#92 - Montana's "Rock of Ages"

Above: Gates of Mountains Recreation Area 20 Miles North of Helena

If Montanans ever decided to select a state rock formation, a pretty good argument could be made for the Madison limestone. This "Madison Formation" (aka Madison Group) can be seen in many of our state's more recognizable landscapes (listed below). The photo above shows cliffs of Madison limestone near the southern end of the Gates of the Mountains Recreation Area. Below is a list of some other places where you can see outcroppings of Madison Limestone.

1. The Bridger Mountains (photo near Sacajawea Peak) and the Horse Shoe Hills (Bozeman area)
2. The Little Rockies in north-central Montana, including Mission Canyon
3. Lewis and Clark Caverns and Jefferson Canyon near Three Forks
4. The Sawtooth Range and Sun River Canyon between Helena and Glacier Park
5. The Little Belt Mountains, including Sluice Boxes State Park southeast of Great Falls and along the Smith River
6. Bighorn Canyon and the Pryor Mountains south of Billings
7. The mountains north and south of Lewistown (Judith and Snowy Mtns.)
8. The Jefferson Canyon south of Whitehall and the Headwaters State Park near Three Forks
9. The Castle Mountains southeast of White Sulphur Springs
10. Beaverhead Rock near Dillon

What is a "formation"? . . .
To a geologist a "formation" (or "group") is a thick layer (or series of layers) of a particular type of sedimentary rock covering a large geographic area. Some of Montana's more famous formations include the Hell Creek Formation, the Judith River Formation, and the Two Medicine Formation, which have all yielded significant dinosaur fossils. The familiar"Rimrocks" of Billings and the White Cliffs east of Ft. Benton are both exposures of the Eagle Formation. The Madison Formation consists of sediment laid down during the Mississippian Period of the Paleozoic Era (roughly 350 million years ago). It was laid down over most of Montana, eastern Idaho, northern Wyoming, and the Dakotas. The Madison is between 1,000 and 2,000 feet thick throughout most of the Montana.

Reading the rocks . . .
One reason geologist get so excited about rock formations is that they provide clues about what the environment was like in that location when the sediments were being deposited. For example the Eagle Formation consists of sand deposited near the shore of a shallow inland sea, whereas parts of the Judith River Formation indicate the presence of deltas much like the one at the mouth of the Mississippi River. Based on the types of fossils contained in the Madison Formation, geologists think its sediments accumulated on the floor of a tropical shallow sea. The map below shows where geologists believe Montana was located during this time (the Mississippian Period). Click on the map to enlarge it.

A deposit of calcite sediment . . .
Although it comes in many forms, all limestone is primarily made up of the mineral called calcite (calcium carbonate; CaCO3). This mineral is produced as marine organisms draw calcium carbonate out of the water in order to build shells or other hard parts. As these organisms die their soft tissues decay, but the shells, etc. made of calcite build up as sediment on the sea floor. Tropical waters also support abundant seaweeds that excrete calcite, which precipitates onto the seafloor as a light-colored lime mud, especially during periods of evaporation. Judging by the thickness of the Madison Limestone, this tropical marine environment persisted for many millions of years.

Terms: excrete, precipitate

#93 - Slabs of Belt Rocks Tell of an Ancient Basin

Above: "Belt Rocks" can be seen throughout much of western Montana, but one of the most interesting exposures can be seen along the Pintler Scenic Highway between Philipsburg and Anaconda. Here (above photo) the layers have been tilted, making the layers look like books leaning against a wall. Sedimentary rocks are formed in horizontal layers, but in northwestern Montana the horizontal layers of Belt rocks were messed up as colliding plates formed the northern Rocky Mountains roughly 80 million years ago. The mountains of Glacier Park formed as a large portion of Belt layers were thrust up and over younger sedimentary rock. Not only did this collision form many of the mountains of western Montana, but it also exposed layers that were buried deep beneath the Earth, giving geologists a look farther back in time.

Map courtesy of Mountain Press Publishing in Missoula, Montana: This map was borrowed from Northwest Exposures, a great resource for anyone wishing to learn more about the geologic past of the Northwest. The yellow X indicates where the photo at the top of this page was taken. The authors David Alt and Donald Hyndman contend that the Belt Basin (a.k.a. the Belt Sea) was a roughly circular area found on the continent, and that it continued to subside as sediments accumulated. Recent radiometric measurements indicate that the sediment was deposited from 1.470 billion years ago to 1.400 billion years ago (Evans et al., 2000; Ross and Villeneuve, 2003). This is a far shorter time interval than previous interpretations from the late 1960s, which suggested that the sediment was deposited between 1.5 billion years ago and 800 million years ago.

Must have been quite the basin . . . .
Over 15,000 feet (about 3 miles) of sediment was deposited in the basin. Its layers of beach sands, clear water limestones, shallow water sands, fine sand flats, and mudstones indicate that a variety of environments existed in the basin during the Precambrian Era. The Belt Basin covers a large region in Montana, Idaho, Washington, British Columbia, and Alberta. It is one of the deepest, best exposed, most accessible, and well-studied Mesoproterozoic basins in the world. Geologists refer to its thick series of rock layers as the "Belt Supergroup".

Clues in the layers . . .
The Belt rocks exhibit great examples of some features typically found in sedimentary formations, but other characteristics are missing. These features provide more clues about the basin, and in some cases raise even more questions about the Belt Sedimentary Basin.

1. mudcracks: Abundant between layers of Belt rocks; These were caused by surfaces that dried in the sun before being flooded by water that brought more mud or silt, which preserved the delicate features.

2. ripple marks: These suggest the presence of gentle waves in shallow water.

3. no animals: Although the Belt Supergroup includes plenty of extremely primative plants and bacteria, the rocks offer no trace of animal life. This helps explain why features such as mudcracks and ripple marks are so well preserved. There were no burrowing organisms in the Precambrian Era as there was in the more recent Paleozoic Era when sea floor sediments where more likely to be "bioturbated" by mollusks and the like.

4. no windblown sand: Although the basin was dry at times, there is no windblown sand. This puzzles geologists and begs the question, "did the wind not blow hard enough to carry sand?"

5. diabase sills: In places molten rock from below forced its way between layers forming sills of hardened magma. Unlike sedimentary rock, scientists can determine the age of igneous rock using radiometric techniques. These dates can help establish the ages of various layers in the Belt Formation.

6. reds and greens: Many of the layers of mudstone in the Belt Supergroup are colored interesting shades of green or reddish purple. The red, purple, and maroon color is due to the a red mineral called "hematite" (Fe2O3, which is formed as iron reacts with oxygen from the atmosphere. It is believed that the green rocks were formed in deeper water where oxygen was less available. Instead the iron combined with silica to form iron silicate compounds that changed into a green mineral called chlorite as heat and pressure altered the rock. Alternating green and red layers suggest fluctuations in the depth of the Belt Sea (a.k.a. Belt Basin).

Below: All of the rocks in Glacier Park are part of the Belt Supergroup, including the colorful layers of the Grinnell formation shown in this photo by Jeff Kuhn of the Glacier Institute.

Term: subside, basin, Precambrian Era, bioturbated

Sources:

Alt, David, and Donald W. Hyndman. Northwest Exposures. Missoula, Montana: Mountain Press Publishing, 1995.

Evans, K.V., Aleinikoff, J.N., Obradovich, J.D., and Fanning, C.M., 2000, SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana; evidence for rapid deposition of sedimentary strata: Canadian Journal of Earth Sciences, v. 37, no. 9, p. 1287-1300.

Ross, G.M., and Villeneuve, M., 2003, Provenance of the Mesoproterozoic (1.45 Ga) Belt Basin (western North America): Another piece in the pre-Rodinia paleogeographic puzzle: Geological Society of America Bulletin, v. 115, p. 1191-1217. .

#94 - Where Cement Comes From

Limestone Quarry . . .
The photos on this page were both taken near Montana City, a community located 5 miles southeast of Helena. Here a light-colored rock, called Madison Limestone, can be found just beneath the soil, making this a convenient place for a quarry. Ash Grove Cement Company removes limestone from the quarry and hauls it to a plant just a few miles away where it is used to make cement. A quarry is sort of like a mine. However, at a mine the rock (called ore) is removed because it contains a valuable metal, which must then be removed by processes such as milling, leaching, and smelting. On the other hand, at a quarry it is the rock itself that the company is after.

A deposit of calcite sediment . . .
The Madison limestone is primarily made up of the mineral called calcite (calcium carbonate; CaCO3). This rock is made of sediment that was deposited on the floor of a shallow ocean about 340 million years ago. Marine organisms took calcium carbonate out of the water and built shells or other hard parts. As these organisms died their soft tissues decayed, but the shells, etc. made of calcite built up as sediment on the sea floor. Judging by the thickness of the Madison Limestone, found here and in other parts of Montana, this tropical marine environment persisted for many millions of years.

Why limestone . . .
Limestone is primarily made up of the mineral calcite whose chemical formula is CaCO3. As Ashgrove heats the limestone (CaCO3 aka calcite) to 1000 C lots of CO2 is given off as the calcite changes to CaO. This step releases so much CO2 into the atmosphere that cement production is one of the top five sources of the CO2 emissions in the USA. In the next step, the CaO is heated to 1500 C and it reacts with silica to form calcium silicate. The most common combination of materials used to make cement is limestone, clay and sand. These materials are crushed and then processed in a furnace called a kiln where temperatures reach 1500 C (2730 F). The intense heat causes chemical reactions that convert the partially molten raw materials into pellets of calcium silicate called clinker. After adding some gypsum and other key materials, the mixture is ground into the extremely fine gray powder that we call "cement".

The carbon cycle . . .
Limestone stores huge amounts of carbon for millions of years. As Ashgrove heats the limestone, this carbon returns to the atmosphere in the form of CO2. Another (natural) way that the carbon can return is through volcanism. If this limestone were ever to be melted into a magma, the carbon may be expelled as CO2 when the volcano erupts. Based on the current motion of Earth's tectonic plates, it doesn't look like this will be happening to the limestone in Ashgrove's quarry any time soon.

What's the difference between cement and concrete? . . .
Although the terms cement and concrete often are used interchangeably, cement is actually one of the ingredients used to make concrete. The other ingredients are sand and/or gravel (aggregate), and water. Typically, concrete is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water.

If it can't be grown it has to be mined . . .
No one likes to see the land torn up, but we all benefit from buildings, roads, and sidewalks made of concrete. Although 93 % of U.S. highways are paved with asphalt, 40 % of our interstate highways are made of concrete.

Above: Here is a closer view of Ash Grove's limestone quarry at Montana City.

Term: aggregate

#95 - Shock Wave Frozen in Stone on Rim of Impact Crater

A blast from the past . . .
In 1990 geologists discovered evidence of an ancient asteroid impact in the mountains of southwestern Montana. The team found rocks with distinct cone-shaped fracture patterns resembling horsetails. They recognized that these were “shatter cones”; the unique fracture patterns formed as intense shock waves generated by an asteroid impact travel through bedrock.

Right: This photo, which was provided by Mike Plautz, Science teacher at Hellgate Elementary in Missoula, shows a classic shatter cone at a location scientists refer to as the "Beaverhead Site”. The presence of shatter cones here puzzled scientists because the area lacked visible evidence and other characteristics typically associated with impact craters.

Solving the mystery . . .
Scientists have identified over 160 impact craters on Earth. Unlike the surface of the Moon where impact craters dominate the surface, most of Earth’s craters are much more difficult to identify. On Earth there are many processes that wear away or hide the craters, including erosion, vegetation, seafloor sediments, lava, and plate tectonics. So, in order to find impact craters scientists search for the following clues.

1. Evidence of rocks that have been changed by shock waves (shatter cones)

2. Craters, or geology that indicates the presence of a crater

3. Geophysical anomalies: variations in gravity and/or magnetism that stand out as “unusual”

4. The presence of meteorites: fragments from the asteroid (or comet)

The Beaverhead Impact Structure . . .
Although the shatter cones found in southwestern Montana were the first clue that there had been an impact, scientists eventually determined that the actual crater is centered about 50 miles southwest of the Beaverhead Site around the Challis, Idaho area. The mystery of the crater’s location was solved when scientists found that some of the rocks in the area around Challis had different properties when it came to magnetism and gravity readings. As it turns out these geophysical anomalies revealed a crater-shaped pattern indicating that the crater was probably about 100 km in diameter (65 miles). Radiometric dating done in 1999 suggested that the impact happened about 900 million years ago. . . . No wonder it was so hard to find! The crater, which was named the Beaverhead Impact Structure, is one of only eight known impact craters over 50 km in (32 miles) diameter.

Below: These diagrams are from a masters thesis paper by A. E. McCafferty, Colorado School of Mines. The top one shows a (proposed) cross-section of what the geology of the crater area might have been like soon after the impact 900 million years ago. The bottom one shows a cross-section of the area today. Compare the location of the points labeled “Grouse Peak” and “Beaverhead Site” on the two diagrams. Geologists believe that, since the impact happened, the top part of the crust moved several miles to the east causing these two points to be offset from their original position on oposite sides of the crater. This type of "thrust faulting" was common in the area during the formation of the Rocky Mountains. The yellow area indicates location of bedrock that provided the unusual magnetic and gravity readings.

Sources:

McCafferty. A.E. , 1995, Assessing the presence of a buried meteor impact crater using geophysical data, south-central Idaho: Masters Thesis, Colorado School of Mines, 88p.

Carr, J., and Link, P.K., 1999, Neoproterozoic conglomerate and breccia in the formation of Leaton Gulch, Grouse Peak, northern Lost River Range, Idaho: Relation to Beaverhead Impact Structure, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of natural History, p. 21-29.

Term: anomaly