by David Baker
Here in the Little Belt Mountains in Central Montana we have creeks in which a portion goes dry for a while in the summer and in the winter. One example is Dry Fork (of Belt Creek), which joins Belt Creek at Monarch. In dry summers and in periods of low flow in the winter Dry Fork is empty for several miles upstream from Monarch. In very dry summers several miles below Monarch all of Belt Creek disappears into the ground and there is a dry stretch of empty creek bed; however, both upstream and downstream the creek has water. Every summer and winter there are times when Belt Creek dries up for several miles above Armington Junction.
The U.S. Geological Survey measured stream flow (or discharge) in creeks in the Little Belt Mountains and found that the creeks lost significant amounts of water where they crossed carbonate rocks--limestone or dolomite. The largest loss occurs in the 1700 foot thick Madison limestone. Monarch Canyon and the Sluice Boxes are carved in Madison limestone. Water disappears into this limestone and flows underground through a connected network of cracks, fissures, crawlways, tunnels, and caves.
The Little Belt Mountains are formed as a large fold in the layered rocks--about 30 miles wide and 80 miles long (Figure 1). The axis goes through Neihart. Beds north of Neihart are tilted north towards Great Falls. As shown in the cross section the groundwater flows downhill in the Madison limestone aquifer towards Great Falls where there is a leak in the system, known as Giant Springs.
At Giant Springs the Madison limestone is less than 400 feet below the surface and groundwater from the Little Belts escapes upwards through cracks in the overlying Kootenai sandstone (Figure 2).
(These cracks are easily seen from the concrete walkway over the springs.) [The USGS also suggested a contribution from groundwater entering the Madison limestone along the old Missouri River channel through Gibson Flats, south of Great Falls. However, the water chemistry is consistent with a source in the Little Belt Mountains, not the Missouri River.]
Although much water flows out at Giant Springs, a significant leak in the Madison limestone
aquifer, water in this aquifer flows eastward from the Little Belt Mountains across Eastern
Montana towards the Williston Basin (Figure 3). The groundwater is deflected around the north
side of the Williston Basin and flows into Canada. Groundwater from the Little Belt Mountains
finally comes to the surface in a series of saline seeps along the shores of Lake Winnipegosis and
Lake Manitoba in the Province of Manitoba after traveling 1000 kilometers or 600 miles
underground.
In Central Montana groundwater in the Madison limestone contains mostly calcium (and magnesium) carbonate which was dissolved from the limestone. However, in the Williston Basin and in Canada the groundwater in the Madison aquifer becomes a saturated brine solution containing mostly sodium chloride.
It takes several thousands of years for groundwater to reach Giant Springs, but tens of thousands
of years to travel underground to Manitoba.
Neihart is a (lead, zinc, silver) mining town in the center of the Little Belt Mountains. The
second photo (Neihart Baldy) shows 2.8 billion year old basement rocks. The top of the
mountain is capped by a 1.5 billion year old metamorphosed sandstone, called the Neihart
quartzite. As indicated in Figure 1, the Madison limestone has long since been eroded off the
top of the Little Belts. The peak adjacent to Kings Hill Pass is Porphyry Peak, a small 50 million
year old igneous intrusion, which is the site of the Showdown Ski Area. Click here for a local
area map showing location of Belt Creek, Monarch, Neihart, Kings Hill, Sluice Boxes, and
Smith River. Armington Junction is shown as the intersection of U.S. 89 and MT 200. Dry
Fork is a tributary that enters Belt Creek at Monarch. [Webpage courtesy of Bob's Bar, Dining,
and Motel in Neihart.]
Cliff
Sluice Boxes--the location of a "slot" cut into Madison limestone by Belt Creek. The "slot" is about a hundred feet deep and 30 feet wide and is about a mile above the railroad trestle shown in the photo. Sluice Boxes State Park.
Giant Springs is located next to the Missouri River, east of Great Falls. It is a "leak" or
discharge point for the Madison Limestone aquifer. The site is now a State Park and the source
for bottled water.
Karst features, such as
The Williston Basin is a roughly circular-shaped area in North Dakota, Montana and
Saskatchewan that has been subsiding very slowly for the last half billion years. The bowl shape
of this basin is apparent in a structure contour map, showing the configuration (in feet below sea
level) of beds about 66 million years old (= top of the Cretaceous). [Map courtesy of U.S. Dept.
of Energy]
The
Deflection of groundwater. Groundwater in Madison limestone in the Williston Basin is a
completely saturated brine--a saltwater solution that contains so much sodium chloride that no
more salt will dissolve in it.. The salt comes from thick salt beds (evaporites) at the top of the
Madison limestone. The brine has a higher density than fresh water coming from Central
Montana. The fresh water stays high and flows around the Williston Basin rather than going
down through the Basin. Water coming from the Little Belt Mountains flows around the north
side of the Williston Basin, whereas water coming from recharge areas in the Bighorn
Mountains and the Black Hills of South Dakota flows around the south side of the Williston
Basin.
Saline seeps
Introductory Level Q & A (suitable for a 9th grade earth science class):
1. Question: From Central Montana to Central Manitoba is 1000 kilometers or 600 miles. That is a long ways for water to travel underground. What makes it flow so far? Answer: Gravity. Water flows downhill. The elevation of Belt Creek below Monarch where water goes into the Madison limestone is about 4500 feet (1370 meters) above sea level. The elevation at Giant Springs is 3230 feet (985 meters) and at Lake Winnepegosis, 830 feet (253 meters). On the topographic map of Montana note how the Little Belt Mountains in the center of the state are high and the northeast corner of Montana is low. If you pour water into the high end of a long garden hose, the water will run out the low end, provided that the hose is not plugged. The Madison limestone aquifer has the geometry of a porous sheet rather than an open tube.
2. Question: Why doesn't the water leak out to the surface? What keeps it underground for so long and for such a great distance? Answer: There are layers of shale above and below the Madison limestone which keep the water confined so that it has to stay in the limestone. Shale, which was deposited originally as mud, makes a good barrier for underground water. Water can not flow through shale very easily, whereas the limestone contains interconnected holes in a large range of sizes. Water flows easily through the limestone. To see a diagram showing the Madison limestone with the confining layers of shale, click on the Groundwater Atlas of the U.S., then on "Regional Summary", then scroll down in the text to "(Fig. 14)". The Madison aquifer is the one in the middle.
3. Question: Limestone is supposed to be a solid rock. How come it has so many holes? Answer: The limestone was originally deposited when sea shells or skeletal remains fell on the floor of a shallow sea. Most of the animals were one-celled animals living near the surface of the sea. When they died the hard parts, made of calcium carbonate or calcite, "rained" on the seafloor, like a continual snowfall. The space between the shells, which is known as the pore space, contained seawater. However, during compaction and diagenesis most of the pore space was filled by calcite. Eventually the seafloor was uplifted, and the shoreline retreated. Vertical fractures, called joints, formed in the limestone during uplift. Fresh water started to flow very slowly through the fractures in the limestone. The fresh water contained dissolved carbon dioxide, which made it slightly acidic. This weak acidity slowly dissolved some of the limestone. The longer that groundwater flowed through the limestone, the more limestone was dissolved. A network of passages that follow the systematic joints is a common configuration in limestone caves. The map of over 100 miles of passages in Jewel Cave shows the pattern of several sets of intersecting systematic joints.
4. Question: How do geologists know which way water flows underground? Answer: A special map called the potentiometric surface is the main tool for determining the pattern of underground flow. It documents how water flows downhill, even when it is underground. Many water wells and oil wells have been drilled into the Madison limestone. It is easy to measure how deep the water is in each well when there is no pumping going on or the height that water would rise if a pipe were attached above ground. (In the latter case one measures the water pressure and converts the value to the height of a column of water that would generate that pressure.) By plotting this data (expressed as elevations above sea level) on a map and contouring, one can generate a potentiometric surface for the aquifer. The map looks like a topographic map. Groundwater flows in the downhill direction, perpendicular to the contour lines. See Sharon Gabel's lecture notes on potentiometric maps with an example and cross section. (On her home page, click on "hydrogeology", then "potentiometric maps", then on the 3rd and 4th "PTT slides".) The potentiometric surface for the Madison limestone aquifer in central and eastern Montana is available from the Montana Bureau of Mines and Geology. The potentiometric surface follows the topography in central and eastern Montana but is more subdued. In the Little Belt Mountains the potentiometric surface is below the ground, but out on the prairie of eastern Montana it is above the ground.
5. Question: How do geologists know how long it takes for water to travel underground? Answer: Radiocarbon dating is used to determine how long the groundwater has been underground. Cosmic rays bombard the atmosphere converting some nitrogen into radioactive carbon. In particular, when a slow neutron hits a nitrogen atom, 7N14, the neutron replaces a proton to produce the carbon isotope, 6C14. This isotope has a half life of about 5730 years. The carbon reacts with oxygen in the atmosphere to make carbon dioxide. Carbon dioxide dissolves in rain water and surface water. The ratio of radioactive 6C14 to ordinary carbon 6C12 remains about the same as long as the water is in contact with the atmosphere. However, as soon as water goes underground the carbon 14 concentration decreases. By measuring the carbon 14 concentration (and correcting for the non-radiogenic carbon in carbonate that came from dissolved limestone and other effects), the age of the groundwater can be determined. Groundwater at Giant Springs has been dated as older than 2900 years, but younger than 6300 years. The flow rate is roughly 50 feet per year from the Little Belt Mountains to Giant Springs compared with
6. Question: Why are the Little Belt Mountains the recharge area for the Madison limestone aquifer? Answer: To see the maps and diagrams in this section click on the Groundwater Atlas of the U.S., then on "Regional Summary", then scroll down in the text to the relevant figure "(Fig. ??)" and click. East of the Rocky Mountains there are a series of dome-shaped uplifts (Fig. 12), separated by down-dropped basins. These basement-cored uplifts are known as the foreland ranges or the Laramide ranges. Erosion has removed the top of the domes, revealing basement rocks in the core and the sequence of sedimentary formations on the flanks, including Madison limestone (Fig. 14). Surface water enters the Madison limestone where the limestone is exposed on the surface. Recharge areas for Madison limestone are labeled on the map in (Fig. 7) as "Paleozoic aquifers". In central Montana these are the Little Belt and Big Snowy Mountains with minor contributions from the Little Rocky, Moccasin, and Judith Mountains. The other major recharge areas are the Bighorn Mountains and the Black Hills of South Dakota.
7. Question: How can groundwater flowing underground cross the continental divide? Answer: The Missouri River does indeed carry surface water to the Gulf of Mexico. However, groundwater that has traveled through the Madison limestone aquifer around the north side of the Williston Basin is discharged in saline seeps that flow into Lake Winnipegosis or Lake Manitoba and then into Hudson Bay. The groundwater flow, which normally follows the surface flow pattern, is deflected by the dense brines in the Williston Basin. The Missouri River flows across the Williston Basin without hindrence, but groundwater in the Madison limestone aquifer is mostly deflected around the basin.
9. Question: How have plate tectonic motions determined the flow of groundwater in the Madison aquifer? Answer: [UNDER CONSTRUCTION]
10. Question: Discuss the hydrologic model for the Madison limestone aquifer and its numerical simulation. Answer: [UNDER CONSTRUCTON]
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