JUNIATA GEOLOGY TEAM EXAMINES SHALE OUTCROPPINGS
(Posted June 13, 2001)
HUNTINGDON, Pa. -- Which mountain range is older, the mighty peaks of the Rockies in Colorado or the rolling terrain of Pennsylvania? A team of Juniata College students and a Juniata geologist already know the answer, but they are trying to use clues within local rock to pinpoint when the Appalachian mountains of Pennsylvania were formed.
By the way, Pennsylvania's peaks are much older, by hundreds of millions of years.
David Lehman, assistant professor of geology at Juniata, assisted by geology students Donna Beares a senior from Baldwin, Md., and Matt Hoffer, a senior from Ephrata, Pa., are looking to refine the history of the mountains of Pennsylvania by examining a deposit of shale near Martinsburg, Pa.
The Juniata research team did not have to hike deep into a mountain range to find their shale deposits. They have done most of their sampling work on an outcrop less than 100 yards from Martinsburg's main drag, Route 164 on a commercially developed bluff overlooking the roadway.
"We were looking for an area where a specific shale deposit overlays a deposit of limestone, which signifies a mountain building event," Lehmann explains. "The shale at the Martinsburg site is part of what geologists call the Antes Shale."
Lehmann explains that mountains form when a continental plate, such as the edge of North America, collides with something else -- in the case of Pennsylvania, a chain of volcanoes. The collision caused an upthrust of land that formed a mountain range, while at the same time creating a deep, water-filled basin at the western front of the mountains.
Prior to this mountain-building event, known as the Taconic orogeny, most of North America -- including Pennsylvania -- was covered by a shallow tropical sea, in some respects similar to the modern day Bahamas. In this sea, algae and a wide variety of shellfish produced lots of limey sediment on the seafloor. The resulting limestone forms the bedrock for some of the richest agricultural areas in Pennsylvania. However, the development of the deep basin out in front of the growing mountain range put an end to the tropical climate. As the seafloor subsided to depths below which sunlight could penetrate and dark, organic-rich, mud washed into the basin from the growing mountains to the east of Harrisburg, most of the tropical plants and animals could no longer survive.
Finding areas where scientists can examine a continual section of rock from the limestone through the overlying, organic-rich, black shale is a rarity, Lehmann says, and these areas give insight as to how mountain building affected the marine animals living to the west of the growing mountains. "There are only a handful of sites in Pennsylvania where geologists can examine the abrupt change from the tropical to deep marine fossil fauna, and the Martinsburg exposure is the westernmost locality where the contact between the limestone and shale is exposed," he adds.
"There are a number of fossils that are commonly found in shale that allow geologists to estimate the age of the shale within a couple of million years," Lehmann says. "That sounds like it's not very accurate, but for geologists looking at rocks 450 million years old or so -- more than twice as old as the oldest dinosaurs, being a million years off is pretty good."
To date these rocks and gain insight towards the growth of the Appalachian mountains, the researchers are looking for two fossil types in particular. In shale, they are looking for graptolites, an extinct animal with a skeleton about 2 inches long and one millimeter wide -- "They look like little saw blades," Lehmann says. In limestone deposits, the team is seeking conodonts, which are the microscopic tooth-like remnants of another extinct animal that roughly resembled a fish. To search for conodonts, Hoffer is collecting about 50 pounds of samples from each thin layer of limestone that he is studying. Both graptolites and conodonts evolved very quickly, so finding a particular conodont or graptolite allows the geologists to fairly precisely date the rocks in which the fossils occur.
Not that long ago, geologists assumed that black shale deposits like the Antes Shale had relatively few fossils because the shale records oxygen-deprived settings. However, over the past 20 years, paleontologists studying black shale deposits have learned that these rocks contain fossils of animals well adapted for low oxygen settings. "Compared to black shales in other areas, the Antes Shale has a really healthy group of animal fossils in it," Lehmann says. Beares is presently identifying a wide variety of fossils from dense shell beds in the shale. Besides identifying the fossils, she is also examining how the shells were preserved, giving the team insight towards what life and death in the deep basin was like 450 million years ago.
The easy access to the shale's location is an added bonus, says Lehmann. "Usually our sites are in the middle of nowhere," he says. "This research will let us stitch together a fascinating moment in geologic time—the end of a Bahamian paradise and the start of the Appalachian mountains."
Contact John Wall at firstname.lastname@example.org or (814) 641-3132 for more information.