File:Moqui Marbles (ironstone concretions, Late Oligocene, about 25 Ma; southern Utah, USA.JPG

“Moqui Marble” ironstone concretions (each is approximately 2.5 to 3 centimeters in diameter) from southern Utah, USA

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.