Unlocking Earth's Secrets

EARTH'S SECRET PAST

GEOLOGISTS SLEUTH FOR CLUES IN THE GRAND CANYON'S 'BACK ALLEY' text and photographs by GARY LADD

Geologists have funny feet. I know this because I've seen many geologists and their poor "dogs."

This morning much carnage exists in the wake of yesterday's loop hike from our river trip camp to the top of Nankoweap Butte, a 10-mile, 2,600-foot scramble, and return to camp. Tape, foam and moleskin appear everywhere as Ph.D. faculty, graduate and undergraduate stu-dents all practice the art of blister control. We're going back to Chuar Valley today, some of us to the summit of Nankoweap Butte again.

What's so interesting in Chuar Valley? Simple. A lot of great rocks. And a primordial mystery waiting to be solved.

Chuar Valley remains a devil of a place, known well by only a handful of hardened canyoneers. In the 19th century, it attracted horse thieves funneling stolen stock from one Rim to the other. Today it appeals to a somewhat higher grade of professional, the geologist.

From high on the Rim, the 15-mile-long Chuar Valley, camouflaged in muted colors and rolling terrain, draws little notice. From the Colorado River, the valley crouches behind a curtain of buttes and mesas where few river travelers suspect a parallel universe lies behind the western escarpment. It's the back alley of the Grand Canyon.

In the alley's open terrain hide thousands of feet of ancient, tilted strata revealed nowhere else in the Grand Canyon - and, in fact, nowhere else on Earth. This precious package of rock is called the Chuar group.

Preserved within the Grand Canyon lies one of the best geologic records in the worldmore than 1.7 billion years of Earth's history. Gaps, called un-conformities, riddle the record, due to periods of erosion that erased the traces of previous environments.

John Wesley Powell, the Grand Canyon's first river explorer, named the primary gap the Great Unconformity. And great it is: Most everywhere in the Canyon the base-ment rock, Vishnu schist, is overlain by the Tapeats sandstone along a visually unpretentious paper-thin contact. This is the Great Unconformity. Its celebrity comes not from what it is but from what it isn't: It isn't the record of 1.2 billion years, the time that passed between the creation of the schist and the overlying sandstone.

Powell's Great Unconformity represents a stony silence. Beneath it, in the dark schist of the Granite Gorge, life was bogged down in its most primitive forms - slimes, bacteria and algae about 1.7 billion years ago. Yet, immediately above it, in the Tapeats sandstone, life teemed in astonishing diversity about 500 million years ago. The stunning flowering of life was so dramatic it is known today as the Cambrian explosion, a geologic period of such exu-berance that it dwarfed everything in the Earth's previous 4 billion-year history. Not only did the ancestors of most of today's lifeforms appear, so did many others that failed to prosper. It was as if life suddenly went crazy with experiments. Some worked, most didn't. Our ancestors, and those of all our Earth-mates, were the survivors. The time of the trials 540 million years ago.

It was not until shortly after the mysterious proliferation of life, in fact, that animals first left the womb of the oceans to creep onto the continents for the first time. Ninety percent of Earth's years to date had slipped by before life colonized dry land. Until then, no trees, no grasses, no bugs and certainly no cows or politicians lived or crept upon the land. The reason for the sudden flowering of life, after billions of years of bumbling, re-mains a mystery. Why then? What hap-pened in thosemissing years? Was it a plant-induced increase in atmospheric oxygen? Was it the migration of most of the world's land masses away from the frozen South Pole toward the more hospitable equatorial regions? My geologist friends don't know. But they'd sure like to.

Between the Vishnu and the Tapeats, sheaves of pages of rock history, covering 1.2 billion years, have been ripped from the record, pages that would have told of seas and landscapesthat lay here long before the Grand Canyon, environments that might shed light on the explosion enigma.

Think of it, one-quarter of northern Arizona's history gone without a trace, a period more than four times longer than all the millennia represented in all of the overlying 4,000 feet of Grand Canyon strata. But there's a saving grace: a clue called the Chuar group.

History written in Chuar group rocks strata protected in wedge-shaped pockets between the Vishnu and Tapeats constitutes a mother lode. These rocks flash back to the prelude to the Cambrian explosion, from about 700 million to 900 million years ago.

Chuar group rocks, despite their great age, look much today as they did then, silts and sands deposited and massaged by ocean currents. Geologists, carefully measuring and describing the 5,000-foot-thick Chuar sequence, divided it into two formations the Galeros and Kwagunt each of which, for convenience, is further subdivided into members.

Strata within Chuar Valley include from oldest to youngest the Nankoweap, Tanner, Jupiter, Carbon Canyon, Duppa, Carbon Butte, Awatubi, Walcott and Sixty Mile. All but the first and last are part of the Chuar group representing a continuous period of deposition near the shore of an ancient ocean.

I found the rock strata names difficult to recollect until Joel Pederson, a geologist who studies canyon-cutting events, constructed a handy mnemonic, which suggested to me that geologists also possess at least a rudimentary interest in biological processes. Here it is, from oldest to youngest, Nankoweap to Walcott: "Never Touch Juicy Cow Dung Before A Walk."

me that geologists also possess at least a rudimentary interest in biological processes. Here it is, from oldest to youngest, Nankoweap to Walcott: "Never Touch Juicy Cow Dung Before A Walk."

Up in Chuar Valley on the second day, the team splits up to pursue several differ-ent but related projects. Karl Karlstrom and Laura Crossey from the University of New Mexico, who assembled the research group, resume their studies of the Sixty Mile formation on the summit of Nankoweap Butte.

There, in 1999, Mike Timmons sampled a one-inch-thick layer of volcanic ash at the base of the formation that could be traced to an eruption 742 million years ago. That was the first time a member of the Chuar group had been dated with accuracy.

When I join Karlstrom, Crossey and others on Nankoweap Butte, I take time to scout the scenery. The panorama includes the North Rim's Point Imperial, at 8,803 feet, the highest Rim viewpoint at the Grand Canyon. From near there in 1882 Charles Doolittle Walcott first examined the unusual, tilted Chuar rocks. Walcott, a paleontologist, worked under the direction of John Wesley Powell, who by then had become the U.S. Geological Survey director.

Walcott and his small team searched valiantly for fossils in Chuar Valley. They constructed the Nankoweap Trail into the Canyon, built passable routes between side canyons and deciphered the complex movement of the Butte Fault. But in 10 weeks, they found very few fossils, a puzzling outcome.

The problem: The ancient rocks looked far younger than their actual age; Walcott sought lifeforms that did not yet exist. However, he did find a new fossil he named Chuaria, a lifeform that hinted at the coming Cambrian diversification of life.

Years later, Walcott became secretary of the Smithsonian Institution and, in 1909, made startling discoveries in Canada's Burgess Shale, a rock unit seemingly laid down a few weekends after the Cambrian explosion.

Everyone in our group holds Charles Walcott in high regard. I mention Walcott's name to Arlo Weil, our ex-

Pert

Pert in paleomagnetism working out past positions of the wandering continents, and Arlo instantly whoops, "He's the Man!"

Atop Nankoweap Butte, we discuss the implications of the three-quarter-billionyear age for the volcanic strata at the base of the Sixty Mile. The date remains important because it likely was at about that time a mighty supercontinent, composed of essen-tially today's continental material, broke apart, its pieces dispersing far and wide. If correct, this interpretation becomes part of a larger story extending back an additional 500 million years to the Unkar group rocks resting directly below the Chuar group (and atop the Vishnu schist) that record the time of assembly of the supercontinent.

Unlike the Chuar group, the Unkar can be found in several sites in the Grand Canyon, often quite conveniently along the shore of the Colorado River. It may be that the old supercontinent Rodinia (from the Russian word meaning "motherland") was first assembling as the youngest of the Unkar group rocks were settling into place.

Some evidence suggests that in Unkar Time, Australia and Antarctica were cozied up against the west coast of North America. The more we know about the Unkar and Chuar groups, the more we