NORTHWEST TERRITORY : Geologist finds layers of meaning in region’s rocks
Posted on Thursday, August 28, 2008
URL: http://www.nwanews.com/adg/Outdoors/235641/
One wouldn’t guess Van
Brahana to be in his 60 s from the
youthful enthusiasm exuded as he recently inspected the geology of a mountain at the end of a ridge flanking the Kings River in Madison County. From the chert scattered across the top of the “bald knob” and down through the layers of rock forming the bluffs encircling the mountainsides, the professor of geosciences at the University of Arkansas at Fayetteville hustled about happily, filling his hands with rock samples as if they were gold nuggets. “There’s more geology around this place than I would normally see in six different places in the Ozarks,” Brahana said, noting that Madison County is one of his favorite geological areas. There is a reason for that, but first a little background about the beliefs of some scientists regarding the geology of the Ozarks in general would be helpful. At the risk of oversimplification, those scientists believe the layers of rock forming the Ozark Plateau were laid down between 300-480 million years ago, when the far-flung plateau was a marine environment. Around 180 million years ago, the waters rose and fell and came and went to create the distinctive layers of rock seen on many bluffs, such as those along the Buffalo National River.
The thick limestone layers are believed to have been created by the secretions of living marine creatures and the skeletons of dead ones. The beds of sandstone emerged from beaches and dunes around ancient seas or from the banks of lowland rivers. The thin, flaky layers of shale were formed from clay and silt deposited in deltas and marshes.
It is further believed that about 300 million years ago, the movement of tectonic plates uplifted the Ozarks into the folds of ridges and valleys recognizable today. Since then, the plateau has been shaped by erosion, such as wind and rain and the carving effect of streams.
Over time, the erosion shaped the Ozarks into three irregular but distinctive regions known as the Boston Mountains and the Springfield and Salem plateaus, each with characteristic formations of rock.
As the name suggests, the Boston Mountains Plateau extends through the Boston Mountains along the southern edge of the Ozarks. The Springfield Plateau is a long and relatively narrow region extending northeasterly from northwestern Oklahoma to the Missouri River near St. Louis. The Salem Plateau is by far the largest, including much of the White River drainage and the central and eastern Ozarks of Arkansas and Missouri.
While the geology in specific areas of the various plateaus, such as along the Buffalo River, have been well-documented, less is known about the transition zones between the three regions. That’s where geologists like Brahana can find interesting surprises and anomalies.
One such area is Madison County, in the vicinity of the Kings River.
“Right here, we have geology representing elements of all three plateaus — the Boston Mountains, Springfield and Salem; that’s what makes it so interesting,” Brahana said.
He also said that the contours of Northwest Arkansas were not only shaped by erosion, but also by earthquakes along large and once-active fault lines, including one in western Madison County. It extends roughly along Arkansas 23 through the Forum area and is known as the Drake’s Creek Fault.
“We have evidence it extends up to 3, 000 feet deep and up to 50 miles long,” Brahana said.
During the recent mountain inspection, he singled out the uppermost line of bluffs wrapping around the mountainside. About 30-40 feet high, it features a distinctive streak of red limestone running through layers of limestone in colors of white to dark gray.
“This is St. Joe limestone associated with the Springfield Plateau,” Brahana said, noting the red streak as an identifier.
Brahana believes that in geologic time, the fossil-rich limestone developed more than 400 million years ago and is older than the layer known as the Boone Formation of limestone that forms most of the upper layer of the Springfield Plateau.
“The St. Joe limestone is a beautiful former of caves,” Brahana added.
As a matter of fact, a small cave had been discovered in the mountain, and Brahana’s presence during the recent inspection was primarily to help UA’s Eric Pollock plant scientific sensors in the cave as part of a research project.
Like typical Madison County mountains, the St. Joe limestone lies atop a thick layer of sandstone, which tops another bluff line of dense gray limestone 20-30 feet thick.
“The lower bluff line is an Everton sandy dolomite layer, which is what makes a lot of the big bluffs along the Buffalo River,” Brahana said.
The Everton layer also is believed to be the oldest rock in the Ozarks and is associated with the Salem Plateau.
The geologist was especially struck by the abundance of chert found on the surface of the mountain above the St. Joe limestone. Consisting of a mixture of silica and quartz, the flinty chert was present in larger, thicker chunks than he had seen.
“This chert is unusual,” he said, hefting a couple of the big chunks. “It represents that something unique was happening here when the chert was formed in a marine environment.”
Brahana theorized it could have been the pressure of water of unusual depth or the result of volcanic activity.
Brahana pointed out many other geological features during the ramble, and his infectious attitude caused an ordinary mountain to be viewed and appreciated in an entirely new light.