How old is coconino sandstone

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In the Press. DIYSci Activities. September 18, Interestingly, underwater sand dunes that commonly occur today on the continental shelves have dips comparable to what we found in the Coconino.

In modern deserts, sand grains especially the larger ones become rounded as wind blows them about. We collected over different Coconino samples from the breadth and depth of the formation and measured their rounding using a standard tool among geologists—a petrographic microscope. The most rounded Coconino grains, found at Chino Point, had a rho value of only 3. The most angular grains, found at Warm Springs Canyon, had a rho value of 2.

In most cases, the Coconino can best be described as moderately sorted or poorly sorted, based on over samples studied under the microscope. When most people look at the Coconino grains, they use a low-power hand lens as they walk along a hiking trail. Very few have collected samples and cut thin sections for microscopic study. Secular geologists assume that the Coconino sand formed on top of dry, cracked mud.

Underneath the Coconino Sandstone are the brown siltstones of the Hermit Formation, made of mud turned into rock. In many places at the top of the Hermit Formation, the mud has cracks filled with sand Figure 6. The story goes that the Hermit Formation was deposited across a giant river floodplain. The river then dried up, causing the mud to dry and crack.

Subsequently, the Coconino desert blew in, filling the cracks with sand. The observable facts. If wind gradually filled the deep cracks with sand, layer after layer, the fillings should have horizontal layers in them.

But when we studied these features, we could not find any evidence of horizontal layering, and only occasionally did we find vertical layering within the cracks. Further study revealed a very different story. Over a wide area, the cracks had a pattern like you would find resulting from a massive earthquake.

The cracks were deepest near the Bright Angel Fault, a major rupture in the earth that cuts right through Grand Canyon. We found that the cracks shortened and eventually disappeared as we moved away from the fault, probably because the energy from the earthquake was dissipating.

Based upon these data and other features, we have interpreted the sand-filled cracks as a special phenomenon known as sand injectites 12 that form in much the same way in which oil companies perform hydraulic fracking. During intense earthquakes, high-pressure, water-saturated sand flows like a fluid it becomes liquefied. Our team concluded that an earthquake caused the sand at the bottom of the Coconino to become liquefied and flow forcefully down into the Hermit Formation, making the sand-filled cracks.

One of the most interesting things about these footprints is that the animals were almost always traveling up the dune slopes Figure 7. Climbing to escape rising floodwater would explain these features. Leonard Brand, 13 who has done the most field work on these footprints, has also done laboratory studies of salamanders walking on various types of sand—dry, wet, and underwater.

The experimental tracks that best matched the Coconino tracks were made underwater. Flowing water would also explain the sudden appearance and disappearance of many tracks, as the currents picked up animals and they landed in new places. These rare features in the Coconino are often interpreted as fossilized raindrop prints that could have formed only during desert squalls over dry sand. They penetrate up to 1 cm through the sand, or are formed by small round nodules that have fallen out of the rock Figure 8.

In deserts, the wind picks up sand grains and forcefully throws them against each other. The same thing happens to glass when sand is blown against it. Coconino grains are quartz, a mineral both abundant and durable. When rocks weather and erode, many minerals may be present in the resulting sediment.

Some of these minerals dissolve; others erode to dust. Usually, after blowing in the wind for a very long time, only the sturdy quartz grains remain. These dune quartz grains are very mature: uniform composition, small sand size, very well rounded, and all grains about the same size. Sand transported by water never reaches this degree of maturity because water transports sand more slowly than wind, causing less abrasion.

To turn a sediment into rock requires a natural glue: silica and calcite cements are most common.