“MOQUI MARBLE” CONCRETIONS
The term “Moqui Marbles” 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
“blueberry” 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.