ECOS Guide to the Ecology of the Northern Rockies
 
   
 

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Belt Supergroup

Chief Mountain – Belt Supergroup rocks thrusted
over younger Cretaceous rocks
in Glacier National Park

From about 1.5 billion years ago to about 800 million years ago, a thick accumulation of sediments was deposited to the west in a basin over 130,000 km2 between the North American continent and another continent (current research suggests that this continent could be what is now Siberia).  The sediments lithified into what we now call the Belt Supergroup.  It is up to ~16 km thick and is divided into four general groups:

  1. Lower Belt,
  2. Ravalli Group,
  3. Middle Belt carbonate
  4. Missoula Group. 

During the formation of the Rocky Mountains, beginning about 75 million years ago, the Belt Supergroup was thrust out of its basin and onto parts of Idaho and western Montana.  The Belt Supergroup is easily identified in western Montana as the red, purple, or gray exposures of hard quartzites, mudstones, and carbonates. 


Belt Supergroup Layers

Lower Belt-Purcell Supergroup —The lower Belt Supergroup includes the Prichard Formation, is >6 km thick, and is dominated by deep basinal turbiditic muds, silts and sands, with intercalated syndepositional mafic sills. The sills have been U–Pb dated at about 1470 Ma throughout the basin (Anderson and Davis, 1995; Sears et al., 1998 and Hoy et al., 2000).

Ravalli Group —The Prichard Formation grades upward into the Ravalli Group.  The Ravalli Group is predominantly composed of sedimentary rocks deposited in shallow-water and terrestrial environments. 

Middle belt carbonate —The middle` Belt carbonate overlies the Ravalli Group.  It is composed of stromatolitic carbonates with varying amounts of siliciclastic content. 

Missoula Group —The Missoula Group is composed of clastic wedges that become thinner and finer grained northward, (Winston, 1986).  The lower Missoula Group contains the 1443 million year old (U–Pb dating) Purcell lava.  

 
The ECOS program is sponsored by the University of Montana's Division of Biological Sciences, and the College of Forestry and Conservation. Carol Brewer Program Director, Division of Biological Sciences. Paul Alaback Program Co-Director, College of Forestry and Conservation.
NSF LogoECOS is supported by the GK-12 Program of the National Science Foundation.
Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.