Cedar Mesa and White Rim Sandstones

Our main aim today was to examine and, where possible, sample the Cedar Mesa and White Rim Sandstones. Both are Permian cross-bedded sands usually thought to have been deposited in an eolian environment. The Cedar Mesa Sandstone is well exposed in Natural Bridges National Monument, and so we drove to the park and hiked the Kachina Bridge Trail, taking a series of strike and dip measurements on the cross beds as we made our way back up the section (see the photograph left). Outside the park we took further strikes and dips and also collected several samples of both the Cedar Mesa and the overlying Organ Rock Formation, which closely resembles and is probably the equivalent of the Hermit Formation of central and northern Arizona.

Later in the day we drove out to Hite Marina, where the White Rim Sandstone can be seen overlying the Organ Rock Formation. It starts as a thin layer and rapidly thickens until in the Orange Cliffs, near the Colorado River crossing, it is a prominent cliff-forming unit overlain by the Moenkopi Formation. We traced the White Rim along the highway to a roadcut where these formations could be observed by the roadside. We then sampled the whole section from the top of the Organ Rock Formation, right the way through the White Rim Sandstone, and into the Moenkopi. Strikingly, the base of the Moenkopi was marked by a thick, matrix-supported breccia, with large angular clasts of many different kinds. We think this represents some kind of debris flow deposit. Also, we were intrigued by several horizons of sand-filled cracks in the lowermost Moenkopi. Close inspection revealed that many of these cracks connected to sand horizons at their base but not at their tops, and that the overlying sediments were slightly deformed, indicating that the sand was mobilised upwards (see the photograph right). That would seem to rule out the idea that they are desiccation cracks, as might be supposed, but instead they are probably sand injectites of some kind.

We’ll be back looking at the Cedar Mesa and White Rim Sandstones tomorrow.