The IGS Rock Library
About 470 million years ago, a meteor as big as a city block hurtled through the Earth’s atmosphere and struck what is now northeastern Iowa. The enormous impact shattered the bedrock and blasted everything outward. Shallow, brackish seawater rushed into the crater, scooping up the materials that had just blown out and sweeping in marine animals along with the sediments. The crater was almost four miles wide, and its shallow marine environment provided a special place for fossil preservation.
Humans weren’t around to witness the destructive power of the meteorite impact near what is now Decorah, Iowa. The discovery of the second recognized impact structure in Iowa was largely due to the ingenious detective work and meticulous data gathering of researchers at the Iowa Geological Survey (IGS), which is now part of IIHR—Hydroscience & Engineering.
The fossils and other rock samples from the Decorah meteor impact structure are just part of the Iowa Geological Survey’s impressive collection of samples, fossils, records, publications, and other treasures collectively known as the “rock library.” Off the beaten path at the University of Iowa’s Research Park, the rock library offers a glimpse into Iowa’s geologic history.
The Decorah Impact Structure
The Decorah crater, or impact structure, isn’t visible to the eye today. Before it was recognized, unusual phenomena in the area had bothered residents for many years. In the early 20th century, some people thought good coal could be found near Decorah because some of the dark sediments there would burn. Well drillers noticed that wells didn’t produce as much water as others nearby. The normal aquifers seen in other parts of the state just didn’t seem to be there.
Iowa well drillers fill out detailed logs and collect rock chip samples every five feet as they drill for water. They send both the logs and the chip samples to the IGS, where staff members carefully analyze and file the data.
Most of Iowa’s geologic structure is like a flat, predictable layer cake. But around Decorah, nothing seemed normal. As early as the 1950s, IGS geologists knew something wasn’t right in that area. Researchers pored over the samples and delved further into the IGS collection. Through painstaking analysis, they discovered a strange circular feature around the current town of Decorah. Inside the bull’s-eye, the typical Iowa geology just didn’t exist. Outside the bull’s-eye, the normal rules applied.
An aerial geophysical survey confirmed the bull’s-eye pattern around Decorah. Discovery of shocked quartz in the area was indicative of a major impact. Little by little, the geologists were coming to a conclusion: a meteorite had likely struck the area, shattering the normal geology. Geologist H. Paul Liu first presented the discovery to his IGS colleagues in 2007.
Liu, who is a paleontology expert, had uncovered a treasure trove of fossils in the Decorah impact structure. He was awarded a grant to conduct the impact structure research and to divert the Upper Iowa River just long enough to dig a pit and unearth slabs of rock. Derek Briggs of Yale University later joined as co-PI on the project.
The Winneshiek Lagerstätte, as it is known, is a deposit area with excellent fossil preservation. The slabs Liu and IGS colleagues McKay and Brian Witzke brought up were from a new local stratigraphic unit, the Winneshiek Shale. By splitting the shale apart, they revealed a remarkable range of exceptionally well-preserved fossils, including some of the earliest known eurypterids (an extinct group of arthropods related to spiders), phyllocarids (shrimp-like animals), what may be the earliest sporangia (a plant structure in which spores are stored) with in situ spores in the fossil record, and much more. Some slabs even include imprints of soft tissues, which are very rare in the fossil record.
Liu says his colleagues worldwide were excited by this scientific discovery. They called it a “gold mine,” “amazing stuff,” and even the “discovery of the decade” in Ordovician paleontology. “This is something special,” Langel says. “Now it’s on our shelves, and Paul [Liu] knows all about it.”
The discovery of the meteor impact structure started with tiny bags of rock chips. The IGS currently has almost 40,000 sets of rock chip samples in its repository, which works out to about 1.5 million bags of rock chips.
IGS student employees carefully wash and process the rock chip samples, and then put the prepared samples into small envelopes. Austin Potthoff, an undergraduate student in geology, says he enjoys the work, even if it is a bit repetitive. “You usually find one or two little treasures,” he says.
Professional staff later analyze the cleaned rock chips under the microscope. “You’re looking at samples that haven’t seen the light of day in 500 million years!” says IGS Geologist Jason Vogelgesang. Each set of chips is translated onto a strip log, which uses different colors, symbols, and abbreviations to visually represent what the well driller turned up.
The strip logs can be quite beautiful. Each one is unique, and IGS staff create them by hand with colored pencils. Tiny, neat handwriting is a must. “It’s the hardest part!” says IGS Research Specialist Ryan Clark. The IGS hopes to recreate the strip logs electronically, but the complexity makes it challenging.
Core Samples Tell Iowa’s Geological History
Useful as the rock chips are, IGS staff members get much more excited about drill core samples — long cylinders of soil, sand, and rock drilled from beneath the Earth’s surface.
“Core for me is like the holy grail,” Langel says. Core yields more detail and context than chip samples do. Researchers can see the finer points — sand grains, shale layers, fossils, and fractures that allow groundwater to percolate through the rock, and much more.
The IGS has core from more than 1,200 sites; the core samples would stretch more than 100 miles if put end to end. Using the chip and core samples, researchers can model Iowa’s geology in three dimensions across the entire state. This information is important not only to scientists, but also to industry, communities, and anyone who drinks water.
Reading Iowa’s Groundwater
In Iowa, the primary use of geologic information is to better understand groundwater. The IGS can provide a well forecast for any area of the state based on data in the system. A community or company that needs to drill a new well that will produce a certain amount of water can turn to the IGS to learn about their options: where they should drill, how deep they will have to go, and the quality and quantity of water they will likely find when they get there. “That’s probably the greatest resource,” Vogelgesang says.
The applications of the rock library’s collection are many and wide-ranging. From predicting water quantities to finding anomalies in Iowa’s geology, the rock library is ready and waiting for use.
“What could be in the samples?” Langel asks. Another meteor impact structure, perhaps? “You never know until you start looking.”