June 13. The Redwall Limestone

In the Grand Canyon area, the rocks equivalent to the Madison Limestone that we talked about yesterday are called the Redwall Limestone. It forms one of the highest near-vertical cliffs in the canyon, as much as 500 to 800 feet high. The Redwall is a typical gray to tan limestone – so why is it called Redwall? It is stained red by erosion of the overlying rocks, especially the Supai group and Hermit Shale, which is red because of oxidized iron in the shale. The shale was deposited in a low mud flat that was periodically exposed to air and at other times underwater, which allowed the iron in the rock to oxidize. Shale erodes quite easily so it washed down the cliff faces and stained the underlying Redwall. 

Redwall Cavern, Grand Canyon

Fairly soon after the deposition of the Redwall – and this applies to the Madison limestone in many places, too – gentle uplift brought the limestones above sea level. Then the action of moving water, percolating through the rock, did what it does to limestone – it dissolved out caves. In a good number of places, caves collapsed, and sinkholes and other features typical of karst topography developed. This was a pretty widespread development, because we can see the evidence of the collapses in the Black Hills, in Montana, and in the top of the Redwall limestone too. This was not remotely related to the cave formation that made the modern Lewis and Clark Caverns we talked about yesterday, which are at most a couple million years old, but was something that happened probably quite soon after these Mississippian limestones were lithified. How do we know that? Because the collapse structures don’t include much of the overlying rock layers. The collapsing had to take place before those layers were laid down.

I can assure you that the Redwall does make really steep cliffs. Probably the most difficult hike of my life was on a perfectly smooth, level trail about three or four feet wide along the Redwall’s face. The problem was there was a drop-off on the left that went straight down about 400 feet, and the wall on my right went straight up another 400 feet. If you’re interested in the travelogue of that 1987 backpack trip, here’s a link to my report on it.

—Richard I. Gibson

Photo in Redwall Cavern, Grand Canyon, by Richard I. Gibson

June 12. The Madison Limestone

The Madison Limestone of the Rocky Mountains and Great Plains is formally a geologic group that includes several formations, including the Lodgepole Formation and Mission Canyon Formation.

It was formed in those warm, shallow Mississippian seas that covered a lot of the interior of North America about 330 to 340 million years ago. The calcium carbonate deposited in those seas accumulated to as much as 2,000 feet thick in parts of Montana, where the Madison forms prominent gray cliffs. The Gates of the Mountains, named by Lewis and Clark for dramatic cliffs along the Missouri River east of Helena, Montana, are made of these Mississippian limestones.  

Madison Limestone at Gates of the Mountains

The Madison was named back in 1893 for outcrops along the Madison River near Three Forks, Montana, or perhaps for the nearby Madison Mountain Range, but it has equivalents with different names as far away as the Black Hills of South Dakota, central Colorado, and Arizona. It was an extensive shallow sea.

You probably won’t be surprised to hear that the warm shallow water supported lots of life. The Lodgepole Formation especially has loads of fossils, ranging from crinoid stem columnals to brachiopods and small horn corals and much more. The Lodgepole Formation has nice bedding sometimes distinguished by a little silt or mud interbedded or mixed with the limestone. The beds are often two to four inches thick and sometimes show color differences that give the rock a distinct striped appearance – but don’t visualize bright colors, we’re talking about shades of tan and gray.

The Mission Canyon formation, younger than and on top of the Lodgepole Formation, is massive. That’s a technical term that means we seldom see the discrete beds like the ones in the Lodgepole, or for that matter in most layered sedimentary rocks. The Mission Canyon is just limestone. Lots of it. Microscopically, the Mission Canyon is called bioclastic – that means “life, broken” – and the rock is often made up mostly of tiny broken pieces of shells and crinoid stems and such, all cemented together by more calcite. It’s usually gray, and because it’s thick and doesn’t have planes of weakness, bedding planes, the Mission Canyon sometimes makes really prominent cliffs like those at the Gates of the Mountains.

Limestone is easily soluble in rainwater, which is normally slightly acidic because of the reaction between water and carbon dioxide in the atmosphere, which makes carbonic acid. Even in arid country like Montana, caves can develop. Lewis and Clark Caverns, a state park along the Jefferson River east of Whitehall, Montana, probably formed when Montana was a lot rainier than it is today, probably during the glacial periods of the past couple million years. Lewis and Clark were on the river just below the caverns, but they never saw them. The caves were discovered by non-Native Americans in 1882, and they were designated our second National Monument in 1908, but because the park service couldn’t manage it, the cave was transferred to the State of Montana in 1938 and became Montana’s first state park.

The porous, easily dissolved limestones of the Madison Group serve as important oil reservoirs in places like the Williston Basin of eastern Montana and western North Dakota, and elsewhere they are valuable groundwater aquifers.

—Richard I. Gibson

Photo of Madison Limestone at Gates of the Mountains by Richard I. Gibson.

June 11. Waulsortian Mounds

During the Mississippian, a special kind of carbonate build up evolved, called Waulsortian Mounds. They were not reefs – they did not contain the kinds of organisms like corals that could build large complex structures. You may recall that the end-Devonian extinction decimated the reef-building organisms, and we don’t find true reefs in the geologic record for a hundred million years after the Devonian extinction. These mounds are the only constructions known during the interval until reef builders recovered. They are not simple piles of sediment – they can reach 200 meters in height with very steep flanks. They seem to really be constructions of some kind.   

And they are found in a restricted geographic setting – the southern margin of the Laurussian Continent, formed by the collision of North America and Baltica, the core of Europe. Today, this zone is in present-day Belgium – the community of Waulsort, Belgium, gives its name to these mounds – southern England, Wales, and Ireland, Illinois, Missouri, and Kansas, and north up to North Dakota and Alberta. During the Mississippian, that margin was very near the paleo-equator, so the mounds probably relate to processes and life in warm, tropical seas. The entire southern margin of Laurussia was probably a relatively narrow but continuous ocean, in part the Tethys Ocean between Gondwana and Eurasia and in part the ocean that remained between Gondwana and southeastern North America.

Waulsortian mounds are mostly lime mud, including some broken skeletal fragments of things like crinoids and bryozoans. It is not clear exactly why the mounds formed, but there may be a connection to the likely storm tracks of hurricanes at the time they formed in the Mississippian Period. Or more specifically, the lack of storms. The mounds seem to be in regions that would likely have been protected from the most common tracks of hurricanes, so that they would have had time to grow by whatever mechanism. David King, at Auburn University, shows a connection between the mounds and low wave energy, another way of suggesting that major storms were rare or absent. I have a link on the blog to King’s interesting report on Waulsortian mounds.

Were these mounds constructed by life processes such as sediment binding by bacteria, or by some inorganic process of selective cementation? As near as I can tell, the answer is not certain. It may have been a combination of processes, made possibly by the fortuitous absence of large storms.

—Richard I. Gibson

Link:
David T. King, Jr.’s report