Chapter 10.6 Stromatolites from the Middle Proterozoic Altyn Limestone, Belt Supergroup, Glacier National Park, Montana

Abstract

Publisher Summary The Belt Supergroup, a thick Middle Proterozoic sedimentary sequence consisting of argillaceous, arenaceous, and calcareous rocks, extends from western Montana and northern Idaho into adjacent parts of Alberta, British Columbia, and Washington State. The Altyn Limestone is of restricted distribution, occurring only in a narrow outcrop band that immediately overlies the Lewis Overthrust on the east side of Glacier National Park and an adjacent part of Canada. The interval exposed in Glacier National Park attains a thickness of several hundred meters and is composed predominantly of silty and sandy dolostone. Stromatolites growing under more closely spaced conditions and forming relatively more horizontal biohermal growth surfaces developed into less divergently branched forms. Lateral expansion of these stromatolites is inhibited by adjacent columns and the growth of relatively more erect branches is favored by the more nearly horizontal nature of the bioherm growth surface. The inclined elongate forms apparently developed under relatively strong current conditions, their elongate nature probably being a streamlining effect caused by unidirectional currents.