ÒMOQUI MARBLEÓ CONCRETIONS
The term ÒMoqui MarblesÓ refers to spherical and
subspherical ironstone concretions weathered from cross-bedded quartz
sandstones of the lower Navajo Sandstone Formation (uppermost Triassic to
lowermost Jurassic). Ironstone concretions are ubiquitous in the Navajo
Sandstone and in subjacent & superjacent stratigraphic units from several
localities throughout southern Utah, USA. They range in size from
millimeters to tens of meters. They vary from spherical shapes to tower shapes
to complexly irregular shapes.
Compositionally, Moqui Marble concretions are principally
mixtures of iron oxides (hematite & goethite) and manganese oxides.
The dark brown surface extends not too far into the concretions - broken
samples show pale orangish-brown quartz sandstone inside.
The origin of Moqui Marble concretions has been
debated, but published research (Chan et al., 2004, Nature 429: 731-734)
has demonstrated that the ironstone concretions in the Navajo Sandstone formed
about 25 million years ago (during the Late Oligocene), which strongly
contrasts with the depositional age of the host rock (Triassic to Jurassic).
The concretions formed principally by chemical
reactions within and along moving groundwater fronts. The Navajo Sandstone
is a highly permeable & porous unit (~10-30% porosity), which permits easy movement
of ground water. The resulting ironstone concretions are much harder than
the surrounding quartzose sandstone matrix, so the concretions weather out
preferentially and accumulate in piles or layers (see
photo in the field).
Moqui Marble concretions have attracted renewed
interest lately, and have been specifically cited as excellent Earth-analogues
to the Martian
ÒblueberryÓ concretions photographed in Eagle Crater on Mars by the
American Martian Rover Opportunity.
"Moqui Marble" Concretions (each is approximately 2.5 to 3 cm in
diameter). The broken specimen at right shows an interior of
concentrically iron-banded quartzose sandstone.