Sandstone Secrets: Quartz Pebbles Reveal Ancient River System in Appalachia
Three hundred million years ago, great rivers cut 200-foot-deep canyons and 20-mile-wide valleys through the Appalachian Mountains.
Imagine great rivers flowing along the Appalachian Mountains. Their waters wind through dense forests and murky swamplands. Their currents, over many years, have cut 200-foot-deep canyons and 20-mile-wide valleys.
This is the picture geologists Stephen Greb and Allen Archer paint of the Appalachian Mountains 300 million years ago. The two researchers have recently discovered evidence that these long-gone rivers drained much of North America, an area almost the size drained by the mighty South American Amazon, and their findings call into question what scientists believe about the climate in this region 300 million years ago.
"These rivers would have made some great tourist attractions," says Greb, a researcher with the Kentucky Geological Survey at the University of Kentucky. "In Western Kentucky you'd have canyonlands -- 200-foot-deep canyons with rivers flowing at the bottom." In what is now Eastern Kentucky, broad valleys were filled with sand deposited by rivers over millions of years. These sands became sandstones which are visible in present-day wonders like Natural Bridge, a 78-foot long sandstone arch, and Cumberland Falls, a 68-foot waterfall.
Archer, a geologist at Kansas State University, and Greb map these rivers as two systems. One system likely began in Southeastern Quebec or Novia Scotia, Canada, traveled through the Appalachians in Eastern Kentucky, and emptied 2,300 miles south near the present-day coastal plain of Mississippi and Alabama. The second system began in Southeastern Ontario, Canada, and flowed through Western Kentucky to Arkansas.
The Quartz Pebble Key
Greb and Archer traced the path of these rivers through two-inch quartz pebbles in sandstone deposits in the eastern United States. Previous research by scientists in Kentucky, Illinois, Indiana, Michigan, Arkansas, Ohio, Pennsylvania and West Virginia revealed rocks of the same age, all containing the same size quartz pebbles. These quartz pebbles were the key to connecting deposits and forming a picture of the ancient rivers.
"Quartz pebbles can't come from any of those states at the time these rivers flowed," Greb says. "Quartz pebbles of this size formed with intense pressure and heat, in areas where mountains are forming or at the ancient cores of the continents." No other natural process could have formed those pebbles, Greb says. The core of North America is exposed in Canada, from the U.S. border to the Hudson Bay area, and he says that is where the quartz originated.
In what is now Eastern Kentucky, broad valleys were filled with sand deposited by rivers over millions of years. These sands became sandstones which are visible in present-day wonders like Natural Bridge, a 78-foot long sandstone arch, and Cumberland Falls, a 68-foot waterfall.
Greb says another clue that these pebbles are far from home is the purity of the sandstones. "These sandstones are 90 to 95 percent quartz grains with very little clay," he says. "The more ocean waters or river currents rework a sand deposit, the more it weathers and erodes, the more dirt you get rid of. These quartz pebbles had to be cleaned many times before they reached this purity.
"We're not the first to connect the similar rocks in each state," Greb says, "but we're the first to quantify the whole drainage area and make comparisons to form what we believe is an accurate picture of what these rivers looked like."
Rivers that Rival the Amazon
The South American Amazon is the largest river in the world in terms of the amount of water it carries. At 3,900 miles long, it's a good modern comparison to these ancient rivers, says Greb. "The ancient rivers rivaled the Amazon in terms of the size of the drainage basin -- all of the little rivers flowing in the same direction that fed into the big river," Greb says.
In the United States, the ancient rivers drained an area even larger than the modern Mississippi, which flows from Northern Minnesota to Louisiana. "American geologists tend to view the Mississippi River as big, but in fact, it carries only about one tenth as much water as the Amazon," Archer says.
Greb and Archer say the ancient rivers didn't look like the Mississippi. "The Mississippi, Ohio and Kentucky rivers are snake-like, with little twists and loops," says Greb. The ancient rivers were braided rivers with tear shaped islands where the wide river splits and reconnects. "Some of the individual rivers may have been small, but there were hundreds of them interconnecting and forming a very broad river system," Greb says. Some of the rivers may have been 35 to 40 feet deep and more than a kilometer wide.
But how do they know how the ancient rivers looked? "Modern rivers are our basis for understanding how ancient rivers worked," Greb says. "We study how the sand and mud move in a modern river and apply the layers, grain sizes and textures of those sediments to what we see in the ancient rocks."
Stephen Greb says that a significant application to his river system research, carried out with Allan Archer from Kansas State University, is the ability to predict the location of oil, natural gas, water, and coal in similar sandstone deposits.
Charting the course of an ancient river takes thousands of measurements, says Greb. "From the layers you see in sandstones, you can determine which way the river was flowing, how fast it was flowing, and how deep the water was." Some of these layers are at an angle. "We measure that angle and it tells us the way the ancient sandbars were moving," Greb says. "The rivers in both systems had sandbars all over the place, with the current changing their shape and depositing the quartz pebbles they contained," Greb says. "We take hundreds of those sandstone measurements to form a picture of how the sandbars were moving."
Greb and Archer used hundreds of measurements taken by other researchers as well, including very detailed research on Kentucky sandstones by Donald Chestnut, Greb's colleague at the Kentucky Geological Survey. "By also combining data from researchers in other states, we can present some interesting interpretations," Archer says.
Greb and Archer's goal was to find the minimum and maximum size of the drainage basin in order to get an accurate picture of the potential size of these river systems. "The first step in finding the minimum is figuring out the closest locations the quartz pebbles could have come from," Greb says. "Then we look at geological structures like mountains, things that would be obvious barriers to the river." To determine the maximum extent of these rivers, Greb and Archer looked at where the sandstone deposits end -- where the rivers met the coast mid-way through Alabama. The coast of 300 million years ago was inland from today's coast, says Greb. Ocean covered Louisiana, and parts of Alabama, Georgia, South Carolina, Texas, and Oklahoma.
A Question of Climate
"Kentucky was a tropical paradise 300 million years ago," Greb says. According to widely accepted scientific models, Kentucky was on the equator. "We took the drawing of the equator that other scientists did based on the theory of plate tectonics, commonly called continental drift," says Greb.
"Continental drift is supported by many different types of geological evidence," says Greb. "For instance, you can see something that was deposited in a coral reef in a warm tropical climate is now in Siberia. That tells you the continent has moved."
"When these sandstones were formed in Kentucky, most of the earth's continents were part of a super-continent named Pangea," Archer says.
The fact of Kentucky's tropical past is supported by today's mineable coal resources. "The thick coals we have in Eastern and Western Kentucky needed a tropical climate to form. The best modern comparisons to those coals are in swamps in Indonesia," Greb says.
"We know Kentucky was tropical because these coals formed, but we also found evidence that the climate wasn't continually wet," Greb says. In fact, Greb and Archer's findings challenge much of what scientists believe about the climate 300 million years ago. While other scientists hold that the climate was purely tropical, Greb and Archer and a growing group of researchers believe at times the climate may have been temperate or even semi-arid. Greb sums up their theory in one word: oscillation, a cycle of rising and falling oceans.
Greb and Archer traced the path of these rivers through two-inch quartz pebbles in sandstone deposits in the eastern United States. These quartz pebbles were the key to connecting deposits and forming a picture of the ancient rivers.
"When we analyzed these sandstones, and the rocks they cut through, we found sharply sloped valleys," says Greb. "With a very humid, tropical climate we'd expect the valley walls to get rounded with the constant water weathering. To cut a valley 200 feet deep, sea level had to drop 200 feet. To fill the valley sea level had to rise." The valleys themselves support the idea that the ocean was repeatedly rising and falling over time.
Greb and Archer explain that tropical conditions prevailed as sea levels rose, moving the coast more than 200 miles inland until it reached Kentucky. "We found new evidence that tides and waves reworked sandstones near the top of the valleys, so we were much closer to the shore than previously thought," Greb says. As ocean waters washed in, the sand-filled valleys became shallow swamps with frequent rain, humidity, and abundant plant life. These plants decayed forming peat, the precursor to coal.
"The fact that we can find evidence of many different swamps in this time period tells us that the ocean continued rising and falling, and sometimes Kentucky was covered by shallow seas, sometimes it was a swamp, and sometimes it was river plains," says Greb. "It just kept cycling through time."
A Fuel Gauge in the Sand
The coal fields where Greb and Archer met may now benefit from their river research. Greb met Archer, who at the time worked with the Indiana Geological Survey, during a field trip in Indiana in 1989. By trading sandstone images electronically on the Internet and sharing ideas through long-distance phone calls, these two researchers aren't discouraged by the 800 miles between them. "We try to meet in person several times a year, but with e-mail and next-day mail the distance really doesn't matter," Greb says. "In fact, in some ways the distance helps. In geology it's very easy to get too focused on your own backyard. When you work with someone who is in another state or even another country, that person is being exposed to different research, which often helps to test hypotheses you may take for granted or to put your own work in a different light.
"A very significant application to our river system research is the ability to predict the location of oil, natural gas, water and coal in similiar sandstone deposits," says Greb.
Some of the ancient river sands were very porous, filled with tiny holes that held oil, natural gas and water. For industries that rely on underground core drilling, knowing where these sands are located within the ancient Kentucky valleys will eliminate hit-and-miss drilling in other valleys. "Drilling cores is very expensive so you can't just keep drilling holes into the ground hoping you'll come up with oil or water or natural gas," Greb says. "If you find a spot with oil or natural gas, knowing the probable pattern of these resource-rich sands in the valley will tell you where to put your next hole."
Greb and Archer's findings about these oil-rich sands have attracted international interest. "Many countries have similiar valleys, but in other countries these types of deposits exist thousands of feet beneath the surface, where the only information geologists can get about the sands are from core samples, often spaced kilometers apart," Greb says. To find out what happens between these points, geologists need to find a similar standstone exposed at the surface, where they can measure changes in sandstone layers. "Because the quartz-pebbled sandstones are so easily accessible in Kentucky, geologists from other countries can learn about details of the sandstones from our naturally occurring outcrops at the surface and apply that information below ground in other parts of the world," Greb says.
For the coal industry in Kentucky, Greb and Archer's research can tell where coal won't be found. "These sands filled valleys that cut through coal-bearing rocks," Greb says. "In the future when we concentrate more on the deep underground coals, we can warn mine planners where these sands are and where the coal is not." Greb says knowing where these sands are also has safety applications for underground mining. "Some of these sandstones are very hard -- the spaces between grains and quartz pebbles were filled with natural cement," he says. "If you're in a coal mine and that's your roof, it's very hard to drill holes for support bolts."
Beyond the fuel benefits, this research could impact global warming theories. "Understanding how the ancient climates changed in response to rising sea level has important implications for predicting what might happen if global warming is a reality and global sea levels continue to rise," Archer says. "We certainly don't understand how climate has varied over time and how it is changing today. There is quite a bit of conjecture in some of the more dire predictions of global climate change that is not necessarily supported by the available data. Studies like ours might help clear the air a bit."
One thing is clear: the ancient sandstones hold secrets to Kentucky's past as well as keys to the future. The picture Greb and Archer have painted of these great rivers reveals new questions for scientists exploring climate and fresh opportunities for energy resources on a global scale.