Navajo Sandstone is a hot new reservoir target in Utah, so you'll
want to know about the blueberries on Mars.
expect to read THAT sentence, did you?
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.
the astrogeology theme session at the AAPG Annual Convention in
Calgary, Chan will discuss "Analogs of Earth Marbles to Mars Blueberries."
requires some explanation.
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.
Arches, Capitol Reef and Bryce Canyon national parks and the Grand
Staircase-Escalante National Monument were all within easy reach.
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
her to see some odd hematite-cemented "pipes" -- rock columns sticking
up from the ground.
examined iron-oxide concretion "marbles," which were scattered across
the Navajo Sandstone in parts of the Grand Staircase-Escalante National
ranged from golf ball- to pea-size, and were usually round but sometimes
that interested me was their variability," Chan said.
of the marbles and the geologic mechanism for forming them, however,
remained a puzzle.
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.
away, I knew the "C" word (chemistry) was going to be involved,"
clue came from the sandstone's rich coloration," Chan said. "Pink
Navajo Sandstone contains 1 to 2 percent hematite (Fe2O3).
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
of sand that make up the sandstone are mostly colorless quartz.
Parry knew the sandstone's reddish color came from thin hematite
films coating the quartz grains.
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.
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."
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
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."
Their Thrill ...
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.
intrigued Jens Ormö, one of Chan's research collaborators.
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).
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 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 the Martian spherules were "somewhat expected," given
the model of marbles in Utah -- but they were still thrilled by
the rover discoveries.
say it just kind of blows your socks off when you see the similarities,"
photographed by the Opportunity rover were about half a centimeter
or less in diameter, smaller than many Earth marbles, she said.
rover later sent photos of more hematite nodules from the other
side of Mars.
shown in the Opportunity photos, like berries embedded in a muffin,
could be a key indicator of origin, according to Chan.
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.
distribution is characteristic of concretions formed by the secondary,
diagenetic movement of fluids through the porous host rock," she
World of Color(ation)
to Utah -- but 200 million years ago.
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.
of sand eventually became the Navajo Sandstone, the most porous
formation on the Colorado Plateau, up to 2,500 feet thick in places.
excellent reservoir rock.
bands in the Navajo sands show past movement of reducing fluid through
the rock, according to Chan.
case, reducing fluids are hydrocarbons.
geologists, the processes that formed marbles in the Navajo Sandstone
can help reveal the pattern of petroleum migration from source to
the exciting things about all this is that these spherical concretions
accrue from hydrocarbons that flush through porous sandstone and
mobilize iron," Chan said.
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.
its application to Mars, this model holds significant value for
petroleum geology on Earth, Chan noted.
of the coloration, from the micro-scale of deformation bands up
to reservoir scale, can yield important information about fluid
migration," she explained.
age-dating, including potassium-argon analysis and field relationships,
suggests that bleaching in the Navajo Sandstone probably began 50-65
million years ago.
of the iron concretions may have happened as recently as six to
25 million years ago.
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.
already have asked about the possibility of using sandstone coloration
patterns as an exploration tool, according to Chan.
her, the hydrocarbon model has special meaning in its application
to those blueberries on Mars.
story helped us understand the relationships to develop a model
that we can compare to Mars," she said.
the host rock, chemistry and mobilizing fluid may be a bit different,
we've learned some of the process lessons from the terrestrial hydrocarbon
samples to examine in the lab, the origin of Mars blueberries remains
an unproven theory.
have put forward competing theories to account for the formation
of rock spherules on Mars, Chan acknowledged.
true, the Utah analog provides one more compelling piece of evidence
that fluid flowed on Mars in the past.
might be more.
bleaching patterns and apparent "rings" in the Colorado Plateau
show similarities to high-albedo rings on Mars, Chan said.
scientists already are thinking about the possibility that large
amounts of methane existed on Mars, she noted.
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,"
Chan is content with the link that ties planetary geology to a terrestrial
it was just serendipitous," Chan said. "As geologists, we always
are excited when predictive models work."