromatolite unit in the upper Siyeh Limestone, which contains Baicalia and Conophyton and appears to represent a prograding stromatolite reef, with Baicalia originating in a moderate-energy reef-front setting, and Conophyton originating in a lower energy back-reef setting. Individual units in these cycles can be correlated for 90 km. Many of the Conophyton in these cycles are inclined, probably as a result of gentle wave action, and the direction of inclination is relatively constant for 90 km, with the axes trending SW-SSW and plunging 30–60° SW.