Milwaukee Public Museum

Deep Tunnels, Desert Peaks and Snow-Covered Mountains:
Connecting rocks thousands of miles apart and hundreds of millions of years old
by Paul Mayer, Geology Collections Manager

EDITOR'S NOTE: This article is reprinted from LORE magazine, a benefit of museum membership. ©2000 Milwaukee Public Museum, Inc.

Geologists mention at times something they call the Picture. In an absolutely unidiomatic way, they have often said to me, 'You don't get the Picture.' The oolites and dolomite - tuff and granite, the Pequop siltstones and shales - are pieces of the Picture.' The stories that go with them - the creatures and the chemistry, the motions of the crust, the paleoenvironmental scenes - may well, as stories, stand on their own, but all are fragments of the Picture. ... Multidimensional, worldwide in scope and in motion through time it is sometimes called the Big Picture."

Sponge spicules, magnified (frame 30, microslide, lower in page)

What do deep tunnels drilled under Milwaukee, scorched desert peaks of Nevada and snow-covered mountaintops in the Canadian Rockies have in common? The simple answer is the rock. In all of these areas, carbonate rocks rich in fossils of the same age can be found. The rock in these places is separated by thousands of miles, and the geologic forces that acted upon them are very different, yet they are linked by the fossils contained within them. In all three places the animals that made the fossils all lived and died during the geologic timeframe known as the Devonian Period.

Devonian fossils are very old, older than the oldest dinosaur fossils by more than 100 million years. The earth was a very different place during the Devonian, and it is intriguing to imagine what life on earth was like back then.

North America was positioned over the equator and Wisconsin was in the tropics. A shallow tropical sea, teeming with life, covered much of North America, including Milwaukee. The most common fossils found in the Devonian bedrock below Milwaukee are marine invertebrate fossils including corals, bivalves, gastropods, cephalopods, trilobites, brachiopods, bryozoans and crinoids.

No mammals lived during the Devonian; amphibians and fishes were the dominant vertebrates. Placoderm fishes, over 20 feet long and armored with thick, bony plates and spines swam through these tropical seas. Fossils of these fishes and small sharks can be found in the bedrock underneath Milwaukee. No flowering plants lived during the Devonian Period, but distant relatives to ferns, horsetails and club mosses grew as large as trees.

Near the end of the Devonian a mass extinction destroyed over 70 percent of all species living on the planet. Scientists debate what caused this mass extinction. Was it an asteroid impact? Many craters around the world have been dated to the Devonian, including possible impact sites in Chicago and Missouri, but these impact sites seem too small to have caused a mass extinction. Was it a climate change or did the oceans suddeniy turn anoxic? No one can say for certain, but scientists are studying the fossil clues left in the rocks to try to solve this mystery.

Research

During the past eight years I have collected and studied Devonian fossils from Wisconsin, Nevada and Canada. My research on these fossils helps me better understand what life on earth was like 360 million years ago.

Wisconsin

Wisconsin It is a crisp, clear-blue autumn day: the Milwaukee River shivers with ice, and brilliant red and yellow leaves carpet the ground. Fishermen in chest-high waders brave cold waters in pursuit of spawning salmon. I put on my own rubber boots, grab a white bucket full of rock hammers, chisels, measuring tapes and burlap sample bags and head down the stone stairway at Estabrook Park to the Milwaukee River. I may look like a fisherman, but I am here to collect fossils - clues from the Devonian.

Fishing from Devonian bedrock in Estabrook Park, Milwaukee.
Close-up of Devonain bedrock at Estabrook Park.

Here along the banks of the Milwaukee River the bedrock underlying northern Milwaukee and Ozaukee counties makes a rare appearance. Generally the bedrock is buried 20-200 feet below sand, clay, soils and other rock debris dumped upon it by thousands of years of glaciation. Less than I million years ago, glacial ice scraped rock and soil from northern Wisconsin, Michigan and Canada and carried it southwards where it finally melted, dumping its ground-up, pulverized cargo here and across much of the Midwest.

The small, scattered bedrock outcrops in Estabrook Park occur in several spots along the river where the rushing water has removed the glacial float and is now slowly carving its way through the bedrock. The rock forms small ledges 4-5 feet high and 10-30 feet long. I study these exposed sections bv measuring each layer or bed, and recording the type of rock, fossils and anv sedimentary structures I can observe.

In the Museum's laboratory the samples are split apart, sawed and polished, and, finally, dissolved in acid in an attempt to recover as many fossils as possible from the rock. The rock yields many fossils, especially brachiopods, a two-shelled organism that superficially looks like a clam or bivalve but are a separate group of animals unrelated to mollusks.

Brachiopods have a beautiful symmetry not found in bivalve shells. In the Paleozoic Era brachiopods were very common and widespread, inhabiting many different environments. Today brachiopods are restricted to very deep water or dark cracks and crevices in rocky shorelines. The fossil brachiopods at Estabrook Park lived in a shallow tropical sea and correlate with the middle part of the Devonian.

Even in these small outcrops I notice changes in the composition and diversity of brachiopods, reflecting changes in ancient marine paleocommunities and possibly in the paleoenvironment. The goal of my research is to explain why these marine paleocommunities changed. Can changes in the paleoenvironment be linked to changes in the paleocommunities?

To answer this I need to identify depositional environments-primarily deep-water and shallow-water environments-and the different paleocommunities that lived in these environments. Subsequently I correlate these rocks and fossils to Iowa and Michigan. I hope to identify continent-wide sea-level rises and falls, and to track paleocommunities from Iowa to Wisconsin to Michigan with these changing sea levels. This allows me to study the effects of changing environments on marine paleocommunities through time.

The outcrops at Estabrook Park provide just a glimpse of the Devonian and expose about 5 percent of the total thickness of Devonian rock underneath Milwaukee. A more complete picture was available 100 years ago when quarries occupied this site. The Milwaukee Cement Company quarried this rock from 1875-1912. The workers, quarrying by hand, exposed over 25 vertical feet of rock. This allowed them the opportunity to collect and sell fossils to private collectors. One of these collections is on display at the University of Wisconsin-Milwaukee's Greene Gallery.

Geologists in Wisconsin divided the Devonian rock into four formations, naming them after towns in Wisconsin and Michigan: Lake Church, Thiensville, Milwaukee and Antrim. The total thickness of the combined Devonian section was estimated to be 200-250 feet. As the quarries were shut down they were also filled in or flooded, leaving behind only a small fraction of rock exposure for geologists to study.

All of this was recently changed with the completion of the Milwaukee Metropolitan Sewerage District's (MMSD) Deep Tunnel Project. Several tunnels were excavated in the bedrock underneath Milwaukee during the project. These tunnels are designed to fill with the overflow water from Milwaukee's sewer system during storms.

Before the tunnels were excavated, boreholes were drilled and cores from these boreholes were recovered. Some of the holes were drilled to over 800 feet deep, providing a continuous record of the rock below Milwaukee. These cores are vital to my research and will be the focus of my studies for the next few years.

In examining the outside of these cores it is difficult to see anything of substance. The outer surface is curved, and marks left by the drill bit obscure the details. First I saw the core in half with a diamond-bladed rock saw and polish the resulting flat surface of one half. Many sedimentary structures and fossils, previously impossible to observe, are revealed.

Then the other half of the core is dissolved in a weak acid solution. The rock matrix will dissolve, but many of the fossils, clay minerals and quartz sand grains will not. From this acid residue I remove any large fossils and, under a microscope, carefully pick out the microfossils. Microfossils are small fossils that include the tests, or shells, of single-celled animals, small fish teeth and scales, small brachiopods, sponge spicules and conodonts. I am particularly interested in conodonts, small, teeth-like elements from an early vertebrate possibly related to the hagfish or lamprey.

Conodonts are a very useful correlation tool, as the Devonian is divided into 63 conodont zones. By identifying the conodonts in a sample and matching them to a conodont zone, it is possible to make detailed correlations with rocks across North America and the world.

The deep tunnel cores provide a near-complete section through the Devonian rocks of Milwaukee. While the section is very long, it is also quite narrow (only 2.4 inches in diameter), so few large fossils will be found. However, the continuous nature of the core makes it possible to collect many conodont samples and identify sea-level changes. I will have to rely on the outcrop at Estabrook Park for brachiopod and other large fossil collections. By correlating sections from the outcrops to the core I hope to be able to complete my picture of paleocommunities and sea-level changes in Milwaukee and start comparing them to Iowa and Michigan.

Nevada

In 1992 I examined Devonian rocks and brachiopods in cast-central Nevada. In comparison to Wisconsin, Nevada is a very different place to do geologic fieldwork. There are few flowing rivers to watch, and no maple or oak trees to provide shade from the fierce sun. Jack rabbits leap out of your way, small lizards scurry over rocks, scorpions hide underneath them, and the occasional rattle from the tail of a rattlesnake keeps your attention riveted to the ground.

The author collecting brachiopods at Dutch John Mountain, Nevada.
Taking notes while measuring a section at Fox Mountain, Nevada.

However, the rock exposures are spectacular and everything is exposed with only a few sagebrush, Joshua trees and cacti to avoid. Thousands of feet of rock are accessible, as opposed to only 12 feet in Milwaukee. Whole mountains are made of Devonian-aged rock, sandstones, limestones, shales, conglomerates and cherts. I ignored most of it and focused on one formation, the West Range Limestone. This formation correlates with the Late Devonian, and it was deposited after the Devonian mass extinction. The brachiopods and conodonts I collected were related to the lucky ones that had somehow managed to survive the mass extinction.

The rocks were deposited on the edge of a large basin, 60 square miles in area. The seven sections I measured were each over 500 feet thick. I measured them foot by foot instead of inch by inch. This one formation in Nevada was twice as thick as all the Devonian formations in Milwaukee. Again, I used conodonts to correlate all the sections. The base of the West Range Limestone correlates to the upper part of the Antrim Shale in Milwaukee.

Measuring a section of West Range Limestone at Fox Mountain.
Collecting Brachiopods from the West Range Limestone at Alamo, Nevada.

What I concluded was that the West Range Limestone represented one deepening event. Throughout the section the depositional environment gradually changed from a shallow-water environment at the base of the West Range Limestone to a deep-water environment at the top of the formation. The brachiopod and conodont faunas also reflected this change, with a shallow-water fauna occurring in the lower half of the formation and a deep-water fauna occurring in the upper half.

Canada

Recently I had a chance to study Devonian fossils high in the Canadian Rockies with a team of scientists sponsored by the National Geographic Society. The group was investigating global climate changes that occurred during the Devonian Period. Again, fieldwork was very different in the Canadian Rockies. We had to hike through miles of pine and spruce forests-, deer and elk were everywhere and we were always wary of a possible encounter with a grizzly bear. Snow remained in the shadows and gullies of the mountains, and the weather was variable with warm, bright, sunny mornings, cold, strong winds coming down from the mountains, and scattered thunderstorms and hailstorms forcing us to take cover during the afternoons. The exposures were excellent, high above the pine forests on bighorn sheep trails.

Campground at base of Mount Stelfox, Alberta, Canada.
Big Horn sheep near Cripple Creek, Alberta, Canada.

Thousands of feet of Devonian rock are exposed. Our study, however, concentrated on just one formation, the Mount Hawk Formation. This formation was deposited during the Late Devonian, just before the mass extinction. It correlates with the lower and middle parts of the Antrim Shale in Milwaukee. We examined two sections of the Mount Hawk Formation. The first section, measured at Cripple Creek, was mostly clean limestones and dolostones, containing many brachiopods and corals. These brachiopods lived in a very clear, shallow tropical sea. The second section, measured on Mount Stelfox, was composed of carbonates and shales. The rocks and fossils here were deposited in a deep-water channel evidenced by the many different types of brachiopods and lack of corals. Ammonites, a shelled animal related to the modern-day octopus, were also present.

The Cripple Creek section in Alberta, Canada.
Sunset from our Campground at base of Mount Stelfox, Alberta, Canada

At Mount Stelfox we measured a 400-foot-long section of rock and took paleo-magnetic samples every one and a half feet. These samples will be processed in a lab and their magnetic fields measured. This is a new technique which we hope will provide information on the climate at the time that these rocks were deposited, and may also prove useful as a correlation tool. All the samples we collected-paleo-magnetic, brachiopods and conodonts-had to be carried back down the mountains in our backpacks, which made for a long hike back to our campgrounds, but spectacular sunsets rewarded us when we got back to our tents.

The Big Picture

The Devonian rocks from each of the three areas I have written about are separated by thousands of miles and millions of years. All of these rocks have their own separate stories to tell, yet they are all part of a much larger story. As my research continues in the Midwest I hope to be able to fit Wisconsin's Devonian rocks and fossils into a more complete picture: one that includes Iowa and Michigan, and one that will help explain why the brachiopod communities changed and why some brachiopods were doomed to extinction and others flourished. Maybe one day this research will link with another geologist's research on why the sea, level rose and fell. Ultimately as more research is done and linked together, an ever bigger and more detailed picture of the Devonian world can be formed.

Microfossils collected from the acid residue of core sample Loc. 11131-11135 recovered from 233 feet below the surface of Milwaukee in the Deep Tunnel borehole I30-8-NS.

The outside, unpolished view of a piece of core from MMSD's Deep Tunnel Project and the same section of core cut and polished. The specimen number is Loc. 11024 and it was recovered from a depth of 198 feet in borehole I30-8-NS. This borehole is located on the northeast corner of the University of Wisconsin-Milwaukee's campus.