Geological History of Southwest Montana From Mississippian Through the Triassic

Published: 2021-07-08
911 words
4 pages
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Wesleyan University
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The geology of the Dillon area records over 4.5 billion years ago in history. There was no oxygen to breathe over a billion years ago, and the only living creatures were bacteria and mountains that formed due to collide of the continents. According to James & Hedge (1980), a million years of erosion did away with the mountain and submerged them under ocean water as the paleo-pacific opened. The oldest rocks that are found in the bottom of the stack occurred after the formation of layers. It can be noted that the earth evolves over a span of time that geoscientists call the dark time. The oldest rock according to Cowan et al. (2014) in the Dillon is dated from 4.5 Ga about mid-march. Animals were the first creatures to appear in the ocean in May. Then plants followed later in November. The Rockies uplifted and dinosaurs dominated around December. Explosive volcanoes erupted in Mid-December and the basin range topography that human sees each day started to form on the last day of December. The interior of the earth has a core, mantle and crust layers (James & Hedge, 1980). Around 1.5 billion years, Dillon was close to the Southern shore of an inland sea referred to as the Belt basin. The sea covered a depression that was formed by a crustal extension and it was full of sediment eroded from nearby highlands. The belts rocks have a cross bed like structures, and their preservation was because of the disposition of the Belt sediments before the evolution of burrowing animals that destroyed the structures.

The Hostile Time

The geological history of Southwest Montana started in an era that comprises around 80 percent of the history of the earth. As earlier indicated, present evidence suggests that during the first part of the surface, the Archean had less oxygen and large amounts of carbon-dioxide. No animal or plants could survive as only rocks existed. According to Cowan et al. (2014), during the Proterozoic, weathered stones had eroded from the world and layers of sediments were placed in shallow seas taking over most areas. The surrounding remained stable for around 600 million years. Changing layers of silt, sand, and clay resulted in chemically formed limestone, and each layer had changes in the type and source of rocks, rate of erosion and the depth and size of the sea over an extended duration of time. Disposition of every new layer resulted in the disappearance of old segments (Cowan et al., 2014). Finally, the exerted pressure of minerals deposits was entrapped under sea water transformed the sediments into a full sequence of huge rocks known as the Belt. Massive unit of fine-grained sandstones, limestone, and dolomites was formed, and they offer an extraordinary record of the history of the earth.

Paleoproterozoic in Mississippian

The most important metamorphic activity in Dillon was the Big Sky Orogeny. Medicine Hat which is a collision of volcanic resulted in cramping and rising of the crust of the earth into massive mountains. During the Mississippian time, there was the formation of the Conover Ranch, Lombard Limestone, Kibbey Sandstone, Mission Canyon Formation and the formation of Lodgepole. Tropical marine deposits continued to take place in the late Paleozoic with the disposition of limestone Mississippian period. As indicated by Cowan et al. (2014), Fossils of echinoids, corals, crinoids and brachiopods were synonymous in the Lodgepole formation specifically at the South part of Dillon known as the Canyon Reservoir. Since limestone can withstand erosion in arid areas, it results in important landmarks like the Beaverhead Rock.


Paleozoic period ended with the elimination of a lot of marine species, and the Great Dying was a result of environmental changes taking place at the same time (Underwood et al., 2014). Extinction is not recorded in the Dillon era but during the formation of Diywood in Triassic, brachiopods are demonstrated with snails, and squids that lived in shallow seas around 15 million years after their termination. Talc is among the most significant minerals in the Dillon community. Talc is among the purest minerals on earth, and it is utilized to make paints, plastics and cosmetics (Underwood et al., 2014). Talc was formed around 1.4 billion years ago when hot water interfered with the Archean dolomitic marble. The water percolated on faults formed by extensions of crustal which created a basin for the Belt Basin. The energy required for heating water could have come from dark-colored dikes called diabase which resulted in the basement rock when Belt Basin was being formed. Under the Triassic, there was a formation of the things, Woodside, and Dunwoody. Neoproterozoic

1.3 billion years of erosion left most Dillon area a distinct place with no features by the end of Neoproterozoic. Continents were gathered to a supercontinent known as Rodinia, and world cooling enveloped the earth to ice. The continent separation was associated with a rise in the sea level that placed marine and stream sediments on old metamorphic and igneous rocks. The border between the two is the great unconformity since it represents a gap between the rock records of 1.3 billion years (James & Hedge, 1980).


Cowan, D. S., Haugerud, R. A., & Lang, K. A. (2013). Field study of the Dillon area, Southwestern Montana.

James, H. L., & Hedge, C. E. (1980). Age of the basement rocks of southwest Montana. Geological Society of America Bulletin, 91(1), 11-15.

Underwood, S. J., Childs, J. F., Walby, C. P., Lynn, H. B., Wall, Z. S., Cerino, M. T., & Bartlett, E. (2014). The yellowstone and regal talc mines and their geologic setting in southwestern Montana. Field Guides, 37, 161-187.

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