Regional Stratigraphy Field Trip: Little Belt Mountains

Stratigraphy of Central Montana.  A trip through the Little Belt Mountains is a trip back through geologic time.  The Little Belt Mountains are a large domal uplift and erosion has exposed the sequence of stratigraphic formations like layers in an onion.  An Ice-Age (Pleistocene) river channel is seen at the Sand Coulee cutoff, east of Great Falls.  Outcrops of sedimentary rocks range from 100 million years (m.y.) at Belt to 550 m.y. at Monarch.  A Mississippian-age fossil locality is located near Monarch.  There is a billion year gap (disconformity) in the geologic record in Belt Creek Canyon.  Most metamorphic rocks are about 1.8 billion years (b.y.) old, but some are as old as 2.7 b.y.  Belt Supergroup.  Memorial Falls.

Route: Great Falls, Belt, Monarch, Neihart, Kings Hill, White Sulphur Springs

Table 1. The sequence of formations under the Highwood Mountains is:

Period

Age (m.y.)

 Group Formation Localities
Eocene 48-53   Lava flows Highwood Mtns.
56   Wasatch Conglomerate Highwood Mtns.
Cretaceous 78 Montana Judith River Formation Highwood Mtns.
  Clagett shale  
82 Eagle sandstone Square Butte, White Cliffs of Missouri River
  Telegraph Creek shale  
  Colorado Marias River shale  
95 Blackleaf formation Belt Butte, Arrow Creek Bed at Geyser
115   Kootenai Formation Belt, Armington

 

Table 2. The sequence of formations in Kibbey Canyon (between Raynesford and Monarch on Montana Highway 427) is:

Period

Age (m.y.)

 Group Formation Characteristics
Cretaceous 115.   Kootenai Formation sandstone, shale, redbeds
Jurassic 148 Morrison Formation coal, grey mudstone
152

Ellis

 Swift Sandstone yellow sandstone
Piper Shale
165 Rierdon Shale
Mississippian 320 Big Snowy Heath Shale
Otter Shale grey shale
  Kibbey Formation red beds, gypsum
330 Madison Mission Canyon Limestone massive limestone, karst


Table 3. The sequence of formations in the Monarch area is:

Period Age (m.y.) Group Formation Lithology
Mississippian 330 Madison Mission Canyon Limestone massive limestone, karst
350 Lodgepole Limestone limestone with shale interbeds
Devonian 360   Three Forks Shale shale
370   Jefferson Dolomite dolomite and limestone
    Maywood Shale  
Cambrian 510   Pilgrim Limestone includes flat-pebble conglomerate
    Park Shale  
525   Meagher Limestone mottled limestone
    Wolsey Shale  
550   Flathead Sandstone dark red sandstone, source of Monarch Sandstone-a decorative building stone
Precambrian 1,800 Basement complex gneiss and other metamorphic rocks
 

Table 4. Starting at Neihart and extending across western Montana, the Belt Supergroup of Proterozoic (Precambrian) age sedimentary rocks is found beneath the Cambrian-age Flathead sandstone and above highly metamorposed basement rocks. The lower Belt formations were deposited approximately 1½ billion years ago. The sequence of rocks found between Neihart and White Sulphur Springs is:


Era Period Age (m.y.) Group Formation Lithology
Paleozoic Cambrian 530   Flathead sandstone
Precambrian Proterozoic 1,440 Belt Spokane shale
  Grayson shale
  Newland limestone
  Chamberlain shale
1,540 Neihart Quartzite quartzite
1,800   Basement complex gneiss, Pinto diorite
Archean 2,700   Basement complex meta-gabbro, meta-granite


Concepts: stratigraphy, Archean, Proterozoic, Paleozoic, Mesozoic, brachiopods, bryozoa, horn coral, crinoids, limestone, dolomite, unconformity, disconformity, regional metamorphism, retrograde metamorphism, Belt Basin, Helena Embayment, aulochogen

Guidebook

Guidebook to the Stratigraphy of Central Montana.. The 85 page geology guidebook describes the geological features seen on the field trip and the geologic setting in which they occur.

Links to Related Sites

Geology Field Trips and Hikes  Central Montana Geology  Monarch Area  Contact Us

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