The Navajo Sandstone is a hot new reservoir target in Utah, so you'll want to know about the blueberries on Mars.
Didn't expect to read THAT sentence, did you?
But there's a direct connection, according to professor Marjorie Chan, chair of the Department of Geology and Geophysics at the University of Utah in Salt Lake City.
During the astrogeology theme session at the AAPG Annual Convention in Calgary, Chan will discuss "Analogs of Earth Marbles to Mars Blueberries."
Okay, that requires some explanation.
Concretion = Chemistry
It all started eight years ago, as Chan took a sabbatical to do field work in the spectacular national and state parks of the Colorado Plateau in the western United States, where water, wind and time have sculpted sandstone into fantastic shapes.
Canyonlands, Arches, Capitol Reef and Bryce Canyon national parks and the Grand Staircase-Escalante National Monument were all within easy reach.
"I was doing work in Moab, and a friend there who'd done a lot of geology on a lay basis started showing me some things in the field," Chan said.
He took her to see some odd hematite-cemented "pipes" -- rock columns sticking up from the ground.
Next Chan examined iron-oxide concretion "marbles," which were scattered across the Navajo Sandstone in parts of the Grand Staircase-Escalante National Monument.
The marbles ranged from golf ball- to pea-size, and were usually round but sometimes irregular.
"One feature that interested me was their variability," Chan said.
The origin of the marbles and the geologic mechanism for forming them, however, remained a puzzle.
Chan knew that to unravel the mystery of the marbles she would need to understand their geochemistry, so she called in help from Bill Parry, a geochemist on the university faculty.
"Right away, I knew the "C" word (chemistry) was going to be involved," she said.
"An important clue came from the sandstone's rich coloration," Chan said. "Pink Navajo Sandstone contains 1 to 2 percent hematite (Fe2O3).
"When there is a higher concentration of iron oxide cement (5 to 25 percent), the sandstone often looks deep brownish-red," she continued. "Different cement minerals can impart a rainbow of colors to the sandstone. But where the marbles formed, the rock was usually bleached to near white color."
The grains of sand that make up the sandstone are mostly colorless quartz.
Chan and Parry knew the sandstone's reddish color came from thin hematite films coating the quartz grains.
"The Navajo Sandstone was deposited by wind as dunes migrated across a desert," she said. "Weathered silicates release iron that ends up in the thin grain coatings at the time of deposition or soon after burial.
"We formed a hypothesis that reducing waters moving through the rock later removed and remobilized the hematite coatings to bleach the sandstone white," she added. "Whenever the reducing waters carrying the mobilized iron met oxidizing waters, the iron immediately precipitated out."
The iron oxides (e.g., hematite and geothite) formed concretions in the sandstone, producing the buried marbles and other shapes, Chan believes. Erosion of the Navajo Sandstone has exposed, and often releases, the hard cemented marbles.
Why such round shapes?
That's not completely understood, though Chan noted that spheres "are the easiest form to produce in nature -- especially in eolian sands, where the deposits are highly porous and permeable."
They Found Their Thrill ...
Now jump ahead a couple of years, from the red sandstones of Utah to the red planet of Mars, when analysis of spectrographic data revealed a large area of hematite on the Martian surface.
That finding intrigued Jens Ormö, one of Chan's research collaborators.
"He told me, 'I think we should look at this to help explain the hematite on Mars,'" she said.
So a team of researchers already was studying concretions as a possible source of Martian hematite when the first photographs arrived from the Opportunity and Spirit Mars rovers (see March EXPLORER).
Opportunity sent back photos showing spheroids embedded in bedrock on the eroded surface of Mars. NASA scientists quickly dubbed them "blueberries" because of their spacing, like blueberries in a muffin.
"As soon as we saw those, we said, 'Oh, there's been groundwater on Mars.' We can even tell certain things about the properties of the rocks," Chan said.
In fact, Chan said the Martian spherules were "somewhat expected," given the model of marbles in Utah -- but they were still thrilled by the rover discoveries.
"Some people say it just kind of blows your socks off when you see the similarities," she said.
"Blueberries" photographed by the Opportunity rover were about half a centimeter or less in diameter, smaller than many Earth marbles, she said.
The Spirit rover later sent photos of more hematite nodules from the other side of Mars.
Spacing shown in the Opportunity photos, like berries embedded in a muffin, could be a key indicator of origin, according to Chan.
"The in situ distribution having some self-organizing spacing is important, because depositional mechanisms typically place grains or nodules in a bed touching each other," Chan said.
"This spaced-out distribution is characteristic of concretions formed by the secondary, diagenetic movement of fluids through the porous host rock," she added.
Wonderful World of Color(ation)
Now back to Utah -- but 200 million years ago.
During the Jurassic, a giant erg -- a sea of sand dunes -- larger than today's Sahara Desert formed in what is now the western United States.
That sea of sand eventually became the Navajo Sandstone, the most porous formation on the Colorado Plateau, up to 2,500 feet thick in places.
And an excellent reservoir rock.
Bleached-out bands in the Navajo sands show past movement of reducing fluid through the rock, according to Chan.
In this case, reducing fluids are hydrocarbons.
For petroleum geologists, the processes that formed marbles in the Navajo Sandstone can help reveal the pattern of petroleum migration from source to reservoir.
"One of the exciting things about all this is that these spherical concretions accrue from hydrocarbons that flush through porous sandstone and mobilize iron," Chan said.
"This model of sandstone coloration and concretions on the Colorado Plateau is a product of hydrocarbon movements, some probably along blind faults of Laramide structures," she added.
Even without its application to Mars, this model holds significant value for petroleum geology on Earth, Chan noted.
"Understanding of the coloration, from the micro-scale of deformation bands up to reservoir scale, can yield important information about fluid migration," she explained.
Mineral age-dating, including potassium-argon analysis and field relationships, suggests that bleaching in the Navajo Sandstone probably began 50-65 million years ago.
Precipitation of the iron concretions may have happened as recently as six to 25 million years ago.
Flow patterns can vary even on a scale of inches, with thin red layers of sandstone alternating with bleached white layers. This coloration points to microscopic variations in rock texture.
Geologists already have asked about the possibility of using sandstone coloration patterns as an exploration tool, according to Chan.
But for her, the hydrocarbon model has special meaning in its application to those blueberries on Mars.
"The hydrocarbon story helped us understand the relationships to develop a model that we can compare to Mars," she said.
"Even though the host rock, chemistry and mobilizing fluid may be a bit different, we've learned some of the process lessons from the terrestrial hydrocarbon model."
Mystery and Methane
Without samples to examine in the lab, the origin of Mars blueberries remains an unproven theory.
Other scientists have put forward competing theories to account for the formation of rock spherules on Mars, Chan acknowledged.
But if true, the Utah analog provides one more compelling piece of evidence that fluid flowed on Mars in the past.
And there might be more.
Large-scale bleaching patterns and apparent "rings" in the Colorado Plateau show similarities to high-albedo rings on Mars, Chan said.
Planetary scientists already are thinking about the possibility that large amounts of methane existed on Mars, she noted.
"It's possible that either precipitation of certain minerals or bleaching from methane can produce these types of high-albedo patterns on Mars, but at this point we cannot say anything conclusive about biogenic methane on Mars, even though the idea captures our imagination," she said.
For now, Chan is content with the link that ties planetary geology to a terrestrial example.
"For me, it was just serendipitous," Chan said. "As geologists, we always are excited when predictive models work."